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Towards predicting catalytic performances of zeolites
From Zeolites to Porous MOF Materials – the 40th Anniversary of International Zeolite Conference R. Xu, Z. Gao, J. Chen and W. Yan (Editors) © 2007 Elsevier B.V. All rights reserved.
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Towards predicting catalytic performances of zeolites J. A. van Bokhoven* and B. Xu Department of Chemical and Bioengineering, ETH Zurich, HCI E115, Wolfgang-Pauli-Str. 10, 8093 Zurich, Switzerland. Tel: +41-44-632 55 42; Fax: +41-44-632 11 62; Email: [email protected] ABSTRACT The intrinsic activation energies of alkane activation reactions that involve the protonation of the reactant in the rate-limiting step are independent of zeolite structure and of Si/Al ratio. Their observed rates, however, increase with decreasing pore-sizes. Reactions that involve the formation and desorption of alkoxide species in their rate-limiting step strongly depend on the zeolite structure and on the interaction of the adsorbed molecule with the local environment of the acid site. There is a strong dependence on the Si/Al ratio. 1. INTRODUCTION The activity and selectivity of chemical reactions in zeolites vary with pore size and connectivity and with Si / Al ratio. Zeolite treatments, such as heating in their own moisture or steam, stabilize the structures and activate them. Many explanations have been put forward to explain enhanced zeolite activity after activation in moisture. Quite often, differences in activity are directly related to variations in acid strength. However, the various techniques that determine acid strength have not provided a single scale to which zeolites can be held to predict activity. Based on the early works of Haag [1], we have systematically determined the intrinsic activity of Brønsted acid sites in zeolites using a simple reaction, which is the monomolecular cracking of small alkanes. This reaction is first order in alkane, produces simple products, is not diffusion limited, and shows no deactivation. Combining zeolite reactivity and structural determination provides structure / performance relations. In recent years, it has become much clear how the zeolite structures change under different conditions. The aluminum coordination changes as function of temperature [2]. Exposure of acidic zeolites to moisture at room temperature leads to structural collapse, which can be completely recovered [3]. The activity of Brønsted acid sites is determined by a combination of the number of sites and the reactivity of each of them. The work of Haag showed that at low coverage of reactant, the rate expression monomolecular conversion of alkanes is: r = k´[A] = kK[A] (1) in which kc is the observed rate constant; k the intrinsic rate constant, and K the adsorption constant. This latter term describes the enrichment of reactant on the surface of the catalyst compared to the gas phase; k represents the intrinsic reaction parameters, which depend on the intrinsic properties of the active site. In case of zeolites, this is generally assumed to be
1168 related to acid strength. Because, k and K have similar dependencies on temperature, the intrinsic activation energy can be determined using the following equation: act Eapp
act Etrue ¦ ni 'H i
(2) 'H is the heat of adsorption and n the order of reaction and one in case of the monomolecular cracking of alkanes. This paper describes the intrinsic reactivity of Brønsted acid sites in monomolecular reactions and provides insights into the intrinsic reaction rates of different zeolite structures and the stability of Brønsted acid sites in these structures. The stability of silica-alumina structures and the reversibility of structural collapse will be described. Monomolecular conversion of alkanes takes place at high temperatures, at low alkane pressures, and at low conversions. 1.1. Intrinsic activity of zeolitic Brønsted acid sites 1.2.1. Cracking of alkanes of different length Haag has shown that the differences in rates of cracking of alkanes of increasing length are determined by the different sorption properties of these alkanes [1]. Figure 1a shows the Ahrrenius plots of the monomolecular cracking of alkanes of increasing length. Figure 1b shows the Constable plot of these data; the observed activation energies are plotted against the pre-exponential terms. These figures have been made using the data that were measured in reference [4], which measured the monomolecular cracking of small n-alkanes over HZSM5 at temperatures between 723 K and 823 K at one bar total pressure. The partial pressures of nalkane varied from 0.1 to 10 kPa. The data in Figure 1a were extrapolated to
Fig. 1. a. Extrapolated Ahrrenius plots of the monomolecular cracking of alkanes showing an isokinetic point; b. Linear Constable plot of the data in 1 a
higher temperatures. Although the data were extrapolated over a large interval, they intersect in a single point, which is the isokinetic point. According to Bond et al. [5], Ahrrenius plots that intersect in a single point show a true compensation relation. The linear correlation between observed activation energies and pre-exponential terms is also showing this. Compensation relations have been often observed in the literature and various explanations had been given. The review article by Bond et al. [5] summarized all these explanations and provided an overall interpretation. Because the heat and entropy of adsorption show compensation relations, which means that high heats of adsorption between reactant and catalyst correlate to high losses in entropy, first-order reactions also show a linear relation between observed activation energy and pre-exponential term. In a first-order reaction, the heat of adsorption affects the observed (apparent) activation energy and the entropy the preexponential term. Equation 2 shows the correlation between the observed and true activation
1169 energies and the heats of adsorption. The compensation relation indicated that the true activation energies for the monomolecular cracking of alkanes over zeolite HZSM5 were independent of the length of the alkane. Because the stronger adsorption and consequently higher heats of adsorption, longer alkanes showed higher turnover frequencies than shorter alkanes. The longer alkanes show lower observed activation energies, but after correction of the heats of adsorption, the intrinsic activation energies were identical (vide infra). The different rates of reaction are determined by the different sorption characteristics, caused by the interaction of alkane with the zeolite pore wall. 1.2.2. Cracking of alkanes over different zeolite types A few studies have compared monomolecular cracking over different zeolites, including MFI, MOR, FAU, and BEA [6-9]. A Constable relation was observed for monomolecular cracking of n-hexane for zeolites of different structures and post-synthesis treatments [7]. The implication of this study was that the intrinsic kinetic parameters of the different zeolites are very similar, contrary to the earlier proposals for zeolites with enhanced acid strength. The linear Constable correlation also implies that there is a linear compensation relation between the entropy and enthalpy of adsorption, which was observed for the hexane adsorption in all these zeolites [10]. Table 1 Apparent and true activation energies of monomolecular cracking of alkanes over zeolites. Reactant / zeolite Propane / HZSM5a Butane / HZSM5a Pentane / HZSM5a Hexane / HZSM5a Propane / ZSM5b Propane / MORb Propane / Yb Propane / Betab a b
Apparent activation energy (kJ/mol) 155 135 120 105 147 149 165 156
Heat of adsorption Intrinsic activation (kJ/mol) energy (kJ/mol) 43 198 62 197 74 194 92 197 46 193 41 190 31 196 42 198
Data from reference Data from reference [9]
Table 1 compiles the observed activation energies, the experimental heats of adsorption, and the intrinsic activation energies, determined using equation 2, of the monomolecular cracking of alkanes of different length on HZSM5 and of propane over various zeolites [9]. Alkanes of increasing length show lower activation energies with increasing length (Figure 1) and the measured activation energies of cracking of propane over various zeolites varied with pore size [9]. After correction for the heat of adsorption, the intrinsic rates of reaction are very similar for all these systems. The linear compensation relation in Figure 2 and the very similar intrinsic activation energies show that the origin of the differences is the sorption of the alkane into the pores of the zeolite. A higher heat of adsorption results in a higher concentration of the reactant in the pores, which is responsible for higher turnover frequencies in the monomolecular cracking of alkanes. Because the rate-limiting step in this reaction is the protonation of the alkane [11], the ability of an acid site in any of the measured zeolites to donate a proton to a reactant is independent of the pore structure. When assuming that the ability to donate a proton to a weak base depends on the acid strength, the differences in acid strength between zeolites of various structures and in zeolites after post-symnthesis treatments are very small.
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Fig. 2. Compensation relation of the cracking of hexane on differently treated zeolites ZSM5 and Y
1.2.3. The dehydrogenation pathway The monomolecular conversion of alkanes has two possible pathways, the first is cracking forming an olefin and an alkane, which was discussed above; the second is dehydrogenation forming hydrogen and an olefin. The rate-limiting step in the monomolecular cracking of alkanes is the protonation of the alkane and that in the dehydrogenation is the desorption of the alkoxide that is formed in the dehydrogenation [12]. The observed activation energies of this latter reaction therefore reflect the differences in stability of the alkoxide species [9]. The stabilities of alkoxide species on different acid sites in zeolites have been shown to vary by over 100 kJ/mol [13], which correlated to experimentally observed value [9]. Unlike the cracking path, the observed activation energies of dehydrogenation varied with Si/Al ratio, which might reflect the flexibility and / or the ionicity of the framework. The rates of both reactions decreased with the aluminum content, which indicated that the Brønsted acid sites are responsible for the activity. Figure 3a shows the number of acid sites determined by the decomposition of n-propyl amine in zeolite HMOR with Si/Al ratio of 9.9 that has been progressively poisoned with sodium. The linear relation suggests a linear decrease in number of acid sites with exchanged sodium. Figure 3b shows the Ahrrenius plots of monomolecular cracking of propane over the H,NaMOR. In the cracking pathway, the lines were parallel and activation energies of about 145 kJ/mol were observed. Lower rates were observed with increased poisoning. The rates for dehydrogenation also decreased, however, the activation energies of dehydrogenation increased with sodium poisoning and varied between 134 and 155 kJ/mol (Table 2). The rates of reaction of both reaction paths decreased with poisoning, which shows that the Brønsted acid sites are the reactive centers for both reaction paths. Because the rate-limiting steps of the reaction paths are different, the observed activation energies show different trends. The cracking pathway represents the ability to donate a proton to the reactant and its intrinsic reactivity is particularly determined by the sorption of the alkane; the dehydrogenation pathway represents the stability of the alkoxide species. The former depends on the acidity of the material, the latter on the stability of the alkoxide species. 1.2.4. Stability of silica-aluminas Acid zeolites are unstable when exposed to moisture from the air [14,15]. In acidic zeolite, octahedrally coordinated aluminum is generally observed. Its amount depends on zeolite type
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Fig. 3. a. Number of acid sites in zeolite H, Na MOR of Si/Al ratio of 9.9 with various contents of sodium; b. Ahrrenius plots of the monomolecular cracking of popane over the zeolite H, Na MOR
Table 2 Kinetic parameters of the conversion of propane over zeolite H, Na MOR. Sample
9.9NH4MOR-Na5 9.9NH4MOR-Na21 9.9NH4MOR-Na34 9.9NH4MOR-Na70 a
Ratecracking *10-6 (mol/g.s.bar) 12.5 11.3 6.0 0.5
cr RateE app dehydrogenation (kJ/mol) *10-6 (mol/g.s.bar) 5.2 147 4.1 144 2.5 146 0.9 145
de
E app (kJ/mol) 134 147 150 155
100*cr /(cr+de)a
71 73 71 36
Selectivity to cracking
and Si/Al ratio [3,16]. However, when these zeolites are exposed to a strong base in the presence of moisture, there is a complete recovery of structure and activity. When heating an acidic zeolite that had been exposed to moisture, its structure will show further degradation and loss of activity [15]. To illustrate this behavior, the acidic form of mordenite had been exposed to moisture of the air. 27Al MAS NMR shows the formation of octahedrally coordinated aluminum (Figure 4). Treatment with ammonia at 150qC shows a complete
Fig. 4. 27Al MAS NMR of zeolites mordenite: NH4 MOR,HMOR and ammonia treated H MOR
recovery of the tetrahedrally coordinated aluminum; this sample is called H(NH3)MOR. The behaviors of zeolite Beta, Y, and amorphous silica-alumina are completely analogous [15-17]. The activation energies of the cracking of propane of these catalysts are compiled in Table 3. Except for zeolite Beta with Si/Al ratio of 100, higher activation energies were observed
1172 when the acidic samples had been exposed to moisture prior activation in the reactor. It has previously been shown that high silica zeolite Beta is stable in moisture and will not form octahedrally coordinated aluminum [16]. Higher activation energies are generally paralleled with a loss of activity. In all zeolitic samples, the ammonia treatment caused full recovery of the original kinetic parameters. Table 3 Apparent activation energies of monomolecular conversion of propane over zeolites and amorphous silica-alumina. Sample 9.9NH4MORa 9.9HMOR 9.9H(NH3)MOR 10.5NH4Beta 10.5Hbeta 10.5H(NH3)Beta 100NH4Beta 100HBeta NH4Y HY H(NH3)Y 15NH4ASA 15HASA 15H(NH3)ASA a
cr
de
E app
E app
(kJ/mol) 145 151 144 157 168 157 156 158 165 208 162 182 206 235
(kJ/mol) 134 148 136 161 213 169 138 143 173 212 176 172 172 235
the numbers before the sample names represent Si/Al ratios
Fig. 5. Nitrogen physisorption over amorphous silica alumina: NH4 ASA, H ASA, and H(NH3) ASA
This shows that the structures of the active sites in the ammonia treated samples are identical to those in the parent material and that structural collapses, such as visible in the 27Al MAS NMR spectra, are fully reversible. However, in case of amorphous silica-alumina, the kinetic parameters did not return to their original values, although the 27Al MAS NMR indicated that the ammonia treated sample only contained tetrahedrally coordinated aluminum. The nitrogen physisorption isotherms (Figure 5) indicate that the exposure of the H ASA to moisture changed the pore structure compared to the parent sample. The treatment with ammonia did not result in recovery of the original pore structure, but further changed the sorption isotherm and thus the pore structure. A further loss in intrinsic activity of the material in the monomolecular conversion of propane was also observed. Thus, although aluminum and silicon oxygen bonds are formed in the ammonia treatment, the original structure is not
1173 recovered. This behavior differs from zeolites, probably because of the crystalline nature of their framework. In the acidic zeolites that had been exposed to moisture, silicon and aluminum oxide bonds have been broken, however, because a crystalline network of such bonds forms the zeolite structure, the bonds that remain unbroken cause the memory effect of the original structure. The amorphous nature of the amorphous silica-alumina framework will not have such memory effect, which does not result in the recovery of the original structure. 2. CONCLUSION In first-order reactions in which the rate-limiting step is the protonation of the reactant, the sorption of the reactant dominates the rates. Identical intrinsic rates of reactions were observed for the cracking of alkanes over zeolites of different structure types and after postsynthesis treatments. Compensation relations were observed, which shows that the differences in kinetic parameters are caused by the sorption characteristics of the reactants. A better fit between reactant and pore wall increases the heat of adsorption, and decreases the apparent reaction barrier. Reactions that depend on the stability of the adsorbed reactive intermediates will have a different dependence on zeolite structure and Si/Al ratio, which is shown by the kinetic parameters of the dehydrogenation of alkanes. There is no simple dependence of zeolitic acid strength and the rate of reaction in either of these types of reactions. The stability of zeolites depends on their structure and Si/Al ratio, and crystalline structures (zeolites) have a memory effect, which is absent in amorphous silica-aluminas. The already low activity of these amorphous structures is decreased after exposure of these materials to moisture and, unlike zeolites, the structural changes are irreversible. REFERENCES [1]
[2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
W.O. Haag, in: J. Weitkamp, H.G. Karge and W. Hölderich (Eds.), Zeolites and Related Materials: State of the Art 1994, in: Stud. Surf. Sci. Catal., vol. 84, Elsevier Science B.V., Amsterdam, 1994, p. 1375. van Bokhoven, J. A., van der Eerden, A. M. J., Koningsberger and D. C., Stud. Surf. Sci. Catal., 142 (2002) 1885. E. Bourgeat-Lami, P. Massiani, F. Di Renzp, P. Espiau and F. Fajula, Appl. Catal., 72 (1991) 139. T.F. Narbeshuber, H. Vinek and J.A. Lercher, J. Catal., 157 (1995) 388. G.C. Bond, M.A. Keane and J.A. Lercher, Catal. Rev.Sci. Eng., 42 (2000) 323. S.M. Babitz, B.A. Williams, J.T. Miller, R.Q. Snurr, W.O. Haag and H.H. Kung, Appl. Catal. A,179 (1999) 71. J.A. van Bokhoven, B.A. Williams, W. Ji, D.C. Koningsberger, H.H. Kung and J.T. Miller, J. Catal. 224 (2004) 50. S. Kotrel, M.P. Rosynek and J.H. Lunsford, J. Phys. Chem. B, 103 (1999) 818. B. Xu, C. Sievers, S.B. Hong, R. Prins, and J.A. van Bokhoven, J. Catal., 244 (2006) 163. C.E. Ramachandran, , B.A.Williams, J.A. van Bokhoven and J.T. Miller, J. Catal., 233 (2005) 100. T.F. Narbeshuber, H. Vinek and J.A. Lercher, J. Catal., 157 (1995) 388. T.F. Narbeshuber, A. Brait, K. Seshan and J.A. Lercher, J. Catal., 172 (1997) 127. V. Nieminen, M. Sierka, D.Y. Murzin and J. Sauer, J. Catal., 231 (2005) 393. G.T. Kerr, J. Catal., 15 (1969) 200. B. Xu, F. Torunno, S. Bordiga, R. Prins and J.A. van Bokhoven J. Catal., 241 (2006) 66. A. Abraham, S.-H. Lee, C.-H. Shin, S.B. Hong, R. Prins and J.A. van Bokhoven, Phys. Chem. Chem. Phys., 6 (2004) 3031. B. Xu Ph.D. thesis, ETH Zurich, Nov 8 2006.
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Towards predicting catalytic performances of zeolites J. A. van Bokhoven* and B. Xu Department of Chemical and Bioengineering, ETH Zurich, HCI E115, Wolfgang-Pauli-Str. 10, 8093 Zurich, Switzerland. Tel: +41-44-632 55 42; Fax: +41-44-632 11 62; Email: [email protected] ABSTRACT The intrinsic activation energies of alkane activation reactions that involve the protonation of the reactant in the rate-limiting step are independent of zeolite structure and of Si/Al ratio. Their observed rates, however, increase with decreasing pore-sizes. Reactions that involve the formation and desorption of alkoxide species in their rate-limiting step strongly depend on the zeolite structure and on the interaction of the adsorbed molecule with the local environment of the acid site. There is a strong dependence on the Si/Al ratio. 1. INTRODUCTION The activity and selectivity of chemical reactions in zeolites vary with pore size and connectivity and with Si / Al ratio. Zeolite treatments, such as heating in their own moisture or steam, stabilize the structures and activate them. Many explanations have been put forward to explain enhanced zeolite activity after activation in moisture. Quite often, differences in activity are directly related to variations in acid strength. However, the various techniques that determine acid strength have not provided a single scale to which zeolites can be held to predict activity. Based on the early works of Haag [1], we have systematically determined the intrinsic activity of Brønsted acid sites in zeolites using a simple reaction, which is the monomolecular cracking of small alkanes. This reaction is first order in alkane, produces simple products, is not diffusion limited, and shows no deactivation. Combining zeolite reactivity and structural determination provides structure / performance relations. In recent years, it has become much clear how the zeolite structures change under different conditions. The aluminum coordination changes as function of temperature [2]. Exposure of acidic zeolites to moisture at room temperature leads to structural collapse, which can be completely recovered [3]. The activity of Brønsted acid sites is determined by a combination of the number of sites and the reactivity of each of them. The work of Haag showed that at low coverage of reactant, the rate expression monomolecular conversion of alkanes is: r = k´[A] = kK[A] (1) in which kc is the observed rate constant; k the intrinsic rate constant, and K the adsorption constant. This latter term describes the enrichment of reactant on the surface of the catalyst compared to the gas phase; k represents the intrinsic reaction parameters, which depend on the intrinsic properties of the active site. In case of zeolites, this is generally assumed to be
1168 related to acid strength. Because, k and K have similar dependencies on temperature, the intrinsic activation energy can be determined using the following equation: act Eapp
act Etrue ¦ ni 'H i
(2) 'H is the heat of adsorption and n the order of reaction and one in case of the monomolecular cracking of alkanes. This paper describes the intrinsic reactivity of Brønsted acid sites in monomolecular reactions and provides insights into the intrinsic reaction rates of different zeolite structures and the stability of Brønsted acid sites in these structures. The stability of silica-alumina structures and the reversibility of structural collapse will be described. Monomolecular conversion of alkanes takes place at high temperatures, at low alkane pressures, and at low conversions. 1.1. Intrinsic activity of zeolitic Brønsted acid sites 1.2.1. Cracking of alkanes of different length Haag has shown that the differences in rates of cracking of alkanes of increasing length are determined by the different sorption properties of these alkanes [1]. Figure 1a shows the Ahrrenius plots of the monomolecular cracking of alkanes of increasing length. Figure 1b shows the Constable plot of these data; the observed activation energies are plotted against the pre-exponential terms. These figures have been made using the data that were measured in reference [4], which measured the monomolecular cracking of small n-alkanes over HZSM5 at temperatures between 723 K and 823 K at one bar total pressure. The partial pressures of nalkane varied from 0.1 to 10 kPa. The data in Figure 1a were extrapolated to
Fig. 1. a. Extrapolated Ahrrenius plots of the monomolecular cracking of alkanes showing an isokinetic point; b. Linear Constable plot of the data in 1 a
higher temperatures. Although the data were extrapolated over a large interval, they intersect in a single point, which is the isokinetic point. According to Bond et al. [5], Ahrrenius plots that intersect in a single point show a true compensation relation. The linear correlation between observed activation energies and pre-exponential terms is also showing this. Compensation relations have been often observed in the literature and various explanations had been given. The review article by Bond et al. [5] summarized all these explanations and provided an overall interpretation. Because the heat and entropy of adsorption show compensation relations, which means that high heats of adsorption between reactant and catalyst correlate to high losses in entropy, first-order reactions also show a linear relation between observed activation energy and pre-exponential term. In a first-order reaction, the heat of adsorption affects the observed (apparent) activation energy and the entropy the preexponential term. Equation 2 shows the correlation between the observed and true activation
1169 energies and the heats of adsorption. The compensation relation indicated that the true activation energies for the monomolecular cracking of alkanes over zeolite HZSM5 were independent of the length of the alkane. Because the stronger adsorption and consequently higher heats of adsorption, longer alkanes showed higher turnover frequencies than shorter alkanes. The longer alkanes show lower observed activation energies, but after correction of the heats of adsorption, the intrinsic activation energies were identical (vide infra). The different rates of reaction are determined by the different sorption characteristics, caused by the interaction of alkane with the zeolite pore wall. 1.2.2. Cracking of alkanes over different zeolite types A few studies have compared monomolecular cracking over different zeolites, including MFI, MOR, FAU, and BEA [6-9]. A Constable relation was observed for monomolecular cracking of n-hexane for zeolites of different structures and post-synthesis treatments [7]. The implication of this study was that the intrinsic kinetic parameters of the different zeolites are very similar, contrary to the earlier proposals for zeolites with enhanced acid strength. The linear Constable correlation also implies that there is a linear compensation relation between the entropy and enthalpy of adsorption, which was observed for the hexane adsorption in all these zeolites [10]. Table 1 Apparent and true activation energies of monomolecular cracking of alkanes over zeolites. Reactant / zeolite Propane / HZSM5a Butane / HZSM5a Pentane / HZSM5a Hexane / HZSM5a Propane / ZSM5b Propane / MORb Propane / Yb Propane / Betab a b
Apparent activation energy (kJ/mol) 155 135 120 105 147 149 165 156
Heat of adsorption Intrinsic activation (kJ/mol) energy (kJ/mol) 43 198 62 197 74 194 92 197 46 193 41 190 31 196 42 198
Data from reference Data from reference [9]
Table 1 compiles the observed activation energies, the experimental heats of adsorption, and the intrinsic activation energies, determined using equation 2, of the monomolecular cracking of alkanes of different length on HZSM5 and of propane over various zeolites [9]. Alkanes of increasing length show lower activation energies with increasing length (Figure 1) and the measured activation energies of cracking of propane over various zeolites varied with pore size [9]. After correction for the heat of adsorption, the intrinsic rates of reaction are very similar for all these systems. The linear compensation relation in Figure 2 and the very similar intrinsic activation energies show that the origin of the differences is the sorption of the alkane into the pores of the zeolite. A higher heat of adsorption results in a higher concentration of the reactant in the pores, which is responsible for higher turnover frequencies in the monomolecular cracking of alkanes. Because the rate-limiting step in this reaction is the protonation of the alkane [11], the ability of an acid site in any of the measured zeolites to donate a proton to a reactant is independent of the pore structure. When assuming that the ability to donate a proton to a weak base depends on the acid strength, the differences in acid strength between zeolites of various structures and in zeolites after post-symnthesis treatments are very small.
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Fig. 2. Compensation relation of the cracking of hexane on differently treated zeolites ZSM5 and Y
1.2.3. The dehydrogenation pathway The monomolecular conversion of alkanes has two possible pathways, the first is cracking forming an olefin and an alkane, which was discussed above; the second is dehydrogenation forming hydrogen and an olefin. The rate-limiting step in the monomolecular cracking of alkanes is the protonation of the alkane and that in the dehydrogenation is the desorption of the alkoxide that is formed in the dehydrogenation [12]. The observed activation energies of this latter reaction therefore reflect the differences in stability of the alkoxide species [9]. The stabilities of alkoxide species on different acid sites in zeolites have been shown to vary by over 100 kJ/mol [13], which correlated to experimentally observed value [9]. Unlike the cracking path, the observed activation energies of dehydrogenation varied with Si/Al ratio, which might reflect the flexibility and / or the ionicity of the framework. The rates of both reactions decreased with the aluminum content, which indicated that the Brønsted acid sites are responsible for the activity. Figure 3a shows the number of acid sites determined by the decomposition of n-propyl amine in zeolite HMOR with Si/Al ratio of 9.9 that has been progressively poisoned with sodium. The linear relation suggests a linear decrease in number of acid sites with exchanged sodium. Figure 3b shows the Ahrrenius plots of monomolecular cracking of propane over the H,NaMOR. In the cracking pathway, the lines were parallel and activation energies of about 145 kJ/mol were observed. Lower rates were observed with increased poisoning. The rates for dehydrogenation also decreased, however, the activation energies of dehydrogenation increased with sodium poisoning and varied between 134 and 155 kJ/mol (Table 2). The rates of reaction of both reaction paths decreased with poisoning, which shows that the Brønsted acid sites are the reactive centers for both reaction paths. Because the rate-limiting steps of the reaction paths are different, the observed activation energies show different trends. The cracking pathway represents the ability to donate a proton to the reactant and its intrinsic reactivity is particularly determined by the sorption of the alkane; the dehydrogenation pathway represents the stability of the alkoxide species. The former depends on the acidity of the material, the latter on the stability of the alkoxide species. 1.2.4. Stability of silica-aluminas Acid zeolites are unstable when exposed to moisture from the air [14,15]. In acidic zeolite, octahedrally coordinated aluminum is generally observed. Its amount depends on zeolite type
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Fig. 3. a. Number of acid sites in zeolite H, Na MOR of Si/Al ratio of 9.9 with various contents of sodium; b. Ahrrenius plots of the monomolecular cracking of popane over the zeolite H, Na MOR
Table 2 Kinetic parameters of the conversion of propane over zeolite H, Na MOR. Sample
9.9NH4MOR-Na5 9.9NH4MOR-Na21 9.9NH4MOR-Na34 9.9NH4MOR-Na70 a
Ratecracking *10-6 (mol/g.s.bar) 12.5 11.3 6.0 0.5
cr RateE app dehydrogenation (kJ/mol) *10-6 (mol/g.s.bar) 5.2 147 4.1 144 2.5 146 0.9 145
de
E app (kJ/mol) 134 147 150 155
100*cr /(cr+de)a
71 73 71 36
Selectivity to cracking
and Si/Al ratio [3,16]. However, when these zeolites are exposed to a strong base in the presence of moisture, there is a complete recovery of structure and activity. When heating an acidic zeolite that had been exposed to moisture, its structure will show further degradation and loss of activity [15]. To illustrate this behavior, the acidic form of mordenite had been exposed to moisture of the air. 27Al MAS NMR shows the formation of octahedrally coordinated aluminum (Figure 4). Treatment with ammonia at 150qC shows a complete
Fig. 4. 27Al MAS NMR of zeolites mordenite: NH4 MOR,HMOR and ammonia treated H MOR
recovery of the tetrahedrally coordinated aluminum; this sample is called H(NH3)MOR. The behaviors of zeolite Beta, Y, and amorphous silica-alumina are completely analogous [15-17]. The activation energies of the cracking of propane of these catalysts are compiled in Table 3. Except for zeolite Beta with Si/Al ratio of 100, higher activation energies were observed
1172 when the acidic samples had been exposed to moisture prior activation in the reactor. It has previously been shown that high silica zeolite Beta is stable in moisture and will not form octahedrally coordinated aluminum [16]. Higher activation energies are generally paralleled with a loss of activity. In all zeolitic samples, the ammonia treatment caused full recovery of the original kinetic parameters. Table 3 Apparent activation energies of monomolecular conversion of propane over zeolites and amorphous silica-alumina. Sample 9.9NH4MORa 9.9HMOR 9.9H(NH3)MOR 10.5NH4Beta 10.5Hbeta 10.5H(NH3)Beta 100NH4Beta 100HBeta NH4Y HY H(NH3)Y 15NH4ASA 15HASA 15H(NH3)ASA a
cr
de
E app
E app
(kJ/mol) 145 151 144 157 168 157 156 158 165 208 162 182 206 235
(kJ/mol) 134 148 136 161 213 169 138 143 173 212 176 172 172 235
the numbers before the sample names represent Si/Al ratios
Fig. 5. Nitrogen physisorption over amorphous silica alumina: NH4 ASA, H ASA, and H(NH3) ASA
This shows that the structures of the active sites in the ammonia treated samples are identical to those in the parent material and that structural collapses, such as visible in the 27Al MAS NMR spectra, are fully reversible. However, in case of amorphous silica-alumina, the kinetic parameters did not return to their original values, although the 27Al MAS NMR indicated that the ammonia treated sample only contained tetrahedrally coordinated aluminum. The nitrogen physisorption isotherms (Figure 5) indicate that the exposure of the H ASA to moisture changed the pore structure compared to the parent sample. The treatment with ammonia did not result in recovery of the original pore structure, but further changed the sorption isotherm and thus the pore structure. A further loss in intrinsic activity of the material in the monomolecular conversion of propane was also observed. Thus, although aluminum and silicon oxygen bonds are formed in the ammonia treatment, the original structure is not
1173 recovered. This behavior differs from zeolites, probably because of the crystalline nature of their framework. In the acidic zeolites that had been exposed to moisture, silicon and aluminum oxide bonds have been broken, however, because a crystalline network of such bonds forms the zeolite structure, the bonds that remain unbroken cause the memory effect of the original structure. The amorphous nature of the amorphous silica-alumina framework will not have such memory effect, which does not result in the recovery of the original structure. 2. CONCLUSION In first-order reactions in which the rate-limiting step is the protonation of the reactant, the sorption of the reactant dominates the rates. Identical intrinsic rates of reactions were observed for the cracking of alkanes over zeolites of different structure types and after postsynthesis treatments. Compensation relations were observed, which shows that the differences in kinetic parameters are caused by the sorption characteristics of the reactants. A better fit between reactant and pore wall increases the heat of adsorption, and decreases the apparent reaction barrier. Reactions that depend on the stability of the adsorbed reactive intermediates will have a different dependence on zeolite structure and Si/Al ratio, which is shown by the kinetic parameters of the dehydrogenation of alkanes. There is no simple dependence of zeolitic acid strength and the rate of reaction in either of these types of reactions. The stability of zeolites depends on their structure and Si/Al ratio, and crystalline structures (zeolites) have a memory effect, which is absent in amorphous silica-aluminas. The already low activity of these amorphous structures is decreased after exposure of these materials to moisture and, unlike zeolites, the structural changes are irreversible. REFERENCES [1]
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