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The characterization of zeolite acidity by dynamic methods
2922 Table 1. Characterization of the zeolite samples Structure type FAU MOR MFI BEA
Sample ID Y M Z B1 B2
PQ Product ID CBV 720 CBV 30A CBV 3020 CP811-BL-25 CP814-E22
Si/AIo NH4§ b mmol/g 11.4 0.586 13.7 0.688 16.2 0.598 14.0 0.772 11.6 0.927
Nd ' mmol/g 0.009 0.002 0.001 0.002 0.019
I;AI ~ mmol/g 1.335 1.129 0.965 1.101 1.305
EFAI ~ mmol/g 0.749 0.441 0.367 0.329 0.378
Si/AIF 26.0 22.4 26.1 19.9 16.3
9 Zeolite was rendered soluble and the solution was analysed for Na, AI and Si. b Determined from the amount of NH3 evolved upon thermal decomposition of the NH4-form. c The concentration of extra-framework aluminum (EFA1) was obtained as a difference of the total (EAI) and the framework aluminum concentration (AIF). AIF was taken as equivalent with the NI-I4§ content of the sample. The sorption of ammonia was studied by the FR method. Examinations were carried out on the H-form of these samples prepared by in situ thermal treatment of the NH4-form. The FR "spectra"of ammonia sorption were recorded in the 373-873 K temperature region at various equilibrium NH3 pressures in the range of 50-200 Pa range. A batch-type FR system was used. The volume of the system was perturbed by a + 1 % square wave. The perturbation frequency was changed between 0.01 and 10 Hz. The pressure response was recorded in the presence and in the absence of the sorbent zeolite. Data were processed by computer programs to derive plots of the response function vs. perturbation frequency, i. e., to derive the FR spectrum. A detailed description of the method is given in ref. [5]. The acidity of the zeolites were characterized using them as acid components of bifunctional catalysts. The hydroconversion of n-octane was used as a test reaction. Catalysts containing 0.5 wt % Pt were prepared. The NH4-forms of the zeolites were impregnated with a Pt(NH3)4CI2 solution. The solid was dried at 353 K. Prior to the catalytic measurements a standard in situ pre-treatment was given to each preparation. Each sample was calcined first in flowing dry air at 723 K for 1 hour and then reduced in pure flowing hydrogen at 623 K and 15 bar H2 pressure for 1 hour. A flow-through microreactor was used in the temperature range of 473-673 K under 15 bar total pressure. The flow rate of H2 was 2400 cm3 h"t. The space velocity of the reactant n-octane was 21 moln-octane mol~F"t. h"1. The A1F, was assumed to be equivalent to the exchange capacity for NH4§ and to give the number of Brrnsted acid sites, i. e, the space velocity was adjusted to be the same for the different zeolites relative to the number of the acid sites. Reactor effluent was analyzed by an on-line GC. 3. RESULTS AND DISCUSSION In bifunctional zeolite catalysts metal and acid functions co-operate to activate the alkane molecules for hydroconversion. The activity of the catalyst is best characterized by the rate constants of the reactions, such as hydrogenation/dehydrogenation, isomerization and cracking. When metal and hydrogen are present in sufficient excess, as in the catalytic systems of this study, it can be assumed that under hydroconversion conditions rapid metal catalyzed
2923 hydrogenation/dehydrogenation establishes equilibrium olefin to alkane ratios. For such a case the kinetic analysis shows that, the rate limiting step of the reaction is the acid catalyzed transformation [6]. This simplifies matters, since differences in the activities and selectivities can be interpreted as result of different catalyst acidities. However, the correlation between the activity and the acidity of the catalyst has not been demonstrated as yet. 100 The steady-state n-octane conversion arO-and product yields are plotted as a function 80 /other cracks of reaction temperature in Figure 1. a / O~ 0 60 Isomerization prevails at lower n-Ca / 0 i.C4 temperatures while at higher temperatures 4oi cracking is the favored reaction. As a result 201 the i-C8 yield passes through a maximum as til ,, ~ 0 - - ~ A .... q .--7-."~ . . ~ .-Z 9 ~ 6 a function of temperature. For the different 100 catalysts tested the maximum occurred at / o o ~ r cracks m 80 M different temperatures but generally at about 70-80 % total conversion. The n - C a ~ ~ - i-C4 0 2 reactant feed was about the same for the Q. 40 / .,-c,\\ different catalysts relative to the number of 9~ 20BrOnsted acid sites. Thus, the similar E conversions correspond to nearly the same "6 lOOi turnover frequency of the reacting 80 cracks molecules on the active sites. For less active 60 "u sites, i.e. for sites of weaker acidity, higher 1= temperatures are required to get the same 40 c/ Me-C7 \V. turnover frequency. It is suggested, o /o/o ~-c~ 920 therefore, that the temperature of identical conversions can be used to characterize the .'~-. ,' g . . . . . . . . . . . . . . . . . ~> 100 9-8- .--<-.Y! .'r cactivity, and indirectly the acid strength of o / C) 80 z the active sites. We selected the a other cracks 60 temperature where the yield of branched Cs n'C8 ~ ~ ~ . C 4 isomers is a maximum. This characteristic 40 temperature (Tch) is the highest for catalyst 20 Pt/Y. Low and similar T~h were obtained for Pt/B2 or Pt/Z. These data suggest that 473 523 573 hydroconversion may proceed on the active sites of P t ~ - Y which are less acidic than Temperature/ K Fig. 1. The conversion and product yields in n- those of Pt/H-Beta and Pt~-ZSM-5. No correlation was found, however, between octane hydroisomerization over Pt/H-zeolites. Tch and the selectivity substantiating that The area under the curve of total conversion is selectivity is controlled also by factors such divided to sections by curves to give the as zeolite structure, which are not or only product distribution. indirectly related to the acidity of the active sites. The sequence of activity suggested by the values of Tch of the hydroconversion catalysts is H-ZSM-5 ,= H-Beta > H-Mordenite >H-Y. This sequence corresponds to that suggested for the acid strength of such zeolites by theoretical and by experimental studies [7, 8]. Even the
B2
Aer
o=,.
n-C,/\\
2924 qualitative correspondence is surprising if we consider that the parameter Toh is characterizing a catalyst under steady state reaction conditions. The steady-state catalytic activity is a property of the solid which is developed in interaction of the neat solid acid with the reactants and products [9]. The Br6nsted acid sites adsorb alkanes or alkenes and depending on the extent of proton transfer from the solid to the bound molecule alkene can be converted to carbenium ion or to covalently bound alkoxy species. The operating sites of the working acid catalyst are protons, alkyl and alkenyl carbenium ions (acids) bound to their conjugated base sites (sites of the anionic zeolite framework). The adsorption of hydrocarbons increases the negative charge on the framework anion, thereby the acid strength of the BrOnsted sites available for reaction decreases. The catalytic significance of the enhanced anionic character of the framework was demonstrated by poisoning experiments [8, 10, 11 ]. If less than about 10 % of the protons of H-zeolites were exchanged for Na + or NH4+ the activity for alkane conversion decreased strongly or was completely lost. Obviously, the inherent acidity of the zeolite must be different of the acidity under steady state reaction conditions. A bimolecular hydride transfer reaction can occur between a reactant alkane molecule and the carbenium ion. As a result an alkane is released which preferably, but not necessarily, is different from the reactant itself. Simultaneously a new carbenium ion is formed. If this hydride transfer is more frequent than the proton transfer between the hydrocarbon and the zeolite (the initiation and termination of reaction chains) long reaction chains are providing the products and determine conversion and selectivity. The contribution of the direct proton-hydrocarbon interactions to the the n-octane hydroconversion is minor. The reaction proceeds primarily in long reaction chains which are affected by catalyst acidity indirectly as the acid strength of the Br6nsted sites determines the stability of the carbenium ions. 1.0 The objective of the present work was to demonstrate the similarity of the zeolite 05 bound carbenium ion/alkane and ammonium t#} e-" ion/ammonia systems. It was found, vide 0 infra, that the dynamic parameters of weak O0 ammonia sorption over the zeolites t'r" containing "protonated ammonia", i.e., NH4§ 1.o ions can be correlated with the catalytic and, most probably, with the acidic properties of 0.5 the zeolites. Representative ammonia FR spectra are shown in Fig. 2. The theory of o.o O.Ol o.1 1 lO lOO the FR method suggests that the spectrum F r e q u e n c y / Hz profile obtained for the H-ZSM-5/ammonia Fig. 2. FR spectra of ammonia sorption on H- system is characteristic for mass transport having a rate-controlling sorption step. ZSM-5 at different pressures. Spectra were recorded at 700 K. Symbols (1"i) and (O) Similar FR spectra were recorded for all the zeolites studied. A common characteristic of represent the in-phase and out-of-phase these spectra is that they show a strong outcomponents of the response pressure wave, of-phase peak at about 10 Hz. Using a curve respectively. Lines give the best-fit fitting procedure a less intense second peak characteristic curves determined according to was resolved at lower frequency. It has been Yasuda's sorption model [ 15]. suggested previously [3], that the low~1~
...................
2925 frequency peak reflects the resonance between the perturbation and the process of IT + NH3 r NH4+, while the highfrequency peak belongs to the interaction of ammonia with weak acid sites, most probably with NH4§ ions formed in the above mentioned sorption equilibrium. Quantum-chemical cluster calculations
[12] and MAS-NMR [13, 14] and IR spectroscopic results [14] provide ample of
k, 250f 200"
s-1 pa-1 s-~ [] Y 0.89 18 0 M 0.62 12 A B1 0.60 3 v B2 0 z
o62 o.
4
I
/
'7r 150"
evidence for the presence of ~ NH3] + species over the surface of solid Br6nsted 100acids if ammonia coverage is not very low. It was shown that a rapid proton transfer prevails between the NH4§ ion and the NH3 bound to it by a hydrogen bond. The 50situation is similar to that occurring in the hydride transfer step of the hydroconversion where hydrogen is O . . . . exchanged between the positively charged o go 16o 1go 200 carbenium ion and the neutral alkane. Of Pe / Pa course, no ammonia conversion is Fig. 3. Pressure dependence of the time constant proceeding in this latter case. Using the Langmuir rate equition for the of the ammonia sorption (r.j) giving the highsorption process on sorption site j, the frequency FR resonance. The adsorption- and sorption time constant, r_j, can be desorption rate constants (k, (i) and 1~(i)) were expressed as r_j=k,(i~P= + kd(i) , where P= is obtained as the slope and the intercept of a lines, the equilibrium pressure, k, (i) and k~(i) are respectively. the adsorption and desorption rate constants, respectively. The good linear r_j v s . P= correlation shown in Fig 3. suggests that the Langmuir model gives a reasonable representation of the weak ammonia sorption process for each zeolite. The values of the desorption rate constants follow the same order as T~ indicating that the dynamic parameters of the ammonia sorption over the NH4 § sites can be used to characterize the active sites. Data suggest that the properties of the sorption sites NH~§ ions are reflecting to some extent the acidity of the Br6nsted site whereupon the NH~§ cations were formed. 4. CONCLUSIONS It has been substantiated that the weak interaction of ammonia with ammonium ion sorption sites in zeolites reflects the acid strength of the BrOnsted site upon which the ion was formed. The similarities of the NH4+/NH3 and the carbenium ion/alkane systems have been pointed out. Rate constant of ammonia desorption over the NH4 § ions show good correlation with the activity of the H-zeolites in catalytic alkane conversion reactions.
2926 ACKNOWLEDGEMENTS Thanks is due to the Royal Society, CEE Project Grant and to the Hungarian Academy of Sciences for supporting this work. The valuable technical assistance of Mrs. Agnes Wellisch are gratefully acknowledged.
REFERENCES
1. P.B. Venuto, Microporous Materials, 2(1994) 297. 2. J. Fraissard and L. Petrakis (eds.), Acidity and Basiciy of Solids: Theory, Assessment and Utility, Kluwer Academic Publisher, London, 1994. 3. J. Valyon, Gy. Onyestyfik and L. V. C. Rees, J. Phys. Chem. B,. 102 (1998) 8994. 4. Gy. Onyestyfik, D. Shen and L. V. C. Rees, J. Chem. Faraday Trans., 92 (1996) 307. 5. L.V.C. Rees and D. Shen, Gas. Sep. Purif., 7 (1993) 83. 6. T.F. Degnan and C. R. Kennedy, AIChE Journal, 39 (1993) 607. 7. D. Barthomeouf, Materials Chemistry and Physics, 17(1987) 49. 8. B. Umansky, J. Engelhardt, and W. K. Hall, I. Catal., 127 (1991) 128. 9. N.M. Rice and B. W. Wojciechowski, Canad. J. Chem. Eng., 69 (1991) 1100. 10. W. K. Hall, J. Engelhardt and G. A. Sill, in Zeolites: Facts, Figures, Future (P. A. Jacobs and R. A. van Santen, eds.) p. 1253. Elsevier, Amsterdam, 1989. 11. J. Engelhardt, J. Catal., 164 (1996) 449. 12. E. H. Teunissen, R. A. van Santen, A. P. J. Jansen, and F. B. van Duijneveldt, J. Phys. Chem., 97 (1993) 203. 13. W. P. J. H. lacobs, J. W. de Haan, L. J. M. van de Ven and R. A. van Santen, J. Phys. Chem., 97 (1993) 10394. 14. W. L. Earl, P. O. Fritz, A. A. V. Gibson and J. H. Lunsford, J. Phys. Chem., 91 (1987) 2091. 15. Y. Yasuda, Heterog. Chem. Rev., 1 (1994) 103.
Sample ID Y M Z B1 B2
PQ Product ID CBV 720 CBV 30A CBV 3020 CP811-BL-25 CP814-E22
Si/AIo NH4§ b mmol/g 11.4 0.586 13.7 0.688 16.2 0.598 14.0 0.772 11.6 0.927
Nd ' mmol/g 0.009 0.002 0.001 0.002 0.019
I;AI ~ mmol/g 1.335 1.129 0.965 1.101 1.305
EFAI ~ mmol/g 0.749 0.441 0.367 0.329 0.378
Si/AIF 26.0 22.4 26.1 19.9 16.3
9 Zeolite was rendered soluble and the solution was analysed for Na, AI and Si. b Determined from the amount of NH3 evolved upon thermal decomposition of the NH4-form. c The concentration of extra-framework aluminum (EFA1) was obtained as a difference of the total (EAI) and the framework aluminum concentration (AIF). AIF was taken as equivalent with the NI-I4§ content of the sample. The sorption of ammonia was studied by the FR method. Examinations were carried out on the H-form of these samples prepared by in situ thermal treatment of the NH4-form. The FR "spectra"of ammonia sorption were recorded in the 373-873 K temperature region at various equilibrium NH3 pressures in the range of 50-200 Pa range. A batch-type FR system was used. The volume of the system was perturbed by a + 1 % square wave. The perturbation frequency was changed between 0.01 and 10 Hz. The pressure response was recorded in the presence and in the absence of the sorbent zeolite. Data were processed by computer programs to derive plots of the response function vs. perturbation frequency, i. e., to derive the FR spectrum. A detailed description of the method is given in ref. [5]. The acidity of the zeolites were characterized using them as acid components of bifunctional catalysts. The hydroconversion of n-octane was used as a test reaction. Catalysts containing 0.5 wt % Pt were prepared. The NH4-forms of the zeolites were impregnated with a Pt(NH3)4CI2 solution. The solid was dried at 353 K. Prior to the catalytic measurements a standard in situ pre-treatment was given to each preparation. Each sample was calcined first in flowing dry air at 723 K for 1 hour and then reduced in pure flowing hydrogen at 623 K and 15 bar H2 pressure for 1 hour. A flow-through microreactor was used in the temperature range of 473-673 K under 15 bar total pressure. The flow rate of H2 was 2400 cm3 h"t. The space velocity of the reactant n-octane was 21 moln-octane mol~F"t. h"1. The A1F, was assumed to be equivalent to the exchange capacity for NH4§ and to give the number of Brrnsted acid sites, i. e, the space velocity was adjusted to be the same for the different zeolites relative to the number of the acid sites. Reactor effluent was analyzed by an on-line GC. 3. RESULTS AND DISCUSSION In bifunctional zeolite catalysts metal and acid functions co-operate to activate the alkane molecules for hydroconversion. The activity of the catalyst is best characterized by the rate constants of the reactions, such as hydrogenation/dehydrogenation, isomerization and cracking. When metal and hydrogen are present in sufficient excess, as in the catalytic systems of this study, it can be assumed that under hydroconversion conditions rapid metal catalyzed
2923 hydrogenation/dehydrogenation establishes equilibrium olefin to alkane ratios. For such a case the kinetic analysis shows that, the rate limiting step of the reaction is the acid catalyzed transformation [6]. This simplifies matters, since differences in the activities and selectivities can be interpreted as result of different catalyst acidities. However, the correlation between the activity and the acidity of the catalyst has not been demonstrated as yet. 100 The steady-state n-octane conversion arO-and product yields are plotted as a function 80 /other cracks of reaction temperature in Figure 1. a / O~ 0 60 Isomerization prevails at lower n-Ca / 0 i.C4 temperatures while at higher temperatures 4oi cracking is the favored reaction. As a result 201 the i-C8 yield passes through a maximum as til ,, ~ 0 - - ~ A .... q .--7-."~ . . ~ .-Z 9 ~ 6 a function of temperature. For the different 100 catalysts tested the maximum occurred at / o o ~ r cracks m 80 M different temperatures but generally at about 70-80 % total conversion. The n - C a ~ ~ - i-C4 0 2 reactant feed was about the same for the Q. 40 / .,-c,\\ different catalysts relative to the number of 9~ 20BrOnsted acid sites. Thus, the similar E conversions correspond to nearly the same "6 lOOi turnover frequency of the reacting 80 cracks molecules on the active sites. For less active 60 "u sites, i.e. for sites of weaker acidity, higher 1= temperatures are required to get the same 40 c/ Me-C7 \V. turnover frequency. It is suggested, o /o/o ~-c~ 920 therefore, that the temperature of identical conversions can be used to characterize the .'~-. ,' g . . . . . . . . . . . . . . . . . ~> 100 9-8- .--<-.Y! .'r cactivity, and indirectly the acid strength of o / C) 80 z the active sites. We selected the a other cracks 60 temperature where the yield of branched Cs n'C8 ~ ~ ~ . C 4 isomers is a maximum. This characteristic 40 temperature (Tch) is the highest for catalyst 20 Pt/Y. Low and similar T~h were obtained for Pt/B2 or Pt/Z. These data suggest that 473 523 573 hydroconversion may proceed on the active sites of P t ~ - Y which are less acidic than Temperature/ K Fig. 1. The conversion and product yields in n- those of Pt/H-Beta and Pt~-ZSM-5. No correlation was found, however, between octane hydroisomerization over Pt/H-zeolites. Tch and the selectivity substantiating that The area under the curve of total conversion is selectivity is controlled also by factors such divided to sections by curves to give the as zeolite structure, which are not or only product distribution. indirectly related to the acidity of the active sites. The sequence of activity suggested by the values of Tch of the hydroconversion catalysts is H-ZSM-5 ,= H-Beta > H-Mordenite >H-Y. This sequence corresponds to that suggested for the acid strength of such zeolites by theoretical and by experimental studies [7, 8]. Even the
B2
Aer
o=,.
n-C,/\\
2924 qualitative correspondence is surprising if we consider that the parameter Toh is characterizing a catalyst under steady state reaction conditions. The steady-state catalytic activity is a property of the solid which is developed in interaction of the neat solid acid with the reactants and products [9]. The Br6nsted acid sites adsorb alkanes or alkenes and depending on the extent of proton transfer from the solid to the bound molecule alkene can be converted to carbenium ion or to covalently bound alkoxy species. The operating sites of the working acid catalyst are protons, alkyl and alkenyl carbenium ions (acids) bound to their conjugated base sites (sites of the anionic zeolite framework). The adsorption of hydrocarbons increases the negative charge on the framework anion, thereby the acid strength of the BrOnsted sites available for reaction decreases. The catalytic significance of the enhanced anionic character of the framework was demonstrated by poisoning experiments [8, 10, 11 ]. If less than about 10 % of the protons of H-zeolites were exchanged for Na + or NH4+ the activity for alkane conversion decreased strongly or was completely lost. Obviously, the inherent acidity of the zeolite must be different of the acidity under steady state reaction conditions. A bimolecular hydride transfer reaction can occur between a reactant alkane molecule and the carbenium ion. As a result an alkane is released which preferably, but not necessarily, is different from the reactant itself. Simultaneously a new carbenium ion is formed. If this hydride transfer is more frequent than the proton transfer between the hydrocarbon and the zeolite (the initiation and termination of reaction chains) long reaction chains are providing the products and determine conversion and selectivity. The contribution of the direct proton-hydrocarbon interactions to the the n-octane hydroconversion is minor. The reaction proceeds primarily in long reaction chains which are affected by catalyst acidity indirectly as the acid strength of the Br6nsted sites determines the stability of the carbenium ions. 1.0 The objective of the present work was to demonstrate the similarity of the zeolite 05 bound carbenium ion/alkane and ammonium t#} e-" ion/ammonia systems. It was found, vide 0 infra, that the dynamic parameters of weak O0 ammonia sorption over the zeolites t'r" containing "protonated ammonia", i.e., NH4§ 1.o ions can be correlated with the catalytic and, most probably, with the acidic properties of 0.5 the zeolites. Representative ammonia FR spectra are shown in Fig. 2. The theory of o.o O.Ol o.1 1 lO lOO the FR method suggests that the spectrum F r e q u e n c y / Hz profile obtained for the H-ZSM-5/ammonia Fig. 2. FR spectra of ammonia sorption on H- system is characteristic for mass transport having a rate-controlling sorption step. ZSM-5 at different pressures. Spectra were recorded at 700 K. Symbols (1"i) and (O) Similar FR spectra were recorded for all the zeolites studied. A common characteristic of represent the in-phase and out-of-phase these spectra is that they show a strong outcomponents of the response pressure wave, of-phase peak at about 10 Hz. Using a curve respectively. Lines give the best-fit fitting procedure a less intense second peak characteristic curves determined according to was resolved at lower frequency. It has been Yasuda's sorption model [ 15]. suggested previously [3], that the low~1~
...................
2925 frequency peak reflects the resonance between the perturbation and the process of IT + NH3 r NH4+, while the highfrequency peak belongs to the interaction of ammonia with weak acid sites, most probably with NH4§ ions formed in the above mentioned sorption equilibrium. Quantum-chemical cluster calculations
[12] and MAS-NMR [13, 14] and IR spectroscopic results [14] provide ample of
k, 250f 200"
s-1 pa-1 s-~ [] Y 0.89 18 0 M 0.62 12 A B1 0.60 3 v B2 0 z
o62 o.
4
I
/
'7r 150"
evidence for the presence of ~ NH3] + species over the surface of solid Br6nsted 100acids if ammonia coverage is not very low. It was shown that a rapid proton transfer prevails between the NH4§ ion and the NH3 bound to it by a hydrogen bond. The 50situation is similar to that occurring in the hydride transfer step of the hydroconversion where hydrogen is O . . . . exchanged between the positively charged o go 16o 1go 200 carbenium ion and the neutral alkane. Of Pe / Pa course, no ammonia conversion is Fig. 3. Pressure dependence of the time constant proceeding in this latter case. Using the Langmuir rate equition for the of the ammonia sorption (r.j) giving the highsorption process on sorption site j, the frequency FR resonance. The adsorption- and sorption time constant, r_j, can be desorption rate constants (k, (i) and 1~(i)) were expressed as r_j=k,(i~P= + kd(i) , where P= is obtained as the slope and the intercept of a lines, the equilibrium pressure, k, (i) and k~(i) are respectively. the adsorption and desorption rate constants, respectively. The good linear r_j v s . P= correlation shown in Fig 3. suggests that the Langmuir model gives a reasonable representation of the weak ammonia sorption process for each zeolite. The values of the desorption rate constants follow the same order as T~ indicating that the dynamic parameters of the ammonia sorption over the NH4 § sites can be used to characterize the active sites. Data suggest that the properties of the sorption sites NH~§ ions are reflecting to some extent the acidity of the Br6nsted site whereupon the NH~§ cations were formed. 4. CONCLUSIONS It has been substantiated that the weak interaction of ammonia with ammonium ion sorption sites in zeolites reflects the acid strength of the BrOnsted site upon which the ion was formed. The similarities of the NH4+/NH3 and the carbenium ion/alkane systems have been pointed out. Rate constant of ammonia desorption over the NH4 § ions show good correlation with the activity of the H-zeolites in catalytic alkane conversion reactions.
2926 ACKNOWLEDGEMENTS Thanks is due to the Royal Society, CEE Project Grant and to the Hungarian Academy of Sciences for supporting this work. The valuable technical assistance of Mrs. Agnes Wellisch are gratefully acknowledged.
REFERENCES
1. P.B. Venuto, Microporous Materials, 2(1994) 297. 2. J. Fraissard and L. Petrakis (eds.), Acidity and Basiciy of Solids: Theory, Assessment and Utility, Kluwer Academic Publisher, London, 1994. 3. J. Valyon, Gy. Onyestyfik and L. V. C. Rees, J. Phys. Chem. B,. 102 (1998) 8994. 4. Gy. Onyestyfik, D. Shen and L. V. C. Rees, J. Chem. Faraday Trans., 92 (1996) 307. 5. L.V.C. Rees and D. Shen, Gas. Sep. Purif., 7 (1993) 83. 6. T.F. Degnan and C. R. Kennedy, AIChE Journal, 39 (1993) 607. 7. D. Barthomeouf, Materials Chemistry and Physics, 17(1987) 49. 8. B. Umansky, J. Engelhardt, and W. K. Hall, I. Catal., 127 (1991) 128. 9. N.M. Rice and B. W. Wojciechowski, Canad. J. Chem. Eng., 69 (1991) 1100. 10. W. K. Hall, J. Engelhardt and G. A. Sill, in Zeolites: Facts, Figures, Future (P. A. Jacobs and R. A. van Santen, eds.) p. 1253. Elsevier, Amsterdam, 1989. 11. J. Engelhardt, J. Catal., 164 (1996) 449. 12. E. H. Teunissen, R. A. van Santen, A. P. J. Jansen, and F. B. van Duijneveldt, J. Phys. Chem., 97 (1993) 203. 13. W. P. J. H. lacobs, J. W. de Haan, L. J. M. van de Ven and R. A. van Santen, J. Phys. Chem., 97 (1993) 10394. 14. W. L. Earl, P. O. Fritz, A. A. V. Gibson and J. H. Lunsford, J. Phys. Chem., 91 (1987) 2091. 15. Y. Yasuda, Heterog. Chem. Rev., 1 (1994) 103.