摘要:

J. CHEM. SOC'. F'ARADAY TRANS., 1994, 90(6), 899-903 High-temperature Diffusion of Hydrogen and Deuterium in Palladium Takeshi Maeda, Shizuo Naito," Masahiro Yamamoto, Mahito Mabuchi and Tomoyasu Hashino Institute of Atomic Energy, Kyoto University, Uji, Kyoto 61I, Japan The diffusivity of hydrogen and deuterium in palladium has been measured at temperatures between 773 and 1373 K and at hydrogen and deuterium concentrations less than lop4 H/Pd and D/Pd (atomic ratio). The mea- sured diffusion coefficients for hydrogen (DH)and deuterium (D,) showed Arrhenius behaviour. The depen- dence of the ratio DD/D, on temperature has been explained by a model of diffusion in which a hydrogen atom in an octahedral site of the palladium lattice can jump into the adjacent octahedral site only when local deforma- tion of the palladium lattice assists the jump.The palladium-hydrogen system has long attracted interest and has been the most extensively studied of all the metal- hydrogen systems.' This is primarily because palladium can absorb a large amount of hydrogen and the hydrogen atoms in palladium have high mobility; also, in the palladium-hydrogen system reliable experimental data such as accurate solubility and diffusivity can easily be obtained from measurements of the rate of absorption of gaseous hydrogen by solid palladium with simpler surface treatments than in other metal- hydrogen systems. Nevertheless, there remains some uncertainty in the details of the measured diffu- sion coefficients and of the model to be applied to the diffu- sion of hydrogen in palladium.The measured diffusion coefficients for hydrogen (D,) are in good agreement.6 'However, there have been fewer mea- surements of difTusion coefficients for deuterium (0,) and the agreement between the reported values at high ternperat~res'*'~is not good enough to form a basis for a diffusion model. Models of hydrogen diffusion in metals, especially those applicable to low-temperature diffusion, have been extensively discussed.3-5911-l4 Despite a variety of models presented, no conclusion has been reached as to which is the most appropriate for describing the high-temperature diffusion of hydrogen in fcc palladium. A clue to the most appropriate model may be the follow- ing: the pre-exponential factor of the measured D, is of the same order of magnitude as that calculated from the Debye temperature of palladi~rn~,'~ and the values of the measured ratio DD/D, at high temperatures are larger than the classical value i.e.the ratio is rather isotope independent 1/,/2,77"7'0 compared with the classical case. We thus consider a model in which the motion of palladium atoms, rather than the vibration of a hydrogen atom, plays a key role in the diffusi- vity. Then, to calculate D, and D, from the model, we need, if we apply rate theory,' 'pl 5.16 partition functions and conse- quently energy levels of hydrogen and deuterium in palla- dium. However, the available energy levels obtained experimentally' '9'' and the~retically'~ 22 are insufficient for calculating the partition functions and it is difficult to find the analytical expression for the energy levels.We therefore employ the partition functions that can reproduce the mea- sured solubility of hydrogen and de~terium.~.~.~~-~~ In this study we measure the diffusivity and solubility of hydrogen and deuterium in palladium. We next consider a model of diffusion and obtain, using rate theory, an expres- sion for the diffusion coefficient. The partition functions appearing in the diffusion coefficient are determined from the solubility data. The diffusion coefficient is then computed and compared with the measured one. Finally, we show that the model considered can be successfully applied to the diffusion of hydrogen in palladium at high temperatures.Applicability of other models is also briefly discussed. Experimental The diffusion coefficient was obtained from measurements of the rate of absorption of gaseous hydrogen by palladium. The details of the apparatus and procedures for measurements were identical to those described previo~sly.~~~~~ The specimen was a spherical, polycrystalline sample of palladium of diameter 15 mm and had a nominal purity of 99.9%. It was heated to 1173 K in a vacuum chamber for 4 h until the ambient pressure reached 1 x lop7Pa. Before each measurement the specimen was heated to a temperature 50 K higher than that at which the measurement was made. The absorption rate was measured in the temperature range 773-1373 K and at hydrogen pressure 6.7 Pa.This pressure corresponds to hydrogen concentrations less than 1 x H/Pd (atomic ratio) in this temperature range. The hydrogen pressure, and consequently the hydrogen concen-tration, should be as small as possible to measure the tracer diffusion ~oefficient,',~ which is simpler to interpret than the chemical diffusion coefficient and, also, the hydrogen pressure should be as large as possible to reduce the effect of disso-ciative adsorption of hydrogen molecules onto the surface of the specimen, which is necessarily involved in the measured absorption rate and becomes large as the pressuredecrease^.^^.^^ (The details of the procedure for separating this effect from the measured absorption rate have been described in ref.9, 27 and 30.) By choosing the pressure to be 6.7 Pa the absorption rate was found to be limited only by diffusion of hydrogen into the palladium bulk. We could thus obtain the diffusion coefficient by comparing the amount of absorbed hydrogen obtained experimentally with that com- puted from the solution of the diffusion equation together with its boundary c~ndition.~~.~~ To determine the partition functions involved in the expression for the diffusion coefficient, we obtained the solu- bility from the amount of hydrogen absorbed when the absorption rate became negligibly small. Results and Discussion Experimental Results Fig. 1 shows the Arrhenius plots of the measured diffusion coefficients, D, and D,, together with those reported in the literature.*-" The measured values of D, and D, fell on straight lines.The measured D, almost coincided with that reported by Katsuta et aL8 It was impossible to observe the upward deviations from the straight lines and some hysteresis on cooling as reported by Gol'tsov et al." for temperatures above ca. 950 K; below ca. 950 K their data are in agreement with those in the present study. Extrapolation of the mea- sured values of D, to room temperature reproduced the J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 .-v) N E--. Q 1o-8 -'\ I I I I I I I 0.8 1.o 1.2 1.4 103 KIT Fig. 1 Arrhenius plots of the diffusion coefficients DH and D,. (0) D, and (a)D,, this study; (--) Gol'tsov et al." (upper line: DH and lower line: D,; the result of their measurements is shown here only for heating); (.* .) Katsuta et a1.* for D, and (-. -) Powell and Kirkpatrick' (upper line: D, and lower line: OD).The thick solid line has been computed using eqn. (1). For details see the text. reported values of DH.6*7From Fig. 1 we can obtain graphi- cally the activation energies: 219 meV for hydrogen and 215 meV for deuterium and the pre-exponential factors : 2.8 x lod7m2 s-l for D, and 2.4 x lop7m2 s-l for D,. These values are in general agreement with those report- ed.5-1 0,31-34 Fig. 2 shows the temperature dependence of the measured ratio DD/DH together with those rep~rted.~*~*'*"*~'-~~ The measured values agreed with those of Gol'tsov et ~1 for.~ temperatures below ca.950 K, where they observed neither the deviations nor the hysteresis. The data of Jost and Widmann31 show lower values than those in the present study and the values of Powell and Kirkpatrick' are, to a lesser extent but systematically, smaller than those reported. General agreement, however, is good, allowing for the com- parison of DD/DH,which is more critical than the comparison of quantities such as solubility. The values obtained experi- mentally are larger than the classical value 1/,/2 and unlikely to approach it even at high temperatures. The large values at low temperatures are, as will be mentioned later, due mainly to the difference in the zero-point energies of hydrogen and deuterium.Model of Diffusion of Hydrogen in Palladium As systematic discussions have been presented for models of diffusion of hydrogen in metal^^^^*''-^^ we are here con- cerned with finding among them a model that can be applied, with minor modifications, to the diffusion of hydrogen in pal- ladium at high temperatures. A characteristic of the measured D, is that its pre-exponential factor is 2.8 x lo-' m2 s-l, which compares with the values 4.3 x lop7m2 s-l calculated from the Debye temperature (274 K) and the distance (0.27 nm) between the 2000 1000 500 300 I I I 1 1 1.5 I Q 1.o 0.5 I I I 0 1 2 3 4 103 KIT Fig. 2 Temperature dependence of DJDH. (@) Present study and (--) reported N umbers on the dashed lines refer to references.The result of Gol'tsov et al." is shown only for T < 950 K. The thick and thin solid lines have been computed using eqn. (1) and (2), respectively. For details see the text. Note that the scale of the inverse temperature axis has been compressed compared with that in Fig. 1. adjacent octahedral (0)sites of the palladium lattice (Fig. 3). The value 2.8 x lo-' m2 s-l, however, is considerably smaller than the value 1.3 x m2 s-l calculated from the Einstein temperature (800 K) of hydrogen in palladium. This indicates that the hydrogen atom cannot jump over the fixed potential of the palladium lattice with the frequency of vibra- tion of the hydrogen atom. Another characteristic is that the measured ratio DD/DH has values larger than those expected from the classical value l/J2 at high temperatures (Fig.2); the diffusion coefficient is rather isotope independent. At high temperatures the jump of the hydrogen atom from one site to another over the fixed potential barrier would lead to DD/DH x 1/J2.5J1 Both these characteristics imply, as a model of high-temperature diffusion of hydrogen in palladium, the jump of ~ the hydrogen atom assisted by the local deformation of the palladium lattice. In this model the hydrogen atom can only make the jump from one site to another by jumping over the potential barrier when the palladium lattice is deformed. Here, we assume that the change in the electronic state of the palladium-hydrogen system is much faster than the motions n Fig.3 Diffusion path for hydrogen in the palladium lattice. Circles: palladium atoms, 0:octahedral sites, S: saddle points, T: tetrahedral site and dotted line: a possible diffusion path. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 of the palladium and hydrogen nuclei and the change in the motion of the hydrogen nuclei is faster than that of the palla- dium nuclei, although the validity of this assumption has not yet been proved for paliadi~rn.~*'~ The jump process is shown schematically in Fig. 4. Although models that incorp- orate tunnelling of the hydrogen atom through the potential barrier into the jump rate have been propo~ed,'~.~~we cannot apply them directly to the interpretation of the experi- mental results because they include the tunnelling probabil- ity, which is difficult to estimate for the locally deformed palladium lattice.We note here that, according to the present model, the pre-exponential factor may be smaller than that calculated from the Debye temperature because coincidence of two uncorrelated vibrations of palladium atoms in direc- tions perpendicular to each other is needed to cause the local deformation that permits the jump of the hydrogen atom.12 The diffusion coefficient can now be formulated on the basis of the model described above. Using rate theory15 we can write it in the form where d' is a constant, v is the effective jump frequency,f&, andfH(i, are the partition functions for the hydrogen atom in the activated state and at the 0 site (Fig.3), respectively, and E~ is the activation energy for diffusion. Eqn. (1) resembles the familiar form of the diffusion coefficient, but the following should be noted. First, v is usually taken to be the frequency with which the hydrogen atom passes through the activated state along its diffusion path.15 In the present model, however, we assume v to be the frequency of occurrence of the local deformation of the palladium lattice, and therefore to be independent of hydrogen isotopes, for the following reason. The hydrogen atom in the activated state is on a rather flat potential-energy surface along the diffusion path as can be expected from the result of the first-principles20*2' and molecular-dynamics14 calculations and has a lifetime prob- ably longer than that of the local deformation of the palla- dium lattice.Thus, if the hydrogen atom in the activated state always succeeds in making the jump, v corresponds to the frequency of occurrence of the local deformation and not to the vibrational frequency of the hydrogen atom in the acti- vated state. Secondly, the activation energy is the energy needed to make a configuration of the locally deformed palla- dium lattice with the hydrogen atom in it. Thirdly,fi(i, has the same degree of freedom asfH(i). Eqn. (1) is often used in w Fig. 4 Schematic diagram of the jump of the hydrogen atom in the palladium lattice. The hydrogen atom (H) is self-trapped in the 0 site (a)jumps over the potential barrier lowered by the local deformation of the palladium lattice (b)into the adjacent 0 site and again is self-trapped (c).901 the f~rm"*'~*'~ In this form the partition functionf&, has one less degree of freedom thanf&i,. To compute eqn. (1) and compare it with the measured values of D, we next need to determinef&i, andfH,i,. Partition Functions for Hydrogen in Palladium Partition FunctionfH(j) The partition function for the hydrogen atom in the 0 site is written, if the energy levels Enx,n,,n,of the hydrogen atom vibrating in the x, y and z directions with the quantum numbers n, ,nyand n, are given, as Enx,ny, ",measured with inelastic neutron scattering (INS)'7*1 indicate harmonic oscillation of the hydrogen atom with anharmonicity. It seems difficult to obtain from the measured Enx,ny, n, the analytical expression for Enx, that permits cal- ny, n, culation of the Enx,ny,n,beyond those measured, i.e.beyond 190 meV. The first-principles calculation20,2' suggests that the analytical expression for Enx,ny, n, is difficult to obtain because of the great complexity of the potential-energy surface for the hydrogen atom and the resulting delocal- ization of the hydrogen atom beyond one 0 site. Fortunately, however, both the experimental and the calculated results show that the values of the ratio of En=,ny, n, -Eo, o, for deu- terium to that for hydrogen lie around 0.67.'8,'9Taking this fact into account we try to find the Enx,ny,nzsuch that .fH(i) computed from eqn.(3) reproduces the measured solubilities of hydrogen and deuterium, as described below. The relationship between the hydrogen pressure pH and the hydrogen concentration 8, (H/Pd, atomic ratio) is, for small 8, ,given by' 5727 PH = kHuh (4) (5) where k' is a constant,f,,,, is the partition function for hydro- gen molecules in the gas phase, and E, is the heat of solution per hydrogen atom and is assumed to be the same for hydro- gen and deuterium. fH(g) and fD(g) can readily be calcu-lated.2.'5.16*27.28 We consider fH(i, and fDci, that are determined by the Enx,ny, n, of harmonic oscillators of hydro- gen and deuterium whose energy-level spacings hw, and haD are related by hoD/hoH # 1/,/2.Fig. 5 shows the comparison of the values of kD/kH computed using these Enx, with theny, n, experimental results. The experimental values are shown only for those obtained in this study and those of Lasser and because their measurements are the most systematic and because most reported values3,23-26 are in good agree- ment with each other. The thick solid line has been computed using fH(i) and fD(i) for three-dimensional isotropic harmonic oscillators with h~,= 69 meV and hwD= 47 meV,17*'8i.e. hwD/hwH = 0.68, which is slightly smaller than 1/42. The thin solid line shows the values computed for the model used by Rush et a/.' i.e. the three-dimensional isotropic harmonic oscillators with anharmonicity, whose energy levels are given =by Enx,ny.n,hw(nx +ny+n, +3/2) +p(n: +n: +nf +n,+n,, +n, + 3/2), nx, n,,, n, = 0, 1, 2, ...with h(oH= J2hwD = 50 meV and PH (the magnitude of the J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 TIK 2000 1000 500 300 4 I r Y 2 0 0 1 2 3 4 103 KIT Fig. 5 Temperature dependence of kdk,. (0)Present study and (- -) Lasser and The thick solid line has been computed for f&) with houhwf, = 0.68 and the thin solid line for fH(i) with the energy levels given by Rush et a/.' 'For details see the text. anharmonicity) = 2pD = 9.5 meV. Fig. 6 shows the Arrhenius plots of the same k, and k, as those in Fig. 5. From Fig. 5 and 6 we find the solubility can be well repro- duced by f&) and f&) computed for the harmonic oscillators with ho,,/h~~,= 0.68.We will use these fH(i) and fD(i) to compute D, and D,. The value hwD/ho, = 0.68 compares well with those obtained experimentally18 and expected from the cal~ulations'~-~~ and with the value 0.65 introduced to explain the isotope effect for the solubility of hydrogen and deuterium in palladium24 although doubt over the used value 0.65 has been rai~ed.~ TIK 2000 1000 500 300 loio I I I I 10' I 1 1 I 0 1 2 3 4 lo3 KIT Fig. 6 Arrhenius plots of k, and k,. (0)k, and (0)k,, present study; (--) Lasser and Powellz6 (lower line: k, and upper line: kD).The solid lines correspond to those in Fig. 5. Partition Functionf&,) The hydrogen atom in the 0 site jumps into the adjacent 0 site through the tetrahedral (T) site'3*'4*20,21*23 as shown in Fig.3. A saddle (S) point comes between the 0 site and the T site. The diffusion path for the hydrogen atom is thus O-S-T-SO. We assume the activated state to be the state of the hydrogen atom occupying some point near the T site. Since we are dealing with the model of hydrogen diffusion assisted by the local deformation of the palladium lattice and it is difficult to determine unambiguously the potential energy at the S point, it does not seem to be of crucial importance to fix the point for the activated state strictly at the S point. We assume the hydrogen atom in the activated state to be a three-dimensional harmonic oscillator with two identical vibrations in the directions perpendicular to the diffusion path and one slower vibration along it.The energies of the vibrations will be determined when eqn. (1) is computed and compared with the experimental result. Comparison of the Experimental and Calculated Results Fig. 1 shows the comparison of the measured D, and DD with the computed values. The thick solid lines represent the values of eqn. (1) computed using the following partition functions:fH(i) computed for the three-dimensional isotropic harmonic oscillator with ho, = 69 meV and hoD/ho, = 0.68 and fk,, computed for the strongly anisotropic three-dimensional harmonic oscillator with hof, = 1.82hwH for the two directions perpendicular to the diffusion path, Aof, = 0.02hoH for the remaining direction and hoD/ho, = 0.68.The values hof, = 0.02hoH and hof, = 1.82hoH indicate that the hydrogen atom in the activated state is on a rather flat energy surface along the diffusion path and in a relatively steep potential well perpendicular to it, respectively. The acti- vation energy for diffusion has been found to be ed = 195 meV for both hydrogen and deuterium, which is different from the values 219 meV for hydrogen and 215 meV for deu- terium obtained graphically from Fig. 1. This difference comes from the fact that in eqn. (1) the zero-point energies of the vibrations of the hydrogen and deuterium atoms have been included in fH(i), fDCi), f&(i) and fb, and not in Ed. The results of the computations using other models are not shown in Fig.1 because the difference in the results calculated for different models cannot be clearly distinguished in Fig. 1. Fig. 2 shows the comparison of the measured ratio DJD, with the computed ratio. DD/DHincreases beyond unity as the temperature decreases. This is due to the difference in the zero-point energies of the vibrations of the hydrogen and deuterium atoms and to the values of hof, and hob for the activated state" that are larger than those of ho, and Am, for the 0 site. The thick solid line has been computed for the same fH(i), fqi,, f&i, and as those used in Fig. 1. The values of hof,and hob have been chosen so that they repro- duce the values of DD/DH obtained in this study and by Volkl .~~et ~1 The result of Powell and Kirkpatrick' can be repro- duced by choosing the values of hof, and hob.A line almost identical to the thick solid line, which for clarity is not shown in Fig.2, has been obtained by using for fH(i) andfqi, the energy levels given by Rush et a1.,'* i.e. ha, = ,/2ho, = 50 meV and BH = 28, = 9.5 meV and for f&i) and f&i) the energy levels of three-dimensional harmonic oscillators with hok = 2.57h0, for the two directions, hof, = 0.03h0, for the remaining direction and hoh/hwf, = 0.68. The thin solid line shows eqn. (2) computed using for f&) and f&) the energy levels of two-dimensional harmonic oscillators with ha$, = 2.25hwH and AoD/ho, = 0.68. The values of hof, etc. have been chosen so that the experimental result at high tem- peratures could best be reproduced, but the computed values J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 903 approaches 1/J2 as the temperature increases, We can see 5 Y. Fukai, The Metal-Hydrogen System, Springer, Berlin, 1993. the following from the comparison of the three computations above (the solid line, the line almost identical with the solid line and the thick solid line in Fig. 2): the model of diffusion described by eqn. (1) gives a possible explanation of the ratio DD/D, observed at high temperatures as well as at low tem- peratures whereas eqn. (2) can only poorly reproduce the 6 7 8 9 H. K. Birnbaum and C. A. Wert, Ber Bunsenges. Phys. Chem., 1972, 76, 806. J. Volkl and G. Alefeld, in Hydrogen in Metals I, ed. G. Alefeld and J.Volkl, Springer, Berlin, 1978. H. Katsuta, R. J. Farraro and R. B. McLellan, Acta Metall., 1979,27, 11 11. G. L. Powell and J. R. Kirkpatrick, Phys. Reo. B, 1991,43,6968. experimental results, especially those at high temperatures. Similar behaviour of DD/DH has been observed for hydro- gen and deuterium in fcc nickel and ~opper'~,~~ and Katz et ~1.'~have explained it in terms of anharmonicity. The values of haHthey used, however, were 116 meV for nickel and 138 meV for copper, which are larger than the value 88 meV 10 11 12 13 V. A. Gol'tsov, V. B. Demin, V. B. Vykhodets, G. Ye. Kagan and P. V. Gel'd, Phys. Metals Metallog., 1970, 29, 195. K. W. Kehr, in Hydrogen in Metals I, ed. G. Alefeld and J. Volkl, Springer, Berlin, 1978. H. Kronmiiller, G.Higelin, P. Vargas and R. Lasser, Z. Phys. Chem. NF, 1985,143,161. M. J. Gillman, Phil. Mag. A, 1988,58, 257. obtained by INS3' and considerably larger than the value 69 meV for palladi~m.'~~'~ It can be shown from numerical cal- culations that the values of DJDH larger than 1/,/2 are obtained more easily for larger values of ha,; the values 116 meV and 138 meV are large enough to reproduce, without having recourse to eqn. (l), the ratio DD/DH observed at high temperatures in terms of the anharmonicity by using eqn. (2), which corresponds to the model used by Katz et all6 Eqn. (1) is capable of reproducing the nickel result by using the value ho, = 88 meV obtained by INS. Thus, the model described by eqn. (1) explains the observed D, and DD for nickel better than other models such as that described by eqn.(2). However, more experimental and theoretical investi- gations are necessary to discuss D, and DD for a wider range of the fcc metals. 14 15 16 17 18 19 20 21 22 23 24 25 Y. Li and G. Wahnstrom, Phys. Rev. B, 1992,445,14528, S. Glasstone, K. J. Laidler and H. Eyring, The Theory of Rate Processes, McGraw Hill, New York, 1941. L. Katz, M. Guinan and R. J. Borg, Phys. Reo. B, 1971,4, 330. W. Drexel, A. Murani, D. Tocchetti, W. Kley, I. Sosnowska and D. K. Ross, J. Phys. Chem. Solids, 1976,37, 1135. J. J. Rush, J. M. Rowe and D. Richter, Z. Phys. B, 1984,55, 283. L. R. Pratt and J. Eckert, Phys. Reu. B,1989,39, 13170. C. Elsasser, PhD Thesis, Universitat Stuttgart, 1990. C. Elsasser, K. M. Ho, C. T. Chan and M. Fahnle, J.Phys: Condens. Matter, 1992,4, 5207. B. M. Klein and R. E. Cohen, Phys. Rev. B,1992,4S, 12405. E. Wicke and G. H. Nernst, Ber. Bunsenges. Phys. Chem., 1964, 68,224. J. D. Clewley, T. Curran, T. B. Flanagen and W. A. Oates, J. Chem. SOC., Faraday Trans. I, 1973,69,449. W. A. Oates and T. B. Flanagan, J. Chem. SOC., Faraday Trans. I, 1977, 73,407. 26 R. Lasser and G. L. Powell, Phys. Rev. B, 1986,34, 578. Conclusion 27 28 S. Naito, J. Chem. Phys., 1983,79, 3113. S. Naito, T. Hashino and T. Kawai, J. Chem. Phys., 1984, 81, The measured diffusion coefficients for hydrogen and deute- rium in palladium showed Arrhenius behaviour in the tem- perature range 773-1373 K. A model of diffusion in which the hydrogen atom can jump from one 0 site into the adja- cent 0 site assisted by local deformation of the palladium lattice well describes the behaviour of the high-temperature diffusion of hydrogen and deuterium in palladium. 29 30 31 32 33 3489. S. Naito, M.Yamamoto and T. Hashino, J. Phys: Condens. Matter, 1990, 2, 1963. T. Maeda, S. Naito, M. Yamamoto, M. Mabuchi and T. Hashino, J. Chem. SOC., Faraday Trans., 1993,89,4375. W. Jost and A. Widmann, 2. Phys. Chem. B, 1940,45,285. G. Bohmholdt and E. Wicke, 2. Phys. Chem. NF, 1967,56, 133. H. Ziichner and N. Boes, Ber Bunsenges. Phys. Chem., 1972,76, 783. 34 J. Volkl, G. Wollenweber, K. H. Klatt and G. Alefeld, 2. Natur- References forsch., A, 1971,26,922. F. A. Lewis, The Palladium Hydrogen System, Academic, London, 1967. E. Wicke and H. Brodowsky, in Hydrogen in Metals 11,ed. G. Alefeld and J. Volkl, Springer, Berlin, 1978. R. Lasser, Tritium and Helium3 in Metals, Springer, Berlin, 35 36 37 H. Eyring, Trans. Faraday SOC., 1938,34,41. W. Eichenauer, W. Loser and H. Witte, 2. Metallk., 1965, 56, 287. J. Eckert, C. F. Majkzrak, L. Passell and W. B. Daniels, Phys. Rev. B,1984,29,3700. 1989. T. B. Flanagan, Annu. Rev. Mater. Sci., 1991,21,269. Paper 3/06593F; Received 3rd November, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000899

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(6), 905-910 Surface Characterization and Catalytic Activity of Co,Mg, -xAl,O, Solid Solutions Oxidation of Carbon Monoxide by Oxygen Franco Pepe* Dipartimento di Chimica della Terza Universita di Roma, Via Ostiense 159,00154-Roma, ltalia Manlio Occhiuzzi Centro Struttura e Attivita Catalitica di Ossidi (CNR), c/o Dipartimento di Chimica dell'Universita di Roma 'La Sapienza ', P. le A. Moro 5,00185-Roma, ltalia Spinel solid solutions of Co,Mg, -xAl,O, with x = 0-1 have been studied as catalysts for CO oxidation by molec- ular 0,. The experiments were performed either with or without CO, condensation. The apparent activation energies varied depending on the cobalt content and the procedure adopted. The activity per ion (turnover frequency) was fairly constant over the whole range of cobalt contents, when surface cobalt ion concentrations, as measured by X-ray photoelectron spectroscopy (XPS), are considered.It is inferred that each Co2+ ion is active irrespective of its concentration and configuration (isolated, in dimers or in clusters) and that the coordi- nation (tetrahedral or octahedral) plays a minor role in the activity per ion. The reaction appears to be structure insensitive as opposed to the N,O decomposition which appears to be concentration dependent. A mechanism of CO oxidation is proposed in terms of the nature and reactivity of the surface species formed on adsorption of 0, and/or CO. The effect of CO, on the reaction rate is also discussed.In a previous paper' it has been shown that cobalt ions fully Experimentalisolated and dissolved in the MgAl,O, spinel matrix are inactive for dinitrogen oxide decomposition. Comparison Sample Preparation and Characterization with oLer solid solutions containing led to the The catalysts used were those investigated in a previous conclusion that this behaviour had to be correlated with the paper' where the notation adopted, preparation, chemical non-assistance of the matrix MgAl,O, in the kinetically analysis, structural and magnetic properties are described. important step of oxygen migration and desorption. If, Note that the Co,Mg, -xA120, solid solutions are designated however, pairs of cobalt ions were created simply by increas- as SAMCo and that the figure after the symbol SAMCo indi-ing the guest ion concentration, the activity strongly cates the nominal number of cobalt atoms per 100 (Mg + Co) increased with respect to that of the matrix.This finding was atoms. Pure MgAI,O, and CoAl,O, are designated SAM correlated both with (i) the presence of cobalt ions octa- and SACo, respectively. In the present investigation surface hedrally coordinated and (ii) the isolation of pairs of the analysis was performed by XPS. The catalysts were treated +Co2 species. for different times in air at various temperatures (873, 753, The effect on the activity of the different coordinations of 573 K), immediately transferred to the sample holder and transition metal ions (tmi), in the spinel phase (site symmetry) then evacuated at room temperature to better than has been a matter of controversy. Here it is sufficient to recall 1.3 x lo-' N rn-, in the XPS analysis chamber. It was the classical work by Schwab et al.on ferrites for CO oxida- checked that a standard treatment of 3 h at 753 K was suffi- ti~n,~where the divalent ions were found to be completely cient to achieve reproducible results. The spectra were inactive and the normal ferrites more active than the inverse recorded at room temperature on a Leybold Heraeus LHS 10 ones. An opposite result was found by Boreskov et al. for spectrometer (FAT mode) equipped with an HP 2113 com- methane and hydrogen oxidation.6 puter for data analysis. Mg-Ka (1253.6 ev) and Al-Ka (1486.6 As a further aspect, several model reactions appear to be eV) radiations (12 kV, 30 mA) were used.The Co 2p, 0 Is, A1 'structure sensitive' or 'demanding' because the activity per 2s, A1 2p, Mg 2s and Mg 2p peaks were recorded. The 0 1s ion (turnover frequency) varies with ion concentration. peak at 529.5 eV was taken as reference. The data analysis However, the stucture sensitivity does not necessarily arise procedure involved smoothing, backgound subtraction by a from a particular geometry of the active site. In fact, struc- non-linear integral profile and curve-fitting (DS4X program tural sensitivity may also result if the strengths of bonds by Leybold Heraeus). The surface composition of SAMCo involved in the step controlling the molecular mechanism solid solutions was obtained using the sensitivity factor depend on the active ion concentration or are directly linked appr~ach.~."The atomic sensitivity factors were determined to the reaction mechanism.The N,O decomposition and the on SAC0 pure compound by means of the expression: H,-D, equilibration on Coo-MgO solid solutions can be quoted as examples:' in the former, the steps involved are SCd+ = (&o 2plb 1s)SACo (1) oxygen adsorption-desorption ; in the latter, the proposed The following equation was applied to evaluate the cobalt reaction mechanism requires at least two neighbouring Co2 + surface content in the SAMCos ions. It seemed therefore, of interest to investigate, on the same catalytic system, the roles of the active ion concentration and coordination in a simple reaction, such as CO oxidation, in order to elucidate the nature of the active site, to compare the Catalysis activity of cobalt ions in spinel solid solutions with that in CO oxidation was carried out in a circulating system with a pure oxides and to suggest a reaction mechanism.total volume of 0.36 1 with two traps at 77 or 194 K placed before and after the reactor. The catalysts were initially con- ditioned in uucuo (p = 0.0013 N m-,) at 753 K for 4 h and at the same temperature for 0.5 h between runs. The CO : O2 ratio in the mixture was 2 : 1 and the initial pressure usually around 2 x 10, N m-,. The extent of the reaction was fol- lowed by means of a pressure transducer. The absolute first-order rate constant was calculated by the expression : (3) where po is the initial pressure of the mixture, p the pressure at the time t, I/ the reaction volume and A the catalyst surface area.The assumption of a first-order law was proved satisfactory by plotting ln(p/pO) us. time. In all cases, provided that the reaction extent was mantained below 30%, a straight line was obtained. Two sets of experiments were performed by changing the freezing mixture in the traps. By using liquid nitrogen (LN), the CO, produced was immediately condensed after the reactor: these experiments will be labelled as A. The second set with the traps at 194 K and without CO, condensation is labelled as B. The reaction order, n, with respect to the total pressure was calculated by performing experiments at different initial pres- sures, po, of the stoichiometric mixture and plotting the log of initial velocities as a function of the log of the initial pres- sures. Adsorption Experiments Static adsorption experiments were performed in a conven-tional BET apparatus with an LN trap placed in series with the adsorption loop.A fresh portion of the catalyst was evac- uated at a pretreatment temperature of 753 K for 4 h and oxygen and carbon monoxide were added at a given adsorp- tion temperature T, according to the scheme: (a)evacuation at 753 K; (b) oxygen adsorption at T; (c) evacuation at 7';(d) oxygen readsorption at T; (e) evacuation at T; (f) CO adsorption at T;(g) evacuation at T; (h) measurement of the pressure of the condensed gas; (i) evacuation at T; u)oxygen readsorption at T.The steps (a)and (b) allow measurement of the total amount of oxygen adsorbed. The amount of J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 reversibly adsorbed oxygen was measured uia steps (c) and (d). The steps (a)-(e) were repeated until constant values were obtained at each selected temperature T; then a known volume of CO was admitted and the decrease in pressure monitored. The pressure of the condensed gas was then mea- sured [step (h)]. A further admission of oxygen [step (j)] allowed calculation of the amount of oxygen readsorbed. The condensable gas was assumed to comprise CO, only. Results Sample Characterization and Surface Analysis The main features of the sample characterization (experimental cobalt molar fraction, X, , fraction of cobalt ion in tetrahedral sites, a, and BET surface areas, A) are reported in Table 1.The relevant XPS parameters for SAM (1473), SAC0 (1473) and SAMCo (1473) samples are listed in Table 2. In SAMCol the cobalt signal was hardly detectable because its intensity was just a little stronger than the ground noise. However, the peak binding energy (Eb) and the inten- sities of Mg, A1 and 0 were in agreement with those of SAM. For the other cobalt-containing samples, the Co 2p region was very similar in the SAMCos and in SACo. The param- eters are typical of cobalt ions in the 2 + oxidation state.' In the SAMCo samples the 0 1s region showed a small shoul- der at an Eb of ca.2 eV lower than that of the main peak. The shoulder area was estimated by a curve-fitting procedure to be a few per cent of that of the main peak. The cobalt surface contents of the solid solutions were obtained by the areas of the Co 2p and 0 1s peaks reported in Table 3 apply- ing eqn. (1) and (2) quoted in the Experimental section. The results show that with reference to the surface cobalt content of SAC0 (taken equal to l), SAMCo5O has x,, = 0.48, SAMColO has x, = 0.12 and SAMCoS has x, = 0.084. Comparison with the experimental Co content quoted in Table 3 suggests that cobalt enrichment occurs on the dilute samples and the phenomenon is particularly evident on SAMCo5. Catalytic Activity The catalytic activity for carbon monoxide oxidation was investigated in the 420-740 K temperature range on the solid Table 1 Experimental Co2+ mole fraction, fraction of tetrahedral cobalt, BET surface area and apparent activation energies for procedure A and B of Co,Mg, -,Al,O, samples sample a A/m2 g-I SAM (1473) 0.00 1.9 67 f5 113 f5 SAMCol (1473) 0.0033 2.9 38 f5 88 & 5 SAMCo5 (1473) 0.01 6 0.60 1.7 38 f5 52 k5 SAMColO (1473) 0.0323 0.75 1.9 33 f5 50 f5 SAMCo5O (1473) 0.164 0.77 1.o 29 f5 29 & 5 SAC0 (1473) 0.333 0.77 0.7 29 f5 29 f5 SAMCo5 (1073) 0.016 0.69 2.2 42 f5 70 & 5 SAC0 (1073) 0.333 0.82 0.8 33 f5 33 & 5 Table 2 XPSparameters of Co,Mg, -,Al2O, samples equilibrated at 1473 K Sample sat4/eV Mg2p4/eV A1 2s4/eV SAM SAMCo 1 -C -C 537.6 537.4 48.6 48.6 117.6 117.6 SAMCoS 780.2 786.4 0.36 796.1 537.2 48.6 117.6 SAMColO 780.1 785.9 0.43 796.3 537.1 48.5 117.6 SAMCoSO 780.0 785.2 0.36 796.0 536.4 48.3 117.6 SAC0 780.0 785.4 0.39 795.6 536.5 117.6 " Referred to E, (01s) = 529.5 eV.'As observed before charging correction. Hardly detectable. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 3 XPS intensities of Co,Mg, -xAl,04 samples sample XColl1: I* I,, 2: ico 2pb SAM 18.5 2.09 SAMCo 1 0.010 16.3 2.28 -C SAMCoS 0.048 16.9 2.19 1.65 SAMCo 10 0.097 15.4 1.98 2.17 SAMCoSO 0.49 15.9 2.30 8.8 1 SAC0 1 15.2 2.48 17.7 " Experimental cobalt content in the spinel formula Co,Mg,-,A1,04. Intensities are expressed in lo3 eV counts s-'.Hardly detectable. solutions specified in Table 1, which gives the apparent acti- vation energy, E, ,for both sets A and B. The main results are: (i) the catalytic activity increases with increasing cobalt content irrespective of the procedure adopted and (ii) the E, values are higher in procedure B for SAM as well as for the dilute specimens (SAMCol-lo), whereas difference in E,s between procedures A and B are found for SAMCOSO and SACo. The kinetic constants ktbS and k:bs at 500 and 667 K as a function of cobalt content are listed in Table 4. Comparison shows that: (i) at 667 K, ktbs and k;bS are roughly equal for each cobalt concentration with the exception of SAM and SAMCol and (ii) at 500 K the same behaviour is obeyed by SAC0 and SAMCo5O only, whereas for all the dilute samples, kt&/k;b, x 3.The role of the cation distribution has been investigated on the catalysts SAC0 and SAMCo5 equilibrated at different temperatures (see Table 1).The results for SAMCo5 are pre- sented in Fig. 1 as Arrhenius plots. It turns out that the speci- mens from both procedures A and B with a larger amount of octahedral cobalt are more active. Also the sample SAC0 equilibrated at 1473 K and, hence, with a larger percentage of octahedral cobalt, is slightly more active. However, for SAC0 no difference either in activity or in E, was observed between procedures A and B. Reaction orders with respect to the total pressure were investigated in the 560-750 K range and calculated from the initial velocities.They ranged from 0.85 to 1 for procedure A. For procedure B the samples SAMCo5O and SAC0 exhibited the same value as for procedure A, whereas for diluted samples and SAM a negative effect of the CO, was estimated as a negative order in CO, in the range 0.3-0.5. 1 I I I 1.5 1.7 1.9 103 KIT Fig. 1 Catalytic activity of SAMCo5 with different cation distribu- tions. (0)SAMCo5 (1473) expt A; (0)SAMCo5 (1473) expt B; (0) SAMCo5 (1073) expt A; (0)SAMCoS (1073) expt B. Adsorption Experiments The adsorption experiments were performed on SAM, SAMCo5 (1473) and SAMCo5O (1473) at 570 and 670 K. No oxygen adsorption was monitored on pure SAM and in Table 5 the results are reported for SAMCo5 and SAMCo5O in terms of atoms of oxygen and molecules of CO or CO, adsorbed per 1 nm2.It turns out that adsorption is activated on both of the samples and that the oxygen is irreversibly adsorbed on SAMCo5, but only partially reversibly adsorbed on SAMCo5O. Assuming that the total number of cations exposed on the averaged (loo), (111) and (110) is 7 x 1Ol8 m-, and that in oxygen adsorption one oxygen atom is adsorbed per cobalt ion, it follows that the number of cobalt ions exposed on SAMCo5 and SAMCo5O would be 0.20 and 1.12 nrn-,, respectively and that two thirds of cobalt ions would be covered by irreversibly adsorbed oxygen atoms at 670 K on SAMCo5; by contrast, only one third of the Co2+ surface in SAMCoSO would be covered at the same tem- perature (Table 5).The subsequent adsorption of CO is in Table 4 Absolute first-order rate constants at 500 and 667 K for both procedures A and B sample ktbS(500)/lO8m s-' k~,,,(500)/108m s-' k,",,(667)/108 m s-' k:b,(667)/108 m s-' SAM (1473) SAMCol (1473) SAMCo5 (1473) SAMColO (1473) SAMCo5O (1473) SAC0 (1473) SAMCo5 (1073) SAC0 (1073) 0.2 2.7 1.8 13.8 14.5 0.9 13.0 0.8 0.6 13.0 14.5 13.0 0.2 1.5 20.0 13.2 63.0 79.4 10.9. 79.b 0.1 0.6 22.8 12.0 62.7 79.4 12.5 79.0 Table 5 sample T/K SAMCo5 (1473) 570 SAMCoS (1473) 670 SAMCo5O (1473) 570 SAMCo5O (1473) 670 ., 0, adsorption is reported as atoms nm-' &0.03. Oxygen and carbon monoxide adsorption at 570 and 670 K" 02.tot 02.rev 02,irrcv co co2 OZ,reads 0.07 0.00 0.07 0.37 0.00 0.00 0.13 0.00 0.13 0.44 0.40 0.47 0.3 1 0.22 0.09 1.03 1.oo 0.9 1 0.43 0.10 0.33 2.27 2.30 2.00 and CO adsorption as molecules nm-'. The experimental error of the measured quantities was large excess compared with the irreversible adsorption of oxygen on both of the samples. It can be inferred that CO adsorption also involves matrix sites, in addition to Coz+ sites. The amount of CO, evolved by SAMCoSO corresponds to the amount of CO admitted at both temperatures, i.e. adsorption was fully reversible in the 570-670 K range. For SAMCo5 no CO, was desorbed at 570 K. If, however, the temperature was raised to 670 K, the amount of CO, desorbed and collected in the LN trap coincided with the amount of CO adsorbed.This suggests the formation of a strong C0,-surface bond, as a consequence of which, at 570 K successive oxygen adsorption is hindered on SAMCoS. The readsorption of oxygen on SAMCoSO, by contrast, seems to replace totally the surface oxygen which reacted with the CO admitted. Discussion Turnover Frequencies and Cobalt Concentration The catalytic activity of the SAMCo solid solutions and SAC0 for CO oxidation is distinctly higher than that of the pure matrix SAM. The increase in activity is present over the entire cobalt concentration range and suggests that the active centres responsible for the activity of SAC0 and SAMCos involve cobalt ions. In addition, the presence of cobalt is also responsible for the lower E, compared with that of the pure matrix.Indeed it is possible to envisage three distinct sce- narios: (i) the pure matrix SAM has the highest E, irrespec-tive of the procedure adopted; (ii) SAC0 and SAMCo5O have the lowest E, values irrespective of the procedure adopted; (iii) the dilute samples show decreasing values of E, on increasing the cobalt content (SAMCo1 to SAMColO). However, the variation in E, is quite smooth for procedure A in contrast with procedure B, where the variation is rather drastic (Table 1). Since the reaction order deviates from unity as the cobalt content decreases and since the adsorption experiments showed a larger reversibility of CO, adsorption on SAMCo5O as opposed to SAMCoS, it can be inferred that the variation in E, observed in passing from SAC0 to SAM is due to an increase in the adsorption heat of CO,.Indeed a strong adsorption of CO, can take place if basic sites 0;-are present (see mechanism section) and this situation is shown more markedly for dilute samples and SAM, and for procedure B. Turning now to the specific activities, the values of ksbs indicate that isolated Coz+ ions, such as those in SAMCol, are active per se and it may be asked whether the isolation of the ion is a fundamental prerequisite for the reaction to occur. The problem of identifying the surface configurations required for a given reaction may be investigated by inspect- ing the dependence of the turnover frequencies, PIion, as a function of the tmi ~oncentration.~ In fact, if it is assumed that the same mechanism is operating over the whole range of active ion concentration and that the same active complex is involved, as suggested by the constancy of E, in the reac- tion, then the only parameter affecting kabs is the active site concentration.Now the active site concentration in few cases is accessible experimentally; XPS studies on COO-MgO solid solutions" have supported the assumption that the surface Coz+concentration is equal to the Coz+ bulk concentration and that all the cobalt ions or a constant fraction of them are active sites for CO oxidation. The latter assumption deserves some comment. The real surface concentration of active sites may be less than the surface ion content, because the active site could require a special configuration such as Co2+-Co2+ dimers or Co2+clusters. Such a configurational factor is con- centration dependent and one has to calculate the amount of J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 each configuration as a function of the cobalt concentration. However, if it is assumed that the active configuration is simply that containing one cobalt ion, either isolated or belonging to a cluster, the turnover frequency is directly pro- portional to the kabs values divided by the real surface cobalt concentration. An extended study on the quantitative XPS surface analysis in ternary systems 1s in progress and the results presented in the Experimental suggest that the surface cobalt content of SAMCo5 is about twice the analytical one whereas a negligible enrichment is found for SAMColO.For SAMCol it is possible to suggest only that the surface cobalt concentration is similar to the analytical one. This hypothesis is based on the simulation, through the sensitivity factor, of the Co2+signal intensity of samples with x = 0.01 and 0.02. In Fig. 2(a),log Nionvs. log (cobalt surface mole fraction) is reported at two temperatures (500 and 667 K) for procedure A on the assumption that the cobalt surface content of SAC0 is equal to the analytical one. It appears that the trend shown is not influenced by the variation in E,. It may also be added that for procedure B the trend is substantially unmodified. Inspection of Fig.2(a) shows that Nionvalues are scattered around values of 2 x lo-' and 3 x molecules s-' per ion at the considered temperatures. Then during CO oxida-tion, cobalt ions, either isolated as sn dilute samples or in clusters as in concentrated samples, have constant activity. The trend parallels that previously found for Co2+dispersed either in MgO or in other oxide matrices; in addition, the absolute values of Nionfor the Co2'-containing solid solu- tions previously investigated are in good agreement with the present value^,'^^^^ even if the system COO-MgO was the most active for reasons related to the cobalt coordination and/or to the special ability of the matrix in oxygen transfer. It turns out that the cobalt ion activity has to be attributed to the electronic structure of the pair Co-0 only and the reaction appears to be facile and not configuration depen- dent. Finally, note that the slightly higher activity per ion of SAMCoS is in line with the fact that the SAMCo5 (1473) sample contains the largest amount of octhahedral cobalt (see Table 1) and, hence, a higher activity per ion has to be expected as discussed in the next section.A different situation is envisaged for N20 decomposition as shown in Fig. 2(b). In fact, the inactivity of SAMCol points to non-assistance of the spinel matrix in the oxygen SAMCo 1 5 10 50 SAC0 -1 I I +l t I 1 1 - 16 17 18 19 log[C02+] Fag. 2 Turnover frequency as a function of cobalt concentration for two reaction.(a) CO oxidation at 500 K (a),and at 667 K (m); (b) N,O decomposition at 500 K (a),and at 667 K (a). J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 desorption process, and therefore the active site for N,O decomposition must be a configuration of two or more cobalt ions by which neighbouring adsorbed oxygen atoms can be desorbed. On statistical grounds, isolated cobalt ion pairs are formed in the cobalt ion concentration range 1-10% and the highest values of Nionfor SAMCoS and SAMColO reason- ably suggest that isolated Co2+ pairs are the active surface configuration for the decomposition. The reaction is therefore concentration dependent and hence 'demanding' because it is configuration dependent.Catalytic Activity of Octahedral and Tetrahedral Cobalt For N,O decomposition it was proved that small differences in bulk cation distribution, as exist between the (1073) and the (1473) series, resulted in a large change in activity.' In the reaction investigated here, by contrast, the differences in cata- lytic activity due to different bulk cation distributions are less relevant as shown in Fig. 1. In addition, the E, values in the present case vary from 29 kJ mol-' [SACo (1473)l to 33 kJ mol-' [SACo (1073)l and from 38 kJ mol-' [SAMCoS (1473)l to 42 kJ mol-' [SAMCoS (1073)l. In N,O decompo-sition, the increases for the same pairs of samples were from 84 to 121 kJ mol-' and from 88 to 121 kJ mol-', respec-tively. It can be concluded that in both reactions a larger amount of bulk octahedral cobalt favours a higher activity even if to a different extent.An attempt to explain the different role of the tetrahedral cobalt in the two reactions must consider: (i) the actual tetra- hedral cobalt on the surface; (ii) the role of the tetrahedral cobalt in the specific reaction; and (iii) the presence of carbon monoxide and its effect on the reduction of the surface. For point (i) it was proposed that a randomization effect as a function of the equilibration temperature would render the catalyst surface with a given cobalt concentration more rich in octahedral cobalt with respect to the bulk' and, hence, more active when equilibrated at 1473 K. The present results would be in agreement with this hypothesis.However, while tetrahedral cobalt was found to be completely inactive in N,O decomposition because of a strong cobalt-oxygen bond formed upon decomposition, for the present reaction results published el~ewhere'~ have shown that the cobalt ion tetra- hedrally coordinated in zinc oxide is only slightly less active with respect to the octahedral one. Such a behaviour may find support by considering the reducing power of CO able to overwhelm the difference in oxygen bond stengths deriving from different cation coordinations. A simil-ar explanation was, in fact, proposed for manganese ions dissolved in MgO where both Mn3+ and Mn4+ exhibited the same activity for CO oxidation irrespective of the difference in oxygen bond ~trength.'~It transpires that the difference in activity between pairs of samples of equal cobalt content and equilibrated at different temperatures is expected to be less for CO oxidation than for N,O decomposition as, in fact, is found.In conclusion, cobalt ion coordination plays a role in the present reaction. However, many factors, such as the presence of a randomization effect, the activity of tetrahedral cobalt and the reducing ability of the CO, tend to smooth the picture. Reaction Mechanism The oxidation of CO on cobalt-based catalysts generally occurs uia a redox mechanism and the oxygen involved in the active surface complex comes from surface lattice ~xygen.'~." The reaction between CO and 0;-in the SAMCo solid solutions takes place at high temperatures only (above 570-670 K depending on cobalt concentration, Table 909 5).Therefore, a mechanism which involves CO oxidation oia the extraction reaction 0:-+ CO+CO, + 0, + 2e (1) followed by oxidative adsorption of O,, thus restoring the 0;-via a Mars-Van Krevelen mechanism, is likely in the temperature range investigated. In addition, the dependence of turnover frequency on the cobalt concentration suggests that a cobalt-containing intermediate is involved in the reac- tion mechanism during sustained catalysis. With reference to an IR study on the spinel CoAl,04 and on NiO/A1,03,'8*19 it is possible to envisage the surface location of Co2 + on the (1 11) plane as: P uAv \IAv \IAV I 9CO Al CO CO Al I CO CO /I\ I In I A\ 1 A\ where Mg2+ can replace tetrahedral cobalt in the present spinel solid solutions.The unsaturated cationic sites would be the adsorption sites for 0, and CO. In fact, the adduct Co3+.* -0;has been identified as an adsorption intermediate in the MgO-CoO system" while combined electron para- magnetic resonance (EPR) and IR experiments" suggested the coordination of a CO molecule to a Co3+*-.0;adduct and the formation of a hydrogencarbonate-like comple~.'~*~~ On cobalt aluminate the CO adsorption gives rise to IR bands assigned to CO on Co2+ centre^.'^*^^ Moreover, on increasing the contact time, bands in the carbonate region grew up and these modifications were associated with redox reactions giving CO, and carbonates." A tentative reaction intermediate would therefore be 0, co The evolution of the suggested intermediate implies the for- +mation of a carboxylate species Co3 * -CO;.However, the formation of a true carbonate species cannot be excluded : co3+. . .co; + 0:--,CoZ+...co2-(11) CO, desorption might take place as follows: CO~+.-CO; -,co, + co2+ (111) and/or as : co~+-*co~-co2++ co, + 0:-(Iv)-b Monodentate and/or bidentate carbonate complexes have frequently been identified by IR on cobalt-containing ~arnples~~*~~-~'and, in general, their stability would depend on the basic character of the 0;-environment. CO, effect on the reaction rate The main result of the adsorption cycles was that CO, adsorption appeared to be reversible on SAMCoSO at all temperatures, but on SAMCoS only at high temperature 670 K.In addition the activity was generally lower for procedure B and deviation from a first-order kinetic law was observed. These findings suggest that strong adsorption of CO, can involve the basic sites 0'-of the matrix MgA1204 and can arise to a greater extent when CO, is not frozen. The two forms of adsorbed CO,, the former produced via reaction and the latter directly adsorbed from the gas phase, may be not in equilibrium, as reported for CO oxidation on Tho, .29 In our case the first species would be that formed on the Co2+. .Oz-centres in concentrated samples and constitutes the active complex. The second species, which inhibits the reaction, would be related to the increasingly basic character of 02-in the Co2+*..02-sites in dilute samples, where, on statistical grounds, the 0’-ions will be shared more with Mg2+ ions.In fact, on MgA120, surface carbonates are found to be more stable than on CoA120, by IR investiga- tion~.~~ Finally, note that the activity of SAC0 in the present work is one order of magnitude less than that of C00.l~ Therefore, the large decay in activity found when surface CoA1204 is f~rmed~l.~~is not confirmed, while the E, values are substan- tially in agreement. Conclusions The cobalt surface analysis proved that a tmi enrichment exists, which is a function of the cobalt concentration. The knowledge of the tmi real surface content has revealed dis- tinctly differance activity patterns for N20 decomposition and CO oxidation. The trend of the turnover frequency as a function of cobalt concentration suggests that the surface configurations active for the specific reactions are different.It turns out that the former reaction is configuration dependent or ‘demanding’ while the latter is a configuration indepen- dent or ‘facile’ reaction. As a consequence in each reaction, both the coordination of the tmi and the matrix environment play a very different role in the activity per ion. References C. Angeletti, F. Pepe and P. Porta, J. Chem. Soc., Faraday Trans. I, 1978,74,1595. A. Cimino and F. Pepe, J. Catal., 1972,25,362. F. Pepe, M. Schiavello and G. Ferraris, 2.Phys. Chem. NF, 1975, %, 297. C. Angeletti, A. Cimino, V. Indovina, F. Pepe and M. Schiavello, J. Chem. SOC., Faraday Trans. I, 1981,77,641. G. M. Schwab, E. Roth, Ch. Grintos and N. Mavrakis, Structure and Properties ofsolid Surfaces, ed. R. Gomer and C. S. Smith, Chicago, 1953, p. 464. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 6 G. K. Boreskov, V. V. Poposki, N. El. Lebedeva, V. A. Sazonov and T. V. Andruchkevitch, Kinet. Catal., 1970, 11, 1093. 7 V. Indovina, A. Cimino, M. Inversi and F. Pepe, J. Catal., 1979, 58,396. 8 C. Angeletti, F. Pepe and P. Porta, J. Chem. SOC., Faraday Trans. 1, 1977,73, 1972. 9 C. D. Wagner, L. E. Davis, M. V. Zeller, J. A. Taylor, R. H. Raymond and L. H. Gale, SurJ: Interjuce Anal., 1981,3,211.10 B. L. Yang, S. F. Chan, W. S. Chang and Y. Z. Chen J. Catal., 1991, 130, 52. 11 B. A. Sexton, A. E. Hughes and T. W. Turney, J. Catal., 1986, 97, 390. 12 A. Cimino, Mater. Chem. Phys., 1985,13,221. 13 V. Indovina, A. Cimino and F. Pepe, Gazz. Chim Ital., 1980,110, 13. 14 V. Indovina, A. Cimino and F. Pepe, Proc. ZX Zberoamerican Symposium on Catalysis, Lisbon, 1984. 15 A. Cimino and V. Indovina, J. Catal., 1974,33,493. 16 J. K. Dixon and J. E. Longifield, Catalysis, Reynold, New York, 1960, vol. 7, p. 303. 17 J. M. Thomas and W. J. Thomas, Introduction to the Principles of Heterogeneous Catalysis, Academic Press, London, 1967, p. 367. 18 G. Busca, V. Lorenzelli, V. S. Escribano and R. Guidetti, J. Catal., 1991, 131, 167. 19 E. Borello, A. Cimino, G. Ghiotti, M. Lo Jacono, M. Schiavello and A. Zecchina, Discuss Faraday So(:.,1971,52,149. 20 D. Cordischi and V. Indovina, Adsorption and Catalysis on Oxide Surfaces, Elsevier, Amsterdam, 1985, p. 209. 21 V. Indovina, D. Cordischi, M. Occhiuzzi and A. Zecchina, per- sonal communication. 22 V. G. Amerikov and L. A. Kasatkina, Kinet. CataL, 1971, 12, 137. 23 C. Morterra, G. Ghiotti, F. Boccuzzi and S. Coluccia, J. Catal., 1978,51, 299. 24 G. Busca, R. Guidetti and V. Lorenzelli, J. Chem. SOC.,Faraday Trans. I, 1990,86,989. 25 G. Busca, F. Trifiro and A. Vaccari, Langmuir, 1990,6, 1440. 26 W. Hertl, J. Catal., 1973,31,281. 27 A. J. Goodsel, J. Catal., 1973,30, 175. 28 S. Matsushita and T. Tanaka, J. Chem. Phys., 1962,36, 665. 29 M. Breisse, B. Claude1 and J. Veron, Kinet. Katal., 1973, 14, 102. 30 G. Busca, personal communication. 31 Y. Y. Yao, J. Catal., 1974,33, 108. 32 M. A. Wheller and M. Bettman, J. Catal., 1975,40, 124. Paper 3/05781J; Received 24th September, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000905

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994,90(6), 91 1-917 91 1 Mechanism of Branched Carbon-chain Formation from CO and H, over Oxide Catalysts Part 1.-Adsorbed Species on ZrO, and CeO, during CO Hydrogenation Ken-ichi Maruya," Akihiro Takasawa, Makiko Aikawa, Takashi Hataoka and Kazunari Domen Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta , Midori-ku, Yokohama 227, Japan Takaharu Onishl Tokyo Polytechnic College ,232-I Oga wanishi, Kodaira City, Tokyo 187, Japan The adsorbed species on ZrO, and CeO, during CO hydrogenation forming branched carbon chains, especially isobutene, (over ZrO,), have been investigated by chemical trapping, in situ IR, and solid-state NMR methods. These indicated that methoxide and formate were present as the surface species on ZrO, .CO hydrogenation on ZrO, with pre-adsorbed methoxide or formate showed that the pre-adsorption of methoxide promotes the forma- tion of the higher hydrocarbons containing mainly isobutene but that formate retards the reaction. Chemical trapping experiments on CeO, after CO hydrogenation at 523 and 673 K led to the formation of methane together with methanol, indicating the presence of methyl, p-methylene, or carbene as a surface species. q2-Formaldehyde was suggested to be the precursor of the methyl, p-methylene, or carbene species, the latter of which in turn gives C, species by insertion of CO. Following the products of CO hydrogenation and chemical trapping on CeO, at 523 K with time indicated that an aldol-condensation-type reaction leads to C, and branchedchain C, products from C, oxyhydrocarbon.We have previously reported that isobutene,' 2-met hylpropanal,2 isobutyl alcohol2 and isoprene3 are formed from hydrogenation of CO under mild reaction conditions over ZrO,, CeO,, In,O,-CeO, and Ln,O,(Ln = lanthanide)-CeO, catalysts, respectively. It has also been shown that branched alkanes are formed from CO hydro- genation over Tho, :La,O, '* and Dy,O, 'catalysts under very severe conditions. All of these products have branched carbon chains and all the catalysts are oxides. These results seem to indicate that the formation of branched carbon-chain compounds from CO hydrogenation is a characteristic of oxide catalysts which are difficult to reduce such as ZrO,, lanthanide and actinide oxide^.^ The mechanism of branched-hydrocarbon formation is not yet clear, even though there have been a few proposal^.^-'^ On the other hand, there have been many reports on the synthesis of iso- butyl alcohol with modified catalysts for methanol synthe- sis.The synthesis of this branched higher alcohol has generally been explained by a conventional aldolic conden- sation mechanism'' and recently Nunan et al. proposed a modified aldol-condensation-type mechanism on the basis of 13Ctracer experiments.', However, the mechanism of forma- tion of C, from C, species is still a matter of controversy. In this paper the species adsorbed on ZrO, and CeO, catalysts during CO hydrogenation are investigated by means of chemical trapping, in situ IR and solid-state NMR methods, and the mechanism of branched carbon-chain formation, especially of C, species, on the oxide catalysts will be dis- cussed.Experimental The catalysts were prepared by the precipitation of hydrox- ides from the aqueous nitrates of Zr and Ce with ca. 3% aqueous ammonia solution and calcination of the precipitates at 773 K for 3 h. CO hydrogenation was carried out typically in a gas-circulation system of total volume 470 cm3 and reactor volume of 55 cm3. Catalysts, except for CeO, used for CO hydrogenation at 523 K, were evacuated at 973 K for 3 h before the reaction. For CO hydrogenation at 523 K, the CeO, catalyst was evacuated at 973 K for 3 h and then treated with H, at 67 kPa and 773 K for 16 h before the reaction.The pre-adsorption of methoxide on ZrO, was carried out by treatment with dimethyl ether at 35 Torr and 643 K. The pre-adsorption of formate on ZrO, was achieved by evacuation at 973 K for 3 h, treatment with water vapour at room temperature and at 15 Torr, CO treatment at 643 K for 30 min, and then evacuation for 1 h at 643 K. After evac- uating the gas-circulating system, except for the reactor, CO hydrogenation over ZrO, with pre-adsorbed methoxide or formate was carried out at 643 K. Chemical trapping was carried out by modification of the method described in the 1iterat~re.I~ During CO hydro-genation the catalyst was rapidly cooled by liquid N, ,slowly warmed to room temperature under evacuation, and treated r'O-I P, 0 .-I I I I I I OO 20 40 60 chemical trapping time/h Fig.1 Dependence of the HCO,CH, formation by chemical trap- ping with dimethyl sulfate on the chemical trapping time with a vapour of diluted aqueous HC1 solution at room tem- perature or of dimethyl sulfate at 453 K under circulation of He. The products were collected at liquid-nitrogen tem- perature together with chemical trapping reagents. The iden- tification of products was carried out using GC-MS. IR spectra of the catalysts, which had been treated for 1 h with a vapour of diluted aqueous HCl solution, showed >90% dis- appearance of methoxide, indicating the rapid completion of chemical trapping.However, it takes much longer to com- plete the treatment with dimethyl sulfate, as shown in Fig. 1. The catalysts for the CP MAS NMR measurement, which were rapidly cooled after CO hydrogenation, were evacuated while warming slowly, and then placed in a Pyrex tube in a glove box under Ar. CP MAS NMR spectra were recorded on a JEOL GX 270 at room temperature. Methoxide and carbonate species were assigned on the basis of adsorbed species obtained by the adsorption of l3CH3QH and 13C0, on fresh ZrO,. Formate resonances were assigned according to species obtained by the adsorption of "CO on ZrO, which was treated with a water vapour at room temperature and then evacuated at 573 K for 10 min. Results Adsorbed Species on ZrO, during CO Hydrogenation 13C CP MAS NMR Spectra 13C CP MAS NMR spectra of adsorbed species on ZrO, are shown in Fig. 2.Measurements of the same sample a week later showed a 10% decrease in the peak intensities. In situ IR measurements showed no change of the peak intensities upon evacuation at room temperature. These results are indicative of the stability of the adsorbed species. The chemi- cal shifts of methoxide, formate and carbonate species adsorbed on ZrO, are presented in Table 1. The 13C NMR spectrum of ZrO, after reaction at 523 K shows only two peaks due to methoxide and formate species at 652.7 and 170.0, respectively. However, the spectrum at 643 K is more complex. The methoxide peaks consist of a main peak at 656.5 accompanied by a shoulder of CQ.653. Two peaks are observed for the formate species. A new peak at 6173.1 appears in addition to the peak at 6169.2. II -l""l""r-.'.l~"',"-,I. *..I 250 200 150 100 50 0 -50 6 Fig. 2 CP MAS MNR spectra of ZrO, treated with a mixture of CO and H, at (a)523 K and (b) 643 K.* spinning side band J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 Chemical shifts of methoxide, formate, and carbonate species adsorbed on ZrO, ~~~ ~~~ ~ species chemical shifts (63 ref. 'CH 30-55.1" this work 52.7' this work 56.5, 53.0 (sh)'Pd this work "CH,O-Zr complex 60.1 13~0,-163.0 this work ~13~00-169.5 this work 170.p this work 193.1, 169.2'' this work " Methanol adsorption. 'At 523 K. sh = shoulder. At 643 K.CO, adsorption. CO adsorption on H,O-treated ZrO, . Table 2 Amount of foxmate and methoxide species obtained by chemical trapping after CO hydrogenation over ZrO, " amount of adsorbed species/pmol g-T/K formate methoxide CO, ' 523 46 140 250 & 50 643 6 30 110 f30 Chemical Trapping of Adsorbed Species on ZrO, Table 2 shows the products and the amounts of species obtained in chemical trapping experiments. Only methyl formate and methanol were detected. (MeO),CO from the carbonate species was not detected. The total amount of carbon species on the catalyst surface was estimated by the amount of CO,, which was formed upon treatment of the catalyst with 0, at 973 K after CO hydrogenation. The sums of the amounts of methyl formate and methanol are 186 pmol g-' at 523 K and 36 pmol g-' at 643 K, while at these tem- peratures 250 & 50 and 110 _+ 30 pmol g-' CO, is formed. Therefore, the respective sums of amounts of formate and methoxide species are 74 f20% and 33 27%, based on the total amount of surface carbon species. The XPS measure-ments showed no clear difference between peak height of carbon species on the fresh and used ZrO, surfaces.CO Hydrogenation on ZrO, with Pre-adsorbed Methoxide or Formate Fig. 3 shows the product yield with time for CO hydro- genation over ZrO, with pre-adsorbed methoxide. The reac- tion times of 10, 30 and 50 min refer to product collection 81 4 6-4--0 20 40 reaction time/rnin Fig. 3 Dependence of hydrocarbon yield on the reaction time of CO hydrogenation on ZrO, with pre-adsorbed methoxide: C, (O), c3 (A) and c, (0) J.CHEM. SOC. FARADAY TRANS.,1994, VOL. 90 cb, 1.c \ h5-O 0.8 -.5 2 '5. 0.6 se8 0.4 0c> 0.2 0 20 40 reaction time/min Fig. 4 Dependence of the yields of hydrocarbon, (a)CO, (O),and H,O (A) on the reaction time of CO hydrogenation over ZrO, with pre-adsorbed formate w I IO0L 20 40 60 reaction ti me/m in Fig. 5 Dependence of hydrocarbon selectivity (based on carbon) on the reaction time of CO hydrogenation on ZrO, with pre-adsorbed formate: C, (O),C, (A),C, (0)and C, (0) times from 0-20, 20-40, 40-60 min, respectively, at liquid- nitrogen temperature. The product yield within the first 20 min is high and within the next 20 min has already reached the steady state. The hydrocarbon distribution seems to be almost constant over the whole reaction time.Fig. 4 shows the yields of hydrocarbons, H, and CO, formed with time during CO hydrogenation over ZrO, with pre-adsorbed formate. The yield of CO, is very high in the first 20 min, after which it decreases rapidly. Fig. 5 shows the hydrocar- bon distribution with time. C, and C, hydrocarbon selec- tivities decrease with increasing reaction time, while those of C4 and C, hydrocarbons increase. CO Hydrogenation and Chemical Trapping of Adsorbed Species on CeO, Catalysts CO Hydrogenation at 673 K over CeO, The products of CO hydrogenation at 673 K over CeO, are presented in Table 3. The main species are C,, C, and C, hydrocarbons which formed with similar yields.The selec- tivities of isobutene in C4 hydrocarbons and of isoprene in 913 Table 3 Chemical trapping products at 523 and 673 K on CeO, amount of products product selectivity from chemical in CO hydrogenation trapping/pmol (%)" product 523 K 673 K 523 K 673 K hydrocarbon c, 0.47 0.23 26 0.001 0.018 11 28c2 tr 0.008 11 9c3 c4 tr 0.018 20 21 c5 tr 0.011 20 8 C6 + b b 38 8 oxyhydrocarbon bCH,OH 0.36 0.18 56b bCH,CHO C b 1 bC,H,CHO 0.001 b(CH,),CHCHO 0.04 b 26 b 5 b(CH,),CHCH,OH b b 1 b(CH,),CHCOCH c b 7 b(CH ,),CHCOC,H b b b(CH,),CHCOCH(CH,), b 55 2.5 ~~~~~~~~~ a Formation rates of hydrocarbons are 0.55 x lo7 mol g-' h-' at 523 K, (surface area = 11 m2 g-I), and 427 x lo7 mol g-l h-' at 674 K (surface area = 21 m2 g-').Formation rates of oxyhydrocarbons are 2.5 x lo7 at 523 K and almost zero at 673 K. Not detected. Trace amount although the overall selectivity (21%) of C, hydrocarbons at 673 K with CeO, is much lower than that with ZrO, (63.4%).' CO hydrogenation on CeO, with pre-adsorbed methoxide resulted in the increase of methane alone and had almost no effect on the other hydrocarbons, which is different to the case for ZrO, . Chemical Trapping on CeO, after CO Hydrogenation at 673 K The chemical trapping products from CeO, after CO hydro-genation at 673 K are mostly methane and methanol as shown in Table 3. +c6+ hydrocarbons form in much smaller amounts and the product distribution is similar to that of CO hydrogenation. CO Hydrogenation over CeO, at 523 K The catalyst for CO hydrogenation at 523 K was treated with H, at 773 K for 16 h to decrease the induction time, which was, however, still long as shown in Fig.6 and 7. Hydrogen pretreatment leads to very high initial yields of hydrocarbons, which rapidly decrease and reach the steady state after 72 h, as shown in Fig. 6. Initially methane forms at the highest rate, however, higher hydrocarbons become the main pro- ducts in the steady state. Fig. 7 shows the rate of formation of oxyhydrocarbons with time. Methanol and c6 ketone pass through maximum yields. Propanal increases significantly after 2 days and then rapidly decreases.The rate of formation of 2-methylpropanal increases gradually over 5 days. Diiso- propyl ketone, which forms in the highest yield among the oxyhydrocarbons, reaches steady yield within 2 days. Chemical Trapping on CeO, after CO Hydrogenation at 523 K The yield of chemical trapping products is different from that of the products of CO hydrogenation, as shown in Fig. 8. Methanol and propanal reach maximum values after 1 day. The maximum yield of methane occurs a little later, while 2-methylpropanal starts to form after 2 days, and is still increasing after 4 days, C, and C5+ aldehydes were not C, hydrocarbons at 673 K are 66 and 71%, re~pectively,~,~detected. 914 J. CHEM. SOC. FARADAk' TRANS., 1994, VOL.90 n 0,-0.14 0 r 5.I '=-r 37 0.12 D CT, 2 h n0, cn O.lOl\ .-QP 2 +.' iQ .-EE + -D .-al > reaction ti me/days Fig. 8 Dependence of product yield from chemical trapping on the reaction time of CO hydrogenation on CeO, at 523 K: methane (a);methanol (0);C, ,propanal (A) and C,, 2-methylpropanal (0) 0 1 2 3 4 5 reaction time/days methoxide. However, treatment of catalysts with 0, after CO hydrogenation indicates that the sum of the amounts of Fig. 6 Dependence of the rate of hydrocarbon formation on the reaction time of CO hydrogenation over CeO, at 523 K: C, (O), C, formate and methoxide species is 74 and 33% of the amount (01, and c6 (0) of CO, corresponding to the total carbon species on the cata- C, (V), C, (O),C, (0) lyst at 523 and 643 K, respectively.In order to confirm the amounts determined in the chemical trapping experiments, 0.1: the ratio of the amount of formate or methoxide species at 523 K to that at 643 K was compared to the ratio of the IR band intensity at 523 to that at 673 K.', The ratio of inten- c sity of the 0-C-0 asymmetric stretching band due to the I r formate species at 523 (1566) to that at 673 K (1560 cm-') r I cn / was 7.4, while the ratio of the amount of methyl formate h 0, obtained at 523 K to that formed at 643 K is 7.7 from Table -2. Similarly, the ratio of intensity of the C-0 stretchingO 0.1( band due to methoxide at 523 (1144) to that at 673 K (11345--. i c cm-') was 5.4, while the ratio of the amount of methanol at 2 523 K to that at 643 K is 4.7 from Table 2.This indicates C .-that the amounts of methanol and methyl formate deter- c E mined by chemical trapping are near to the actual amounts *b of adsorbed methoxide and formate species, although it is C presumed that the difference between the molar absorption 5 O.O! coefficients of the bands at 523 and 673 K is negligible and iQ 2 that the difference between the peak intensities at 643 and 673 K are within experimental errors. Therefore, the large z r>. difference between the amount of CO, and the sum of methyl 0 formate and methanol suggests the presence of species other than formate and methoxide on the catalyst surface. The species most likely to escape detection by NMR and chemical I trapping measurements is coke, because (i) the catalyst was 0 1 2 3 4 5 dark grey and grey after CO hydrogenation at 643 and 523 reaction time/days K, respectively, (ii) the NOE effect for coke will be small Fig.7 Dependence of the rate of oxyhydrocarbon formation on the because of the lack of hydrogen, and (iii) the reaction with reaction time of CO hydrogenation over CeO, at 523 K: C,, meth-aqueous diluted HCl solution vapour and dimethyl sulfate anol (0)C, ,propanal (A) C,, 2-methylpropanal (0);C, , methyl would give no gaseous products. Since a broad peak around isopropyl ketone (0);c6, ethyl isopropyl ketone (0);isopropyl ketone (m) and C,, di- 60 was also observed on fresh ZrO,, it could not be due to carbon species from CO hydrogenation, though the species could not be identified.Thus, only formate and methoxide can be possible intermediates in the formation of isobutene Discussion from CO. On the other hand, the pre-adsorption of methox- Adsorbed Species on ZrO, ide leads to the initial high yield of' hydrocarbons with high C, selectivity as shown in Fig. 3, while the treatment of CeO, In situ IR spectra of CO hydrogenation over ZrO, at 523 with pre-adsorbed formate with a mixture of CO and H, and 673 K showed only the presence of formate and methox- results initially in the formation of a large amount of CO, ide species.14 Solid-state NMR and chemical trapping mea- and only small amount of hydrocarbons, and lower C, selec- surements also show no species other than formate and tivity as shown in Fig.4 and 5. Since the decrease of CO, is J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 accompanied by an increase in hydrocarbons and the large amount of CO, is formed from the decomposition of the adsorbed formate species, it is unlikely that surface-adsorbed formate is an intermediate in hydrocarbon formation, espe- cially C, hydrocarbons. Therefore, only methoxide may be an intermediate in the formation of C, hydrocarbons from CO. Reaction Mechanism Selective Ethene Formation on CeO, As described above, the formation of 2-methylpropanal and isobutene seems to be accounted for by the aldol-condensation-type reaction from acetaldehyde. The forma- tion of the isobutyl group in the synthesis of isobutyl alcohol has also been described by the aldol-condensation type reac- Adsorbed Species onCeO,'9, tion.' However, the formation of acetaldehyde, which Since formate is not an intermediate but rather a poison for hydrocarbon formation, chemical trapping by a vapour of aqueous diluted HCl solution was carried out only for trap- ping adsorbed molecules such as alkoxide, acyl, q2-aldehyde, alkyl, p-methylene, and carbene species.The main products from chemical trapping on the CeO, catalyst are methanol and methane after CO hydrogenation at both 523 and 673 K. Methanol is presumably derived from methoxide, as in ZrO, . 30 pmol g-' methoxide is adsorbed on ZrO, at 643 K, i.e. 0.53 pmol m-,, while 0.18 pmol g-' is absorbed on CeO, at 673 K, i.e.8.6 x lo-, pmol m-,, and 0.36 pmol g-' at 523 K, i.e. 33 x lop3pmol m-2. Therefore, twice as much methoxide is adsorbed on ZrO, than on CeO, , even at higher temperatures. The pre-adsorption of methoxide only enhances the formation of methane for CeO,, but the formation of higher hydrocarbons with high C, selectivity for ZrO, . This may indicate that the propaga- tion step from C, to C, species is slower for CeO, than for ZrO, . The formation of methane during chemical trapping on CeO, is the most notable point. The reaction of methoxide with protons gives methanol but not methane. It is known that CeO, is partially reduced by H, and reoxidized by H,O to form H, .l5 The reaction of methoxide with H, ,which is formed by the reaction of the partially reduced CeO, with H,O during chemical trapping, should be negligible, because the coexistence of H,O in a mixture of CO and H, retards CO hydrogenation.16 Therefore, the surface species giving methane during chemical trapping could be a C, species such as methyl, p-methylene or carbene.0.23 pmol g-' of C, species is adsorbed at 673 K (Table 3), i.e. 0.011 pmol md2, as the surface area is 23 m2 g-'. There are 5.4 x lo-'' mol surface Ce ions on the fluorite structure and the unit cell con- tains two Ce ions on the ~urface.'~ Therefore, the surface concentration of the C, species is estimated to be ca. 0.1% of the surface Ce ion concentration. Similarly at 523 K, the con- centration of C, species is ca.0.043 pmol rn-, and therefore the surface concentration is about 0.4%.The high concentra- tion of C, species is characteristic of CeO, but not the ZrO, catalyst. Chemical trapping on CeO, after hydrogenation at 523 K yields aldehydes such as 2-methylpropanal in addition to methanol and methane (Table 3). Adsorbed molecules which would give such aldehydes on treatment with acid could be q2-aldehyde or acyl species. Treatment of the latter with acidic water usually gives carboxylic acid,'* however, it has been reported that the treatment of an acyl-zirconium complex with hydrochloric acid gives an aldehyde product.lg Therefore, adsorbed acyl species may be the source of alde- hyde upon chemical trapping. The plot of chemical trapping products us.time (Fig. 8) shows that methanol is the first should be the starting aldehyde for aldol condensation is not clearly understood. The following results are notable when considering a possible mechanism for C, aldehyde formation : (i) the high selectivity of ethene during CO hydrogenation over CeO, at 673 K and (ii) the formation of methane during chemical trapping. First, as reported previously,20 CeO, is a good catalyst for the formation of ethene from CO and H,, i.e. it has higher C, than C, selectivity (29% us. 9%) in total hydrocarbons and gives a higher proportion of ethene (96%) in C, hydrocarbons than propenes (84%) in C, hydrocar- bons. Transition metals" and ZrO, catalysts usually form ethene with lower C, hydrocarbon selectivity than that expected from the Schulz-Flory distribution and with higher ethene selectivity in C, hydrocarbons than propenes in C, hydrocarbons.These results seem to suggest that there could be one particular reaction path which forms only ethene in the course of CO hydrogenation over CeO, . Considering the reaction to give ethene alone, coupling of either p-methylene,' or carbene species,, could explain the unusually high ethene selectivity. Secondly, methane formation upon chemical trapping indicates the presence of methyl, p-methylene or carbene species on the catalyst during CO hydrogenation as seen above. This indicates that there is a p-methylene or carbene species on CeO, ,which induce high ethene selectivity.On the other hand, it has been reported that the thermolysis of a trimeric (q2-formaldehyde)zir-conocene compound leads to the expulsion of methylene to form hydrocarbons [eqn. (l)] and 'metal oxide' fragments,, and that the q2-methoxymethylzirconocene,species decom- poses at 523 K to give ethene and the methoxyzirconium complex shown in eqn. (2).,"4 /"\"'$, -(CPZ~O)~ hydrocarbons (1)+ Cp2Zr-0 However, Toreki et al. reported that [{Ta(silox),Cl) ,(pH),] (silox = BuiSiO) and [{Ta(silox),H,},J reacts with CO to form [{Ta(silox),Cl) ,@-H)(p-CHO)] and [{Ta(silox),Cl) ,@-CH,)] respectively, as in the following reaction^.^ HI r. co /:Ap-H),] lTa: ,TaC I(silox),] (3)[{Ta(silo~)~CI}~( [(si10x)~C H H\ ,Hco product, followed by propanal, and then finally 2-[{Ta(~iIox)~H2)~] methylpropanal.This is the order of the species accumulated on the CeO, surface, suggesting chain growth by an aldol- condensation-type reaction as 2-methylpropanal is the end H They also showed that aqueous degradation of p-formyl and product from acetaldehyde in this type of reaction. The p-formaldehyde species produces methanol, and that the p-results of CO hydrogenation at 523 K (Fig. 7) support this methylene species is quenched by H,O to give methane in .~~hypothesis, although the formation of ketone cannot be so quantitative yield. Also, Casey et ~ 1 and Gladysz et easily explained. have shown that o-formyl and hydroxycarbene species react [(sil~x)~Ta~~~Ta(silox)~(4) '0'1 IH with protons to give methanol.26 These reactions, and the in situ IR measurements during CO hydrogenation at 473 K over ZrO, showing the presence of formyl, dioxymethylene and methoxide specie~,'~*~~ and the formation of formalde- hyde with no induction time2' suggest that similar reactions occur for ZrO, and CeO, catalysts as shown in Scheme 1.r! A Scheme 1 Proposed mechanism of selective ethene formation Formation of Isobutenefrom C,Species In the above discussion an aldol-condensation type reaction for the formation of 2-methylpropanal from acetaldehyde and the thermolysis of q2-formaldehyde to form carbene and then ethene have been proposed. The next question concerns the C, precursor of acetaldehyde, since the latter is a starting compound in an aldol-condensation-type reaction to form isobutene, for which selectively formed ethene is not the pre- cursor.29 The formation of acetaldehyde or ethanol on oxide cata- lysts, or those for isobutyl alcohol formation, has been explained by the insertion of CO into the carbon-metal bond of the q2-formaldehyde species7p8 and the reaction of formal- dehyde with adsorbed formyl species.12 However, the carbon atoms of the carbonyl group of q-and/or pformaldehyde, J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 y3 CH2 M-H y3 co -A c=o -M -0-M --M -0-M __ -L+-I\ -0-M I I It I1 Scheme 3 Formation of acyl species from q2-formaldehyde for alkene hydrogenation,20 supporting the above suggestion. The formation of acyl species leads to the formation of 2- methylpropanal and then isobutene.The formaldehyde for the aldol-condensation-type reaction could result from a large amount of adsorbed methoxide species. The selective formation of isobutene is due to the 2-methylpropanal being an end product in the aldol-condensation-type reaction. Formation of Diisopropyl Ketone CO hydrogenation over CeO, at 523 K results in the forma- tion of ketones, although they are hardly detected by chemi- cal trapping. Diisopropyl ketone can be formed in two ways: (i) the ketonisation reaction of 2-methylpropanal or (ii) the aldol-condensation-type reaction of acetone, which could be formed oia ketonisation of acetaldehyde. As methyl isopropyl ketone and ethyl isopropyl ketone are formed and diiso- propyl ketone reaches a steady state faster than 2-methylpropanal ketones are likely to be formed by the aldol condensation-type reaction of acetone with formaldehyde as shown in Scheme 4.Diisopropyl ketone is the end product from the reaction of acetone with formaldehyde. This is the reason for the high selectivity of diisopropyl ketone com- pared to other ketones. CO 0:: PCH3C-H A2CH3-g-CH3 CH20 H2 CH3-CHZ-C-CH340 H2O I I1CHo CH30---$>CH3-CH-C-CH3 H!0 ketone and adsorbed formyl species are ele~trophilic.~~*~~.~' Therefore, the above indication that p-methylene or carbene species are present on the CeO, catalyst could naturally suggest that these species or their precursors may participate as a key intermediate in the formation of acetaldehyde, and, therefore, 2-methylpropanal and isobutene.Where p-methylene or carbene are assumed to be precur- sors of acetaldehyde, carbonylation followed by reaction with hydrides gives acyl species as shown in Scheme 2. H3C. EH2 -'k,-,O -M-CH3-M-I M-H c=o or or L-4-0-y-uu ?\ I I Scheme 2 Formation of acyl species from p-methylene or carbene On the other hand, the reaction of q2-formaldehyde with hydrides gives adsorbed methyl species.31 The presence of p-methylene or carbene species on CeO, suggests that the mechanism outlined in Scheme 2 is pre- dominant for this catalyst. The experimental results that CO hydrogenation over CeO, results in the selective formation of ethene at 673 K and the formation of aldehyde and ketones at 523 K indicate that the thermolysis of p-methylene or carbene species has a higher activation energy than carbon- ylation or the aldol-condensation-type reaction. On the other hand, no detection of ethene on ZrO, may suggest the forma- tion of acyl species through the path-way shown in Scheme 3.When hydrogenation of p-methylene or carbene to form methyl species is fast, the pathways in Scheme 2 and 3 are hard to differentiate. ZrO, is much more active than CeO, CH30CH3-CH-C-CH2-CH3I II CH20 v H2$ H20 t CH,O CH3 I1 I CH3-AH-C-CH-CH3 Scheme 4 Formation pathway for ketones Conclusions This study shows that: (i) The formation of branched carbon- chain compounds from CO hydrogenation over oxide cata- lysts may be attributed to the aldol-condensation-type reaction of aldehydes and ketones with formaldehyde.2-Methylpropanal is the end product from the reaction of acetaldehyde with formaldehyde and diisopropyl kentone is that from acetone with formaldehyde. 2-Methylpropanal undergoes hydrogenation and dehydration yielding iso-butene. (ii) Acetaldehyde and acetone are produced by hydro- genation and the hydration, respectively, of an acyl intermediate. The acyl species is assumed to be formed by the carbonylation of methyl or carbene species which are thermal decomposition products of q2-formaldehyde. References 1 T. Maehashi, K. Maruya, K. Domen and T. Onishi, Chem. Lett., 1984,747.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 917 2 3 4 5 K. Maruya, T. Arai, K. Domen and T. Onishi, in Catalytic Science and Technology, ed. S. Yoshida, N. Takezawa and T. Ono, Kodansha, Tokyo, 1991, vol. 1, p. 457. K. Maruya, K. Ito, K. Kushihashi, Y.Kishida, K. Domen and T. Onishi, Catal. Lett., 1992, 14, 123. H. Pichler and K. H. Ziesecke, Brennst. Chem., 1949,30,13. R. Kieffer, J. Varela and A. Deluzarche, J. Chem. SOC., Chem. 17 18 19 T. Arai, K. Maruya, K. Domen and T. Onishi, J. Catal., 1993, 141, 533. P. Pino, F. Piacenti, and M. Bianchi, Organic Synthesis via Metal Carbonyl, ed. I. Wender and P. Pino, Wiley-Interscience, New York, 1997, p. 233; J. Falbe, ‘New Syntheses with Carbon Monoxide,’ Springer, New York, 1980. C. A. Bertelo and J.Schwartz, J. Am. Chem. SOC.,1975,97,228. 6 7 8 Commun., 1983,763. K. Maruya, A. Inaba, T. Maehashi, K. Domen and T. Onishi, J. Chem. SOC., Chem. Commun., 1985,487. T. J. Mazanec, J. Catal., 1986,98, 115. S. C. Tseng, N. B. Jackson and J. G. Ekerdt, J. Catal., 1988, 109, 284. 20 21 22 23 T. Arai, K. Maruya, K. Domen and T. Onishi, J. Chem. SOC., Chem. Commun., 1987,1757; Bull. Chem. SOC. Jpn., 1989,62,349. W. A. Hermann, Ado. Organomet. Chem, 1982,20,159. E. 0.Fisher, Adv. Organomet. Chem, 1976,14, 1. K. Kropp, V. Skibbe, G. Erker and C. Kruger, J. Am. Chem. SOC.,1983,105,3353. 9 10 R. Kieffer, G. Cherry, J. Varela and R. Touroude, J. Chim. Phys., Phys. Chim. Biol., 1987,84,901. H. Idriss, R. Kieffer, P. Chumette and D. Durand, Znd. Eng. Chem. Rex, 199 1,30,1130.24 25 G. Erker, C. Kruger and R. Schlund, 2.Natuqorsch., Teil B, 1987,42,1009. R. Toreki, R.E. LaPointe and P. T. Wolczanski, J. Am. Chem. SOC.,1987,109,7558. 11 12 13 14 J. P. Hindermann, G. J. Hutchings and A. Kienemann, Catal. Rev. Sci. Eng., 1993,35, 1. J. G. Nunan, C. E. Bodgan, R. G. Herman and K. Klier, Catal. Lett., 1989, 2, 49; J. G. Nunan, C. E. Bodgan, K. Klier, K. J. Smith, C-W. Young and R. G. Herman, J. Catal., 1992,116, 195. A. Delzarche, R. Kieffer and A. Muth, Tetrahedron Lett., 1977, 38, 3357; A. Deluzarche, J. P. Hindermann and R. Kieffer, Tetrahedron Lett., 1978, 39, 2787; J. Saussay, J. C. Lavalley, T. Rais, A. Chakor-Alami, J. P. Hindemann and A. Kienemann, J. Mol. Catal., 1984, 26, 159. H. Abe, K. Maruya, K. Domen and T. Onishi, Chem. Lett., 1984, 1875. 26 27 28 29 30 31 32 C. P. Casey and S. M. Neumann, J. Am. Chem. SOC., 1978, 100, 2544. J. A. Gladysz and W. Tam, J. Am. Chem. SOC., 1978,100,2545. T. Onishi, H. Abe, K. Maruya and K. Domen, J. Chem. SOC., Chem. Commun., 1985,103. K. Maruya, T. Fujisawa, A. Takasawa, K. Domen and T. Onishi, Bull. Chem. SOC. Jpn., 1989,62, 11. D. M. Roddick and J. E. Bercaw, Chem. Ber, 1989,122, 1579. G. Erker, U. Do$ J. L. Atwood and W. E. Hunter, J. Am. Chem. SOC., 1986,108,2251. T. Arai, K. Maruya, K. Domen and T. Onishi, Bull. Chem. SOC. Jpn., 1989,62, 349. 15 K. Otsuka, M. Hatano and A. Morikawa, J. Catal., 1983, 79, 493. 16 K. Kushihashi, K. Maruya, K. Domen and T. Onishi, J. Chem. SOC., Chem. Commun., 1992,259. Paper 3/056006;Received 16th September, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000911

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(6), 919-929 FTIR Spectroscopic Study of the Zeolitic Adsorption of Hydrogen Cyanide on Acidic Sites Clive J. Blower and Thomas D. Smith* Chemistry Department, Monash University, Clayton, Victoria, Australia 3 168 The uptake of hydrogen cyanide by protonic forms of zeolites: Y, steam-treated Y, X, mordenite, L, beta and ZSM-5, has been studied using Fourier-transform infrared (FTIR) spectroscopic measurements of the -C=N stretch vibration to characterize the binding of hydrogen cyanide by the various Brsnsted acid sites of each zeolite. Diminished pressure and thermal desorption of zeolitically bound hydrogen cyanide, monitored by reduction in FTlR spectral band intensities, have been used to distinguish the strength of binding of hydrogen cyanide by the various acid sites.A relationship exists between the type of Brsnsted acid site, characterized by the -CEN stretch wavenumber of bound hydrogen cyanide, and the zeolite framework structural features in terms of the presence of sodalite units and the salient channel structures. A consideration of the acid sites of greatest strength for each zeolite indicates that the decreasing order of strength of hydrogen bonding of hydro-gen cyanide is ZSM-5 > mordenite > L x Y, X b beta. The IR spectral features of hydrogen cyanide bound by the Bransted acid sites on the protonic form of zeolite Y steamed at various temperatures have been interpreted in terms of acidic sites on extra-framework material and those associated with the zeolite framework.The extra-framework component has been interpreted to be a highly acidic, hydrated alumina-like material of much higher acidic strength than that of the framework acidic sites and whose formation is critically dependent on the temperature of steaming, and an aluminosilicate phase formed throughout the range of steaming temperatures. While there is no correlation between the strength of the various zeolitic Brsnsted acid sites and the IR spectral wavenumber of hydrogen cyanide bound to such sites, the wavenumber serves to characterize the sites and makes possible their identification in various zeolites and the elucidation of their relationship to zeolitic structur- al features. The zeolitic adsorption of gaseous material, whose molecules are capable of interaction with zeolitic functional sites, may serve to probe the internal structure of the zeolitic frame- work.FTIR has proved to be an invaluable investigative tool in the study of zeolitic functional groups and in the identifica- tion of materials within the zeolite.'-3 Therefore, a further requirement of the probe material is that it possesses an FTIR response, within the spectroscopic window of the zeolite, which could be used to monitor its tenancy of zeolitic acid sites. A chemical compound which meets these require- ments is hydrogen cyanide, as it diffuses easily into the zeolite, reacts with the acidic sites at room temperature and gives rise to an easily detected IR spectral response in the 2100 cm-' region. The present investigation deals with FTIR spectral mea- surements of hydrogen cyanide adsorbed by protonic forms of zeolites Y, steam-treated Y, X, L, beta and ZSM-5 and mordenite, which are designed to show the distribution of acidic sites, their relative concentrations and common fea- tures shared by the different zeolitic materials.Experimental Hydrogen cyanide (bp 298.7 K) was prepared by the method described in the literature? It was dried in the vapour phase by passage through anhydrous calcium chloride at 323 K and distillation from anhydrous calcium chloride after collection. The dried hydrogen cyanide was stored at low temperature and freshly distilled and dried before use. Starting with zeolite Y (Linde SK40, Si : A1 = 2.55 : 1)in its sodium form, it was converted to 90% ammonium form pH4(90)Y] by exchange with aqueous ammonium nitrate.5 Almost complete exchange of sodium ion by ammonium ion, NH4(98)Y, was achieved by hydrothermal exchange at 433 K while complete exchange to the ammonium form, NH4( lOO)Y, was accomplished by initial conversion of NaY zeolite to its silver ion form followed by treatment with aqueous ammonium thiocyanate.6 The sodium ion form of zeolite X (Si : A1 = 1.25 : 1) was prepared using the gel composition 1 rnol A1,o3-3.5 mol sio,-4.55 mol Na,0-200 mol H,O by the literature pro- ~edure.~The sodium form of zeolite X was converted to its 93% ammonium form [NH4(93)X] by treatment with a hot 353 K) 1 : 1 mixture of 0.5 mol dm-3 ammonium chloride and 0.5 mol dm-3 ammonium hydroxide.* The protonic form of zeolite ZSM-5, H(A1)ZSM-5, with Si : A1 = 38.8 : 1, was prepared as outlined previously.' The protonic forms of zeolite L (Si : A1 = 3.2 : 1) and mordenite (Si : A.= 6.0 :11) were kindly received from the Materials Science Division of CSIRO, Melbourne. Steam dealuminated samples (0.2-0.5 g) of NH4(98)Y were prepared by heating (393 K) the zeolite sample contained in a quartz tube in a stream of high-purity dinitrogen (30 cm3 min-') for 1 h before passage of steam through the zeolite at various temperatures (573, 623, 723, 823 and 1023 K). A similar procedure was used to prepare steam-treated alumina using chromatographic grade neutral material (Merck, 70-230 mesh) at 823 K.The analysis of the zeolitic material was carried out as described previo~sly.~ Sodium and aluminium contents were determined by atomic absorption spectroscopy, silicon by a combination of gravimetry and spectrophotometry. The zeo- lites were stored over saturated calcium nitrate solution. The X-ray powder diffraction patterns of the solid materials were recorded by a Rigaku Geigerflex instrument using nickel- filtered Cu-Ka radiation. Zeolite Adsorption of Hydrogen Cyanide and its IR Spectroscopy FTIR spectra were recorded using a Perkin-Elmer 1600 spec-trophotometer: resolution 2.0 cm-', range 4400-450 cm-' and four scans (16 s). Deconvolution, derivatization and mea- surements of the area under the curve on the IR bands of interest were carried out using the instrumental computer and software.Self-supporting wafers of the zeolites (about 15 mg, 15 mm diameter and 0.05 mm thick) were obtained by compression (1500 psi? for 15 min, 3000 psi for 15 min) of the zeolite and mounted in a cell with sodium chloride windows and a region for heat treatment of the wafer." The cell was con- nected to a gas-handling line which enabled gas transfer in and out of the cell to be carried out as well as provision for final drying of hydrogen cyanide and treatment of the wafer. Most of the treatment of zeolite wafers with hydrogen cyanide involved using enough of the reagent to saturate the zeolite at room temperature. This was initially heated to some desired temperature (usually 593 K), before recording the IR spectrum, it having been established that any free hydrogen cyanide in the gas phase within the cell did not contribute to the IR bands of interest.Hydrogen cyanide was removed from the cell by reducing the pressure to 0.0o01 Torr for 2 min (pressure desorption) and allowing the iso- lated cell to stand for 15 min. The IR spectrum was recorded once more by rolling the wafer assembly, after thermal or chemical treatment, into the light path. Spectral Analysis by Derivative Spectroscopy The analysis of IR spectral bands and other spectroscopic data by spectral deconvolution (resolution enhance-ment)' '-I4 and derivatization '5-18 has been described, while the limitations of such procedures have been o~tlined.''-~' Amongst a number of applications, enhanced resolution FTIR spectroscopy has been used to identify the components of the high-wavenumber (HW) band due to the hydroxy groups in zeolites X and Y." In another approach to analys- ing the components of the low-wavenumber (LW) IR spectral band due to the zeolite hydroxy groups, curve resolution was used and critically evaluated.23 To quantify, as far as pos- sible, the relative amounts of hydrogen cyanide bound at various sites by the zeolites, the rather sharp IR band due to zeolite-bound hydrogen cyanide was recorded with scale expansion to delineate the main peak constituents in the band.To identify the peaks further derivative spectra were obtained, the fourth derivative offering the best separation but with some distortion of relative intensities.The main purpose of taking the various derivatives was to establish the wavenumber range of each peak. With the additional aid of deconvolution of the experimental spectra, the areas under the curves of the experimental spectra for each wavenumber range appropriate to a particular peak in the fourth-derivative spectrum were computed instrumentally and used as a measure of the relative amounts of hydrogen cyanide in its various modes of binding. The procedure was used largely to monitor the relative changes in the binding of hydrogen cyanide when the zeolites were subjected to various initial thermal treatments, to sequential exposure to various amounts of hydrogen cyanide reaching saturation and finally in the partial removal of hydrogen cyanide from the zeolite at diminished pressure.A typical example of the segmentation of the experimental spectrum into areas whose wavenumber limits are defined by the fourth derivative of the experimental spectrum is shown in Fig. 1. An alternative approach used to establish the relative con- tributions of constituent bands to the area of the experimen- tal spectrum was to generate constituent band shapes such that their combined areas were close to that of the experi- 7 1 psi (pounds per square inch) z 6.894757 x lo3 Pa. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 mental spectrum and gave a good wavenumber fit. An example of such a procedure is illustrated in Fig.2 which involved the following steps. The fourth derivative of the experimental IR spectrum due to zeolite-adsorbed hydrogen cyanide was used to identify four peaks at 2121, 2116, 2107 and 2097 cm-'. A further peak at 2111 cm-'was identified from the fourth derivative of the experimental spectrum due to zeolite-adsorbed hydrogen cyanide after a short period of diminished pressure pumping. The band centred at 2097 cm-' was constructed first by using the second derivative of the experimental spectrum to measure the separation of the two positive lobes on both sides of the negative lobe at 2097 cm-'from which the half-width of the original bandIg was determined and found to be 9.0 cm-'. This value was used for the construction of the synthetic band centred at 2097 cm-', a value of 8.0 cm-' being finally chosen.Owing to the overlapping of the negative lobes of the second derivative of the experimental spectrum the height and half-width values of the remaining bands centred at 2121, 2116, 2107 and 2111 cm-' were constructed using Lorentzian lineshapes so that their combination gave a good fit of the experimental spec- trum which was judged by a comparison of the four deriv- atives of the experimental and the combined lineshape as shown in Fig. 2. Results Interaction of Hydrogen Cyanide witb Zeolite Y The Brransted acidity of zeolite Y is characterized by the (HW) IR spectral band at 3640 cm-' and (LW) band at 3549 cm-'. The variation in the intensity of these spectral bands as a result of heat treatment of the zeolite is a measure of the thermal stability of the Brransted acid sites.The IR spectral data collected for zeolite H(98)Y over the range 428 to 873 K show a decline in the existence of Brnrnsted acid sites over the temperature range 673-873 K. The IR spectral changes brought about by exposure at room temperature (298 K) of wafers of zeolite H(98)Y, ini-tially heated to 593 K, to increasing amounts (12.9 pmol to 450 pmol) of hydrogen cyanide show that the Br~rnsted acid sites, represented by the HW and LW IR spectral bands, take part in the binding of hydrogen cyanide while the reduction in the intensity of the HW spectral band is more marked compared to that of the LW spectral band when the expo- sures involve small amounts of hydrogen cyanide (77-450 pmol).The binding of hydrogen cyanide by the zeolite H(98)Y gives rise to bands on either side of the 2100 cm-' region which increase in intensity with greater amounts of hydrogen cyanide on the zeolite. Deconvolution of the IR spectral bands due to adsorbed hydrogen cyanide shows that the smaller additions (25 to 180 pmol) of hydrogen cyanide give rise largely to a band in the region 2116 cm-' and to a smaller extent at 2095 cm-' while larger additions (8 mmol) of hydrogen cyanide lead to additional bands in the 2120 cm-' region and a pronounced increase in the bind at 2095 cm-'. The component peaks of the original IR spectrum due to hydrogen cyanide adsorbed by zeolite H(98)Y, made clearer by deconvolution, are more easily discerned by derivatization with prominent peaks at 2097, 2120, 2116, 2108 cm-' and shoulders at 2112 and 2104 cm-'.Partial removal of hydro-gen cyanide by diminished pressure desorption effects a marked reduction in intensities of peak components at 2120 and 2097 cm-' and a diminution of intensities of com-ponents at 2116, 2112, 2108 and 2104 cm-' with the main form of hydrogen cyanide binding being represented by the peak at 2112 cm-'.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 2150 2100 2050 2150 2100 2050 wavenumber/cm-' wavenumber/cm -' Fig. 1 Illustration of IR signals differentiation applied to the vCN region of hydrogen cyanide adsorbed onto H(98)Y zeolite prepared at 593 K, before pressure desorption.The shaded regions indicate the segment used as a measure of the area of bands at 2114 and 2096 cm-' in the experimental spectrum. The segments were determined from the fourth-derivative spectrum. (a) Experimental spectrum, (b) first-derivative spectrum, (c) second-derivative spectrum, (d) third-derivative spectrum, (e) fourth-derivative spectrum and (f)experi-mental spectrum including segments. The IR spectra at room temperature due to hydrogen cyanide adsorbed by wafers of zeolite H(98)Y, each pre- viously subjected to heating at a particular temperature and reduced pressure for 1 h, along with those after partial desorption of hydrogen cyanide by diminished pressure were recorded.Treatment of the IR data by deconvolution and derivatization to identify peak components followed by curve segmentation as described previously gives a measure of the variation of the modes of binding of hydrogen cyanide with initial temperature treatment of each zeolite wafer. The results are shown in Table 1. A similar sequence of IR mea-surements of hydrogen cyanide adsorption by heat-treated wafers of zeolite H( 1OO)Y and Y(90)Y were carried out and a similar treatment of the experimental data provided the infor- mation summarized in Table 2 for zeolite H( 1OO)Y and Table 3 for zeolite H(90)Y. Interaction of Hydrogen Cyanide with Steam-treated Zeolite H(98)Y Zeolite H(98)Y was heated in steam at 823 K, washed with water and finally washed with dilute (0.1 mol dm-3) hydro- chloric acid.After each treatment wafers of the zeolite material, after heating to 593 K, were exposed to hydrogen cyanide at room temperature, the IR spectra recorded and compared with those of wafers of the starting zeolite H(98)Y and chromatographic-grade neutral alumina heated in steam at 823 K (both heated to 593 K before being exposed to 921 I cu al C+ v).I) 0.30 21 50 2100 2150 2100 2150 2050 2150 wavenumber/cm -' wavenumber/cm-' Fig. 2 Comparison of vCEN regions of (a) H(98)Yprepared at 593 K before pressure desorption of hydrogen cyanide (total area 3.4152 A cm-' mg-') and (b) a synthetic peak system constructed on a back- ground of H(98)Y (593 K), before hydrogen cyanide adsorption, using the following Lorentzian peaks (total area 3.3212 A an-' mg-'): (i) 2097 cm-', abs 1.30, width 8.0 an-';(ii) 2121 cm-',abs 1.20, width 13.0 an-'; (iii) 2116 cm-' abs 0.70, width 12.0 cm-' an-'; (iv) 2107.5 cm-', abs 0.20, width 12.0 an-'; (v) 2111.5 an-', abs 0.225, width 10.5 cm-'.Fourth-derivative spectra of vCrN regions of (c) H(98)Y (593 K) before pressure desorption of hydrogen cyanide and (d) the total synthetic peak system described in (b).hydrogen cyanide at room temperature). The IR spectra after partial removal of hydrogen cyanide from the steamed, water-washed and dilute acid leached zeolite samples were also recorded and compared with those obtained after partial removal of hydrogen cyanide from the starting zeolite H(98)Y and steamed alumina. The effect of steaming the zeolite H(98)Y at various temperatures on the IR spectral character- istics of hydrogen cyanide adsorbed by wafers of steamed H(98)Y with the temperature of steaming of the zeolite, is shown in Fig. 3, from which it is evident that the peak area at 2226 cm- ' is little affected by diminished pressure pumping, while the peak area at 2147 cm-' is substantially reduced.The components of the IR bands due to hydrogen cyanide on the zeolite H(98)Y steamed at various temperatures were identified by derivatization. The variations in the peak areas at 2120 and 2097 cm-', both of which are substantially influ- enced by diminished pressure pumping, with temperature of steaming, as well as similar variations in peak areas with tem- perature and diminished pressure for peaks at 21 14,2108 and 2104 cm- ',are summarized in Table 4.Interaction of Hydrogen Cyanide with Zeolite HX The variation, with temperature, in the intensity of the com- bined peak areas of the IR bands at 3650 cm-'(HW) and 3572 cm-' (LW) due to the hydroxy groups responsible for the Brsnsted acidity of zeolite HX is given in Table 5, which J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 Areas of constituent bands of the vCnN envelope due to room-temperature adsorption of hydrogen cyanide onto zeolite H(98)Y zeolites prepared at various temperatures (A,) after diminished pressure pumping for 2 min (A,) 2120 cm-' 2097 cm-' 2116 cm-' 2108 cm-' 2104 cm-' T/K A1 A2 A1 A2 A1 A2 A2 A2 A1 A2 ~ 430 0.65 0.10 1.37 0.57 0.34 0.11 0.4 1 0.15 0.45 0.21 535 1.03 0.26 0.90 0.20 0.44 0.28 0.55 0.12 0.37 0.15 595 0.95 0.16 0.80 0.15 0.44 0.28 0.46 0.12 0.33 0.10 680 0.85 0.15 0.82 0.15 0.48 0.30 0.38 0.14 0.33 0.12 870 0.10 0.05 0.05 0.05 0.12 0.10 0.26 0.06 0.18 0.08 ~ ~~~ Table 2 Areas of constituent bands of the vCsN envelope due to room-temperature adsorption of hydrogen cyanide onto zeolite H(100)Y zeolites prepared at various temperatures (Al), and after diminished pressure pumping for 2 min (A,) 2120 cm-' 2097 cm-' 2116 cm-' 2108 cm-' 2104 cm-' T/K A' A2 A1 A2 A' A2 A1 A2 A1 A2 428 0.61 0.06 1.10 0.42 0.34 0.18 0.26 0.10 0.38 0.10 532 0.78 0.12 0.70 0.10 0.45 0.28 0.28 0.08 0.25 0.08 595 1.05 0.13 0.82 0.13 0.62 0.32 0.33 0.08 0.25 0.08 675 0.67 0.10 0.72 0.13 0.43 0.30 0.3 0.08 0.18 0.08 Table 3 Areas of constituent bands of the vCZN envelope due to room-temperature adsorption of hydrogen cyanide onto zeolite H(90)Y zeolites prepared at various temperatures (Al), and after diminished pressure pumping for 2 min (A,) ~ ~~ ~ ~~ 420 0.40 0.04 0.87 0.22 0.27 0.12 0.45 0.12 0.43 0.3 1 540 0.70 0.07 0.97 0.12 0.42 0.15 0.58 0.18 0.44 0.2 1 598 0.65 0.07 0.96 0.16 0.45 0.20 0.56 0.20 0.42 0.26 670 0.35 0.07 0.97 0.2 1 0.38 0.25 0.72 0.25 0.50 0.37 870 0.07 0.02 0.36 0.02 0.13 0.07 0.28 0.07 0.19 0.08 also shows similar variations in terms of the combined peak particular temperature for 1 h at diminished pressure, along areas of the HW and LW bands for zeolite H(90)Y.with the effect of partial removal of' hydrogen cyanide by The IR bands due to hydrogen cyanide adsorbed at room diminished pressure pumping for a short time, were recorded. temperature on wafers of zeolite HX, previously heated at a Table 6 summarizes this variation in terms of the reductions of the total IR band areas due to adsorbed hydrogen cyanide with diminished pressure pumping and initial heat treatment 0.50 r of the zeolite wafer. With the aid of deconvolution and derivatization of the IR bands due to adsorbed hydrogen cyanide, the constituent bands were identified at 2120, 2114, 2107, 2104 and 2095 cm-'.The variation in the segmental peak areas derived from Table 6, where the wavenumber range of each segment is defined by the derivative peaks, with initial heat treatment of the zeolite wafer, is shown in Table 7, which illustrates that E 0.30< 0.40 (b) '\ the hydrogen cyanide represented by segments at 2120, 2114 and 2095 cm-' is easily removed by lowering the pressure, 2? I \ while that represented by segments at 2107 and 2104 cm- is 3 rather more firmly held. 0.20 -.-0 Thermal Desorption of Hydrogen Cyanide from Zeolite C H(98)Y and Zeolite HXi? Diminished pressure desorption of hydrogen cyanide from 0.10 -(a wafers of zeolites X and Y provides a means of identifying the more tenacious binding sites of hydrogen cyanide.Thermal desorption of hydrogen cyanide from wafers of the zeolite which have been subjected to a short period of reduced pres- """""""""""' sure pumping to remove the less firmly held hydrogen cyanide provides additional information on the relative strength of binding of hydrogen cyanide by the various stronger Brsnsted acid sites. The loss of constituent IR band areas due to hydrogen cyanide initially adsorbed at room temperature and after 2 min of reduced pressure on a wafer of zeolite H(90)Y, previously heated to 593 K for 1 h at J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 923 Table 4 Areas of constituent bands of the vCIN envelope due to room-temperature adsorption of hydrogen cyanide onto zeolite NH,(98)Y zeolites prepared at various temperatures (Al), after diminished pressure pumping for 2 min (A,) 2120 cm-' 2097 cm-' 2114cm-' 2108 cm-' 2104 cm-' T/K A1 A2 A1 A2 A1 A2 A1 A2 A1 A2 570 0.20 0.02 0.52 0.11 0.34 0.13 0.20 0.18 0.25 0.18 620 0.15 0.08 0.47 0.12 0.27 0.09 0.21 0.18 0.24 0.14 720 0.18 0.08 0.49 0.12 0.29 0.11 0.3 1 0.18 0.24 0.07 820 0.18 0.06 0.54 0.08 0.17 0.09 0.3 1 0.13 0.24 0.06 1020 0.09 0.02 0.47 0.04 0.14 0.05 0.22 0.06 0.19 0.05 diminished pressure, with increasing temperature of the tion enables constituent peaks of the spectra to be identified wafer, is summarized in Table 8.Similar information con-at 2144, 2108, 2101 and 2094 cm-'.The thermal desorption cerned with the loss of IR spectral peak areas due to hydro- of hydrogen cyanide from the wafer of zeolite HL after gen cyanide adsorbed, in the first instance at room reduced pressure pumping, in terms of the loss of spectral temperature followed by pumping for 2 min, by a wafer of segmental areas for the ranges 21 16-2106, 2106-2097 and zeolite HX initially heated to 593 K for 1 h at diminished 2097-2082 cm-', defined from the derivatized spectra at each pressure, with increasing wafer temperature is summarized in temperature, is summarized in Table 10.Table 9. Interaction of Hydrogen Cyanide with Mordenite Interaction of Hydrogen Cyanide with Zeolite L The IR spectrum due to hydrogen cyanide adsorbed at room The IR spectrum due to hydrogen cyanide adsorbed at room temperature by the protonic form of mordenite fabricated temperature by zeolite HL, initially heated to 593 K at into a wafer, which was initially heated at 593 K for 1 h at diminished pressure for 1 h, was recorded along with the loss diminished pressure, was recorded after partial removal of of IR spectral intensity after reduced pressure pumping for 2 hydrogen cyanide by reduced pressure pumping.A com-min. A combination of spectral deconvolution and derivatiza- bination of spectral deconvolution and derivatization results Table 5 Total peak areas of IR hydroxy-group region of H(90)Y and HX zeolites prepared at various temperatures Table 8 Decreases in areas of constituent bands in the vCEN envelope arising from thermal desorption of hydrogen cyanide from zeolite H(90)Y band area zeolite HX band area zeolite H(90)Y initially prepared at 593 K and pumped at diminished T/K (3675-3423 cm-') (3678-3423 cm-') pressure for 2 min 430 5.1 1.9 490 5.8 2.0 540 6.0 2.0 300 0.10 0.17 0.42 0.26 560 6.0 1.9 325 0.08 0.14 0.34 0.23 595 5.8 1.6 375 0.02 0.06 0.13 0.09 670 3.6 0.2 420 0.01 0.03 0.06 0.04 720 2.8 0 480 0 0 0.01 0.02 870 0.4 0 520 0 0 0 0.01 Table 6 Total peak area of the vCIN region after room-temperature Table 9 Decreases in areas of constituent bands in the vCEN adsorption of hydrogen cyanide onto HX zeolite prepared at various envelope arising from thermal desorption of hydrogen cyanide from temperatures zeolite HX initially prepared at 593 K and pumped at diminished pressure for 2 min A1 A2 T/K 2095 cm-' 2107 cm-' 2104 cm-' 430 1.40 0.67 540 1.40 0.62 300 0.24 0.09 0.12 590 1.25 0.61 325 0.20 0.06 0.10 680 0.94 0.45 370 0.09 0.02 0.04 870 0.05 0 430 0.03 0.01 0.01 480 0.01 0 0For A,, A,, see Table 1.Table 7 Areas of constituent bands of the vCIN envelppe due to room-temperature adsorption of hydrogen cyanide onto zeolite HX prepared at various temperatures (Al), and after diminished pressure pumping for 2 minutes (A,) 2120 cm-' 2095 cm-' 2114 cm-' 2107 cm-' 2104 cm-' T/K A1 A2 A1 A2 A1 A2 A1 A2 A1 4 -430 0.11 0.01 0.5 1 0.32 0.19 0.02 0.13 0.02 0.16 -535 0.10 0.01 0.60 0.23 0.19 0.02 0.18 0.07 0.19 595 0.05 0.01 0.56 0.23 0.13 0.03 0.16 0.09 0.20 -675 0.02 0.01 0.42 0.16 0.07 0.02 0.13 0.06 0.20 --870 0 0.04 0 0.01 0 0.01 0 0.01 924 Table 10 Decreases in areas of constituent bands in the vCpN envelope arising from thermal desorption of hydrogen cyanide from zeolite HL initially prepared at 593 K and pumped at diminished pressure for 2 min T/K 2111 an-' 2101 m-' 2093 an-' 300 0.13 0.18 0.09 320 0.11 0.15 0.07 370 0.05 0.08 0.04 420 0.0 1 0.2 0.01 in the identification of constituent peaks of the spectra, which indicates that the major band constituents, which represent the binding of hydrogen cyanide by the zeolite, occur at 2121, 2118, 2114, 2110, 2106, 2102 and 2098 cm-' with major binding sites being represented at 2118 and 2114 cm-'.The thermal desorption of hydrogen cyanide from the more tena- cious binding is summarized in Table 11, which shows the decreasing segmental areas of the IR bands with increasing temperature of the zeolite wafer exposed to hydrogen cyanide at room temperature followed by pumping for 2 min.Interaction of Hydrogen Cyanide with Zeolite Beta The adsorption, at room temperature, of hydrogen cyanide by a wafer of the protonic form of zeolite beta, initially heated to 573 K for 1 h at diminished pressure, give rise to the IR spectrum which shows a dramatic decrease in spectral intensity as a result of lowering the pressure for a short time. Spectral deconvolution and derivatization shows that the spectrum after pumping possesses constituent spectral bands Table 11 Decreases in areas of constituent bands in the v,--~ envelope arising from thermal desorption of hydrogen cyanide from the protonic form of mordenite, initially prepared at 593 K, pumped at diminished pressure for 2 min T/K 2118 cm-' 2114 an-' 2110 an-' 2106 an-' 298 0.15 0.10 0.07 0.07 322 0.13 0.09 0.06 0.06 372 0.08 0.06 0.03 0.04 420 0.04 0.03 0.02 0.02 443 0.0 1 0.02 0.01 0.0 1 Table 12 Decreases in areas of constituent bands in the vC-., envelope arising from the temperature desorption of hydrogen cyanide from the protonic form zeolite beta, initially prepared at 563 K, pumped at diminished pressure for 2 min T/K 2115 cm-' 2106 cm-' 300 0.024 0.010 3 20 0.019 0.007 350 0.007 0.003 375 0.003 0.002 Table 13 Decreases in areas of constituent bands in the vCpN envelope arising from the temperature desorption of hydrogen cyanide from the protonic form of zeolite ZSM-5,initially prepared at 563 K, pumped at diminished pressure for 2 min T/K 2122 m-' 2114 cm-' 300 0.070 0.029 325 0.058 0.018 378 0.040 0.0 12 440 0.018 0.004 470 0.006 0.002 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 at 2115 and 2106 cm-' with the latter band accounting for most of the adsorbed hydrogen cyanide, the thermal desorp- tion of which from the zeolite is summarized by Table 12. Interaction of Hydrogen Cyanide with Zeolite ZSM-5 The IR spectrum due to hydrogen cyanide adsorbed at room temperature by a wafer of the protonic form of zeolite ZSM-5, previously heated to 573 K for 1 h at diminished pressure, was recorded. The constituent bands of this spec- trum, identified by deconvolution and derivatization, occur at 2122, 2115, 2107 and 2098 cm-'.Diminished pressure pumping for a short time removes nearly all of the hydrogen cyanide represented by the peaks centred at 2098 and 2107 cm-'.The thermal desorption of the more firmly bound hydrogen cyanide from the zeolite, most of which may be accounted for by peaks at 2122 and 2114 cm-', is sum- marised in Table 13. Discussion Hydrogen-bonding Interactions by Brensted Acid Sites of Zeolite Y Gas-phase hydrogen cyanide possess three IR bands in the mid-range of wavenumbers typical of a linear (Cmv)triatomic molecule, namely: v3, C-H stretch, 331 1 cm-';vl, -CEN stretch, 2097 cm-' and v2, bond-angle deformation, 712 cm-l .24 Solid hydrogen cyanide exists in a linear hydrogen- bonded chain form crystallizing with an orthorhombic unit cell, 1 mm (C2v20with two molecules in the unit cell).25 In the solid form the wavenumber of the C-H stretch mode, v3, is reduced while that of the bending mode, v2, is increased. Even in the gas phase hydrogen cyanide shows a tendency towards the formation of hydrogen-bonded aggregates with about 10%in dimeric and 3% in trimeric form.26 The multi- mer forms, particularly that of the dimer, become increasingly important in low-temperature matrices. The effect of hydro- gen bonding in the dimeric aggregates is to shift the vl, v2 and v3 band wavenumbers, typically to values27 of v3, 3306 cm-'; vl, 2093 and 2112 cm-' and v2, 731 and 793 cm-' and, importantly, to enhance greatly the intensity of the v1 -CEN stretch band.27-34 The basicity of hydrogen cyanide is exemplified by its com- bination with hydrogen fluoride when in monomeric or superficially linear dimeric form3 to give the theoretically predi~table~~increase in the -CzN stretch wavenumber (vl) with intensity enhancement of the band.37-39 Hydrogen cyanide adsorbed by surface hydroxy groups on silicate glass surfaces gives rise to absorption bands due to single molecules as well as polymers.40i41 The adsorption of hydrogen cyanide on silica and alumina has been monitored by far-IR spectroscopy where hydrogen bonding between the surface hydroxy groups and hydrogen cyanide results in a hindered rotation of hydrogen cyanide.42 An important IR spectral band which occurs in all cases of zeolitic adsorption of hydrogen cyanide has a wavenumber in the 2095 cm-' region.On the grounds that the profile of band intensity with variation in temperature treatment of the zeolite parallels that of the spectral intensity changes in the hydroxy-group region, this band is attributable to Brmsted acid site binding of hydrogen cyanide rather than gas-phase hydrogen cyanide, which has a low spectral intensity in this wavenumber region, or a dimeric form of hydrogen cyanide which would give a band of high spectral intensity in concert with a weaker band at 21 12 cm-'. Although a band is obser- vable in the 2112 cm-' region, its spectral intensity is not coupled with that at 2095 cm-' in circumstances of partial removal of hydrogen cyanide from the zeolites.The remain- J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 ing IR spectral bands are attributable to hydrogen cyanide bound to Brsnsted acid sites whose relative strengths of binding have been differentiated by reductions in spectral band intensity brought about by diminished pressure or increasing zeolitic temperature desorption. However, there is no simple relationship between the strength of the hydrogen cyanide interaction with Bronsted acid sites and the IR spec- tral wavenumber shift from that of the free-gas value, some of the largest wavenumber shifts being associated with the less firmly held hydrogen cyanide. The hydrogen form of the near-faujasite Y zeolite, whose structural features are well known 43*44 and which contains various Brernsted acid sites, is formed as a result of heating to 593 K, with reversible evol- ution of ammonia and loss of physically bound water.A sig-nificant decrease in Bronsted acidity occurs as a result of slow water loss from neighbouring hydroxy groups at 593-823 K with little structural change. A faster dehydroxylation takes place at 823-903 K, which involves migration of framework aluminium into the intracrystalline pore system with com- plete disruption of the crystal The existence of several composite IR spectral bands due to zeolitically bound hydrogen cyanide is in keeping with the recognition of the various spectral bands which make up the HW and LW IR band due to the hydroxy gro~ps.~O-~~ The incremental addition of hydrogen cyanide to zeolite H(98)Y shows that the HW spectral band due to hydroxy groups is more affected by relatively small additions of hydrogen cyanide, this band being associated with the stronger acidity.A reduction in the intensity of the HW hydroxy-group is paralleled by the appearance of constituent bands due to hydrogen cyanide at 2116, 2108 and 2104 cm-' with a pre- dominance of the first two. Reductions in the intensity of the LW band, which requires the presence of larger amounts of hydrogen cyanide, coincide with the growth of constituent bands at 2120 and 2097 cm-'. Bearing in mind the uncer- tainties involved in comparing IR spectral band intensities, the greater intensities of the bands at 2120 and 2097 cm-' compared with those at 2116, 2108 and 2104 cm-' indicates that the former bands arise from a greater number of acid sites, albeit of lower strength than those of the latter.The variation of the intensities of the bands at 2120 and 2097 cm-' with initial thermal treatment of the zeolite indicates that both weaker acid sites become less numerous with increasing temperature of heat treatment of the zeolite. At lower temperatures of heat treatment the weaker acid sites, represented by the band at 2097 cm-', are present in greater amount while the full formation of the other weaker acid binding site (2120 cm-') requires zeolite temperatures of about 573 K.The variation of the spectral intensity of the constituent peaks at 2114, 2108 and 2104 cm-' due to the more firmly held hydrogen cyanide bound by the zeolitic Bronsted acid sites characterized by the HW hydroxy-group, with the tem- perature of prior thermal treatment of zeolite H(98)Y, shows a rise and fall which parallels that of the HW spectral band intensity with temperature. In conditions of diminished pres- sure desorption the greater part of the hydrogen cyanide is bound at a single site represented by the constituent band at 21 12 cm-'. A largely similar picture emerges for the binding of hydrogen cyanide by zeolite H(100)Y with some variations arising from the different synthetic route used to obtain the protonic form of the zeolite.However, the incomplete conver- sion of the protonic form of the zeolite, as in traditional zeolite H(90)Y, results in marked differences. For example, the more weakly held hydrogen cyanide, which is represented by the constituent peak at 2120 cm-', accounts for less of the bound hydrogen cyanide while amongst the more firmly held forms of hydrogen cyanrde, rather more is held by the site 925 Table 14 Percentage fall in vCrN IR segmental band areas with pumping for 2 min at 0.001 Torr for partial removal of adsorbed hydrogen cyanide wavenumber/cm -zeolite 2120 2097 2116 2108 2104 H(98)Y 80 64 36 76 69 H(100)Y 82 85 43 54 36 H(90)Y 85 82 56 64 36 HX 91 64 79 35 44 dealuminated HY 80 87 60 -30 represented by the constituent band at 2108 cm-' which remains the more dominant binding site throughout the tem- perature range of thermal treatment of zeolite Y.The amount of hydrogen cyanide on the other two sites (2116 and 2104 cm-') remains about the same as in zeolite H(98)Y and H(1OO)Y. Consideration of the percentage falls in IR segmental band areas as a result of diminished pressure pumping, which gives a measure of binding of hydrogen cyanide at the acid sites characterized by the IR spectral wavenumbers due to adsorbed hydrogen cyanide, the smallest fall indicating the highest degree of hydrogen bonding, are summarized in Table 14. It shows that zeolites H(90)Y and H(1OO)Y possess an abundance of sites of relatively low degree of hydrogen- bonding interaction (2120 and 2097 cm-'), sites of stronger hydrogen-bonding interaction (21 16 and 2108 cm-') and sites of a relatively high degree of hydrogen bonding (2104 cm-'). Assuming that the molar absorption coefficients at each wavenumber in Table 14 have comparable values, the amount of hydrogen cyanide bound at the acid site giving rise to the peak at 2104 cm-' is reasonably constant at 13.8% for H(90)Y, 11.5% for H(100)Y and 12% for H(98)Y of the total uptake of hydrogen cyanide.The interaction of hydrogen cyanide with silica gives rise to a band at 2105 cm-' which may indicate that the band at 2104 cm-',due to uptake of hydrogen cyanide on zeolite Y, is due to a distribu- tion of silica within the zeolitic structure whose hydrogen- bonding capacity for the most part shows a steady decline with increasing temperature of initial heat treatment of the zeolite.Influence of Steaming on the Uptake of Hydrogen Cyanide by Zeolite Y The dealumination of zeolite Y results in changes to unit-cell size, crystallinity, acid site strengths and spatial distribution, pore-size distribution and the occurrence of extra-framework material, all of which influence the zeolite in fluid hydrocar- bon cracking processe~.~~-~~High-temperature (1 173 K) steaming of zeolite HY results in zeolite crystal damage, a surface accumulation of an alumina-like material and a reduction of zeolitic acid sites6* which are important in hydrocarbon cra~king.~~?~'Framework dealumination results in a change of the Brsnsted acid ~ites.~' Steaming in the lower-temperature range (773-873 K) results in the for- mation of alumin~silicate,~~-~~which may play a role in hydrocarbon cracking.75 Other work has provided evidence for the formation of aluminium oxide in the sodalite as well as changes in zeolitic porosity.79 The aluminosilicate amorphous phase may reside largely in the supercages8' and possess Lewis acidity.81 The variation in the intensities of the IR spectral constitu- ent bands due to hydrogen cyanide adsorbed by steamed zeolite H(98)Y with the temperature of steaming may be used as a guide to the changes in zeolite structure which occur as a result of the steaming process with the aid of the following assignments of the bands.(i) The IR band at 2226 cm-'is in the same region as that at 2210 cm-' which characterizes hydrogen cyanide binding by similarly steamed hydrated alumina. It may be concluded that the band at 2226 cm-' represents easily removed, highly dispersed, water-solubilized material which is close in composition to hydrated alumina. The intensity of this band is critically dependent on the tem- perature of zeolite steaming. While small amounts of this material occur at lower steaming temperatures, there is a marked increase when the temperature of zeolite steaming reaches 723 K rising to a maximum at 923 K; thereafter the number of its acidic sites declines rapidly up to 923 K. By the yardstick of hydrogen cyanide binding, this material pos- sesses acid sites much stronger than those of the zeolite framework.(ii) The IR spectral band at 2147 cm-' represents material formed within the pore system of the zeolite, being removed by acid leaching and possessing an acidity compara- ble to that of the zeolite framework sites. The formation of this material is quite variable with temperature of steaming, the number of its acidic sites declining at higher steaming temperatures. (iii) The band at 2120 cm-' arises from the uptake of hydrogen cyanide by a zeolitic framework site, much reduced in intensity throughout the range of steaming temperatures compared with that due to the starting material. This band, whose growth is paralleled by a reduction in the intensity of the LW band of the hydroxy group, is most affected by the presence of sodium ion, being of lower intensity in H(90)Y compared with H(98)Y and H(100)Y, and is therefore thought to be associated with the migratory protons from the sodalite or prism cages.(iv) The band at 2097 cm-' which occurs due to hydrogen cyanide uptake by H(98)Y prior to steaming (though at an increased level), remains at a remarkably steady level throughout the temperature range of steaming while resisting the rapid decline in intensity in the case of H(98)Y at higher initial tem- perature of heat treatment. (v) The IR spectral band at 2104 cm-' is thought to be due to hydrogen cyanide bound to the acidic site of silica within the zeolite structure, its persistence throughout the temperature range of steaming being compa- rable with that observed with increasing initial temperature treatment of H(98)Y. Its most important features are its sur- vival with acid leaching and its assignment to the site of strong acidity.(vi) The band at 2114 cm-' is due to an acid site on the zeolite framework. The downward trend in its intensity with increasing steaming temperature beyond 573 K is comparable with the behaviour observed for the starting material. (vii) The band at 2108 cm-' represents the binding of hydrogen cyanide by a Brarnsted acid site on the zeolite structure. The intensities of this constituent band for H(98)Y and H(90Y) are similar at 523 K, but with increasing tem- perature the intensity falls for H(98)Y and increases for H(90)Y.The steep rise in the intensity of the 2108 cm-' band with temperature of H(90)Y in the range 573-673 K is paral- leled by an equally steep drop in the band at 2120 cm-', whereas the intensity of the 2108 cm-' band in this same temperature interval for H(98)Y falls by about 20% and the fall in the intensity of the 2120 cm-' band is rather less. The steaming process results in an increasing band intensity at 2108 cm-and depressed values of intensities of the band at 2120 cm-' throughout the temperature range. The growth of the band at 2108 cm-' due to the progressive uptake of hydrogen cyanide effects the demise of the HW hydroxy-group band of zeolite H(98)Y; the former band is assigned to hydrogen cyanide bound to a Brarnsted acid site located in the supercage.The temperature profiles of the IR spectral constituent band intensities which monitor the formation of extra-J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 framework material and the structural changes of the zeolite framework may be used to form a picture of the changes in these components which occur as a result of the steaming process. For simplicity, the IR spectral band intensities are referred to by their wavenumbers. At the lowest temperature of steaming there is a breakdown of the zeolite framework involving sodalite cages and prisms (lowered 2120 cm-'), the structural fragments of aluminosilicate possessing acidic sites (2147 cm-') causing some blocking of the supercage (high 2147 cm-', lower 2097, 2116, 1208 and 2104 cm-').This process continues at higher temperatures with the initial surface of the supercage being extended or cleared of debris, thus exposing more acid sites (increasing 2108 cm-'). At steaming temperatures just in excess of 723 K there is a sudden and substantial formation of hydrated alumina-like phase, possibly from dealumination of the zeolite framework, highly dispersed and easily removed, collecting on the outer surface and pore openings so as not to impede gas flow into the microporous system and possessing very strong acid sites compared with those of the zeolitic framework (sharp increase at 2230 cm-'). At still higher steaming temperatures (1023 K) the numbers of Brarnsted acid sites of all kinds are substantially reduced, particularly those associated with non- framework material.The optimal steaming temperature of zeolite H(98)Y for a good mix of extra-framework and zeo- litic framework acidic binding sites is in the region of 823 K where the presence of the outstandingly strong acidic sites due to the hydrated alumina-like material could be expected to influence hydrocarbon cracking greatly, particularly in that part of the process involving relatively high molecular mass components which crack on the outside surface of the zeolite crystals just where this highly acidic material is thought to accumulate. Uptake of Hydrogen Cyanide by Zeolites X, L, Mordenite, Beta and ZSM-5 Compared with zeolite H(98)Y there are fewer Brarnsted acid sites in zeolite HX, since their viability at increased tem- perature is impaired, while the occurrence of hydroxy groups represented by the LW band is reduced.Since the IR spectral band positions representing the binding of hydrogen cyanide by zeolite HX are in the same wavenumber regions as those found in zeolite Y, clearly the nature of the acidic sites in both zeolites is closely similar. In making comparisons of the binding capacity for hydrogen cyanide at the various acidic sites it is appropriate to compare the interaction of hydrogen cyanide with zeolite HX and zeolite H(90)Y. Considering first the relatively weak acid sites represented by the bands at 2095 and 2120 cm-', the spectral contribution of the former is reduced by about one third while that due to the latter is much reduced compared with that found for zeolite H(90)Y in keeping with the reduced intensity of the LW hydroxy- group band with which these binding sites are associated.Comparison of the relatively stronger acid sites represented by spectral constituents centred at 2107, 2104 and 2114 cm-', reveals that their variations with temperature of initial thermal treatment are remarkably similar for both zeolites, but their contributions are reduced for zeolite HX. The simi- larity in the nature of the acid sites in zeolites X and Y is further illustrated by the thermal desorption of the more tenaciously bound hydrogen cyanide from the relatively strong acid sites which, for example, have an IR spectral con- tribution centred at 2108 cm-'.While the starting material, NH4(93)X is highly crystalline, there is an appreciable loss of crystallinity below 473 K and the zeolite is amorphous at 673 K.8 These earlier findings are in keeping with the results sum- marized in Tables 5-7. The loss in crystallinity influences J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 15 Percentage fall in vCZN IR segmental band areas for thermal desorption of hydrogen cyanide from various zeolites zeolite temperature range/K vc.&m -' band area decrease per K ("4) H(98)Y 125 2109 0.73 HX 125 2107 0.73 2093 0.78 HL 125 2111 0.74 2101 0.7 1 2093 0.79 H-mordenite 125 21 18 0.60 21 14 0.57 21 10 0.70 2106 0.72 H-beta 75 2115 1.37 2106 1.16 H-ZSM-5 175 2122 0.51 greatly the capacity of zeolite HX for hydrogen bonding of hydrogen cyanide.However, the value of the present results lies in establishing that both zeolites X and Y employ the same binding sites for hydrogen cyanide. The acidity, catalytic activity and thermal stability of various L zeolites have been studied and the effect of cation content on acidity and catalytic activity determined.82 The total amount of hydrogen cyanide bound by the protonic form of zeolite L by all its acidic sites is considerably less than that bound by zeolite H(98)Y or zeolite HX. The rela- tively weak acid binding site is represented by spectral con- stituent bands at 2095 cm-'.A major relatively stronger acid binding site for hydrogen cyanide is represented by the con- stituent spectral band centred at 2101 cm- ' whose wavenum- ber, persistence at diminished pressure and thermal desorption profile suggest that this site is distinctive. Rela- tively strong acid sites marked by spectral constituent bands centred at 2108 and 2116 cm-' are present, but in smaller amounts. The acidity and superacidity of mordenite have been The total amount of hydrogen cyanide taken up by the protonic form of mordenite is similar to that bound by zeolite HL. While the relatively weak acid sites for binding of hydrogen cyanide may be identified by the spectral constit- uent bands at 2094 and 2120 cm-' and the relatively strong- er acid sites by bands at 2114, 2110, 2106 and 2102 cm-' in descending order of relative intensity, the stronger binding of hydrogen cyanide is dominated by a distinctive spectral con- stituent band centred at 2118 cm-'.The structural features of zeolite beta have been described89 91 and its use in hydrocarbon conversions out- lined.92 The relatively strong acid site is characterized by the spectral constituent band centred at 2106 cm-' and zeolitic temperature desorption accounts for the greater part of this zeolitically adsorbed hydrogen cyanide. The remainder of the more tenaciously held hydrogen cyanide is represented by the spectral constituent band centred at 2115 cm-'. A very minor amount of hydrogen cyanide is held by the weak acid site monitored by the spectral constituent band centred at 2098 cm- '.The Brczrnsted acidity of zeolite ZSM-5 has been character- ized by ammonia adsorption93 95 and IR spectros-copy.96-'O4 Th e acidity depends on the conditions of deammoniation, 'O5 and temperature-programmed desorption of ammonia from zeolite H-ZSM-5 shows the existence of three types of acid site: weak, medium and strong.'06 In keeping with the lower occurrence of acidic sites on ZSM-5 zeolite compared with near-faujasite zeolites, much less hydrogen cyanide is retained by zeolite ZSM-5 than with zeolite H(98)Y. The IR spectral changes which occur due to partial removal of hydrogen cyanide by diminished pressure pumping points to adsorption in terms of weakly held and more bound hydrogen cyanide is represented by the IR spec-tral band centred at 2098 cm-'.The more strongly held hydrogen cyanide is accounted for by the spectral constituent band at 2122 cm-'. This acid site achieves distinction on the grounds of the IR spectral wavenumber of the bound hydro- gen cyanide and due to the retention of a significant amount of its hydrogen cyanide above 423 K, a temperature at which a proportionately larger amount of hydrogen cyanide would have been lost from all the other zeolites studied here. The percentage fall in IR band area with temperature, obtained from the thermal desorption data of hydrogen cyanide from the strongest acid site, as summarized in Table 15, decreases in the order ZSM-5 mordenite < L z X, Y $ beta which is in their order of decreasing acid strength.Conclusions While there is no correlation between the acid strength of the various zeolitic Brransted acid sites and the IR spectral wave- number of hydrogen cyanide bound to such sites, the wave- number serves to characterize the sites and makes possible their identification in various zeolites and the elicudation of their relationship to zeolitic structural features. Therefore, it would come as no surprise to find that the IR spectral wave- numbers of adsorbed hydrogen cyanide were the same for isostructural zeolites X and Y (2120, 2097, 21 14 and 2108 cm-'). The channel structure of mordenite, though con-structed differently, has hydrogen-bonding sites (21 18, 2 1 14, 2110 and 2106 cm-') which, by this criterion, are not very different from those found in the near-faujasites.In zeolite L the structural difference in channel construction becomes greater and the most important hydrogen-bonding site in the channel of ZSM-5 is obviously different (2122 crn-.') from that of the other zeolites, though the less important site (21 14 cm-') is similar to one found in the channel of mordenite. Despite differences in structure, the hydrogen-bonding site of relatively low strength (cu. 2095 cm- ') occurs in all of the zeolites, in the supercage of the near-faujasites and tubular channels of zeolite L, ZSM-5 and mordenite. The relatively weak hydrogen-bonding site (2120 cm-') occurs only in the near-faujasites which possess sodalite and prism cages.References I A. Janin, J. C. Lavalley, A. Macedo and F. Raatz, in Per-spectives in Molecular Sieve Science, A.C.S. Symp. Ser. 368, ed. W. H. Flank and T. E. Whyte, (Am. Chem. Soc.. Washington DC, 1988, p. 11 7. J. L. White, A. N. Jelli, J. M. Andre and J. J. Fripiat, Trans. Faraday Soc., 1967,63,461. L. M. Kustov, V. B. Kazansky, S. Beran, L. Kubelkovi and P. Jiru, J. Phys. Chem., 1987,91, 5247. 0. Glemser, in Handbook qf Preparative Inorganic Chemistry, ed. G. Brauer, Academic Press, New York, 2nd edn., 1943, vol. 1, p. 658. 5 J. H. Lunsford, J. Phys. Chem., 1968, 72,4163. 6 T. L. M. Maesen, H. van Bekkum, T. G. Berburg, Z. 1. Kolar and H. W. Kouwenhoven, J. Chem. Soc., Farudap 7ran.s.. 1991, 87, 787.7 P. E. Hathaway and M. E. Davis, J. Catal., 1989, 116, 263. 8 P. Chu, and F. G. Dwyer, J. Catal., 1980, 61,454. 9 G. P. Handreck and T. D. Smith, J. Chem. Soc., Furaday Trans. I, 1988,84,4191. 10 S. H. Moon, H. Windawi and J. R. Katzer, lnd. Eny. Chern. Fundam., 198 1,20,396. 11 D. A. C. Compton, J. R. Young, R. G. Kollar, J. R. 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ISSN:0956-5000    

DOI:10.1039/FT9949000919

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(6), 931-934 931 FTIR Study of the Interaction of Hydrogen Cyanide with Alkali-metal Ion, Silver(i) and Nickel (11) Ion-exchanged Near-Faujasite Zeolites Clive J. Blower and Thomas D. Smith* Chemistry Department Monash University, Clayton, Victoria, Australia 3168 The uptake of hydrogen cyanide by alkali-metal ion-exchanged near-faujasite zeolites involves Lewis acid bind- ing of hydrogen cyanide by the alkali-metal ions such that the -CeN stretch wavenumber depends on the ionic radii. In the case of lithium and sodium ion-exchanged X and Y zeolite there is a significant formation of charac-teristic Brsnsted acid sites resulting from protonation of the zeolite along with formation of cyanide ion by a reversible process. The uptake of hydrogen cyanide by silver(\) ion-exchanged Y zeolite results in the reversible formation of silver cyanide and zeolite 8rsnsted acidity. Similar treatment of nickel(i1)-exchanged zeolite Y involves Lewis acid complexation of hydrogen cyanide at room temperature and irreversible formation of nickel(i1) cyanide within the zeolite at elevated temperatures.An understanding of the chemical modifications which occur as a result of the occupation of metal ion binding sites of the near-faujasite zeolites X and Y by alkali-metal ions has been achieved by using a diversity of physico-chemical information which focusses attention on their structural features using X-ray and neutron diffra~tion,'-~ cation location by ion- exchange5-' and carbon monoxide adsorption measure-ments,8 dielectric and electrical conducti~ity,~*'~far-IR spectroscopy," 7Li NMR,12 heats of adsorption of water," lattice vibration^,'^ heats of immersion in solutions contain- ing alkene~,'~ Various mea- and their role in cataly~is.'~*~~ surements of the basicity of alkali-metal ion-exchanged zeolites have been described, and the importance of the ability of the probe molecule to distinguish between basic and acidic sites as well as exchanged metal ions has been established.' It has been found that the IR spectral measurements of the uptake of hydrogen cyanide by the protonic forms of a number of zeolites are sensitive to the variety of acid sites and structural features of the zeolite.lg Additionally, hydro- gen cyanide meets the criteria required for the detection of basic sites and its potential use in this role is described in the present work.Experimental Zeolites Y (Linde SK40)and X, prepared as described pre- viou~ly,'~were in their sodium ion-exchanged form. Preparation of Alkali-metal Ioo-excbanged Zeolites To obtain alkali-metal ion-exchanged zeolites, 1.O g portions of NaY or NaX zeolite were stirred at room temperature for 24 h in 25 cm3 volumes of the appropriate 99.5% purity alkali-metal ion chloride solution (1 mol dm-3) a total of four times using fresh exchange solution on each occasion. The exchanged zeolite samples were washed thoroughly with distilled water (negative chloride test), air-dried and finally dried at 393 K for 18 h.Storage was over saturated ammon- ium chloride solution. Exchange of the original NaY and NaX zeolites with sodium chloride solution was conducted to ensure pure NaY and NaX zeolite samples. Exchange levels were determined by atomic absorption spectroscopy and were found to be 100% for lithium and potassium ion- exchanged X and Y zeolites, 70% for RbY and CsY zeolite samples and 78% for RbX and CsX zeolites. These compared well with literature values2' Preparation of Silver, Nickel and Ammonium Ion-exchanged Y Zeolites AgY zeolite was prepared in darkness by stirring 1.0 g of NaY zeolite in 40 cm3 of 1.0 mol dm-3 silver nitrate solution for 24 h at room temperature followed by three successive exchanges with fresh 40 an3quantities of 0.1 mol dm-3 silver nitrate solution for 12 h each.The zeolite product was washed thoroughly with distilled water, air-dried at room temperature and stored in darkness. The level of exchange was 100% according to atomic absorption spectroscopy. To obtain N(")Y zeolite with Ni2+ ions exchanged into the sodalite cage and hexagonal prism sites,21 a repetitive exchange-heating cycle was adopted.22 Hence, 1.0 g of NaY zeolite was stirred in 50 cm3 of 0.1 mol dm-3 nickel nitrate solution for 24 h at room temperature. After filtration and washing, the NiNaY zeolite was heated at 393 K for 2 h, 493 K for 2 h and finally 673 K for 14 h under a stream of nitro- gen gas (30 cm3 min- '). This complete treatment of exchange and heating was repeated.A 90% exchange level was found by atomic absorption spectroscopy. 98% exchanged NH4Y zeolite, NH4(98)Y, was prepared as described previously.' Hydrogen Cyanide Adsorption Experiments The procedures for the preparation of hydrogen cyanide and zeolite wafers, adsorption of hydrogen cyanide, recording of the FTIR spectra and data processing (including derivatization) have been detailed previously. Dehydration of wafer forms of metal ion-exchanged X and Y zeolites and deammoniation of the NH,(98)Y zeolite, to give HY zeolite [H(98)Y], were performed at 593 K for 1 h at 0.001 Torr (prior to hydrogen cyanide adsorption. 2,6-Lutidine (2,6-dimethylpyridine, BDH, i.r.) was dried over sodium hydroxide pellets and distilled ( 4.1 1-418 K).Adsorption of 0.50 mmol of 2,6-lutidine onto H(98)Y and NaY zeolites was conducted fqr 1 h at room temperature in the evacuated cell after wafer dehydration but before hydro- gen cyanide adsorption by the normal procedure. Results The FTIR spectra in the CEN stretch region due to hydro- gen cyanide adsorbed by lithium-, sodium-, potassium-, rubidium-and caesium-exchanged sodium X, sodium Y zeolite and silver(1)- and nickel(I1)-exchanged sodium Y zeolite were recorded. Removal of the less tightly bound hydrogen 932 cyanide by diminished (0.001 Torr) pressure pumping for 2 min allowed the measurement of the CEN stretch wavenum- ber (vC=") due to hydrogen cyanide bound to the metal ion centres, as summarized in Table 1.In potassium cyanide the CEN stretch band occurs at 2078 cm-' and for sodium cyanide at 2092 cm-'. 23 The uptake of hydrogen cyanide by potassium ion-exchanged zeolite Y results in an IR band due to the -C=N stretch centred at 2092 cm- ' (for simplicity referred to as the 2092 cm-' band) which undergoes some bandwidth reduction on partial removal of hydrogen cyanide at dimin- ished pressure. Spectral derivatization shows that most of the partially removed hydrogen cyanide was bound to Bransted acid sites on the zeolite framework (2106 and 2114 cm-'), leaving a single band at 2090 cm-' due to hydrogen cyanide bound to exchanged potassium ion. Similarly, for rubidium and caesium ion-exchanged Y zeolite, there is some binding of hydrogen cyanide to Brsnsted acid sides, which after pres- sure desorption shows hydrogen cyanide bound to rubidium ion and caesium ion characterized by spectral bands at 2088 cm-' for RbY and 2082 cm-' for CsY.The IR spectral bandwidth due to hydrogen cyanide adsorbed onto sodium ion-exchanged zeolite Y shows a much greater IR spectral band narrowing after partial removal of hydrogen cyanide, indicating a greater amount of hydrogen cyanide bound to Brsnsted acid sites. Partial removal of hydrogen cyanide leaves that portion bound to exchanged sodium ion characterized by a band at 2101 cm-'. Since the sodium ion-exchanged zeolite does not possess Bransted acid sites the interaction with hydrogen cyanide must result in their formation, thus the treatment of sodium Y containing 2,6-lutidine (1601, 1578 cm- ') with hydrogen cyanide results in its conversion to the lutidinium ion (1650, 1678 cm-').The reversibility of the whole process is demon- strated by the fall in relative intensity of bands due to 2,6- lutidinium ion and rise in the corresponding 2,6-lutidine bands on partial removal of hydrogen cyanide. The other changes brought about by the addition of hydrogen cyanide to silver(1)-exchanged NaY are also reversible, since the bands due to the formation of silver cyanide (2166 cm-'; c$ AgCN, 2160 cm-', Ag+(HCN), 2138 cm-', Ag+(HCN),, 2147 cm-1)24 along with bands due to hydrogen cyanide at dimin- ished pressure. The IR bands due to hydrogen cyanide adsorbed by the nickel@) ion-exchanged NaY zeolite, which show little change on pressure reduction, are attributable to hydrogen cyanide bound by nickel@) (2133 cm- '), Bransted acid sites (2108 cm- ') and residual sodium ion (2094 cm- ').An increase in temperature (373-573 K) causes a reduction of these band intensities with the appearance of a band at 2172 an-' due to formation of nickel cyanide [cf. nickel@) cyanide tetrahydrate 2175 cm-'].25 The thermal desorption of hydrogen cyanide from alkali- Table 1 vCEN stretch wavenumbers for HCN adsorbed by metal ion-exchanged zeolites after removal of the less tightly bound HCN at diminished pressures (0.001 Torr); vCEN for HCN in the gaseous phase, 2097 cm-metal ion zeolite Y zeolite X Li 2 107 2101 Na 2101 2096 K 2090 2082 Rb 2088 2082 cs 2082 208 1 AgNi 2108,2166 2108,2133 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 metal ion-exchanged Y zeolite showed that the retention of hydrogen cyanide in each case was greater than that of the protonic form of the zeolite. Discussion Ionic Radius Considerations of Hydrogen Cyanide Uptake X-Ray crystallographic studies of hydrated and dehydrated alkali-metal ion and silver(1) ion-exchanged zeolite Y show that there is a preferential occupation of the hexagonal prisms (site SI)26 along with exchange into the more acces- sible sodalite (sites SI*, SII' and SII,) and easily accessible, supercages (site SII) which contains the greatest number of exchange sites.27 In the lithium ion-exchanged X zeolite, the lithium ions are located in site SI' and site SII.,* The crystal structure of dehydrated nickel@) ion-exchanged natural fau- jasite shows that nickel@) ions occupy two-thirds of the SI sites while the remaining nickel(@ ions are distributed in the region of the SI', SII' and SII sites.29 The nickel@) ions achieve octahedral coordination in the hexagonal prisms while others have trigonal symmetry in sites SII and SII, and in the sodalite sites.30 As in the case of the uptake of carbon m~noxide,~'metal ions in the hexagonal prisms are inahcess- ible to hydrogen cyanide.In the uptake of hydrogen cyanide by the various Brsnsted acid sites there is a shift of the -CZN stretch wavenumber to higher values compared to the free-gas value, although there is no clear relationship between the acid strength of each site and the magnitude of the wavenumber shift which may also depend on configu- rational features of the site symmetry and hydrogen bonding to the nitrogen atom of bound hydrogen cyanide.lg Again, as in the case for carbon monoxide bonding to metal centre^,^' the increase in IR spectral wavenumber of the -C'N group compared with that of the free-gas value points to a with- drawal of electron density from the nitrogen atom of the -C=N group by the hydrogen atom of the Brernsted acid site, leading to a strengthening of the carbon-nitrogen bond.Similar considerations have been made for the interactions of carbon monoxide with zeolite Y in protonic and sodium- exchanged forms.'* On the other hand, in circumstances where hydrogen cyanide is bound to an anionic site33 (e.g. ClHCN-, BrHCN-) there is an IR spectral wavenumber shift to values less than that possessed by the free gas, owing to a weakening of the -C=N bond. As shown in Fig. 1, the binding of hydrogen cyanide to the alkali-metal ion-exchanged zeolites induces an ionic radii-dependent IR spec- tral shift of the -CSN band to higher wavenumbers for binding to lithium and sodium ions, compared with the free- gas value, as well as a shift to lower wavenumbers when pot- assium, rubidium or caesium ions are present. The increase in spectral wavenumber arises from coordination of the hydro- gen cyanide by lithium and sodium ions with an orientation of the nitrogen atoms towards the metal ion centre, the strength of the interaction being determined by the ionic radii.The IR spectral shifts to lower values, correlated by a somewhat different slope in Fig. 1, may be thought to arise from an orientation of the hydrogen bond of hydrogen cyanide towards the framework oxide anion of the zeolitic framework, whose effective negative charge depends on shielding by the positive charge on the cation, the effect of which diminishes with increasing cation size. 34 Existence of Bransted Acidity in Metal Ion-exchanged Zeolites Early studies showed that alkali-metal ion-exchanged zeolites X and Y do not possess Brsnsted a~idity~~.~~ while a con- J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 L\ 2100 b i' 2090f i1 \ Rb 2085 r1 t 2080 0.6 0.8 1.0 1.2 1.4 1.6 1.8 crystal ionic radtus/A Fig. 1 Change in wavenumber of vCzh with alkali-metal crystal ionic radius for hydrogen cyanide adsorbed at room temperature on various alkali-metal ion-exchanged zeolites initial11 dehydrated at 593 K. after pressure desorption for 2 min at 0.001 Torr: (a) Y zeolite, (b)X zeolite temporary study of the time3' showed that lithium and sodium ion-exchanged zeolites X and Y have significant Brransted acidity, the presence of which has been used to explain the different products obtained in the alkylation of toluene by formaldehyde on alkali-metal ion-exchanged zeo- lites X and Y, where the products obtained from the lithium ion-exchanged zeolite catalyst were those expected of acid catalyst^.^' Dissociative adsorption of hydrogen cyanide by silica-supported iron catalysts generates surface hydroxy groups on the silica support,39 while a similar phenomenon leading to the generation of hydroxy groups has been observed in the adsorption of hydrogen sulfide on zeolite ZSM-5.40 The Brransted acidity generated by reversible disso- ciative adsorption of hydrogen cyanide by alkali-metal ion- exchanged zeolite Y can be measured by the appropriate IR band area due to hydrogen cyanide bound to these sites, excluding the contribution made by the hydrogen cyanide associated with the metal ion centres. A comparison of such areas with that due to hydrogen cyanide adsorbed by zeolite H(98) is shown in Table 2.The greatest level of acidit) is found for the lithium ion-exchanged zeolite followed by the silver(r) ion and sodium ion-exchanged forms. Since there is no evidence for the formation of 2,6-lutidinium ion with these metal ion-exchanged forms, it is concluded that these ions Table 2 Comparison of vCN area, A [using the range 2131-21 12 (k1.5 cm-')I of hydrogen cyanide adsorbed on H(98)Y zeolite with that for aikali-metal ion-exchanged Y zeolites and AgY zeolite, and excluding the HCN bound to the metal ion zeolite 4cm-l mg-' area ('61 H(98)Y 1.39 100 LiY 1.39 100 NaY 0.27 19 KY 0.04 3 RbY 0.07 5 CSY 0.04 3 AgY 1.12 81 933 take part in dissociative adsorption on subsequent exposure to hydrogen cyanide.Basicity of Alkali-metal Ion-exchanged X and Y Zeolites It has been shown that the basicity of oxides and alkali-metal ion-exchanged zeolites can be characterized by the IR N-H stretching wavenumber of chemisorbed pyrr01e.~ '-43 In the case of the alkali-metal ion-exchanged zeolites X and Y, a distinction has been made between the weak acidity of sodium and lithium ion-exchanged zeolite Y and the increas- ing basicity of potassium, rubidium and caesium ion-exchanged zeolite Y; note that similarly exchanged zeolite X is more basic. The correlation between the N-H wavenum-ber shifts of pyrrole, using the literature data,42*43 with those of hydrogen cyanide when adsorbed by alkali-metal ion- exchanged zeolites X and Y is shown in Fig.2. The wave- number shifts due to the -C=N stretch are over a smaller wavenumber range compared with those of the -NH group, in keeping with electronic changes occurring in a multiple bond compared with those in a single bond. However, the correlation of the IR wavenumber shift data for ion-exchanged zeolite Y is good, with both sets of data showing a grouping of lithium and sodium, and potassium, rubidium and caesium on lines of different slopes. The difference is accentuated in the IR wavenumber shift data for alkali-metal ion-exchanged zeolite X which, in addition, shows a more basic character with less effective shielding by the positive charge on the cation.This latter effect is illustrated in Fig. 3, which correlates the literature values of the partial charge on the oxygen of the basic site42,43 with the IR wavenumber shift of the -CSN stretch when hydrogen cyanide is taken up by alkali-metal ion-exchanged forms of zeolites X and Y. In both cases, this effect is similar to those shown in corre- lations of the --NH stretch IR spectral shifts for the chemi- sorption of pyrr~le.~~ The increased basicity of zeolites which arises as a result of alkali-metal ion exchange may have a profound effect on 3450 r 3350 //@ 1 3200 1 cs 1 A!.-3150 ' "" 2080 2085 2090 2095 2100 2105 2110 vcs ./crn-' Fig. 2 Comparison of the change in wavenumber of vNH due to pyrrole adsorbed on alkali-metal ion-exchanged zeolites, with wave- number of vcE due to hydrogen cyanide pressure-desorbed from similar zeolites initially dehydrated at 593 K.Pyrrole wavenumber data by Barth~rneuf:~~ (a)Y zeolite, (b)X zeolite. Pyrrole wavenum- ber data by Huang and Kaliag~ine:~~ (c) Y zeolite, (d)X zeolite. 934 2110 IL 2105 -I;\Li 2100 -2095 -cs Fig. 3 Comparison of the change in wavenumber of vcI N, due to hydrogen cyanide adsorbed on alkali-metal ion-exchanged zeolites, with oxygen charge of similar zeolites prepared by Huang and Kalia- g~ine.~~Solid lines indicate oxygen charge calculated by approx- imating the local composition as Si,-nAlnO,,M, for cation M+ and dashed lines indicate the calculated oxygen charge of the bulk com- position.(a)and (c),Y zeolite, (b)and (d), X zeolite. their catalytic properties. For example, the relative amounts of citronellal and isopulegol formed in an alkali-metal ion- exchanged zeolite X-catalysed Meerwein-Ponndorf-Verley reaction are greatly influenced by the nature of the alkali- metal ion, with the caesium ion favouring the formation of citronellal, and the lithium ion, isop~legol.~~ The results here suggest that the selectivity in product formation may arise from the increased basic character of the zeolite as a result of caesium ion exchange. References 1 W. J. Mortier, Compilation of Extra-Framework Sites in Zeolites, Butterworth, Guildford, 1982. 2 J. V. Smith, Chem. Rev., 1988,88, 149. 3 D. W. Breck, J.Chem. Educ., 1964,41,678. 4 L Broussard and D. P. Shoemaker, J. Am. Chem. SOC., 1960 82, 1041. 5 P. P. Lai and L. V. C. Rees, J. Chem. SOC., Faraday Trans. 1, 1976,82,1809. 6 P. P. Lai and L. V. C. Rees, J. Chem. SOC., Faraday frans. 1, 1976,72, 1827. 7 H. S. Sherry, J. Colloid Interface Sci., 1968,28, 288. 8 T. S. Egerton and F. S. Stone, Trans. Faraday SOC., 1970, 66, 2364. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 9 D. Vucelic, J. Chem. Phys., 1977,66,43. 10 R. A. Schoonheydt and J. B. Uytterhoeven, in Molecular Sieve Zeolites I, ed. R. F. Gould, Ado. Chem. Ser. 101 Am. Chem. SOC., Washington D.C., 1971, p. 456. 11 W. M. Butler, C. L. Angell, W. McAllister and W. M. Risen, J. Phys. Chem., 1977,81,2061. 12 H. Herden, W.-D.Einicke, R. Schollner and A. Dyer, J. Inorg. Nucl. Chem., 1981,43,2533. 13 0.M. Dzhigit, A. V. Kiselev, K. N. Mikos, G. G. Muttik and T. A. Rahmanova, J. Chem. SOC.,Faraday Trans. 1, 1971,67,458. 14 V. M. Radak, T. S. Cerancic and I. M. Zivadinovic, 2. Natur-forsch., Teil B, 1978,33, 11 16. 15 H. Herden, W-D. Einicke, V. Messow, K. Quitzsch and R. Schollner, J. Colloid Interface Sci., 1984,97, 565. 16 P. B. Venuto and P. S. Landis, Adu. Catal., 1968, 18,259. 17 P. E. Hathaway and M. E. Davis, J. Catal., 1989,116,263. 18 D. Barthomeuf, G. Coudurier and J. C. Vedrine, Mater. Chem. 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ISSN:0956-5000    

DOI:10.1039/FT9949000931

出版商:RSC    

年代:1994

数据来源: RSC