摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(9), 1301-1306 I8O Tracer Studies of CO Oxidation with 0, on MOO, Part 1.-Diffusion of *O Atoms from Active Sites during the Catalysis and the Determination of the Number of Active Sites Yasuo lizuka Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto, 606,Japan The catalytic oxidation of CO with l80,has been carried out on MOO, at 753 K. The isotopic composition of the product CO, was examined as a function of time. The amount of C '*02was always negligible. The percentage of C'80'60 increased rapidly at the beginning of the reaction then gradually reached a plateau value. Inter- ruption of the oxidation did not significantly affect the percentage of C'80160in the CO, produced.Oxygen vacancies produced by the reduction of MOO, with CO apparently remained unchanged on the surface after evacuation for 120 min at 718 K. The incorporation of 0, into oxygen vacancies took place rapidly. A small fraction of "0 atoms taken up into oxygen vacancies were recovered in subsequent CO reduction processes, while the greater part of the l80diffused into the bulk of the MOO,. The catalytic oxidation of CO with l80,op MOO, could be explained in terms of mobile and immobile sites with regard to ''0 atoms taken up into vacancies of active oxygen. This model enables calculation of the time dependence of the yield of C l80l60in the product CO, and estimation of the number of immobile sites. "0 tracer techniques have frequently been used for eluci- dation of the role of surface oxide ions in the catalytic oxida- tion of CO and hydrocarbons on metal 0~ides.l-l~ Keulks found that only a small fraction of oxygen in oxidized pro- ducts is "0 in the oxidation of propene with "0, over a bismuth molybdate catalyst.' He inferred that oxide ions in several subsurface layers participate in the oxidation, and the diffusion of oxygen from the surface into the bulk and from the bulk to the surface must be rapid.Wragg et al. showed, using '0-enriched gaseous oxygen and "0-enriched bismuth molybdate catalysts, that the oxygen atoms in acrol- ein are derived from the metal oxide in the catalytic oxida- tion of propene., Similarly, the participation of lattice oxide ions in the formation of acrolein was demonstrated in the selective oxidation of propene on multicomponent metal oxidesg and on multicomponent bismuth molybdate cata- lyst~.",'~ The participation of lattice oxide ions was also shown in catalytic oxidation of CO on V205,4 V205 doped with Mo6+,, y-Fe,O, and Pr6011." cussion of the isotope exchange is reported in the following paper. Experimental Analytical-grade MOO, (99.5%) from East Merck Company was used as the catalyst.According to the manufacturer's data, its principal impurities are C1, SO,, Pb, Fe and NH,; each is present at levels < 100 ppm. The specific surface area of MOO, was determined to be 0.40 m2 g-' by the BET met hod. CO, 0, and CO, , obtained commercially, were purified by fractional distillation at low temperature."0, ("0-atom fraction, 98.6%) from Commissariat a L'Energie Atomique was used without further purification. All reaction mixtures of CO and 0, or ''0, had a molar ratio of CO : 0, of 2 : 1. The details of the procedures employed have been described previou~ly.'~.' However, the apparatus was modi- fied slightly in order to combine the isotope tracer technique Krenzke and Keulk~,~ with kinetic measurements, as shown in Fig. 1. The circula- and Moro-oka and co-worker~~.~~ determined the amount of lattice oxide ions entering the oxi- dized products from variations in the "0 content in the oxi- dized products with the progress of the catalysis. However, the number of surface oxide ions that are active sites in the course of the catalysis has rarely been determined to date.The kinetics and mechanism of CO oxidation with 0, on MOO, were elucidated in our previous paper by using a usual kinetic method combined with electrical conductivity measurements of the catalyst during the The object of this paper is to determine the number of surface oxide ions that are active sites during the catalysis of CO oxidation with 0, on MOO,. An l80tracer technique was combined with the kinetic method for this purpose. Two problems are encountered when estimating the number of active sites from the variation in the "0 content in C02 with the progress of the catalytic oxidation of CO with "0,. One problem is the diffusion of l80atoms from active sites into the bulk during the catalysis and the other is the occurrence of oxygen exchange between "0 atoms in the product CO, and surface lattice l60oxide ions of the catalyst. The diffu- sion of ''0 from active sites into the bulk is considered in the present study.Oxygen isotope exchange between CO, and surface lattice oxide ions is briefly described. A detailed dis- tion loop of a reaction mixture had two pathways. One passed through metal cocks B and C, and trap TI; another passed through metal cocks F and G, and trap T, . Hereafter, the former is referred to as the T, pathway and the latter as the T, pathway. A quadrupole mass spectrometer (ULVAC MSQ-150) was connected to the T, passway through a Granville-Phillips variable-leak valve.MOO, powders (1.0-9.9 g) were pretreated at 800 K with circulating 0, (ca. 7.0 kPa) for ca. 3 h. Subsequently, the catalyst was cooled to the reaction temperature and evac- uated. Then a reaction mixture of CO and 0, (ca. 5 kPa) was admitted into the loop (TI pathway). The product CO, was always condensed in trap T,, which was cooled with liquid nitrogen. Having confirmed the establishment of steady cata- lytic activity, the mixture was removed from the loop. Fresh MOO, was usually subjected to this pretreatment prior to catalytic studies. In a typical isotopic experiment, a mixture of CO and I8O2 (1.33 kPa) was introduced into the loop. T, was always cooled with liquid nitrogen, while T, was cooled, if necessary. The variation of the isotopic composition of CO, with reac- tion time was examined at regular intervals (usually every 20 min) by switching the circulation loop from TI to T, for a J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Fig. 1 Schematic representation of the reactor portion of the recir- culatory reaction apparatus. The circulation loop has two circulation pathways. A-H, metal stop cocks; QR, quartz tube reactor; EF, elec-tric furnaces; PG Pirani vacuum gauge; CP, circulation pump; M, U-shaped mercury manometer; TI-T, ,liquid-nitrogen-cooled traps ; V, variable-leak valve; IP ion pump; QM, quadrupole mass spec- trometer. fixed period (usually, 2 min) and analysing the CO, con-densed in trap T, mass spectrometrically. During the cata- lytic reaction, the pressure was maintained at 1.33 kPa (_+3%) by supplying an adequate amount of the mixture through cock E. Before the next isotopic experiment, the catalyst was evacuated at 800 K and exposed to a mixture of CO and 0, for 5 h until the l80content in the product CO, became negligible.The reaction system was changed into a static one for the examination of isotope exchange between CO, and MOO,, where the static system was enclosed with metal cocks A, B, G and H (Fig. 1). The catalyst was exposed to CO, contain-ing l80. The CO, used was obtained from the oxidation of CO with “0, in the course of the catalysis. The isotopic composition of CO, was followed at intervals of 15 min. The consumption of CO, in the mass analyses was negligible compared to the total CO, in the system. Electrical conductivities of the catalyst were measured as described previ0us1y.l~ Rates of uptake of 0, into the reduced catalyst were determined from pressure changes mea- sured with a Pirani gauge.Traps T, and T, were cooled con- tinuously by using liquid nitrogen or a dry ice-acetone mixture to protect the catalyst from contamination due to grease and mercury vapour. Results CO Oxidation with ‘*O,on MOO, and Oxygen Isotope Exchange between CO, and MOO, The catalytic CO oxidation with 1802on MOO, at 753 K was performed for 122 min at a total pressure of 1.33 kPa. The yield of C 1802 in the product CO, was always negligi- ble in comparison to those of C l80l60and C 1602.The yield of C l80l60is plotted against reaction time in Fig. 2. Almost all CO, was C 1602at the beginning of the catalysis. The percentage of C “0 l60increased rapidly with time in the early stages of the catalysis but showed a tendency to saturation, with a plateau value of ca. 13%. The time depen- dence of the yield of C l80l60on the left-hand side of Fig. 2 I I I 0 30 60 90 120 150 180 210 240 t/min Fig. 2 Variation of the yield of C l8Ol6O in CO, as a function of time. The catalysis between CO and “0,on MOO, was performed at 753 K, maintaining the pressure of the reaction mixture at cu. 1.33 kPa for 122 min. CO, collected for 2 rnin at the end of the reaction was exposed to the catalyst after evacuation at the same temperature.was independent of the amount of catalyst (1.0-9.9 g) as long as the pressure of the mixture was maintained at 1.33 kPa. The product CO, was collected in trap T, for 2 rnin at the end of the catalysis, i.e., from 120 to 122 min. The amount of oxygen in the CO, collected corresponded to ca. 1% of the total surface lattice oxide ions. The total number of surface lattice oxide ions was calculated from the surface area of MOO, and the number of oxide ions per unit surface area of MOO,, i.e. 1.7 x 10’’ rn-,.14 The fraction of C l80l60in the CO, was ca. 8%. After evacuation of the catalyst, the CO, in trap T, was vaporized and passed over the catalyst at 753 K. The variation of C l80l60yield with time is shown on the right-hand side of Fig.2. The isotopic composition of CO, barely changed with time during its contact with the catalyst. Time Dependence of the Number of Oxygen Vacancies Previously we reported that the rate of incorporation of oxygen into the reduced MOO, catalyst can be expressed by the equation -d(O,)/dt = k,, Po,(l -O), (1) where k,, is the rate constant for incorporation of oxygen into the catalyst and Po, is the pressure of oxygen. The term (1 -0) denotes the ratio of the amount of deficient active oxygen sites to the total amount of active oxygen atoms on the ~ata1yst.l~ The oxidized catalyst was first reduced at 718 K with circu- lating CO (ca. 2.0 kPa for 30 min) and then evacuated. The amount of oxygen vacancies was 3.6 x mol (3.4 x 10” m-,) from the amount of CO, collected in trap T,.However, the total amount of active oxygen atoms on the catalyst was estimated to be 1.3 x 10l8 m-,, assuming that the total amount agreed with the limiting amount of CO, produced in the reduction of the catalyst with CO for a sufficiently long time.14 The estimated value corresponded to 7.6% of the total surface lattice oxide ions on MOO,. Therefore, (1 -8) was 0.26. The catalyst was then exposed to oxygen (0.54 Pa for 3 min) 14 rnin after the beginning of evacuation in the static system. The pressure of 0, , which decreased rapidly owing to 0, uptake, was measured every 30 s. After the 0, had been completely removed, the catalyst was evacuated for 43 rnin and exposed to oxygen once more (0.54 Pa for 3 min).The decrease in 0, pressure was measured. The same pro- cedures and measurements were repeated after a further evac- uation for 57 min. Fig. 3 shows the decrease in 0, pressure J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 0123456 t/min Fig. 3 Decrease of the oxygen pressure due to incorporation into MOO, that had been reduced by CO at 718 K. Oxygen (0.54 Pa) was admitted for 3 min (n) 14, (b) 60 and (c) 120 rnin after evacuating the co. with time for the three cycles of oxygen dosing. The total times elapsed after starting the first evacuation were 14, 60 and 120 min. Eqn. (1) and the slopes of the straight lines in Fig. 3 gave (1 -6) values at 60 rnin and 120 min. The two (1 -6) values obtained were corrected for oxygen incorpor- ation.Fig. 4 shows the corrected values of (1 -0) at 14, 60 and 120 min. There is no appreciable change in (1 -6) with time, implying that the oxygen vacancies produced by CO reduction are not diminished in number even after 120 min. Electrical Conductivity Changes due to Oxygen Uptake MOO, , previously oxidized with circulating oxygen, was reduced at 713 K with CO (6.6 kPa for 30 min). After evac- uating the reduced catalyst for 10 min, oxygen (0.13 Pa) was introduced into the reactor for 4 min. Then the catalyst was evacuated again for 75 rnin at the same temperature, and sub- sequently exposed to 0, (0.13 Pa for 4 min) again. The same exposure of the catalyst to oxygen was repeated after a further evacuation for 60 min.Fig. 5 shows that the electrical conductivity of MOO, increased significantly after reduction. Evacuation did not alter the electrical conductivity, but introduction of oxygen (0.13 Pa) caused a slight decrease. At the end of the experi- ment, i.e., at 205 min, 0, (2.6 kPa) was introduced into the system. The electrical conductivity of the catalyst returned immediately to the value before reduction. The reduction of active oxide ions on the surface with CO leaves behind charges in the catalyst. The charge left behind may be responsible for the electrical conductivity. However, the decrease in the electrical conductivity is caused by the incorporation of 0, into the oxygen vacancies. 0 30 60 90 120 150 t/min Fig.4 Variation of the deficient ratio of active oxygen, (1 -O), with time -evac evac io 60 90 1;o 150 180 t/m in Fig. 5 Variation of the electrical conductivity of the catalyst during reduction, evacuation and reoxidation of MOO, at 713 K Effect of Interruption on the Catalytic Oxidation of CO with 1802 CO oxidation with "0,was carried out on MOO, at 753 K for 120 min. The pressure of the mixture was 1.33 kPa. After the first evacuation (60 min), a mixture was introduced again into the catalytic system, and the oxidation of CO was per- formed for 70 rnin at the same temperature and pressure as before. The second evacuation was for 110 min, after which catalysis was carried out for 60 rnin under the same condi- tions. Fig. 6 shows the variation of the C "0 l6O yield in the product CO, throughout these cycles.The decrease in the percentage of C'80160 in the product CO, caused by the two interruptions is relatively small and independent of the length of interruption. The percentage of C "0 l60rapidly returned to the level it had attained before the interruption and then increased gradually with time. Recovery of "0 from the Active Sites by CO PulseReduction MOO, was first reduced for 30 min with CO (2.64 kPa) at 753 K. This yielded many vacancies of active oxygen on the surface. After removal of CO, the reduced catalyst was oxi- dized with "0, (2.64 kPa) for 5 min. The results in Fig. 5 show that "0atoms are instantaneously taken up by almost all the vacancies. After continuous evacuation of the sample for 10 min, the catalyst was again reduced with CO (27 Pa) for 5 min to recover "0 from the active sites.The amount of l8O recovered was determined from the amount of CO, col-lected in trap T, and the proportion of C "0 l6O in it. This CO pulse reduction for 5 rnin was repeated twice more at intervals of 30 min. The "0 recovery experiment was per- formed twice on catalysts that had previously been oxidized 30.0 1 1 -CO+'802 co + 1 802 co + 1e02 c 3 evac e--+ evac ( 3 n ( 3 ( ) 20.0-v 0 -.-0 ..---------_--.-. 10.0-9 1 -0.oJ 1 I I b 6.0 E 0 30 60 -90 120 150 180 tlmin Fig. 7 Variation of the amount of l80recovered in each CO pulse reduction with time.Successive CO pulse reduction experiments were started 10,60 and 120 rnin after evacuating the ''0,at 753 K. with 180, and continuously evacuated for 60 rnin and 120 min before starting the CO pulse reduction. Fig. 7 shows the amount of l80recovered in each CO pulse reduction process as a function of the time elapsed after the evacuation of . The amount of l8O recovered decreased markedly with the repetition of the CO pulse reduction. However, the amount recovered after the first reduction was almost inde- pendent of the elapsed time. The total amount of l80recov-ered was only a fraction of the total amount of taken up and was almost independent of evacuation time in each case. Total Amount of Recoverable "0on the Active Sites The CO oxidation with 180, was carried out for 240 rnin on MOO, at 753 K and 1.33 kPa.After the yield of C l80l60 reached saturation, the reaction mixture was removed com- pletely and the CO, in trap T, was pumped off. A mixture of CO and 1602 was introduced into the circulation loop and the catalytic oxidation of CO was performed at 1.33 kPa for >600 min. All the product CO, was collected in trap T,. The catalysis was continued until the amount of C "0 l6O was negligible. The total amount of l80recovered from the active sites was 1.6 x lop6mol (4.0 x 1017 m-') from the amount of CO, in trap T, and its C l80 l60content. Discussion Oxygen Isotope Exchange between CO, and MOO,during Catalysis Miura et al. investigated the catalytic oxidation of CO with 1802 on Bi, Ca, Ni and La molybdate catalysts by measuring l80-atom fractions in CO, .6 The distribution of in CO, was in good agreement with the value calculated on the assumption that isotope mixing between CO, and several layers of surface lattice oxygens of the catalyst was rapidly completed.6 The formation of C l80,was always negligible in the present experiment, suggesting that isotope mixing is slow on MOO,.Our previous study showed that the rate of CO oxidation with 0, on MOO, was independent of the pressure of COZ.l4 This means that the adsorption of CO, on the active sites is extremely weak. The weak adsorption may be responsible for the slow isotope mixing between CO, and the surface oxide ions of MOO,. We confirmed that the time dependence of the C l80l6O yield shown on the left-hand side of Fig.2 was independent of the amount of catalyst. The contact time of the product GO, with the catalyst bed during the catalysis varied with the amount of catalyst. The right-hand side of Fig. 2 shows that the exposure of the CO, produced at the end of the catalysis to the catalyst surface does not change the percentage of J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 C l60in it appreciably. This suggests that the isotopic composition of the product CO, is hardly altered by contact with the surface lattice oxide ions of MOO, in the course of the catalysis. Therefore the observed isotope composition must be that of the CO, just produced at active sites and it reflects the isotopic composition of the active sites.Since the yield of C "0, is always negligible compared to those of C "0 l60and C 1602, we conclude that the percentage of C l80l6O in the product CO, reflects the fraction of l8O in total active oxygen. Diffusion of l8OAtoms from the Active Sites during Catalysis It was reported previously that the catalysis of CO oxidation with 0, on MOO, proceeds by the cyclic repetition of the following two steps: CO(g) + O(S)+CO,(g) + (s) (1) 2(s) + O,(g) 20(s) (11)+ where O(s) refers to active oxygen on the surface of MOO, and (s) refers to an active oxygen mcancy on the ~urface.'~ A CO molecule in the gas phase reacts with abtive oxygen on the catalyst to produce CO,, which is desorbed, leaving an oxygen defect on the surface in step (I).An oxygen molecule in the gas phase is dissociatively and irreversibly adsorbed on two paired defects in step (11). Oxygen adsorption on paired defects is supported by the quantitative relation expressed by eqn. (1). The rate constant of step (11)is much larger than that of step (I). Accordingly, almost all the active sites are covered with oxygen during catalysis [the (1 -0) value for active oxygen is only ca. l-2%].14 The rate of reduction of the active oxygen by CO is dynamically balanced with the rate of uptake of oxygen by the paired active oxygen vacancies at the steady state.I4 The I8O2used was 98.6%pure; however, Fig. 2 shows that hardly any CO, molecules produced at the beginning of the catalytic oxidation contained atoms.Thus the active oxygen with which CO molecules react must be lattice oxide ions on the surface of MOO,. Since the pressure of the C0-180, mixture was always constant during the catalysis, the amount of l80incorpor-ated into MOO, must increase in proportion to the reaction time. Previously we reported that the rate constant for the production of CO, at 753 K is 6.0 x lo-' mmol kPa-' min-' rn-,.14 The amount of l8O atoms taken up during catalysis up to 122 rnin corresponds to 3.6 times the number of active oxide ions on MOO,. The amount of active oxide ions, determined by the CO reduction method, is 1.6 x 10l8 m-' at this temperat~re.'~ If we assume that all active oxide ions work equally well as active sites and do not migrate, the percentage of C l8Ol60in the product CO, should reach 96% after 120 rnin from the beginning of the catalysis.If all surface lattice oxide ions work equally well as active sites and do not migrate, the yield of C l80l6O should be 28% at 120 min. Fig. 2 shows that the yield of C180160 increases rapidly at the beginning of catalysis and levels off gradually. The yield of C l80 l60 is 8% at 120 min, showing that the diffusion of l80atoms from active sites into the bulk must occur simultaneously with the diffusion of l6O oxide ions from the bulk to the active sites. However, the oxide ions at the active sites cannot be in equilibrium with the oxide ions in several subsurface layers in the bulk because (1) the yield of C'80160 shows a tendency to saturation at a value much lower than 98.6%(Fig.2) and (2) the amount of l80in CO, produced in the CO pulse reduction decreased markedly with repetition of the pulse (Fig. 7). If oxide ions in several sub- surface layers equilibrate with those at the active sites and J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 participate equally in the catalysis, the percentage of C l80 l60should reach 98.6% after a sufficiently long reac- tion time and the amount of l80recovered should be inde- pendent of the number of CO pulse reductions. Oxidation and Diffusion of Vacancies of Active Oxide Ions Keulks suggested that in the oxidation of propene with l80, over bismuth molybdate catalysts the adsorption of 1802 occurs elsewhere than the site for propene adsorption.2 Bielanski and Haber stated that oxygen vacancies produced during the catalytic oxidation of hydrocarbon molecules on surfaces of oxysalts such as molybdates and tungstates are reoxidized not only by oxygen molecules from the gas phase but also by oxide ions diffusing from the bulk.16 However, Fig.4 shows that oxygen vacancies produced by CO reduction remain without any diminution on the surface even after 120 min. On the other hand, the adsorption of 0, on reduced MOO, occurs rapidly at 718 K independent of the elapsed time under evacuation (Fig. 3). The adsorption of 0, probably occurs at active oxygen vacancies produced by CO reduction. Fig. 5 shows that a sharp decrease in the conductivity is accompanied by the adsorption of 0, on oxygen vacancies.Eqn. (1) and the decrease of the conductivity suggest that the oxidation of paired vacancies of active oxygen by an 0, mol-ecule takes place according to 02(g)+ 2(s) + 4e --+ 2o2-(s) where the excess charges in the reduced MOO, may be fixed on oxide ions on active sites. On the other hand, Fig. 5 also shows that the electrical conductivity of the reduced MOO, does not change when the catalyst is evacuated. The decrease in (1 -0) is small, as shown in Fig. 4. This probably means that the reoxidation of oxygen vacancies on the surface by oxide ions diffusing from the bulk must be accompanied by the diffusion of oxygen vacancies into the bulk of MOO,.Fig. 4 suggests that this might be an extremely slow process at this temperature. Diffusion of Oxide Ions and Characteristics of Active Sites We can conclude from these results that oxygen vacancies produced during the catalytic oxidation of CO with 1802 are oxidized by 1802 molecules in the gas phase and not by l60 oxide ions diffusing from the bulk. Consequently, Fig. 2 shows that a large fraction of the l80atoms taken up into vacancies of active oxygen must subsequently diffuse from active sites into the bulk. The diffusion of l80must occur prior to reaction with CO and must be accompanied by replacement with l60oxide ions diffusing from the bulk, because a large part of the CO, produced is C 1602 even at 120 min. The diffusion of l80oxide ions into the bulk and their replacement with l60oxide ions does not need to produce vacancies in the bulk.Fig. 6 shows that the interruption of the catalysis does not affect the percentage of C l80 l60in CO, . This suggests that the fraction of l80 on active sites does not change during the interruption. Accordingly, a fraction of atoms taken up into active sites during catalysis must remain at active sites. A greater part of CO, produced in the CO pulse reduction pre- sented in Fig. 7 is C 1602, indicating that a large part of "0 atoms taken up into oxygen vacancies diffuse into the bulk and are replaced by l60oxide ions diffusing from the bulk. However, Fig. 7 also shows that there is a small amount of "0 atoms remaining on the active sites and these can be recovered by the CO pulse reduction. // "0 mobile sites Q c1602is constantly Vtotal < produced "0immobile sites The percentage of C'80'60 varies with time from 0 to 98.6% Fig.8 Reaction model of CO oxidation with "0,on MOO, based on immobile and mobile active sites. The reaction rate is composed of two parts which proceed independently on two kinds of active sites. All the present results can be explained in terms of two kinds of active sites: mobile sites and immobile sites. Fig. 8 shows a schematic diagram for mobile and immobile sites in the catalytic CO oxidation with 180, on MOO,. Both sites contribute independently to the catalytic oxidation of CO. The atoms taken up on mobile sites during the catalysis immediately diffuse into a huge amount of l60oxide ions in the bulk prior to their reaction with CO and, therefore, cannot be recovered during the CO reduction step.In other words, l80 atoms at mobile sites are immediately replaced by l60oxide ions supplied from the bulk. As a result, the mobile sites are constantly covered with l60oxide ions and only Cl60, is produced there. On the other hand, l8O atoms taken up into immobile sites during the catalysis do not diffuse and can be recovered by the CO reduction step, resulting in the formation of C l80l60.Although all immo- bile sites are covered with l60before the beginning of the catalytic oxidation of CO with the percentage of l80 at the immobile sites gradually increases with time and finally reaches 98.6%.Concomitantly the percentage of C l8O l60 in CO, produced on the immobile sites also increases grad- ually and finally reaches 98.6%. The percentage of C l80l60 in the CO, produced on all the active sites increases grad- ually and shows a tendency to saturation at a value much lower than 100% (Fig. 2). Time Dependence of the Yield of C "0 l60and Determination of the Number of Immobile Sites According to the proposed reaction model, the yield of C l8O l60in CO, produced over all the active sites can be expressed as follows : C l80 l60(Yo)= (ratio of the rate of production of CO, at immobile sites to the total rate of production of CO,) x (percentage of in active oxide ions at immobile sites) =RxC (2) The ratio R is estimated to be 0.13 from the percentage of C l80 l60at saturation in Fig.2. The "0 percentage at the immobile sites, C,varies with time. The rate of uptake of l80atoms by immobile sites is equal to the rate of production of CO, at the immobile sites at steady state.14 The rate of consumption of l80atoms at the immobile sites is equal to the rate of production of C l80 l60.The difference between uptake and consumption corresponds to the change in the fraction of l80at the immobile sites. The material balance gives 98.6V dt -VC dt = N dC (3) 15001 0 20 40 60 80 100 120 140 t/min Fig. 9 Time dependence of the C '*O l60yield in CO, calculated according to the proposed reaction model. Various numbers of immobile sites N were assumed and calculated theoretical curves were compared with the observed data (0).(a) 1.0%,(b) 1.8%,(c) 2.0%,(d)5.0%.where N is the number of immobile sites, 98.6 is the percent- age of l80in and V is the rate of production of C02 at the immobile sites. Integration of eqn. (3) yields C = 98.6[ 1 -exp(-Vt/N)] (4) where t is in min and the initial condition of integration is C = 0 at t = 0. The combination of eqn. (2) with eqn. (4) gives the time dependence of the yield of C l80l60 in CO, . The rate of production of CO, at immobile sites, i.e. V can be obtained from the product of the total rate of production of CO, and the ratio R. However, the number of immobile sites, i.e. N, is unknown. We attempted to calculate the time dependence of the yield of C l80l60in CO,, assuming N to be 1.0, 1.8, 2.0 and 5.0% of the total surface lattice oxide ions.The results are com- pared to typical experimental results in Fig. 9. The number of immobile sites N can be estimated accurately because theo- retical curve changes sensitively with N. The best fit was obtained with N = 1.8%. The number of immobile sites is quite small on the surface of MOO,. After the oxidation of CO with "0, has been performed for a sufficient time, e.g. for 240 min on the catalyst, all immobile sites are expected to completely covered with l80 atoms. These l80atoms may be recovered by subsequent oxidation of CO with 1602 for sufficiently long, e.g. >600 min. The total amount of "0 atoms recovered corresponds to the number of immobile sites.The observed amount of l80 atoms recovered was 1.6 x mol (4.0 x 1017 rn-,). On the other hand, the number of immobile sites calculated J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 from the time dependence of the yield of C l80 l60using the method presented in Fig. 9 was 1.7 x mol (4.3 x 1017 rn-,). This agreement not only confirms the model of immo-bile and mobile sites presented in Fig. 8 but also means that "0 atoms incorporated into mobile sites do not participate in the catalytic reaction at immobile sites. The percentage of l80 at immobile sites does not change during the interruption of the catalysis. The solid curve in Fig. 6 shows the calculated result according to the proposed reaction model. The curve reproduces the experimental results fairly well.The percentage of C "0l60 just after each interruption is slightly lower than the calculated curve. However, the extent of lowering is independent of the length of interruption. A small limited portion of l80atoms at immobile sites might be replaced very slowly with l60oxide ions from the bulk during the interruption. The author thanks Professor Shigeyoshi Arai and Plenary Professor Takuya Hamamura of Kyoto Institute of Tech- nology for their helpful discussions and advice. References 1 A. Roiter, Kinet. Catal., 1960, 1, 63. 2 G.W. Keulks, J. Catal., 1970, 19,232. 3 R. D. Wragg, P. G. Ashmore and J. A. Hockey, J. Catal., 1971, 22, 49. 4 H. Kakioka, V. Ducarme and S. J. Teichner, Kinet. Catal., 1973, 14, 78. 5 T. Otsubo, H. Miura, Y. Morikawa and T. Shirasaki, J. Catal., 1975,36,240. 6 H. Miura, Y. Morikawa and T. Shirasaki, Nippon Kagaku Kaishi ,1975, 11, 1875. 7 L. D. Krenzke and G. W. Keulks, J. Catal., 1980,61,316. 8 K. Sakata, F. Ueda, M. Misono and Y. Yoneda, Bull. Chem. SOC. Jpn., 1980, 53, 324. 9 Y. Moro-oka, W. Ueda, S. Tanaka and T. Ikawa, Proc. 7th Znt. Congr. Catal., Tokyo, 1980, ed. T. Seiyama and K. Tanabe, Kodansha, Tokyo, part B, p. 1086. 10 W. Ueda, Y. Moro-oka and T. Ikawa, J. Catal., 1981,70,409. 11 Y. Takasu, M. Matsui and Y. Matsuda, J. Catal., 1982,76,61. 12 I. Brown and W. R. Patterson, J. Chem. SOC., Faraday Trans. I, 1983,79,1431. 13 I. Matsuura, H. Hashiba and I. Kanesaka, Chem. Lett., 1986, 533. 14 Y. Iizuka, Y. Onishi, T. Tamura and T. Hamamura, J. Catal., 1980,64,437. 15 Y. Iizuka, Y. Onishi, T. Tamura and T. Hamamura, J. Catal., 1981,70,264. 16 A. Bielanski and J. Harber, Catal. Rev. Sci.Eng., 1979,19, 1. Paper 3/02846A; Received 19th May, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949001301

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(9), 1307-1312 '*O Tracer Studies of CO Oxidation with 0, on MOO, Part 2.-Active Sites for CO Oxidation with 0, and for Oxygen Isotope Exchange between CO, and MOO, Yasuo lizuka," Haruyuki Tanigaki, Masakazu Sanada, Junji Tsunetoshi, Naruki Yamauchi and Shigeyoshi Arai Depa rtmen t of Chemistry and Ma te ria Is Techno logy , Kyoto Ins titu te of Technology, Matsugasaki , Sakyo-ku, Kyoto, 606,Japan Oxygen isotope exchange between C 1802and MOO, has been examined in connection with the catalytic oxida- tion of C l60with "02on MOO,. Oxide ions at mobile and immobile sites on MOO,, which oxidize CO in the catalytic oxidation, were found to work also as active sites in the oxygen isotope exchange between C 1802and Mo "03.The observed time dependences of the yields of C 1802,C '*O'6Oand C '602in the exchange agreed satisfactorily with those calculated from the mobile and immobile site model.In the calculation, the number of immobile sites was estimated to be 1.8% of the total surface lattice oxide ions. The exchange rate at immobile sites was 13% of the total exchange rate. Catalytic oxygen isotope exchange on metal oxides has fre- quently been investigated by using l80tracer technique. Miura et al. examined the oxidation of CO with on Bi, Ca, Ni and La molybdate catalysts by measuring l80atom fractions in the product CO,.l The isotope mixing between C02 and surface oxygen occurred very rapidly over these catalysts. Therefore, they concluded that the observed l80 atom fraction in CO, does not give direct information about the active oxygen species on the surfaces of these catalysts.' Chaudhri et al.studied the oxidation of CO on nickel oxides by using the l80tracer technique.' They calculated the amount of exchangeable or reactive oxygen in the catalyst on the assumption of equilibrium between the product CO, and exchangeable or reactive oxygen atoms.' In general, oxygen isotope exchange between CO, and the metal oxide proceeds much more rapidly than does the cata- lytic oxidation of CO.'-' Muzykantov et al. studied the isotopic exchange of oxygen between CO, and MOO, in the range 573-873 K.4 The rate for oxygen exchange per unit surface area of MOO, at 753 K is 20 times larger than the rate for the catalytic CO oxidation with 0, on MOO, mea- sured in our laboratory.' Morikawa and Amenomiya studied oxygen isotope exchange between C'802 and y-alumina in the range 300-973 K.6 They calculated the number of exchangeable oxygen atoms on the surface from the material balance of l6O, by assuming equilibrium between oxygen atoms in the gas phase and the exchangeable oxygen atoms on the surface.6 Peri studied oxygen isotope exchange between C1802 and several high-area oxides such as silica, y-alumina, silica-alumina and ~eolite.~ He found that these oxides have some unusually exchangeable surface oxide ions at low concentrations and the order of activities of these cata- lysts for cracking of n-heptane agrees with that for the esti- mated numbers of exchangeable oxide ions.7 This suggests that the active sites for oxygen exchange between CO, and these oxides are involved in the n-heptane cracking reaction.In the previous paper, on the catalytic oxidation of CO with "0,on MOO,, we attempted to determine the numbers of active oxide ions from the variation of the iso- topic composition of the product CO, with time.' There are two kinds of "0 atoms on active sites. l80atoms taken up into mobile sites during the catalysis immediately diffuse into the bulk and are replaced by l60oxide ions from the bulk. As a result, only C1602 is produced at mobile sites. On the other hand, "0 atoms taken up into immobile sites do not diffuse and can be recovered by the CO reduction step, resulting in the formation of C'80160.Furthermore, we confirmed that the isotope composition of the product CO, is not changed by its contact with the surface lattice oxide ions of MOO, in the catalysis. This finding leads us to the conclusion that the observed isotopic composition of the product CO, must reflect the isotopic composition of active sites. However, as shown by Muzykantov et a/., the product CO, must undergo oxygen isotope exchange with surface lattice oxide ions of MOO, during the catalytic oxidation of C0.4 This means that the oxygen isotope exchange between the product CO, and MOO, which proceeds during the cata- lytic oxidation of CO with 1802does not change the isotopic composition of the product CO,.Therefore we decided to examine the oxygen isotope exchange between C"0, and MOO, and to elucidate the relation between active sites for the oxygen isotope exchange and those for the catalytic oxi- dation of CO with 0,. Experimental Analytical-grade MOO, (99.5%)from East Merck Company was used as the catalyst. The specific surface area of MOO, was 0.40 m2g-' as determined by the BET method. CO and 0, obtained commercially were fractionally distilled at liquid-nitrogen temperature for purification. Both C "0 (180-atom fraction, 99.7%) and C 1802 (180-atom fraction 99%) were purchased from Commissariat a L'Energie Atom- ique and used without further purification. The experimental apparatus, which had both circulating and static reaction systems, was described elsewhere.8 MOO, powder (ca.6.0 g) was at first oxidized at 800 K for 3 h with circulating 0, (7.0 kPa). The catalyst was then treated with a mixture of CO and 0, (CO: O2= 2: 1) until it showed steady catalytic activity. After the catalyst had been cooled to the reaction temperature and completely evacuated, a known amount of C1802 was introduced into the static reaction system (225 cm3). The change in the isotopic composition of CO, was followed at regular intervals (usually 15 min) by using a ULVAC MSQ-150 mass spectrometer. The total amount of CO, consumed in mass analyses was so small that the decrease in CO, pressure was negligible. Before each iso- topic experiment, MOO, was treated at 800 K with a mixture of CO and O2 until the amount of C "0l6O in the product C02 was negligible.1308 In order to examine whether l80atoms that occur at active sites through oxygen isotope exchange are involved in the catalytic oxidation of CO with O,, oxygen isotope exchange between Cl80, and MOO, was succeeded by the catalytic oxidation of C l60with 1602 or C l60with 1802. The catalytic oxidation of CO with "0, was followed by oxygen exchange between C1602 and MOO, in order to examine whether I80 atoms taken up into active sites through "0, dissociative adsorption in the catalytic oxida- tion are involved in oxygen exchange between C1602 and MOO,. The catalytic oxidation of C l80with 1602 was per- formed on MOO, to examine the exchange between the product CO, and MOO, which proceeds during the catalytic oxidation.The time dependence of the yield of isotopic species of CO, in the oxygen isotope exchange between Cl80, and MOO, was calculated using an NEC 9801-VX type personal computer. Results Oxygen Isotope Exchange between C 1802 and MOO, MOO, was exposed to C "0, (95.1 Pa) at 753 K in the static system. The total amount of l80atoms in Cl80, corre-sponded to 26% of all surface lattice oxide ions. Oxygen isotope exchange between C and Mo 1603produced C l80I60 and C 1602 during the reaction. Fig. 1 shows the time dependence of the yields of CI8O2, CI80 l60and C 1602 in CO,. C l80 l60 is the primary product in the exchange and CI60, is produced in a secondary step uia c l80l60.A logarithmic plot of the yield of C 1802us. time is shown in Fig. 2. A linear relationship is observed at the beginning of the reaction. Therefore, the rate of decrease in the yield of Cl80, is proportional to the concentration of C1802, at least initially. loo K'*O n s V C 0.6 6 0.4 0.2 0.0 0 30 60 90 120 150 tlmin Fig. 1 Time dependences of the isotopic composition of CO, in the oxygen isotope exchange between C '*02and MOO, at 753 K. The pressure of CO, was 95.1 Pa. Dashed curves were calculated accord- ing to the following two-step irreversible and successive first-order mechanism : c 180, -P c l80l60--+ c 160,. Solid curves were calculated according to the proposed immobile and mobile sites model.The dotted curve shows the time dependence of 6. a,c ,602 ; 0,c l8Ol60;0,c '80,. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 0.10 0 30 60 90 120 150 r/min Fig. 2 Logarithmic plot of yield of C'80, as a function of time. The specific rate constant calculated from the slope of the straight line was k, = 4.0 x loT2min-'. Oxygen Isotope Exchange between C 1802 and MOO, followed by the Catalytic Oxidation of C l60with 1602 Oxygen isotope exchange between C "0, (47 Pa) and MOO, was continued for 2 h at 753 K. The amount of "0 atoms transferred to the surface was estimated to be 9.6% of all surface lattice oxide ions of MOO, from the material balance of l80in CO, . After the sample had been evacuated for 20 min, the catalytic oxidation of Cl60 with 1602was per- formed for 2 rnin at a total pressure of 1.33 kPa. The catalyst was then evacuated again.This cycle was repeated three times at 60, 120 and 180 min after the exchange reaction in order to examine the change in the l80concentration at the active sites with time. The percentage of C l80l60in CO, produced in each pulse oxidation is shown in Fig. 3 as a function of time elapsed after the exchange reaction. The CO, produced in the first pulse oxidation contained 3.7% C l80l60. This amount did not decrease appreciably throughout the oxidation and evacuation cycles. Oxygen Isotope Exchange between C 1802 and Mo l6O3 followed by the Catalytic Oxidation of CO with 1802 The oxygen isotope exchange between C1802 (95 Pa) and Mo 1603was allowed to continue for 60 rnin at 753 K, then 0 0 0.0 0 60 120 180 t/min Fig.3 Change in the yield of C l8O l6O in the product CO, as a function of time. After completion of the oxygen isotope exchange between C I8O2 and MOO, at 753 K for 2 h, the catalytic oxidation of Cl6O with I6O2 for 2 min was intermittently carried out four times after continuous evacuation. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 20.0 h ,\" Y -0 9 10.0 0 0.0 IIIII1 I I 20 40 60 80 t/min Fig. 4 Change in the yield of C l80l60in the CO, produced in the catalytic oxidation of C l6O with 1802 performed at 753 K and 1.33 kPa. The oxygen isotope exchange reactions between C"0, and MOO, for 60min were repeated three times prior to this experiment.the CO, was' evacuated. This procedure was repeated three times over the same catalyst in order to transfer l80atoms into MOO, to a large extent. The total amount of "0 atoms transferred to the surface of MOO, reached 50% of all surface lattice oxide ions. Afterwards, the oxidation of Cl60 with 1802 was carried out for 100 min on the same catalyst. The total pressure of the mixture of Cl60 and 180,was main- tained at 1.33 kPa and the temperature of the oxidation was 753 K. The isotopic compositions of the CO, produced were examined at 0, 20, 40 and 60 min after starting the catalytic oxidation. The CO, produced at the beginning of the cataly- sis already contained 18% of C l80l60, as shown in Fig. 4. The yield of C "0 l60did not change appreciably with the progress of the catalysis.Catalytic Oxidation of C l60with lSO2on MOO, followed by Oxygen Isotope Exchange between C "02and the Catalyst The catalytic oxidation of Cl60 with l80,was performed over Mo 1603 for 3 h at 753 K, until the yield of C l80l60 in the product CO, reached saturation, i.e. nearly 13%. The total pressure was maintained at 1.33 kPa throughout. The time dependence of the yield of C l8Ol60is shown on the left-hand side of Fig. 5. After the sample had been evacuated, C1602 (3.72 Pa) was introduced into the reactor, where the amount of l60atoms in CO, corresponded to 1% of the total surface lattice oxide ions. C l80 l60was present. The change in the yield of C l80l60in CO, is shown as a func- tion of time on theright-hand side of Fig.5. The yield grad- ually increased with time up to a maximum and then decreased slightly. The maximum value coincided with the evac -c0+'*02 \b Moo3+ Cl602 saturation level of C l80l60observed in the catalytic oxida- tion of CO with "0,. Oxygen Isotope Exchange between Product C02 and MOO, during the Catalytic Oxidation of C ''0 with 1602 C l80was oxidized with 1602over Mo I6O3at 753 K, main-taining the total pressure of the reaction mixture at 1.33 kPa. All CO, was expected to be C l80l60immediately after its formation at active sites, because C l8O is oxidized with l60 oxide ions on the catalyst, at least, at the beginning. C 1602 may be produced in the subsequent oxygen isotope exchange between C l80l60and l60oxide ions on the surface.The yields of C l80l60,C 1602 and C 1802 in the product CO, are plotted against time in Fig. 6. CO, produced at the beginning of the oxidation contained ca. 80% C l80l60and ca. 20% C 1602, suggesting that ca. 80% of C l80l60pro-duced in this catalysis left the catalyst bed without first trans- ferring its l80atom to the surface. On the other hand, 20% of the product C'80160 exchanged its l80atom with surface oxide ions before leaving the catalyst bed. The yield of Cl80 l60increased slightly with time, while that of C 1602decreased. A small amount of C "0,appeared after 60 min. Discussion Oxygen Isotope Exchange between C lSO2and MOO, and Diffusion of "0 Atoms from the Active Sites The primary process in oxygen isotope exchange between C1802 and MOO, is the formation of C'80160 and the secondary exchange process between C l80l60and MOO, gives C 1602, as seen in Fig.1. The decrease in the C 1802 yield, i.e. the concentration of C 180,fits a first-order kinetic law at the beginning of the exchange, as shown in Fig. 2. The first-order kinetics may be applied to the secondary exchange between C180160 and MOO,. Therefore, the oxygen isotope exchange may proceed, at least initially, according to the following irreversible and successive two-step process : c 180, k c 180 160 LCW, where k (4.0 x lo-, min-') denotes the specific rate for the first step calculated from the slope of the straight line in Fig.2. The specific rate for the second step is assumed to be half 0 60 120 180 240 300 360 420 480 t/min Fig. 5 Change in the yield of C "0l60in CO, as a function of elapsed time. The catalytic oxidation of CO with "0,was carried out at 753 K for 3 h and 1.33 kPa. Subsequently, the oxygen isotope exchange between C 1602 and MOO, was performed. The pressure of CO, was 3.72 Pa. The amount of oxygen atoms in CO, corres-ponded to 1% of all surface lattice oxide ions of the catalyst. -(c) Fig. 6 Change in the isotopic composition of CO, produced in the catalytic oxidation of C l8O with 1602 with time. (a) C l8Ol60, (b) c lag,, (c) c 180,. of that for the first step, because each C l80l60molecule has only one l80atom. The dashed curves in Fig.1 show the time dependences of the yields of C 1802, C l80 l60 and C 1602 calculated according to the abovementioned simple model.' The dashed curves agree well with the observed data in the initial part of the exchange reaction. However, the observed yields of C 1802and C l80l60gradually deviate upwards from the calculated curves and that of Cl60, decreases as the exchange proceeds. This irreversible and successive first-order two-step model assumes that the surface of MOO, is always covered with only l60oxide ions during the exchange. In other words, all l80oxide ions transferred from either C 1802 or C l80l60 to the surface immediately diffuse into the bulk to be replaced by l60oxide ions. Consequently, Fig. 1 shows that a large proportion of the l80atoms transferred from C1802 and C'80160 probably diffuse into the bulk of the catalyst, while a small fraction of the l80atoms seem to remain on the surface, as the exchange proceeds.Active Sites for Oxygen Isotope Exchange between CO, and MOO,and for CO Oxidation with 0, on MOO, Fig. 3 shows that the CO, produced in the first pulse oxida- tion of C l60with 1602 after the exchange between C 1802 and Mo 1603contains 3.7% C l80l60.The CO, produced in the same procedure after 180 min contains almost the same amount of C l80l60. Our previous study showed that l80atoms taken up at immobile sites during the catalytic oxidation do not diffuse and can be recovered in the CO reduction step. Therefore, C'80160 is produced only at immobile sites.The production of C l80l60in the catalytic oxidation of C l60with 1602shown in Fig. 3 therefore clearly indicates that a part of l80atoms transferred from C 1802in the exchange must be taken up by immobile active sites. In other words, a certain part of oxide ions at immobile sites act both in CO oxidation with 0, and in oxygen isotope exchange between C 1802and MOO,. On the other hand, the results of Fig. 4 show that the per- centage of C l80l60in the product CO, already reaches saturation at the beginning of the catalytic oxidation of CO with 1802. Prior to the oxidation, however, MOO, was repeatedly exposed to C "0,. This shows that all oxide ions at immobile sites contribute to both CO oxidation with 0, and oxygen isotope exchange with CO,.However, a large part of l80atoms transferred from C1802diffuse into the bulk, as shown in Fig. 1. Therefore, oxide ions at mobile sites are expected also to work as active sites both in the CO oxi-dation with 0,and the oxygen isotope exchange with CO, . Fig. 5 shows that the yield of C l80l6O in CO, introduced into the reactor after the catalytic oxidation of CO with 1802 gradually increased up to a maximum which coincided with the saturation level of C I80 l60observed in the preceding catalysis. If CO, exchanges its 0 atoms only with l80oxide ions at immobile sites, the percentage of C 180'60must become much larger than the saturation level, i.e. nearly 13% because the amount of oxygen atoms in dosed C02 is only 1% of the total surface lattice oxide ions, while the number of '80-oxidized immobile sites was 1.8% of the total surface lattice oxide ions of If CO, exchanges its 0 atoms with surface lattice oxide ions more readily than those at mobile and immobile sites, the yield of C l80I60 must show a maximum much lower than the saturation level.Therefore, we conclude that (1) oxide ions at mobile sites, as well as those at immobile sites, work as active sites in oxygen isotope exchange with CO, and (2) surface lattice oxide ions other than immobile and mobile sites do not exchange l80atoms with CO, nor do they oxidize CO in catalytic oxidation. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Reaction Model of Oxygen Isotope Exchange between C "0, and MOO, and the Number of Immobile Sites Oxygen isotope exchange between C "0, and oxide ions at mobile and immobile sites may proceed as follows.After isotope exchange, l80atoms at mobile sites are immediately replaced with l60oxide ions diffusing from the bulk. As a result, mobile sites are constantly covered with l60oxide ions during the exchange reaction. The oxygen isotope exchange between C "0, and oxide ions at mobile sites can be regarded as an irreversible and successive first-order two- step reaction. On the other hand, l80atoms at immobile sites do not diffuse into the bulk and therefore can again undergo oxygen exchange with CO,. In other words, the oxygen isotope exchange at immobile sites is considered to be a reversible and successive first-order reaction.All immobile sites are covered with l60oxide ions before the beginning of the exchange reaction. The coverage ratio of immobile sites with l80oxide ions may vary with time. When "0 oxide ions at immobile sites cause an exchange reaction with C'80160 or Cl60, in the gas phase, C1802 or C l80l60is formed. Therefore, the yields of C "0, and C l80l60deviate upwards from the dashed curves, while that of Cl6O, decreases with the progress of the exchange reaction (Fig. 1). l80 atoms can be recovered from immobile sites as C l80l60or C "0,. They can, of course, exchange again with l60oxide ions at mobile sites. This means that l80 oxide ions at immobile sites move to mobile sites gradually through CO, in the gas phase and then diffuse into the bulk irreversibly.This irreversible transfer of "0 atoms from immobile sites to mobile sites can explain the slow decrease in the yield of C l80l60in dosed CO, on the right-hand side of Fig. 5. If 8 is the coverage ratio of immobile sites with "0 oxide ions, the oxygen isotope exchange reaction between C I8O2 and oxide ions at mobile and immobile sites can be expressed as follows: immobile sites (2) where k, denotes the specific rate of oxygen isotope exchange at mobile sites and k, denotes the specific rate of oxygen isotope exchange at immobile sites. The rate of oxygen isotope exchange at mobile sites does not vary throughout the exchange reaction. On the other hand, the rate of oxygen isotope exchange at immobile sites depends upon 8 and therefore changes with time. Differential equations which give time dependences of the yields of isotopic species of CO, are expressed using k,, k, and 8 as follows: d[C 1802]/dt = -kl[C 1802]-k,(l -e)[c180,] + k, e/2[c 180 1601 (3) d[C l80'"O]/dt = kl[C '802]-k1/2[C l80160] + k,(i -e)c18023-k,(i -e)/2[c 180 1601 +k, ecc 160,1-k2e/2[c 180 1601 (4) d[C 1602]/dt = k1/2[C l80160] + k,(l -8)/2 x [C l80160] -k,B[C 160,] (5) J.CHEM. SOC. FARADAY TRANS.,1994, VOL. 90 The time dependence of 6 is given by the following differen- tial equation : dO/dt = A/N{k,(l -6)[C "O,] + k,(l -6)/2[C "0 160] -k, e/2[cl80 1601-k, ecc 160,1) (6) where A is the amount of CO, in the reactor and N is the number of immobile sites.We attempted to calculate the time dependences of yields of isotopic species of CO, and 0 by solving eqn. (3)-(6) by a numerical integration method. In this case, the isotopic com- position of CO, before starting the exchange reaction is C"02 = 99% and C"0 l60= 1%. 6 is zero at t = 0. A does not change throughout the reaction. The sum of k, and k, ,i.e. k = 4.0 x lop2min-', can be obtained from the slope of the initial straight line of C "0, presented in Fig. 2. However, N and the values of k, and k, are unknown. There- fore, we assumed various combinations of N and the ratio of k, to k (=k, + k,) and calculated the time dependences of the yields of C1'02, C'80160 and C1602 in CO, to compare them with the observed results. The time interval of numerical integration was 1 s and numerical calculations were accumulated up to 150 min with the aid of a computer.A ratio of k,/k, + k, = 0.13, i.e. k, = 3.48 x lo-, min-' and k, = 0.52 x lo-' min-' and N = 1.8% of the total surface lattice oxide ions gave the best fit with the observed data. The results of the calculations (shown in Fig. 1 by solid curves) agree well with the observed data up to CQ. 90 min and therefore support the proposed reaction model, at least at the beginning of the exchange. Our previous study on the catalytic oxidation of CO with "0, on MOO, showed that the ratio of the rate of pro- duction of CO, at immobile sites to the total CO, pro-duction rate is 0.13 and the number of immobile sites is 1.8% of the total surface lattice oxide ions of MOO,.Thus, both the number of immobile sites which work in the exchange and the ratio of the rate of exchange at immobile sites to that of the whole active sites agree with the corresponding number and the ratio of the rate of oxidation at immobile sites determined in the catalytic oxidation of CO with "0,. This might suggest that the reaction mechanism of oxygen exchange between CO, and oxide ions at mobile and immo- bile sites is closely associated with that of CO oxidation with 0, which proceeds on common active sites. The kinetics and reaction mechanism of oxygen exchange between C 180,and MOO, will be discussed in the next paper in connection with the catalytic oxidation of CO with 0, on MOO,. Fig.1 also shows the variation of 6 with time. The time dependence of 8 is similar to that of the yield of C "0 l60. Note that MOO, prepared by the thermal decomposition of (NH,),Mo,O2, -4H,O at 827 K in an O2 stream has been shown also to possess mobile and immobile active sites for catalytic CO oxidation with "0,. X-Ray diffraction patterns revealed that both MOO, powders have orthorhombic crystal form. However, the MOO, from Merck showed intense dif- fraction peaks at (020), (040) and (060) and had the morphol- ogy of flat hexagonal plates. On the other hand, MOO, prepared by thermal decomposition (1.3 m2 g-') exhibited maximum diffraction at (021) and had the morphology of rec- tangular prisms.Furthermore, we confirmed that WO, and ZrO, , which have different crystal structures from that of MOO,, also possess mobile and immobile active sites for the catalytic oxidation of CO with "0,. Oxygen Isotope Exchange between the Product CO, and MOO, under the Catalytic Oxidation of CO with 0, In the catalytic oxidation of C"0 with 1602 on Mo160, we observed the formation of 20% CI6O2even at an early stage (Fig. 6). This indicates that 20% of C "0 l60 molecules produced initially exchange their "0 atoms with l60oxide ions at mobile and immobile sites before leaving the catalyst bed. This means that "0 oxide ions at immobile sites increase with time during the catalytic oxidation of C lSO with I6O2.This increase results in an increasing opportunity for "0 oxide ions at immobile sites to take part in exchange with the product C l80l60.Thus, the opportunity for the formation of Cl6O, decreases, while that for C"0, increases.Fig. 6 shows slight increases in C "0 l6O and C ''0, with time as well as a slight decrease in C 1602. It is-natural that the CO, produced in the catalytic oxida- tion of Cl60 with "0, undergoes similar oxygen isotope exchange with active oxide ions at both mobile and immobile sites before it leaves the catalyst bed. In this case, the percent- age of C'80160 decreases due to "0 exchange with l60 oxide ions at mobile and immobile sites, while a fraction of the C 1602changes into C l80l6O through oxygen exchange with l80oxide ions at immobile sites.As a result, the decrease and increase in the yield of C "0 l6O concur in the catalyst bed. Fig. 5 shows the variation of C "0 l60with time during the exposure of the catalyst surface to C I6O2 after the cata- lytic oxidation of Cl60 with "0,. The percentage of C l8Ol60increased rapidly at first. However, the rate of the increase diminished with time and finally C "0 l6O reaches to a plateau. The rate of the decrease of C "0 l60was larger with increasing yield of C "0 l60in the gas phase. There- fore, the plateau must result from a dynamic equilibrium between the increase and decrease of Cl80l6O. The dynamic equilibrium lasts for more than 30 min, as shown in Fig. 5. CO, produced in the oxidation of C l60with "0, must undergo a similar oxygen exchange in the catalyst bed to that at the plateau on the right-hand side of Fig.5. In other words, CO, produced at the catalyst surface contains C l80l6O at the same level as CO, in the plateau. Pre- viously, we found that the exposure of the CO, produced at the end of the catalysis to the catalyst surface did not appre- ciably change the percentage of C "0 l60.'The time depen- dence of the yield of C'80160 in CO, produced in the oxidation of Cl6O with "0, was independent of the amount of catalyst.' These experimental facts can be explained satisfactorily by the dynamic equilibrium. It was also reported in the previous paper that C"0, is always negligible in the catalytic oxidation of Cl60 with 1802.8C1*0, is produced by oxygen exchange between C "0 l60and "0 oxide ions at immobile sites. The negligi- ble production of C "0, may be caused by the low content of C "0 l60in the reaction mixture, the short time it spends in the catalyst bed and its possible change into C "0 l60etc.We conclude that the subsequent oxygen isotope exchange does not affect the percentage of C "0 l60in the catalytic oxidation of Cl6O with 1802 on MOO, and the percentage is almost equal to the "0 atom fraction at the active sites. The authors thank Plenary Professor Takuya Hamamura of the Kyoto Institute of Technology for his helpful discussions and advice. References 1 H. Miura, Y.Morikawa and T. Shirasaki, Nippon Kagaku Kaishi, 1975,11,1875. 2 S. Chaudhri, Y.Kera and K. Hirota, Bull. Chem. SOC.Jpn., 1972, 45, 3301. 3 G. W. Keulks and C. C. Chang, J. Phys. Chem., 1970,74,2590. 4 V. S. Muzykantov, K. Ts. Cheshkova and G. K. Boreskov, Kinet. Katal., 1973, 14,432. 1312 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 5 6 7 8 Y. Iizuka, Y. Onishi, T. Tamura and T. Hamamura, J. Catal., 1980,64,437. Y. Morikawa and Y. Amenomiya, J. Catal., 1977,48, 120. J. B. Peri, J. Phys. Chem., 1975,79,1582. Y. Iizuka, J. Chem. SOC.,Faraday Trans., 1994,90,1301. 9 A. A. Frost and R. G. Pearson, Kinetics and Mechanism, John Wiley, New York, 2nd edn., 1953, p. 166. Paoer 3/05655D;Received 20th September, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949001307

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(9), 1313-1322 Conformational and Vibrational Properties of a,w-Dihalogenoalkane/Urea Inclusion Compounds :A Raman Scattering Investigation Sharon P. Smart Department of Chemistry, University of St. Andrews, St. Andrews, Fife, Scotland, UK KY16 9ST Abdelkrim El Baghdadi and FranGois Guillaume* Laboratoire de Spectroscopie Moleculaire et Cristalline, Universite de Bordeaux I, CNRS URA 124, 33405 Talence Cedex, France Kenneth D. M. Harris* Department of Chemistry, University College London, 20 Gordon Street, London, UK WCIH 0AJ Raman spectroscopic investigations of urea inclusion compounds containing a,o-dihalogenoalkane [X(CH,),X ; n = 8 for X = CI; n = 7-11 for X = Br; n = 8 for X = I] guest molecules are reported. In these inclusion com- pounds, the urea molecules form a tunnel structure within which the guest molecules are located.Vibrational modes due to the urea confirm the structural identity of the inclusion compounds, and lattice modes of the urea host structure are assigned tentatively. Investigations of the vibrational properties of the X(CH,),X guest mol- ecules included within this host structure have focused on the longitudinal acoustic mode (LAM-l) and the C-X stretching vibrations. Bands in the Raman spectrum due to the v(C-X) mode have been studied as a function of: (i)the length (n) of the guest molecule; (ii) the identity of the terminal substituent X; (iii) temperature and (iv) pressure. From these results, trends in the relative amounts of gauche and trans end-groups for the X(CH,),X guest molecules in their urea inclusion compounds have been assessed.Urea inclusion compounds have been widely investigated from the viewpoint of their fundamental physico-chemical properties.’ The ‘host’ structure in these crystalline solids is constructed from an extensively hydrogen-bonded arrange- ment of urea molecules, and this structure contains a regular arrangement of one-dimensional, parallel tunnel^.^.^ Guest molecules of appropriate dimensions can be accommodated within these tunnels. Because the cross-section of the tunnels (defined by the van der Waals surface of the tunnel wall) is ca. 5.3-5.7 A, only guest molecules based on a sufficiently long n-alkane chain, with an appropriately limited degree of sub- stitution, can fit within these tunnels.Of relevance to the present paper is the fact that a,o-dihalogenoalkanes [X(CH,),X] are known2-’ to form inclusion compounds with urea. In such a ‘constrained’ environment, these guest mol- ecules must adopt a linear, extended conformation and, as a consequence, urea inclusion compound formation has often been used as a means of isolating molecules in such confor- mations, which may differ substantially from the preferred conformations of the same molecules in other solid-state environments or in dispersed phases. The urea inclusion com- pounds have thus been exploited as prototypical materials for understanding the structural, dynamic and spectroscopic properties of molecules in linear, extended conformations.The physico-chemical properties of urea inclusion com-pounds containing alkane guests have been studied extensively’ via a wide range of experimental techniques, although much less is known, at present, about the corre- sponding properties of urea inclusion compounds containing functionalized alkane guests. We are currently addressing this question by carrying out extensive studies of the structural and dynamic properties of urea inclusion compounds con- taining guests such as a,o-dihalogenoalkanes, diacyl per- oxides, and carboxylic acid anhydrides. In this paper, we consider the vibrational properties of the host and guest com- ponents in a,o-dihalogenoalkane/urea inclusion compounds. The alkane/urea inclusion compounds undergo a phase transition from a high-temperature (HT) phase in which the host structure is hexagonal to a low-temperature (LT) phase in which the host structure is orthorh~mbic.~*~ Differential scanning calorimetry has shown that the a,o-dibromoalkanel urea inclusion compounds undergo a similar phase tran-sition, and powder X-ray diffraction* has shown that this transition is associated with the same distortion of the host tunnel structure as that established previously6~’ for the alkane/urea inclusion compounds. For all of the guest mol- ecules considered here, the phase-transition temperature is in the range CQ.145-170 K. At sufficiently high temperature, urea inclusion compounds decompose to produce the ‘pure’ crystalline phase of urea (which differs in structure from the host structure in urea inclusion compounds, and is not a tunnel-containing structure).The decomposition temperature is a function of the length of the guest molec~le;~ urea inclu- sion compounds with short guests such as octane are not stable if left in the atmosphere at room temperature. The ‘pure’ crystalline phase of urea is often referred to as ‘tetragonal urea’ and the conventional urea inclusion com- pounds are often referred to as ‘hexagonal urea’ (as a conse- quence of the different crystal systems of these structures). We have discovered recently,4*10 by X-ray diffraction and other techniques, that urea inclusion compounds containing functionalized alkane guests exhibit new structural proper- ties, particularly concerning the three-dimensional packing arrangement of the guest molecules.In particular, single- crystal X-ray diffraction4 has shown that, at room tem-perature, the guest molecules in the urea inclusion compounds containing Br(CH,),Br guest molecules with n = 7-10 exhibit a characteristic three-dimensionally ordered packing arrangement with A, = ~$3, where c, denotes the periodic repeat distance of the guest molecules along the tunnel and A, denotes the offset, along the tunnel axis, between the positions of guest molecules in adjacent tunnels (Fig. 1). This guest structure is rhombohedral, and a given single crystal of the inclusion compound usually contains two domains of this guest structure, differing in orientation with respect to the host structure.Furthermore, the M Br(CH,),Br/urea inclusion compounds also contain regions in which the guest molecules are ordered only along the tunnel axis; the periodic repeat distance is the same (within experimental error) for these one-dimensionally ordered regions and the three-dimensionally ordered regions dis- cussed above. It is relevant to note that, in contrast to this situation for the Br(CH,),Br guest molecules, the molecular packing in the three-dimensionally ordered regions of the guest structure in alkane/urea inclusion compounds corre- sponds to A, = 0. It is important to consider whether the presence of terminal bromine atoms uis-ci-uis methyl groups on the guest molecule gives rise to a similarly marked differ- ence in the local structural properties of the guest molecule, with particular interest, within the context of this paper, in the conformational properties of the guest molecule.It is important to stress that the X-ray diffraction studies dis- cussed above have elucidated only the relative positioning of guest molecules within the urea tunnel structure, and have not led to structure determination of the guest structure (i.e. determination of atomic coordinates, from which the confor- mational properties could clearly be deduced). Finally, we note that the periodic repeat distance of the guest molecules along the tunnel (c,) is usually incommensurate with the periodic repeat distance of the host structure along the tunnel ;a detailed discussion of incommensurate uersus com-mensurate behaviour in one-dimensional inclusion com-pounds (typified by the urea inclusion compounds) has been given elsewhere.11-' The periodic repeat distance c, of the guest molecules in the alkane/urea inclusion compounds is always ca. 0.5 %i shorter than the 'van der Waals length' of the alkane mol- ecule in its all-trans conformation, and similar behaviour has also been observed for Br(CH,),Br/urea inclusion com-pound~.~This arises" from the fact that, in the energetically most stable state of the inclusion compound, there is a repul-sive interaction between adjacent guest molecules in the same tunnel. It is clearly relevant to consider whether the propor- tion of conformational defects (i.e.differing from a trans end- group) at the ends of the guest molecules is influenced by this repulsive intejaction between adjacent end-groups.Our recent incoherent quasielastic neutron-scattering studie~'~.'~of the dynamic properties of the guest molecules in Br(CH,),Br/urea inclusion compounds have shown that both reorientational motions of the guest molecules about the tunnel axis and translational motions of the guest mol- ecules along the tunnel axis are effective on the picosecond timescale. The translation length depends critically upon temperature and is ca. 2.3 %i for 1,9-dibromononane/urea at 280 K. The motivation underlying the work described in this paper was to investigate, uia Raman spectroscopy, the confor- mational and vibrational properties of the a,u-dihalo-genoalkane/urea inclusion compounds. One specific aim was J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 to derive an understanding of the effect of the dense packing of guest molecules, and the translational motions of the guest molecules, on their conformational properties. The aspects of the Raman spectrum that can be used to probe the structural properties of the host are discussed, and the Raman-active vibrational modes of the guest molecules, which depend criti- cally upon the molecular conformation, are analysed. In this regard, two specific vibrational modes have been considered in detail: (a)the longitudinal acoustic mode LAM-1 and (b) the v(C-X) stretching mode, the frequency of which is partic- ularly sensitive to the conformation of the -CH,-CH,-X end-group.These vibrational properties have been studied as a function of: (i) the identity of the halogen atom (X); (ii) the length (n)of the guest molecule [X(CH,),X] ;(iii) temperature and (iv) hydrostatic pressure. Because our previous X-ray diffraction4 and neutron ~cattering'~? l5 investigations were focused on Br(CH,),Br/urea inclusion compounds, particu- lar emphasis [concerning points (ii), (iii) and (iv)] has been given here to the study of Br(CH,),Br guest molecules. Experimental Urea inclusion compounds containing X(CH,),X guest mol- ecules (n= 8 for X=C1; n=7-11 for X=Br; n= 8 for X = I) were prepared by slowly cooling warm solutions of X(CH,),X and urea in methanol.Powder X-ray diffraction confirmed that all samples had the characteristic tunnel host structure of the conventional urea inclusion compounds. The phase-transition temperatures for the Br(CH,),Br/urea inclu- sion compounds, established by differential scanning calorim- etry, are in the range ca. 145-170 K. Polarized Raman spectra were recorded for these X(CH,),X/urea inclusion compounds on a triple-mono-chromator Dilor 224 spectrometer and on a double-mono- chromator Jobin-Yvon Ramanor spectrometer. The specific polarizations used were (in the Porto notation) X(ZZ)Y, X(ZX)Y, X(YZ)Y and X(YX)Y in direct goemetry. Unless otherwise stated, the samples used in these experiments were single crystals of the urea inclusion compounds, with typical crystal dimensions of ca.0.5 x 0.5 x 3 mm3. Each crystal was sealed in a thin glass tube with its long axis [which corre- sponds to the urea tunnel axis (crystallographic c axis)] parallel to the Z axis of the laboratory reference frame (see Fig. 2). The incident radiation was the 514.5 nm line of an Ar' ion Spectra Physics laser, with power ca. 150-300 mW at the sample. Spectra were generally recorded between 20 and 2000 cm-'. The spectral resolution (full width at half maximum height) was 1.5 cm-' for the spectral range 20 < V/cm-' < 300 and 2.8 cm-' for the spectral range V > 300 cm-'. A =r I incoming laser beam I INKII y X \IT-X Fig.2 Configuration of a single crystal of a urea inclusion com- pound in the polarized Raman experiments. The axes (X,Y, Z) are fixed in space (laboratory reference frame), whereas the axes (x, y, z) are associated with the guest molecule within the inclusion com- pound. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 liquid-nitrogen cryostat was used to perform experiments in the temperature range 80-300 K (temperature stability ca. +3 K). Raman spectra were also recorded at non-ambient pressure (at room temperature) using a home-built device described in detail elsewhere. l6 These experiments were performed on polycrystalline samples, and only the Br(CH,), ,Br/urea inclusion compound was studied. The pressure range was 1 bar to 6 x lo3 bar and helium gas was used as the compres- sing fluid.Background Vibrational Modes due to the Urea Host The symmetry of an isolated urea molecule is described by the point group C,,. In the high-temperature phase of the urea inclusion compounds, the space group of the urea host structure is P6,22 and there are six urea molecules per unit ell.^.^ The site symmetry of the urea molecule is C, and the factor group is D, . There are 45 Raman-active internal vibra- tions and 13 Raman-active external vibrations: I'fiman9Al= + 18E1 + 18E2; = 2A1 + 5E1 + 6E2. Below the phase-transition temperature for the urea inclu- (using superscript g to represent gauche and superscript t to represent trans): These intensity ratios have been used in our assessment (uide infia) of the relative amounts of the gauche and trans end- groups in the guest molecules. A potential problem in the study of Raman bands associ- ated with the LAM-1 modes of the guest molecules is that these can be hidden beneath bands due to urea lattice vibra- tions.In order to derive a precise estimate of the LAM-1 fre- quencies, the LAM-1 bands in the spectra were fitted using several Lorentzians convoluted with a triangular instrumen- tal resolution function. The vibrational modes of the X(CH,),X guest molecules will necessarily reflect the geometrical constraints imposed upon them by the confined character of their one-dimensional host environment. In the all-trans conformation, the symmetry of the molecules with odd n is described by point group C,, and the symmetry of the molecules with even The space group is P2,2,2,, with 12 urea molecules per unit cell (and three independent urea molecules in the asymmetric unit).In this structure, the site symmetry of the urea molecule is C, and the factor group is D,.There are 72 Raman-active internal vibrations and 2 1 Raman-active external vibrations : = 18A + 18B1+ 18B2 + 18B3; I'&Lan= 6A + 5B1+ 5B2 + 5B3. Vibrational Modes due to the a,a-Dihalogenoalkane Guest For the scattering geometries used in this work, the following components of the derivative of the polarizability tensor for the X(CH,),X guest molecules can be obtained:17 From X(ZZ)Y experiments: (alzz)* = (1) From X(2X)Yand X(Y2)Yexperiments: = 3c<a:z)2 + (2) From X(YX)Yexperiments: In these expressions, a:j = (2)Qo where Q is the normal coordinate of the vibration.As shown in Fig. 2, Oz represents the main axis of the guest molecule (assumed, on average, to be parallel to the urea tunnel axis). The Raman intensity (IIJ) is proportional to the mean (averaging over rotation about the z axis) of the square of the relevant component of the derivative of the polarizability tensor: '1, cc (4) The relative intensities of the bands due to the v(C-X) stretching vibrations for the gauche and trans end-groups (-CH,CH,CH,X) were determined by numerical integra- tion. The intensity ratios Yftand 9$were evaluated from the spectra recorded in the different polarizations as follows sion compounds, the urea host structure is orth~rhombic.~,~ n is described by point group C,, .LAMS correspond to stationary vibrational waves with one or more nodes in the longitudinal displacements of the skeletal carbon atoms." Only LAM vibrations with an odd number of nodes are Raman active, and the Raman intensity decreases rapidly as the number of nodes is increased. The LAM-1 vibration gives rise to the most intense LAM band in the Raman spectrum and is of A, symmetry when n is odd and A, symmetry when n is even. The LAM-1 vibration is an intramolecular mode, and the term 'longitudinal accordion motion' is probably a more accurate description. LAM-1 vibrational modes have been modelled (primarily for polymers) using various different approaches (see later).In this paper, we consider whether the LAM-1 frequency is affected by the structural constraints imposed upon the guest molecules by the urea tunnel structure. Considering now the end-group conformations of the X(CH,),X guest molecules, we refer to situations in which the end-group is in the gauche conformation as 'end-gauche defects'. For a molecule in the all-trans conformation, the symmetries of the v(C-X) stretching modes are A, and B, when n is even, and A, and B, when n is odd. The point symmetry of a molecule containing one gauche end-group is C, and all vibrations are thus Raman active. The v(C-X) stretching modes for the conformations with trans and gauche end-groups have different characteristic frequencies in the 300-1000 cm-' region of the Raman spectrum," and these modes can therefore be used to probe the existence of end- groups in the gauche conformation.Results and Discussion Assessment of Crystal Integrity As shown previously,20 Raman spectroscopy can be used to determine the integrity of urea inclusion compound single crystals (hexagonal urea). In our work, this has been assessed from the following features of the spectrum recorded at 300 K: (i) At low wavenumbers, the pure crystalline phase of urea (tetragonal urea) is characterized by a sharp, strong band at 60 cm-'. As shown in Fig. 3A(b), this band was observed in the spectrum of the 1,7-dibromoheptane/urea inclusion com- pound, indicating partial decomposition of this inclusion A I 40 60 80 100 120 140 160 180 200 980 990 1000 1010 1020 1030 1040 Fig.3 Raman spectra [recordedin X(Z2)Ypolarization] for (a)the Br(CH,),,Br/urea inclusion compound and (b) a partially decom- posed Br(CH,),Br/urea inclusion compound. The spectral regions shown are for: A, the lattice modes and By the v(C-N) stretching mode. The asterisks indicate the Ar' emission lines (77 an-' and 117 cm-' for A, = 514.5 nm). The arrows indicate bands due to tetragonal urea. compound. This band was not observed for the other urea inclusion compounds studied [see e.g. the spectrum of 1,lO- dibromodecane/urea shown in Fig. 3A(a)]. (ii) In the spectral region corresponding to the bending modes 6(NCO) of urea, a strong band at 556 cm-' and a weak band at 570 cm-' are characteristic of tetragonal urea, whereas the corresponding bands for the urea inclusion compounds are at 530 cm-' and 608 cm-'.(iii) The v,(C-N) stretching vibration is very intense and occurs at 1010 cm-' for :etragonal urea and at 1024 cm-' for hexagonal urea.,' As shown in Fig. 3, the integrity of the inclusion compounds can be checked readily by considering this spectral region. As expected, the polarization properties of a urea inclusion compound single crystal are lost if it decomposes to tetrago- nal urea. Modes due to the Urea Host In general, the total number of bands observed for urea vibrational modes is consistently lower than the number pre- dicted theoretically, both in the HT and LT phases.The number of observed Raman bands is the same below and above the phase-transition temperature, and is consistent with the fact that the changes in the host structure associated with the phase transition are small. Polarization of the Raman bands is also unaffected by the phase transition. These observations are probably related to the fact that a J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 single crystal in the HT phase (hexagonal structure) becomes triply twinned in the LT phase (orthorhombic structure). The frequencies of the internal vibrational modes of the urea are essentially independent of the length of the guest molecule. As discussed previously,21 only the highest-frequency internal vibrational modes [v(N-H) stretching region above 3200 cm-'1 have a significant temperature dependence. Some of the lattice modes reflect changes in the tunnel structure which occur at the phase transition (see Fig.4). The band at 185 cm-' in the LT phase (at 93 K), which is pol- arized X(YX)Y (i.e.E, symmetry), moves considerably to 164 cm-' in the HT phase (at 298 K). E, modes arise from coupled translations and rotations about different axes, and although this band cannot be assigned definitively to a par- ticular lattice vibration, it may be compatible with an oscil- lation of the urea tunnel with a large component in the plane perpendicular to the tunnel axis. At the phase transition there is a major change in the frequency of this mode, as a conse- quence of the structural changes in the urea tunnel at the phase transition.The band at ca. 135 cm-'in the LT phase (at 93 K) and at ca. 130 cm-' in the HT phase (at 298 K) has A, symmetry and is strongly X(ZZ)Y polarized; A, modes result from coupled translations along and rotations about the axis of the C-0 bond of each urea molecule. In contrast to the E, mode discussed above, the frequency of this A, mode is not sensitive to change of temperature, and may represent a vibration with a substantial component along the tunnel direction. The broad band at ca. 101 cm-' [symmetry El; polarized X(ZY)Y] does not shift significantly with change of tem- perature and is also assigned as a urea lattice vibration. A f/cm -' BiI ~~ ~ 10 68 126 184 242 300 f/cm-' Fig.4 Raman spectra showing the spectral region for lattice modes of the Br(CH,),Br/urea inclusion compound at different polariza- tions: (a)X(Zz)Y,(b)X(2X)Yand (c)X(YX)Yfor: A, the LT phase (T = 93 K) and B, the HT phase (T = 298 K) J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Other bands below 100 cm-’ arise from lattice vibrations, although definitive assignments of these modes are difficult. Modes due to the a,o-Dihalogenoalkane Guest LAM-1 Mode LAM-1 vibrations have been widely studied for polyethylene and alkanes in the all-trans conformation, and have been identified for alkane guest molecules in their urea inclusion compounds.22 A variety of different approaches have been used to model these vibrational modes.For example, Mizushima and Schiman~uchi~~approximated the all-trans polyethylene molecule by an elastic rod, and several worker^'^.^^ have retained this approximation to study the effect of end-groups on LAM-1 frequencies. The model of Minoni and Zerbi2’ is more realistic and more appropriate in view of the aims of the work discussed here. In this model, the system is treated as an infinite, one- dimensional crystal, the repeat unit of which is a single molecule (Fig. 5). In employing this model for the Br(CH,),Br/urea inclusion compounds, we have represented each Br(CH,),Br molecule by a strictly one-dimensional arrangement of point masses, with each terminal substituent (in this case CH,Br) having mass M and each CH, unit having mass rn.Interactions are considered only between adjacent point masses, and only longitudinal displacements are considered. The intramolecular CH,-CH, force con- stant is denoted F and the intermolecular force constant is denoted f: For simple harmonic oscillator behaviour, the fol- lowing dispersion relation can be derived :” (7) where V is the LAM frequency (in wavenumbers) and 8 is the molecular phase. A value of F = 5.2 mdyn A-’ is obtained -by equating eqn. (7) to the empirical function[?(a)= A8 + ... where A = 2495 cm- ’and 8 = 8/n]given by Schaufele and Schimanouchi26 for polyethylene. The molecu- lar phase relation for Raman scattering with wavevector k z0 can be shownz5 to be: sin[(N -1)8](cos 8 -1) x [(2K -1 -2K2)cos 8 + 2K2 -2K + R -2RK] + cos[(N -1)Blsin 8[(1 -~K)(cos8 -1)-R] + R sin 8 where R =flF, K = M/m, and N is the number of point masses per molecule.This equation has N solutions in 8. Since F,f,rn and M are known, the molecular phase relation can be solved to give values for 8. LAM-1 wavenumbers can then be calculated from eqn. (7). Using the value f = 0.05 mdyn A-determined24 for alkanes at low temperature, a value F = 4.2 mdyn A-1 was determined27 by fitting the calculated wave- numbers to experimental results28 at 168 K for the range of alkanes CH,(CH,),CH, with n = 6-26. In other work,,’ MmmmmM M M Fig. 5 Definition of the parameters used in the infinite one-dimensional model of a linear chain developed by Minoni and Zerbi’’ force constants have been calculated for various alkanes CH,(CH,),CH, and a,o-disubstituted alkanes X(CH,),X with even n, with the aim of investigating the effect of the end-group X on the LAM-1 frequency for these molecules in urea inclusion compounds.Experimental data on LAM-1 vibrations for a,o-dibromoalkanes in their pure crystalline phases have been reported.27 In this work, theoretical LAM- 1 wavenumbers (Table 1) for Br(CH,),Br molecules with n = 7-10 were determined via the Minoni-Zerbi model, using an intramolecular force constant F = 4.2 mdyn A-’ and an intermolecular force constantf = 0.05 mdyn A-’. Raman spectra recorded in X(ZZ)Y polarization in the present work for Br(CH,),Br/urea inclusion compounds illus- trating the LAM-1 bands in the LT phase (at 93 K) and the HT phase (at 298 K) are shown in Fig.6 and 7, respectively. The experimental wavenumbers (f)for LAM- 1, deduced from fitting these bands, are reported in Table 1. The uncertainties in V are greater at 298 K owing to the larger bandwidths, and the error in the measurement of V at 93 K is estimated to be ca. &0.5 cm-’. Furthermore, for 1,lO-dibromodecane/urea at both 93 and 298 K and for 1,7-dibromoheptane/urea at 298 K, there is a larger uncertainty in V since the band due to the LAM-1 vibration overlaps bands due to urea lattice modes. Comparison between our results and those for the pure crystalline a,o-dibromoalkanest7 suggests that, in the Br(CH,),Br/urea inclusion compounds, the Br(CH,),Br guest molecules are predominantly in the all-trans conformation, and the proportion of guest molecules containing kinks in the middle portion of the molecule is negligible.For the Br(CH,),Br molecules with even n, the theoretical and experi- mental wavenumbers are in acceptable agreement (Table 1). However, for the Br(CH,),Br molecules with odd n, the theo- retical values [calculated via eqn. (7) and (S)] are significantly higher; even reducing the value off to zero does not lower the theoretical wavenumbers sufficiently to give agreement with the experimental wavenumbers. The failure of the Minoni-Zerbi model to predict correctly the observed wave- numbers for the compounds with odd n arises from the fact that the model considers only the longitudinal mode (i.e.lon-gitudinal displacement of atoms).Normal mode calculations on short alkanes CH,(CH,),CH, with odd n (n = 7, 9, ll), however, have shown3’ that the LAM-1 vibration cannot be Table 1 Measured wavenumbers (7)of LAM-1 for Br(CH,),Br mol-ecules in their urea inclusion compounds (UIC) and their pure crys- talline (PC) phases, and calculated values (as discussed in the text) Br(CH2)7 Br UIC (LT phase) UIC (HT phase) PC 132 133 140 calculated 158 Br(CH,),Br UIC (LT phase) UIC (HT phase) PC 151 151 150 calculated 145 Br(CH,),Br UIC (LT phase) UIC (HT phase) PC 115 116 121 calculated 135 Br(CH,),,Br UIC (LT phase) UIC (HT phase) PC 122 126 126 calculated 127 Experimental results for the urea inclusion compounds were deter- mined (in this work) at 93 K (LT phase) and at 298 K (HT phase).Experimental results for the pure crystalline phases are taken from ref. 25 and 29. t 4-.-v) Q,+ C 80 90 100 110 120 130.140 150 160 F/cm-Fig. 6 Raman spectra [recorded in X(Z2)Ypolarization] showing the spectral regions for LAM-1 vibrations (indicated by arrows) of Br(CH,),Br/urea inclusion compounds at 93 K (LT phases). n = (a) 7, (b)8, (c) 9 and (6)10. described by such longitudinal displacements alone, and the origin of this effect is attributed to the difference in mass between the terminal CH, groups and the CH, repeat units in alkanes with odd n. Similarly, the presence of heavy CH,Br end-groups on the Br(CH,),Br molecules will have an important effect on the LAM-1 vibration.In order to give a physical interpretation to the calculated LAM-1 frequencies for alkanes with odd n, it has been suggested3’ that coupling between an ‘unperturbed’ LAM-1 mode (as described by the linear chain model) and an ‘unperturbed’ transverse acoustic mode (TAM-4) is effective. Evidence for such coupling has also been obtained from normal mode calculations on 1-br~moalkanes.~~Unperturbed LAM and TAM modes, as defined by a linear chain model, can interact provided they belong to the same symmetry species and provided their unperturbed frequencies are sufficiently close to each other. This coupling therefore cannot occur for the molecules with even n since LAM-1 has symmetry A, and TAM-4 has sym- metry B, (although LAM-1 and TAM-3 do have the same symmetry and, in principle, they could interact if their fre- quencies were sufficiently close).For the molecules with odd n, on the other hand, LAM-1 and TAM-4 both belong to symmetry species A,. Assuming that coupling occurs through a Fermi-type resonance3’ (anharmonicity due to difference in masses), the maximum coupling for the alkanes occurs when n = 11. In our work, low intensity bands were observed at wavenumbers between 150 and 300 cm-’ only for Br(CH,),Br/urea inclusion compounds with odd n [as shown in Fig. 8 for Br(CH,),Br/urea] and can be assigned tentati- J. CHEM. SOC. FARADAY TRANS., 1994, VOL.90 80 90 100 110 120 130 140 150 160 f/cm -Fig. 7 Raman spectra [recorded in X(2z)Y polarization] showing the spectral regions for LAM-1 vibrations (indicated by arrows) of Br(CH,),Br/urea inclusion compounds at 298 K (HT phases). (a)-(d) as Fig. 6. vely as TAM modes (on the basis of comparing the Raman spectra of the Br(CH,),Br/urea inclusion compounds with those of the pure solid phases of a,o-dibr~moalkanes).’~-~~ From Table 1, it is clear that there is no significant tem- perature dependence of the LAM-1 frequency for the Br(CH,),Br/urea inclusion compounds. Furthermore, our experimental LAM-1 frequencies are similar to those re-for theported previou~ly~~*~~ corresponding a,o-dibro-moalkanes in their pure crystalline phases.Thus, the 100 140 180 220 260 300 ;/cm -’ Fig. 8 Raman spectrum [recorded in X(22)Ypolarization] for the Br(CH,),Br/urea inclusion compound at 93 K (LT phase) J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 ' ?/cm- I B 500 540 580 620 660 700 ?/cm-' Fig. 9 Raman spectra [recorded in (a)X(ZZ)Y,(b) X(ZX)Yand (c) X(YX)Ypolarizations] showing the spectral regions for the v(C-Br) stretching vibrations of the Br(CH,),Br/urea inclusion compound :A, at 108 K (LT phase); B, at 298 K (HT phase) 'constrained ' environment of the urea tunnel structure appar- ently has little effect on the frequency of the LAM-1 mode of the Br(CH,),Br molecule, in comparison with the same mol- ecule in its pure crystalline phase. C-X Stretching Mode Study US a Function of the Length of the Guest Molecule and Temperature.Raman spectra were recorded in the X(ZZ)Y, X(2X)Y and X( YX) Y polarizations for Br(CH,),Br/urea inclusion compounds with n = 7-10 in both the LT phase and the HT phase. Representative spectra illustrating the v(C-Br) stretching modes in the X(ZZ)Y, X(2X)Y and X( YX)Y polarizations for Br(CH,),Br/urea at 108 K (LT phase) and at 298 K (HT phase) are shown in Fig. 9. The v(C-Br) band for the trans end-group is polarized X(Z2)Y (asexpected for symmetry A,) and is at ca. 656 cm-at 298 K. The v(C-Br) band for the gauche end-group is at ca. 570 cm-' at 298 K; this band is depolarized and is much less intense than the corresponding band for the trans end-group. The ratios 97 for each sample at 90 K (LT phase) and at 298 K (HT phase) are reported in Table 2.Despite our limited knowledge regarding the relationship between 9ftand the Table 2 Values of 97 [defined in eqn. (5)] measured for Br(CH,),Br/urea inclusion compounds at 90 K (LT phase) and at 298 K (HT phase) sample LT HT Br(CH ,),Br/urea 0.17 0.15 Br(CH,),Br/urea 0.05 0.08 Br(CH ,),Br/urea 0.09 0.13 Br(CH,),,Br/urea 0.10 0.13 proportion of end-groups in the gauche conformation, we assess that the proportion of end-groups in the gauche con-formation is in the range 5-14% for the LT phase and in the range 7-13% for the HT phase. From Table 2, no well defined relationship between 9;'and molecular length (n) is apparent. Raman spectra were recorded as a detailed function of temperature for Br(CH,),Br/urea in polarization [X(ZZ)Y + X(2X)Y)(in this configuration, both horizontally and vertically polarized components of the scattered radi- ation are analysed together).From the results (Fig. lo), the intensity ratio 9f[defined in eqn. (6)]increases slightly with temperature but exhibits no detectable discontinuity at the phase-transition temperature, and we conclude that the pro- portion of end-groups in the gauche conformation has only a weak temperature dependence. Study as a Function of the Terminal Functional Group on the Guest Molecule. The Raman spectra recorded in the region of the v(C-X) stretching modes for 1,8-dichlorooctane/urea, 1,8-dibromooctane/urea and 1,8-diiodooctane/urea in the X(Zz)Y, X(ZX)Y and X(YX)Y polarizations at 298 K are shown in Fig.11. From the mea- sured intensity ratios 9r,the proportion of end-groups in the gauche conformation is calculated to be ca. 51% for Cl(CH,),Cl/urea, 7% for Br(CH,),Br/urea and 1% for I(CH,),I/urea. Thus, the proportion of end-groups in the gauche conformation decreases as the size of the terminal substituent increases. Presumably a major factor here is that the gauche conformation becomes relatively more difficult to accommodate within the urea tunnel structure on moving from Cl to Br to I as the end-group. In deriving this conclu- sion from the Raman results, we have assumed that the mea- sured intensity ratios 9f*[defined in eqn. (5)] for C1(CH2),C1/urea, Br(CH,),Br/urea and I(CH,),I/urea can be compared directly. This is considered justified in view of the fact that the depolarization ratio (measured for the pure liquid) for the v(C-X) stretching vibration of the trans con-formation is similar in magnitude for the X(CH,),X mol-ecules containing the different end-groups X, and the depolarization ratio for the v(C-X) stretching vibration of the gauche conformation is similar in magnitude for the X(CH,),X molecules containing the different end-groups X.Study as a Function of Pressure. Experiments to probe the pressure dependence of the Raman spectrum were carried out on a polycrystalline sample of Br(CH,),,Br/urea (as a conse- quence of using a polycrystalline sample, the concept of polarization in these experiments is not relevant).With increase of pressure, the urea vibrational modes shift to 0.05 o.04/ 0.03t ;,., I I I I I 50 100 150 200 250 300 TIK Fig. 10 9;'us. temperature for the Br(CH,),Br/urea inclusion com- pound 1320 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 F A h(c) 1 r\1 trans 1 500 540 580 620 660 700 trans Jc) I 450 490 530 570 610 650 i/cm-' Fig. 11 Raman spectra [recorded in (a)X(ZZ)Y,(b)X(2X)Yand (c) X(YX)Y polarizations] showing the spectral regions for the v(C-X) stretching vibrations of the X(CH,),X/urea inclusion com-pounds at 298 K (HT phases): A, X = C1; B, X = Br; C, X = I higher frequencies (Fig. 12), and the intensity of the band for the v(C-Br) stretching vibration of the gauche end-groups increases markedly (Fig.13). The results of these experiments were reproducible, and the variation of the spectrum with change of pressure was identical for the low-to-high and high-to-low pressure cycles. From consideration of the v(C-N) stretching vibration and the bands at ca. 530 and 610 cm-' (characteristic of urea in the urea inclusion compounds), it is concluded that there was no pressure-induced decomposition of the Br(CH,), 'Br/urea inclusion compound under the pressures exerted in these experiments. We now consider in more detail the v(C-Br) stretching mode [at V = 570 cm-'for the gauche end-group, and at V x 660 cm-' for the trans end-group (Fig. 13)]. As seen from 129 cm-' 165 cm-' I I *II(1:141 cm-' 183 cm-l- L...#...,...,,, ,...y 80 104 128 152 176 200 i/cm-Fig.12 Raman spectra showing the spectral region for the lattice modes of the Br(CH,),,Br/urea inclusion compound at 298 K (HT phase). p = (a)1 and (b)6 x lo3bar. Fig. 13, the intensity ratio: p-Z(570 cm -') 3-I(660 cm -I) (9) increases markedly as the applied pressure is increased. Con- sidering 99 as a measure of the equilibrium constant for trans s gauche interconversion, and using AVE to denote the difference in molar volume between the gauche end-group and the trans end-group (AVE = V:gauche -V?"'), it can be shown (assuming that AVE is independent of pressure over the restricted range of pressures considered here) that Sg varies with the applied pressure (p) according to : where C is a constant independent of p.Fig. 14 shows a graph of ln(ff) us. p at T = 298 K; the value of AVE deter-mined from the best-fit line is AVE = -7.4 cm3 mol-'. It has been suggested32 that the cross-sectional area of the urea tunnel decreases with increase in pressure, and it is also prob- able that the periodic repeat distance (ch) of the urea tunnel structure along the tunnel axis also decreases with increase in pressure. In view of this reduction in the space available within the urea tunnel structure on increasing the pressure, it may be expected that reduction of the volume of the guest molecule (for example, by conversion from a trans end-group to a gauche end-group) will be favoured on increasing the pressure.While this explanation is consistent with the observed pressure dependence of Yf,we make no attempt in this paper to pursue this argument further in view of the fact that the different shapes (as well as the different volumes) of the gauche and trans end-groups must be taken into account in assessing the feasibility of fitting these different conforma- tions of the guest molecule within the urea tunnel structure at increased pressure. This issue clearly requires a more detailed future investigation. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 I I trans I 500 540 580 620 660 700 i/crn-' Fig. 13 Raman spectra showing the change in spectral region for the v(C-Br) stretching vibration, as a function of pressure, for the Br(CH,),,Br/urea inclusion compound at 298 K (HT phase): p = (a) 1, (b)4 x lo3and (c)6 x lo3bar.Finally, we stress that the results reported here refer to urea inclusion compounds under hydrostatic pressure gener- ated by helium gas as compressing fluid. Further experiments using other compressing fluids (e.9. N,, CO,, Ar, Xe) are 0.0 .a , h 'lo -0.5-B' '"s v L' -C -1 .o -I' , I -----I--L I 140i---l ' 1 0 1000 2000 3000 4000 5000 6000 7000 p/bar Fig. 14 ln(Yf) us. pressure for the Br(CH,), Br/urea inclusion com- pound at 298 K (HT phase) required before a full understanding of the effect of the com- pressing fluid on the results from such experiments can be obtained; at this stage, we cannot rule out the possibility that atoms of the compressing fluid enter the tunnels of the inclu- sion compound during these experiments.Concluding Remarks From the Raman investigations reported here, some general conclusions can be drawn regarding the structural and con- formational properties of the a,o-dihalogenoalkane/urea inclusion compounds. The fact that changes in spectral characteristics on passing through the phase transition are small suggests that the LT phase is structurally very similar to the HT phase (as sug- gested previ~usly~~~ for alkane/urea inclusion compounds). Two strongly polarized urea lattice modes were identified (in agreement with other authors);21,33.34 an X( YX)Y polarized mode at ca.180 cm-' (LT phase) and ca. 163 an-' (HT phase), and an X(Z2)Y polarized mode at ca. 130 cm-' (LT phase) and ca. 129 cm-' (HT phase). It is important to con- trast the strong temperature dependence of the X(YX)Y pol-arized mode with the weak temperature dependence of the X(Z2)Y polarized mode. In view of the known changes in the urea tunnel structure upon crossing the phase transition temperature, the differing temperature dependences of these modes may imply that the mode which is polarized X(YX)Y may involve translations of the urea molecules in a plane almost perpendicular to the tunnel axis whereas the mode which is polarized X(Z2)Y may represent a vibration with a large component along the tunnel direction. The latter band has been attributed34 to the 'totally symmetric breathing mode of the lattice', with the proposal that this mode relates to the periodic change in the cross-section of the urea tunnel; such an assignment would imply that the frequency of this mode should have a larger temperature dependence than that observed in our experiments.The LAM-1 modes due to the a,o-dihalogenoalkane guest molecules [X(CH,),X] were identified in the LT phase and less readily in the HT phase. As expected, the LAM-1 fre- quency decreases as the length of the guest molecule is increased within the series with odd n, and decreases as the length of the guest molecule is increased within the series with even n. On applying the Minoni-Zerbi model2' to these systems, the calculated frequencies were in reasonable agree- ment with the experimental values for the Br(CH,),Br guest molecules with even n, suggesting that a large proportion of the included Br(CH,),Br molecules are in the fully extended all-trans conformation.However, the experimentally observed LAM- 1 frequencies for the Br(CH,),Br molecules with odd n cannot be described by the Minoni-Zerbi model. Substantial further work is required to interpret fully the observed bands in the low-frequency region of the Raman spectra of the Br(CH,),Br/urea inclusion compounds and also the pure crystalline phases of the a,o-dibromoalkanes. The C-X stretching modes due to the trans and gauche end-groups were identified for all of the X(CH,),X/urea inclu- sion compounds studied.The intensity ratios suggest that the proportion of end-groups in the gauche conformation is ca. 5-14%. Studies of the evolution of the measured intensity ratios as a function of terminal substituent, temperature and pressure show the following: (a)9":decreases with increase in the size of the terminal substituent; (b)Yf increases slightly with increasing temperature [for Br(CH,),Br/urea inclusion compounds]; (c) 9!t increases markedly with increase in applied pressure [for Br(CH,), ,Br/urea]. In the current study, changes in the measured intensity ratio 9y were used 1322 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 to assess changes in the proportion of end-groups in the gauche conformation. In view of the differences in polariza- tion of the v(C-X) modes for the trans and gauche end-groups, this approach for assessing trends in the proportion 6 7 8 Y.Chatani, H. Anraku and Y. Taki, Mol. Cryst. Liq. Cryst., 1978,48,219. K. D. M. Harris, I. Gameson and J. M. Thomas, J. Chern. SOC., Faraday Trans., 1990,86,3135. A. E. Aliev, I. J. Shannon, S. P. Smart and K. D. M. Harris, in of gauche end-groups has substantially more justification than the use of this approach to assess the absolute value of the proportion of gauche end-groups. It is clear from the results reported here that intercon- version between the gauche and trans conformations of the end-group is slow with respect to the timescale of the Raman measurement at all temperatures investigated here, and other techniques are required to elucidate quantitative dynamic information on this conformational exchange process.In this regard, it is relevant to note that our high-resolution solid- state ' 3C NMR studies of a,o-dihalogenoalkane/urea inclu-sion compounds35 are consistent with the guuche-trans interconversion being rapid with respect to the timescale of 9 10 11 12 13 14 15 16 preparation. H. G. McAdie, Can. J. Chem., 1962,40,2195. K. D. M. Harris and M. D. Hollingsworth, Proc. R. SOC.London A, 1990,431,245. A. J. 0. Rennie and K. D. M. Harris, Proc. R. SOC. London A, 1990,430,615. A. J. 0. Rennie and K. D. M. Harris, J. Chem. Phys., 1992, %, 7117. I. J. Shannon, K. D. M. Harris, A. J. 0. Rennie and M. B. Webster, J. Chem. SOC.,Faraday Trans., 1993,89,2023. S. P. Smart, F.Guillaume, K. D. M. Hams, C. Sourisseau and A. J. Dianoux, Physica B, 1992,180,181,687. F. Guillaume, S. P. Smart, K. D. M. Harris and A. J. Dianoux, J. Phys. : Condens. Matter, in the press. J. J. Martin, R. Cavagnat, J. C. Cornut, M. Couzi, G. Daleau, J. this technique at all temperatures investigated. The existence of end-groups in the gauche conformation in alkane/urea inclusion compounds has recently been the source of considerable controversy. IR spectroscopy2' and molecular dynamics simulation36 have both suggested that the proportion of end-groups in the gauche conformation is small (less than 3% and 5%, respectively). Raman s~attering~~?~~(on the basis of bond polarizability model calculation^^^) and 2H NMR3' experiments have shown that the amount of gauche end-groups in a series of alkane/urea inclusion compounds is ca.5%. In contrast to these results, it has been propo~ed,~'.~' on the basis of 2H NMR, 13C NMR, 17 18 20 21 22 19 Devaure, M. Maissara and R. Mokhlisse, Appl. Spectrosc., 1986, 40,217. R. G. Snyder, J. Mol. Spectrosc., 1971,37, 353. The Theory of Vibrational Spectroscopy and its Application to Polymeric Materials, ed. P. C. Painter, M. M. Coleman and J. L. Koenig, Wiley, New York, 1982, p. 336. Characteristic Raman Frequencies of Organic Compounds, ed. F. R. Dollish, W. G. Fateley and F. F. Bentley, Wiley, New York, 1974. H. L. Casal, J. Phys. Chem., 1990,94,2232. J. Le Brumant, M. Jaffrain and G. Lacrampe, J. Phys. Chem., 1984,95, 1548. V. Fawcett and D. A. Long, in Ado. Raman Spectrosc., 1973, 1, 570.molecular mechanics (MM2) and molecular dynamics simula- tions, that the amount of gauche end-groups may be as large as 40%. Despite the diversity of results obtained from these experi- ments, it is nevertheless clear that alkane and a,o-dihaloge- noalkane guest molecules within the tunnels of urea inclusion 23 24 25 26 S. Mizushima and T. Schimanouchi, J. Am. Chem. SOC.,1949,71, 1320. S. L. Hsu and S. Krimm, J. Polm. Sci., Polym. Phys. Ed., 1977, 15, 1769. G. Minoni and G. Zerbi, J. Phys. Chem., 1982,86,4791. R. F. Schaufele and T. Schimanouchi, J. Chem. Phys., 1967, 47, 3605. compounds contain some amount of end-groups in the gauche conformation, and it is proposed that the existence of these conformational defects depends strongly on the longitu- dinal packing of the guest molecules within the tunnels.There is clearly considerable scope for future work to extend further our understanding of the conformational properties of the guest molecules in urea inclusion compounds. 27 28 29 30 31 32 K. Viras, F. Viras, C. Campbell, T. A. King and C. Booth, J. Phys. Chem., 1989,93,3479. H. G. Olf and B. Fanconi, J. Chem. Phys., 1973,59,534. H. G. M. Edwards, V. Fawcett and M. T. Lung, J. Inclusion Phenom. Mol. Recognit. Chem., 1991,11,267. J. Mazur and B. Fanconi, J. Chem. Phys., 1979,71,5069. F. Viras, K. Viras, C. Campbell, T. A. King and C. Booth, J. Polym. Sci., Part B, Po1ym.-Phys., 1991,29, 1467. K. Fukao, T. Horiuchi, S. Taki and K. Matsushige, Mol. Cryst. The authors wish to thank Dr R. Cavagnat (CNRS, LSMC) for technical assistance and Dr C. Sourisseau (CNRS, LSMC) for fruitful discussions. The SERC is thanked for financial support (studentship to S.P.S. and general support to K.D.M.H.),and the University of St. Andrews is thanked for 33 34 35 Liq. Cryst., 1990, lWB, 405. V. Fawcett and D. A. Long, J. Raman Spectrosc., 1975,3,263. M. Kobayashi, H. Koizumi and Y. Cho, J. Chem. Phys., 1990, 93,4659. A. E. Aliev, S. P. Smart and K. D. M. Harris, unpublished results. the award of an Ettie Steele Travel Scholarship to S.P.S. 36 K-J. Lee, W. L. Mattice and R. G. Snyder, J. Chem. Phys., 1992, 96,9138. References 1 K. D. M. Harris, J. Solid State Chem., 1993,106,83. 2 A. E. Smith, Acta Crystallogr., 1952,5,224. 3 K. D. M. Harris and J. M. Thomas, J. Chem. SOC., Faraday Trans., 1990,86,2985. 4 K. D. M. Harris, S. P. Smart and M. D. Hollingsworth, J. Chem. 37 38 39 40 A. El Baghdadi, Ph.D. Thesis, University of Bordeaux I, 1993. Y. Kim, H. L. Strauss and R. G. Snyder, J. Phys. Chem., 1989, 93,485. G. M: Cannarozzi, G. H. Meresi, R. L. Vold and R. R. Vold, J. Phys. Chem., 1991,95,1525. F. Imashiro, D. Kuwahara, T. Nakai and T. Terao, J. Chem. Phys., 1989,90,3356. SOC.,Faraday Trans., 1991,87,3423. 5 S. P. Smart, Ph.D. Thesis, University of St. Andrews, 1993. Paper 3/07016F; Receioed 25th November, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949001313

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(9), 1323-1328 Conformational Properties of Monosubstituted Cyclohexane Guest Molecules Constrained within Zeolitic Host Materials A Solid-state NMR Investigation Abil E. Aliev and Kenneth D. M. Harris* Department ofChemistry, University College London, 20 Gordon Street, London, UK WCIH OAJ Raphael C. Mordi Department of Chemistry, University of St. Andrews, St. Andrews, Fife, UK KY16 9ST The conformational properties of monosubstituted cyclohexane guest molecules (C6H1, X with X = CH, , OH, CI, Br and I) included within microporous solid host materials (silicalite-I, H-ZSM-5, NH,-mordenite and zeolite NH,-Y) have been elucidated via high-resolution solid-state '3C NMR spectroscopy. For all of the inclusion compounds investigated, the fraction of monosubstituted cyclohexane molecules in the equatorial conformation is similar to that in solution, suggesting that these host materials do not impose any significant constraints upon the conformational properties of the monosubstituted cyclohexane guest molecules.For the mono-halogenocyclohexane guest molecules (C6H11X with X = CI, Br and I), this result is in marked contrast to the situation for the same guest molecules in the thiourea host structure, for which the conformational properties of the guest molecules are substantially different from those of the same molecules in solution. For cyclohexanol (C6H1 OH) in H-ZSM-5, some amount of dicyclohexyl ether (C6H1 OC6H1 1) is observed, and is analogous to the proposed production of dimethyl ether in the first stage of methanol-to-gasoline conversion on this zeolite.The comparatively low temperature (ambient temperature) at which this conversion from cyclohexanol to dicyclo- hexyl ether occurs is noteworthy. In addition to our high-resolution solid-state 13C NMR studies of these materials, 'H MAS and 27AI MAS NMR spectra have also been recorded, and are discussed. One major impetus underlying current research on solid inclusion compounds is the desire to investigate the proper- ties that may be conferred upon an organic 'guest' molecule by virtue of embedding it within a crystalline 'host' material and to understand the extent to which the properties of the guest molecule may be altered from those of the same mol- ecule in dispersed phases or in its 'pure' crystalline phase.Many solid host materials are known, encompassing aluminosilicates, aluminophosphates, organic solids and many other classes of material. These host materials possess a wide variety of different inclusion topologiesy1v2 such as linear tunnels, isolated cages, networks of intersecting tunnels and/or cages, and two-dimensional regions within layered hosts. Monohalogenocyclohexanes (C6HllX, with X = C1, Br, I) generally exist as an equilibrium between axial and equato- rial conformations (with a chair conformation of the cyclo- hexane ring). In the liquid and vapour phases there is a considerable excess of the equatorial conf~rmer,~-~ whereas in the solid state it has been reported6 that only the equato- rial conformation exists.However, when included as guest molecules within the thiourea host structure (the inclusion topology of which comprises uni-directional tunnels'), C6HllC1, C6Hl,Br and C6H111 have been shown to exist predominantly in the axial conformation. These results have been established from IR,8 Raman' and high-resolution solid-state 13C NMR1'*' techniques. High-resolution solid-state 13C NMR investigations' of thiourea inclusion compounds containing monosubstituted cyclohexane guest molecules (C6H1 ,X) have shown that these guest molecules can be subdivided into two classes: those with X = C1, Br and I have a predominance of the axial con- former (fraction of equatorial conformer CQ.0.05-0.15), whereas those with X = CH,, NH,, and OH have a pre- dominance of the equatorial conformer (fraction of equatorial conformer ca. 0.82-0.97). The fact that the conformational properties of the C6HllX guest molecules within their thio- urea inclusion compounds depend critically upon the identity of the substituent X reflects the fine and subtle energetic bal- ances that exist for these inclusion compounds, and impor- tant insights into the reasons underlying the preference for the axial conformation of the C6Hl,C1 guest molecules in thiourea have been obtainedI2 from the application of a theoretical approach that has been developed for the predic- tion and rationalization of structural properties of one-dimensional inclusion compounds.In this paper, we have extended our studies of the confor- mational properties of organic guest molecules in constrained solid-state environments to encompass monosubstituted cyclohexane guest molecules within several crystalline zeolitic host materials. From the results, direct comparisons can be drawn between the inclusion compounds of these micro- porous hosts and the inclusion compounds containing the same guest molecules within the thiourea host structure. Such comparisons are particularly interesting in view of the pro- spect that the thiourea inclusion compounds (and other solid organic inclusion compounds) may, in many respects, rep- resent model systems for structurally similar (i.e. possessing uni-directional tunnel topologies) zeolitic host materials. In this work, the following microporous host materials have been considered : silicalite-I, H-ZSM-5, NH,-mordenite and zeolite NH,-Y.ZSM-5 is a medium-pore zeolite, the structure of which consists of a set of sinusoidal tunnels inter- secting a set of straight tunnels, each with 10-membered ring openings. The diameter of the straight tunnels is CQ. 5.3-5.6 A, and the diameter of the sinusoidal tunnels is ca. 5.1-5.5 A. The framework structure of silicalite-I is the same as that of ZSM-5, and can therefore be regarded as the purely siliceous version of ZSM-5. Zeolite Y has 12-membered rings of ca. 7.4 8, diameter leading to a supercage with diameter ca. 13 A, whereas mordenite has 12-membered rings forming one-dimensional tunnels with diameter CQ.6.5-7.0 A. The conformational properties of monosubstituted cyclo- hexane guest molecules (C6H1 ,X with X = CH3, OH, Cl, Br and I) included within these host materials have been investi- gated uia high-resolution solid-state '3C NMR spectroscopy. The results are discussed in the light of our previous studies" of the conformational properties of the same guest molecules included within the thiourea host tunnel structure and in the solution state. Experimental The following host materials were used in this work: silicalite-I, H-ZSM-5 (Laporte Inorganics, RD 1136/88), NH,- mordenite (Si/Al =lO.l), and zeolite NH,-Y (Strem Chemi- cals, Inc.). All of these samples were calcined in a mume furnace at 773 K for at least 24 h before use. The mono- subs ti tuted cyclohexanes were obtained commercially and were used without further purification, with the exception of C,H,,Cl which was distilled at 415 K before use.Two different methods for including the guest molecules within the host materials were considered. In method A, the monosubstituted cyclohexanes were adsorbed into the host materials by contacting about 5 cm3 of the liquid mono- substituted cyclohexane with ca. 0.6-1.0 g of the powdered host material in a round-bottomed flask under vacuum for ca. 3 days. After this period, the excess liquid was removed under vacuum and the solid allowed to dry. The flask was sealed and removed to a dry box, in which the solid was transferred to the rotor to be used in the solid-state NMR experiments.In method B, the host material was exposed to the liquid monosubstituted cyclohexane for ca. 20-40 h in an ultrasonic bath at ca. 303 K. After this treatment, the excess liquid was removed and the solid washed with 2,2,4-tri- methylpentane and then allowed to dry. The solid was packed into the NMR rotor in the open laboratory (ie.not in a dry box). It is clear that the amount of water present within the host materials may be higher for those materials prepared via method B. In order to confirm that the monosubstituted cyclohexanes were adsorbed on the internal (rather than the external) sur- faces of the host materials, a control experiment was carried out by subjecting a sample of quartz to the same preparation procedures (with C6H1 ,C1 as the potential adsorbate).High- resolution solid-state I3C NMR spectra of the samples of quartz recovered following these preparation procedures revealed no detectable amounts of C,H,,C1 (which, if present, would necessarily have been adsorbed on the exter- nal surfaces of the quartz). On the basis of the results of these control experiments, it was concluded that the amounts of monosubstituted cyclohexanes on the external surfaces of the host materials, subjected to the same preparation procedure, would be insignificant, and that any detected amounts of these molecules must be adsorbed on the internal surfaces of the host materials. Solid-state 'H, 13Cand 27Al NMR spectra were recorded at 500.13, 125.76 and 130.32 MHz, respectively, on a Bruker MSL500 spectrometer using a standard Bruker magic-angle sample spinning (MAS) probe with double-bearing rotation mechanism.The samples were studied as polycrystalline powders in zirconia rotors (4 mm external diameter) and MAS frequencies between 2 and 12 kHz (with stability better than ca. k10 Hz) were used. Single-pulse and cross-polarization (CP) techniques were used to record the 13C NMR spectra, under conditions of MAS and with inverse-gated 'H decoupling applied during acquisition. Although the CP technique is intrinsically non-quantitative (since the efficiency of polarization transfer may vary from one carbon environment to another), our experiments have shown that single-pulse and CP (contact time =1 ms) tech- niques give the same relative intensities for the resonance lines for the cyclohexane derivatives at room temperature.I3C and 'H chemical shifts are given relative to tetra-J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 methylsilane and 27Al chemical shifts are given relative to the signal assigned to Al(H,0)63+ in the sample. The stability and accuracy of the temperature controller (Bruker B-VT1000) were ca. k2 K. Results and Discussion Fig. 1 shows I3C CP-MAS NMR spectra of C,H,,Br/H-ZSM-5 (prepared by method A), recorded at 293 and 200 K. At 200 K, there are two sets of signals with approximate inte- grated intensity ratio 4 :1. The I3C NMR resonance lines with chemical shifts 39.4, 25.0 and 27.9 ppm are assigned as carbons C(2), C(3) and C(4) in the equatorial conformer and the resonance lines at 34.7 and 21.1 ppm are assigned as carbons C(2) and C(3) in the axial conformer.These chemical shifts are in close agreement with those found for the C,H 1,Br/thiourea inclusion compound.' The resonance line for C(l) is broad (particularly at low temperature), and it is possible that second-order quadrupolar effects from bromine contribute to this broadening. l3 The I3C CP-MAS NMR spectrum of C,H, ,CI/H-ZSM-5 (prepared by method A) recorded at 200 K also contains two sets of signals, assigned to equatorial [6,(,, =38.2 ppm; SCO, =24.8 ppm; bC(,)=27.1 ppm] and axial [6c(2)=34.7 ill"l'""ll""ll'''~~i' 60 50 40 30 20 10 6 Fig.1 13C CP-MAS NMR spectra of C6H,,Br/H-ZSM-5 (pre- pared using method A) recorded (a)at 200 K and (b)at 293 K J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 ppm; 8c(3)= 20.7 ppm] conformers, with approximate inte- grated intensity ratio 4 :1. As in the case of C6H1 ,Br/H- ZSM-5, the resonance line for C(l) is broad (particularly at low temperature), and it is possible that second-order quad- rupolar effects from chlorine contribute to this broadening.' Although such low-temperature '3C NMR spectra provide direct identification of the axial and equatorial conformers, the low loadings of guest molecules within the host materials and the fact that the resonance lines are comparatively broad (as discussed in more detail below) complicate considerably the application of 13C NMR for an accurate study of the conformational properties of the guest molecules in these inclusion compounds. To determine the relative proportions of equatorial and axial conformers at ambient temperature, we have employed a technique which considers NMR param- eters averaged by the axial-equatorial exchange.This tech- nique has been applied successfully to study conformational equilibria for substituted cyclohexanes and other systems from solution-state NMR data. l4 For the conformational equilibrium between the axial and equatorial conformers in monosubstituted cyclohexanes (i.e. a simple two-site exchange process), the fraction (peq)of molecules in the equa- torial conformation is determined from : where 6,, and Sax are the chemical shifts for a given carbon in the equatorial and axial conformers, respectively, and (6) is the averaged chemical shift observed for the same carbon in the measured spectrum (recorded under fast-exchange conditions), see Table 1.Although this approach requires the intrinsic temperature dependence of the chemical shifts for the axial and equatorial conformations to be known, it has been shown14 that this technique provides acceptable results for the C(2) and C(3) carbons in monosubstituted cyclo- hexanes in the solution state; furthermore, the temperature dependence of the isotropic chemical shifts for these mol- ecules in the solid host materials considered here may be expected to be less than in solution, in view of the compara- tively small coefficients for thermal expansion of these materials.For the C(2) carbon of the C6H11X guest mol- ecules considered here, the 13C NMR chemical shift can be determined with an accuracy of ca. k0.2 ppm, leading to a percentage error of ca. 10% in the estimate of peq; although this error is comparatively large, the results obtained by this method are nevertheless sufficiently accurate to distinguish whether the conformational properties of the guest molecules I325 in these host materials resemble those of the same molecules in the thiourea host structure or in solution. Using this tech- nique, the values of peq for C6HllCl/H-ZSM-5 and for C,H, ,Br/H-ZSM-5 were estimated [from the chemical shift values for C(2)] to be ca.0.8 in both cases. Values of peq for C6HI1C1/H-ZSM-5 and for C6HllBr/H- ZSM-5 have also been determined directly from the inte- grated intensity ratios for the peaks due to C(2) in the 13C CP-MAS NMR spectra recorded at 200 K, and it is inter- esting to note that these values are in close agreement (see Table 2) with those determined at 293 K via the method described above. It should be noted that our assessment of peq from integrated peak areas in the 13C CP-MAS NMR spectra recorded at 200 K is based upon the assumption that the CP efficiency for a given carbon environment [specifically C(2)] is the same in the axial and equatorial conformations. This assumption is reasonable in view of the fact that the protons dGtly bonded to carbon will dominate the polar- ization transfer from protons to carbon in the CP experi- ment, together with the fact that the geometry of the CH, group is essentially the same for the axial and equatorial con- formations.For all the other inclusion compounds studied in this Table 2 Fraction (peq) of equatorial conformer, as a function of temperature, for monosubstituted cyclohexane (C,H ,X)molecules in solid host materials and in solution X CH3 OH I Br c1 environment thiourea solution H-ZSM-5 thiourea solution H-ZSM-5 thiourea solution H-ZSM-5 thiourea solution H-ZSM-5 H-ZSM-5 thiourea solution H-ZSM-5 H-ZSM-5 silicalite-I NH,-mordenite NH,-Y T/K Peq 208 0.97 200 0.99 293 1 198 0.82 200 0.96 303 0.9 177 0.15 220 0.76 293 0.8 208 0.05 200 0.75 200 0.83 293 0.8 200 0.08 200 0.81 200 0.76 293 0.8 293 0.8 293 0.8 293 0.8 Table 1 13C NMR chemical shifts for monosubstituted cyclohexane (C,H, ,X)guest molecules within microporous host materials CH3 H-ZSM-5 293 33.5 36.3 27 27 21.1 OH H-ZSM-5 293 72.5 36.4 25 25 I H-ZSM-5 293 34 40.9 25.0 29.0 Br H-ZSM-5 293 53 38.6 27 27 Br H-ZSM-5 200 * 39.4 (eq) 25.0 (eq) 27.9 (eq)* 34.7 (ax) 21.1 (ax) * c1 H-ZSM-5 293 60 37.5 26 26 c1 H-ZSM-5 200 * 38.2 (eq) 24.8 (eq) 27.1 (eq)* 34.7 (ax) 20.7 (ax) * c1 silicalite-I 293 60 37.5 26 26 c1 NH,-mordenite 293 63 37.6 26 26 c1 NH,-Y 293 63 37.6 25.5 27.0 ~~ Data for the equatorial and axial conformations are indicated by (eq) and (ax), respectively; entries marked * in the table refer to data that cannot be established from the spectra.1326 paper, the boundary values for a,,and 6,, in eqn. (1) were taken as the values determined from the low-temperature high-resolution solid-state '3C NMR spectrum of the appro- priate C6H ',X/thiourea inclusion compound.' 'The values of pes determined via this approach are presented in Table 2, which also contains the corresponding data (taken from ref. 11) for the C6H, ,X/thiourea inclusion compounds and for C6Hl1X in solution. From Table 2, the proportion of mol- ecules in the equatorial conformation is greater than ca. 0.8 for all C6H,,X guest molecules in the zeolitic host materials at ambient temperature, and the conformational properties of the C,H, ,X guest molecules in these inclusion compounds are thus very similar to those of the same molecules in solu- tion. The uncharacteristic conformational behaviour found for the C6H,,X guest molecules with X =c1, Br and 1 in their thiourea inclusion compounds is not reproduced for these guest molecules in the zeolitic host materials investi- gated here.Fig. 2 shows I3C CP-MAS NMR spectra, recorded at 293 K, of C6Hl,Cl/H-ZSM-5 prepared by method A [Fig. &I)] and by method B [Fig. 2(b)]. The linewidths of the reson- ances are in the range 180-400 Hz, and the chemical shifts (Table 1) are in good agreement with those expected for the equatorial conformer of C6H1 ,C1 (on the basis of substituent chemical shift parameters for the C1 substituent determined from low-temperature solution state 13C NMR studies' ',13,14 and from low-temperature solid-state 13C NMR studies of the C,H,,Cl/thiourea inclusion compound' ').These linewidths are larger than those (16-27 Hz) observed previously' 'for the C6H1 ,Cl/thiourea inclu- sion compound at 293 K.The linewidth of the resonance (at ca. 60 ppm) due to the C(1) carbon (directly bonded to C1) is particularly large in comparison with the other resonances. For the other inclusion compounds studied in this paper, the 13C NMR linewidths are also larger than for the same guest molecule in its thiourea inclusion compound. There are several possible explanations for this observation.One pos- sible explanation is that each broad resonance line observed in the 13C CP-MAS NMR spectra of C6HllC1/H-ZSM-5 comprises a superposition of several isotropic peaks (with dif- ferent isotropic chemical shifts), each representing C6H 1,X guest molecules in a different environment with respect to the H-ZSM-5 host structure. The non-Lorentzian lineshape observed (Fig. 2) for the C(1) carbon in C6HllC1/H-ZSM-5 is consistent with this proposal. Specifically, the signal due to the C(l) carbon in Fig. 2(a) and (b) can be considered as a superposition of at least two components: a 'broad' com-ponent at higher frequency and a 'narrow' component at lower frequency. In samples prepared by the two different methods, the ratio of the 'broad' and 'narrow' components is different, resulting in the different lineshapes for this signal in Fig.2(a)and (b). For C6Hl1C1/H-ZSM-5 (prepared by method B) and C6Hl,Br/H-ZSM-5 (prepared by method B), a weak signal was detected at ca. 73.1 ppm in the 13C NMR spectrum [Fig. 2(b)]; this signal is assigned to the C(1) carbon of C6Hl,0H (with C6HllX :C6H110H z7 for X =C1 and Br). A signal at ca. 74.5 ppm was also detected for C6Hl,C1/NH,-Y, and is also assigned to the C(1) carbon of C6Hl10H (with C6H11C1 C6Hl10H X 2). The presence Of C6Hl10H is attributed to the occurrence of a hydration reaction of C H -!L-6 C1 and-C6H_Br within the H-ZSM-5 and NH,-Y host materials. In this regard, it should be recalled that the amount of water present within the host materials is likely to be higher for those materials prepared uia method B.This assignment of the signal at ca. 73-75 ppm in the 13C NMR spectra of C6H1 ,Cl/H-ZSM-5, C6H1 ,Br/H-ZSM-5 and C&i1 ,Cl/NH,-Y samples was confirmed by recording the J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 !I IIII I,,, I,,, I,,/ ,Ill ,,I, ,I,, I1' 80 70 60 50 40 30 20 10 I I,,, ,I,, ,,,I I,,, ,,It I,,, ,(,,,, 80 70 60 50 40 30 20 10 s Fig. 2 13CCP-MAS NMR spectra of C,H,,Cl/H-ZSM-5 recorded at 293 K: (a)for the sample prepared using method A and (b)for the sample prepared using method B 13C NMR spectra for C6H1 ,OH/H-ZSM-5 samples prepared using methods A and B [Fig. 3(a) and (b)]; in these spectra, the chemical shift for the C(l) carbon is 73.2 ppm.For C6H, ,OH/H-ZSM-5, however, there is an additional signal at 79.2 ppm in the 13C NMR spectrum. This signal is assign- ed as the C(1) carbon of dicyclohexyl ether (C6H1,0C6H11), with C,HllOH :C6H,1OC6Hll x 4 for the sample prepared by method A and 1.3 for the sample prepared by method B. I3C NMR chemical shift increments reported" for alkyl groups R in ethers C6H110R corroborate our assignment of this signal to the c(1) carbon of C6Hl,0C6Hl,. Other signals [C(2), C(3) and C(4)] for C6H,,OC6Hl1 are in the region 24-36 ppm and overlap the signals due to C6H,,0H. It is interesting to note that an analogous conversion of methanol into dimethyl ether has been proposed as the first stage of methanol-to-gasoline conversion on H-ZSM-5.l6 As for C6H, ,X/thiourea inclusion compounds,' 'there is substantial line narrowing in the 'H MAS NMR spectra of C6H1,X/H-ZSM-5 inclusion compounds. Analogous line narrowing has also been observed for zeolite and cyclo- phosphazene inclusion compounds containing various substi- tuted benzenes as the guest species.17*'* It has been suggested' that, for such systems, all the dipolar interaction tensors have their principal axes in the same direction, either J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 1"''I~"'I'"'I""1"''1"''1""1'"'I""I 90 80 70 60 50 40 30 20 10 6 Fig. 3 13C CP-MAS NMR spectra of C6H,,0H/H-ZSM-5 record- ed at 293 K: (a) for the sample prepared using method B; (b)for the sample prepared using method A because of special features of the crystal stru~ture,'~ or because the whole molecule performs rotational motions which substantially average the intermolecular dipolar inter- actions.'8 In the case of C6HllC1/H-ZSM-5, the narrow 'H NMR resonance lines observed for the guest molecules (Fig. 4) suggest that 'H-'H dipoleaipole interactions (including the intermolecular dipole-dipole interactions between the 'H of the host and the 'H of the guest) are aver- aged substantially as a result of high conformational, rota- tional and translational mobility of the guest molecules at room temperature. For C6H1 ,CI/H-ZSM-5, 'H NMR reson- ances at 6 =1.7 ppm [linewidth at half-height AvIl2 x 0.6 ,..I' ,I..,. .,. .I. .,. . . 10.0 8.0 6.0 4.0 2.0 0.0 -2.0 4.0 s Fig. 4 'H MAS NMR spectrum of C6HllCl/H-ZSM-5 (prepared using method B) recorded at 293 K (MAS frequency =9.3 kHz). The sharp signal at 0.2 ppm is due to tetrakis(trimethylsi1yl)silane (added as an internal standard); this signal does not overlap with any signals from C6Hl ,Cl/H-ZSM-5. -. 100 80 60 40 20 0 -20 -Fig. 5 27Al MAS NMR spectra of C,H,,Cl/H-ZSM-5 (prepared using method A) recorded (a) at 200and (b)at 293 K kHz; assigned as H(2), H(3) and H(4) of C,H,,Cl], 6 =4.0 ppm [Avl12 x 0.4 kHz; assigned as H(l) of C6HllC1], 6 =2.2 ppm (AvlIz xO.3 kHz; assigned as 'H in H-ZSM-5) and 6 =5.6 ppm (Avl12 x 0.7 kHz; assigned as 'H in H-ZSM-5) are observed in the 'H MAS NMR spectrum recorded at 293 K (Fig.4). For unloaded zeolites,20 the line at 2.2 ppm can be assigned to the non-acidic OH groups (e.g. terminal OH groups at the outer surface of the zeolite or at structural defects). The line at 5.6 ppm can be assigned to the acidic bridging OH groups in the zeolite framework and/or to water molecules present within the structure; the comparatively large linewidth may arise from the presence of both of these types--of OH group. 27Al MAS NMR spectra were also obtained for the inclu- sion compounds with H-ZSM-5 as the host material. Fig. 5 shows 27Al MAS NMR spectra of C,HllC1/H-ZSM-5 recorded at 293 and 200 K. The broad line at 54.9 ppm (Av,,, x 690 Hz at 293 K and Avl12 x 860 Hz at 200 K) is assigned the the four-coordinated aluminium in the host framework. The narrow line at 0 ppm (Avl,, x 60 Hz) in the spectrum recorded at 293 K is assigned to the six-coordinated aluminium in Al(H,0),3+.21 The line at 0 ppm is considerably broader at 200 K (Avl,, x 1 kHz) than at 293 K, consistent with the suggestion that the Al(H20)63+ has less motional freedom at low temperature.Concluding Remarks The high-resolution solid-state '3C NMR results reported here suggest that the solid host materials silicalite-I, H-ZSM- 5, NH,-mordenite, and zeolite NH,-Y do not impose any major constraints upon the conformational properties of the monosubstituted cyclohexane guest molecules studied (C,H,,X with X = CH, , OH, C1, Br and I); the relative pro- portions of the axial and equatorial conformations of these guest molecules are the same, within experimental error, as those of the same molecules in solution (at the same temperature).It is interesting to speculate on the reasons underlying the difference in behaviour for the C,H,,X guest molecules with X = C1, Br and I in the thiourea host structure (for which the axial conformation predominates) compared with the zeolitic host structures considered here (for which the equatorial con- formation predominates). One major difference concerns the effective loading of guest molecules within these host struc- tures. For thiourea inclusion compounds, the host structure is stable only when there is a dense packing of guest mol- ecules within the tunnels, and this tunnel structure collapses to a more compact structure if the guest molecules are removed ; thus, the thiourea inclusion compound containing a particular type of guest is known at only one specific guest : host ratio (corresponding to ‘saturation’).Zeolitic hosts, on the other hand, generally remain stable if the guest molecules are removed, and, as a consequence, inclusion compounds can be formed between a particular zeolitic host and a particular guest species with a range of guest concen- trations [ranging from zero (‘empty’ host) to some maximum value (corresponding to saturation)]. For the preparation methods employed in this work, the loading of guest mol- ecules is actually rather low [in the range 0.1-1.0 guest mol- ecules per loo0 A3 of the host material, determined from elemental analysis (carbon percentage) results], which is con- siderably lower than saturation.It is, therefore, reasonable to assume that, in the zeolitic inclusion compounds investigated in this paper, the guest molecules are essentially ‘isolated’ from each other. As discussed in detail elsewhere,22 there are major funda- mental differences in considering the optimum structural properties of guest molecules for cases (such as the thiourea inclusion compounds) in which the inclusion compound can exist with only one specific guest : host ratio (corresponding to saturation), in comparison with those cases (such as the zeolitic inclusion compounds) in which the guest : host ratio is an experimental variable.If the inclusion compound is of the former type, and if the host structure is a strictly one- dimensional tunnel structure, it is possible to predict and rationalize the structural properties of the inclusion com-pound by applying a theoretical approach that has been developed For the inclusion compounds with zeolitic hosts (that can be prepared with essentially arbitrary guest : host ratio), on the other hand, this theoretical approach is not valid. Furthermore, the question of predict- ing and rationalizing the structural properties of the guest molecules on the basis of computed potential-energy func- tions would, in any case, become considerably more difficult for host Structures (such as ZSM-5) that do not consist of independent one-dimensional tunnels, and the methodology (analogous to that developed previously for the strictly one- dimensional inclusion compounds) required for such systems has not yet been developed.Nevertheless, for zeolitic hosts containing low loadings of guest molecules, it is qualitatively clear that the host-guest interaction energy and tne intramol- ecular potential energy of the guest molecule are the major determinants of the structural and conformational properties of the guest molecules (since the guest molecules in the inclu- sion compounds with low loadings of guest probably behave as essentially isolated molecules, the guest-guest interaction can be considered negligible). The constraints imposed upon the guest molecules by the host environment (quantified by a consideration of the host-guest interaction) could have a crucial influence in controlling the conformational properties J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 of the guest molecules, and could, in principle, outweigh the effect of the intramolecular potential energy in dictating the observed conformational behaviour of the guest molecules. The presence of C6H1 ,OC,H, ,in the inclusion compound formed between C,H,,OH and H-ZSM-5 is interesting, par- ticularly in view of the proposal that the corresponding ether (dimethyl ether) is produced in the first stage of methanol-to- gasoline conversion on this zeolite. It is particularly inter- esting that, in the case of C,H,,OH/H-ZSM-5, significant amounts of C,H,,OC,H,, are produced even at ambient temperature.Control experiments were performed to prove that the C6HllOC6Hl1 is produced within the zeolite, and not in the liquid C,H,,OH phase during preparation of the inclusion compound (i.e. during adsorption of C,H1 ,OH from the liquid phase). It is interesting that the relative amount of C,H,,OC,H,, produced is higher for the inclu- sion compound prepared by method B than for the inclusion compound prepared by method A, although the exact reasons underlying this fact remain to be investigated in detail. We are grateful to the Royal Society and the SERC for the award of postdoctoral fellowships (to A.E.A.) and to the SERC for general support (to K.D.M.H.).Professor Sir John Meurig Thomas and colleagues at the Royal Institution are thanked for providing some of the zeolite samples used in this work. References 1 Inclusion compounds, ed. J. L. Atwood, J. E. D. Davies and D. D. MacNicol, Academic Press, London, 1984, vol. 1-3. 2 K. D. M. Harris, Chem. Br., 1993,29,132. 3 M. Larnaudie, Compt. Rend., 1952,235, 154. 4 P. Klaeboe, J. J. Lothe and K. Lunde, Acta Chem. Scand., 1956, 10, 1465. 5 K. Kozima and K. Sakashita, Bull. Chem. SOC. Jpn., 1958, 31, 796. 6 P. Klaeboe, Acta Chem. Scand., 1969,23,2641. 7 K. D. M. Harris and J. M. Thomas, J. Chem. SOC., Faraday Trans., 1990,86, 1095. 8 K. Fukushima, J. Mol. Struct., 1976,34, 67. 9 A. Allen, V. Fawcett and D. A. Long, J. Raman Spectrosc., 1976, 4, 285. 10 M. S. McKinnon and R. E. Wasylishen, Chem. Phys. Lett., 1986, 130,565. 11 A. E. Aliev and K. D. M. Harris, J. Am. Chem. SOC., 1993, 115, 6369. 12 P. A. Schofield, K. D. M. Harris, I. J. Shannon and A. J. 0. Rennie, J. Chem. SOC., Chem. Commun., 1993,1293. 13 R. K. Harris and A. C. Olivieri, Prog. Nucl. Magn. Reson. Spec- trosc., 1992,24,435. 14 0.A. Subbotin and N. M. Sergeyev, J. Am. Chem. SOC., 1975,97, 1080. 15 H-0. Kalinowski, S. Berger and S. Braun, Carbon-13 NMR Spectroscopy, Wiley, Chichester, 1988. 16 M. W. Anderson and J. Klinowski, J. Am. Chem. SOC., 1990,112, 10. 17 S. Sekine, A. Kubo and H. Sano, Chem. Phys. LRtt., 1990, 171, 155. 18 J. Forbes, C. Husted and E. Oldfield J. Am. Chem. SOC., 1988, 110,1059. 19 J. P. Yesinowski and M. J. Mobley, J. Am. Chem. SOC.,1983, 105, 6191. 20 H. Pfeifer, D. Freude and M. Hunger, Zeolites, 1985,5,274. 21 J. M. Thomas and J. Klinowski, Adu. Catal., 1985,33, 199. 22 A. J. 0. Rennie and K. D. M. Harris, Proc. R. SOC. London, A, 1990,430,615. 23 A. J. 0. Rennie and K. D. M. Harris, J. Chem. Phys., 1992, %, 7117. 24 I. J. Shannon, K. D. M. Harris, A. J. 0. Rennie and M. B. Webster, J. Chem. SOC.,Faraday Trans., 1993,89,2023. Paper 4/0116H;Received 10th January, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949001323

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(9), 1329-1334 Solid-state Ion Exchange in Zeolites Part 53-N H,-Y-Iron( 11) Chloride Karoly Lazar Institute of Isotopes, Hungarian Academy of Sciences, Budapest, Hungary Gabriella Pal-Borbely and Hermann K. Beyer Central Research Institute of Chemistry, Hungarian Academy of Sciences, Budapest, Hungary Hellmut G. Karge* Fritz Haber Institute of the Max Planck Society, Berlin, Germany Solid-state ion-exchange reactions proceeding upon grinding of hydrated NH,-Y zeolite and FeCI, 4H,O in air and subsequent heating in vacuum have been followed by X-ray diffraction (XRD) and temperature-programmed evolution of volatile reaction products (H,O, NH, and HCI) monitored by mass spectrometry (MS). Changes in valence state and coordination environment of the exchanged iron ions upon heat treatment up to 720 K in vacuum and air, as well as in hydrogen after treatment in air, were studied by Mossbauer spectroscopy.Partial ion exchange and oxidation to cationic hydroxy-iron(iii) species proceed upon mere grinding. Heat treatment in vacuum results in partial autoreduction and characteristic environmental changes of di- and tri-valent Fe species, especially in the temperature range between 620 and 720 K. In an oxidizing atmosphere, autoreduction is suppressed; however, at temperatures >520 K, iron(ii1) oxide is formed in increasing amounts. Reduction by hydrogen subsequent to heat treatment in air at 720 K results in the formation of a magnetite-like oxide phase and some metallic a-iron.The phenomenon of solid-state ion exchange in zeolites was first observed by Rabo et ~1.'~~and Clearfield et aL3 In recent years, insertion of cations into zeolites via solid-state ion exchange has attracted increasing attention. Recently, the results of systematic investigations in this field obtained by several research groups were reviewed., Generally, solid-state ion exchange occurs at relatively high temperatures (550-1000 K) in intensively ground mix- tures consisting of (1) a hydrogen zeolite and (2) a metal halide or oxide. The process results in the formation of the respective metal zeolite and evolution of hydrogen halide and water. In contrast to conventional ion exchange proceeding in aqueous media, any desired ion-exchange degree can be achieved simply by applying the corresponding amount of salt, and, in many cases, even an ion-exchange degree of 100% can be reached easily in the solid state.Furthermore, this method does not require the handling of large volumes of salt solutions and long exchange times. Recently, the phenomenon of 'contact-induced' ion exchange between hydrated Na-Y zeolite and chlorides of Li, Na, K, Ca or and between NH,-Y zeolites and LaCl, has been reported. This reaction proceeds upon mere grind- ing of zeolite-salt mixtures at ambient temperature. Evidence of the exchange of cations under these experimental condi- tions included the formation of a separate crystalline phase of NaCl and NH,Cl, detected by XRD.In contrast to conven-tional ion exchange and similar to the solid-state variant, this type of ion exchange proceeds in solid mixtures of zeolites and salts, although intracrystalline water molecules adsorbed in the zeolite are obviously involved in the mechanism. Nevertheless, this process also provides a simple and, in many respects, advantageous method for cation insertion into zeolites and, after dehydration by subsequent heat treatment, it can be combined with 'true' solid-state ion-exchange reac- tions. For numerous chemical reactions, efficient mono- and bi- functional catalysts can be obtained by incorporation of -f Part 4: ref. 5. transition-metal ions into zeolites. The catalytically active species may be the zeolitic cation itself and, after reduction, highly dispersed transition-metal particles.Solid-state ion exchange has been shown to provide excellent possibilities for insertion of transition-metal ions (Pt, Pd, Cu, Fe, V, Cr etc.) into zeolites., The incorporation of easily oxidizable cations (e.g. Fe2+) in their lower valence state requires the exclusion of oxygen during the whole ion-exchange procedure. This leads to experimental complications, especially in the initial step, i.e. in the preparation of the ground zeolite-salt mix-tures. On the other hand, it is well known that, for example, Fe'" species formed during ion exchange of Fe2+ ions into zeolites in the presence of oxygen undergo, upon evacuation at higher temperatures, 'autoreduction' under evolution of molecular oxygen.8-' O The present study deals with the insertion of iron ions into Y zeolite by contact-induced ion exchange in mixtures of NH,-Y with iron@) chloride. In particular, changes of oxida- tion and coordination states of the iron cations during grind- ing of the mixtures in the presence of oxygen and upon successive heat treatments are investigated.Experimental Materials The parent Na-Y zeolite was provided by Union Carbide (Tarrytown, USA) and converted to NH,-Y by repeated ion exchange (15 times) with 1 mol dm-, NH,Cl solution. The (idealized) unit-cell composition of the exchanged zeolite was 10.3(~~4)54.iNa0.5 [Ai64.9Si 127.1O3841. Mechanical mixtures of this zeolite with crystalline iron(@ chloride tetrahydrate were prepared by intensive grinding in an agate mortar.The iron@) salt was applied in amounts equivalent to the aluminium content of the zeolite, i.e. the atom ratio of Fe to zeolitic framework aluminium was 0.5. No precautions were taken to exclude atmospheric moisture and oxygen during the grinding procedure. Methods X-Ray diffraction patterns of the mixture of NH,-Y zeolite and FeCl, before and after heat treatment at different tem- peratures were obtained with a Philips PW lo00 diffractome- ter using a graphite monochromator and Cu-Ka radiation. Temperature-programmed evolution (TPE) of volatile pro- ducts (HCl, H,O, NH, and 0,)during heating of the samples in high vacuum (< 5 x 10-Pa) up to lo00 K was measured by mass spectrometry.The method and apparatus are described in detail elsewhere." In situ Mossbauer measurements were carried out in a cell the description of which may be seen in ref. 12. The two series of 300 K spectra were recorded after various treatments (evacuation at 3 x lop2 Pa and calcination followed by reduction) at different temperatures (420, 520, 620 and 720 K) for 4 h. The calcination was carried out in air; reduction was performed in a flow of purified hydrogen. The isomer shift values are related to the centre of the a-iron spectrum mea- sured at room temperature. The lines are fitted with a Lor- entzian shape without constrained positional parameters. The accuracy of the positional data is kO.03mm s-'. Results X-Ray Diflkactometry Schematic XRD patterns of the parent NH,-Y zeolite and of its mixture with FeC1,.4H2O as prepared by grinding at ambient temperature and after heat treatment at 730 K are presented in Fig.1. Upon introduction of iron cations, the intensity of most of the XRD reflections considerably decreases. However, this effect does not indicate lattice damage. It is due partly to changes of the respective structure factors upon incorporation of iron cations, and partly to the higher X-ray absorption factor of the Fe component. The intensity re-increase of several reflections upon heat treat- ment and the lack of the broad peak at 20 = 20-30°, indica- tive of amorphous silica, show that the lattice cannot be seriously affected.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Contact-induced ion exchange proceeds upon grinding the NH,-Y-FeCl, mixture as evidenced by the appearance of the 110 reflection of crystalline NH,Cl at 28 = 32.68". However, the reflections indicative of FeCl, .4H20 do not completely disappear [Fig. l(b)]. Therefore, only partial ion exchange occurs. Upon heat treatment, the incorporation of iron cations gradually proceeds and at 520 K no separate phase of iron chloride can be detected. This ion exchange is accompa- nied by a steady decrease of the cubic unit-cell parameter and typical intensity changes of some reflections [c$ Fig. l(b) and (41.Using the relationship between the lattice parameter and the framework Si :A1 ratio of faujasite-type zeolites given by Breck and Flanigen,', the thus obtained Si : A1 ratio of the parent NH,-Y zeolite (1.8 :1) agrees well with the value determined by wet chemical analysis (2.0 :1).A small decrease of the lattice constant occurs upon grinding with FeCl, * 4H,O and a considerable shrinkage of the unit cell is observed after heating the mixture up to 720 K [see the cell parameters indicated in Fig. l(u)-(c)]. This unit-cell contrac- tion is due to the incorporation of iron lattice cations rather than to aluminium release from the framework and, hence, to an increase of the Si : A1 ratio of the lattice. TPEExperiments The intensities of the MS signals of the main products, HCl (mass number 36), NH, (mass number 17 minus the contribu- tion of H,O) and H,O (mass number 18), evolved from the FeC1, -4H2O-NH,-Y mixture during heating to lo00 K with a rate of 10 K min-were measured. The lower-temperature region of these evolution curves is shown in Fig.2(u). For comparison, the decomposition curves of crystalline ammon- ium chloride are presented in the same figure [Fig. 2(b)]. The maximum rate of release of ammonia and HCl is observed at 't n 10 20 30 28/degrees 400 500 600 Fig. 1 Schematic presentation of XRD patterns of (a) the parent temperature/K NH,-Y zeolite, a = 24.785 A; (b) the mixture of NH,-Fig. 2 TPE curves of hydrogen chloride (m/e = 36) (-), ammonia Y-FeCl, 1 4H,O ground in air at ambient temperature, a = 24.742 A; (m/e = 17) (---) and water (m/e = 18) (---*) evolved (a) from a (c) material (b) heated in air up to 720 K (heating rate: 10 K min-'), ground mixture of NH4-Y-FeC1,.4H,O (Fe : A1 = 0.5 :1); (b) from a = 24.542 .$; * FeCl, .4H,O; ** NH,Cl crystalline ammonium chloride J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 about 510-520 K and coincides with the maximum decompo- sition rate of ammonium chloride suggesting a preceding contact-induced ion exchange between the zeolitic com-ponent and the salt. A characteristic feature of the TPE pattern of the FeC1,-containing system is the small but sig- nificant and well reproducible delay of HCl evolution shifting the respective TPE curve to temperatures about 10 K higher than that for the ammonia evolution. In TPE patterns of mixtures containing other metal chlorides (e.g.CaCl, ,Fig. 3), ammonia and HCl evolution completely coincide, at least up to 570 K. As expected, the same is observed for the decompo- sition of ammonium chloride (Fig. 2). The water TPE curves of all NH,-Y-salt mixtures exhibit a maximum at about 400 K due to the desorption of water coordinatively bound to the metal cations. Among the systems investigated to date, only the mixture prepared with FeCI, * 4H,O evolves a second water species, revealed by an additional maximum at about 510 K coinciding with the ammonia release. This water probably originates from hydroxy groups which may have formed by hydrolytic pro- cesses involving iron ions and intracrystalline zeolitic water during the grinding procedure and then, at higher tem-peratures, were eliminated by dehydroxylation.At higher temperatures (>570 K for CaC1,-NH,-Y and 670 K for FeC1,-NH,-Y) the ammonia evolution declines faster than the HCl release. Therefore, part of the ammonium ions must be thermally decomposed and the resulting bridged hydroxy groups are eliminated only at higher temperatures by true solid-state ion exchange with the remaining CaC1, and FeCl, ,respectively, resulting in lattice metal cations and HC1. In summary, the main features of the TPE pattern of FeC1,-NH,-Y clearly reveal that, in this system, a complex reaction process is going on with a maximum rate at about Miissbauer Spectroscopy Mossbauer spectra (measured at 300 K) of the FeC1,-NH,-Y mixture heat-treated in vacuum at different temperatures are presented in Fig.4. Based on recently reported assign- ment~,'~the oxidation and coordination states of iron species were deduced from the Mossbauer parameters (isomer shift and quadrupole splitting) of the signals obtained by spectrum deconvolution (see Fig. 5). Depending on the pretreatment temperature, two Fe"' species differing in their coordination state (octahedral and trigonal, respectively) and six different Fe" species [viz. FeCl, xH,O, two types of iron(rr) ions in tetrahedral and trigonal coordination, respectively, and three Fe" species differing somehow in their octahedral environ- ment] could be distinguished. The presumption of the exis- tence of various iron components is proven by the low 2' values of the fits (x2 < 1.8).Fitting with smaller peak numbers resulted in a significant increase of x2. The results are summarized in Table 1; Fig. 6 illustrates the dependence of the spectral area of the individual Fe species on the pretreatment temperature of the zeolite-salt mixture. After grinding of hydrated NH,-Y and FeCl, .4H20 (Fe : Al,,,, = 0.5) at ambient temperature, the greater part (57%) of the iron applied is oxidized to the trivalent state. The Mossbauer parameters of this Fe"' species point to a filled (close to octahedral) coordination. The remaining Fe" is present in the form of three distinct species, one of which, amounting to about 21%, has been identified by its Moss-bauer parameters as partially dehydrated iron(1r) chloride.Another one (13%) is characterized by parameters similar to those of partially hydrated, tetrahedrally coordinated Fe" ions located in sodalite cages and large ~avities,'~ probably with one molecule water in its coordination sphere (Fei:tr). The third Fe" species (Fe:ct-l) has been shown to be in an octahedral environment probably provided by water and OH ligands [Fe(OH)+].' 400 500 600 -5 -3 -1 1 3 5 temperature/K velocity/mm s-' Fig. 3 TPE curves of hydrogen chloride (-), water (-.--) evolved from a groundY-CaCl, .2H20 ammonia (---) and mixture of NH,- Fig. 4 Mossbauer spectra of (a) NH4-Y-FeCl, * 4H20 ground at ambient temperature in air and material (a) after heat treatment in vacuum at (b)420 K, (c) 520 K, (d) 620 K and (e)720 K ,$ip'II I I -b I-5 I I veiocity/mm s-' Fig. 5 Mossbauer signals of individual iron species giving the best fit to the spectrum Fig.4(e)of ground NH,-Y-FeCl, heat-treated at 720 K. (e)Fe;;, ;(a)Fe:::, ;( El) Fe::,, ;(V) Fe:,-, ;(@I) Fe:t-3 Heating to 520 K results in a gradual autoreduction of the Fe:Et species as indicated by the decrease of its relative area to 44% at 420 K and to 38% at 520 K, accompanied by the formation of an Fe"' species (11Yo)with Mossbauer param- eters similar to those found for oxidized Fe-A zeolite and Table 1 Mossbauer parameters of iron species present in NH,-Y-FeCl, vacuum species parametef as prepared 0.37 0.63 57 ---0.69 0.35 13 ---0.83 1.95 9 ------1.13 1.83 21 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 -60 --40 U--20 -300 400 500 600 700 temperature/K Fig. 6 Relative intensity (I,) of iron species found by Mossbauer spectroscopy in NH,-FeC1, ground at ambient temperature in air us. heat-treatment temperature. (e)Fe;, ;(W) Fe::, ;(0)Fe:it, ;( El) Fe:,-,F&,; (0) ;(V)Fe!c,-2 ;(0)Fe;c,-3 ;(+) FeC1, * xH,O attributed to trigonally coordinated Fe3 residing in the six- + membered window site SI." Furthermore, it has been shown by X-ray analysis16 that in oxidized samples of FeY zeolites iron is preferentially located in site SI'. Thus, the Fen' species appearing at 520 K (Fe:::J is also assigned to Fe3+ three-fold coordinated to three oxygen atoms of the D6R units.However, only part of the decrease of Fe!:t is due to conver- sion to Fei5,. A new Fen species (Fe&,J appears, in part due to autoreduction of Fe:;,, but also to full dehydration of the Fe::tr. Based on the interpretation of the Mossbauer spectra of Fe"'-A zeolite^,'^^'^ this species is attributed to Fe2+ ions coordinatively bound to three oxygen atoms of six-membered rings. Dramatic changes in the oxidation and coordination state of the iron species occur in the temperature range from 520 to 620 K. The concentration of Fe:Et abruptly decreases to 5% and new Fetct species appear. The processes are obvi- .4H,O mixtures after grinding and subsequent heat treatment in high after high-vacuum treatment 420 K 520 K 620 K 720 K 0.35 0.31 0.30 0.33 0.72 0.75 0.72 0.58 44 38 5 2 - 0.25 0.23 0.22 - 1.53 1.71 1.73 - 11 11 2 0.72 - - - 0.36 - - - 11 - - - - 1.08 0.88 0.92 - 0.78 0.62 0.68 - 21 21 30 0.79 - - - 2.00 - - - 22 - - - - - 0.95 0.99 - - 2.20 2.13 - - 37 45 - 1.06 1.23 1.27 - 2.60 2.19 2.16 - 9 26 20 1.04 1.06 1.79 1.90 23 21 a Parameter: 6, isomer shift; A, quadrupole splitting, I,, relative intensity.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 ously accompanied by the evolution of ammonia, hydrogen chloride and water observed at about 520 K, when the tem- perature of the zeolite-salt mixture is continuously increased at a rate of 10 K min-' (see Fig.2). The parameters of the new FeLL,-, component do not exactly correspond either to those observed for iron@) ions in zeolite sites or to those for iron in crystalline aluminates (FeAl,O,).' Nevertheless, this species characterized by the parameters given in Table 1 may be related to iron(I1) ions interacting with oxidic extra-framework A1 species released from the framework during autoredu~tion.'~In line with this assumption, the amount of this species increases with increasing treatment temperature. The data for the Fefc,-, species are characteristic of ions located in the hexagonal prisms of the faujasite ~tructure.~.'~ The Mossbauer signal with parameters typical of FeCl, .xH,O suddenly disappears at temperatures above 520 K indicating that the part of FeCl, not incorporated in the zeolite lattice by contact-induced ion exchange during grind- ing (about 20%) is now involved in a 'true' solid-state ion- exchange reaction.The previous observations clearly demonstrate that signifi- cant autoreduction of Fe"' ions has taken place during treat- ments in vacuum. Therefore, the presumption that the autoreduction processes are probably suppressed under treat- ments in air seems plausible. However, a series of 300 K Mossbauer spectra has also been recorded after calcination in air and reduction at 520, 620 and 720 K. The spectra are displayed in Fig. 7. They exhibit characteristic features. As it is seen, reduction at 520 K has no severe consequences; the overwhelming part of the iron remains in the Fef;, state, only 12% of the spectral area belongs to a reduced iron(@ (Fefc,) component.The 620 K treatments have more severe effects ;after calcination even Fe203 lines can be detected (only the inner two lines of the sextet fall into the velocity range of the measurement; note the shoulders on the central doublet of spectrum (b). Reduction by hydrogen at the same temperature results in the appearance mainly of Fete, components. Fe" ions in tetra- hedral or trigonal coordination are hardly detected, indicat- ing low ion-exchange degrees. Calcination at 720 K results in the pronounced formation of a separate iron oxide phase; 45% of the spectral area belongs to the characteristic sextet.After the reduction performed at the same temperature, 19% of the spectral area can be assigned to a magnetite-like phase2' (with characteristic magnetic field values of 48.9 and 45.9 T for A and B sites, respectively, and isomer shift values of 0.32 and 0.67 mm s-'), and 11.5%of the area belongs to metallic a-iron (H,,, =33.0, 6 =0.0 mm s-'). Furthermore, this spectrum contains two doublets of Fefct (with relative -' ' ' ' ' ' ' ' ' :""""'-""""'""-3 -1 1 3 -3 -1 1 3 -6 0 6 velocity/mm s-' Fig. 7 In situ Mossbauer spectra of NH,-Y-FeCI, mixtures record- ed at 300 K after calcination [(a),(b)and (c)] and reduction in hydro- gen [(a'),(b')and (c')]at temperatures of (a)520, (b)620 and (c) 720 K areas of 15.5 and 40%)and one doublet of Fe:fi, (8%).Note here that for the appearance of magnetic sextets in the spectra a certain threshold particle size must be exceeded. In our case, depending on the particular compound, a critical size of about 4-8 nm can be estimated at room tem-perature.'l This value is definitely larger than any of the pore diameters characteristic of Y zeolite. The formation of a magnetite-like oxide phase upon reduction may indicate that the aluminium ions penetrate into the oxide phase and mixed (Fe, A1),0, is formed. (On the silica support, the reduction of Fe,O, results in the formation only of metallic iron, while on the alumina support, a mixed oxide is easily formed since A13+ and Fe3 + ions have similar sizes.,,) Discussion The NH,-Y-FeCl, mixture releases the greater part of the ammonia and HC1 in the temperature range typical of the decomposition of crystalline NH,C1 (Fig.2). Thus, ion exchange resulting in the formation of ammonium chloride probably proceeded to a high degree during the grinding process or during the successive heating operations in the temperature range in which decomposition of NH,C1 does not yet occur. NH,-Y-FeCl, differs from other NH,-Y-metal chloride systems in the release of water with increasing temperature. The peak at about 400 K associated with water coordi-natively bound to cations appears more or less pronounced in the TPE pattern of all zeolite-salt mixtures. The system containing iron chloride, however, shows a distinct second water desorption peak with a maximum at about 510 K coin-ciding with the release of ammonia.A further peculiarity of this system is the small but significant delay of the HC1 evol- ution compared with the ammonia release (Fig. 2). This unique TPE behaviour of NH,-Y-FeCl, mixtures points to the hydrolytic formation of hydroxy-iron cations during the grinding process or successive heat treatments : Fe2+ +H,O +Fe(OH)+ +H+ (1) As evidenced by Mossbauer spectroscopy, most of the iron ions applied are oxidized during grinding to Fe"' species and may be also present as hydroxy-cations, e.g. Fe(OH),+ and/or Fe(OH),+. The water release at 520 K, which coin- cides in the TPE experiments with the decomposition of ammonium chloride formed during grinding of the mixture by contact-induced ion exchange, may be due to the reaction: where n is the valence state of the cationic iron species.FeC1("-1)+ undergoes (at somewhat higher temperatures) solid-state ion exchange according to : FeCl("-')+ +H+ +Fen++HCl (3) The consumption of at least some of the HCl [reaction (2)] formed by decomposition of the ion-exchange product NH,Cl and its release at higher temperatures [reaction (3)] consistently explain the shift of the HCl evolution curve to higher temperatures. It is striking that the decomposition of the ammonium chloride observed in TPE experiments around 520 K and the abrupt change in the oxidation state of iron revealed by Mossbauer spectroscopy somewhere between 520 and 620 K proceed in nearly the same temperature range.The slight dif- ference may be due to the much higher vacuum in the TPE apparatus favouring the decomposition of NH,CI. The auto- reduction of Fe"' spontaneously proceeding in this tem-perature interval is associated with the appearance of an equivalent amount of a new octahedrally coordinated Fe" species (Fe$t-2), with Mossbauer parameters not typical of lattice iron cations in faujasite. This new species has been tentatively assigned to Fe" cations interacting with alu-minium oxide. The coincidence of these processes is probably not an acci- dental one. A plausible explanation consistent with all experi- mental findings can be given if it is assumed that stable Fe'" forms of zeolites contain hydroxy-cations [Fe(OH)2 or+ Fe(OH), '1 or, after dehydroxylation, dimeric cations con- nected via an oxygen bridge (Fe-00-Fe)~' (i.e.cationic FeIn species with less than three charges per Fe atom). Auto- reduction of these Fe'" zeolites results in the formation of the respective Fe" form and oxidation of the oxygen bound to Fe"' species (in hydroxy groups or oxygen bridges) to molec- ular oxygen. The formation of Fe3+ cations is believed to result in unstable or less stable structures, especially in zeo- lites with higher framework Si :A1 ratios and, hence, larger distances between the negatively charged tetrahedral struc- ture units with aluminium as the central atom. Therefore, it may be assumed that, at least at 520 K in Y zeolite, Fe3 cations are unstable and change their oxidation + state by autoreduction immediately after their formation according to eqn.(3). In this case, however, autoreduction must involve the oxidation of framework oxygen atoms to molecular oxygen associated with the release of oxidic alu- minium species from the framework: (A104/2}3-Fe3f-,(A104i2}2-Fe2++ A1,0, + 1/20, (4) where fA104,,)- denotes the part of the zeolite structure containing one tetrahedron with A1 as the central atom. Interactions between the simultaneously formed Fe" cations and aluminium oxide species are reflected by the Mossbauer parameters assigned to Fe:ct-2. Thus, the Fe" form of faujasite containing only very low amounts of two Fe"' species (2% each) can be obtained by contact-induced ion exchange and subsequent heat treatment at 720 K.However, the complex process involves reactions resulting in a partial release of framework aluminium in the form of oxidic species and, hence, affects the structure. It can be deduced that, although the evacuation results in the disturbance of the framework to a limited extent, the vacuum is essential for the solid-state ion-exchange processes, providing a fast removal of the small molecules formed (NH,, HC1 and H,O). The autoreduction can be suppressed with the treatment in air at 520 K, but at higher temperatures separate iron oxide phases form and the extent of the solid- state ion-exchange processes is, even at 720 K, rather limited.The size of the oxide particles is large and exceeds the size of the pores. Thus, lattice rupture also proceeds with treatments in air. The chlorine ions (and the various FeCl, species) have a distinguished role; their presence probably causes an inter- mediate step to be inserted among the ion-exchange pro- cesses [as is proposed in eqn. (2)]. Further studies of J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 solid-state ion exchange of iron salts containing other anions are in progress. Partial funding provided by the Hungarian National Foun- dation for Scientific Research (OTKA grant T7364), as well as financial support by the Ministry of Technology and Research of the Federal Republic of Germany (BMFT, Project No. 03C 252 A7), is gratefully acknowledged.References 1 J. A. Rabo, M. L. Poutsma and G. W. Skeels, in Proc. 5th Znt. Congr. Catal., 1972, ed. J. W. Hightower, North-Holland, Amsterdam, 1973, p. 1353. 2 J. A. Rabo, in Zeolite Chemistry and Catalysis, ed. J. A. Rabo, ACS Monograph 17 1, American Chemical Society, Washington, DC, 1976, p. 322. 3 A. Clearfield, C. H. Saldariagga and R. C. Buckley, in Proc. 3rd Znt. Con$ Molecular Sieoes, 1973, Recent Progress Reports, ed. J. B. Uytterhoeven, University of Leuven Press, Leuven, 1973, p. 241. 4 H. G. Karge and H. K. Beyer, in Zeolites and Catalysis, Stud. Surf. Sci. Catal., ed. P. A. Jacobs, N. I. Jaeger, L. Kubelkova and B. Wichterlova, Elsevier, Amsterdam, 1991, vol. 69, p. 43. 5 G. Borbely, H.K. Beyer, L. Radics, P. Sandor and H. G. Karge, Zeolites, 1989,9,428. 6 H. K. Beyer, G. Pal-Bortkly and H. G. Karge, Zeolites, sub-mitted. 7 H. G. Karge, G. Borbkly, H. K. Beyer and Gy. Onyestyak, in Proc. 9th Znt. Congr. Catal., 2988, ed. M. J. Philips and M. Ternan, Chemical Institute of Canada, Ottawa, 1988, p. 396. 8 J. A. Morice and L. V. C. Rees, Trans. Faraday SOC., 1968, 64, 1388. 9 R. L. Garten, W. N. Delgass and M. J. Boudart, J. Catal., 1970, 18,90. 10 J. Novakovi, L. Kubelkova, B. Wichterlova, T. Juska and Z. Dolejsek, Zeolites, 1982,2, 17. 11 H. G. Karge and V. Dondur, J. Phys. Chem., 1990,94,765. 12 K. Lhzar, K. Matusek, J. Mink, S. Dobos, L. Guczi, A. Vizi-Orosz, L. Mark6 and W. M. Reiff, J. Catal., 1984,87, 163. 13 D. W. Breck and E. M. Flanigen, in Molecular Sieoes, Society of Chemical Industry, London, 1968, p. 47. 14 K. Lazar, Struct. Chem., 1991,2,245. 15 Z. Gao and L. V. C. Rees, Zeolites, 1982,2,215. 16 J. R. Pearce, W. J. Mortier and J. B. Uytterhoeven, J. Chem. SOC., Faraday Trans. 1, 1981,77,937. 17 2.Gao and L. V. C. Rees, Zeolites, 1982,2,205. 18 B. I. Dickson and L. V. C. Rees, J. Chem. SOC.,Faraday Trans. 2, 1974,70,2038. 19 G. Connell and J. A. Dumesic, J. Catal., 1986, 102, 216. 20 N. N. Greenwood and T. C. Gibb, Mossbauer Spectroscopy, Chapman and Hall, London, 197 1. 21 B. S. Clausen, H.Topsoe and S. Morup, Appl. Catal., 1989, 48, 327. 22 A. F. H. Wielers, A. J. H. M. Kock, C. E. C. A. Hop, J. W. Geus and A. M. van der Kraan, J. Catal., 1989,117, 1. Paper 3/05715A; Received 21st September, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949001329

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(9), 1335-1344 Manganese-promoted Rhodium/NaY Zeolite Catalysts An IF?Spectroscopic Study Tilman Beutel and Helmut Knozinger" lnstitut fur Physikalische Chemie, Universitat Munchen, Sophienstr. 1I,80333 Miinchen, Germany Horacio Treviiio, 2. Conrad Zhangt and W. M. H. Sachtler V. N. lpatieff Laboratory, Center for Catalysis and Surface Science, Departments of Chemistry and Chemical Engineering, Northwestern University, Evanston, IL, USA 60208 Carlo Dossi, Rinaldo Psaro and Renato Ugo CNR Center and Dipartimento di Chimica lnorganica , Metallorganica e Analitica , Universita degli Studi, Via G. Venezian, 21-20133 Milano, Italy Manganese-promoted rhodium/NaY catalysts have been prepared by ion exchange and subsequent H, reduction and by chemical val ' deposition (CVD) of [Rh(CO),(acac)] and Mn,(CO),.followed by thermal decomposition. Carbonyl FTIR spectra reveal significant differences between monometallic Rh/NaY and Mn/NaY and their bimetallic counterparts and they demonstrate profound effects of the synthetic methodology. The H,-reduced Rh/NaY samples prepared by ion exchange contain an appreciable amount of protons. As a consequence Rh+(CO), complexes are formed in the presence of CO. In contrast, samples prepared by CVD contain only Rh,(CO),, and Rh,(CO), 6 which can easily interconvert. The samples prepared by ion exchange and containing both Rh and Mn, exhibit bands at 1800 and 1830 cm-' characteristic of bridging CO ligands which are always accompanied by low-frequency bands at 1684 and 1700 cm-'.These bands are attributed to an Rh2-CO-Mn2+ complex. It is suggested that these q2-C0 species are formed by interaction between bridging CO ligands and Rh,-carbonyl clusters (n = 4 and 6) with Mn2+ ions via the oxygen end. This complex is totally absent in the bimetallic samples prepared by CVD of neutral organometallic precur- sors. These materials do not contain Mn2+ ions since the Mn,(CO),, precursor is decomposed on previously deposited Rho particles, thus forming bimetallic particles, the surfaces of which are presumably enriched in Mn. The number of Rh, ensembles is therefore low and bridging CO ligands are not formed in the presence of CO. The possible relevance of these results for the catalytic conversion of CO-H, mixtures on manganese-promoted Rh/NaY catalysts is discussed. The selectivity of supported rhodium catalysts towards oxygen-containing products in CO + H, reactions can be enhanced by transition-metal oxide promoter^.'-^ Man-ganese oxide has been shown to be an active promoter, and C-and 0-bonded carbon monoxide species (Rh-C-0 -+ MnX+) have been detected by IR spectros- cop^.^.^ It has been argued that these species might be crucial intermediates in oxygenate f~rmation.~q~.~ The supports used in these studies were exclusively high-surface-area oxides such as SiO, or A1,0,, while the application of zeolites has not been reported as yet.Zeolites are known for a variety of well defined cage/ channel structures and for a wide range of controllable acid- ities.The unique structural and acidic properties have led to some important industrial applications of zeolites. Metals located in the open channels or pores of zeolites are particu- larly attractive, as with these materials the metal and acid functions can be controlled independently. Acid protons interact chemically with metal clusters and impede their migration ;''-I3 moreover, the metal-proton adducts are able to act as bifunctional sites.I4 While zeolite-encaged noble metals are mostly used as bifunctional catalysts, the accessibility of metal particles to reactant molecules during catalysis is strongly determined by zeolite structures and pretreatment conditions. For faujasite zeolites, such as Y, the high tendency for metal ion migration into the small cages, such as sodalite cages and hexagonal prisms, and low accessibility of these cages to reactants under reaction conditions create the undesirable situation of ineffi- t Present address : Catalytic System Division, Johnson Matthey, 456 Devon Park Drive, Wayne, PA 19087, USA.cient use of the metal catalysts. For this reason multivalent ions such as Ca2+ or Cr3+ have been applied to block the small cages. Manganese ions can also be accommodated in the small cages; however, the extraordinary stability of the MnZ+(H,0)6 complex retains most of these ions in the super- cages of zeolite Y even after extended heating in dry helium, as demonstrated by Pearce et ~1.'~Accordingly, it is assumed that also in bimetallic samples that have been calcined at 773 K, most of the Mn2+ ions will reside in large cages.In the present study the effect of MnZ+ ions on Rh clusters in sugercages of zeolite Y is followed by FTIR spectroscopy using carbon monoxide as a probe. CO was adsorbed at liquid-N, temperature at reduced pressures so as to probe the initial state of the metal-containing zeolite free from any CO-induced morphological or structural changes. This was subsequently followed by increasing the temperature in the presence of CO. When the conventional ion-exchange tech- nique is used in the preparation of zeolite-encaged metal par- ticles, protonic acidity will be generated upon hydrogen reduction.'6 In the presence of CO, the intrazeolitic rhodium chemistry was dependent on surface acidity.The problem of obtaining non-acidic metal-in-zeolite cata- lysts has been approached recently in a completely different way, using neutral organometallic complexes as metal precur- sors. The organometallic molecule can be introduced selec- tively into zeolite cages uiu chemical vapour deposition (CVD), without altering the distribution of intrazeolitic cations. Metal particles are then simply formed oia thermal removal of the volatile ligands under reducing conditions.' ' For comparison purposes with the ion-exchanged materials, volatile organometallic complexes, such as [Rh(CO),(acac)] (acac = acetylacetonate) and Mn,(CO),, , were deposited into zeolite via CVD.These systems were characterized by diffuse reflectance FTIR spectroscopy (DRIFTS) of adsorbed CO at room temperature. Experimental Sample Preparation by Ion Exchange The starting zeolite was NaY (LZ Y-52, Linde, lot No. 968087061020-S-8) containing 56 Na+ ions per unit cell. The Mn/NaY zeolite was prepared by dropwise addition of a 0.20 mol dm-, aqueous solution of Mn(NO,), * 4H20 (Aldrich) to a 200 ml g-' slurry of NaY and doubly deion- ized water at room temperature for 24 h. The pH of the slurry at the end of the exchange was 6. This was followed by calcination in a flow of pure oxygen (250 ml min-') with a temperature ramp of 0.5 K min- 'from room temperature to 773 K and keeping the temperature at this value for 6 h.This sample is denoted MnY. From ICP analysis, it contains ca. 15 Mn2+ and 25 Na+ ions per unit cell (see Table 1). Rh-containing NaY samples were prepared by dropwise ion exchange in an aqueous solution of [Rh(NH,),Cl]Cl, for 24 h at 355 K. The equilibrium between [Rh(NH,)5C1]2+ and [Rh(NH,),H20l3+ is shifted toward the latter at this temperature, thus minimizing the content of C1 in the zeolite.'* The [Rh(NH3)H,0I3+ ions are retained in the supercages because of their size. Two samples with Rh con- tents 0.4 and 3 wt.% were prepared. They were denoted RhY04 and RhY3, respectively. These samples were air dried at room temperature for 12 h, but not precalcined. For preparation of the bimetallic sample, NaY was first loaded with manganese and calcined as described above.Subsequently, the MnY sample was exchanged with [Rh(NH,),C1]2+. This sample will be denoted RhMnY, while the same sample after reduction at 523 K will be indicated by the acronym RhMnY523. Sample notations and metal contents are summarized in Table 1. Sample Preparation by Chemical Vapour Deposition [Rh(CO,)(acac)] (acac = acetylacetonate) and Mn,(CO),, have been purified by sublimation in oucuo (lo-, mbar) at 363 and 343 K, respectively. The NaY zeolite has been pretreated in flowing argon at 673 K prior to the deposition of the organometallic precur- sor. Deposition of [Rh(CO),(acac)] and Mn2(CO),, has been conducted under CVD condition^'^ in a U-tube glass reactor in flowing argon at 353 and 378 K, respectively.After deposi- tion, the supported precursors were decomposed in flowing hydrogen from room temperature to 673 K with a heating rate of 10 K min-'. The two CVD-based monometallic samples, both having a final metal loading of 1 wt.%, will be denoted throughout the text as RhY(CVD) and MnY(CVD). A bimetallic Rh-Mn/NaY sample has been similarly pre- pared by a two-step procedure. First, [Rh(CO),(acac)] was deposited from the vapour phase into NaY zeolite and subse- Table 1 Catalyst notations and compositions sample Mn content (wt.%) Rh content (wt.%) RhY04 - 0.4 RhY3 RhY (CVD) -- 3 1 MnY 4.9 - MnY (CVD) RhMnY 1 4.6 -2.8 RhMnY (CVD) 1 1 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 quently decomposed to metallic particles in flowing hydrogen as described above.After purging in argon to remove physi- sorbed hydrogen, Mn,(CO),, was introduced from the vapour phase, and the resulting material heated in flowing hydrogen with the same temperature profile as previously indicated. The final catalyst, with a 1 wt.% metal loading both for rhodium and for manganese, will be denoted as RhMnY(CVD). Transmission IR Spectroscopy For transmission IR measurements the samples were pressed at 200 bar into self-supporting wafers and placed in a cell which can be cooled to 83 K and which can be evacuted to the pressure of mbar. The samples were treated in situ in 0, at a flow rate of 100 ml min-' at 773 K using heating rates between 0.5 and 8 K min-'.After 20 min purging with N, at 773 K the samples were cooled to 330 K and then reduced for 20 min in a flow of H, (100 ml min-') at 523 K at a heating rate of 8 K min-'. Prior to adsorption of COY the wafer was evacuated for 1 h at the temperature of reduction. When CO was dosed at 85 K, the sample adsorbed the first portions almost completely such that the total pres- sure dropped below 0.1 mbar. In order to control the amount of CO added, a volume of 489 ml was filled at a given pres- sure of CO at room temperature and then expanded into the cold cell (total volume 883 ml). The corresponding pressures within the cell as room temperature were measured; these are denoted as room-temperature CO pressures. Transmission spectra were recorded on a Bruker IFS-66 FTIR spectrom- eter.64 scans were accumulated, corrected for background absorptions and gas-phase contributions. Diffuse Reflectance IR Spectroscopy (DRIFTS) In situ diffuse reflectance spectra were recorded on an FTS-40 Digilab spectrophotometer fitted with a Harrick DRA-2CI diffuse reflectance attachment and a Harrick HVC cell which allows the spectra of the sample in granular form to be recorded under controlled temperature and pressure condi- tions. Samples prepared by CVD of the organometallic precursor were transferred into the DRIFT cell and reduced in a flow of H, at 673 K at a heating rate of 10 K min-'. DRIFTS mea- surements of adsorbed CO were carried out at atmospheric pressure in a flow of CO at room temperature. All spectra were recorded against a KBr standard at 4 cm-' resolution with accumulation of 100 scans per spec- trum.The spectra were converted into the Kubelka-Munk function and plotted against wavenumber. Results and Discussion Ionexchanged Zeolites Hydroxy-group Stretching Region Fig. 1 shows transmission FTIR spectra of Rh- and Mn- containing zeolite samples after in situ reduction in flowing H, at 523 K for 20 min followed by evacuation at 523 K for 1 h. The spectra were recorded at 87 K. The NaY zeolite does not contain any characteristic zeolitic hydroxy groups, except silanol groups located at the external surface of the crys- tallites. These groups give rise to a band at 3749 cm-'. Reduction in H, results in the formation of protons which are attached to bridging oxygens and give rise to the typical 0-H stretching frequencies at 3649 (high frequency, HF) and 3550 (low frequency, LF) cm-' that are attributed to OH groups located in supercages and sodalite cages, respec- J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 d-3645 I 21771 /I 10.050.5 10 t10M I I I I 3900 3700 3500 3300 3100 wavenumber/cm- Fig. 1 Hydroxy-group stretching spectra of RhY04 (a), RhY3 (b), MnY (c) and RhMnY523 (d)recorded at 87 K. Sample pretreatment: oxidation at 773 K for 1 h followed by reduction in flowing H, at 523 K for 20 min and evacuation at 523 K for 1 h. ti~e1y.l~The development of these bands can be seen in Fig. l(a) and (b).Sharp bands are formed at 3653 and 3649 cm-' for samples RhY04 and RhY3, respectively.The intensity ratio of these bands is very close to the ratio of the Rh load- ings of the two materials. In addition, a weaker broad LF band at 3560 cm-' is observed for RhY3. The 0-H stretching spectra observed for the Mn2+-containing samples are also shown in Fig. l(c) and (4. Besides the characteristic bands of the SiOH groups, the Rh-free sample, MnY, exhibits a band of Al(0H)Si groups at 3648 cm- ', with relatively low intensity. In addition, very weak bands can be detected at 3694, 3673 and 3609 cm-'. These bands do not appear in any Mn2+-free samples but are consistently observed (although with varying relative intensities) in all samples containing Mn2+ ions.Their occurrence therefore suggests that they are related to the presence of Mn2 + ions, probably in different coordination environments. The position of the weak band at 3609 cm-' is in the frequency range characteristic of bridging OH groups. This band may therefore tentatively be attributed to Mn(0H)Mn or Mn(0H)Al species. The reduced samples containing Rh in addition to Mn [Fig. l(d)], clearly develop the characteristic HF and LF bands at 3642-3645 and 3545 cm-' of Al(0H)Si groups in supercages and soldalite cages, respectively. The behaviour of the 0-H stretching bands on low-temperature adsorption of CO is complex and will be re-ported independently. Carbonyl Stretching Region Fig. 2 shows the carbonyl stretching region of CO adsorbed on MnY at 88 K.The major feature observed at the lowest CO pressures is a narrow band at 2177 cm-'. This band must be attributed to CO coordinated to Na+ cations, as proposed by Bordiga et a1.*' who observed a carbonyl band at 2178 em-' on Na-ZSM-5. The band shifts to lower wave- numbers (2173 cm-') and becomes broader as the CO con-centration in the zeolite pores increases. This may be due to the formation of Na+. -CO complexes with Na+ ions in dif- ferent positions, but solvent-like effects may also contribute. Minor contributions to this band may also be due to H- bonded CO molecules;" the OH population of this sample, however, is very low. , I I 2230 2190 2150 2110 2070 wavenumber/cm- Fig.2 Carbonyl stretching spectra of CO adsorbed on MnY at 87 K and increasing pressures: (a) 0.05, (b) 0.3, (c) 0.55, (6)0.8, (e) 1.05 mbar In addition to the dominant band at 2177 mi-', a weak band at 2209 and a shoulder at 2188 cm-' can be discerned. These are presumably due to A13+- -CO complexes" which result from the presence of some extraframework A13+ sites. A weak pair of bands at 2128 and 2118 cm-', which coalesce at increasing CO pressures, is also seen in the spectra of Fig. 2. The 2128 cm-' component can be interpreted as the 13C0 satellite to the principal band at 2177 cm-' at natural I3C abundance (theoretical isotope shift, -49 cm-'). The origin of the second component at 2118 cm-' is still an open ques- tion. Bands close to this band position have been observed by Angel1 and S~haffer~~ who proposed an interaction of CO via the oxygen end with lattice oxygen atoms of the zeolite.Bordiga et al." suggested an q2-interaction of CO with Na+ ions and a second Lewis acid site. Fig. 3 shows the carbonyl spectra of RhY04 as they develop in 50 mbar CO at increasing temperatures. At 85 K there is a broad feature around 2100 cm-' in the region of terminally bonded CO on rhodium. No bridging CO is al C mf! $P 2250 2150 2050 1950 1850 1750 1650 1550 wavenumber/cm-Fig. 3 Carbonyl spectra of RhY04 after 1 h oxidation at 773 K and 20 min reduction in H, at 523 K followed by 1 h evacuation at 523 K: (a) after admission of 0.05 mbar CO at 85 K. (6)-(f)50 mbar CO with rising temperature: (6)223, (c) 263, (d)283, (e) 303, (f)323 K.detectable which indicates high metal dispersion. When the sample is warmed in 50 mbar CO significant changes occur only as the temperature rises beyond 223 K. At 263 K [Fig. 3(c)] bands at 2122, 2102, 2086, 2050, 2026 and 1831 cm-' are formed. The bands at 2102 and 2026 cm-' can be as- signed to an Rh gem-dicarbonyl structure.24 The formation + of these species in the zeolite pores is consistent with the high Rho dispersion and the presence of protons. As the tem- perature was raised to 323 K the band at 2122 cm-' is reduced in intensity and a new band at 2117 cm-' becomes evident. The bands at 21 17 and 2050 cm-'can be interpreted as a second dicarbonyl species, which is in line with the results of Bergeret et dZ5who claimed the existence of two rhodium dicarbonyl structures with different support inter- actions in the zeolite framework.Bands at 2122 and 2086 cm-',which start to develop at 223 K and decrease at tem- peratures higher than 263 K, may be assigned to Rh"+(CO),.26*27 Near room temperature this species loses one CO ligand thereby being transformed into an Rh+(CO), species. The narrow absorbance at 1831 cm-' is character- istic of bridging CO on metal clusters. Generally, the number and position of bands for a distinct carbonyl cluster depend strongly on the support. A band at 1830 cm-l has been assigned by Gelin et dz8to a hexanuclear rhodium cluster in Nay, whereas Rode et aL2' ascribed a band at 1834 cm-' to a tetranuclear rhodium cluster in Nay.Ichikawa and co- worker~~~assigned the band at 1830 cm-' to edge-bridging CO of the unstable isomer Rh,(CO),,(p(,-CO)flaY. It was thermally transformed into the isomer Rh,(CO)' 2@3-CO)4, in which the triply bridging CO ligands were characterized by a band at 1760 cm-'. Terminal bands centred at 2088 and 2086 cm-' were con- sidered to be characteristic of the hexa- and tetra-nuclear cluster species, respectively. In Fig. 3, we observed a shoulder around 2090 cm-',which grows in concert with the bridging band at 1831 cm-'. It seems plausible to assign these bands to the same cluster molecule. The antisymmetric stretching vibration of the Rh+(CO), species at 2050 cm-' is much more intense than the symmetric vibration at 2117 cm-'.Moreover, the band at 2047 cm-' in Fig. 3(c) is shifted by 3 cm-' downwards when the temperature is raised from 263 to 283 K. This is reasonable if one assumes that there is a second contribution to that band from the cluster species. A band between 2040 and 2050 cm-' is often reported to be characteristic of rhodium carbonyl cluster^,^ 'v3 1-33 although it is not specific for a certain nuclearity. Hanlan and O~in,~~using matrix-isolation techniques, reported bands at 2060 (s), 2040 (s, sh), 1852 (m) and 1830 (m) cm-' as being characteristic of an Rh,(C0)8 cluster in its bridge-bonded form. The formation of this species requires either high CO pressures, as studied by Wh~man,~' or low temperatures.At 220 K this species is transformed into Rh,(CO) ,under high-vacuum condition^.^^ We find bands at 2050, 2026, 1879 and 1830 cm-' which grow in parallel. However, these bands start to develop at 263 K. This tem- perature is rather more in the stability regime of Rh,(CO),, than of Rh,(CO), . Therefore, the cluster transformations of Rh,(CO),, into Rh,(CO), 6 occur readily around room temperature under vacuum condition^.^' In the presence of CO, however, the tetranuclear species is stable.33 The nuclearity of the cluster species in RhY04 cannot be determined unambiguously, but Rh4(C0),, is the most likely candidate. Fig. 4 shows the carbonyl spectra of RhY3 with increasing temperature. In contrast to RhY04 there is a broad feature at 1928 cm-',which is typical for doubly bridging CO on larger Rho particles.At 85 K a weak band is observed at 1830 cm-' which might be indicative of triply bridging CO. Its intensity J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 AIr"" 2250 2150 2050 1950 1850 1750 1650 1550 wavenumber/cm-' -IH $1 I l'1'1'1'I'I'I'l 2250 2150 2050 1950 1850 1750 1650 1550 wavenumber/cm-' Fig. 4 Carbonyl spectra of RhY3 after 160 min oxidation at 773 K followed by 20 min reduction at 523 K and 1 h evacuation at 523 K. A: (a) after admission of 0.05 mbar CO at 85 K, (b)at 0.7 mbar CO pressure, (c) 2.5 mbar at 143 K, (d) 3.7 mbar at 183 K, (e)50 mbar at 213 K, (f)50 mbar at 233 K. B, Warm-up in 50 mbar CO: (a) at 233 K as in A(f), (b)at 243 K, (c) at 253 K, (6)at 263 K, (e)at 273 K, (f) at 283 K and (9)at 296 K.decreases and a new band grows in at 1810 cm-' when the temperature is increased to 233 K. On further warming, the latter band disappears at 263 K and a bridging cluster band at 1830 cm-' develops, which keeps growing with rising tem- perature. The corresponding linear CO band at 2091 cm-' is clearly visible at 323 K when the tricarbonyl band intensities at 2122 and 2088 cm-' have been depleted. At the same time the evolution of two pairs of twin bands located at 2101 and 2024 cm-' and at 2114 and 2047 cm-' is observed, which are attributed to two Rh+(CO), species with different support interactions as mentioned above and reported earlier.24*2 Fig.5 demonstrates the influence of a further temperature increase (323-523 K) on the cluster chemistry of RhY3. When the sample is heated to 423 K in 50 mbar CO for 10 min [Fig. 5(c)], the bands at 1830 cm-' and 2090 cm-' disappear and new cluster bands at 1760 cm-' and 2086 cm-' grow in. The latter bands seem to belong to one species, most prob- ably an Rh6(CO),, cluster. The hexanuclear cluster is still stable at 523 K [Fig. 5(d)] but is decarbonylated under vacuum at that temperature. Hence, the temperature- and J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 2250 2150 2050 1950 1850 1750 1650 1550 wavenumber/crn -Fig. 5 Carbonyl spectra of RhY3 after 10 min exposure to 50 mbar CO at (a) 323, (b) 373, (c) 423, (d) 523 K pressure-dependent changes observed on RhY3 confirm the assignment of the tetranuclear cluster bands in RhY04. Fig.6 shows the carbonyl spectra of RhMnY523 as the sample is warmed from 85 to 333 K in the presence of CO. Besides the bands at 2168 and 2121 cm-' due to CO inter-acting with Na+ cations and/or acidic protons there is a broad feature in the bridging carbonyl region around 1870 cm-', which indicates bridging CO ligands on Rho metal particles. The corresponding linear CO species give rise to broad bands at 2079 cm-' and at 2050 cm-'. At CQ. 183 K [Fig. 6A(c)], new peaks at 2125 and 2089 cm-' appear and grow in parallel when the system is heated to 333 K [Fig. 6B(f)]. This behaviour is in contrast to that of unpromoted Rh catalysts for which the respective bands at 2122 and 2088 cm-' vanished at room temperature.This is evidence for the presence of additional species in the Mn-promoted Rh samples giving rise to the bands at 2125 and 2089 cm-'. In the bridging carbonyl region we observe the evolution of bands at 1798 and 1684 cm-'. While the former band is typical for bridging CO on rhodium carbonyl clusters, the second band might be attributed to a bridging CO group which forms an additional bond to an adjacent Lewis acid site via the oxygen end. Such tilted CO species have been suggested to exist on silica-supported Mn-promoted rhodium catalyst^.^.^^^^ At 253 K [Fig. 6B(b)], the band at 1799 starts to decrease and a new absorption at 1831 cm-' grows in, together with a small peak at 1869 cm-'.Simultaneously, the band at 1689 cm-' is shifted up to 1700 cm-'. The growth of the band at 1830 cm-' is accompanied by the evolution of sharp peaks at 2098,2052 and a shoulder at 2039 cm- '. The sharpness of the bands at 2125 and 2089 cm-' sug-gests the formation of a well defined carbonyl species. Their assignment to a cluster species, however, is doubtful for several reasons. First, a band as intense and high in energy as that at 2125 cm-' has not yet been observed to our know- ledge for a rhodium carbonyl cluster. Secondly, several changes in the bridging carbonyl region in Fig. 6 indicate that the cluster size and/or geometry is altered with increas- ing temperature. This should influence the position of the linear CO ligands as well.The bands at 2125 and 2089 cm-', however, do not shift. Note that the intensities of the pair of bands at 2052 and 2039 cm-' begins to increase when the bridging CO species at 1831 cm-' stops growing. This sug- gests that the linear bands at 2052 and 2039 cm-' may belong to another cluster species which does not contain I 2250 2150 2050 1950 1850 1750 1650 1550 wavenurnber/cm-t B 0.2 2250 2050 1850 1650 wavenumber/cm-' Fig. 6 A, Carbonyl spectra of RhMnY523 after 1 h oxidation at 773 K, 20 min reduction at 523 K and 1 h evacuation at 523 K: (a) after admission of 0.05 mbar CO (room-temperature pressure) at 85 K, (b) at 0.7 mbar pressure at 85 K, (c) at 2.8 mbar and 183 K, (d)at 5 mbar and 203 K, (e) at 50 mbar and 223 K.B, Warm-up spectra in the carbonyl region of RhMnY523 at a CO pressure of 50 mbar: at (a) 243, (b) 255, (c)273, (6)293, (e) 313, (f)333 K. bridging CO ligands. Besides, symmetry lowering may lead to additional bands. On the other hand, a stepwise carbon- ylation of a given cluster framework can be ruled out. The adsorption of linearly bonded CO onto a bridging carbonyl cluster should exert significant energy shifts on the bridging CO ligands. The band position of the latter, however, does not change. Fig. 7 demonstrates the effect of cooling the sample in the presence of 50 mbar of CO. When the temperature is decreased from 333 to 233 K [Fig. 7C(u)],the bands at 2125 and 2088 cm-' keep growing, while the peaks at 2052 and 2039 cm-l decrease in intensity.Only further cooling to 123 K [Fig. 7B(b)] leads to the depletion of the band intensity at 2088 cm-'. On the other hand, the bands at 2098, 1865, 1830 and 1700 cm-' do not change in intensity or position during the entire cooling procedure. From this fact it may be con-cluded that the cluster species which is characterized by these bands has a constant nuclearity, symmetry and coordination. This, in turn, proves that the bands at 2088, 2052 and 2039 cm-' cannot be attributed to one cluster species alone. 2250 2150 2050 1950. 1850 1750 1650 1550 wavenumber/cm-2250 2150 2050 1950 1850 1750 1650 1550 wavenumber/cm-l Fig. 7 A, Cooling spectra of RhMnY523 at 50 mbar CO pressure at (a) 333, (b) 293, (c) 233 K.B, Cooling spectra of RhMnY523 at 50 mbar CO pressure at (a)233, (b) 123,(c) 173 K. Samples prepared by CVD IR carbonyl spectra of the samples prepared by CVD have been measured by the DRIFTS technique, since it is hardly possible to prepare self-supporting wafers for transmission spectroscopy without exposing the samples to air. It has been demonstrated in advance using standard Rh catalysts that both techniques gave identical carbonyl spectra when compa- rable treatment conditions were applied. It might be argued that the precursor complexes used in the CVD technique would be deposited on the external zeolite surface only. However, the methodology described earlier17 has been shown to produce materials with organometallic precursor molecules being selectively depos- ited in the cages of large-pore Y-zeolites.Evidence for this is also provided by the carbonyl spectra of Rh, and Rh6 car- bonyl clusters shown in Fig. 9 (see later). These spectra are practically identical to those reported by Rao et ~1.~'who prepared their samples by ion exchange. Rhodium clusters located on the external surface of zeolite crystals have totally different spectra, which closely resemble those of rhodium carbonyl clusters adsorbed on amorphous silica supports.33 Hence, we are confident that the organometallic precursor complexes and the species derived thereof are located inside the zeolite cages in the present samples. J. CHEM. SOC. FARADAY TRANS., 1994, VOL.90 MnY(CVD) The carbonyl spectra obtained for the MnY(CVD) sample after exposure to CO at 1 bar and 300 K are significantly different from those described above for the ion-exchanged sample MnY. In particular, bands characteristic of man-ganese carbonyl species are developed in contrast to the ion- exchanged sample, suggesting an entirely different manganese location in the zeolite cages. After 10 min contact with CO, two bands at 2044 and 1948 cm-' begin to appear. Within a 60 min period, these two bands are fully developed, together with additional weaker features at 1959, 1985, 2014 and 2060 cm-' (Fig. 8). The appearance of such bands of the recarbonylated MnY(CVD) sample is likely to be attributed to the formation of zeolite- entrapped Mn(O)(CO), and Mn+(CO), species (see Table 2).However, from simple comparison with the IR spectra of pure reference compounds, it is still not possible to identify unequivocally the exact chemical nature of the entrapped car- bonyl being formed. RhY(CVD) Decomposition to metal of the [Rh(CO),(acac)] complex, vapour-deposited inside the zeolite, was easily performed in an H, atmosphere by thermal removal of ligands without the formation of protonic acidity, i.e. 4 -[Rh(CO),(acac)/NaY + 3 H, 1 Rhi/NaY + Hacac + 2CO n (1) On contacting this sample with CO at room temperature (Fig. 9), bands at 2095 and 1832 cm-' start to appear in the DRIFT spectrum, together with weaker features around 2060 and 2020 cm-'. With increasing time in flowing CO, the 2095 cm-' band grows in concert with other weaker bands at 2060, 2043, 2020, 1987 and 1947 cm-'.In parallel, a new bridging band at 1763 cm-' appears after 180 min.This spectrum is then stable in flowing nitrogen or hydrogen at room temperature. Note that the bands typical for rhodium dicarbonyl structures (2102,2026 cm- 'and 21 17,2050 cm-') are totally absent with this material. This confirms the pre- vious suggestion that the 2090 cm-' shoulder in the IR spectra of Fig. 3 and 4 is related to the bridging CO bands at 1830 and 1763 cm-', being attributed to Rh, and Rh6 entrapped carbonyl clusters. As shown previously, the CO-induced disruption of Rh clusters is, in fact, an oxidation of Rho by protons yielding Rh' ions and H,.37 The Rh+ ions react with CO to form Rh+(CO),.Evidently, this process is impossible in the absence of protons; it is therefore quite reasonable that in the CVD samples only neutral carbonyl clusters are formed. Their interaction with cage walls is a likely cause for the upward shift in the IR bands of the terminal CO groups with respect to the spectra of the unsupported clusters, and also for the downward shift of the bands in the bridging CO region. Table 2 Stretching frequencies (cm-') of some Mn carbonyls" Mn,(CO) 10 2044 m, 2012 s, 1982 m Mn,(CO),(PPhMe,) 2094 w, 2016 s, 1993 vs, 1969 sh, 1938 m Mn,(CO),(PPh,Me), 1983 w, 1954 vs Mn(CO),+ 2090 CMn(CO)sINa 1912 s, 1883 s, 1803 s Mn(CO),Cl 2139, 2083, 2055, 1998 Mn(W,(?-C,H,) 2028, 1945 " Ref.38; my medium intensity; s, strong; vs, very strong; sh, shoul-der; w, weak. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 60-50-C .-*.' C= 40-Y C ?2 30-0)9 3 Y 20-10-t I I I I I I I I I I I 2300 2250 2200 2150 2100 2050 2000 11950 1900 1850 1800 1750 wavenumber/cm-Fig. 8 DRIFT spectra of MnY(CVD) in flowing CO at room temperature taken after (a)10, (b)30, (c) 60min After recarbonylation of RhY(CVD) a temperature-programmed desorption experiment in flowing H, was carried out by monitoring the intensities of the three most significant features in the DRIFT spectrum, namely the ter- minal band at 2094 cm-' and the two bridging ones at 1834 and 1763 cm-', as a function of increasing temperature (Fig.10). The rapid decrease of the 1834 cm-' band is paralleled by the increase in intensity of the 1763 cm-' band. This transformation does not alter significantly the intensity of the terminal band at 2094 cm-', thus precluding the possibility that an extensive decarbonylation of the intrazeolitic carbon- yl clusters is taking place at these low temperatures. At about 373 K, the transformation of the 1834 cm-' band into the 1763 cm-' band is complete. A new IR spectrum with bands at 2094, 2069 and 1763 cm-' is obtained, in very good agree- ment with that of pure Rh,(CO),, inside the Con-version of Rh,-carbonyl to Rh,-€arbOnyl is much easier in u, m 0 N 8-0 0C Nfl-.-tj 6-2 Y-Y Cr' CL 4-r2 Y 2-an atmosphere of H, or N, than in CO, since it is accompa- nied by evolution of carbon monoxide: 3Rh4(C0),, -+ 2Rh6(CO),, + 4CO.(2) By increasing the temperature further, both bands decrease in intensity, until they disappear completely at cu. 470 K. RhMnY(CVD) After flowing CO over the sample at room temperature for 5 min, a set of bands starts to appear at 2071, 2036, 2005 and 1946 cm-' (Fig. 11). At this temperature, the carbonylation process proceeds slowly and requires almost 120 min until the spectrum is fully developed, with additional small bands appearing at 2059, 2014 and 1962 cm-' and shoulders at 2042 and 2020 cm -'. rgI\ h \/ Al II 2300 2250 2200 2150 2100 2050 2000 1950 1900 1850 1800 1750 wavenumber/cm-' Fig.9' DRIFT spectra of RhY(CVD) in flowing CO at room temperature taken after (a)20, (b)90, (c 180 min J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 12*00I"5 9.60 2.40 cr \ ,0.00 I I OYUO 250 300 350 400 450 500 ternperature/K Fig. 10 IR band intensities as a function of increasing temperature in flowing H, of the RhY(CVD) sample after contacting with CO at room temperature for 180 min: (+) 2094, (A) 1834, (0)1763 cm-' 0 A comparison of these spectra with those of the ion- exchanged RhMnY sample of Fig. 6B immediately shows that both the high-frequency bands at 2125, 2088 cm-', and the bridging band at 1831 cm-' are dramatically reduced in intensity. In addition, absorption bands due to C0.a .MnX+ interactions were completely absent.This substantial change in the IR spectrum thus indicates that a completely different chemical interaction between Rh and Mn is taking place in the CVD-based RhMnY sample. The Mn,(CO),, precursor is introduced and decomposed onto the entrapped rhodium clusters previously formed inside NaY zeolite. The effect of metal-promoted decomposi- tion of vapour-deposited carbonyls is well documented in the literat~re.,~In order to ascertain the stability of the carbonyl species giving rise to the complex band pattern in the 2080- 1900 cm -region, a temperature-programmed desorption experiment was carried out, under the same experimental conditions as used for sample RhY(CVD) (Fig. 12). Decarb-P 0 0N 2300 2250 2200 2150 2100 2050 2000 1950 1900 1850 1800 1750 wavenurnber/crn -' Fig.11 DRIFT spectra of RhMnY(CVD) in flowing CO at room temperature taken after (a) 5, (b) 10, (c) 60, (d) 120 min m70. me mo ON 60. .-5 50-c C m2 hw-40-j 30-I]3 Y 20-10-t I I I I I I I I I I I 2300 2250 2x10 2150 2100 2050 2000 1950 1900 1850 iaoo 1750 wavenumber/cm-' Fig. 12 DRIFT spectra as a function of increasing temperature in flowing H, of the RhMnY(CVD) sample after contacting with CO at room temperature for 120 min: at (a) 318, (b) 343, (c) 353, (d) 363, (e) 373, and (f) 423 K J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 onylation of the supported species begins around 343 K and is terminated at around 373 K.In addition, from plotting the band intensities against temperature, it can be seen that the intensities of all bands decrease with increasing temperature, without any substantial frequency shift occurring during the decarbonylation process. This behaviour indicates that a single carbonyl species is formed by contacting the RhMnY(CVD) sample with CO at room temperature. This suggests to us that Rh-Mn bimetallic particles are being formed. The reactivity of the CO-covered Rh-Mn particles is therefore very different from that of the pure Rh clusters, indicating once again the intimate interaction between man- ganese and rhodium in the RhMnY(CVD) sample, and perhaps the formation of bimetallic clusters. Unfortunately, no molecular Rh-Mn carbonyl clusters are known in the lit- erature; a direct assignment of the intrazeolitic carbonyls is therefore impossible at present.The very low intensities of the IR bands at 2094, 1834 and 1763 cm-' in Fig. 9 and at 2044 and 1948 cm-' in Fig. 8, i.e. of the characteristic features for CO-covered Rh only and Mn only samples indicate that no discrete monometallic aggregates are present upon decompo- sing Mn,(CO),, onto prereduced Rh particles inside Y zeolite. Conclusions The CO FTIR spectra presented in this paper reveal signifi- cant differences between the monometallic zeolite-supported samples Rh/NaY or Mn/NaY and their bimetallic counter- part Rh-Mn/NaY; they also demonstrate profound effects of the preparation methodology. As for Rh/NaY, the samples prepared by ion exchange, fol- lowed by reduction, contain an appreciable concentration of protons; by consequence Rh ions coexist in equilibrium + with Rhi clusters.Upon admission of CO this equilibrium is shifted in favour of the Rh+ ions as a consequence of the high stability of the gem my1 ion, Rh+(CO),. While this complex ion is form lution of H, takes place, as was shown previou~ly.~~ ected, the gem-dicarbonyl ion is completely absent in the spectra of the proton-free samples prepared by CVD; only the neutral clusters Rh,(CO),, and Rh,(CO),, are visible. These clusters can interconvert rather easily; the Rh,(CO),, cluster with CO : Rh = 3 : 1 prevails at high CO pressure, but in the absence of gaseous CO the Rh,(CO),, cluster with co :Rh = 8 :3 predominates.As for Mn/NaY, only carbonyl clusters of Mn" are detected; these can be prepared only by the CVD technique, because it is impossible to reduce zeolite-entrapped Mn2+ ions with H, under the conditions used here. As mentioned in the Introduction, the main focus of the present work is on the bimetallic samples, containing both Rh and Mn. A remarkable feature of the samples prepared by ion exchange is the interconversion of bridging CO with a band at 1800 cm-',into a species absorbing at 1830 cm-'. These bands are always accompanied by low-frequency bands at 1684 and 1700 cm-'. As the latter bands are totally absent from the CO FTIR spectra of the monometallic samples, they apparently reveal some chemical interaction of Rh, Mn2+ and CO.Moreover, the absence of these bands from the spectra of the samples prepared by CVD shows that these bands discriminate between Mn" (or Mn+?) and Mn2+. The latter ion is absent in the CVD-prepared samples but predominates in the samples prepared via ion exchange and reduction. This gives high potential relevance to the bands at 1684 and 1700 cm-'. From the work of Pearce et a2.l' it follows that the majority of the Mn2+ ions will be located in super- cages and sodalite cages; only a minority is trapped in hex- agonal prisms. As the reduced Rh, clusters are also located in supercages, it appears that carbon monoxide probes for adducts of Rh, clusters with Mn2+ ions; the IR bands that are so exclusive for the bimetallic system prepared by ion exchange are typical for this interaction. The 1700 cm-' band is supposed to be characteristic of an q2-C0 species which bridges two atoms of an Rh, cluster and simulta- neously coordinates to an Mn2+ cation via the oxygen.Since there is a Coulomb interaction between Mn2+ and the uncompensated negative charge of the zeolite wall, this Rh2-C-O-Mn2+ complex will be anchored to the cage wall. An alternative attribution of these low-frequency bands to formyl species, which have characteristic vibrational modes near 1700 an-', must be ruled out since the corresponding C-H stretching mode near 2720-2760 cm-' could never be detected. The nuclearity of the Rh, clusters is an open question at this stage, but EXAFS work is in progress to clarify this aspect.The conversion of Rh, to Rh6 clusters is expected to cause a downward shift of the bands due to bridging CO at 1800 and 1684 cm-'. FTIR spectra of Rh6(CO),, in NaY have been reported to include a strong band at 1756 cm-', suggesting the presence of a three-fold bridging C0.28*29*31-40 In the present work, a band at 1760 cm-' was observed only for Rh/Y at 273 K. The fact that the 1684 and 1800 cm-' bands increase in intensity in concert suggests that the Rh,CO and Rh,-C-O-Mn2+ entities are parts of the same complex. It therefore appears that the Rh,(CO),, complex migrates from one absorption site in the zeolite to another. The observed changes in the number and position of the carbonyl bands probably reflect the different chemical environments at these sites, in addition to changes in sym- metry.The major bands at 2125 and 2088 cm-' in the Mn- promoted sample increase when the system is brought to room temperature under CO partial pressure. The bands at 2122 and 2088 cm-' evident in RhY3 at low temperature vanished at 300 K. The latter bands, indicating a rhodium tricarbonyl species, happen to be located very close to the former bands typical for RhMnY523. The features at 2125 and 2088 cm-' in RhMnY523 may thus be ascribed to CO on top of isolated rhodium ions in different oxidation states. A band near 2130 cm-' has been stressed to be indicative of Rh3+-C0 complexes by Rice et aL4' Since the absorptions at 2125 and 2088 cm-' are con- fined to Mn-exchanged RhY samples it may be inferred that Mn ions might play a role as adsorption sites for these species.Alternatively, metal atoms that are strongly polarized by protons may serve as coordination centres for CO, as was evidenced by Zhang et al.,, for Pd-exchanged zeolite Y. Note that all of these distinguishing features, both in the high- frequency (2130-2080 cm- ') and in the low-frequency (1850- 1600 cm-') region are almost completely absent from the IR spectrum of the RhMn(CVD) sample; they are replaced by a totally new set of bands between 2075 and 1940 cm-'. Rhodium and manganese thus experience a totally different structural and electronic situation when they are introduced via the vapour phase as volatile organometallics. The very low intensity of the 2095 and 1832 cm-' bands is significant in this respect.The decomposition of Mn,(CO),, on the pre- formed Rh, particles inside the supercages of NaY strongly resembles the decomposition of Re,(CO),, on Pt particles in the same zeolite, which was reported previ~usly.~~ In this case no Mn2+ ions are formed, as is actually confirmed by the present IR spectra. The bimetallic particles formed in this way will presumably have surfaces rich in Mn. As Mn prefer- entially ligates with CO in the linear mode and as its pres- 1344 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 ence at the surface of the bimetallic particles strongly reduces the number of Rh, ensembles, no bridging CO is expected for these systems. This is confirmed by the absence of the bands typical of bridging CO.Summarizing, comparison of mono-and bi-metallic samples, and the concomitant comparison of samples pre- 15 16 17 18 J. R. Pearce, W. J. Mortier, J. B. Uytterhoeven and J. H. Luns- ford, J. Chem. SOC., Faraday Trans. 1, 1979,75,898. T. T. T. Wong, Z. Zhang and W. M. H. Sachtler, Catal. Lett., 1990,4, 365. C. Dossi, R. Psaro, A. Bartsch, E. Brivio, A. Galasco and P. Losi, Catal. Today, 1993, 17, 527. R. D. Shannon, J. C. Vedrine, C. Naccache and F. Lefebvre, J. pared by ion exchange and by chemical vapour deposition, provides evidence for the chemical interaction of Mn2+ ions with Rh, clusters. This interaction, possibly leading to CO molecules that are C-bonded to Rh and O-bonded to Mn2+, is likely to be at the root of the propensity of Mn to ‘promote’ Rh catalysts to become highly selective for the for- 19 20 21 Catal., 1984,88,431.P. A. Jacobs and J. B. Uytterhoeven, J. Chem. SOC., Faraday Trans. 1, 1973,69, 359. S. Bordiga, E. E. Platero, C. 0.Arean, G. Lamberti and A. Zec-china, J. Catal., 1992, 137, 179. N. Echoufi and P. Gelin, J. Chem. SOC., Faraday Trans., 1992, 88,1067. mation of oxygenates in the syngas conversion reaction. 22 H. Knozinger, in Acid-Base Catalysis, ed. K. Tanabe, H. Hattori, T. Yamaguchi and T. Tanaka, Kodansha, Tokyo, 1989, The work performed in Munich was financially supported by the Deutsche Forschungsgemeinschaft (SFB 338) and by the Fonds der Chemischen Industrie.The group at Northwestern University acknowledges support from the Director of the Chemistry Division, Basic Energy Sciences, U.S. Department of Energy, grant number DE-FGO2-87ERAl3654. The work performed in Milan was financially supported by the Italian National Research Council (Progetto Finalizzato Chimica Fine). The assistance of Dr. S. Recchia with DRIFTS mea- 23 24 25 26 27 p. 147. C. L. Angel1 and P. C. Schaffer,J. Phys. Chem., 1966,70,1413. M. Primet, J. Chem. SOC.,Faraday Trans. I, 1978,74,2570. G. Bergeret, P. Gallezot, P. Gklin, Y.Ben Taarit, F. Lefebvre, C. Naccache and R. D. Shannon, J. Catal., 1987,104,279. H. Miessner, I. Burkhardt, D. Gutschick, A. Zecchina, G. Mor-terra and G. Spoto, J. Chem. SOC., Faraday Trans.I, 1989, 85, 2113. H. Miessner, I. Burkhardt, D. Gutschick, A. Zecchina, G. Mor- terra and G. Spoto, J. Chem. SOC., Faraday Trans., 1990, 86, 2321. surements is gratefully acknowledged. Collaboration between the three research groups was made possible by NATO 28 29 P. Gelin, Y. Ben Taarit and C. Naccache, J. Catal., 1979,59,357. E. J. Rode, M. E. Davis and B. E. Hanson, J. Catal., 1985, 96, research grant number 900056. 30 574. L-F. Rao, A. Fukuoka, N. Kasugi, H. Kuroda and M. Ichikawa, J. Phys. Chem., 1990,94,5317. References 31 32 M. Ichikawa, J. Catal., 1979,59, 67. T. Shido, T. Okazaki, M. A. Ulla, T. Fujimoto and M. Ichikawa, 1 M. M. Bhasin, W. H. Bartley, P. C. Ellgen and T. P. Wilson, J. Catal., 1978,54, 120. 33 Catal. Lett., 1993,17,97.A. Theolier, A. K. Smith, M. Leconte, J-M. Basset, G. M. Zan- 2 T. P. Wilson, P. H. Kasai and P. C. Elgen, J. Catal., 1981, 69, derighi, R. Psaro and R. Ugo, J. Organomet. Chem., 1980, 191, 193. 415. 3 4 5 W. M. H. Sachtler, D. F. Shriver, W. B. Hollenberg and A. F. Lang, J. Catal., 1983,92,429. W. M. H. Sachtler, D. F. Shriver, W. B. Hollenberg and A. F. Lang, J. Catal., 1985,93, 340. F. G. A. van den Berg, J. H. E. Glezer and W. M. H. Sachtler, J. Catal., 1985,93,340. 34 35 36 37 L. A. Hanlan and G. A. Ozin, J. Am. Chem. SOC., 1975,%, 6324. R. Whyman, J. Chem. SOC., Dalton Trans., 1972,1375. M. Ichikawa and T. Fukushima, J. Phys. Chem., 1985,89, 1564; M. Ichikawa, P. E. Hoffmann and A. Fukuoka, J. Chem. SOC., Chem. Commun., 1989,1395. T. T. T. Wong, A. Yu. Stakheev and W. M. H. Sachtler, J. Phys. 6 7 M. Ichikawa, T. Fukushima and K. Shikakura, in Proc. 8th Int. Congr. Catal., Berlin, Dechema-Verlag Chemie, Frankfurt- Weinheim, 1984, vol. 2, p. 69. S. A. Stevenson, A. Lisitsyn and H. Knozinger, J. Phys. Chem., 1990,94,1976. 38 39 Chem., 1992, %, 7733. G. Wilkinson, F. G. A. Stone and E. W. Abel, Comprehensive Organometallic Chemistry, Pergamon Press, Oxford, 1980, vol. 4, p. 1. T. T. T. Wong, A. Yu. Stakheev and W. M. H. Sachtler, J. Phys. 8 H. Knozinger, in Homogeneous and Heterogeneous Catalysis, ed. YG. Yermakov and V. Likhololov, VNU Science Press, Utrecht, 40 Chem., 1992,%, 7733. E. Mantovani, N. Palladino and A. Zanobi, J. Mol. Catal., 1977/ 9 10 11 1986, p. 789. W. M. H. Skhtler and M.Ichikawa, J. Phys. Chem., 1985, 90, 4752. W. M. H. Sachtler and Z. C. Zhang, Ado. Catal., 1993,39,129. Z. C. Zhang, T. T. T. Wong and W. M. H. Sachtler, J. Catal., 1991,128,13. 41 42 43 78,3,285. C. A. Rice, S. D. Worley, C. W. Curtis, J. A. Guin and A. R. Tarrer, J. Chem. Phys., 1981,74,6487. Z. Zhang, T. T. T. Wong and W. M. H. Sachtler, J. Catal., 1991, 128,13. C. Dossi, J. Schaefer and W. M. H. Sachtler, J. Mol. Catal., 12 L. Xu, Z. C. Zhang and W. M. H. Sachtler, J. Chem. SOC., 1989, 52, 193. Faraday Trans., 1992,88,2291. 13 Z. C. Zhang, B. Lerner, G-D. Lei and W. M. H. Sachtler, J. Catal., 1993, 140,481. Paper 3/072725; Received 8th December, 1993 14 X. L. Bai and W. M. H. Sachtler, J. Catal., 1991,129, 121.

ISSN:0956-5000    

DOI:10.1039/FT9949001335

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(9), 1345-1350 1345 Zinc-exchanged Y Zeolites studied with Carbon Monoxide and Xenon as Probes Bruno Boddenberg" and Andreas Seidel Lehrstuhl fur Physikalische Chemie 11, Universitat Dortmund, Otto-HahnStr. 6, 0-44227 Dortmund, Germany The adsorption isotherms of carbon monoxide and xenon, as well as the 12'Xe NMR chemical shifts of xenon, in 55 and 74% zinc-exchanged Y zeolites with various pretreatment conditions, such as temperature of dehydra- tion, oxidation and preloading with CO, have been measured. The sets of experimental data can be explained quantitatively with a unifying approach which considers localized adsorption of both CO and xenon on well defined adsorption sites consisting of two types of zinc cations in different environments, and sodium cations.The two different zinc cation sites are characterized by their individual adsorption constants and isosteric heats of CO adsorption (63 and 46 kJ mol-') as well as by the '*'Xe NMR chemical shifts of the xenon atoms accommodated on them (185 and 135 ppm). The concentrations of these sites as a function of the degree of zinc exchange and pretreatment conditions were determined. Transition-metal-exchanged zeolites are interesting catalysts for a variety of chemical reactions.' An important question related to the performance of such catalysts concerns the location, oxidation state and distribution of the transition- metal species in the zeolite voids accessible to reactant mol- ecules. It is well known that only species in certain oxidation states and in well defined geometrical and chemical environ- ments within the aluminosilicate framework of zeolites provide catalytic activity.Although at present zinc-exchanged faujasite-type zeolites do not seem to be catalysts of great importance and have only scarcely been explored with respect to catalytic activ- ity,2-8 they appear well suited for basic studies aimed at the determination of the concentration and characteristic adsorp- tion behaviour of -active sites in zeolites. The literature reports investigations of the adsorption of CO in zinc-exchanged Y-type zeolite^,^.' which indicate the presence of a limited number of Zn2+ species at or near the supercage walls with different CO adsorption capabilities.The method employed here consists of the combined appli- cation of CO and xenon adsorption techniques as well as '29Xe NMR spectroscopy. The latter, which was introduced by Ito and Fraissard," sensitively probes the physical and chemical properties of zeolite voids accessible to xenon, whereas CO adsorption is a standard technique for locating the transition-metal cation centres that exhibit enhanced CO bonding capability. The combination of both methods should therefore provide more detailed information about cation concentration and distribution in transition-metal-exchanged zeolites. Experimental Zeolite NaY (LZ-Y52, Union Carbide, Si : A1 = 2.37 :1) was treated twice with 0.1 mol dm-, NaCl aqueous solution with subsequent intense washing, dried at 80"C and then rehydrated over saturated NH,Cl solution.The ion exchanges were performed with 0.1 mol dm -Zn(NO,), aqueous solutions at pH 5.7-5.9. After drying at 80°C (6 h) the zeolites were stored over saturated NH,Cl solution. The samples prepared are designated Zn(55)Y and Zn(74)Y, indi- cating the percentage of Zn2+ exchanged for Na+. The results of the chemical analysis for zinc (ICP/OES), sodium (FAES) and water are collected in Table 1. A differential thermal analysis/thermogravimetry (DTA/TG) investigation showed that the dehydration is completed at 300 "C and that the zeolites are stable up to 890 [Zn(55)Y] and 910°C CZn(74)Yl.The zeolites were dehydrated under high vacuum at 25 "C (ca.2 h), 120°C (6 h) and 400°C (16 h) with heating at rates of 20 and 60 K h-' in the intervals between. Subsequently, the samples were cooled to ambient temperature after either no contact or a 16 h contact with oxygen (400 hPa) at 400°C. In a similar way, samples dehydrated at a final temperature of 200 "C were prepared. For a precise characterization of the variously pretreated zeolites, the final temperature and treat- ment with oxygen (if applied) are indicated in the sample designation, e.g. Zn(74)Y/673ox. In their rehydrated form the samples were examined with X-ray diffraction (XRD) as well as with 29Si and 27Al magic- angle spinning (MAS) NMR spectroscopy. Sharp XRD lines characteristic for faujasite' ' with lattice constants a = 2.468 [Zn(55)Y] and 2.475 nm [Zn(74)Y], single 27Al NMR lines at 6 = 60.6 [Zn(55)Yl and 59.4 ppm [Zn(74)Y] relative to 0.1 mol dm-3 AlCl, solution, and 29Si NMR quintets with amplitude ratios corresponding to Si : A1 ratios of 2.2 :1 [Zn(55)Y] and 2.4 :1 [Zn(74)Y]12 were obtained.These results prove that crystallinity is retained after the zinc exchange and the subsequent dehydration/oxidation pro-cedures. A 75-80% zinc exchange seems to be the critical limit for framework stability since >80% zinc-exchanged Y zeolites have been reported to lead to framework collapse.2 The adsorption isotherms of CO and xenon were measured volumetrically using conventional glass and all-steel equipments, respectively. The adsorption was reversible, and equilibrium was attained within 30-60 min in both cases.The 29Xe NMR measurements were performed at ambient tem- perature and at the resonance frequency, 0,/27t = 21.4 MHz, using a Bruker (Karlsruhe, Germany) spectrometer type CXP 100. In each case studied, the '29Xe NMR spectrum was found to consist of a single resonance line. The chemical shifts were evaluated according to 6 = 1OB(vprobc-V,,~)/V,~~, with xenon gas at vanishing pressure taken as the reference. Table 1 Cation and water concentrations of the investigated zinc- exchanged Y zeolites number of species per unit cell Zn2+ for Na+ sample Zn2+ Na+ H20 exchange level (%) ~~~~~ ~ ~~ Zn(55)Y 15.8 20.7 253 Zn(74)Y 21.0 10.6 244 55 74 J. CHEM. SOC.FARADAY TRANS., 1994, VOL. 90 Results Fig. l(a)and (b) show the adsorption isotherms of CO in the zeolites Zn(x)Y/673ox (x = 55 and 74) at temperatures between 273 and 309 K. For comparison purposes, the ambient temperature CO adsorption isotherm of zeolite NaY dehydrated at 693 K is included.13 After initial steep rises up to pressures of several hPa, the isotherms merge into straight lines of slopes that decrease with temperature and also with the degree of zinc exchange. Fig. 2(a) and (b) show the ambient temperature adsorption isotherms of CO in the zeolites Zn(x)Y/673 (x = 55 and 74) and in the zeolite Zn(74)Y/473ox. For comparison purposes, the ambient temperature isotherms of the 673 K oxidized zeolites and of NaY are taken from Fig.1. Basically, the iso- therm shapes of the zinc-exchanged zeolites are the same as described before. Interestingly, the oxidation following the dehydration at 673 K leads to a decrease of the CO adsorp-tion. At the lower dehydration temperature (473 K) the amount of adsorbed CO is drastically reduced. Fig. 3 and 4 show the ambient temperature xenon adsorp- tion isotherms and the '"Xe NMR chemical shifts of xenon in the zeolites Zn(55)Y and Zn(74)Y dehydrated and oxidized at 673 K, as well as in Zn(74)Y/673ox preloaded with 1.7 CO per unit cell (uc) (0.21 CO per supercage). The corresponding data for zeolite NaY dehydrated at 693 K13-" are included for comparison. In contrast to Nay, the isotherms of the zinc-exchanged zeolites are each concave to the pressure axis initially and merge into straight lines at higher pressures.The preloaded zeolite exhibits lower xenon adsorption than both non-preloaded samples over the whole pressure range investi- gated. Of the latter zeolites, the sample with the higher zinc concentration exhibits larger xenon adsorption at pressures 5 4 u 53 P 0 $2 1 3 0 52 P 0Y 21 0 2 4 6 8 10 P/hPa Fig. 1 Adsorption isotherms of CO in the zeolites Zn(74)Y/673ox (a),Zn(55)Y/673ox (0)and NaY/693 (x): (a) in the range 0-300 hPa, (b) in the range 0-10 hPa. The symbol diameters in ascending order refer to the measuring temperatures 273, 298 and 309 K. The solid lines are fittings according to the model discussed in the text.5 4 0 53P 0 $2 1 0 3 0 z2 0 0 0, p1 Fig. 2 Adsorption isotherms (298 K) of CO in the zeolites Zn(74)Y/ 673 (m), Zn(74)Y/673ox (O), Zn(55)Y/673 (O), Zn(55)Y/673ox (O), Zn(74)Y/473ox (A)and NaY/693 (x). (a) and (b) as for Fig. 1. The solid lines are fittings according to the model discussed in the text. Fig. 3 Adsorption isotherms (298 K) of xenon in the zeolites Zn(55)Y/673ox (O),Zn(74)Y/673ox (O), Zn(74)Y/673ox preloaded with CO (+) and NaY/693 ( x ). The solid lines are fittings according to the model discussed in the text. 160 140 120 100 80 0 5 10 15 N/Xe per uc Fig. 4 12'Xe NMR chemical shifts (298 K) of xenon in the zeolites Zn(55)Y/673ox (0),Zn(74)Y/673ox (O), Zn(74)Y/673ox preloaded with CO (+) and NaY/693 ( x ).The solid lines are fittings according to the model discussed in the text. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 up to about 500 hPa, but the reverse is true at higher pres- sures. Interestingly, each of the adsorption isotherms of the zinc-exchanged samples intersects the NaY curve. The '29Xe NMR chemical shifts of xenon in each of the zinc-containing zeolites are higher than those in Nay. With increasing xenon loading they first decrease and then increase again, approaching straight lines that run almost parallel to the line found for xenon in Nay. Discussion Adsorption of Carbon Monoxide Carbon monoxide is known to be much more strongly adsorbed in zinc-exchanged zeolites Y than in their precursor form This behaviour has been attributed to the for- mation of strong complexes of CO with the accessible Zn2+ cations within the faujasite ~tructure.~~~ From IR2*16 and adsorption isotherm' studies it has been concluded that two types of Zn2+ species interact with CO molecules, which at ambient temperature cannot penetrate into the small j?-One of these Zn2+ cations was found to be present to an appreciable extent at zinc exchanges of 60%.The initial steep increase of the adsorption isotherms of CO in the zinc-exchanged zeolites (Fig. 1 and 2) and their subsequent almost linear courses indicate the presence of a limited number of strong adsorption sites that become satu- rated at pressures of several hPa, and of weak adsorption sites with characteristics similar to those of Nay.Assuming a 1 :1 correspondence between CO and the strong adsorption sites, the concentration of these sites can be estimated by extrapolating the linear isotherm portions to zero pressure. For instance, the ordinate intersections in the case of the zeolite Zn(74)Y/673ox at the three different temperatures investigated (Fig. 1) yield a common value, Np+O,of three CO molecules uc- '. Fig. 5 shows the isosteric heats of adsorption, 4s1,of CO in the zeolite Zn(74)Y/673ox as function of CO concentration. The data points shown were obtained with the aid of the well known equation" d ln(p/hPa))4,=,,.( dT from isosteres constructed by interpolating the adsorption isotherm data at the three measuring temperatures.Their accuracy was estimated to be fi5 kJ mol-'. The results obtained suggest a three-plateau feature with plateau heights at about 65, 45 and 20 kJ mo1-'. The latter value agrees quite well with literature data (23-25 kJ mol-') obtained for CO adsorbed at low concentration in the zeolite NaY.9918919 80 r 60 20 I I 1 0 1 2 3 4 N/CO per uc Fig. 5 Isosteric heats of CO adsorbed in the zeolite Zn(74)Y/673ox. Dotted lines show plateau levels at 20,46 and 63 kJ mol-'. The solid curve represents the heats calculated from the model discussed in the text. This indicates that the linear portions of the adsorption iso- therms are due to the supercage Na+ cations. Remarkably, the former two values are of the order of the low-coverage heats of adsorption of CO on the surface of ZnO (42-50 kJ mol -1)20-23 suggesting that these heats are associated with the zinc species in the zeolites under study.The discussion carried through so far suggests that the adsorption isotherms of Fig. 1 and 2 can be represented by a superposition of three isotherms, of which one is of the Henry type. Owing to the rather low concentration of the strongly adsorbing sites, it seems appropriate to choose Langmuir- type adsorption isotherms to represent the CO adsorption on them. So we write N=-+-nlklP n2kzP 1 + k,p 1 + k,p +K,P In this equation ni and ki (i = 1, 2) are the concentrations and adsorption constants of the strong adsorption sites which are assumed to accommodate one CO molecule each.Any attempt to reproduce the adsorption isotherms with one Langmuir and one Henry type expression remained unsuc- cessful. The fitting of the adsorption isotherms of Fig. 1 and 2 with eqn. (2) requires the determination of five parameters in each case. K, and the sum n, + n2 can be obtained from the slopes and ordinate intersections of the higher-pressure linear isotherm portions. Starting from appropriately chosen values of the remaining three unknown parameters, namely k,, k, and n,, an iteration procedure was applied which accom- plished optimum fits of the complete sets of the experimental isotherm data of Fig. 1 and 2 simultaneously. It was required that (i) samples with the same pretreatment exhibit identical values of n, and n2irrespective of the measuring temperature, and (ii) samples with different pretreatments exhibit the same values of k, and k, at a given measuring temperature.The main concern was to reproduce as faithfully as possible the low-pressure portions of the adsorption isotherms which required slight corrections of the values of K, and (n,+ n,) introduced initially. The results of the fittings are shown in Fig. 1 and 2 (solid curves) and in Table 2 where the optimum-fit parameter values are collected. These data are estimated to be correct within +_1digit of the last relevant figure. Obviously, the experimental results can be reproduced very well, especially as far as the low-pressure range is concerned.This lends strong support for the appropriateness of the underlying three-site model. Introducing the fitting parameters for the zeolite Zn(74)Y/ 673ox (Table 2) into eqn. (2) and constructing isosteres, iso- steric heats of adsorption at any CO concentration were calculated. These are represented by the solid curve in Fig. 5 which exhibits the three-plateau feature very well. From the data presented in Table 2 several interesting con- clusions may be drawn. The concentrations, n, and n, ,of the zeolites pretreated at 673 K both increase with zinc content. This observation indicates that the type 1 and 2 sites involve Zn2+ ions at or near the walls of the supercages of the fauja- site structure. The low concentration of these sites at the 55% zinc exchange level (ca.10% of the overall zinc content) and its considerably larger value at 74% exchange (ca. 15% of the overall zinc content) is in accordance with the previously mentioned findings of Otsuka et a!.' The treatment with oxygen of the zeolites dehydrated at 673 K decreases n2but leaves n, unchanged. This observation indicates that the type 1 ZnZ+ cations are in stable positions whereas some of the type 2 cations become displaced, probably into the sodalite cages, under high-temperature oxygen contact. The low J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 2 Parameter values used for the fits of the experimental adsorption isotherms of carbon monoxide Nay1693 298 Zn( 55)Y/673ox 298 0.26 1.3 Zn(55)Y/673 298 0.26 1.5 Zn(74)Y/673ox 273 1.2 1.8 Zn(74)YY/673ox 298 1.2 1.8 Zn( 74)Y/673ox 309 1.2 1.8 Zn( 74)Y/673 298 1.2 2.2 Zn(74)Y/473ox 298 0.18 0.18 values of n, and n2 obtained for the zeolite dehydrated and subsequently oxidized at 473 K are considered to be due to a screening of most of the Zn2+ ions by residual water mol- ecules and/or OH groups which prevent access of the CO molecules to these cations.At each measuring temperature, the adsorption constants k, and k2 differ from each other by more than one order of magnitude, indicating a much different strength of bonding of CO to these sites. We may write the temperature dependence of ki(i = 1, 2) as ki = ki, exp(q,lRT) (3) where 4iis a parameter associated with the heat of adsorp- tion on the respective site.17 Fig.6 shows the logarithmic plot of k, and k2 us. reciprocal temperature for the zeolite Zn(74)Y/673ox. Also shown is the corresponding plot of K, for this zeolite. Obviously, straight lines are obtained from which values of ki, and qiwere evaluated. The data obtained are collected in Table 3. The heats of adsorption of CO on the three types of sites considered, namely 63, 46 and 20 kJ mol-', are represented by the dashed lines in Fig. 5. Most satisfactorily, they are just at the plateau heights of the iso- steric heats, which lends further support to the three-site assumption introduced. The appropriateness of the Langmuir model to describe CO adsorption on type 1 and 2 sites requires that the values of the pre-exponential factors k,, and k,, (Table 3) are of physically reasonable magnitude.In order to prove this, we use the expression for the Langmuir constant k (for conve- nience we drop here the lower index i) provided by statistical mechanics24 In this equation 4' and U,, are the partition function and the potential energy of the adsorbed molecule, respectively, and q&* is the internal partition function of the molecule in the gaseous state. h and k, are Planck's and Boltzmann's con- stants, respectively. The two partition functions may be decomposed further according to (5) Here ql, qI1and q, are the partition functions of the vibra- tions perpendicular and parallel to the surface, and of the wagging-type motions, respectively.The latter motions corre- spond to the rotational degrees of freedom of the CO mol-ecules in the gas phase for which the partition function is 4,,,= 8n2Zk, T/h2.Here I is the moment of inertia. Finally, q:ib and 4Vpibare the stretching vibration partition functions of CO in the adsorbed and gaseous state, respectively. For the purpose of estimation we put qW= 1 and q1 =-ql1, and express the latter partition functions in the high-6.6 13 0.74 6.5 13 0.74 6.5 125 3.6 9.4 13 0.74 4.8 5.0 0.33 3.4 13 0.74 4.8 13 0.74 4.2 -5w7-5= I,.,,!,,,,,,l 3.2 3.3 3.4 3.5 3.6 3.7 103 KIT Fig. 6 van't Hoff plots of the constants k, (a),k, (0)and K, (x) of CO adsorbed in zeolite Zn(74)Y/673ox temperature approximation, i.e.(7) Here v, is the vibration frequency of the molecule with respect to the surface. The stretching frequencies of CO in the gaseous state (2143 cm-1)25 and adsorbed on Zn2+ in Y zeolites (in the range 2120-2220 cm-1)216 are close together so that the respective partition functions are the same within 1Oh. Introducing the approximations considered into eqn. (4), the pre-exponential factor is obtained to be from which relation the vibrational frequencies vSi (i = 1, 2) can be calculated from the respective values of k,. The resulting data are collected in Table 3. The frequency values obtained are of the expected magnitude for physisorbed molecules.24 The higher value of the frequency and, hence, of the force constant associated with the type 1 sites in compari- son to type 2 reflects the stronger bonding of CO to the former.This result is in accordance with the conclusions drawn from analysis of the heat of adsorption data. The nature of the sites of types 1 and 2 remains to be eluci- dated. Undoubtedly, these sites are associated with Zn2 + ions at the supercage walls of the faujasite structure. We imagine that type 1 zinc ions exhibiting the highest heat of adsorption towards CO (63 kJ mol-') have lower coordination to sur- Table 3 Parameters obtained from analysis of the temperature dependenceof the adsorption and Henry constants 1 k,/hPa -VSi/lO'~ s -qJkJ mol-' 1 9 x lo-" 1.6 63 2 5 x 10-9 0.43 46 3 --20 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 rounding framework oxygen atoms than the type 2 species. This is because the cations with lower coordination to oxygen atoms are expected to exhibit higher Lewis acidity towards the weakly Lewis basic CO molecules and/or, even- tually, provide easier access for the adsorbate species. These considerations suggest that type 1 zinc cations are located at or near the crystallographic S I11 positions, and type 2 zinc ions reside at or near the octahedral positions between the successive oxygen three-rings connecting the supercages with the sodalite cages. The latter positions are known to be occupied by, e.g., Cu2+ ions in copper-exchanged zeolite Y.26 Conceivably, under contact with CO, the zinc ions residing here may be pulled slightly towards the S I1 position.This effect of cation displacement is a known behaviour of, e.g., Cu' 27 and Ni2+ in faujasite-type zeolites. '29Xe NMR and Adsorption of Xenon It has repeatedly been demonstrated that cation sites which strongly adsorb CO also provide strong adsorption centres for xenon.13,28,29 The shape of the xenon adsorption iso- therms of the zeolites Zn(x)Y/673ox (x = 55 and 74) with initial concave portions and high-pressure straight lines of slopes decreasing with increasing zinc content (Fig. 3) is remi- niscent of the shape characteristics of the corresponding CO adsorption isotherms. This suggests that the rule cited at the beginning may be valid also for the zinc-exchanged zeolites studied here.In order to prove this, we use eqn. (2) likewise to treat the xenon adsorption data. As in the previous case of the adsorp- tion of CO, we assume a 1 : 1 correspondence between the xenon atoms and each of the sites available. Under the condi- tion of rapid exchange of the xenon atoms among the adsorp- tion sites available, the observable chemical shift, 6, may be formulated as where ai (i = 1-3) is the local chemical shift of a xenon atom accommodated on site i, and FN,, is a collective term taking into account the xenon density dependence of the chemical shift. 3*30,31The rapid-exchange condition is considered to be valid in the present case because a single resonance line is always observed. The fitting of the experimentally determined adsorption isotherm and chemical shift data of xenon in the zeolites Zn(55)Y/673ox and Zn(74)Y/673ox requires the determi-nation of k,, k,, K,, F and 6, -6,.The site concentrations, n, and n,, are taken over from the analysis of the CO adsorption results (Table 2). The parameter F is assumed to exhibit the same value as for Nay, namely F = 1.84 ppm uc, which is evaluated from the slope of the straight 6 us. N line of NaY (Fig. 4). Except for K,, the further parameters are assumed to be identical for both zinc-exchanged zeolites under consideration. Starting from appropriately chosen values of the parameters, an iteration procedure was applied to yield a simultaneous optimum fit of the sets of adsorption isotherm and chemical shift data.The solid curves in Fig. 3 and 4 demonstrate that both the adsorption isotherms and the chemical shifts of xenon in the zeolites without preadsorbed CO can be reproduced excel- lently. The values of the parameters, with which these fits were accomplished, are collected in Table 4. It is recognized that the adsorption constants of xenon follow a gradation similar to those of CO. However, k, and k, for xenon are about two orders of magnitude smaller than the correspond- ing values for CO, whereas the Henry constants describing the strength of adsorption on type 3 sites show contrasting behaviour, but are of a similar magnitude in both cases. Importantly, the procedure applied here, allows an accu- rate extrapolation of the chemical shifts to zero xenon con- centration (6,= o), and also the determination of site-specific chemical shifts for cases where the shifts increase at low xenon concentrations.The extrapolated values are found to be S,=, = 158 and 120 for the zeolites Zn(74)Y/673ox and Zn(55)Y/673ox, respectively, and the site characteristic shifts are 6, = 185 and 6, = 135. The shift 6, = 64 obtained for the type 3 sites is close to a,=, = 58 for xenon in NaY.'0*'3,'5 This result supports the previous conclusion that type 3 sites are sodium cations at the supercage walls. The fact that the adsorption constants k, and k, for CO are both much larger than either of the corresponding values for xenon, suggests that CO preadsorbed on sites 1 and 2 effectively blocks these sites for the xenon atoms. Knowing the number of blocked sites and assuming that the blocking does not create new sites, the adsorption isotherm and 129Xe NMR chemical shift data of xenon adsorbed in the preloaded zeolite should be predictable quantitatively with eqn.(2) and (9), and the parameter data of the unpreloaded zeolite Zn(74)Y/673ox, but with n, and n, replaced by the corre- sponding concentrations of the non-blocked sites. These con- centrations are obtained from the CO adsorption data as follows: The concentration, 1.7 CO uc-', of preadsorbed CO in the zeolite Zn(74)Y/673ox is maintained at an equilibrium pressure 0.71 hPa at 298 K. At this pressure the concentra- tions of type 1 and 2 sites occupied by CO molecules are calculated to be 1.1 and 0.6 sites uc- I, respectively, using the Langmuir-type expressions of eqn.(2) with the values of the respective adsorption constants k, and k, of Table 2. Conse- quently, the concentrations of the type 1 and 2 sites still accessible to xenon are n, = (1.2-1.1) uc-' = 0.1 uc-l and n2 = (1.8-0.6) uc-l = 1.2 uc-l, respectively. These values are listed in Table 4. The xenon adsorption isotherm and chemical shifts with the data thus obtained (last line of Table 4) and with F = 1.84 ppm uc given before, are represented by the solid curves in Fig. 3 and 4. Obviously, excellent agreement with the experimental data is obtained. Conclusions The study of zinc-exchanged zeolites with adsorption of xenon and carbon monoxide in combination with 12'Xe NMR spectroscopy has turned out in this investigation to be very useful for discriminating between zinc cations in differ- ent environments at or near the supercage walls of Y-type Table 4 Parameter values used for fits of the experimental adsorption isotherms of xenon and the I2'Xe NMR chemical shifts NaY/693 0 0 Zn(55)Y/673ox 0.26" 1.3" Zn(74)Y/673ox 1.2" 1.8" Zn(74)Y/673ox (1.7 CO uc-') 0.1 1.2 " The site concentrations were adopted from Table 2. 58 2.16 185 135 64 0.05 0.008 1.72 185 135 64 0.05 0.008 1.44 185 135 64 0.05 0.008 1.44 1350 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 zeolites. The whole body of experimental data of zinc-exchanged Y-type zeolites with different degrees of zinc exchange and various sample pretreatments comprising tem- perature of dehydration, oxidation and preloading with CO can be reproduced quantitatively on the basis of a simple adsorption model.Adsorption constants and 129Xe NMR 8 9 10 11 M. Ziolek and J. Kujawa, Zeolites, 1990, 10, 657. T. A. Egerton and F. S. Stone, J. Chem. Soc., Faraday Trans. I, 1973,69, 22. T. Ito and J. Fraissard, Proc. 5th Int. Con$ Zeolites, Naples, 1980, p. 510. R. von Ballmoos and J. B. Higgins, Collection of Simulated XRD Powder Patterns for Zeolites, Butterworth-Heinemann, Stone- chemical shifts characteristic for xenon atoms adsorbed on ham, 2nd edn., 1990. well defined adsorption sites were obtained. This is of great importance for the analysis of adsorption and chemical shift data of zinc-containing zeolites, or other microporous materials of unknown cation concentrations, and of the dis- tribution of zinc cations within them.The method should be applicable to other cations in zeolites, and thus an atlas of site-characteristic adsorption constants and 12'Xe NMR chemical shifts could be compiled. The 129Xe NMR chemical shifts of xenon adsorbed on the two zinc cation sites detected, presumably Zn2+ ions at the crystallographic S I11 and S II/S 11' positions, were deter-mined to be + 185 and + 135 ppm with respect to xenon gas at vanishing pressure. 12 13 14 15 16 17 18 19 20 21 G. Engelhardt and D. Michel, High Resolution Solid State NMR of Silicates and Zeolites, Wiley, Chichester, 1987. M.Hartmann and B. Boddenberg, Microporous Materials, 1994, 2, 127. J. Watermann, Diploma, University of Dortmund, 1989. T. Geilke, Diploma, University of Dortmund, 1990. C. L. Angel1 and P. C. Schaffer, J. Phys. Chem., 1966,70,1413. A. W. Adamson, Physical Chemistry of Surfaces, Wiley, New York, 5th edn., 1990. T. A. Egerton and F. S. Stone, J. Colloid Interface Sci., 1972, 38, 195. T. A. Egerton and F. S. Stone, Trans. Faraday SOC., 1970, 66, 2364. R. R. Gay, M. H. Nodine, V. E. Henrich, H. J. Zeiger and E. I. Solomon, J. Am. Chem. SOC., 1980,102,6752. A. A. Tsyganenko, L. A. Denisenko, S. M. Zverev and V. N. Financial support of this work by Fonds der Chemischen Industrie is gratefully acknowledged. 22 Filimonov, J. Catal., 1985,94, 10. E. Giamello and B. Fubini, J. Chem. SOC., Faraday Trans. I, 1983,79,1995. 23 G. L. Griffin and J. T. Yates, J. Chem. Phys., 1982,77,3751. References I. E. Maxwell, Adu. Catal., 1982,31, 1. K. Otsuka, J. Manda and A. Morikawa, J. Chem. SOC., Faraday 24 25 T. L. Hill, An Introduction to Statistical Thermodynamics, Addison-Wesley, Reading, 1960. D. H. Rank, D. P. Eastman, B. S. Rao and T. A. Wiggins, J. Opt. SOC. Am., 1961,51,929. Trans. I, 1981,77,2429. M. A. Wassel, E. A. Sultan and F. M. Tawfik, Asian J. Chem., 26 27 I. E. Maxwell and J. J. de Boer, J. Phys. Chem., 1975,79, 1874. J. Howard and J. M. Nicol, Zeolites, 1988,8, 142. 1992,4, 891. 28 R. GroDe, A. Gedeon, J. Watermann, J. Fraissard and B. Bod- M. F. Menoufy, E. A. Sultan and A. K. El-Morsi, Trans. Egypt SOC. Chem. Eng., 1990,16,33. H. Mori, N. Mizuno, T. Shirouzu, S. Kagawa and M. Iwamoto, 29 30 denberg, Zeolites, 1992, 12,909. Y-Y. Huang, J. Catal., 1974, 32,482. J. Fraissard and T. Ito, Zeolites, 1988, 8, 350. Bull. Chem. SOC. Jpn., 1991,64,2681. Y. V. S. Narayana Murthy and C. N. Pillai, Synth. Commun., 31 C. J. Jameson, A. K. Jameson and S. M. Cohen, J. Chem. Phys., 1973,59,4540. 1991, 21, 783. M. Ziolek, H. G. Karge and W. NieDen, Zeolites, 1990,10, 662. Paper 3/05015G; Received 18th August, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949001345

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(9), 1351-1354 Adsorption of Benzene on the Acidic and Basic Sites of KH B Zeolite studied by in situ IR Spectroscopy? Jian-ping Shen,* Jun Ma, Tie Sun and Da-zhen Jiang Department of Chemistry, Jilin University, Changchun , 130023,China En-ze Min Research Institute of Petroleum Processing, Beijing , 100083,China In situ IR studies of benzene adsorbed on several KH B zeolites, where the extent of potassium exchange, K/AI (YO),varied from 0 to 88.32%, have been undertaken under different conditions. The bands of adsorbed benzene on KH B zeolites in the range 2050-1700 cm-' were assigned. KH fl zeolite exhibits basicity only when K/ Al d 88.32%, where its basicity is stronger and its acidity weaker. The order of stability of the various adsorbed benzene species on the samples is as follows: 0-(benzene) > H+(benzene) > K+(benzene) > benzene./?Zeolite has been demonstrated to be useful as a catalyst for K and 0.2 Pa for 40 min, and then cooled. Benzene vapour acid-catalysed reactions such as hydroisomerization and was then admitted at room temperature. IR spectra were hydrocracking'V2 and as a sorbent in the separation of recorded with Nicolet FTIR-5DX spectrometer at different aromatics3 since it was first ~ynthesized.~ Its structure, which times, pressures and temperatures. The spectrum of the comprises channels with a three-dimensional 12-membered adsorbed phase was obtained after subtraction of the zeolite ring, has been p~blished.~ However, the acid-base properties contribution.of /? zeolite and its catalytic behaviour have received little attention. In our previous work6 we reported the effects of exchange conditions on the degree of exchange of H /3 zeolite Results and the isomerization reactions of a-@-)methylnaphthalene The main observation is that the frequencies (2050-1700 over a series of KH B zeolites. The IR spectra of adsorbed benzene on some zeolites are cm-') of the overtone and combination bands of C-H It is of interest to elucidate the adsorp- out-of-plane vibrations are shifted to higher wavenumbers already kn~wn.~-'~ tion mode of hydrocarbons, since it may explain the behav- compared with those of liquid benzene. iour and reactivity of the molecules in the pores.To date Fig. 1 shows the IR spectra in the 2050-1700 cm-' region some workers have at one extreme studied the interaction of of benzene adsorbed on KH j3 -3 zeolite at different adsorp- benzene with parent zeolites like Y,' X,12 /?13 and at the other tion times. Compared with the pure liquid [Fig. l(g)] the extreme with highly exchanged NaHY. However, few C-H out-of-plane vibrations of benzene on the sample are attempts have been made to detail the changes that occur in disturbed by the adsorption time. The bands at 1756, 1816, the spectra of adsorbed benzene molecules as the proton 1840 and 1960 cm- are observed at an adsorption time of 1 content in the zeolite increases and to study these zeolites min. With increasing time the intensities of the 1816 and 1960 cm -' bands decrease rapidly, and simultaneously shoulders using the method of in situ IR spectroscopy.In this paper, the are observed at 1840 and 1973 cm-l. From comparison with adsorpton of benzene on the acidic and basic sites of KH B zeolites was studied by in situ IR spectroscopy on the basis of [Fig. lcf)], it can be seen that adsorption of benzene reaches our previous work. Experimental The starting material is a B zeolite of composition, for 64 T atoms per unit cell,' H3.50Nao.olA13~5,Si60~~~~~~~.The preparation of KH B zeolite sample with different degrees of exchange was conducted as in ref. 6. The chemical composi- tions of four samples are given in Table 1. The in situ IR studies were carried out on self-supporting wafers.The wafers placed in an IR cell were evacuated at 673 Table 1 Chemical composition of the samples sample chemical composition K/Ala (%) HB H3.50Na0.01A13.51si60.490128 0 KH B -H1.82K1.88Na0.01A12.52si80.480128 53.40 KH B -2 H1.05K2.45A13.52si60.480128 69.60 KH B -3 H0.41K3.10A13.51Si60.490128 88.32 2050 1875 1700 a Molar ratio. wavenumber/cm-' Fig. 1 IR spectra of benzene adsorbed on KH /? -3 zeolite at 300 Part 2 of 'Properties of Alkali-metal Ion Exchange of H p K after (a) 1 min, (b) 5 min, (c) 15 min, (430 min, (e) 1h, 012 h, (Q) Zeolite'. liquid equilibrium after 1 h. Note that the spectrum of adsorption equilibrium (1 h) is very different from that of initial adsorp- tion: the intensities of the bands at 1960 and 1816 cm-' are lower, the band at 1840 cm-' is shifted to lower wavenum- bers (1838 cm-') and is sharper, and four new bands at 1854, 1876,1973 and 2004 cm-' are formed.From Fig. 2 it can be seen that all of the samples exhibit the bands at 1960 and 1816 cm-' observed in the IR spec- trum of liquid benzene. Similar IR spectra are obtained when the K/A1 of KH /3 zeolite <88.32%, except for a small band shift at 1838 cm-'. With decreasing K/Al of KH #? zeolite the band at 1877 cm-' increases in intensity and is shifted to higher wavenumbers. The IR spectra of adsorbed benzene on KH #? -3 zeolite under different pressures are shown in Fig. 3. With decreasing pressure the intensities of the bands at 1960 and 1816 cm-' decrease rapidly and the peaks at 1973, 1877, 1853 and 1840 cm-' become sharper.Comparison of Fig. 3(u) and (c) reveals the selective desorption of benzene from KH /3 zeolite. IR spectra of the samples with different K/Al at the same pressure are presented in Fig. 4. The band at 1877 cm-' cuco 2 2050 1875 1700 wavenumber/crn-' Fig. 2 IR spectra at equilibrium of benzene on the samples at 300 (d)H /9-1,BKH-2,(~) /!? -3,(6) KH /9KH(a)K: J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 2050 1875 1700 wavenumber/cm-' Fig. 4 IR spectra of benzene adsorbed on the samples at 3.2 Pa and 300K: (a)KH B -3,(b) KH /3 -2, (c) KH fi -1, (d)H B increases in intensity with decreasing potassium content of ICH /3 zeolite and is shifted to higher frequencies [1888 cm-in Fig.4(d)]. The band at 1853 cm-', however, disappears when K/Al < 88.32%. The band at 1844 cm-', not observed in the spectrum of benzene adsorbed on H /3 zeolite, increases in intensity with increasing potassium content and is shifted to lower wavenumbers. Simultaneously, the band at 1984 cm-' is also shifted to lower wavenumbers. Fig. 5 shows for KH /I-3 the progressive change in the spectra as the desorption temperature increases. The inten- sities of all of the bands, except that at 1853 cm-', decrease with increasing temperature. The adsorbed benzene is com- pletely desorbed from the sample at 453 K. The behaviour of adsorbed benzene is reported in Fig. 6 for 3.2 Pa and T < 323 K. The bands at 2004 and 1853 cm-' are observed only for KH -3 zeolite.The band at 1840 cm-' totally disappears on H #?.On the other hand, the band at 1877 cm-' increases with decreasing potassium content. The maximum intensity of this band is observed for H #? zeolite. I 1 v I I 2050 1875 1700 2050 1875 1700 wavenum berlcm- ' wavenumber/cm-' Fig. 3 IR spectra of adsorption of benzene on KH /? -3 zeolite at Fig. 5 IR spectra of benzene adsorbed on KH B -3 zeolite at dif- 300 K and different pressures: (a)1.0 x lo5 Pa, (6) 13.3 Pa, (c) 3.2 Pa ferent temperatures: (a) 300 K, (6) 323 K, (c)423 K, (d)453 K J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Discussion In liquid benzene bands in the range 3050-1400 cm-' have been a~signed'~ to C-H stretching and a combination of C-C bendings in the range 3050-3000 cm-', to overtone and combination bands of the C-H out-of-plane deforma- tion vibration at 1960 and 1816 cm-' and to ring deforma- tion at 1479 cm-' .The 1960 and 1816 cm-' bands are assigned to v5 + v17 and vl0 + ~17," respectively. Upon adsorption of benzene on alkali-metal zeolite^,^*^*'^ a shift in frequencies and a split of this pair of bands usually occur. The bands resulting from the split of the starting pair are assigned to the low-frequency (LF) and high-frequency (HF) pair. The LF pair of bands can be assigned to the interaction of benzene with cations through the n-electron cloud and the HF pair of bands would result from the interaction with framework oxygen. Note that the extent of each type of inter-action depends on the acidity of the cations and on the oxygen basicity.Fig. 1-6 reveal some similarities in the behaviour of benzene adsorbed on the zeolites. The LF and HF pairs consist of the bands at 1840-1973 and 1853-2004 cm-', respectively. Similar trends are observed in the adsorption of benzene on the alkali-metal, Na /I and Cs B zeolites,I3 which lends further credence to an adsorption mechanism that per- turbs the 7c bonds of the aromatic ring while simultaneously locking the ring into a fixed configuration which does not favour out-of-plane vibrations. The frequencies of these bands are affected by the microenvironment, e.g. the presence of cations and framework oxygen. The intensity of the LF pair at 1840 cm-' increases with increasing content of pot- assium; this band, however, is not observed for the adsorp- tion of benzene on H /I zeolite, and can be assigned to the interaction with potassium cations through the nelectron a 2050 1875 1700 wavenumber/cm -' Fig.6 IR spectra of benzene adsorbed on the samples at 323 K and 3.2 Pa:(a) KH f? -3,(b)KH /3 -2,(c) KH f? -l,(d) H f? cloud. The intensity of the other LF pair at 1980 cm-' which reflects of the effects of the alkali metals in the parent zeolites on ben~ene,~.~ does not change obviously when K/A1 of KH /I zeolite is < 88.32%. Its intensity, however, decreases rapidly for the sample with K/Al = 88.32%. This may result from the interaction of benzene with K+ and H+ cations. It is pro- posed that the HF pair of bands (2004 and 1853 an-') results from interaction between the framework oxygen of the 12-membered ring and benzene on KH 8-3 zeolite.This pair of bands, on the other hand, cannot be observed when K/Al is <88.32%. The basicity of the conjugate acid-base pairs in the zeolites suggests that it is associated with framework o~ygen.'~.'~In other words, KH /I shows some basicity when K/A1 is high. Note that in our experiment a new band at 1877 cm-' (higher than that of 1840 cm-') is observed in a series of KH /3 zeolites. With increasing proton content the intensity of this band increases and its frequency is shifted to higher values. Upon adsorption, the greater the interaction of the adsorbed molecules with the zeolite surface, the greater will be the alternation in the force constant of the C-H bond and fluctuating dipole; as a consequence, the bands associ- ated with these vibrations should appear at higher fre-quencies and with greater intensities.Compared with K', the electrostatic field of H+ is very strong and consequently the interaction of H+ with benzene is stronger than that of K+. Moreover, with increasing proton content of the zeolite, the strength of the acid sites on the zeolite surface increases, and the interaction of H+ with benzene becomes strong. There- fore, we suggest that the band at 1877 cm-' should be assigned to the interaction of benzene with a proton through the n-electron cloud. The assignment of these bands is given in Table 2.Table 3 gives the ratios of integrated absorbances of the adsorbed benzene species on the samples. The results for 0--benzene (0-b) and H+-benzene (H'b) show that the basicity of KH /I -3 is stronger and its acidity weaker. This result is in good agreement with that of NH,(CO,) temperature-programmed desorption.6 Dzwigaj et a1.I3 pointed out that the basic strength of Na /I, Cs /I samples is comparable to that of NaY and rather higher than that of ZSM-5. The basicity is stronger than one would predict. The absorbance ratio of IH+b/ZK+bincreases with increasing Table 2 Assignment of bands at 2050-1700 cm-' species of wavenumber liquid assignment interaction /cm - assignment K+ba 1840 '10 + '17 1816 vl0 +v17 0-b 1853 '10 + '17 H+b 1877 v10 + v17 ?/(C-H) 1960 v5 + vl, (K+,H+)b 0-b 1980 2004 '5 + '17 v5 + '17 'b, benzene.Table 3 Ratios of integrated absorbances for four samples -0.80 1.70 -1.08 0.73 0 2.47 0.10 0" 0.72 1.22 0.17 -1.41 0.06 0 4.16 0.04 ob 3.59 1.57 0.10 -3.22 0 0 12.81 0 (Y 300 K, 1.0 x lo5 Pa. 300 K, 13.3 Pa. 323 K, 3.20 Pa. desorption temperature, decreasing pressure and increasing H+ content. It is proposed that the benzene interacting with potassium is preferentially desorbed, which may result from the fact that the electrostatic field intensity of H is stronger + than that of K+. An increase in Ib/IK+b is observed at lower pressures and temperatures. This shows that liquid benzene is desorbed first from the zeolite.The order of stability of the adsorbed benzene species is as follows: 0-b > H+b > K+b > benzene. References J. A. Martens, J. Perez-Pariente and P. A. Jacobs, Proc. Int. Symp. Zeolite Catalysis, Siofok, Hungary, 1985,p. 487. M. M. J. Treacy and J. M. Newsam, Nature (London), 1988,332, 249;J. M. Newsam, M. M. J. Treacy, W. T. Koetsier and C. B. De Gruyter, Proc. R. SOC.London, Ser. A, 1988,420,375. D.Barthomeuf, U.S. Pat., 4 584 424, 1986. R. L. Wadlinger, G. T. Kerr and E. J. Rosinski, (Mobil), U.S. Pat., 3 308 069, 1967. J. B. Higgins, R. B. Lapierre, J. L. Schlenker, Zeolites, 1988,8, 446. J. CHEM. SOC. FARADAY TRANS., 1994,VOL. 90 6 J-P. Shen, T. Sun, J. Ma, D-Z.Jiang and E-Z. Min, 34th IUPAC Congress, Beijing, China, 1993. 7 B. Coughlan, W.M. Carroll, P. OMalley and J. Nunan, J. Chem. SOC., Faraday Trans. 1,1981,77,303. 8 A. D.Mallmann and D. Barthomeuf, Proc. 7th Int. Zeolite Conf., Kodansha-Elsevier, Tokyo, 1986. 9 M. Primet, E. Garbowski, M. V. Mathieu and B. Imelik, J. Chem. SOC., Faraday Trans. I, 1980,76,1942. 10 A. D.Mallmann and D. Barthomeuf, J. Chem. SOC., Chem. Commun., 1986,90,1311. 11 A. D.Mallmann and D. Barthomeuf, Proc. 7th Int. Zeolite ConJ, Kodansha-Elsevier, Yokyo, 1986,p. 609. 12 A. D.Mallmann and D. Barthomeuf, Zeolites, 1988,8,292. 13 S. Dzwigaj, A. D. Mallmann and D. Barthomeuf, J. Chem. SOC., Faraday Trans., 1990,86,431. 14 R. D.Mair and D. F. Horning, J. Chem. Phys., 1949,17, 1236. 15 E. B. Wilson, Phys. Rev., 1934,45, 706. 16 D.Barthomeuf, J.Phys. Chem., 1984,88,42. 17 D.Barthomeuf and A. D. Mallmann, in Innovation in Zeolite Material Science, Stud. Sutf Sci. Catal., ed. P. J. Grobry, Else- vier, Amsterdam, 1987,vol. 37,p. 365. Paper 3/05716J;Received 21st September, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949001351

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(9), 1355-1356 FARADAY COMMUNICATIONS Solubility of Hydrogen and Deuterium in Ti,AI Masao Kimura Advanced Materials & Technology Research Laboratories, Nippon Steel Corporation, 1618 Ida, Nakahara-ku, Kawasaki 211, Japan Tatsuo Tsuchiyama, Shizuo Naito* and Masahiro Yamamoto Institute of Atomic Energy, Kyoto University, Uji, Kyoto 61I,Japan The solubility of hydrogen and deuterium in Ti,AI has been measured and found to obey Sieverts law. The difference in the solubility between hydrogen and deuterium can be explained by a model in which hydrogen and deuterium atoms perform harmonic oscillations in the tetrahedral site of the Ti,AI crystal lattice. The intermetallic compound Ti,Al has attracted much research interest owing to its excellent mechanical properties and corrosion resistance at high temperatures.Ti3Al, however, has a potential problem of hydrogen embrittlement when used as a structural material. A quantity of prime importance to this problem is the solubility of hydrogen in Ti3A1. Knowledge about the sites occupied by hydrogen atoms in the Ti,Al crystal lattice' is no less important because it underlies an understanding of the solubility. Only a limited number of studies have been reported on the solu- bility of hydrogen in Ti,A12v3 and its alloy^,^ there are no reports on the sites for hydrogen occupation although there are some discussion^^*^ about the sites in intermetallic com- pounds other than Ti,Al. In this study we measure the solu- bility of hydrogen and deuterium in Ti,Al at temperatures between 823 and 1323 K and at concentrations, 8 [H/(Ti3A1/4) and D/(Ti,A1/4)] between 0.001 and 0.1, and determine the likely sites for hydrogen occupation from the difference in the measured solubility of hydrogen and deute- rium.The solubility was obtained by measuring the amount of hydrogen and deuterium gases absorbed by a Ti,Al sample. The apparatus and procedure for measurements were essen- tially the same as those reported previo~sly.~.~ The sample was a 512 mm3 cube of polycrystalline Ti,Al. Chemical analysis showed that its purity was better than 99.9%, the main impurities being oxygen, carbon and nitrogen. X-Ray diffraction measurement showed that the sample had exactly the DOl9 structure.' To ensure homogeneity of the structure of the sample we heat-treated the sample in a vacuum of ca.5 x lo-' Pa at 1323 K for 48 h. Temperature was measured with a calibrated W5%Re-W26%Re thermocouple spot-welded to the sample. Pressure was measured with ionization gauges calibrated for hydrogen and deuterium against a cali- brated diaphragm gauge. The time for hydrogen to be distrib- uted uniformly over the sample was longer than in Ti, e.g. 24 h at 823 K because the diffusivity of hydrogen in Ti3Al is less than in Ti; this will be discussed elsewhere. Fig. 1 shows plots of the measured equilibrium hydrogen pressure p us. 8 at various temperatures. The solid lines are the computed result, which will be discussed later. The plots for hydrogen and deuterium both lay on straight lines with a slope of ca.2. This shows that Sieverts law can be applied to the relationship between p and 8 at small 8 in the form: p = ke' (1) where k is a function of temperature and independent of 8. Deviations from Sieverts law have been reported for the Ti,Al-hydrogen system and explained on the basis of block- ing of the sites by substitutional A1 atoms.' Small deviations from Sieverts law observed in this study (Fig. 1) may be due to this blocking factor. From the Arrhenius plot of the values of k obtained graphically in Fig. 1, the heat of solution per hydrogen atom was found to be 0.57 eV for hydrogen and 0.56 eV for deuterium. The value 0.57 eV for hydrogen is in agreement with the reported value' and, within experimental error, the same as that for a-Ti.' A possible cause may be the similarity of their electronic structures: the structure of the Ti d partial density of states (DOS) below the Fermi level is similar to that of the total DOS of a-Ti,' which is mainly contributed by the Ti d states.On hydrogen absorption the hydrogen s states couple with the Ti d states as in the Ti- hydrogen system" if hydrogen atoms occupy tetrahedral (T) sites, and the resulting DOSs for the Ti,Al-hydrogen and Ti- hydrogen systems are unlikely to be much different, implying a similar behaviour in the heat of solution. We now consider the ratio of k for hydrogen and deute- rium, kD/k,, in order to find the states of hydrogen and deu- terium in the Ti,Al crystal lattice.Fig. 2 shows the temperature dependence of kD/k, obtained from measured p 10-'/0 I 1 I ' 1 I"' I I I I I Ill. 1356 TIK 1400 2.01 I 1200 I 1000 1 800 1.o 0.7 0.9 I .1 1.3 lo3 KIT Fig. 2 Temperature dependence of kdk,. (0)Present study. The solid lines show eqn. (3) computed for haHci,= ,/(2)hoD(,,= (a) 0.07, (b)0.10, (c)0.13 and (d)0.16 eV. and 8. Values of kD/kH were found to decrease as the tem- perature increased and to be in the range of values observed for some other hcp metals.8~” Since k in eqn. (1) may be written, using the partition func- tions for hydr0genfH(,) in the gas phase andfH(i, in the metal, as7*8 for hydrogen and similarly for deuterium, the expression for kD/kH can easily be obtained.Here k’ is a constant and E, is the heat of solution per hydrogen atom, which is assumed to be the same for deuterium. Using the explicit form for the partition function^^.^ we have (3) J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 fourth, sixth and seventh terms in the right-hand side of eqn. (3) are corrections for the anharmonicity. The solid lines in Fig. 2 show eqn. (3) computed for the values of = ,/(2)h~,(~,shown in the figure caption and =for 4H(i)24D(i)= 0. The values used for fH!g) and fD(g) have been taken from standard tables. The experimental result is best reproduced for h~~(~)= 0.13 eV, which is consistent with haHci,= 0.141 eV obtained for a-Ti.I2 It can be seen from numerical calculations that the increase in 4H(i)has the same effect on the values of kD/kH as the increase in ho,(,,.We have assumed that c#I~(~)= 24D(i)= 0 because it is difficult to determine and 4H(i)simultaneously. The solid lines in Fig. 1 have been computed for =. ,/(2)ho,(, = 0.13 eV and 4H(i)24D(i)= 0. The heat of solution has been found to = be E, = 0.58 eV. It should be noted that E, does not include contributions from the partition functions and therefore does not coincide with the heat of solution obtained graphically from Fig. 1, i.e. 0.57 eV for hydrogen and 0.56 eV for deute- rium. The sites occupied by hydrogen atoms in Ti,Al can now be discussed. The similar values of the heat of solution observed for Ti,A1 and Ti suggest hydrogen occupation of tetrahedral (T) sites, which is the case for Ti.12 The fact that the value of ~oI~(~).for Ti,Al estimated in this study is consistent with that for Ti also suggests the T-site occupation.Octahedral (0)-site occupation, for example, would result in a smaller value of Aco,(~)because a larger space is available for the oscillation of the hydrogen atom in the 0 site. For example, = 0.069 eV for palladium,6 in which the 0 sites are preferentially occupied, but this value of AmH(,, can only poorly reproduce the measured values of kD/kH, as can be seen in Fig. 2. In conclusion, the solubility of hydrogen and deuterium in Ti,Al at high temperatures and at small 8 has been found to obey Sieverts law.It is suggested from the measured values of kD/kH and the energy of the hydrogen atoms in Ti,Al that hydrogen atoms occupy the T sites, in which they perform harmonic oscillations. References 1 J. L. Murray, Phase Diagrams of Binary Titanium Alloys, ASM, Metals Park, 1987, p. 12. 2 P. S. Rudman, J. J. Reilly and R. H. Wiswall, Ber. Bunsenges. Phys. Chem., 1977,81,76. 3 P. S. Rudman, J. J. Reilly and R. H. Wiswall, J. Less-Common Met., 1978,58,231. 4 W. Y. Chu, A. W. Thompson and J. C. Wiliams, Acta Metall. Mater., 1992,40,455. 5 K. Yvon and P. Fischer, in Hydrogen in Intermetallic Com- pounds, ed. L. Schlapbach, Springer, Berlin, 1988, vol. I, p. 87. 6 D. Richter, R. Hempelmann and R. C. Bowman Jr., in Hydrogen in Intermetallic Compounds, ed.L. Schlapbach, Springer, Berlin, 1992, vol. 11, p. 97. 7 S. Naito, J. Chem. Phys., 1983,79,3113. 8 T. Maeda, S. Naito, M. Yamamoto, M. Mabuchi and T. Hashino, J. Chem. Soc., Faraday Trans., 1993,89,4375. 9 T. Hong, T. J. Watson-Yang, X. Q. Guo, A. J. Freeman and T. Oguchi, Phys. Rev. B, 1991,43,1940. M. Gupta and L. Schlapbach, in Hydrogen in Intermetallic Com- 10where mH is the mass of the hydrogen atom, o~(~)and o~(~) are the angular frequencies of hydrogen atoms in the hydro- gen molecule and in Ti,Al, respectively, and 4H(g)and 4H(i) are the magnitudes of anharmonicity in the oscillation of hydrogen atoms in the hydrogen molecule and in Ti,Al, respectively. The subscript D indicates deuterium. The third, pounds, ed. L. Schlapbach, Springer, Berlin, 1988, vol. I, p. 139. 11 S. Naito, T. Hashino and T. Kawai, J. Chem. Phys., 1984, 81, 3489. 12 R. Khoda-Bakhsh and D. K. Ross, J. Phys. F: Met. Phys., 1982, 12, 15. Communication 4/01233J; Received 28th February, 1994

ISSN:0956-5000    

DOI:10.1039/FT9949001355

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(9), 1357-1361 Physical Methods of Chemistry. Second Edition. Volume IX 6. Investigations of Surfaces and Interfaces-Part 6. Ed. by B. W. Rossiter and R. C. Baetzold. Wiley-lnterscience, New York, 1993. Pp. xi + 745. Price f206.ISBN 0-471-54405-1. Review series are now commonplace. Notwithstanding the growth of published material in the sciences and the derived technologies, the quality, value and hence the need for some of the series currently available is certainly debatable. However, the model series of which this particular volume is part, in spite of the high cost of its elements (Parts A and B of Volume IX, together: f321/$449) has shown what is attain- able by the circumspect selection of topics, scrupulous choice of contributing authors and meticulous editorial attention. Thus, the publication of the most recent volume of the second edition of the series concerned, Physical Methods of Chemistry, under the editorial stewardship of Rossiter and Baetzold, continues the standards of excellence one has come to expect, i.e.excellence in topic choice, selection of contrib- uting authors, in presentational quality, and consistency of balance between theory, experiment and applications. In the present volume, coverage chapter-by-chapter, within the topics chosen is comprehensive and, given the normal lead time to publication, up to date. Included are eight chap- ters and an index which, while not exhaustive, is certainly more than adequate.Chapter 1 (G. A. Somorjai and M. A. Van Hove, ca. 40 pages) deals with low-energy electron dif- fraction (LEED) in terms of experimental advances, an excel- lent review of 2D structures studied and of the development and application of 3D surface structure determination. Chapter 2 (L. L. Kazmerski, ca. 81 pages) addresses the analysis of surfaces by Auger electron spectroscopy (AES) and related techniques. Overall, this is one of the better recent reviews of Auger-based methods. Almost all one needs to know about the possible applications of AES, energy selec- ted mapping (SAM), chemical speciation and the like is pro- vided, with what can be done set in context. While the emphasis is essentially on current practice and on the techni- cally attainable, the essential fundamentals are not over-looked.Also included is a useful comprehensive bibliography of monographs and texts devoted to surface analysis. Not surprisingly, Chapter 3 (N. H. Turner, ca. 48 pages) follows on dealing, as it does, with the analysis of surfaces by X-ray photoelectron spectroscopy (XPS/ESCA). A short section on basic principles leads into instrumental aspects of the method. An extended consideration of the possible fea- tures which can contribute to an XPS spectrum and of their significance is then given. Quantification, depth profiling, surface derivatisation, angular resolved techniques are all dealt with in some detail. The chapter finishes with a brief but coherent account of the theory of binding energy and energy shifts.Chapter 4 (A. C. Miller, R. B. Irwin and H. F. Helbig, ca. 156 pages) considers low-energy ion scattering (LEIS) and Rutherford backscattering spectroscopy (RBS). The coverage given spans a general introduction, scattering theory in some depth, experimental and instrumental con-siderations, qualitative and quantitative features of LEIS and RBS. Practical applications of both techniques are included, with these drawing on a useful and informative range of examples. Synchrotron sources literally surface in Chapter 5 (D. Norman, ca. 43 pages) which elaborates from first principles the use of X-ray absorption spectroscopy (surface-extended and near-edge X-ray absorption fine structure) at surfaces, i.e. SEXAFS and NEXAFS, respectively. From the basic physics, experimental techniques, sources and detection regimes, are considered, the latter in particular detail.Data analysis, inter- pretation and a representative selection of results complete the chapter. Should the reader wish to see whether their surface science problem can or might be solved using syn- chrotron radiation this would be where to start. Chapter 6 (H. Kuhn and D. Mobius, ca. 167 pages), provides one of the most extensive and thorough reviews available of studies of monolayer assemblies. As such it should be compulsory reading for those already working in the field or wishing to so do. Detailed consideration, both experimental and theo- retical, is given of studies of monolayers at the air/water interface, monolayer dye assemblies, monolayer-substrate interactions, energy transfer phenomena in assembled mono- layers, photoinduction of electron transfer, electrical, electro- optical effects and optical effects, per se.In its turn Chapter 7 (D. A. Scherson and E. B. Yeager, ca. 88 pages) covers the application of spectroscopic techniques to the in situ study of electrochemical interfaces. An adequate introduction to the essential electrochemical background and the optics of interfaces is provided. The short section included on experimental methods usefully tabulates the perceived attributes of the twelve in situ spectroscopic methods reviewed and currently in use. This is followed by sections reviewing adsorption-free interfacial charging and, in more detail, adsorption at the metal/electrolyte interface.Chapter 8 (E. S. Brandt and T. M. Cotton, ca. 85 pages) appraises the surge in recent years of studies of, and involving, the surface- enhanced Raman scattering (SERS) phenomenon. In so doing the relationship between normal and surface-enhanced Raman spectroscopy is dealt with concisely and well. Systems showing SERS are then elaborated, roughened electrodes (nicely complementary to the contents of the preceding chapter), colloidal metal sols, thin metal films and photo- generated silver, in terms of the phenomenon itself and its applications. The requisite instrumentation is also critically discussed. A short resumt: of patented SERS-active substrates and commercial applications is also provided.From the foregoing it is clear that I welcome this volume with enthusiasm. It should be, with its companion volume, Volume IX, Part A, in every scientific library claiming the name. Moreover, it is likely to be in every library serving those with peripheral as well as main stream interests in studies of surfaces and interfaces. Finally, laboratories working specifically in the area of interest, i.e. of surfaces and interfaces, their properties, analysis and characterisation will accept, I believe, that the high purchase price necessary would be money well spent. N.M. D. Brown Received 4th November, 1993 Monographs on the Physics and Chemistry of Materials. 49. Dynamic Light Scattering.The Method and Some Applications. Ed. W. Brown. Clarendon Press, Oxford, 1993. Pp. xvi + 735. Price f95.00 (hardcover). ISBN 0-19-853942-8. ~~ Dynamic light scattering measurements on macromolecular solutions and other systems have been made ever since continuous-wave lasers became commercially available in the mid sixties. The spectral broadening due to phase fluctua- tions resulting from macromolecular motion and changes in configuration is measured, and yields diffusion coefficients and potentially, rotational diffusion coefficients, internal relaxation times and other information. The spectral broadening is of the order of kilocycles and cannot be mea- sured by conventional spectroscopic techniques. It is not to be confused with the much larger shifts due to non-linear effects resulting from vibrational or rotational oscillations in polarizability (Raman scattering).Light scattered from spa- tially random scatterers within a volume having dimensions small compared with the coherence length consists of a random speckle pattern, which changes continuously if the scatterers are moving. If an area of the order of one speckle is observed by the detector, temporal fluctuations in signal are observed which represent homodyne beating between the variously displaced components of the spectrum, and in this way the spectrum can be measured. It has generally been found most convenient to measure the autocorrelation func- tion of the signal. In principle it is not necessary to use a laser, but in order to get sufficient light flux into the necessar- ily small scattering volume, a laser is invariably used. Also because of the low light flux being measured it has been usual to correlate photon counts, since the advent of suitable com- puters in the early seventies, and the technique has become known as ‘photon correlation spectroscopy’. The present volume is concerned, at a fundamental level, with very recent developments, and is suitable only for those who already have some experience in the field.One of the main applications of dynamic light scattering has been in the field of biochemistry, where the technique is often used simply to obtain diffusion coefficients so as to monitor the sizes of biological macromolecules.Many of these workers obtain their results by ‘rule of thumb’, using commercially available equipment, and are unlikely to benefit by reading this book, especially if they have an antipathy to mathe- matics. The book does however contain a short review of applications to biological systems, with over three hundred references to just some of the more interesting recent work. Reference to biological systems is also made in the context of polyelectrolytes (DNA, polysaccharides), rod like macro-molecules (DNA fragments) and rotational diffusion coeffi- cients by depolarized light scattering using Fabry-Perot interferometry (globular proteins); In format the book consists of sixteen chapters on differing subjects, each with a different author.Inevitably there is some overlap, and conventions of mathematical representation are far from standardized, as fashions are continually changing. One quarter of the book is devoted to current methodology, with chapters on photon correlation techniques, noise and data analysis, which should prove useful to those wishing to be brought up to date in these matters. New theories and experiments with polymers and polymer mixtures in dilute solution, semi-dilute solution, and in the bulk, are dealt with extensively in variously entitled chapters, and there are chap- ters on depolarized light scattering (misleadingly titled) and polyelectrolytes. Other systems dealt with (one chapter each) are gels, rod-like macromolecules (with some reference to liquid crystals) and micelles.There are also chapters describ- ing investigation of the critical dynamics of fluids and liquid mixtures by dynamic light scattering, and multiple scattering by dense media (diffusing wave spectroscopy). In conclusion the book is to be recommended to workers with some experience in the field as a reference to recent developments. D. B. Sellen Received 8th November, 1993 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Reflections on Symmetry. By El Heilbronner and J. D. Dunitz. VCH, Weinheim, 1993. Pp. iv + 154. Price €22. ISBN 3-527-28488-5. Symmetry is the strongest unifying concept in chemistry. Consciously or unconsciously, it pervades most of the activ- ities of chemists. Of ancient origin, the power of symmetry arguments began to be realised by physicists and chemists in the 1920s, but it was not until the early 1960s that textbooks readily accessible to undergraduates and non-specialists began to appear.Today there are dozens of texts dealing with symmetry at all levels of detail. This is not a textbook. It had its origins in a lecture given by Heilbronner in 1980 as the first of a series dealing with the border areas between art and science and aimed at a non- specialist audience. This lecture has been expanded and lav- ishly illustrated. The result has something of the style of a ‘coffee table’ browser; it is aimed at chemists but with at least an eye on a wider audience. Somewhat curiously, the publi- sher’s dust jacket suggests that a likely audience is ‘older chemists (who) may be at a disadvantage’ because they ‘feel that their encounter with symmetry concepts during their student days was inadequate’.It seems to me that this audi- ence is precisely one where the necessary limitations of the style adopted would be most frustrating. Such a readership would be better advised to read one of the standard texts. The authors recognized the problem inherent in their approach : “One difficulty we have to face is that the interconnection between the playful aspect of symmetry on the one hand and an exact science is not at all simple. ... This makes it difficult to steer a secure course between the Scylla of irre- sponsible superficiality and the Charybdis of unintelligible jargon.. . . A treatment rigorous enough to satisfy finicky criticism of the experts and at the same time gentle enough for the non-specialist is just not possible in the limited space available”. As a result, the choice of material is idiosyncratic and the phrasing sometimes curious. For example ‘the reflection in a calm mountain lake’ is referred to as ‘fortuitous symmetrization’ (p. 5) and one wonders why Rafael’s ‘Alba Madonna’ was chosen (p. 9) to illustrate the underlying com- positional symmetries in paintings in preference to the same artist’s ‘Madonna of the Meadows’ or a work of Piero della Francesca. The symmetry of atoms and the deeply satisfying relationship of the periodic table to rotational symmetry is not mentioned.Indeed how the symmetry of s-and p-orbitals (p. 139) relates to the spherical symmetry of atoms will leave some readers confused. A recurring theme is that sometimes nature adopts the highest possible symmetry, whereas in others the experimen- tally determined symmetry is lower than one might (naively) have expected. One example quoted is benzene and cyclo- octatetraene, but the underlying symmetries of the orbital structures are not explained, which might have found a place in Chapter 10. By contrast Chapter 9 on chirality seems unnecessarily extended, but still fails to explain the relation- ship between the rotation of the plane of polarised light and the molecular (lack of) symmetry. Overall the fears of the authors seem justified, this book does not succeed in achieving its very ambitious object.It is too long and too detailed (in parts) to be merely a stimu- lation, but too short and too superficial for the reader to obtain any real understanding. It is, however, a brave attempt and will benefit two groups of readers. First, teachers in higher education will find the examples (with some less J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 than well known references), and especially the figures, a welcome source of different ways to present symmetry to stu- dents. Secondly, students meeting symmetry for the first time and working through a formal treatment might find the ‘playful’ approach a welcome diversion. It will certainly raise tutorial questions.The cost of the book is very reasonable considering the very high standard of production. C.D. Flint Received 18th November, 1993 ~~ Advances in Chemistry Series 229. Magnetic Reson- ance of Carbonaceous Solids. Ed. R. E. Botto and Y. Sanada. American Chemical Society, Washington, DC, 1993. Pp. xiv + 664. Price $149.95. ISBN 0-8412-1866-8. This book is an excellent compilation of research papers on ‘H and ’3C NMR and electron paramagnetic spectroscopies. Although based on papers to a Chemical Congress of the Pacific Basin Societies in 1989, these versions of the papers are more up to date. Most of the manuscripts were submitted in mid-1990, with acceptance of revised versions up to the end of 1991 and with publication in 1993.There are very few typographical errors and the production is to the high quality typical of the Advances in Chemistry Series of the ACS. One minor irritation (for this reviewer) which might have been avoided in the editorial process is the inclusion of the elemental analyses of the Argonne Premium Coals in eight of the 33 papers. In some of the eight papers, these data are quoted as from the handbook and given to two decimal places for carbon; in others, they are given to the same apparent accuracy but with no clear comment that they were taken from the handbook; two papers quote dmmf basis (dry, mineral matter free) and the other six quote daf basis (dry, ash free), but do not all make clear which basis applies, while in only one paper are the carbon daf values rounded to one decimal place.Most coal scientists will of course be able to cope with this, but the data could have been summarized conveniently into one table in an Appendix, if it was felt vital to include it at all. Of the 33 papers, seven are devoted to overviews of the techniques: quantitation in I3C NMR of solids; ‘H NMR of solids ; NMR and dynamic nuclear polarization spectros- copy; electron nuclear double resonance spectroscopy; elec- tron spin relaxation measurement ; multifrequency electron paramagnetic resonance spectroscopy; and high-temperature electron paramagnetic resonance and NMR methods. Fifteen papers cover more specific aspects of NMR and coal, such as, the submicroscopic structure of bituminous coals, pyridines sorbed onto coal, principal component analysis of relaxation times, coal liquifaction residues, oxida- tion effects, problems of quantitation, magic-angle-spinning and high-field studies.Ten papers cover electron paramagnetic resonance spec- troscopy applied to the Argonne coals and solvent-swelled coals, using variable microwave frequencies and electron spin-echo methods. The final paper is a thoughtful conclusion on the state of the art of the application of magnetic resonance methods to coal. The importance of the Argonne set of coals, free from drying and oxidation up to the point of opening the sealed ampoules, is emphasised and these studies reveal much about the coal structure. However, there is still a lot to be done on low-rank coals, as this paper makes clear, with a need for a larger number of coals, particularly low-rank coals, as a data- base for the kind of comparison undertaken by this volume.This book is not a light read; it contains a great deal of detail about the techniques, their drawbacks and their appli- cation to coal. The contrast between what they reveal about coal structure and what remains to be investigated, as seen by this collection of distinguished scientists, should ensure that this book will find a place in both the institutional libraries and in the personal libraries of coal scientists, even if they are not active in the particular fields of magnetic resonance covered. A. A. Herod Received 24th November, 1993 Spectroscopy of the Earth’s Atmosphere and Inter-stellar Medium.Ed. K. N. Rao and A. Weber. Aca-demic Press, London, 1992. Pp. xi + 526. Price $129.5. ISBN 0-12-580645-0. The Editors of this book, the fourth volume of ‘Molecular Spectroscopy: Modern Research’, have endeavoured to choose a set of contributed articles relevant to the theme of atmospheric and interstellar spectroscopy. The articles are weighted heavily in favour of atmospheric spectroscopy, with authoritative reviews of far infrared and microwave spectros- copy by Carli and Carlotti and of infrared spectroscopy of Brown, Farmer, Rinsland and Zander. Since Farmer has been one of the pioneers in this area as the driving force behind the ATMOS infrared Fourier transform spectrometer, Chapter 2 makes fascinating reading.The third and fourth chapters on intensities and collisional broadening parameters, and on collisional line mixing, while not devoted to atmospheric spectroscopy directly, discuss the information which is necessary to derive concentration data from the type of spectra produced by current high-resolution spectrometers. The first of these by Smith, Rinsland, Malathy Devi, Rothman and Rao contains an extensive list of param- eters for use in concentration retrievals, whereas the second by Levy, Lacombe and Chackerian discusses the effects of collisions on redistributing energy level population and its effects on intensities, the popular topic of collisional line mixing.The final chapter in the compilation, spectroscopy among the stars, by Winnewisser Herbst and Ungarechts gives a fas- cinating introduction to current uses of rotational spectros- copy for probing the interstellar medium, showing clearly the interplay between spectroscopy, chemistry and astrophysics. By using one or two molecules as vehicles for the discussion, for example carbon monoxide as a tracer for molecular clouds, the authors have shown clearly how the detective work necessary to unravel the physics of these regions is being conducted. The penultimate chapter on spherical top spectra by Champion, Loete and Pierre gives an authoritative survey of spherical tensor methods and of the contact transformation approach to our understanding of the spectra of spherical top molecules.However, although methane is a molecule which is found in the atmosphere, I feel that this contribution, although well written, does not fit in very well with the theme of the rest of this review volume. In summary this book is an up to date and authoritative review of the role of molecular spectroscopy in furthering the understanding of the atmosphere of our planet and of the interstellar medium. It is clearly written and well illustrated and will serve as a useful introduction to current ideas about these aspects of earth observation and molecular astro-physics. G. Duxbury Received 24th November, 1993 Magnetism and Optics of Molecular Crystals. By J. W. Rohleder and R. W. Munn.John Wiley & Sons Ltd., Chichester, 1992. Pp. x + 139. Price f29.95. ISBN 0-47 1-93 17 1-3. As the title indicates, this short text (three chapters and 137 pages) is a monograph on a specialised aspect of molecular physics, limiting itself to the diamagnetism and non-dispersive optical properties of molecular crystals. The book is in part based on graduate lectures given by the authors, and as well as providing a suitable background to the topics discussed, it contains much of the authors’ original contribu- tions to this research area. The feature of molecular crystals explored in this mono- graph is the relationship between properties of molecules and the macroscopic magnetic and optical properties of their crystals. There are two aspects to this problem: (i) the spatial distribution of molecules and their relation to crystal sym- metry axes and principal axes of the magnetic susceptibility and optical permittivity, and (ii) the dependence of the molec- ular properties on crystal environment, either through long range or short range interactions.By focussing on dia-magnetic properties, complications of intermolecular inter- actions can be ignored, and the oriented gas model is a valid basis for the molecular interpretation of the diamagnetism of molecular crystals. Both magnetic susceptibility and electric permittivity are second rank tensors, and Chapter 1 provides a useful discussion of their symmetry and transformation properties. Chapter 2 on diamagnetism includes a section on classical methods of measurement, but this is too brief to be of any real value, and misses an opportunity to introduce more modern methods, such as SQUID magnetometry, which may lead to a revival of interest in magnetic properties of molecules.Examples are given of the analysis of crystal susceptibilities to give components of the molecular dia-magnetic susceptibility, and the chapter concludes with a table presenting data for 40 compounds. In contrast to magnetic properties, the interpretation of the electric response of materials in terms of molecular properties is seriously complicated by the internal field problem. This is important even for optical properties, because the corre-sponding permittivity is at least twice the value for free space.Anisotropy of molecular crystals adds a further difficulty, since the long-range polarization which is responsible for the internal electric field now depends on orientation, and indeed crystal shape. This problem has been actively researched over the past 20 years by the authors and others, and is essentially solved, at least in a formal sense. Chapter 3 gives a detailed description of wave propagation in crystals, and contains a clear account of double refraction together with a brief review of some classical methods for measuring birefringence. The major part of this chapter is devoted to the determi- nation of components of the molecular polarizability tensor from principal refractive indices using a proper representa- tion of the internal electric field and plausible models for the local molecular polarization ;results are given for a number of well studied ’ molecular crystals.The chapter concludes with a short section on the non-linear optics of molecular crystals, accounts of which are, as the authors remark, ‘paradoxically more commonly available than those of the linear optics ’. The objects of study for chemists and molecular physicists are molecules: in molecular crystals they are trapped in a regular array, so their features can be examined in detail, without recourse to statistical averaging. In the pursuit of molecular electronics, molecular engineering, nano-(ie. molecular-scale) technology, it might be supposed that molec- J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 ular crystals would be the focus of applied research, but the reality is different, and molecular crystals remain in the prov- ince of theoreticians and dedicated experimentalists. Perhaps the situation will change, and if it does this small book will be a useful starting point for the understanding of the proper- ties of molecular crystals. At present because of the special- ised nature of its subject matter, it is unlikely that this book will have a wide appeal, but it can be recommended to grad- uate students and other researchers in the field as a useful source book and valuable teaching aid. D. A. Dunmur Received 24th November, 1993 Photodissociation of Simple Molecules in the Gas Phase. By H.Sato. Bunshin, Tokyo, 1992. Pp. v + 158.Price (incl. postage) %2,060, $17.50 (USA); % 2,290, $19.50 (Europe). ISBN 4-89390-092-7. The last two decades have witnessed an extraordinary growth both in the range and in the sophistication of experimental techniques available for unravelling the intimate details of simple gas-phase reactions. Molecular photodissociations represent one major subset of this area of research. For con- firmation that this remains an active, exciting and challenging research area, one need do no more than scan the contents pages of any of the latest issues of the major physical chemistry/chemical physics journals. What are the require- ments of those researching molecular photodissociation dynamics? Good equipment (if experimentalists), computer time (if theoreticians), a sound understanding of molecular spectroscopy and molecular quantum mechanics, of photo-chemistry and reaction dynamics, and a good knowledge of what has gone before. For modern photochemists, Okabe’s book Photochemistry of Small Molecules (Wiley), published in 1978, has been an invaluable source of background know- ledge.However, much has been discovered in the intervening years and there is now a pressing need for an updated version of this book. Suto’s book goes some way to fulfilling this need. It pro- vides a comprehensive bibliography of (largely experimental) papers published in the period 1970 through 1991 in the area of gas-phase molecular photodissociation dynamics.It pro- vides a distillation of the contents of more than one thousand references covering the primary photochemistry of 284 small molecules and 70 or more van der Waal’s species. Entries concerning any one molecule are grouped together, and appear in (roughly) chronological order. As a representative illustration of the information provided in each entry I select the classic paper on the near UV photolysis of HONO by Vasudev, Dixon and Zare (J. Chem. Phys., 1984, 80, 4863). This one or more man years of work is compressed to: HONO 369,355,342 LIF OH(X ’H): V(cold), R(cold, N < 7), F*(F2 preference), A, alignment, split Doppler profile thereby giving information on the photolysis wavelengths studied, the particular fragment species probed and the probing method employed, the deduced internal energy dis- posal within the probed fragment and an indication of the vector properties (alignment, recoil velocity) measured.The compilation is comprehensive, but not critical. For example, the early time-of-flight study of ICN photolysis at 266 nm (which concluded that the two observed peaks indi- cated the formation of both ground and electronically excited CN fragments) is listed in just the same way as the many subsequent papers which have shown that the two peaks are actually due to electronic branching in the companion atomic J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 iodine photofragment. The book thus provides a source of references in much the same way as would be obtained from searching a data base. The present work is, of course, much more than just a bibliography, since it provides an (accurate) summary of the results reported in each paper. However, it cannot be viewed as a replacement for Okabe’s book, or for Herzberg’s classic text, Molecular Spectra and Molecular Structure III, Electronic Spectra and Electronic Structure, because it offers no critical overview of the various cited works. These reservations notwithstanding, Suto’s book is a valu- able addition to the photochemical literature. It is accurately produced and very reasonably priced. Many will share my hope that the author will wish to offer periodic updates to this compilation. Maybe the next edition will appear on disk; the material seems ideally suited to distribution in this way. M. N.R. Ashfold Received 25th November, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949001357

出版商:RSC    

年代:1994

数据来源: RSC