摘要:

J. Chem. SOC., Furuduy Trans. 1, 1987, 83 (lo), 3161-3165 An X-Ray Photoelectron Spectroscopy Study of the Influence of Hydrogen on the Oxygen-Silver Interaction Leon Lefferts, Jan G. van Ommen and Julian R. H. ROSS* Twente University of Technology, Laboratory for Inorganic Chemistry, Material Science and Catalysis, PO Box 217, 7500AE Enschede, The Netherlands Hydrogen treatment of a pure silver catalyst before adsorption of oxygen influences the position and form of the O(1s) peak of the X-ray photoelectron spectra. It is possible that both the formation of sub-surface OH groups and an increase in the concentration of sub-surface oxygen, found to be of importance in earlier work, are responsible for this effect. Furthermore, when oxygen adsorption is carried out with extremely high exposures and at elevated temperature, pronounced differences occur in spectra obtained at temperatures between 298 and 723 K as compared with the results of ultrahigh vacuum studies reported in the literature.These differences are again most probably caused by the presence of the sub- surface oxygen in the present study. Silver is widely used as a catalyst for the oxidative dehydrogenation of methanol to formaldehyde. As reported by Sperber,' it is possible at temperatures around 873 K to reach 90 O/O selectivity to formaldehyde at almost complete conversion. Research is in progress in our laboratory on the influence of reaction conditions on the selectivity of the process2 and on the interaction of silver with oxygen;3 the latter has been studied using temperature-programmed reduction (t.p.r.), temperature-programmed desorption (t.p.d.) and temperature-programmed reaction (t.p.r.s.) with methanol.This has led us to propose2 a reaction mechanism in which dehydrogenation takes place at sites associated with strongly bound surface oxygen and in which the most important side reaction, to give CO,, takes place via weakly bound surface oxygen. Furthermore, it has been found3 that hydrogen treatment of a silver sample before oxidation brings about the formation of a new sub-surface OH species as well as an increase in the amount of observable sub-surface oxygen. Finally, another new surface species develops, probably from sub-surface oxygen, during desorption measurements ; we have suggested3* that this species is strongly bound surface oxygen adsorbed at those parts of the surface, such as grain boundaries and dislocations, which are associated with the formation or desorption of sub-surface oxygen.The present work, using X-ray photoelectron spectroscopy (X.P.S.), was performed to obtain more information on these phenomena and concerns a study of the silver-oxygen interaction after long oxidation treatments at elevated temperatures on a real silver catalyst. Experimental The X-ray photoelectron spectra were recorded on a Kratos ES 200B spectrometer, using A1 K, radiation (1487 eV), in a vacuum chamber with a background pressure < lo-' Torr.? The sample holder was attached to an electrical heating device which could reach 723 K. Calibration of the X-ray photoelectron spectra was performed using the Ag(3d5,,) peak at 368.0 eV, as reported by Joyner and Roberts;5 no significant shift 1 Torr = 101 325/760 Pa.31613162 X . P.S. Study of the Oxygen-Silver Interaction I 538 532 52 6 binding energy/eV Fig. 1. The O(1s) X.P.S. peak of a sample of hydrogen-free oxidized silver measured at various temperatures. of this peak was observed during the heating cycles reported here. The apparatus allows determination of the peak positions with an accuracy better than 0.1 eV. The pure (unsupported) silver material used has been described elsewhere.2 Two samples of this material were oxidized and reduced many times and were then treated in air at 1 173 K for 10 min to remove any hydrogen. One of the samples was then treated with 25 % H, in N,, heating at a rate of 50 K min-l up to 1173 K; it was then kept at this temperature for 10 min. Both samples were finally oxidized in air at 483 K for 17 h and were then kept at room temperature in air at atmospheric pressure for one day before insertion into the vacuum system.Results The X-ray photoelectron spectra for both samples held at room temperature showed a clear C( 1s) peak and a small Si(2s) peak in each case. It was not possible to remove these by normal cleaning procedures without also influencing the oxygen peaks whose behaviour was of interest. Spectra were obtained for both samples at a series of temperatures in the range 298-723 K. The samples were kept at the specific temperature for ca. 15 min before starting the measurement at that temperature ; each measurementL. Leflerts, J.G. van Ommen and J. R. H. Ross 3163 723 K 6 7 3 K 4 7 3 K 298 K . . I . . , , . I , . 5 3 8 532 526 binding energy/eV Fig. 2. The O(1s) X.P.S. peak of a sample of silver treated in hydrogen and subsequently oxidized, measured at various temperatures. lasted for ca. 70 min. In no case did the Ag(3d5,,) peak show any shift in binding energy upon heating. However, the O( 1s) peaks, measured simultaneously, did show some differences; the spectra recorded at different temperatures are shown in fig. 1 for the hydrogen-free sample and in fig. 2 for the hydrogen-treated sample. The results given in fig. 1 show clearly that a shift of the O(1s) binding energy from 531.8 to 532.3 eV occurred when the sample not treated in hydrogen was heated.Fig. 2 shows that a much broader peak at 532.2 eV, having a clearly distinguishable shoulder at ca. 531 eV, was found for the sample pretreated in hydrogen. This shoulder was found to decrease in magnitude when the temperature was raised. In both cases, the intensity of the O(1s) peak decreased very slightly on heating, whereas the intensity of the Ag(3d5,,) peak (not shown, see above) increased by ca. 25%. Discussion T.p.r. and T.p.d. experiments3 have shown that oxidized silver that had not been pretreated with hydrogen contains two types of oxygen species : atomic surface oxygen (which can be reduced and desorbed at ca. 570 K) and sub-surface oxygen (which can be reduced at ca. 11 10 K but cannot be desorbed directly to a measurable extent), Fig.1 shows X-ray photoelectron spectra of oxidized silver that had not been pretreated with3164 X . P.S. Study of the Oxygen-Silver Interaction hydrogen. The results measured at 723 and 673 K must thus be caused by the presence of sub-surface oxygen alone. The X.P.S. result measured at 298 K shows a broader peak; we suggest that this is caused by a contribution to the O(1s) peak of atomic surface oxygen in addition to that from sub-surface oxygen and that this is also responsible for the fact that the peak measured at 298 K has shifted to lower binding energy. The O( 1s) peak measured at 523 K resembles those measured at higher temperatures; however, the t.p.r. and t.p.d. measurements3 have shown that atomic surface oxygen is stable at 523 K. We suggest that this inconsistency is caused by the limited accuracy of the control of the temperature of the sample in this work; the actual temperature of the sample in the sample holder might be higher than indicated. On the other hand it might be possible that slow desorption of weakly bonded oxygen already takes place at 523 K.The fact that the proportion of the oxygen contributing to the signal of electrons with low binding energy decreases upon heating suggests that oxygen with a more negative charge is less stable. The bonding of oxygen on or in the sub-surface of heterogeneous silver would therefore appear to have rather more covalent than ionic character. Our suggestion that mainly sub-surface oxygen is involved in the present results can explain the fact that our data differ in two respects from those given in the literature. First we do not observe peaks in the region of 528 eV, as reported by Campbell and Paffett'j after moderate oxygen exposure (lo3 L).It has been shown by Joyner and Roberts5 and Grant and Lambert' that a peak at ca. 530 eV becomes dominant after high oxygen exposures ( lo7 L). In our experiments, the peaks are at even higher binding energies, in a region attributed by other workers5-' to the presence of weakly bound 0,. However, in our case the oxygen appears to be extremely stable; thus, 0, cannot account for the higher binding energies observed. The extremely high oxygen exposure used in the present work (1013 L) is likely to give more sub-surface oxygen and the latter probably gives rise to the high binding energy that is observed in this work.Secondly, it is clear that we do not find complete removal of the oxygen from the silver surface upon heating up to 723 K. Au et al.' and Arakawa et al.' have observed with X.P.S. that oxygen is almost completely desorbed at ca. 600 K after less extensive oxidation treatments. We suggest that the formation of sub-surface oxygen during extensive oxidation treatments is responsible for the thermal stability of the oxygen species observed in this work. This suggestion is supported by other literature data : Peukert," also using X.P.S., has observed stable adsorbed oxygen species at temperatures up to 873 K after high oxygen exposures. This stability of the oxygen species is also reported by Schon;l1 although he gives no description of his pretreatment procedure, it is clear that his silver was, as in this work, allowed to take up oxygen at atmospheric pressure.The results in fig. 2 were obtained on oxidized silver that was pretreated with hydrogen. From t.p.r. and t.p.d. experiments3 it appeared that three oxygen species are present on silver treated in this way: besides atomic surface oxygen and sub-surface oxygen, sub-surface OH was found to be present. This OH species was found to be stable at temperatures up to 673 K3 The X-ray photoelectron spectra measured at high temperatures and shown in fig. 2 are broader than those in fig. 1 because two species are now responsible for this peak (sub-surface oxygen and sub-surface OH). The peaks at higher temperatures shown in fig. 2 are broadened, especially on the low-energy side.This suggests that the sub-surface OH is a species with a weaker covalent character than that of sub-surface oxygen; this is reasonable because an 0-H bond has a considerable polar character. The shoulder observed in the O( 1s) peak measured at 298 K is probably again caused by the presence of atomic surface oxygen; this peak thus probably has three components, the third again arising from atomic surface oxygen. When the peaks measured at 298 K of fig. 1 and 2 are compared, it is obvious that more sub-surface species are present relative to atomic surface oxygen in fig. 2. As discussed above, this might be due to the formation of sub-surface OH; it is, however,L. Leflerts, J. G. uan Ommen and J . R. H.Ross 3165 also possible that the concentration of sub-surface oxygen is enlarged, as was observed in our t.p.r. s t ~ d y . ~ We thus attribute the low-energy part of the peaks measured at high temperatures in fig. 2 to sub-surface OH. This is in agreement with the results of Au et a1.8 who reported an O(1s) peak of surface OH groups on silver in the same binding- energy region. However, surface OH is found' to be far less stable than is sub-surface Because of the rather poorly defined conditions of the pretreatment of our sample, we cannot definitely rule out the possibility that a contamination effect has influenced the results. Carbon was observed to be present on the surface; thus it is possible that the presence of CO,(ads) might account for a peak in the region of b.e.around 530 eV.6 However, CO,(ads) desorbs as CO, from a silver surface at temperatures below 473 K [leaving O(ads)], and no change which could correspond to such desorption is observed here on raising the sample temperature. Contamination effects are very unlikely to be involved in the O(1s) peaks, as these are found to be rather thermo-stable. Thus, the influence of hydrogen pretreatment on the oxygen species present on silver is evident from the present work. It is not possible to compare the peak intensities of the results given here with the related data from the t.p.r. results given previ~usly,~ which gave evidence for the sub- surface oxygen and OH species. This is because the X.P.S. technique is surface sensitive; it will therefore not allow observation of that part of the sub-surface species which are concentrated along grain boundaries of defect planes deeper in the structure.OH.^ Conclusions Hydrogen treatment before oxidation of silver influences the position and character of the O(1s) X.P.S. peak. This is explicable in terms of the formation of OH groups in the sub-surface and of an increase of the concentration of sub-surface oxygen. Treatment of silver with oxygen at high temperatures and exposures give rise to different values of O(1s) binding energy than are found in experiments carried out in situ at lower exposures. We thank Mr A. Heeres and Prof. G. A. Sawatasky (State University of Groningen) for their assistance in performing the X.P.S. measurements and for useful discussion of the results. The financial support of Methanol Chemie Nederland is gratefully acknowledged. References 1 H. Sperber, Chem.-Zng.-Tech., 1969, 41, 962. 2 L. Lefferts, J. G. van Ommen and J. R. H. Ross, Appl. Catal., 1986, 23, 385. 3 L. Lefferts, J. G. van Ommen and J. R. H. Ross, Appl. Catal., in press. 4 L. Lefferts, J. G. van Ommen and J. R. H. Ross, to be published. 5 R. W. Joyner and M. W. Roberts, Chem. Phys. Lett., 1979, 60,459. 6 C. T. Campbell and M. T. Paffett, Sur- Sci., 1984, 143, 517. 7 R. B. Grant and R. M. Lambert, SurJ Sci., 1984, 146, 256. 8 C. T. Au, S. Sing Boparai, M. W. Roberts and R. W. Joyner, J. Chem. SOC., Faraday Trans. I , 1983, 9 T. Arakawa, A. Saito and J. Shiokawa, Chern. Phys. Lett., 1983, 94, 250. 79, 1779. 10 M. Peuckert, Surf Sci., 1984, 146, 329. 1 1 G. Schon, Acta Chem. Scand., 1973, 27, 2623. Paper 612315; Received 1st December, 1986

ISSN:0300-9599    

DOI:10.1039/F19878303161

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I , 1987, 83 (lo), 3167-3175 Binary Systems of 1,2-Dichloroethane with Benzene, Toluene, p-Xylene, Quinoline and Cyclohexane Part 3.-Dielectric Properties and Refractive Indices at 308.15 K Jagan Nath* and Geeta Singh Chemistry Department, Gorakhpur University, Gorakhpur 273 009, India Measurements of dielectric constants, E, and refractive indices, n, have been made for binary liquid mixtures of 1,2-dichloroethane (CH,ClCH,Cl) with benzene, toluene, p-xylene, quinoline and cyclohexane at 308.15 K. The values of the quantity AE, which refers to the deviations of the dielectric constants of the mixtures from the values arising from volume fraction mixture law, have been calculated. The values of A& have been found to be positive for CH,ClCH,ClLquinoline, and negative for CH,ClCH,Cl- benzene, CH2C1CH,C1-toluene, CH2C1CH,C1-p-xylene and CH,ClCH,Cl- cyclohexane.The positive values of AE for CH,ClCH,Cl-quinoline are attributed to the existence of a specific interaction leading to the formation of a strong complex between CH,ClCH,Cl and quinoline in the liquid state. Values of the equilibrium constant Kf for the formation of a 1 : 1 complex between CH,ClCH,Cl and quinoline have also been calculated using the dielectric constant data, and it has been found that these values of Kf are in accord with the theory of Barrio1 and Weisbecker, which is based upon the electrostatic interactions of the solute with the liquid. The values of the apparent dipole moments papp of CH,ClCH,Cl at various mole fractions in the non-polar solvents benzene, p-xylene and cyclohexane have also been calculated. The values of papp suggest that there exists a specific interaction between CH,ClCH,Cl and the aromatic hydrocarbons.Binary systems of 1,2-dichloroethane (CH,ClCH,Cl) with aromatic hydrocarbons, quinoline and cyclohexane are of considerable interest from the viewpoint of the existence of specific interaction, leading to the formation of adducts between the components of the various systems in the liquid state. The specific interaction of CH,ClCH,Cl with aromatic hydrocarbons and quinoline can be visualised to be due to the presence of two C1 atoms and four H atoms in CH,ClCH,Cl on account of which it can act as a a-acceptor toward, and be involved in the formation of hydrogen bond with aromatics and quinoline.The aromatics in their interaction with CH,ClCH,Cl will act as 7r-donors, whereas quinoline will act as an N-donor. The system CH,ClCH,Cl- cyclohexane, in which only dispersion, dipolar and induction forces are believed to be present between the components, can be used as a reference system. Extensive studies of the interactions between the components of such systems have not been made. Recently,' however, the measurements of excess volumes for binary liquid mixtures of CH,ClCH,Cl with benzene, toluene, p-xylene, quinoline and cyclohexane were carried out at different temperatures and the ultrasonic velocities and adiabatic compressibilities of binary liquid mixtures of CH,ClCH,Cl with benzene, toluene, p-xylene, quinoline and cyclohexane have also been determined., These studies have indicated the existence of specific interaction between the components of the binary systems of CH,ClCH,Cl with aromatic hydrocarbons and quinoline.Dielectric constant measurements of binary liquid mixtures are k n ~ w n ~ - ~ to give reliable information concerning the existence of specific interaction between the components. Furthermore, the values of the apparent 31673168 Dielectric Properties of Binary Systems dipole moments, pUapp, have also provided evidence concerning the existence of specific interactions between the Values of papp can be obtained from dielectric constant measurements of binary liquid mixtures. Hence, in order to gain conclusive evidence about the existence of a specific interaction between CH,ClCH,Cl and aromatic hydrocarbons or quinoline, we have measured dielectric constants for binary mixtures of CH,CICH,Cl with benzene, toluene, p-xylene, quinoline and cyclohexane at 308.15 K.Since the values of the refractive indices of the various pure components and binary mixtures were needed to calculate papp of CH,ClCH,Cl in various solvents, and the molar polarisations of binary mixtures, refractive index measurements have also been carried out at 308.15 K, and the results of present measurements of E and n are interpreted in this paper. Experiment a1 Materials Benzene, toluene, ,p-xylene, quinoline, cyclohexane and 1,2-dichloroethane were purified and their purity was checked as described previously.' Method The measurements of dielectric constants ( E ) were made at 308.15 K and at a frequency of 1.8 MHz, with a dekameter (type DK,,, Wissenschaftlich-Technische, Werkstatten, Germany), using one cell (MFL l/S, no.2078) for mixtures having dielectric constants < 7.0 and another (MFL 2/S, no. 2084) for mixtures having dielectric constants > 7.0, as described previously.6 The cells were thermostatted by the outer jacket using a water thermostat whose temperature was maintained at 308.15&0.01 K. The two cells were first calibrated using purified liquids whose dielectric constants are available from the literature'' and then the measurements of E were made for pure liquids and binary mixtures investigated in the present study. The precision of the measurements of E is of the order of 0.0004 units for dilute solutions of CH,ClCH,Cl in benzene, p-xylene and cyclohexane, and ca.0.001 units for mixtures of CH,ClCH,Cl with quinoline, and for mixtures having higher concentrations of CH,ClCH,Cl in benzene, p-xylene, and cyclo hexane. The measurements of refractive index (n), accurate to within +0.0002, were made using a thermostatted Abbe refractometer at 308.15 K. The values of n were obtained for sodium-D light. Results and Discussion The experimental values of the dielectric constants for the binary systems CH,ClCH,Cl- benzene, CH2C1CH,C1-toluene, CH,CICH,Cl-p-xylene, CH,ClCH,Cl-quinoline and CH,CICH,Cl<yclohexane at 308. I5 K are given in table 1, where x, refers to the mole fraction of CH,ClCH,Cl. The values of E (see table 1) for benzene, toluene, p-xylene, cyclohexane and CH,ClCH,Cl at 308.15 K are 2.2557, 2.3543, 2.2447, 1.9989 and 9.808, respectively, and are in excellent agreement with the corresponding literature values'" of 2.2540, 2.3547, 2.2460, 1.9990 and 9.803, respectively.The present experimental values of n for benzene, toluene, cyclohexane and CH,ClCH,Cl at 308.15 K were found to be 1.491 8, 1.4880, 1.41 80 and 1.4372, respectively, in excellent agreement with the literature values" of 1.49 1 79, 1.487 82, 1.41 8 18 and 1.437 39, respectively. The experimental values of the refractive indices, nl,, for the various mixtures of CH,ClCH,Cl at 308.15 K, as obtained in the present study, were fitted by a least-squares method to the (1) equation n,, = a + hx, + cxf where x, refers to the mole fraction of CH,ClCH,Cl, and a, h and c are constants characteristic of a system at a given temperature.The values of the constants a, h andJ . Nath and G . Singh 3169 Table 1. Dielectric constants for various mixtures of CH,CICH,CI at 308.15 K X l E -x 1 & CH,CICH ,C1-benzene 0.0000 2.2557 0.0060 2.2776 0.0 106 2.2940 0.0323 2.3674 0.0461 2.422 0.050 1 2.428 0.0956 2.603 0. I772 2.934 0.2058 3.079 0.2984 3.546 0.3463 3.815 0.4598 4.58 1 0.5492 5.149 0.6974 6.327 0.7075 6.443 0.7226 6.586 0.8777 8.135 0.91 14 8.537 0.9 190 8.652 0.9539 9.059 0.9889 9.482 1 .0000 9.808 CH ,CICH ,ClLquinoline 0.0000 8.628 0.0930 8.830 0.1 644 8.969 0.4062 9.384 0.4432 9.414 0.4648 9.472 0.5734 9.600 0.6473 9.632 0.8273 9.728 0.8786 9.732 1 .0000 9.808 CH,C1CH,CI-toluene 0.0000 2.3543 0.0768 2.577 0.0823 2.592 0.1539 2.8 19 0.2477 3.157 0.2633 3.238 0.3786 3.802 0.4000 3.936 0.4027 3.948 0.44 12 4.196 0.5460 4.900 0.6 173 5.356 0.8633 7.775 0.9229 8.552 I .0000 9.808 CH,ClCH,Cl -p-xylene 0.0000 2.2447 0.0057 2.2598 0.0089 2.2680 0.01 29 2.2804 0.0362 2.3392 0.0398 2.35 16 0.0448 2.3564 0.0546 2.38 17 0.0922 2.472 0.1049 2.498 0.1234 2.553 0.2225 2.853 0.4344 3.592 0.6745 5.356 0.7629 6.218 0.7875 6.528 0.8334 7.065 0.9693 9.097 0.9802 9.31 1 1 .0000 9.808 CH ,CICH ,Cl-cyclohexane 0 .0000 1.9989 0.0039 2.0 I05 0.0151 2.0321 0.0 199 2.0427 0.0278 2.0641 0.0425 2.0838 0.048 1 2.096 1 0.0842 2.1721 0.1768 2.415 0.3047 2.847 0.3284 2.942 0.3993 3.260 0.4378 3.476 0.51 85 4.043 0.6535 5.214 0.7042 5.641 0.7088 5.713 0.8462 7.328 0.8593 7.561 0.9352 8.682 0.9670 9.166 1 .0000 9.8083170 Dielectric Properties of Binary Systems -1.4 -la2* 0.0 0.2 0.4 O .0.6 0.8 1.0 Table 2. Values of the constants a, b and c of eqn (l), and the standard deviations (T (n) for the various systems of CH,ClCH,Cl at 308.15 K system a b C 44 CH,ClCH,Cl-benzene 1.491 33 -0.060 93 0.006 609 0.000 27 CH,ClCH,Cl-toluene 1.488 15 -0.045 066 - 0.006 08 1 0.000 17 CH ,ClCH ,Cl-p-x ylene 1.487 83 -0.038 218 -0.012 442 0.000 19 CH,ClCH,Clquinoline 1.617 87 -0.105 019 -0.074 836 0.000 79 CH,ClCH,Ckyclohexane 1.418 19 -0.008 403 0.026 714 0.000 37 o.2t O O 0 0 0.01- - 0 . 2 p 6 c along with those of the standard deviations, a(n), for the various systems are given in table 2. The values of the quantity ALE, which refers to the deviation in the experimental values of the dielectric constants of the mixtures from values obtained from the volume- fraction mixture law, have been calculated from the relation ALE = &12-$41&1-$42% (2) where E, and E, are the dielectric constants of the pure components 1 and 2 for which the volume fractions in the mixture are and $4, respectively, and E,, is the dielectric constant of the mixture.The values of ALE for CH,ClCH,Cl-benzene, CH,ClCH,Cl-J . Nath and G. Singh 150 3171 - 0.0 0.2 0.4 0.6 0.8 1.0 X1 Fig. 2. Plot of the apparent molar polarisation, P A , us. mole fraction, xl, of CH,ClCH,Cl for the system CH2C1CH2C1-quinoline at 308.15 K. toluene, CH2C1CH,C1-p-xylene, CH,CICH,Cl-quinoline and CH2C1CH,C1-cyclo- hexane have been plotted us. mole fraction of CH,CICH,CI, x,, in fig.1. It has been shown that the dielectric constants of polar mixtures can be represented as linear functions of the volume fractions of the components.12 Comparison on a volume-fraction basis largely compensates for the 'dipole-dilution' effect, as has been discussed by Franks and Ives.13 Fig. 1 shows that the values of A& are highly positive for CH,ClCH,Cl-quinoline and highly negative for CH,ClCH,Cl-benzene, CH2C1CH2C1--toluene, CH,ClCH,CI- p-xylene and CH,CICH,Cl-cyclohexane. At x, = 0.5, the values of A& for the various systems are in the sequence (AE)quinoline > ('Elbenzene ('&)toluene > (A&), xylene > (A&)cyclohexane. The negative values of A& for CH2C1CH2C1-cyclohexane can be ascribed to the decrease in the degree of alignment of the dipoles with the changing composition of the solu- tion.It has been pointed out that A& is found to be positive in the case of systems in which molecular complexes due to the specific interactions between the components are believed to be f ~ r m e d . ~ The experimental results of measurements of excess volumes, ultrasonic velocities and adiabatic compressibilities for CH,CICH,Cl-benzene, CH,ClCH,Cl-toluene and CH,CICH,Cl-p-xylene have indicated that there exists a specific interaction between CH,ClCH,Cl and aromatic compounds. The negative values of AE for CH2C1CH,C1-benzene, CH,CiCH,Cl-toluene and CH,CICH,Cl-p- xylene can be attributed to the predominance of contributions to A& arising from dipole, induction and dispersion forces over those due to specific interactions.3172 Dielectric Properties of Binary Systems The highly positive values (see fig.1) of AE for CH,ClCH,Cl--quinoline show that CH2ClCH,C1 forms a tight intermolecular complex with quinoline, a consequence which is similar to the case of the system chloroform-pyridine, in which a 1 : 1 intermolecular complex is believed to be formed3 on account of the hydrogen-bond formation between the two components in the liquid state. In order to obtain further evidence concerning the formation of a strong complex between CH,ClCH,Cl and quinoline, we have calculated the values of the total molar polarisations, P, for CH2C1CH,Cl, quinoline and for mixtures of CH,ClCH,Cl with quinoline, by using the Kirkwood-Frohlich equation :14 ( E - n2) ( 2 ~ + n2) V 9 E P = (3) In eqn (3), E , n and V refer to the dielectric constant refractive index and molar volume, respectively.During the present analysis, the density data available in literature" were used to obtain molar volumes for pure liquids, whereas the molar volumes for the various mixtures were obtained from the molar volumes of pure liquids and the measurements on excess The values of the refractive indices, n, of the mixtures used in these calculations were obtained from eqn (1). The total molar polarisations of the mixtures were used to calculate the apparent molar polarisations, PA, of CH,ClCH,Cl at various concentrations in quinoline, in a similar manner to that described by Rastogi and Nath." The values of PA so obtained have been plotted us.mole fraction of CH,ClCH,Cl, x,, in fig. 2, which shows that PA increases sharply with the decreasing mole fraction of CH,ClCH,Cl, thus giving strong evidence that there exists a specific interaction leading to the formation of intermolecular complexes between CH,ClCH,Cl and quinoline in the liquid state. Similarly, the apparent molar polarisations, P,, of quinoline at different compositions of its mixtures with CH,ClCH,Cl were calculated. Considering that a 1 : 1 complex is formed between quinoline (D) and CH,ClCH,CI (A), the value of the molar polarisation, PDA, of the complex formed between D and A was calculated in a manner similar to that described by Earp and G1asstone.l' The value of P,, was found to be 376.36 cm3 mol-l.The values of the equilibrium constant, K,, for the formation of the complex DA, were also calculated from the data on molar polarisations of D, A and DA, and those of the mixtures of D and A, following the procedure described by Earp and Glasstone." The results show that the values of Kf exhibit significant variation with the composition of the mixture. Rivail and Thiebaut3 have also found that in the case of the system pyridine-chloroform, the values of Kf as obtained from the dielectric constant data show a significant variation with the composition of the mixture. It has been pointed out by Rivail and Thiebaut3 that a theory,' based upon electrostatic interactions of the solute with the liquid predicts a linear variation of the logarithm of Kf with the quantity: where E , refers to the infinite-frequency dielectric constant of the mixture.To calculate f i ~ ) from eqn (4), we have taken E, equal to n2. In fig. 3 are plotted the values of logK, us. AE). It is seen that there is a linear variation of logKf with JTE) for CH,ClCH,Cl-quinoline, suggesting that the values of Kf calculated using the simple approach of Earp and Glasstonel' are in accord with the theory17 based upon the electrostatic interactions of the solute with the liquid. Campbell et aL8v9 and Stokes and Marsh7 have shown that the values of the apparent dipole moment, papp, of polar solutes in non-polar solvents furnish useful information concerning the association of the solute molecules with the molecules of the solvent. Hence, in order to know if a specific interaction exists between CH,ClCH,Cl and the aromatic hydrocarbons, we have calculated the values of the apparent dipole moments,J.Nath and G. Singh - 7 -0.60 n .,-I +, 22 5 -0.80 E W --- G - -1.00 Y Kl 3173 - -_____rY___ 0 - I I 1 I I I I 0.15 0.16 0.17 0.18 0.19 0.20 f (€1 Fig. 3. Plot of log [K,/(mole fraction)-'] us. f l c ) for the system CH,ClCH,Cl-quinoline at 308.15 K. papp, of CH,ClCH,Cl at its various mole fractions in the non-polar solvents benzene, p-xylene and cyclohexane using the equation'. l8 where x, is the mole fraction of the polar solute; E is the dielectric constant of the mixture; E , is the dielectric constant of the non-polar solvent (benzene, p-xylene or cyclohexane) and E; is the internal dielectric constant of the polar solute; Vm, Vl and V2 refer to the molar volumes of the solution, the polar solute and the non-polar solvent, respectively ; k is Boltzmann's constant; N is Avogadro's constant; ps,o is the moment of the isolated polar molecule and g is the Kirkwoodl correlation parameter.lg As mentioned by Stokes and M a r ~ h , ~ we have taken (gp,',,)? to be equal to the apparent dipole moment, papp, of the polar solute in tFe non-polar solvent.The values of Vl, V2 and Vm needed for calculations of (gp&)Z from eqn ( 5 ) were obtained from the densities of pure liquids and the values of the excess The value of &; for CH,ClCH,Cl, obtained from the refractive index of CH,ClCH,Cl as described by Stokes and M a r ~ h , ~ was 2.1390. Fig. 4 shows the concentration dependence of pipp for CH,ClCH,Cl in (i) cyclohexane, (ii) benzene and (iii) p-xylene, with a logarithmic scale along the abscissa.Fig. 4 shows that the values of pEpp for CH,ClCH,Cl increase as its concentration in benzene increases. Fig. 4 also shows that the values of pZPp for CH,ClCH,Cl first decrease, reach a minimum and then increase in cyclohexane, whereas the values of pEpp first increase, then decrease to a minimum and increase again in p-xylene. The initial rise in pipp is indicative of the formation of the first, high-dipole-moment species. Fig. 4 therefore indicates that CH,ClCH,Cl monomer is stabilised little by interaction with the non-polarisable solvent (cyclohexane) and stabilised more by association with the solvents benzene and p-xylene. The association of CH,ClCH,Cl with benzene and p-xylene can be considered to be due to the existence of a specific interaction between CH,ClCH,Cl and aromatic compounds, which may be due to the formation of a weak hydrogen bond via interaction of the H atom of CH,ClCH,Cl with the n-electrons of the aromatic ring. However, there is also a possibility that CH,ClCH,Cl is involved in the formation of a weak charge-transfer complex through the interaction of the chlorine atoms of CH,ClCH,Cl with the aromatic n-electrons.Fig. 4 also shows that the initial rise in p:pp for CH,ClCH,Cl in p-xylene solution is delayed more than that in benzene solution, thus showing that the strength of interaction of CH,ClCH,Cl with p-xylene is greater than that with benzene.3174 Dielectric Properties of Binary Systems 4 .O 3.0 N \ CI a N O 2.c 3.1.0 i I I I I -2.0 -1.0 0.0 1 .o log (clmol dm-3) Fig. 4. Plot of ,u&, us. the logarithm of solute concentration for CH,ClCH,Cl in the solvents: (i) cyclohexane, (ii) benzene, (iii) p-xylene. This can be attributed to the fact that the z-electron density of the aromatic ring is increased in p-xylene owing to the presence of two CH, groups. Conclusions In conclusion, we note that the values of ALE show that CH,ClCH,Cl forms a strong intermolecular complex with quinoline in the liquid state. The values of the equilibrium constant Kf for the formation of 1 : 1 complex between CH,ClCH,Cl and quinoline, as estimated from the dielectric constant data, are in accord with the theory of Barrio1 and Weisbecker,l' based upon the electrostatic interactions of the solute with the liquid.The plots of the values of pipp for CH,ClCH,Cl in fig. 4 show that there exist specific interactions of CH,ClCH,Cl with benzene and p-xylene, and that the strength of interaction of CH,ClCH,Cl with p-xylene is greater than that with benzene. This has been attributed to the increased z-electron density of the aromatic ring in p-xylene. The presence of a specific interaction of CH,ClCH,Cl with benzene and p-xylene can be explained as being due to the formation of a weak hydrogen bond between the H atoms of CH,ClCH,Cl and the z-electrons of the aromatic ring. There is, however, a possibility of the existence of a charge-transfer interaction between CH,ClCH,Cl and the aromatic hydrocarbons on account of the interaction of the chlorine atoms in CH,ClCH,Cl with the z-electrons of the aromatic ring.On the other hand, the complexation between CH,ClCH,Cl and quinoline can be attributed to the formation of strong hydrogen bonds between the hydrogen atoms of CH,ClCH,Cl and the lone-pair electrons on the nitrogen atom of quinoline. We are highly thankful to Prof. S. C. Tripathi (Head of the Chemistry Department, Gorakhpur University, Gorakhpur) for providing laboratory facilities. We are also extremely grateful to Prof. R. P. Rastogi (Vice-Chancellor, Banaras Hindu University, Varanasi) for invaluable suggestions. Thanks are also due to the Indian Council of Scientific and Industrial Research, New Dehli, for financial support.J. Nath and G. Singh 3175 References 1 J. Nath and G. Singh, J. Chem. Eng. Data, 1986, 31, 115. 2 J. Nath and G. Singh, J. Chem. Eng. Data, 1986, 31, 327. 3 J. L. Rivail and J. M. Thiebaut, J. Chem. SOC., Faraday Trans. 2, 1974, 70, 430. 4 J. Nath and S. N. Dubey, J. Phys. Chem., 1980, 84, 2166. 5 J. Nath and S. S. Das, Indian J. Pure Appl. Phys., 1981, 19, 343. 6 J. Nath and A. P. Dixit, J. Chem. SOC., Faraday Trans. 2, 1985, 81, 11. 7 R. H. Stokes and K. N. Marsh, J. Chem. Thermodyn., 1976, 8, 709. 8 C. Campbell, G. Brink and L. Glasser, J. Phys. Chem., 1975, 79, 660. 9 C. Campbell, G. Brink and L. Glasser, J. Phys. Chem., 1976, 80, 686. 10 N. A. Lange, Lange’s Handbook of Chemistry (McGraw-Hill, New York, 1973). 1 1 J. Timmermans, Physico-Chemical Constants of Pure Organic Compounds (Elsevier, Amsterdam, 12 T. B. Hoover, J. Phys. Chem., 1969, 73, 57. 13 F. Franks and D. J. G. Ives, Q. Rev., 1966, 20, 1. 14 C. Moreau and G. DouhCret, J. Chem. Thermodyn., 1976, 8,403. 15 R. P. Rastogi and J. Nath, Indian J. Chem., 1967, 5, 249. 16 D. P. Earp and S. Glasstone, J. Chem. SOC., 1935, 1709. 17 J. Barrio1 and A. Weisbecker, C. R. Acad. Sci., Ser. C, 1967, 265, 1372. 18 H. Frohlich, Trans. Faraday SOC., 1948, 44, 238. 19 J. G. Kirkwood, J. Chem. Phys., 1939, 7, 911. 1950). Paper 61241 5 ; Received 15th December, 1986

ISSN:0300-9599    

DOI:10.1039/F19878303167

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

J. Chem. Soc., Faraday Trans. I, 1987, 83 (lo), 3177-3187 Radical Spectra and Product Distribution following Electrophilic Attack by the OH' Radical on 4-Hydroxybenzoic Acid and Subsequent Oxidation Robert F. Anderson,* Kantilal B. Pate1 and Michael R. L. Stratford Gray Laboratory of the Cancer Research Campaign, Mount Vernon Hospital, Northwood. Middlesex HA6 2RN The distribution of electrophilic OH' radical addition to 4-hydroxybenzoic acid (HBA) has been determined by oxidizing the radical intermediates (substituted hydroxycyclohexadienyl radicals) with quinones and viologens, to yield products. It is deduced from product analysis using high- performance liquid chromatography that the fractions of OH' attack at the 1 : 2 : 3 : 4 positions of HBA are 0.16 : 0.04 : 0.65 : 0.15 respectively.Pulse radiolysis studies show that the rate of electron transfer from the radical intermediate formed by OH' addition to position 3 of HBA is dependent on the one-electron reduction potential of the oxidant. This electron-transfer process is in competition with the elimination of water by general acid-base catalysis to yield the phenoxyl radical. Catalysis by OH- proceeds through the formation of the deprotonated species, pK, = 8.4k0.2, followed by the elimination of water, k, = 3.0k0.3 x lo5 s-'. The addition of an OH' radical ips0 to the hydroxy group of HBA also gives rise to the phenoxyl radical through rapid water elimination, k = 2 x lo7 s-l. The addition of an OH' radical to position 3 of HBA gives rise to an absorption band centred at 365 nm.Hydroxylation of substituted benzenes can be achieved in a two-step process in which the electrophilic OH' radical adds to form a substituted hydroxycyclohexadienyl radical, followed by electron transfer to an oxidant to yield products. The most commonly employed oxidants in this process have been metal ions'-5 which often yield a varying range of product distribution, dependent on the nature and concentration of the metal ions. Accurate determinations of the pattern of OH' radical addition to anisole6 and phenol7 have been made using quinones as oxidants, by observing both the kinetics of oxidation of the isomeric radicals and product analysis. A strong preference for addition to occur ortho and para to both the hydroxy and methoxy groups was observed.It was noted in the course of a study on the reaction of OH' radicals with benzoic acid4 in the presence of ferricyanide that hydroxybenzoic acids also react with OH' radicals, but the resulting products were not identified. In this study we have used quinones and viologens as oxidants to study the product distribution in a disubstituted benzene following OH' radical attack on 4-hydroxybenzoic acid (HBA). This compound is a substrate for the enzyme p-hydroxybenzoate hydroxylase, which is a flavin containing mono-oxygenase. Although some intermediates involved in the functioning of this class of enzyme have been identified by comparing U.V. spectra8 with model compounds, the intermediate considered to be closely related to that responsible for oxygen transfer has not been successfully modelled.In this study we also report the spectrum of the OH' radical adduct to HBA. 31773178 Electrophilic Attack by the OH' Radical Experimental 4-Hydroxybenzoic acid (HBA)? was obtained from Aldrich (gold label product) and found to be pure using h.p.1.c. 2,4-Dihydroxybenzoic acid (2,4-DHBA) and 3,4- dihydroxybenzoic acid (3,4-DHBA) were obtained from BDH and hydroquinone (HQ) was obtained from Sigma and repurified by sublimation. Other reagents were of the highest purity commercially available. The pulse radiolysis studies were carried out using a 3.5 MeV Van de Graff accelerator to irradiate [3-20 Gy (J kg-l) in 30 ns] aqueous solutions (21 "C) contained in a 2-cm path-length cell. The associated charge monitoring device and optical detection system have been described.' The absorbed radiation dose was determined by measuring the optical density at 472 nm in aerated KSCN'" (10 mmol drn-,) assuming an extinction coefficient, E of 758 m2 mo1-' and a radiation chemical yield GE$ of 0.29 pmol J-l.Transients were recorded on a Tektronic 7612D digitizer interfaced to a PDP 11/34 computer for data analysis. [Absorptions are presented as the product of the radiation chemical yield, G (mol J-') and the extinction coefficient, E(m2 mol-').I Steady-state irradiations were carried out using a 6''Co pray source. Solutions were prepared and irradiated in either air-tight syringes (25 cm3) from which samples were sequentially removed for analysis or (in the case of DMBQ) in separate sealed h.p.1.c.vials for each dose point. The dose rate (ca. 30 Gy rnin-') was determined using the Fricke dosimeter." Product analysis was carried out by high-performance liquid chromatography (h.p.1.c.) using a Waters 840 chromatograph equipped with a Waters 490 multi- wavelength detector. Detection of the dihydroxybenzoic acids and hydroquinone was at 295 nm and the benzoquinones at 254 nm. The column used was a Waters Novapak C18 cartridge (100 x 8 mm) in a Waters 2-module. Separation was achieved with a binary gradient, the eluants being: A, acetonitrile (95%) and water ( 5 % ) and B; ortho- phosphoric acid (20 mmol dm-3) and sodium dihydrogen orthophosphate (20 mmol drn-,). All the analyses used the same initial linear gradient of 10-20% A in 8 min. For 1,4-benzoquinone and methyl- I ,4-benzoquinone the gradient then returned to 10 % A in 1 min.For the less polar quinones, an additional gradient from 2040% A in 6 min was employed, again returning to 10% A in 1 min. Results and Discussion Reactions initiated by the OH' radical are conveniently studied in irradiated aqueous solution saturated with N,O gas to convert eiq to OH'. H,O+OH', e&, H', H,O,, H,, H30+, OH- (1) (2) N,O + eiq --+ OH' + OH- + N, yielding [OH'] = 0.62 pmol J-' and [H'] = 0.06 pmol J-'.l2 The rate of reaction of OH' with HBA, reaction (3), was determined from the appearance of an absorption band centred at 370 nm following pulse radiolysis (10 Gy in 30 ns). The rate constant was determined using three concentrations of HBA (0.5--2.0 mmol dm-3), yielding k, = 6.0 x lo9 dm3 mol-' s-l.To study the subsequent reaction, (4), t Abbreviations used : HBA (4-hydroxybenzoic acid) 2,4-DHBA (2,4-dihydroxybenzoic acid), 3,4-DHBA (3,4-dihydroxybenzoic acid); Q (quinone), Q'- (semiquinone), QH, (fully reduced quinone) ; pBQ (1,4- benzoquinone), HQ (hydroquinone), MBQ (methyl- 1 ,Cbenzoquinone), DMBQ (2,5-dimethyl- 1,4-benzo- quinone), AQS (an thraquinone-2-sulphonic acid), TQ (7,8-dihydro-6H-dipyrido[ 1,2-a: 2', I'-c][l,4]diaze- pinedi-ium dibromide), V4 1 (6,7,8,9-tetrahydro-2,13-dimethyldipyrido[ I ,2-a : 2', 1 '-c][ 1,4]diazocinedi-ium dibromide), OX (oxidant), OXH, (two-electron reduced oxidant), OX'- (one-electron reduced oxidant), TMPD (NNN"'-tetramethyl-p-phenylenediamine). 1 A G-value of 1 molecule (100 eV)-' corresponds to a radiation chemical yield, G = 0.1036 pmol J-'.R. F.Anderson, K. B. Pate1 and M . R. L. Stratford 3179 HO HO coo- coo- 6 \J OH + OX - products + OX'- coo- (4) it was necessary to employ a high concentration of HBA (10-40 mmol dmP3) relative to the oxidants, OX (50400 pmol dm-3), to ensure complete scavenging of the OH' radicals by HBA, which in turn resulted in some direct scavenging of eiq by HBA, reaction ( 5 ) : The rate constant for reaction ( 5 ) was measured by observing the decay of eiq absorption at 715 nm in the presence of HBA (100400 pmol dm-3) in N,-saturated solutions containing 2-methylpropan-2-01 (0.2 mol dm-3, pH 6). The second-order rate constant, k5 = 2 x lo9 dm3 mol-' s-l, is similar to that reported for benzoic acid.13 This side reaction (< 25 O/O of [e,]) reduced the maximum yield of products arising from OH' radical adducts to 0.545 and 0.595 pmol J-l when using 40 or 10 mmol dm-3 HBA, respectively .Product Yields Addition of OH' to HBA forms isomeric dihydroxycyclohexadienyl radicals which can undergo a variety of unimolecular and bimolecular reactions. From pulse radiolysis studies, see below, it was found that < 400 pmol dm-3 of the quinones of relatively high reduction potential (pBQ, MBQ, DMBQ) resulted in a constant level of electron transfer to form semiquinone as observed on the microsecond time-scale. Yield-dose plots using 400 pmol dm-3 of the oxidants were linear, giving the results presented in table 1. The total radiation dose delivered, 300 Gy (given in 6 serial doses), resulted in < 1 YO conversion of substrate (HBA) and < 25% conversion of the oxidants.With the three most powerful oxidants (highest reduction potentials, table l), the major product, 3,4-DHBA, accounted for 65 2 Yo of the total yield of OH' while 2,4- DHBA (4 & 1 YO) and HQ (1 6 2 YO) were the only other measured products. The results are consistent with the addition of OH' to positions 3,2 and 1 of HBA, respectively, followed by electron transfer to the oxidant to yield products [reaction(4)]. The remaining 15 k 2 '10 of the OH' presumably adds to position 4 of HBA and by analogyw 00 0 er Table 1. Yields" of products and change in oxidantsb following the reaction of hydroxyl radicals with 4-hydroxybenzoic acid oxidant [HBAI/ [products]/pmol J-l [oxidants]/pmol J-l [E(l) at pH 7c] mmol dm-3 pH 3,4-DHBA 2,4-DHBA HQ ox OXH, pBQ (+ 99 mV) MBQ (+ 23 mV) DMBQ (- 66 mV) AQS (-380 mV) TQ (- 548 mV) V41 ( - 740 mV) none none none (N, gas) 40 10 40 40 10 40 10 40 40 40 1 1 1 5 6 5 6 6 6 6 6 6 6 6 9 6 0.358 f 0.01 1 (66 k 2) 0.332 f 0.004 (56 f 1) 0.359 f 0.008 (66 f 2) 0.353 f 0.005 (65 f 1) 0.384f0.010 (65 f 2) 0.351 f 0.004 (64f 1) 0.413 k0.006 (69 f 2) 0.279 f 0.007 (51 f 2 ) 0.184 2 0.007 (34 k 2) 0.162 f 0.003 (30 f 1) 0.168 f 0.009 (27 f 2) < 0.01 < 0.01 - 0.023 f 0.002 ( 4 f 1) - 0.020 f 0.001 ( 4 f 1) 0.015f0.001 (3 & 1) - 0.0 14 f 0.00 1 < 0.01 < 0.01 < 0.01 < 0.01 < 0.01 < 0.01 ( 2 f 1) 0.378 f 0.016 0.358 & 0.010 0.087 & 0.006 (16 k 2) 0.083 k 0.002 (15 f 1) 0.094 f 0.003 (16 & 1) 0.086 & 0.003 (16f 1) 0.098 f 0.003 Wf 1) 0.1 14 & 0.026 (21 f 5) 0.015f0.009 (3 f 2) 0.010 f 0.005 (2f 1) 0.020 & 0.010 (4 f 2) < 0.01 < 0.01 0.308 f0.014 (45 f 2) 0.331 fO.010 (49 f 1) 0.352f0.013 (52 k 2) 0.351 f0.014 (52 f 2) 0.340 f 0.016 (50 f 2) e e d d 0.321 f0.015 0.301 50.015 0.373f0.015 0.275 & 0.01 8 0.250 f 0.007 (47 k 2) (44 k 2) (55 k 2) (40 k 3) (37 k 1) b a Values in parentheses are the percentage yields of (i), products, from a maximum yield of OH' of 0.525 and 0.575 pmol J-' for 40 and 10 mmol dm-3 [HBA] and (ii), the change in oxidants, from a total radical yield ([OH'] + [H']) of 0.68 pmol J-l.The concentration of all oxidants was 0.4 mmol dm-3. One-electron reduction potentials from ref. (19) and (20).Included in the yield of HQ. Unable to be determined using bubbled vials.R. F. Anderson, K. B. Patel and M. R. L. Stratford 3181 with similar studies with anisole6 and phenol7 undergoes rapid water elimination to form a phenoxyl radical coo- coo- The dependence of product yields on the concentration of the oxidants (< 400 pmol dm-3) is understood in that OH' radical adducts in other than the 4 position on HBA can also undergo water elimination in competition to electron transfer [reaction(4)], albeit on a longer time-scale. Electron transfer from the OH' radical adducts of HBA to the quinones form semiquinones which disproportionate [reaction (7)]. 24'- + 2H+ - Q + QH, (7) The loss of pBQ and MBQ and the production of their hydroquinones account for the full radical yield.This indicates that the products arising from phenoxyl radicals are oxidized by pBQ and MBQ (but possibly not by DMBQ). A considerable amount of 3,4-DHBA was formed in the absence of oxidant at pH 6 but not at pH 9, table 1, where only some unidentified less polar products were observed. The 3,4-DHBA product might arise from a radical-radical reaction between the substituted cyclohexadienyl radical and the phenoxyl radical [reaction (S)] HO HO HO coo- coo- coo- coo- in competition with an acid-base catalysed elimination of water similar to that known for phenol14 [reaction (9), see below] to yield the phenoxyl radical, which in turn decays bimolecularly to give unidentified products. HO b coo- coo- No loss of HBA was observed in irradiated N,-saturated solution which implies that the phenoxyl radical formed from the OH' radical adduct oxidises the e& adduct to reform substrate [reaction (lo)] coo- coo- coo-3182 Electrophilic Attack by the OH’ Radical 300 350 40 0 450 500 A/ nm Fig. 1.Spectral changes observed following the pulse radiolysis of a N,O-saturated solution of HBA (5 mmol dm-3) adjusted to pH 7 [NaOH, phosphate (1 mmol drn-”]. Spectra measured : 0, 0.5 ps; 0, 50 ps; A, 200 ps; 0, 800 ps after the pulse (4.7 Gy in 30 ns). Insert: oscilloscope trace of percentage absorption at 360 nm (ordinate) against time (abscissa) for (b), conditions as above and (a), for HBA (10 mmol dm-3) following a radiation dose of 12.7 Gy. Spectrum+was found by subtraction of the spectrum measured at 70 ns (40 ns after a 30 ns pulse) from that measured at 0.5 ,us.Pulse Radiolysis The transient spectrum observed following pulse radiolysis ( 5 Gy in 30 ns) of a N,O- saturated solution [pH 7, phosphate ( 5 mmol dm-3)] containing HBA (5 mmol dm-3) is presented in fig. 1. The transient species exhibiting an absorption band centred at 365 nm (measured 0.5 ,us after the pulse) decayed with first-order kinetics to a new species (isosbestic point at 390 nm) that exhibits a broad absorption band near 400 nm (measured 200 ps after the pulse). A two-stage process was seen in the formation of the 365 nm species where a small increase in absorption (by 0.5 ps) was seen superimposed on the initial OH’ adduct measured at 70 ns (40 ns after a 30 ns pulse) (insert, fig. 1).The rate of build-up of this small increase in absorption is independent of the concentration of HBA (10-40 mmol dm-3) with k x 2 x lo7 s-l. The spectrum corresponding to this increase is found by subtracting the spectrum at 70 ns from that at 0.5 ps, fig. 1. This spectrum has similar features to the spectrum measured at 200 ps and accounts for the observed absorption at 2 400 nm in the spectrum measured at 0.5 ps. The spectrum is similar to known phenoxyl radical ~pectra.’~ Furthermore a two-step oxidation of added NNN”’-tetramethyl-p-phenylenediamine (TMPD) to its radical cation was observed consistent with the known reaction between TMPD and phenoxyl radica1s.l‘ By comparing the formation of the TMPD radical cation following pulse radiolysis (4 Gy in 30 ns) with that formed by the OH’ radical directly it was found that ca.15 % was formed during the 0.5 ps time period (independent of pH), rising to 100 % at pH 3 7.5 when measured within the lifetime of the 365 nm species, fig. 2. This secondary increase in the production of the TMPD radical cation is interpreted as arising from the base- catalysed formation of the phenoxyl radical (see below) in competition with bimolecular reactions of the 365 nm species.R. F. Anderson, K. B. Pate1 and M. R. L. Stratford 3183 Fig. 2. The maximum spectral change observed at 565 nm uersm pH following the pulse radiolysis (4 Gy in 30 ns) of N,O-saturated solutions containing TMPD (400 pmol dm-3) (0) and added HBA (20 mmol dmw3) (0). Insert : oscilloscope trace of percentage absorption (ordinate) against time (abscissa) for pH 6.5. 2.5 2 .o 4 I h N E 1.5 R 0 0.5 0 300 350 400 450 500 Xlnm Fig.3. Spectral changes observed following the pulse radiolysis of a N,O-saturated solution of 3,5-dimethyl-4-hydroxybenzoic acid (5 mmol dm-3) adjusted to pH 7 [NaOH, phosphate (1 mmol dmd3)]. Spectra measured: 0, 2 ps; 0, 50 ps; A, 200 ps; 0, 800 ps after the pulse (2.5 Gy in 30 ns). Insert : oscilloscope trace of percentage absorption at 370 nm (ordinate) against time (abscissa).3184 Electrophilic Attack by the OH' Radical These results support the suggested scheme in which a small proportion of the OH' radicals add ips0 to the hydroxyl group of HBA followed by rapid elimination of water to form the phenoxyl radical, reaction (6). The majority of the OH' radicals add to position 3 of HBA followed also by water elimination, reaction (9).That the observed first-order decrease in the 365 nm peak arises from this pathway was confirmed on using 3,5-dimethyl-4-hydroxybenzoic acid as the substrate, fig. 3. In this case OH' addition to position 3 (and 5) is unfavourable and the observed radical intermediate decayed only with second-order kinetics, 2 k / ~ = 6.2k0.9 x lo4 cm s-l measured at 370 nm. The phenoxyl radical of HBA decayed with second-order kinetics, 2 k / ~ = 1.8 k0.3 x lo5 cm s-l measured at 370 nm and using E,,, = 110 m2 mol-l gave 2k = 2 x lo8 dm3 mol-' s-l. This rate constant is similar to that reported for the decay of the anionic form of the phenoxyl radical formed from hydroquinone. l7 The log plot of the observed first-order rate constant, k, for the conversion of the 365 nm species (the substituted hydroxycyclohexadienyl radical) to the 400 nm species (the phenoxyl radical) against pH exhibits regions that are H+ or OH- catalysed, fig.4. Catalysis by the phosphate buffer also occurs with a second-order rate constant of ca. 5 x lo5 dm3 mol-1 s-l (insert, fig. 4), leading to a region where kis independent of pH, k,. Whereati there are insufficient data at low pH to derive mechanistic conclusions on H+ catalysis a full description of OH- catalysis is possible. The absorbance at 365 nm decreases at high pH (fig. 4), which is consistent with the fast establishment of an equilibrium between the substituted hydroxycyclohexadienyl radical and its de- protonated form, reaction (1 l), before an elimination reaction (k,) which yields the phenoxyl radical, reaction (1 2).HO 0- coo- coo- coo- coo - The kinetic description for when k, = k, and k, > k, is given byla k = k, k,(k, + k,)-l, and for HBA between pH 4 and 9 by k = k, + k , k,,[OH-](K, + Kll[OH-])-l. (13) Fixing k, = 3 x lo3 s-l and performing regression analysis on equation (1 3) gave k, = 3.0 0.3 x lo-'. The value for Kll is in good agreement with that found independently from fig. 4, where a least-squares fit to the optical absorption data gave pK, = 8.4k0.2 and hence Kll = 4.0k2.5 x lo-'. The rate constants for electron transfer from the OH' radical adduct of HBA to oxidants (OX) were determined by following the formation of the absorbance of the one-electron reduced oxidants (OX'-).The second-order rate constants, determined using at least three concentrations of OX, are presented in table 2. The fractions of the total OH' concentration that are associated with the observed electron 0.3 x lo5 s-' and Kll = 2.9R. F. Anderson, K . B. Pate1 and M. R. L. Stratford 3185 3 2 3 4 5 6 7 8 9 1 0 PH Fig. 4. The pH dependence of the initial absorbance at 360 nm (0) formed upon the reaction of the OH' radical with HBA, and the first-order rate constant of its decay (0). Insert: the dependence of the first-order rate constant at pH 6.5 on phosphate concentration. Table 2. Observed rate constants and radical yields for the formation of oxidant radicals following the addition of OH' radicals to 4-hydroxybenzoic acid" ~ _ _ _ _ ~ ~ ~ ~ - - - ~ oxidant 44)/ [OX'-]/ [E(1) at pH 71 pH dm3 mo1-ls-l ['OH], pBQ(+99mV) 5 2.7 x 109 0.72" MBQ(+23mV) 6 1.8 x lo9 0.70" DMBQ(-66mV 6 1 .5 ~ lo9 0.68b AQS (- 380 mV) 7 3.0 x 0.38' TQ(-548mV) 6 4.0 x 107d 0.43' -~ _ _ ~ -~ __._ _ ~ _ -~ ___ ~ 0,(-155mV) 6 5.5 x lox' V41 (-740 mV) 6 6 2 x 107 _________ ~~ ~- " [HBA] = 20 mmol dm-3. ' Plateau value in [OX'-] observed for [OX] d 0.4 mmol dm-3. Rate constant measured for decay of the 360 nm absorption band. Values derived from reciprocal plots. ' Ratio observed using [OX] 0.4 mmol dm-3. transfers were determined against the yield of OX'- formed in solutions containing sodium formate (0.1 mol dm-3) and the OX, table 2. The observed yields ([OX'-] in the presence of pBQ, MBQ and DMBQ (0.70 k0.02)) are similar to the yield of 3,4-DHBA, which implies that the observed rate constants are for the oxidation of the intermediate formed upon the addition of OH' to position 3 of HBA.The rate constants decrease with the one-electron reduction potential of OX, which accounts for the observed decrease in the yields of OX'-, as an increasing proportion of the OH' radical adduct will form the phenoxyl radical.3186 Electrophilic Attack by the OH' Radical Conclusions These results indicate that the OH' radical undergoes electrophilic addition to the ring positions which are activated by the electron donating hydroxy group. This pattern of reactivity is expected from data obtained in similar ~ y s t e r n s . ~ - ~ , ~ * ~ * - ~ ~ However, in the case of HBA clearly steric effects are also important in the pattern of OH' reactivity, where addition to the position ortho to the hydroxy group predominates over the substituted para position.Also a greater percentage of the OH' radicals react ips0 to the hydroxy group than in the case of phen01.~ The spectrum measured 0.5 ps after the pulse is the sum of the absorption bands due to OH' radical addition to all 4 positions on HBA. The fast formation of the fraction of the phenoxyl radical absorption associated with OH' addition to position 4 is observed as an increase in absorbance at all measured wavelengths. This observation implies that the initial OH' radical adduct to position 4 absorbs at A < 320 nm, in the region where the cyclohexadienyl radical The difference in the absorption spectra between addition ( a ) ips0 to the hydroxy group and (h) to position 3 might be related to the different natures of the two substituted cyclohexadienyl radicals produced. Whereas in case (b) a resonance form of the cyclohexadienyl radical can be drawn with a hydroxy-substituted carbon radical, this is not so for case (a). A similar consideration could apply for addition ips0 to the carboxylate group. coo- coo- (a) coo- coo- ( b ) Since OH' addition to position 2 of HBA is ca. 4% of the radical yield it is clear that the band at 365 nm arises from OH' addition at position 3 .During the oxidation of certain substrates for p-hydroxybenzoate hydroxylase it is observed that the absorption spectrum associated with its flavin hydroperoxide, species I (Amax = 350-400 nm, E z 900 m2 mol-'), increases to give species I1 ( E z 1400 m2 mo1-1).8 We are currently investigating these other substrates of p-hydroxybenzoate hydroxylase to see if part of the absorption spectrum of species I1 can be accounted for by the radical mechanism described above.This work is financially supported by the Cancer Research Campaign. We thank Drs S. Steenken and P. Wardman for helpful discussions, Mr B. L. Hall and colleagues for operating the Van de Graaff accelerator and Miss J. P. Mahon for sublimation of the hydroquinone. References 1 0. Volkert and D. Schulte-Frohlinde, Tetrahedron Lett., 1968, 17, 2151. 2 K. Bhatia, J . Phys. Chem., 1975, 79, 1032. 3 M. K. Eberhardt, J . Phys. Chem., 1975, 79, 1913; 1977, 81, 1051. 4 G. W. Klein, K.Bhatia, V. Madhavan and R. H. Schuier, J. Phys. Chem., 1975, 79, 1767. 5 C. Walling, D. M. Camaioni and S. S. Kim, J . Am. Chem. SOC., 1978, 100, 4814. 6 S. Steenken and N. V. Raghavan, J . Phys. Chem., 1979, 83, 3101. 7 N. V. Raghavan and S. Steenken, J . Am. Chem. SOC., 1980, 102, 3495. 8 B. Entsch, D. P. Bailou and V. Massey, J . Biol. Chem., 1976, 251, 2550. 9 B. Vojnovic, Ph.D. Thesis (The University of London, 1983).R. F. Anderson, K. B. Patel and M. R. L. Stratford 3187 10 R. H. Schuler, L. K. Patterson and E. Janata, J. Phys. Chem., 1980, 84, 2088. 1 1 ICRU, Radiation Dosimetry : X-Rays and Gamma Rays with Maximum Photon Energies Between 0.6 and 12 G. W. Klein and R. H. Schuler, Radiat. Phys. Chem., 1978, 11, 167. 13 A. Szutka, J. K. Thomas, S. Gordon and E. J. Hart, J . Phys. Chem., 1965, 69, 289. 14 E. J. Land and M. Ebert, Trans. Faraday SOC., 1967, 63, 118 1. 15 A. Habersbergerova, I. Janovsky and P. Kourim, Radiat. Res. Rev., 1972, 4, 123. 16 S. Steenken and P. Neta, J. Phys. Chem., 1979, 83, 1134. 17 G. E. Adams and B. D. Michael, Trans. Faraday SOC., 1967, 63, 1 17 1. 18 R. P. Bell, in The Proton in Chemistry (Methuen, London, 1959), p. 135. 19 A. J. Swallow, in Functions of Quinones in Energy Conserving Systems, ed. B. L. Trumpower (Academic 20 R. F. Anderson and K. B. Patel, J. Chem. SOC., Faraday Trans. I , 1984, 80, 2693. 21 K. B. Patel and R. L. Willson, J . Chem. Soc., Faraday Trans. I , 1973, 69, 814. 22 D. Meisel and P. Neta, J. Am. Chem. SOC., 1975, 97, 5198. 23 R. F. Anderson, Ber. Bunsenges. Phys. Chem., 1976, 80, 969. 24 S. Steenken and P. O’Neill, J. Phys. Chem., 1977, 81, 505; 1978, 82, 372; 1979, 83, 2407. 25 R. H. Schuler, Radiat. Res., 1977, 69, 417. 26 P. O’Neill, S. Steenken, H. van der Linde and D. Schulte-Frohlinde, Radiat. Phys. Chem., 1978, 12, 27 M. C. Sauer and B. Ward, J. Phys. Chem., 1967, 71, 3971. 50 MeV, Report 14 (Washington D.C., 1969). Press, New York, 1982), p. 59. 13. Paper 6/2416; Received 15th December, 1986 105 FAR 1

ISSN:0300-9599    

DOI:10.1039/F19878303177

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

J . Chern. Soc., Fa-raduy Trans. I , 1987, 83 (lo), 3189-3197 Control of Ni Metal Particle Size in Ni/SiO, Catalysts by Calcination and Reduction Temperatures Hiroyuki Tamagawa, Kyuichi Oyama, Tsuyoshi Yamaguchi, Hiroshige Tanaka, Hideyasu Tsuiki and Akifumi Ueno" Department of Materials Science, Toyohashi University of Technology, Tempaku, Toy ohash i, A ich i 440, Japan It has been found that the particle size of Ni in Ni/SiO, catalysts prepared by the alkoxide technique can be controlled by the calcination and reduction temperatures as well as by the extent of Ni loading in the catalyst. The mechanisms of sintering of the Ni metal and of redispersion of NiO particles during reduction and calcination respectively, are discussed. The sintering occurs by a particle-migration mechanism and the redispersion may occur by direct splitting to small particles. The catalytic property of Ni particles whose sizes were controlled either by their Ni loading or by their calcination and reduction temperatures was examined by using the catalyst for the Boudouard reaction.Since a catalytic reaction over a supported metal catalyst takes place on the surfaces of ' metal particles, the physical and chemical properties of these surfaces will be reflected in their catalytic activity and selectivity. The properties of the surfaces of metal particles are considered to change with additives dissolved in the metal particles and with the supports employed; alloying effects upon the catalysis' are attributed to the former and the so-called SMS12 (strong metal-support interaction) effects are ascribed to the latter.The control of metal particle size is also one of the candidates for modifying the properties of a metal s~rface,~ hence there has been much research effort in this area.* In a previous paper5 we reported that the Ni particle size in silica-supported Ni catalysts can be controlled by adjusting the Ni concentration in the catalyst. The catalyst was prepared by hydrolysis of a mixed solution of ethyl silicate and nickel nitrate dissolved in ethylene glycol (the alkoxide technique). The alkoxide technique has the added advantage of preparing catalysts free from any impurities which will affect catalytic reactions. A few disadvantages have been claimed for catalysts prepared using this technique, one of them being that the metal particle size might not be controlled in the catalyst with a certain concentration of metal.In most of our work the catalysts have been prepared by the alkoxide technique using calcination and reduction temperatures around 500°C. The purpose of the present work is to study the effects of calcination and reduction temperatures upon Ni particle size in Ni/SiO, catalysts prepared by the alkoxide technique. The change in the catalytic activity for CO disproportionation (Boudouard reaction) with the change in Ni particle size is also discussed. Experimental Catalyst Preparation The catalysts employed is Ni/SiO, prepared by the alkoxide technique using ethyl silicate and nickel nitrate dissolved in ethylene glycol. The preparation procedures are summarized in scheme 1.Ni loadings in all the catalysts used are 2.5 wt YO, as revealed by X-ray fluorescence measurements. 3189 105-23190 ethylene glycol- Si( OE t)4 .D H2O h Control of Ni Particle Size v drying at 1 10 "C for 24 h i calcination in air reduction by H, Ni/SiO, In order to study the effects of calcination and reduction temperatures on Ni particle size, the catalysts were prepared by two different methods: (A) the catalysts were calcined at 500 "C in air for 4 h, followed by reduction in flowing hydrogen at 500, 700 and 900 "C for 4 h, respectively; and (B) the catalysts were calcined in air at 500,700 and 900 "C for 4 h, respectively, followed by reduction in flowing hydrogen at 500 "C for 4 h. Hydrogen was supplied from a cylinder and was passed through a cooled trap packed with molecular sieves for removal of impurities.Measurements of Ni Particle Size by T.E.M. The particle sizes of NiO in the calcined catalyst and of Ni metal in the reduced catalyst were monitored by t.e.m. (transmission electron microscopy, Hitachi H-800), operated at an accelerating voltage of 200 kV with a magnification of lo5. The size distribution curves were obtained by measuring the size of more than lo3 paorticles in each catalyst. The lower limit for the measurement of particle sizes is ca. 20 A. Measurements of Ni Particle Size by H, Chemisorption The mean particle size of Ni in the reduced catalyst was also measured by hydrogen chemisorption measurements at room temperature following the procedures proposed by Wilson and The specific surface area of Ni metal particles on theTedwed catalyst was estimated assuming the cross-section of an Ni atom to be 6.8 A2.' The degree of reduction of Ni ions to Ni metal in the reduced catalyst is required for the estimation of the specific surface area and the mean size of Ni metal particles.The saturation magnetization of the reduced catalyst was measured by a vibrating sampleJ. Chem. SOC., Faraday Trans. I , Vol. 83, part 10 ( A ) Plate 1 Plate 1. Micrographs of Ni metal particles in catalysts: (A) for catalysts first calcined at 500 "C and then reduced at (a) 500, (b) 700 and (c) 900 "C; and (B) for catalysts first calcined at (a) 500, (b) 700 and (c) 900 "C and then reduced at 500 "C. H. Tamagawa et al. (Facing p . 3191)H.Tamagawa et al. 3191 magnetometer (VSM-2, Toeikogyo Co.) with a magnetic field at 2 T and a vibrating frequency of 80Hz. The degree of reduction of Ni ions was estimated from the saturation magnetization thus obtained, assuming that the saturation magnetization of Ni metal does not depend upon the particle size, 54.6 emu g-1.8 Ni metal is, of course, a ferromagnetic substance, while NiO is anti-ferromagnetic. Decrease in Ni Metal Surface with Time of Reduction Because of sintering at high temperatures the specific surface area of Ni metal particles decreased with the time of reduction. The change in the Ni surface area with reduction time was monitored by hydrogen chemisorption measurements. The catalyst calcined at 500 "C for 4 h was reduced at 500,700 and 900 "C, respectively, in flowing hydrogen and was then evacuated for 1 h at each reduction temperature, followed by cooling to room temperature for H, chemisorption.Dependence of Ni Metal Catalytic Activity on Particle Size The change in the specific activity of the reduced catalyst for the Boudouard reaction with change in Ni particle size was studied using a closed circulation system made of glass, the total volume of the system being 272 cm3. The dried catalyst (0.5 g) was placed in a reactor made of quartz and was then calcined and reduced at the desired temperatures, followed by introduction of CO gas with an initial pressure of 220 mmHgt. The reaction was carried out at 300°C and the amounts of CO, formed and CO consumed were analysed by gas chromatography using a column packed with active carbon.The activity of the catalyst was expressed in terms of turnover frequency by measuring the amount of CO, formed during the initial 5 or 10 min of the reaction. Results Ni Particle Size measured by T.E.M. Typical micrographs of Ni metal particles in the catalysts calcined and reduced at various temperatures are shown in plate 1. The particles shown in plate 1 (a) are Ni metal particles in the catalysts calcined at 500 "C, followed by reduction at 500, 700 and 900 "C, respectively. In plate 1 (b) are shown the catalysts calcined at 500, 700 and 900 "C, respectively, followed by reduction at 500 "C in flowing hydrogen. The particles in each catalyst have a small spread of size distribution, as shown in fig. 1. The mean particle sizes were calculated using the equation d = C ni di/ni i where di is the particle size measured and ni is the number of particles with the size di.The mean particle sizes thus calculated are shown in fig. 2 and are summarized in table 1. The particle sizes of NiO in the catalysts calcined at 500, 700 and 900 "C, respectively, were also monitored by t.e.m. and the mean particle sizes are shown in fig. 3 with those of Ni metal in the catalysts subsequently reduced at 500 "C in flowing hydrogen. Ni Particle Size estimated by H, Chemisorption By measuring the amount of hydrogen chemisorbed the specific surface area of Ni metal particles was obtained since the degree of reduction of Ni ions in the catalysts have been measured previously (table 1). The degree of reduction was found to increase with t 1 mmHg x 133.3 Pa.3192 Control of Ni Particle Size 50 100 150 50 100 150 50 100 150 I I l 7 5 - ( b 1 50 - 7 - -.25 - - 0 1 1 I 50 100 150 particle size/A 75 50 25 0 75 50 25 0 50 100 150 50 100 150 Fig. 1. Particle size distributions resulting from the micrographs in plate 1 ; the notations of (A) and (B) are the same as given in plate 1. 80 - 1 Z; 60 - 0 .-I 5 LO - 20 - 0 500 700 900 T/"C Fig. 2. Mean particle sizes of Ni in the catalysts measured by t.e.m. : the abscissa gives temperature of reduction (0) and calcination (0) and the notations of (A) and (B) are the same as given in plate 1 . increasing reduction temperature but did not depend on the calcination temperature. The mean size of Ni metal particles in the reduced catalyst was calculated by the equation d = 6/pS assuming that Ni particles are spheres, where p is the density of Ni metal (8.85 g cmP3)' and S is the specific surface area of Ni metal particles in the catalyst.The mean particleH . Tamagawa et al. 3193 Table 1. Characterization of Ni particles in the catalysts (see text for catalyst treatments) calcination temperature/OC : 500 500 500 500 700 900 reduction temperature/OC : 500 700 900 500 500 500 Ni surface area/m2 g-' 88 82 81 88 83 164 d (from H, chemisorption)/A 45 55 71 45 47 24 d (from t.e.m.)/A 34 58 74 34 32 22 reduction (%) 70 80 100 70 69 69 ~~ ~ ~ ~~ 100 80 2 60 .r( a d 40 a 20 0 I I I I I I 500 700 900 T/"C Fig. 3. Mean particle sizes of NiO in the catalysts calcined at 500, 700 and 900 "C and those of Ni produced by reduction of these calcined catalysts at 500 "C. 0, NiO; 0, Ni.sizes of Ni thus obtained are summarised in table 1 and are in good agreement with those monitored by t.e.m. Sintering Mechanism of Ni Metal Particles As shown in fig. 4, the specific surface area of Ni in the silica-supported catalyst decreased with the reduction time, the reducing temperatures being 500,700 and 900 "C, respectively. In order to discuss the sintering mechanism the specific surface area of Ni was converted to the mean particle size using the equation mentioned above. In fig. 5 are shown the plots of log d us. log t at various reduction temperatures. Effect of Ni Metal Particle Size on Boudouard Reaction In order to avoid the change in the surface properties of Ni metal particles during the reaction, the amounts of CO, formed were measured during the initial 5 or 10 min of the reaction and were used to calculate the turnover frequency over the catalysts.The effects of size on the turnover frequency are shown in fig. 6. The open circles in fig. 6 represent the specific rates over the catalysts in which the sizes of Ni metal particles were controlled by Ni loading and solid circles denote the specific rates over the catalyst whose Ni particle sizes were controlled by the calcination and reduction temperatures.3194 Control of Ni Particle Size 100 F 0 7 0 5 10 15 20 reduction time/h Fig. 4. Decrease in the specific surface area of Ni with reduction time. The catalysts were first calcined at 500 "C and then reduced at 500 (O), 700 (0) and 900 "C (a).1.0 I I I 1 I 0.3 0.6 0.9 1.2 1.5 log t Fig. 5. Determination of sintering exponent n. 0, 500; 0, 700; a, 900 "C. Discussion As previously menti~ned,~ one of the features of the alkoxide technique for catalyst preparation lies in the dispersion of metal ions in the dried catalyst. The metal ions may coagulate during the calcination procedure to form the metal oxide clusters with an even size. Thus, the size of the oxide clusters is a function of the calcination temperature as well as the metal concentration in the catalyst. Although the metal oxides are generally considered to increase in size with increase in calcination temperature, a few resultsH. Tamagawa et al. 3195 0- 50 100 particle size of Ni/A Fig. 6. Change in the turnover frequency for Boudouard reaction with the change in mean particle size of Ni in the catalyst.The reaction was carried out at 300 "C. 0, particle size controlled by Ni concentration ; , particle size controlled by calcination and reduction temperatures. indicating the opposite trend have been reported." This phenomenon has been called redispersion and has been used to explain the strong metal-support interaction of the Pt/TiO,'' and Fe/Ti0,12 systems. The process of redispersion has been studied extensively13 and two probable mechanisms have been proposed: one is the actual splitting of particles into smaller ones during oxidation'* and the other is a spread of oxide particles over the support surfa~e.'~ Depending upon the volatility and stability of metal oxides either mechanism will be favourable for the redispersion of the oxide particles.As is shown in fig. 3, tiny NiO particles were well dispersed in the silica support and were not spread over the support surface. Nevertheless the melting point of NiO is not significantly high (1998 "C). The size of NiO particles as obtained by t.e.m. measurements decreased dramatically when the catalyst was calcined at 900 "C (fig. 3). This indicates that very small, stable NiO particles were produced in silica during calcination at high temperature. Ni metal particles were formed by reducing the oxidised catalyst at 500 "C in flowing hydrogen and the size of Ni particles formed was in proportion to that of NiO in the oxidised catalyst (fig. 4). Thus, the Ni particle size can be controlled by varying the calcination temperature.As shown in plate 1 (b) and fig. 1, the particle size was made almost uniform even when the catalyst was calcined at the temperature as high as 900 "C. On the other hand, the Ni particle size increased with increasing reducing temperature, as shown in plate 1 (a) and fig. 1, probably due to the sintering of Ni particles at high reducing temperature. The sintering has also been studied extensively and several mechanisms have been proposed." The mechanism involving vapour transport or particle migration has received much attention. l7 These two mechanisms could be distinguished by measuring the surface area of metal particles as a function of reduction time and by using the sintering kinetics expressed as17 dS/dt = - KS" where S is the exposed surface area of metal per unit area of support and n is the sintering exponent, which can be predicted to be between 4 and 8 for a particle-migration mechanism and to be < 2 and > 13 for a vapour-transport or atomic-migration mechanism." According to Wynblatt and Gjosteinlg the above equation can be modified to (n - 1) log (d/do) = c+ log ( t )3196 Control of Ni Particle Size where do and d are the mean particle sizes of metal particles with the reduction time of 0 and t, respectively, at a certain temperature and C is a constant. Thus, plots of log(d/do) or log(d) us.log(t) should result in straight lines from which the order of sintering, n, can be obtained.20 From the results given in fig. 5, n was found to be 5 for the present silica-supported Ni catalyst. This seems to indicate that the sintering of Ni metal particles in the present catalyst takes place via a particle-migration mechanism, although Bartholomew and Sorensen20 reported a shift in the sintering mechanism from particle migration to atomic migration with increasing temperature and time of sintering. Although the distribution curve of Ni particles in the catalyst reduced at 900 "C was broader than those reduced at 500 and 700 "C, it is still possible to say that the Ni particle size was controlled within a small range [plate ] ( a ) and fig.13. Consequently, the Ni particle size in catalysts prepared by the alkoxide technique can be controlled within a small range and the range can be varied by the catalyst calcination and reduction temperatilres as well as by Ni loading in the catalyst.These conclusions are summarized in fig. 2. The specific rates for the Boudouard reaction were compared to demonstrate the identical catalytic property of Ni particles whose sizes were controlled either by Ni concentration in the catalyst or by the calcination and reduction temperatures. The specific rate, expressed in terms of turnover frequency, increased with increasing Ni particle size (fig. 6). This indicates that the reaction between adsorbed oxygen (originating from dissociatively adsorbed CO) and adsorbed or gaseous carbon monoxide might be the rate-determining step of the reaction,21 since oxygen ions adsorbed on small Ni particles are more strongly bound to the metal atoms than those on large particles and resulted in less reactivity with gaseous or adsorbed CO to form carbon dioxide.As can be seen in fig. 6, the specific rates over Ni particles with sizes controlled either by Ni loading or by the calcination and reduction temperatures fit well on a curve. Thus, the catalytic performances of Ni particles with sizes controlled by Ni loading are identical to those controlled by calcination and reduction temperatures. References 1 J. H. Sinfelt and G. H. Via, J. Catal., 1979,56, 1 ; H. C. de Jongste, V. Ponec and F. G. Gault, J. Catal., 1980, 64, 389; X-Z. Jiang, S. A. Stevenson and J. A. Dumesic, J . Catal., 1985, 91, 11. 2 S. J. Tauster, S. C. Fung and R. L. Carten, J. Am. Chem. SOC., 1980, 100, 180; S. J. Tauster and S.C. Fung, J. Catal., 1978, 55, 29; K. Kunimori, S. Matsui and T. Uchijima, J . Catal., 1984, 85, 253. 3 M. Boudart, J. Catal., 1965, 4, 704; P. H. Otero-Scipper, W. A. Wachter, J. B. Butt, R. L. Burwell Jr and J. B. Cohen, J. Catal., 1978, 53, 414; J. R. Anderson, Structure of Metallic Catalysts (Academic Press, New York, 1975). 4 J. L. Carter, J. A. Cusumano and J. H. Sinfelt, J. Phys. Chem., 1966, 70, 2257; M. Boudart, Adv. Catal., 1969, 20, 153; M. Primet, J. M. Basset, E. Garbowski and M. V. Mathieu, J. Am, Chem. Soc., 1975,97,3655; Y. Takasu, R. Unwin, B. Tesche, A. M. Bradshaw and M. Grunze, Surf. Sci., 1978,77, 219; Y . Takai, A. Ueno and Y. Kotera, Bull. Chem. SOC. Jpn, 1983, 56, 2941. 5 A. Ueno, H. Suzuki and Y. Kotera, J . Chem. SOC., Faraday Trans. I , 1983, 79, 127; K.Tohju, Y. Udagawa, S. Tanabe and A. Ueno, J. Am. Chem. Soc., 1984, 106, 612. 6 G. R. Wilson and W. K. Hall, J. Catal., 1970, 17, 190. 7 D. G. Mustard and C. H. Bartholomew, J. Catal., 1981, 67, 186. 8 Kagaku Binran (Kiso-hen, Zr) (Chem. SOC. of Jpn, Maruzen, Tokyo, 1975), p. 1233. 9 J. R. Anderson, Structure of Metallic Catalysts (Academic Press, New York, 1975), p. 447. 10 M. F. L. Johnson and C. D. Keith, J. Phys. Chem., 1963,67,200; R. M . J. Fiedorow, B. S. Chahar and S. E. Wanke, J . Catal., 1978, 51, 193; S. Takasaki, H. Suzuki, K. Takahashi, S. Tanabe, A. Ueno and Y. Kotera, J. Chem. SOC., Faraday Trans. I , 1984, 80, 803. 11 R. T. K. Baker, E. B. Prestridge and R. L. Carten, J. Catal., 1979, 59, 293. 12 B. J. Tatarchulk and J. A. Dumesic, J. Catal., 1981, 70, 308. 13 K. Forger and H. Jaeger, J . Catal., 1981, 70, 53; 1985, 92, 64; K. Forger, D. Hay and J. Jaeger, 14 E. Ruckenstein and J. L. Malhotra, J . Catal., 1976, 41, 303, D. B. Dadyburjor, J. Catal., 1979, 57, J . Catal., 1985, 96, 154; T. Wang and L. D. Schmidt, J. Catal., 1981, 70, 187; 1980, 66, 301. 504.H. Tamagawa et al. 3197 15 E. Ruckenstein and Y. F. Chu, J. Catal., 1979, 59, 109; J. E. Stulga, P. Wynblatt and J. K. Tien, 16 Sintering and Catalysis, ed. G. C . Kuchynski (Plenum Press, New York, 1975). 17 A. Williams, G. A. Butler and J. Hammonds, J. Catal., 1972, 24, 352; J. T. Richardson and J. G. Crump, J. Catal., 1979, 57, 417; S. E. Wanke and P. C. Fynn, Catal. Rev. Sci. Eng., 1975, 12, 93; P. C. Fynn and S. E. Wanke, J. Catal., 1974, 34, 400. J . Catal., 1980, 62, 59. 18 K-T. Kim and S-K. Ihm, J. Catal., 1985, 96, 12. 19 P. Wynblatt and N. A. Gjostein, Progr. Solid State Chem., 1975, 9, 21. 20 C. H. Bartholomew and W. L. Sorensen, J. Catal., 1983, 81, 131. 21 E. G. M. Kuupers, A. J. H. M. Kock, M. W. C. M. A. Nieuwesteeg and J. W. Geus, J. Catal., 1985, 95, 13. Paper 612486; Received 30th December, 1986

ISSN:0300-9599    

DOI:10.1039/F19878303189

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

J. Chern. SOC., Furaduy Trans. I , 1987, 83 (lo), 3199-3205 Neutron Spectroscopic Study of Polycrystalline Benzene and of Benzene adsorbed in Na-Y Zeolite HervC Jobic" and Albert Renouprez Institut de recherches sur la Catalyse,? 2 Avenue Albert Einstein, 69626 Villeurbanne Cidex, France Andrew N. Fitch Department of Chemistry, University of Keele, Keele, Stafordshire ST5 5BG Hans J. Lauter Institut Laue-Langevin, 156 X , 38042 Grenoble, France The neutron inelastic spectra of solid benzene and of benzene adsorbed in Na-Y zeolite have been obtained between 200 and 2000 cm-l. The intensities of the vibrational transitions of solid benzene are calculated from a force field. Multiphonon processes and an isotropic Debye-Waller factor are also included. After adsorption in Na-Y, two benzene species are observed.The stretching modes of benzene relative to the S,, sodium ions are assigned at 230 and 270 cm-l. Small frequency shifts indicating a weak perturbation of the benzene ring are observed. The vibrational spectrum of benzene has been the subject of numerous optical studies with the aim of obtaining an harmonic force field.' There have been also a few inelastic neutron spectroscopic (i.n.s.) studies which illustrate the major improvements which have been made to the resolution over the past few In this work we present new data for polycrystalline benzene obtained with good resolution and we show that the inelastic neutron spectrum can now be simulated with great accuracy. The adsorption of benzene on zeolites has been followed by several methods : infrared Raman spectroscopy,12 U.V.diffuse reflectance spectroscopy,8*13 n.m.r.,14-18 neutron diffra~tion'~ and small-angle neutron scattering.20 Two types of adsorbed benzene in Na-Y were recently identified by powder neutron diffra~tion.'~ The first benzene molecule (Bzl) is bonded via its n electrons to the S,, sodium ion. The second molecule (Bz2) is centred in the window formed by the 12-membered oxygen ring, between adjacent supercages. The first type of benzene molecule is already well known, and the second type, more unusual, has been corroborated by a comprehensive infrared study." We report in this paper a comparison of the inelastic neutron spectra of solid benzene and of benzene adsorbed in Na-Y. Two types of adsorbed benzene molecules are identified, in agreement with the neutron diffraction work.Experiment a1 The inelastic neutron spectrum of polycrystalline benzene was recorded after quenching a thin sample of liquid (0.2 mm) to 5 K; the container was a rectangular aluminium cell. To study the adsorption of benzene in Na-Y (SK-40), a background run was first performed with the bare zeolite. The zeolite was first pretreated in a glass reactor: the temperature was raised to 700 K under oxygen and then the zeolite was evacuated at 3199 t Laboratoire propre du CNRS, conventionne a 1'Universite Claude Bernard, Lyon I.3200 Neutron Spectra of Benzene in Na-Y Zeolite 720 K for 40 h. It was transferred to a cylindrical aluminium container in a glove box and placed in a helium cryostat. After this measurement, the zeolite was warmed to 300 K and 0.168 cm3 g-l of benzene was adsorbed on the zeolite, which corresponds to an average number of 3 benzene molecules per supercage.The sample was left for 24 h at 330 K to equilibrate and then reintroduced into the cryostat. It took several hours to cool the whole sample, since we used a large quantity of zeolite (9 g). The first runs clearly showed that the benzene molecules were not rigidly fixed, i.e. the Debye-Waller factor was small. The final temperature cannot be given with precision; it is estimated as 20f 10 K. The inelastic neutron spectra were recorded with the beryllium-filter detector spectrometer IN1 B, at the Institut Laue-Langevin in Grenoble. The frequency range 200-1800 cm-l was covered with one monochromator plane, the Cu (220).The frequencies quoted in the paper have been corrected for the beryllium-filter transmission function and are given to a precision of 15 cm-'. The instrumental resolution is ca. 35 cm-l below 800 cm-'; above this frequency, AE/E is better than 5 %, at least up to 1500 cm-l. The resolution function used to simulate the inelastic neutron spectrum of benzene includes the instrumental resolution and the intrinsic width of the modes. A better resolution can be obtained with this spectrometer, but at the expense of the signal intensity. 21 Results and Discussion Polycrystalline Benzene The inelastic neutron spectrum of polycrystalline benzene, recorded at 5 K, is shown in fig. 1 (a). When compared to the spectrum obtained with the old version of IN1 B [see for example ref.4(b)], it can be seen that the resolution has been much improved, especially at high energy transfers. This is due to improved instrumental resolution, since the old (220) reflection of Cu was badly defined. It is also due to the use of a lower sample temperature ( 5 K instead of 80 K), which sharpens the fundamentals by decreasing the effect of the Debye-Waller factor.22 The C-H stretching region is not reproduced in fig. 1 (a) because only one broad band is obtained in this frequency range [e.g. ref. (23)]. Two calculated spectra are also presented in fig. 1. Spectrum (c) was calculated using the one-phonon approximation. 23 The Debye-Waller factor, whether due to internal vibrations or to external modes, was not introduced at this stage since calculation of this factor without consideration of multiphonon processes can lead to erroneous results.The intensities of the peaks at frequencies coA, corresponding to the normal modes A, are related to the proton displacements, which can be obtained from force-field calculations. Numerous force fields have been proposed for the free benzene molecule, recent ones having been reviewed by Pulay et al.' We have tried several force field^^*-^' and have found only small differences between the calculated spectra. This is because all authors used the same vibrational assignments, and thus differences in the force constants imply only small variations in the hydrogen mean-square amplitudes (u'). The spectrum presented in fig. 1 (c) was calculated with the force field of La Lau and Snyder.25 We have used in the calculation the experimental frequencies observed in the liquid phase, and thus small shifts due to the intermolecular forces can be observed in our solid sample.The splittings which have been reported for the internal crystal modes, by optical are smaller than our instrumental resolution and are thus not resolved. The agreement with the experimental spectrum, fig. 1 (a), is satisfactory, but several features are not reproduced: the very broad background centred at ca. 1100 cm-' and extending from ca. 650 to ca. 1700 cm-l, and also the sidebands indicated by the arrows in fig. 1 (a) at 490, 780 and 1260 cm-'. These sidebands correspond to inelastic two- phonon down-scattering processes, from the lattice modes peaked at ca.80 cm-1,395 and from the internal modes corresponding to v16, vll and v9+v15, respectively.H. Jobic, A. Renouprez, A . N . Fitch and H. J. Lauter Elcm-' 500 1000 1500 320 1 15000 - ul I -w I 1 10000- E 2 + 5000- 0 ' I I I I I 1 500 Elcm-' 1000 1 Spo 40 50 10 20E/TH~ 30 I 0.34 R h 0.5 3 0.4 5 0.3 -w .r( v .3 ,x 0.2 5 0.1 .Ei 0 10 20 30 40 50 EITHz Fig. 1. Inelastic neutron spectra of polycrystalline benzene : (a) experimental, two-phonon peaks are indicated by arrows; (b) calculated with multiphonon processes; ( c ) calculated using the one- phonon approximation. It has been shown28*29 that because of the convolution of the internal modes with the lattice modes, the intensity of the fundamentals is progressively transferred to the sidebands as the neutron momentum transfer Q or the temperature T increases.When the lattice Debye-Waller factor (DW) is very small, which may happen for high Q or high T, and with the intermediate resolution which was available with the old version of IN1B,23 it was shown that the shape of the high-frequency modes was Gaussian and that it was multiplied only by the DW due to the internal modes. However, with the new version of INlB, as for other recently built spectrometer^,^^ and at low temperatures, several sidebands can be resolved from the fundamentals, even at high Q values. In order to simulate these multiphonon effects, we have written an approximate method which takes into account the phonon processes involving the internal modes with one or two3202 c 9 10000- 00 2 G 5000- A 2 Neutron Spectra of Benzene in Na-Y Zeolite Elcm-' 500 1000 1500 I 40000- S 30000- s G 00 g 20000- 10000 - 10 2o E/THz3' 40 50 500 E l m - ' 1000 1500 - 1 I I I I I 10 20 30 40 50 5 0 ' EITHz Fig.2. Inelastic neutron spectra of (a) benzene adsorbed on Na-Y and (b) the bare zeolite. external phonons22 (for the benzene molecule, the combinations or harmonics of the internal modes have a lower intensity). The frequency distribution function of the external modes was simulated using published data for solid b e n ~ e n e . ~ , ~ An isotropic B factor defined as B ='8-n2/3 (ul1 + u,, + ~ 4 ~ ~ ) is introduced for the calculation of the Debye-Waller factor, D = exp (- Q2B/8-n2). The contributions from the internal and external vibrations are separated.Mean values for the total B factor of a hyddogen atom of benzene h$ve been determined by neutron diffraction at 218 K (6.5 A,) and at 138 K (3.87 but not at 5 K. For the contribution by the external modes a value of 0.8 A2 is estimated from the ratio of the peak at 405 cm-l to that of the high-frequency sideband centred at 490 cm-', since this ratio corresponds t? D/( 1 - D).29 Including the internal-mode contribution, this gives a total value of 1.9 A2 for B. The resulting spectrum is shown in fig. 1 (h). The agreement with the experimental spectrum of fig. l ( a ) is excellent if one takes into account that higher multiphonon terms yield a background smoothly rising with energy. Benzene adsorbed on Na-Y The inelastic neutron spectrum of benzene adsorbed on Na-Y is shown in fig.2(a). The inelastic neutron spectrum of the bare zeolite [fig. 2(h)] has been subtracted. Since both spectra are shown on the same scale, it can be seen that the background due to the zeolite is fairIy flat, unlike the infrared spectrum, and thus all the vibrational modes of the adsorbed species can be observed in that range. (The small peak at ca. 275cm-' corresponds to phonons of the aluminium container.)H . Johic, A . Renouprez, A . N . Fitch and H . J . Lauter 3203 Comparison of the Inelastic Neutron Spectra of Solid and Adsorbed Benzene If one compares fig. 2(a) with fig. 1 (a), several differences can be noted. A splitting of the out-of-plane C-H bending mode v l l of 25 cm-' is observed in fig. 2(a); it is at the limit of the resolution but it was present in two successive runs.For the same mode, a splitting of 28 cm-l has already been observed in the infrared,' and we make basically the same assignment as these authors. The lower-frequency band at 700 cm-l, which is the least perturbed compared to the free molecule, can be assigned to benzene in the 12-ring window. The high-frequency peak at 725 cm-' is assigned to benzene bonded to the S,, sodium ions. A shift to higher frequency of v l , is well known in the formation of metal n-complexes [e.g. v , , shifts to 785 cm-' in ($-C,H,) C ~ ( C O ) , , ] ~ ~ or on metal surfaces [e.g. it shifts to 745 cm-' on Ni( 1 1 I)]."" This is due to an increase in the diagonal force constant and to changes in the interaction force constants.The other out-of-plane C-H bending modes are thus also expected to be modified. Indeed, vl0, v5 and v,, shift to higher frequency upon adsorption [fig. 2(cr)], the shift for vs and v,, being larger than for vlo. On the other hand vl,, whose potential-energy distribution mainly corresponds to ring torsion, has the same frequency as in the liquid or solid: 405 cm-'. Other peaks at 608 cm-l (v,) and at I 168 cm-' (vls + v,), which correspond to in-plane modes, do not shift in frequency within experimental error, which seems to indicate that the C-C stretching force constant is hardly modified. The preceding results are in agreement with the literature, but we are able to observe two bands at 230 and 270cm-', which were not measured by infrared or Raman spectroscopy.Since the lowest internal mode of benzene is v,,, at 405 cm-I, these bands are assigned to modes of vibration of benzene relative to the zeolite. We know from previous infrared work that the benzene species bonded to the sodium ions (Bzl) is more strongly held than the Bz2 molecule: it appears at lower coverages'' and it has a higher thermal stability.' A higher site occupancy has also been found for the Bzl species by neutron diffraction.19 From studies of the vibrational spectra of arene complexes, e.g. ($ - C,H,)Cr(CO),, the deformations and torsion are generally found at low frequency (< 150 cm-'), whereas the stretching modes are at higher frequency (> 200 cm-l). For these reasons we assign the two bands at 230 and 270 cm-' to the symmetric and antisymmetric stretching modes of benzene relative to the Na+ ion.We expect that the hindered translations and rotations of the Bz2 molecules in the 12-ring windows occur at frequencies < 200 cm-'. This assignment will be tested in a future experiment, at low benzene coverage. The perturbation of the ring and the strength of the sodium-benzene bond are thus weaker than in arene complexes, e.g. in ($ - C,H,)Cr(CO),. Another interesting feature in the inelastic neutron spectrum of adsorbed benzene is the presence of a sideband on the high-frequency side of v,, (and also vll). It arises, as in the solid, from two-phonon scattering, from an internal mode and from the external modes. The existence of this sideband is an indication of interactions between neighbouring molecules and between the benzene molecules and the zeolite (but with a smaller neutron intensity if a cluster of molecules is considered).This is visualised in fig. 3 by comparing the crystal structure of benzene34 with a proposed cluster of benzene molecules within the cavioties of the zeolite. In the solid the molecules are at right angles with contacts of ca. 2.6 A. In Na-Y, where two types of benzene molecules ?re seen,,' the Bzl molecules are at an angle of 109.5" and with contact distances of 2.9 A, yhereas the Bzl and Bz2 molecules are at an angle of 70.5" with average contacts of 2.6 A. This clustering has been postulated from small-angle neutron scattering studies at a coverage of 2.5 molecules per supercage,20 and we have a coverage in this work of 3 molecules per supercage.The mutual attraction between the benzene molecules has also been demonstrated by neutron diffraction at high coverage. l9 The fact that the sidebands3204 Neutron Spectra of Benzene in Na-Y Zeolite Fig. 3. (a) Cross-section through Na-Y showing clustering of benzene molecules ; (b) crystal structure of benzene [from ref. (34)]. are observed at lower frequency than in the solid, e.g. 468 cm-l instead of 490 cm-l, indicates that the intermolecular forces are decreased. This is consistent with a lower Debye-Waller factor, as can be seen from the ratio of the peak at 405 cm-l to that of the sideband centred at 468 cm-l. When compared to the solid, this means that :he contribution of the external modes to the total B factor has almost doubled (1.3 A2), which is due to a large increase in the translational and librational motions of the benzene molecule. Thiso is in agreement with our diffraction work,lg where a mean value for the B factor of 5.8 A2 was found for a deuterium atom at 4 K and for high coverage. This value, which takes into account the effect of fractional site occupancies of benzene, is larger than that deduced from inelastic scattering because it includes both positional and dynamic disorder.Conclusions In this study a comparison of the inelastic neutron spectra of solid and adsorbed benzene has been made. In Na-Y zeolite two types of adsorbed benzene molecules have been found for a coverage of three molecules per supercage. This is in agreement with ourH . Jobic, A .Renouprez, A . N . Fitch and H. J . Lauter 3205 previous neutron diffraction work.lg In the near future we will perform another experiment with better resolution and at a lower coverage, where it is known that the predominant species is that bonded to the sodium ions. It will then be possible to perform a normal-coordinate analysis of this species since all the vibrational modes, including the modes of benzene relative to the sodium ions, can be observed. We thank G. Clugnet for assistance during the preparation of the samples. References I P. Pulay, G. Fogarasi and J. E. Boggs, J. Chem. Phys., 1981, 74, 3999 and references therein. 2 K. W. Logan, S. F. Trevino, H. J. Prask and J. D. Gault, J. Chem. Phys., 1970, 53, 3417. 3 E. L. Bokhenkov, V. G. Fedotov, E.F. Sheka, I. Natkaniec, M. Sudnik-Hrynkiewicz, S. Califano and 4 (a) H. Jobic, J. Tomkinson, J. P. Candy, P. Fouilloux and A. Renouprez, Sure Sci., 1980, 95,496; (b) 5 J. Anderson, J. J. Ullo and S. Yip, Physica, 1986, 136B, 172. 6 V. N. Abramov, A. V. Kiselev and V. I. Lygin, Russ. J. Phys. Chem., 1963, 37, 613. 7 C. L. Angel1 and M. V. Howell, J. Colloid Interface Sci., 1968, 28, 279. 8 M. Primet, E. Garbowski, M. Mathieu and B. Imelik, J . Chem. SOC., Faraday Trans. I, 1980, 76, 9 J. Datka, J . Chem. Soc., Faraday Trans. I, 1981, 77, 511. R. Righini, Nuovo Cimento B, 1978, 44, 324. A. Renouprez, G. Clugnet and H. Jobic, J. Catal., 1982, 74, 296. 1942. 10 B. Coughlan, W. M. Carroll, P. O’Malley and J. Numan, J. Chem. Soc., Faraday Trans. I, 1981, 77, 3037. 11 A.de Mallmann and D. Barthomeuf, J. Chem. Soc., Chem. Commun., 1986,476; Proc. 7th Int. Zeolite Conf., ed. Y. Murakami, A. Iijima, J. W. Ward, (Kodansha-Elsevier, Tokyo-Amsterdam, 1986), p. 609. 12 J. J. Freeman and M. L. Unland, J. Catal., 1978, 54, 183. 13 M. L. Unland and J. J. Freeman, J . Phys. Chem., 1978, 82, 1036. 14 M. Nagel, D. Michel and D. Geschke, J . Colloid Interface Sci., 1971, 36, 254. 15 H. Lechert, W. Haupt and K. P. Wittern, J. Catal., 1976, 43, 356. 16 V. Yu. Borovkov, W. K. Hall and V. B. Kazanski, J. Catal., 1978, 51, 437. 17 A. Germanus, J. Karger, H. Pfeifer, N. N. Samulevic and S. P. Zdanov, Zeolites, 1985, 5, 91. 18 D. L. Hasha, V. W. Miner, J. M. Garces and S. C. Rocke, A.C.S. Symp. Ser., ed. M. L. Deviney and J. L. Gland (Am. Chem. SOC., Washington, 1985), vol. 288, p. 485. 19 A. N. Fitch, H. Jobic and A. Renouprez, J . Phys. Chem., 1986, 90, 13 11. 20 A. Renouprez, H. Jobic and R. C. Oberthur, Zeolites, 1985, 5, 222. 21 H. J. Lauter and H. Jobic, Chem. Phys. Lett., 1984, 108, 393. 22 H. Jobic and H. J. Lauter, unpublished work. 23 H. Jobic, R. E. Ghosh and A. Renouprez, J . Chem. Phys., 1981, 75, 4025. 24 J. R. Scherer, Spectrochim. Acta, 1964, 20, 345; Spectrochim. Acta, Part A , 1967, 23, 1489. 25 C. La Lau and R. G. Synder, Spectrochim Acta, Part A , 1971, 27, 2073. 26 J. Favrot, P. Caillet and M. T. Forel, J. Chim. Phys., 1974, 10, 1337. 27 G. Taddei, H. Bonadeo, M. P. Marzocchi and S. Califano, J. Chem. Phys., 1973, 58, 966. 28 A. Griffin and H. Jobic, J . Chem. Phys., 1981, 75, 5940. 29 M. Warner, S. W. Lovesey and J. Smith, Z. Phys., Teil B, 1983, 51, 109. 30 J. Penfold and J. Tomkinson, Rutherford Appleton Lab. Rep., RAL-86-019 (1986). 31 G. E. Bacon, N. A. Curry and S. A. Wilson, Proc. R. SOC. London, Ser. A, 1964, 279,98. 32 H. Jobic, J. Tomkinson and A. Renouprez, Mol. Phys., 1980, 39, 989. 33 H. Jobic, B. Tardy and J. C. Bertolini, J . Electron Specrrosc. Relat. Phenom., 1986, 38, 55. 34 E. G. Cox, D. W. J. Cruickshank and J. A. S. Smith, Proc. R. Soc. London, Ser. A , 1958, 247, I . Paper 71054; Received 12th January, 1987

ISSN:0300-9599    

DOI:10.1039/F19878303199

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. 1, 1987, 83 (lo), 3207-3222 Hydrogen- 1 Nuclear Magnetic Resonance, Differential Thermal Analysis, X-Ray Powder Diffraction and Electrical Conductivity Studies on the Motion of Cations, including Self-diffusion in Crystals of Propylammonium Chloride and Bromide as well as their N-Deuterated Analogues Shin-ichi Fukada, Hideaki Y amamoto, Ryuichi Ikeda and Daiyu Nakamura" Department of Chemistry, Faculty of Science, Nagoya University, Chikusa, Nagoya 464, Japan 'H N.m.r. measurements and differential thermal analyses have been performed on solid propylammonium chloride and bromide, including their N-deuterated analogues. X-Ray powder diffraction and electrical con- ductivity were measured for the highest-temperature solid phase of NH,(n- C,H,)Cl. The corresponding highest-temperature solid phase of the bromide was determined. These salts have three solid phases below the respective melting temperature. In the highest-temperature phase, having a structure consisting of cation and anion layers, it was found for the chloride that the cations perform self-diffusion in two-dimensional layers.A non-linear log TI us. T1 curve observed in the room-temperature phase of propyl- ammonium chloride and bromide could be well explained by assuming a non-Arrhenius type thermal process for the random reorientation of the cation along the cationic chain axis. The broad Tl minimum of NH,(n- C,H,)Br observed in its low-temperature phase has been assigned to two closely spaced Tl minima due to the C, reorientation of the CH, and NH; groups.Motional parameters for the above cationic motions in the three phases of the present salts have been evaluated, and the features of these motions are discussed. n-Alkylammonium halides with shorter alkyl chains than C,, have been reported mostly to form tetragonal crystals belonging to space group P4/nmm around room temperature. The structure of these crystals consists of two-dimensional layers made by alkylammonium cations and halide anions stacked alternately. In this structure the cations, with regard to their orientation along the chain- or long-axis, are dynamically disordered and are packed between the layers of anions keeping their long-axes perpendicular to the layers. Previously we reported2 that methylammonium iodide, which forms the same tetragonal crystals at room temperature, transforms into CsCl- type cubic crystals with increasing temperature.In this highly symmetric phase the cations are in a more dynamically disordered state in which they perform self-diffusion through the crystalline lattice as well as isotropic rotation about their respective centres of gravity; both types of motion can be observed on an n.m.r. timescale. Tsau and Gilson, carried out measurements of differential scanning calorimetry (d.s.c.) on propylammonium chloride, NH,(n-C,H,)Cl, and found that the tetragonal crystals undergo a phase transition at 408 K with increasing temperature, with the entropy of transition AStr of 10.2 J K-l mol-l, and a high-temperature phase is formed. The value of AStr is considered to be unusually large since it is comparable to the melting entropy AS, of 13.4 J K-' mol-' obtained at 439 K by heating NH,(n-C,H,)Cl in the high-temperature phase.3 The large AStr reported suggests the onset of new cationic 32073208 Hydrogen- 1 N.M.R.of Solid Propylammonium Chloride and Bromide motions in the high-temperature phase, where a more disordered structure is possibly realized in consequence. To obtain detailed information about the dynamical behaviour of the cation in crystals of NH,(n-C,H,)Cl and NH,(n-C,H,)Br over a wide temperature range, the present investigation has been undertaken by examining lH n.m.r., differential thermal analysis (d.t.a.), X-ray powder diffraction and electrical conductivity. Experimental Crystals of NH,(n-C,H,)Cl and NH,(n-C,H,)Br were prepared by neutralizing an aqueous solution of propylamine with hydrochloric acid and hydrobromic acid, respectively, and gradually evaporating water.The crystals thus obtained were recrystallized twice from an ethanol-diethyl ether mixture. Colourless thin plates were obtained for both salts. Partially deuterated analogues, ND,(n-C,H,)Cl and ND,(n- C,H,)Br were prepared by performing repeated crystallizations of purified NH,(n-C,H,)Cl and NH,(n-C,H,)Br, respectively, from heavy water. 'H N.m.r. absorptions4 and d.t.a.5 measurements were carried out using apparatus already reported. The 'H n.m.r. spin-lattice relaxation time TI was measured at the Larmor frequencies of 10.5, 20.0 and 45.5 MHz using pulsed n.m.r. spectrometers described elsewhere.'.The 'H spin-lattice relaxation time in the rotating frame Tlp was measured at 32 MHz for NH,(n-C,H,)Cl under a spin-locking radio frequency magnetic field of 3.7 G.8 The a.c. electrical conductivity of NH,(n-C,H,)Cl was determined at various temperatures using a previously reported homemade apparatusg operating at 1 kHz. The sample pellets were prepared by pressing pulverized crystals. X-ray powder diffraction patterns of NH,(n-C,H,)Cl were recorded at ca. 400 and 420 K employing a Shimadzu model VD- 1 A diffractometer equipped with a Cu anticathode. Results D.t.a. curves recorded for NH,(n-C,H,)Cl and NH,(n-C,H,)Br between ca. 100 K and a temperature above each melting temperature (T,) are shown in fig. 1 and 2, respectively. Their partially deuterated analogues showed approximately the same curves, which were shifted to higher temperatures by deuteration.NH,(n-C,H,)Cl underwent two solid-solid phase transitions at 188 and 406 K, and melted at 436 K. These qr and T, values agree well with those of 188 and 408 K for qr and 439 K for T, reported by Tsau and Gilson3 in their d.s.c. study. As shown in fig. 1, ND,(n-C,H,)CI yielded analogous d.t.a. curves to those of NH,(n-C,H,)Cl. The higher-temperature phase of NH,(n-C,H,)Cl obtained by heating to above 406K could easily be supercooled down to ca. 350 K, while that of ND,(n-C,H,)Cl was supercooled only to ca. 390 K, around which temperature we observed a sudden exothermic transition to the room-temperature phase. When NH,(n-C,H,)Br was heated, a single solid-solid phase transition was recorded at 163 K, a temperature much higher than Tr of 138 K reported by Tsau and Gilson., T, of 457 K observed in the present study agrees well with that of 456 K reported by the same authors.When the melt of NH,(n-C,H,)Br was cooled, it solidified at 419 K. Below this temperature another exothermic peak appeared around 385 K, indicating the appearance of a new solid phase between ca. 385 and 419 K. This phase was so unstable that a sudden exothermic transition to the room-temperature phase took place at any temperature between them when any small shock was given to the sample tube. ND,(n- C,H,)Br exhibited quite similar d.t.a. curves to those of NH,(n-C,H,)Br but Kr and T, differed slightly from those of NH,(n-C,H,)Br. The three solid phases observed in these salts are referred to hereafter as phases I, I1 and 111 for the high-, room- and low- temperature phases, respectively. Temperature dependences of the second moment Mz of lH n.m.r.absorptions inS. Fukada et al. 3209 ca. 351 K (381 -403K 1 436K ( 44OK 1 188 K (188K) I I I1 liquid 1 exo I11 ca.100K - I 188 K (188K) 406K 436K (413K)(44OK) Fig. 1. The d.t.a. curve of NH,(n-C,H,)Cl recorded between ca. 100 K and ca. 470 K. Transition temperatures determined for the partially deuterated analogue, ND,(n-C,H,)Cl, are given in parentheses. 370-400K ~ 1 9 ~ (ca.409K) (421K1 162 K (161 K ) ca.100K 111 I1 liquid 163 K (168K) I 457K (470K) Fig. 2. The d.t.a. curve of NH,(n-C,H,)Br recorded between ca. 100 K and ca. 490 K. Transition temperatures determined for the partially deuterated analogue, ND,(n-C,H,)Br, are given in parentheses.NH,(n-C,H,)Cl and NH,(n-C,H,)Br along with their partially deuterated analogues are shown in fig. 3 and 4, respectively. Values of M, for NH,(n-C,H,)Cl and ND,(n-C,H,)Cl were observed between room temperature and ca. 430 K. Values of M2 for NH,(n-C,H,)Cl in phase I1 were almost constant but decreased slightly from 5.5 to 5.0 G2 on increasing the temperature to Tr = 406 K. Above 406 K M2 decreased to ca. 1 G2. With increasing temperature further, a gradual decrease in M, was observed in phase I. When phase I of NH,(n-C,H,)Cl was cooled to qr (ca. 350 K) for phase 11, M , increased slowly to ca. 2.5 G2. In phase I13210 Hydrogen- 1 N.M.R. of Solid Propylammonium Chloride and Bromide N \ 0 s 5 350 LOO o L i k o ' I I ' I T / K ' I I ' I I ' ' Fig.3. The 'H n.m.r. second moments of NH,(n-C,H,)Cl and its partially deuterated analogue, ND,(n-C,H,)Cl, determined above room temperature. The results of measurements carried out with increasing temperature for NH,(n-C,H,)Cl and ND,(n-C,H,)Cl are indicated by 0 and 0, respectively, whereas those obtained with decreasing temperature for NH,(n-C,H,)Cl are indicated by 0. q,(a) and qr (b) refer to NH,(n-C,H,)Cl, while K, (c) corresponds to ND,(n- C,H,)Cl. 15 10 5 0 100 200 300 T/K 400 Fig. 4. The 'H n.m.r. second moments of NH,(n-C,H,)Br (0) and ND,(n-C,H,)Br(n) observed with increasing temperature. Those determined for NH,(n-C,H,)Br by decreasing the temperature from the temperature of solidification, q, are shown by A.below qr, M, showed the same temperature dependence as that observed on the heating run. Tsau and Gilson" reported M, of 5.0 and 2.0 G2 for NH,(n-C,H,)Cl at 409 and 4 15 K, respectively. These values roughly agree with those observed here in each phase. M, of ND,(n-C,H,)Cl also gradually decreased from 3.8 to 3.2 G2 with increasing the temperature from 295 to 405 K. Around Kr of 413 K, M , decreased significantly to 1.0 G2. In phase I, a gradual decrease in M , was also observed on heating as in the case of NH,(n-C,H,)Cl. Temperature dependences of M, in NH,(n-C,H,)Br and ND,(n-C,H,)Br were determined in the ranges 106-441 and 114-416 K, respectively. In NH,(n-C,H,)Br,S. Fukada et al. 321 1 I I I I 1 10 1 2 4 1 0 3 ~ / ~ 5 Fig. 5.The temperature variation of 'H n.m.r. spin-lattice relaxation time observed for NH,(n- C,H,)CI at two Larmor frequencies, 20.0 (@) and 45.5 Mz (A), and its partially deuterated analogue, ND,(n-C,H,)Cl, at 20.0 MHz (0). Measurements were made mainly in phase I1 of these salts. The Solid line indicates the best fitted calculated curve by assuming non-Arrhenius type temperature dependence for the correlation time of the cationic axial reorientations (see text). M , of ca. 14 GZ was observed at 106 K. When phase 111 was warmed, M, decreased to ca. 11 G2 at ca. 160 K near qr of the transition IIT -+ IT. Around 160 K, M, decreased to 5.3 G2 upon increasing the temperature. With further heating, a gradual decrease in M, was observed in phase I1 and M, of 4.0 G2 was obtained at ca.440 K, the highest temperature attained in this phase. In phase I, prepared by cooling melted NH,(n-C,H,)Br, M , of 1.9 G2 was observed at 417 K just below the temperature of solidification. In this phase, M , increased gradually with decreasing temperature. Below Kr (I -+ TI), this sample exhibited M, values similar to those determined on the heating run. ND,(n-C,H,)Br showed an analogous temperature dependence of M, to that of NH,(n-C,H,)Br. When the temperature was raised from 180 to 416 K in phase 11, M , decreased gradually from 4.7 to 2.9 G2. M, could not be determined for phase I of ND,(n-C,H,)Br because this phase could only be obtained in a narrow temperature range and was extremely unstable. Fig. 5 shows the temperature dependence of 'H q measured for NH,(n-C,H,)Cl at 20.0 and 45.5 MHz and ND,(n-C,H,)Cl at 20.0 MHz in the range 200 K-T,.The temperature dependences of q determined at 10.5, 20.0 and 45.5 MHz are shown in fig. 6 for phase I of NH,(n-C,H,)Cl. The q data observed in phase I1 of both salts agree well with those measured at 25.3 MHz by Albert and Ripmeester.l' q values for phase I of NH,(n-C,H,)Cl increased with increasing temperature in a similar manner to those for phase 11, while the temperature gradients of log us. T1 curves obtained for phase I32 12 Hydrogen- 1 N.M. R . of Solid Propylammoniurn Chloride and Bromide 350 2.3 2.5 2.7 103 KIT Fig. 6. The temperature dependence of 'H n.m.r. spin-lattice relaxation times q and qp for phase I of NH,(n-C,H,)CI. qp was measured at 32 MHz under a radiofrequency magnetic field of 3.7 G.q was measured at 45.5 (A), 20.0 (a) or 10.5 MHz (A). The solid line on the plots indicates the relaxation time Trot originating from a cationic rotational mode and that of the cationic self-diffusion (q,,,,) determined at 10.5 MHz is shown by W and the broken line. The solid line on the qp plots is the best fitted line for the linear portion of the qp data. became steeper than those for phase 11. & values for phase I of ND,(n-C,H,)Cl were about twice those of NH,(n-C,H,)Cl. Temperature dependences of 'H q determined for NH,(n-C,H,)Br at 20.0 and 45.5 MHz and for ND,(n-C,H,)Br at 20 MHz are shown in fig. 7. Each analogue yielded a minimum in phase 111, the values of which were 24 and 35 ms observed at ca.125 K for NH,(n-C,H,)Br and at ca. 120 K for ND,(n-C,H,)Br, respectively. At qr (III+ 11), q of both salts increased discontinuously. In phase 11, almost the same q values were obtained for both analogues, which increased with increasing temperature in a similar way to those of phase I1 for NH,(n-C,H,)Cl and ND,(n-C,H,)Cl. values for phase I, observed in a narrow temperature range for each analogue, were about one fifth of those obtained for phase I1 of NH,(n-C,H,)Br and about one half of those for ND,(n-C,H,)Br. These q values for phase I showed a tendency to increase with increasing temperature. Electrical conductivity, 0, observed for phases I and I1 of NH,(n-C,H,)Cl is shown inS. Fukada et al. 3213 Fig. 7. The temperature dependence of 'H n.m.r. spin-lattice relaxation time observed for NH,(n-C,H,)Br at 20.0 (e) and 45.5 MHz (V) and its partially deuterated analogue, ND,(n- C,H,)Br, at 20.0 Mz (0).The solid line is the best fitted curve calculated by assuming a non- Arrhenius type thermal process for the correlation time of the cationic rotational motion (see text). fig. 8 . At Tr (II+I) the conductivity increased by three orders of magnitude and large values of ca. S m-l were found in phase I. Discussion Crystal Structure King and L i p s ~ o m b l ~ . ~ ~ studied phases I1 and I11 of NH,(n-C,H,)Cl and phase I1 of NH,(n-C,H,)Br by single-crystal X-ray diffraction. Phase I11 of NH,(n-C,H,)Cl forms monoclinic crystals belonging to space group Gh(C2/m) with 2 = 2, in which the carbon and nitrogen atoms of the cation can be located at fixed positions thus suggesting that the structure is not disordered.On the other hand, phase I1 of both salts form tFtragona1 crystalso [space group Dih(P4/nmrn) ; 2 = 21 haying lattice constaonts a = 6.220 A and c = 7.377 A for NH,(n-C,H,)Cl, and a = 6.497 A and c = 7.380 A for NH,(n- C,H,)Br and containing cations disordered about their long axes. The arrangement of heavy atoms in the unit cell of phase I1 can be assumed by using the lattice parameters and is shown as (a) in fig. 9. To obtain information about the structure of phase I of NH,(n-C,H,)Cl, X-ray powder diffraction experiments were carried out at higher temperatures. The powder3214 Hydrogen- 1 N.M.R. of Solid Propylammonium Chloride and Bromide I I I 0 - I E m 1 2.4 2.5 103 K/T 0 0 0 0 0 Fig.8. The temperature dependence of the electrical conductivity, 0, observed for NH,(n-C,H,)Cl at higher temperatures. Experimental results obtained with increasing and decreasing temperatures are shown by and 0, respectively. patterns observed Ft 420 & 5 K coulc be well explained as arising from a tetragonal unit cell with a = 5.23 A and c = 11.70 A. The diffraction angles observed are shown in table 1 together with those calculated using the above lattice constants and the Miller indices assigned. The agreement between observed and calculated angles is quite good. The lattice constants obtained for phase I suggest that the arrangement of ions is different from that of phase I1 because of a large difference in c values between the two phases, although the crystals of both phases have the same tetragonal symmetry.It has been reported’ that there are many alkylammonium halides with normal alkyl chains, which form tetragonal crystals around or above room temperature. All these crystals have the same space group, P4/nmm, whereas their structures can be classified into two groups with different arrangements of ions in each unit cell. As an example of one group, designated the ‘a-form,’ the structure of phase I1 of NH,(n-C,H7)C1 is shown in fig. 9. Crystals of the high-temperature phase of methylammonium bromide4 and the room-temperature phases of NH3(n-C3H,)Br,l2 NH,(n-C,H,)I’* and NH,(CH,)C1,15* l6 for example, are known to have the structure of the a-form. The structure of the other group, the ‘a’-form,’ is shown in fig.9 for butylammonium bromide as an example. The Same structure can be found for the room-temperature phases of NH,(CH,)Br,’7* l8 NH,(CH,)I,” butylammonium halides,lg and many other n-alkylammonium halides with longer n-alkyl chains.19 By comparing the lattice constants determined for phase I of NH,(n-C,H,)Cl withS. Fukada et al. 321 5 L a (a) (a‘) Fig. 9. The arrangement of heavy atoms in the unit cells of NH,(n-C,H,)Cl and NH,(n-C,H,)Br in their a- and a’-forms, respectively. All cations are orientationally disordered about their long axes. Table 1. Observed and calculated 28 values of X- ray powder diffraction patterns recorded at ca. 420 K for NH,(n-C,H,)Cl 15.15 w 18.60 m 22.87 m 24.07 vs 25.26 s 28.56 w 33.49 vw 34.30 vs 35.15 s 39.25 w 41.53 m 49.20 vw 53.10 vw 54.90 vw 15.14 18.58 22.80 24.06 25.25 28.55 33.39 34.29 35.16 39.28 41.58 49.28 53.1 1 54.96 those of the d-fqrms of NH,(CH,)Br (a = 5.09, c = 8.76 and NH,(n-C,H,)Cl (a = 5.02, c = 14.85 A),19 the structure of phase I of NH,(n-C,H,)Cl is reasonably assumed to also have the a’-form.In the a-form, both cations and anions form simple layers which are stacked alternately. On the other hand, both cations and anions form a bilayer structure in the a’-form as shown in fig. 9. The two kinds of crystallographically non-3216 Hydrogen- 1 N.M.R. of Solid Propylammonium Chloride and Bromide Table 2. Calculated second moments (G2) for three motional states of NH,(n-C,H,)+ ions in NH,(n-C,H,)Cl and NH,(n-C,H,)Br" CH,-rot. (CH, + NHi)rot. (CH, + NHi)rot.+chain rot. NH,(n-C,H,)Cl 22.0 14.5 (1 3.6) (13.6) NH,(n-C,H,)Br 21.4 13.9 (13.3) (13.3) 5.4 (4.4) 5.1 (4-2) " Values for the partially deuterated salts, ND,(n-C,H,)Cl and ND,(n-C,H,)Br, are given in parentheses. equivalent cations in a unit cell are packed with their methyl groups facing each other in a similar manner to the lamellar structure of the liquid crystal.20 Note that the ions in the a'-form are more loosely packed than those in the a. This can be proved by calculating the unit-cell volume of phase I, which is larger by % than that of phase I1 evaluated using the lattice constants (a = 6.29 A, c = 7.42 A) obtained from X-ray powder diffraction patterns recorded at ca. 400 K. N.M.R. Second Moments The motional modes of NH,(n-C,H,)+ ions activated in the present salts can be estimated by comparing the observed M, values with the theoretical ones calculated by use of Van Vleck's method.21 Values of M, evaluated for the present NH,(n-C,H,)+ salts having the cations in their several motional states are shown in table 2.These values were calculated in a similar manner to those previously calculated for the NH,(n-C,H,)+ By referring to the calculated values, M, values observed around room temperature for the all salts investigated can be well explained by the following motional model of the cations: the whole cation uniaxially rotates about the long axis together with random reorientations of both CH, and NH: groups by 120" about each C, axis. This explanation is consistent with the crystal structure determined by X-ray analysis, in which orientationally disordered cations exist." The gradual decrease in M, observed with increasing temperature for phase I1 of each salt can be attributed to an increase in the amplitude of lattice vibrations. It is noteworthy that M, values obtained for phase I (a'-form) of NH,(n-C,H,)Cl are much smaller than those of phase I1 (a-form), and that M , for phase I decreases much faster with increasing temperature than those for phase 11.Quite analogous M , behaviour to that of the present phase I was observed for NH,(n-C,H,)Br and NH,(n- C,H,)I in their tetragonal phase (a'-form). For example, NH,(n-C,H,)Br yielded M , of ca. 3 G2 at room temperature and which decreased gradually with increasing temperature to ca. 1.5 G2 at 460 K.,, These low M, values cannot be explained only through the above type of uniaxial rotation of the cations, suggesting the onset of new cationic motions.These may originate from the existence of more movable NH,(n-C,H,)+ ions in the a'-form than in the a-form. This is reasonable in view of the foregoing comparison of unit-cell volumes for these two structures. As a new motion occurring in phase I, translational self-diffusion of the cations, probably of the two-dimensional type, can be considered from the layer structure of the crystal. Experimental evidence for cationic self-diffusion taking place in this phase will be shown in subsequent sections.S. Fukada et al. 3217 Spin-Lattice Relaxation Time lH when nuclear magnetic dipolar interactions are averaged through random motions of molecules.According to this theory, q originating from a single motional process is expressed as can be analysed by the BPP c1 = C [ t / ( 1 + co2z2) + 4t/( 1 + 4c02z2)]. (1) Here, C, z and co denote the motional constant, the correlation time of the motion and the angular resonance frequency, respectively. By assuming an Arrhenius relationship for the motion, z can be written as z = zo exp(E,/RT) (2) where z, and E, are the correlation time at the limit of infinite temperature and the activation energy of the motion, respectively. Phase 111 Albert and Ripmeester" have measured 'H q of NH,(n-C,H,)Cl and ND,(n-C3H,)Cl and detected a q minimum in phase I11 of each analogue. The minimum observed for ND,(n-C,H,)Cl has been attributed to the C, reorientation of CH, groups and that of NH,(n-C,H,)Cl explained in terms of two minima due to the CH, and NH; reorientations overlapping each other.The activation energies for the C3 reorientations of the CH, and NH; groups have been determined as 10.0 and 15.5 kJ mol-l, respectively. NH,(n-C,H,)Br and ND,(n-C,H,)Br of the present study showed quite similar q curves to those of the above chlorides. The q minimum observed for ND,(n-C,H,)Br can be attributed to the C, reorientation of the CH, groups, in accord with the analysis of the results of ND3(n-C,H,)Cl.11 The motional parameters C and E, were evaluated by fitting eqn (1) and (2) to the observed q values of ND,(n-C,H,)Br using a least- squares method. The best-fit values of C and E, were 2.4 x 10' s-' and 6.9 kJ mol-l, respectively.The broad q minimum of phase I11 of NH,(n-C,H,)Br can be explained by the existence of two closely spaced q minima due to the C, reorientations of the CH, and NH; groups. When the two groups can be assumed to reorient independently, the resultant q value is expressed as where qM and qA indicate q components contributing from the CH, and NH; C, reorientations, respectively. Here, TM values were assumed to be the same as those observed for ND,(n-C,H,)Br although a correction factor arising from the difference in the numbers of protons between a ND,(n-C,H,)+ and a NH,(n-C,H,)+ cation was taken into account. The qA curve of NH,(n-C,H,)Br obtained from eqn (3) using the observed < values yielded an asymmetric & curve with a gentler gradient on the low-temperature side of the minimum.From the slope of the log qA us. T1 plots for the low-temperature side of the minimum, E, = 8.2 kJ mol-1 was roughly derived for the NH; C, reorientation by applying eqn (1) and (2) under the condition wz 9 1. The motional constant of the NH; C, reorientation was evaluated as C = 2.5 x lo9 sP2 from the qA minimum value (36 ms) obtained. The value of C for the CH, reorientation in NH,(n-C,H,)Cl can be calculated theoretically by using the equationll 2-y = c:+c; (3) C = (3/10)(9/20) )& A2r-6. (4)32 18 Hydrogen- 1 N.M.R. of Solid Propylammonium Chloride and Bromide Here, yH and r are the gyromagnetic ratio of a proton and th: interprotonic distance in a CH, group. Assuming the C-H bond distance to be 1.096 A22 and a tetrahedral angle between bonds in the -CH, moiety, the motional constant was evaluated as C = 3.34 x lo9 s - ~ .In a similar manner, C for the NH: C3 reorientation in NH,(n-C,H,)Br was calculated to be 3.1 1 x lo9 s-' using the N-H bond distance of 1.045 A.22 The theoretical C values turned out to be larger than the values obtained experimentally from the minima for both CH, and NH: C, reorientations. Another unusual result obtained for the present salts is that Ea values for the CH, and NH,+ C, reorientations are much smaller than those determined for analogous salts by lH n.m.r. measurements. For example, Ea for the CH, C3 reorientation obtained for NH,- (n-C,H,)Cl, NH,(n-C,H,)Br and NH,(n-C,H,)I was reported to be ca. 10 kJ to be in the range 16-27 kJ mol-'.It is known25 that shallower q minima and smaller Ea values than the theoretically expected ones are usually observed when the correlation time of the corresponding motional mode has a distribution over a wide range of time because of the existence of disordered atomic or molecular arrangements in crystals. In fact, analogous q data for NH,(n-C,Hg)Br obtained for its low-temperature phase, roughly equivalent to phase I11 of NH,(n-C,H,)Br, could be well explained by introducing a distribution in the correlation time of the CH, C, reorientation.22 As already described, the orientation of NH,(n-C,H,)+ ions as a whole are dynamically disordered about their long axes in phase 11. If the disordered orientations are partially frozen at Tr (I1 --f 111), it is thought that a distribution in the correlation time for the CH, and NH: C3 reorientations appears even in phase 111.9 and that for the NH,+ C3 reorientation was determined in the same salts mol-l 11,22,23 Phase II In the high-temperature limit given by CUT 6 1, eqn (1) becomes Thus q is independent of co. Combining eqn ( 5 ) and (2), it is found that log us. T1 plots increase linearly with increasing temperature provided that q is attributable to a single motional process. Both NH,(n-C,H,)Cl and NH,(n-C,H,)Br exhibited frequency independent values over the whole temperature range of phase 11. According to the analysis of M2 of this phase, 'H spin-lattice relaxation is mostly governed by a single motional process, namely the cationic reorientation about the long axis.However, the log q. plots showed a non-linear relation against & in this phase. This non-linear behaviour is marked especially for the q curve of NH,(n-C,H,)Br and unexplainable in terms of the Arrhenius type temperature dependence of the correlation time given by eqn (2). Analogous behaviour of the reorientational correlation time has been observed for the supercooled liquid state of substances easily forming glassy states such as g l y ~ e r o l . ~ ~ , ' ~ In this liquid state, the correlation time Z, of molecular reorientations can be expressed by the following non- Arrhenius- type expression : r;' = 5cz. ( 5 ) where z,, and EaR for the molecular reorientation are defined similarly as zo and E,, respectively. To denotes a reference temperature supposed to exist near the glass transition temperature.'The present salts are thought to possibly form an 'orientational glass' like state at low temperatures, if the tetragonal phases can be supercooled. This is because the reorienting low-symmetric NH,(n-C,H,)+ ions would be frozen in the supercooled state with an orientationally disordered state along the crystal axis having ' C4' symmetry. values of phase I1 of NH,(n-C,H,)Br was A fitting calculation for the observedS. Fukada et al. 3219 Table 3. Activation energies (E,) and pre-exponential factors ( T , , ~ ) for motions of NH,(n-C,H,)+ ions in NH,(n-C,H,)Cl and NH,(n-C,H,)Br _____ motional mode of cations phase E,/kJ mol-' 'RO/' G/K" chloride I 2 0 f 2 41 + 2 self-diffusion (q) 4 0 f 2 self-diffusion (qP) I1 6.4& 1 ( 2 .4 f 0 . 3 ) ~ lo-', 55f5 axial rot. bromide I1 4.8 f 1 (4.0 & 0.3) x 60 + 5 axial rot. a Temperatures appearing in eqn (6). performed using eqn ( 5 ) and (6) to estimate the unknown parameters EaR, T,, and To. In the calculation, C in eqn ( 5 ) can be given by2' C = (2/3)& AM2 (7) where AM2 is the reduction in M , due to the onset of the uniaxial reorientation of NH,(n-C,H,)+ ions and is assumed to be 8.8 G2 [( 13.9 - 5. 1)G2] for NH,(n-C,H,)Br from the calculated values given in table 2. The excellently fitted q curve selected by inspection is shown in fig. 7. The most probable values of EaR, zR0 and T, are given in table 3. A small E, value of 4.8 kJ mo1-I for the uniaxial rotation of NH,(n-C,H,)+ ions indicates the presence of cations having quite a low barrier to rotation in phase 11.If the T, values observed and the C value calculated by use of eqn (7) are substituted into eqn (9, z, values for the axial rotation of the cations can be evaluated. The T, values evaluated at 300 and 425 K were ca. 4 x s, respectively. These short correlation times of the cationic rotation also indicate that any barrier to this motion is quite small in this phase. The T, curves observed for phase TI of NH,(n-C,H,)Cl and ND,(n-C,H,)Cl also showed non-Arrhenius type temperature dependence. The same curve fitting calculation for the T, data of NH,(n-C,H,)Br was carried out also for those of NH,(n-C,H,)Cl and the unknown parameters were determined as listed in table 3. The q curve calculated by use of these parameters is shown in fig.5. and 2 x Phase I The fact that the M , values observed for phase T of NH,(n-C,H,)Cl were smaller than those of phase IT of the same salt indicates the onset of a new motion of the cation. The same expectation can be derived from the T, values of this phase, which are shorter by one order of magnitude than those of phase 11. Since increased with increasing temperature in the low-temperature region of this phase, the new motion should have short correlation times satisfying the condition u)z -+ 1. In the high-temperature region of phase I of NH,(n-C,H,)Cl, T,, which was dependent on frequency, decreased with increasing temperature. This can be explained by considering that another magnetic dipolar mechanism appreciably contributes to the value of observed in this temperature region.Self-diffusion of the cations seems to be the most reasonable candidate for the mechanism. This supposition is strongly supported from the electrical conductivity measurements of this phase described below. Consequently, & observed in this temperature region can be written by where Trot and Tdiff represent the relaxation times originating from an unknown cationic rotational mode and the cationic self-diffusion, respectively. Since Trot is I06 FAR I3220 Hydrogen- 1 N.M.R. of Solid Propylammonium Chloride and Bromide independent of w, the former mode should fulfill the condition wz 4 1. However, c d j f f is dependent on w and then the latter should satisfy the condition wz I . Therefore, we assume that Trot can be represented by the solid line shown in fig.6 because q,,,t can be expressed by eqn (5). q d i f f was evaluated by substituting the values observed at 10.5 MHz and the Trot values given above into eqn (8). These results are shown in fig. 6. Since the cation and anion layers are stacked alternately to form a layer structure in phase I, it is expected that the cationic diffusion takes place more frequently in each two- dimensional layer of the cations rather than in the direction perpendicular to the layers. It has been r e p ~ r t e d ~ ’ . ~ ~ that T due to two-dimensional diffusional motion cannot be expressed by the usual BPP relation. However, in the limit of slow motion, wz % 1, almost the same curve is predicted to be obtained as that derived from the BPP theory.,l3 32 Therefore, we determined E, for the two-dimensional self-diffusion of the cations as 41 +2 kJ mol-1 from the gradient of log qdiff us.T1 plots shown in fig. 6. Almost the same E, of 40 2 kJ mol-1 was obtained from the low-temperature gradient of the log qp us. T’ curve. This activation energy is very close to the foregoing value derived from q d i f f , indicating that the observed qp minimum is also attributable to the self-diffusion of the cations. E, for the motion of cations responsible for Trot was obtained from the slope of log Trot us. T1 as 20+2 kJ mol-’. Since the cationic reorientation about the long axis is already quite frequently excited in phase 11, this new motion activated in phase I should be a higher order motion of the cations.For example, a precessional motion of the cationic axis with NH,+ groups fixed on the anionic layer is conceivable as a possible motional model. Similar motions of the cations considered here for NH,(n-C,H,)Cl are expected to exist in phase I of NH,(n-C,H,)Br because the temperature dependences of M, and ‘H & are very similar to those of NH,(n-C,H,)Cl. No further discussion could be made on these data because phase I of NH,(n-C,H,)Br was very unstable and reliable data could be obtained only in a narrow temperature range. Electrical Conductivity in Phase I of NH,(n-C,H,)CI NH,(n-C,H,)Cl yielded a high electrical conductivity, 0, amounting to the order of lo-, S m-l which is the same order of magnitude as those observed for NH,(CH,)NO,’ and NH,(CH,)T2 in their highest-temperature solid phase.The highest-temperature solid phase of various salts of methylamm~nium,~* ’* 33--35 dimethylammonium, 36 trimethylamm~nium~~~~ and g ~ a n i d i n i u m ~ ~ studied to date are known to have similar properties to the plastic phase of molecular crystals in the sense that the constituents of crystals perform rapid self-diffusion. Ionic crystals in such a phase are expected to have a high electrical conductivity as is actually observed for some salts. In these salts, translational jumps of cations in their diffusion mechanism can occur almost isotropically because of the high crystal symmetry of the phase. In phase I of NH,(n- C,H,)CI, however, the cationic self-diffusion is thought to be a two-dimensional type as discussed before.The presence of the two-dimensional- type diffusion of alkylammonium cations in the mixed solid was reported for alkylammonium tetrachlorometallates(II), [NH,(n-C,H,,+l)],[MC14] (M = Mn or Zn) with n equal to 12 and 18.,’ These salts are known to form a layer structure similar to that of NH,(n-C,H,)Cl. The diffusion constant D for moving ions in crystals can be written by the following Nernst-Einstein equation where A, Ze and N denote the spacial correlation factor calculated in the literat~re,~’ the electronic charge of the diffusion ion and the number of ions per unit volume, D = kTAo/(Ze)2N (9)S. Fukada et al. 322 1 * I v) I 03 KIT Fig. 10. The temperature dependence of the diffusion constant defined within the layer D, evaluated from the data of the electrical conductivity measurements in the high-temperature phase of NH,(n-C,H, jCl.respectively. In the crystals of the two-dimensional layer structure, o observed for powder samples can be expressed as where oI and o,, denote electrical conductivities perpendicular to and parallel to, respectively, the cationic long axis. Assuming o, 9 oil, the diffusion constant defined within the layer D, is given by D, = kTA.o/(Ze)2N. (1 1) In the present system, the value of 3, for the two-dimensional square lattice is 0.46694.40 From the observed o values, the temperature dependence of D, can be calculated by using eqn (1 1). The results are shown in fig. 10. From the slope of logD, us. T1 plots, E, for the ionic diffusion could be calculated as 37+3 kJ mol-'.This value agrees very well with E, of 40 and 41 kJ mol-1 derived above from the Tl and TIP data, respectively. This supports our analyses of the relaxation times and electrical conductivity results. In the highest-temperature solid phase of NH,(CH3)N039 and NH3(CH3)I,2 large diffusion constants amounting to the order of m2 s-l were obtained. This order of magnitude is the same as that obtained for phase I of NH3(n-C,H,)Cl. For many plastic crystals, the same order of diffusion constants have also been reported.'l These results imply that the NH3(n-C3H,)+ cations can move quite freely in phase I, in which lattice defects may exist in a high concentration as in some plastic crystals. References 1 R. W. G. Wyckoff, Crystal Structures (Interscience, New York, 2nd edn, 1966), vol.5, chap. XIVA. 2 H. Ishida, R. Ikeda and D. Nakamura, Phys. Status Solidi A , 1982, 70, K151; Bull. Chem. SOC. Jpn, 3 J. Tsau and D. F. R. Gilson, J. Phys. Chem., 1968, 72,4082. 4 H. Ishida, R. Ikeda and D. Nakamura, J. Phys. Chem., 1982, 86, 1003. 5 Y. Kume, R. Ikeda and D. Nakamura, J . Magn. Reson., 1979, 33, 331. 6 L. 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Kristallogr., 1928, 68, 189. 20 G. H. Brown and J. J. Wolken, Liquid Crystals and Biological Structures (Academic Press, New York, 21 J. H. Van Vleck, Phys. Rev., 1948, 74, 1 168. 22 S. Fukada, R. Ikeda and D. Nakamura, Bull. Chem. Soc. Jpn, 1984, 57, 2802. 23 S. Fukada, R. Ikeda and D. Nakamura, Z. Naturforsch., Teil A , 1984, 40, 347. 24 A. Abragam, The Principles of Nuclear Magnetism (Oxford University Press, London, 1961), chap. 25 T. M. Connor, Trans. Faraday Soc., 1964, 60, 1574. 26 M. Wolfe and J. Jonas, J. Chem. Phys., 1979, 71, 3252. 27 P. L. Kuhns and M. S. Conradi, J. Chem. Phys., 1982, 77, 1771. 28 G. Soda and H. Chihara, J. Phys. Soc. Jpn, 1974, 36, 954. 29 A. Avogadro and M. Villa, J. Chem. Phys., 1977, 66, 2359. 30 J. Tabony and J-P. Korb, Mol. Phys., 1985, 56, 1281. 31 M. Vilfan and S. gumer, Phys. Rev. A, 1980, 21, 672. 32 U. Roder, W. Muller-Warmuth, H. W. Spiess and R. Schollhorn, J. Chem. Phys., 1982, 77, 4627. 33 H. Ishida, R. Ikeda and D. Nakamura, Chem. Lett., 1982, 1943; Bull. Chem. SOC. Jpn, 1987, 60, 34 H. Ishida, N. Matsuhashi, R. Ikeda and D. Nakamura, Chem. Lett., 1985, 1859. 35 S. Jurga and H. W. Spiess, Z. Naturforsch., Teil A, 1985, 40, 602. 36 S. Jurga and H. W. Spiess, Ber. Bunsenges. Phys. Chem., 1985, 89, 763. 37 S. Jurga, Phys. Status Solidi A, 1984, 81, 77. 38 S. Gima, Y. Furukawa and D. Nakamura, Ber. Bunsenges. Phys. Chem., 1984, 88, 939. 39 V. Busico and M. Vacatello, Mol. Cryst. Liq. Cryst., 1983, 95, 251. 40 K. Compaan and Y. Haven, Trans. Faraday Soc., 1956, 52, 786; 1958, 54, 1498. 41 J. M. Chezeau and J. H. Strange, Phys. Rep., 1979, 53, 1. 1979), chap. 3. VIII. 467. Paper 71056; Received 12th January, 1987

ISSN:0300-9599    

DOI:10.1039/F19878303207

出版商:RSC    

年代:1987

数据来源: RSC

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Reviews of Books Chemical Industries/21. Catalysis and Surface Science. Ed. by H. Heinemann and G. A. Somorjai. (Marcel Dekker, New York, 1985). Pp. xi + 435. Price $90. This is a particularly good book; one that actually exceeds the advertiser’s blurb on the back cover. It is, as they say, a comprehensive survey of industrially important areas of catalysis and is tackled from a multi-disciplinary point of view which will render it almost essential reading to chemical technologists, catalytic scientists and materials scientists. Not only does it cover the technology and science of catalysis, commercial aspects and future commercial trends are also considered. (The book was compiled from the papers presented at the 2nd Berkeley Catalysis and Surface Science Conference held in January 1984 and it is surprising to see how the techno-commercial prognostications of the early 80s have been buffetted by the recent slump in oil prices.) Although the book purports to treat five subjects: Chemicals from Methanol, Hydrotreating of Hydrocarbons, Catalyst Preparation, Monomers and Polymers and Photocatalysis and Photo- voltaics, the main thrust of the book is on the Hydrotreating of Hydrocarbons.Ostensibly this topic is covered in only four articles, but one of the papers on catalyst preparation relates to preparative techniques for the production of hydrotreating catalysts. [In the opinion of this reviewer this latter article was somewhat disappointing since virtually no correlation was obtained between the preparative method used, the nature of the surface of the catalyst so derived, as determined by surface analysis techniques (e.g.XPS, SEM, TEM and EDX), and catalyst activity.] Two particularly good articles relating to the fundamental science of hydrodesulphurization were contributed by Chianelli and by Topsare and Clausen. In these articles the uniquely different line of investigation that the different authors take is reflected in the fact that Chianelli references Topsare’s work only once in his 34 references, while Topsare affords Chianelli the same number in his 64 references. The Technology of Hydrotreating in general is well reviewed by Dautzenberg and De Deken in an article following those of Chianelli and Topsare. The earlier section of the book on Chemicals from Methanol is less profound than the section on Hydrotreating and the article by Wender degenerates almost to a catalogue of what might be possible in the future.The other particularly strong section of the book is on Monomers and Polymers. Articles by Kaeding et af. on Mobil Zeolite Catalysts for Monomers and by Chien on A Fundamental Study of High Activity Catalyst for Olefin Polymerisation are substantial reviews on their respective subjects. These are followed, as in the format in the Hydrotreating section, by an article on Olefin Polymerization Catalysis Technology by Hsieh which is also of a high standard. The book finishes with articles on Hydrogen Evolving Solar Cells by Heller, The Catalysed Photodissociation of Water by Somorjai et af. and again by their possible future technological application in an article by Wald on The Present Status of Industrial Production and Use of Photovoltaic and Photocatalytic Solar Energy Conversion Devices.The interest in the use of solid cells derived from the oil crisis in 1973 and to an extent the commercial considerations which stimulated this research appear somewhat to have been overtaken by time. They are, nevertheless, extremely good articles. Overall, therefore, I would thoroughly recommend this book as being of keen and abiding interest to both industrial and academic scientists. PhD students in chemistry and chemical engineering will also find much of interest in it, but at $90 (€60) a copy, it may well be out of their reach. K. C. Waugh Received 9th September, 1986 Experimental Physical Chemistry.By G. P. Matthews. (Clarendon Press, Oxford, 1985). Pp. xxvii + 495. Price €30. G. P. Matthews has written a highly structured book which appears to be the companion experimental text to the established physical chemistry text by P. W. Atkins. The broad division of physical chemistry into three familiar parts : equilibrium, structure and change precedes subdivision into 8 topic sections and thence to some 59 individual experiments. Of these, 8 are 32233224 Reviews of Books claimed to be original, whilst the remaining 51 have been ‘gleaned’ from contemporary experimental courses in numerous chemistry departments. In this way Dr Matthews approaches the laudable aim of producing a book that relates to current courses in physical chemistry, whilst, at the same time, ensuring the security of future editions.Following a brief introductory paragraph, each experiment is set out under eight uniform sub-headings : theory, apparatus, procedure, calculation, results, comment, technical notes and references. Although the theoretical sections vary in quality, each gives ample background to the experiment so that it can be both tackled and appreciated by students who have yet to cover the topic in lectures. In this respect the book is considerably better than some experimental texts that are available. Suggestions are made concerning the student level to which each experiment is applicable, and in this way the book covers a complete undergraduate course apart from project work. There is no doubt that Dr Matthews has selected experiments that cover a wide range of both classical and modern physicochemical topics.However, it is disappointing that surface chemistry is represented only by experiments on the surface tension of liquid oxygen and the physisorption of 1,2-dichlorotetrafluoroethane on charcoal, whilst heterogeneous catalysis and colloid chemistry are not represented at all. Seven of the experiments necessitate the use of a computer and Dr Matthews offers to supply programs for these-an act of generosity that he may come to regret! Any teacher with the responsibility of organising undergraduate courses in practical physical chemistry will want a copy of this well produced book. For them the price of &30 is not unreasonable and could justifiably be met from departmental funds, but it is certainly sufficient to preclude the adoption of this hard-back edition as an essential undergraduate text. R.Rudham Received 1st September, 1986 Advances in Polymer Science 68/69. Cationic Ring-opening Polymerization. Part 11: Synthetic Applications. By S. Penczek, P. Kubisa and K. Matyjaszewski. (Springer-Verlag, Berlin, 1985). Pp. xviii+317. Price DM 188. This volume contains a truly comprehensive review of cationic ring-opening polymerisation. An outstanding feature is the effort which is made to resolve uncertainties and conflicting views by critical analysis, and where necessary reanalysis, of published data. Throughout the treatment is authoritative, reflecting the extensive experience which the authors have in this field. The task of synthesising a polymer in a particular molar-mass range is much simplified if the propagating chains are not subject to either termination or transfer reactions.Of the systems under review only a few can be considered to fall into this class of so-called ‘living systems’. The latter would be some of the cyclic ethers, iminoethers, amines and acetals. It is explained in the text why various groups of monomers show other types of more complex behaviour and how this must be taken into account if certain polymers are to be obtained in acceptable yields by the synthetic chemist. On these lines there is a particularly useful discussion regarding the formation of macrocyclic rings versus linear chains. The material is organised in a logical fashion. A short introductory first chapter is followed by a thorough treatment of the thermodynamics of ring-opening polymerisation in chap.2. Peculiarities of cationic ring-opening polymerisations are considered together with the basic parameters that need to be taken into account in choosing the preparation conditions for some particular homopolymers and copolymers. In chap. 3 there is a unified treatment of the thermodynamics and kinetics of macrocyclisation. The probability of cyclisation depends on such factors as ring size, the presence of substituent groups, chain flexibility and the nucleophilicity of the heterochain. The Jacobson-Stockmayer theory is naturally considered first, this being an equilibrium theory which may be used to relate the distribution of cyclic oligomers to the conformational probability of ring closure.In many cases, however, this simple theory is inadequate and the discussion of experimental results emphasises the need to recognise factors which can lead to kinetic control of cyclisation. Chap, 4-14 inclusive give accounts of the polymerisation of various types of monomers: cyclic ethers (including oxiranes, oxetanes, oxolanes, acetals and bicyclic compounds), lactones, cyclic sulphides, cyclic amines, lactams, cyclic iminoethers, siloxanes and cyclic phosphorus-containingReviews of Books 3225 compounds. A wealth of information is presented in these chapters in an interesting and easily digestible form. Some applications of the polymers are also mentioned, along with relevant trade names when commercial products are involved.The book contains 15 chapters, and the final chapter is devoted to copolymerisation. Some of the explanations put forward in the final chapter will be much studied by workers in the field. Included in the treatment is a critical examination of schemes used to interpret statistical copolymerisation. The effect of a reversible propagation stage is considered and it is shown how many of the reactivity ratios reported in the literature were determined using an oversimplified scheme. Finally, consideration is given to the formation of block, alternating and graft copolymers. The authors have carried out what was undoubtedly a difficult task with distinction. Their overall treatment of the subject will be found stimulating by specialists in the field, whilst the blend of theoretical and practical discussion will be much appreciated by synthetic polymer chemists in general.The book is an important addition to the polymer literature and polymer chemists will certainly wish to see it included in their library. C. Price Received 18th September, 1986 Specialist Periodical Reports. Spectroscopic Properties of Inorganic and Organometallic Compounds. Vol. 18. Ed. G. Davidson and E. A. V. Ebsworth. (Royal Society of Chemistry, London, 1985). Pp. xiv+466. Price E95, $171. When receiving this book to review, I was reminded of the critic who, being sent a copy of the telephone directory, remarked that it had too many characters and no discernible plot. The n.m.r. chapter of the present volume has 2792 references. To deal with these in 108 pages of text can only be achieved by resorting to telegrammatic brevity which, combined with the camera-ready typeface now required by Specialist Periodical Reports, makes for a very heavy read.Other chapters deal with vibrational spectra, including those of coordinated ligands, Mossbauer, n.q.r. and rotational spectra, covering the literature in a most comprehensive way up to late 1984. There can be little doubt about the completeness of all these surveys and the authors should be complimented on their assiduity and speed of working. Nevertheless, if such surveys are to be undertaken by people, and not computer-aided retrieval systems, I would have liked to see some commentary signalling the most important developments in each topic and evaluating their significance for the future.Probably, this is too much to ask of busy professionals, but it would make such compilations as the present one more valuable and fun to read. P. Day Received 20th August, 1986 Cambridge Solid State Science Series. Modern Techniques of Surface Science. By D. P. Woodruff and T. A. Delchar. (Cambridge University Press, Cambridge, 1986). Pp. x+453. Price &50, $89.50. This book provides a comprehensive account of experimental developments over the last tfvo decades in the field of surface science. It is difficult to find a surface-sensitive technique that has not been covered and any newcomer to the field will find the book a good source of information on both the principles on which they are based and the kind of information that is likely to emerge from each of them.There are nine chapters, the first is a very brief account of some background information on surfaces and adsorption. This is then followed by chapters dealing in turn with surface crystallography and diffraction; electron spectroscopies ; desorption studies ; field electron and field ion emissions; work function studies, scattering techniques and lastly with vibrational spectroscopies. The book is both well written and produced and I have no doubt that any undergraduate or recent graduate student will find it both readable and a very useful reference book. Although I enjoyed the scholarly approach, where I think it does fall down is the dkjb uu flavour that it has for the more senior researcher. A perhaps obvious example is the chapter on Desorption Spectroscopies which, in various guises, has been on the scene for nearly 30 years with significant contributions made by Ehrlich and Redhead in the 1950s and early 1960s.Over the last decade3226 Reviews of Books there have been numerous books (and series of books) devoted to the subject of surface science embracing both the chemists’ and physicists’ approach. The subject has been treated so well at so many different levels that it would indeed be difficult for any author of a new book, written for the recent graduate, to bring a different slant or perspective to the subject at this stage. In this sense I sympathise with any potential author in a field which has, and rightly so, attracted so much attention over the last decade.Perhaps a book devoted to the impact of the newer experimental techniques on heterogeneous catalysis would be appropriate. M. W. Roberts Received 12th September, 1986 Modelling of Oxidation Processes. By N. M. Emanuel and D. Gal. (Akademiai Kiado, Budapest, 1986). Pp. 436. Price E33. This book traces principles and practices that may be adopted in order to establish a quantitative kinetic interpretation of a complex chain reaction system when mathematical modelling is brought to bear on experimental data. There is approximately equal emphasis on experimental kinetic and computational aspects. This is a timely and appropriate topic in the context of current developments of ‘expert’ systems. The subject for this case study is liquid-phase oxidation, with exclusive reference to the oxidation of ethylbenzene. Every kinetic aspect is presented and discussed in depth.Following an introduction to the main aims, the book opens with an extensive, modern account of experimental methods that have been adopted for the study of liquid-phase reactions. Kinetic features that emerge are illustrated in general terms. The theme then changes to general considerations of the components of a kinetic network to explain a complex chain reaction, now illustrated by ethylbenzene oxidation and expressing all possible elementary reaction modes for it, as perceived by logical deduction. Some preliminary pruning of the scheme is possible on the basis of kinetic knowledge, and is exploited here, but the advice is that, in general, only thermochemical considerations should be invoked as constraints on the multiplicity of the scheme.The authors use a matrix representation for compilation of the ‘possible mechanism’. The heart of the text follows in the next two chapters. These sections constitute a masterly, quantitative account of kinetic deductions from experimental measurements : the different subsets of reactions that comprise the overall kinetic network to explain the experimental features of ethylbenzene oxidation are discussed. The general description of the ‘possible mechanism ’ and the experimental kinetics are brought together in chap. 6 in order to reduce the elementary kinetic model to its most manageable proportions. Validation by numerical simulation is incorporated at this stage and the section closes with the derivation of a ‘main contributory mechanism’ that is capable of explaining the principal features of ethylbenzene oxidation.The penultimate chapter comprises a supplementary discussion of homogeneous and heterogeneous catalysis of this oxidation and how these aspects are incorporated in the numerical analysis. The book closes on a technological note of industrial applications. The combination of experimental data with mathematical analysis and numerical interpretations has, in the last decade, afforded insight into complex kinetic interactions that had been inaccessible hitherto. The computational foundation is presented in a way that would be of particular interest to the computer modeller developing ‘expert’ systems to provide the basis for interpreting general reaction mechanisms.The authors do not address the formal mathematical routes of sensitivity analysis now widely adopted to interpret specific kinetic mechanisms. Above all, this is an authoritative account of liquid-phase oxidation, as befits its most distinguished authorship. Perhaps its greatest strength lies in the lucid and definitive interpreta- tion of experimental, kinetic studies. I can commend it on this account to kineticists who wish to enhance their awareness of the present status of understanding of liquid-phase oxidation. J. F. Griffiths Receiued 8th October, 1986Reviews of Books 3227 Combustion of Polymer Materials. By R. M. Aseeva and G. E. Zaikov. (Hanser, Munich, 1985). Pp. xvi + 389. The authors believe that this book differs from others on the same subject inasmuch as it places emphasis on the general characteristics and principles of polymer combustion. A brief introduction explains that the impetus for research in this area comes largely from the need to develop less-flammable polymeric materials.Chap. 1 first gives a general account of combustion and then goes on to consider theories of the ignition and self-ignition of polymers. Chap. 2 deals with the roles of heat and mass transfer and with the factors affecting the rates at which polymeric materials burn. Chap. 3 is concerned with the chemistry of polymer combustion and considers separately pyrolytic breakdown in the condensed phase, diffusion flames and the formation of smoke; it also deals, in general terms, with the mechanism of action of flame retardants and smoke suppressants.A useful Addendum contains an account of work on the effects of oxygen on polymer decomposition. Chap. 4 describes some of the principal experimental methods used to study the combustion of organic polymers. It deals, in particular, with the determination of rates of combustion, of temperature and product profiles in flames and of smoke and toxic gas formation, as well as with modelling techniques. Chap. 5 considers principally the design and synthesis of thermally stable polymers and the use of flame retardants, fillers and fireproof coatings to reduce the flammability of some of the commoner polymeric materials. The literature references have a strong leaning towards research carried out in the U.S.S.R.and this is valuable since it draws the attention of authors elsewhere to work which might otherwise remain largely unnoticed. It is a pity, however, that the great majority of the references are to research completed by the early 1970s, and that more recent work receives rather scant attention. All in all, however, this book makes interesting reading and has the virtue of stressing the importance of physical, as well as chemical, factors in the combustion of organic polymers. C. F. Cullis Received 25th October, 1986 Annual Reports on the Progress of Chemistry. Volume 82. Section C, Physical Chemistry. Senior Reporter M. C. R. Symons. (Royal Society of Chemistry, London, 1986). Pp. xii + 324. Price E52.50, El3 (members), $102. Never a day goes by but one hears the term ‘information technology’.I think it must be what they call a ‘buzz-word’. I don’t know what it is and, while I have often heard it talked about with erudite-sounding solemnity by such august collections of scholars as the Board of the Faculty of Pure Science and the Senate of my University, I have never yet been able to get a colleague to define it to my satisfaction. I therefore suspect that there is a lot of humbug about it. On the other hand, in my state of blissful ignorance I am completely free to fantasise about it. Thus, it seems to me we are approaching the time when everybody will be an expert at fiddling about with data on computers, but nobody will actually be able to do anything practical; then will come the time when no more computers (or anything else for that matter) are being developed and built, shortly after which all the existing computers will wear out; and that will be that, a singularity in the history of mankind from which we will have to start our pursuit of knowledge and understanding all over again.In contrast to the foregoing liverish hyperbole, I do adore books. For me they are an essence of civilised society; they are the supreme repository of human experience and knowledge and I would not wish to live in a world where they did not exist. Of course, writing a review of a Section of Annual Reports I must have no pretensions to literary criticism, for I do not think they can properly be described as literature. However, this book is a mine of information so I suggest that Annual Reports are representative of a primitive form of information technology.Clearly it is a form of which, at least in principle, I approve. When Annual Reports was reorganised in 1979, and Professor Symons took over the editorship of the newly autonomous Physical Chemistry issue, he asserted that his policy would be to have articles of two sorts : (a) those covering broad areas of mainstream physical chemistry, which would appear at fairly regular intervals and ( b ) reviews of specific topics currently of importance, hopefully of interest to a wide audience of chemists. The authors of one of the articles in this volume begin by defining their report to be in the second category. The report is on Conduction Electron3228 Reviews of Books Spin Resonance in Metallic Systems (Edmonds, Harrison and Edwards) and in my view it succeeds very well: the technique, the systems and representative results are set out in a readable and understandable way.It is interesting that this article has the lowest indigestibility factor, i.e. reference/page ratio, (IF), of any in the book; its value is 1.51. In contrast, the highest IF, 9.34, is achieved by Webb, writing about Nuclear Magnetic Resonance. To be fair, Webb does concentrate on physical applications of n.m.r., he includes useful diagrams illustrating the experiments which can now be performed and he concludes with a glossary of acronyms: thus, in spite of the initial impression of high-density jargon, I felt that the article does give a good feeling for what has been happening in the field. Given my own interests, it is perhaps not surprising that, of the other reports, I particularly enjoyed Matrix Isolation (Perutz, IF = 4.6) and Physical Aspects of Photochemistry (Osborne, IF = 3.19). I found Reactions of Silicon Intermediates (Davidson, IF = 5.80) a bit specialised but 1 felt that each of the three remaining articles covered areas which I really ought to know more about and that they formed suitable bases from which further study could be pursued. These articles are, Isobaric Heat Capacites (Blandamer, Burgess and Scott), Physical Chemistry of Solids (Rao, Rao and Gopalakrishnan), and Electronic Processes in Thin Films and Novel Conductors (Rosseinsky); they all have IF in the range 7.76-7.82. Of course, these are personal views and a different reviewer, with different prejudices and preoccupations, would comment differently. However, I am sure that he would agree with me that there is much of interest here. If one of the Reports is on your special field of interest then you really should not pass it by, but quite apart from that I think it would be time well spent for any physical chemist to browse through this book. It gives stimulating impression of what is happening in a wide range of areas, and it is good for us all to once in a while lift our heads above the battlements of our entrenched research positions and have a look around. N . M. Atherton Received 6th November, 1986

ISSN:0300-9599    

DOI:10.1039/F19878303223

出版商:RSC    

年代:1987

数据来源: RSC