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Zeolite catalysis studies by radiation chemical methods
C.D. Jonah and B.S.M. Rao (Editors) Radiation Chemistry: Present Status and Future Trends 2001 Elsevier Science B.V.
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Zeolite catalysis studies by radiation chemical methods D. W. Werst and A. D. Trifunac
Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439
1 INTRODUCTION Ionizing radiation has found applications in the study of heterogeneous catalysis primarily as a means of producing unstable species and reactive intermediates, many of them paramagnefic, whose study can elucidate fundamental issues of structure, mechanisms, transport and reactivity. Fast detection techniques are needed to observe short-lived species and measure their kinetics. However, solid catalysts and microporous solids, by virtue of specific surface-adsorbate interactions, geometric constraints and strong ionic character, possess the ability to stabilize highly reactive species. Thus radiolysis has been used to produce a variety of paramagnefic species that can be characterized by non-time-resolved methods, and in particular, electron paramagnetic resonance (EPR) spectroscopy, at ambient and sub-ambient temperatures. Intermediates in high-temperature processes have been stabilized at low temperature after ~/irradiation of metal oxides and zeolites. Important early examples were oxygen anions, O-, 0 2- and O3-.~-3 Some of their reactions with small molecules were also elucidated by EPR. Metal cluster ions have also been produced by radiolysis and stabilized in zeolites. Examples include alkali metal cation clusters 3'4 in faujasites and silver cation clusters 5-8 in zeolite A and in silicoaluminophosphate molecular sieves. Detailed information was obtained from EPR studies about their structure, thermal stability and formation of adducts. Radical cations can be stabilized inside zeolites and on some amorphous metal oxide surfaces. This was recognized early by the EPR observation of radical cations generated spontaneously upon exposure of certain solid catalysts to easily oxidized species such as aromatic hydrocarbons and certain olefms. 9-~3 These observations reveal the presence of electron acceptor sites and, whether or not "]'Work at Argonne performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Science, US-DOE under contract number W-31-109-ENG-38.
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the radical cations are actual intermediates in catalysis, their detection can shed light on reaction mechanisms. Similarly, radiolytically produced radical cations can be stabilized in zeolites and related materials. This possibility was exploited by spectroscopists to study the EPR of radical cations and some neutral radicals even before the development of inert matrices such as rare gases and freons for radical cation stabilization. ~4'15 Recently, work in our laboratory has developed the use of inert zeolites as microreactors to control radical cation reactions and to study radiation chemistry in heterogeneous s y s t e m s . 16-26 In the case of active catalysts, radiolysis can potentially produce radical cations of products as well as starting material. 23'27'28 Thus, like the spontaneous oxidation process described above, radiolysis combined with EPR permits a method of post-reaction analysis of products by in situ spectroscopy. It is not practical to cover every topic in this review in which radiation chemical techniques have contributed to the understanding of catalyst function or catalytic reactions. With this introduction as a cursory guide to relevant topics, we move on to a discussion of the radiolyfic spin labeling technique for analyzing products of catalytic reactions in zeolites, which has been the main thrust of experiments directed at fundamental aspects of catalysis in our laboratory. 2 SPIN L A B E L I N G BY IONIZATION Radiolytic spin labeling of molecules adsorbed in zeolites occurs by ionization to form radical cations and by formation of H-adduct radicals by H atom addition. 2628 Ionization of adsorbed molecules is a two-step process, equations (1) and (2). Because the adsorbate loading used in experiments is low (typically one percent or less by weight), energy is absorbed by the matrix and not directly by the adsorbate. Holes (Z "§ created in the zeolite lattice migrate to adsorbate (A) by charge transfer. Stabilization of radical cations is made possible at low temperature by sequestration in the zeolite pores and by trapping of electrons by the matrix. Y Z
~
Z "§ +
Z "+
A
~
+
e-trappe d
Z
+
(1)
A "§
(2)
397
HZSM5
NaZSM5
a)
~
1 -i ~1 i I-I
I '! I
Figure I. Radiolysis/EPR results for a) 1,3,5-cycloheptatriene and b) 2,3-dimethyl2-pentene on HZSM5 and NaZSM5. The radical cations of catalytic products, toluene and tetramethylethylene, were observed only on the acidic zeolite and not on NaZSM5.
It sounds simple, but as mass spectroscopists know, ionization as an analysis technique creates very reactive species. It is not enough to be able to detect ions; one must also have knowledge of how the ions react. Fortunately, many radical cations are quite stable in selected zeolites at low temperature. For example, the excellent stability of radical cations of simple aromatic hydrocarbons and olefins in MFI zeolites has contributed to the successful application of the radiolytic spin labeling technique to catalytic processes of considerable importance to the fuels and petrochemical industries hydrocarbon transformations in zeolite Z S M 5 . 23'27'28 How is it possible to distinguish radical cation reactions from genuine transformations catalyzed by the zeolite, and what types of processes tend to intrude on the identification of products of catalysis? To answer the first question, we rely on the .ion-exchangeability of zeolites. Zeolites are crystalline aluminosilicates, natural or man-made, that can adopt a remarkable range of channel-type and cage-type lattice architectures, depending on the
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connectivity of ring structures built from (SiO4) 4" and (A104) s tetrahedra. 29 Extraframework cations are present in stoichiometric amounts to charge balance the negative lattice. Ion exchange is an important method of varying the acid/base character of zeolites and of incorporating other metal centers (for example, transition metal ions) with catalytic function. In this chapter we limit our discussion to the proton- and Na§ zeolites. The former derive catalytic activity from bridging hydroxyl groups ( --- A10(H)Si =--), that are strong Bronsted acid sites and the latter are inert adsorbents. The availability of both active and inactive, isomorphous zeolites provides a direct control experiment for the radiolytic spin labeling technique because the radical cation reactions can be elucidated on the inactive zeolite. This is best explained by example and two cases are illustrated in Figure 1. In the first example, radiolysis of an HZSM5 sample exposed at room temperature to 1,3,5-cycloheptatriene gave rise to the EPR spectrum of the toluene radical cation, and in the second, the tetramethylethylene radical cation was observed on HZSM5 loaded with 2,3-dimethyl-2-pentene. Alongside each result on the active catalyst the result for the same hydrocarbon loaded on the inactive NaZSM5 is shown. In each case, the EPR spectrum on the inactive zeolite shows only the radical cation of the starting material. Clearly, the transformations observed on HZSM5, ring contraction and cracking, are caused by acid catalysis since the radical cations do not undergo these reactions. When the parent radical cation of a catalysis product is not stabilized, more knowledge is needed to interpret the results of catalysis. For example, adsorption of 1,3- or 1,4-cyclohexadiene on H-Beta gave, upon radiolysis, the EPR spectrum of benzene radical cation (Figure 2a). Control experiments for both olefins on Na-Beta also gave benzene radical cation and no cyclohexadiene radical cation. Proof of catalytic dehydrogenation of cyclohexadiene to give benzene, involving intermolecular hydrogen transfer, an important step in aromatization on zeolites, was revealed by changes in the EPR spectrum upon annealing the samples. In the H-Beta experiment, the benzene radical cation decayed and was replaced by benzene dimer radical cation upon raising the temperature above 120 K (Figure 2b). The exact same behavior was observed on Na-Beta loaded with benzene. In Na-Beta loaded with cyclohexadiene, the benzene dimer radical cation did not appear upon sample annealing - there is no neutral benzene to react with benzene radical cations. From this behavior it can be deduced that extensive conversion of olefin to benzene occurs on H-Beta (catalytically), whereas on Na-Beta only those molecules that are ionized undergo H2 elimination. When the primary species (products of catalysis) must be deduced from knowledge of the radical cation chemistry, interpretation becomes more
399
a)
b)
Figure 2. Radiolysis/EPR result after reaction of 1,3-cyclohexadiene on H-Beta. a) Radiolysis at 77 K gave the benzene radical cation, a = 4.5 G. b) Annealing the sample above approximately 120 K caused the transformation of benzene radical cations to benzene dimer radical cations, a = 2.2 G.
challenging. Not in every case, however, will the radical cation reaction mimic the catalytic reaction as in the preceding example. A last caveat regards quantitative estimates of relative product yields. Since ionization is used as the detection method, then charge migration can skew the representation of relative product yields in the EPR spectrum. That is, hole transfer among different products favors stabilization of radical cations of those species with the lowest ionization potential. We have investigated hole transfer in some detail in mixtures, both in zeolites and in frozen solutions. 24 The degree of bias can be estimated in this way, but the occurrence of hole transfer ultimately makes radiolysis a somewhat selective labeling technique. 3
RADIOLYSIS/EPR METHOD
The advantages of the radiolysis/EPR method for studying mechanisms of zeolite catalysis are due to the sensitivity and structural specificity of EPR, surpassing that of other in situ spectroscopies, such as FTIR and NMR, and the ability to identify products at low temperature. It is often the case that at high temperatures needed to evolve products from the zeolite for ex situ analysis, a
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complex sequence of reactions has already occurred. Therefore, elucidation of the elementary reactions of the sequence necessitates in situ analysis at low temperatures. In addition, product selectivity can be more pronounced for reactions carded out under mild conditions. Product studies under these conditions allow deeper insights into the subtle details of zeolite-reactant interactions. The radiolysis/EPR experiment can be succinctly described as follows. The zeolite powder is first activated to approximately 450~ under vacuum for several hours in a 4 mm o.d. suprasil tube. Under such activation conditions, no Lewis acid sites are created to give rise to spontaneous oxidation of adsorbed species. No EPR signals are observed in loaded zeolites prior to radiolysis. The adsorbate is quantitatively transferred via a glass vacuum manifold to the tube containing zeolite and the tube is sealed. The sealed tube is equilibrated at some temperature Teq for a time t,q. At the end of the reaction period, transfer of the sample tube to a liquid nitrogen storage dewar quenches any further chemistry. Radiolysis is carded out at 77 K and EPR spectra are collected between 4 K and room temperature. Catalytic transformations can be arrested after formation of a single p~oduct or the evolution to many products can be dissected into a sequential development of different intermediates and products by systematically varying Teq and teq. Taking snapshots of the material composition on the zeolite as it incrementally changes greatly aids the interpretation of multic0mponent EPR spectra. Increasing multiplicity of products will eventually defeat any chance of deconvolving the EPR spectrum and one must resort to alternative (ex situ) techniques. However, in the study of isobutene reactions on HZSM5 (vide infra), five different species, including starting material, were identified. 27 4
R E A C T I O N S OF A C Y C L I C OLEFINS ON ZSM5
The investigation of reactions of isobutene and related acyclic olefins on HZSM5 provides a good basis for comparison of the radiolysis/EPR technique and other in situ spectoscopies, such as FTIR, 3~ NMR 32'33 and EPR without radiolysis. 344~ Results obtained by the different techniques are in general agreement, but advantages of the radiolysis/EPR method can be noted. A chief advantage was the ability to distinguish and identify structurally similar products. The loss of isobutene starting material and the build-up of Ca and C6 products, reflecting dimerization, isomerization and cracking processes, were all evident in radiolysis/EPR experiments on HZSM5 at room temperature and
401
I
c)
f-Figure 3. Radiolysis/EPR results after reaction of isobutene on HZSM5 a) for 16 h at 295 K and b) for 16 h at 338 K. c) Radiolysis/EPR result after reaction of propene on HZSM5 for 16 h at 338 K.
below. 27 Above r o o m temperature, products larger than Cs began to form. By appropiate variation of T,q and t,q, the sequence of appearance of products could be partially deduced, adding important chemical insight into the identity of the products.
1 "§
2 .+
Figure 3 shows just two "EPR snapshots" of the evolving material composition on the catalyst loaded with isobutene. The EPR spectrum observed after 16 h equilibration at 295 K consisted of signals from two radical cations with structures 1.+ (broad lines, a = 17 (3) and 2"* (sharp lines,
402 Table 1 Acyclic olefins included in radiolysis/EPRstudies of reactions on HZSM5
\
a = 17 G). There was no evidence of any remaining monomeric starting material; by this stage, it has been consumed in dimerization reactions followed by isomerization and cracking. The radiolysis/EPR study of isobutene was supported by experiments on C3C s olefins on both HZSM5 and NaZSM5 (Table 1). This allowed screening for possible radical cation reactions (minimal for these compounds), and the survey of related compounds aided spectroscopic assignment and tested the catalytic reaction steps from different starting points. For example, 2 was also formed from C7 and Ca feed molecules, corroborating the conclusion that 2 can be formed from isobutene by cracking the dimer as opposed to addition of C4 and C2 units. After reaction at higher temperatures (approximately 338 K), the resulting EPR spectrum evolved frofn that in Figure 3a into that in Figure 3b. The hyperfine structure indicates a radical cation with structure analogous to 1.§ and the sequence of formation of this product suggests that it is a higher molecular weight species. It is clearly a polymeric radical cation, or combination of radical cations, with structure 3"*. Discrimination of different length polymers, n >_ 4, from the EPR data was not possible.
~(CH2)nCH3 3"+
n>4
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A general observation and one that clearly points out the advantage of following catalytic reactions under very mild conditions, was that the radiolysis/EPR result for samples equilibrated for an hour or more at 338 K began to be indistinguishable regardless of feed molecule (for all cases studied, C3-C8). For comparison, the result for propene on HZSM5, equilibrated under the same conditions as the sample in Figure 3b, is shown in Figure 3c. After extensive reaction, the hydrocarbon composition on the catalyst essentially loses memory of the starting material. Experiments under such reaction conditions do not lead to an understanding of the elementary reactions steps, reaction sequence or differences that can account for reaction selectivity. Our conclusions are in general agreement with previous N I ~ R 32'33 and FTIR 3~ studies of olefin reactions on HZSM5. In particular, isobutene isomerization was observed as low as 143 K by NMR. The average size range of Cg-CI2 was reported for reaction up to 300 K, but no evidence of cracking was cited under these conditions. Discrimination of C6, C 8 and C12 species by NMR or FTIR is considerably less certain than by EPR. While chemical shift and vibrational spectra allow the estimation of relative amounts of aliphatic and olefinic protons, vinyl vs. methylene C-H bonds, etc., assignments of unique chemical structures, especially in mixtures, from such data is usually not possible. Finally, radiolysis/EPR results forced the reinterpretation of literature spanning more than 25 years on EPR studies of olefin reactions on zeolites in the presence of Lewis acid s i t e s . 3441 Spontaneous oxidation of olefins and products of their Bronsted acid-catalyzed reactions on dehydroxylated (to form Lewis acid sites) ZSM5 and Mordenite (vide infra) provides an alternative method of spin labeling, and the technique gives results mostly consistent with those from radiolysis/EPR. However, in the spontaneous oxidation method, the limited degree of control over the ionization process and progress of the catalysis has led to erroneous conclusions (mainly in the earlier work). Interpretation of the radiolysis/EPR experiments is greatly aided by the ability to methodically vary the relative contribution of different products to the EPR spectrum. 5
SHAPE SELECTIVITY-
ZSM5 AND MORDENITE
One of the paramount advantages of microporous reactors is the influence of geometry/spatial constraints on the reactions of guest species. This gives rise to shape selectivity which is vividly displayed in the products of cyclic olefin reactions on zeolites ZSM5 and Mordenite. 28 The largest contrast is that Mordenite, with its system of 12-ring channels (Table 2), is capable of
404 Table 2 Diameters of elliptical channels in ZSM5 and Mordenite [ref. 42]
Diameter (A) ZSM5
10-ring, straight channels 10-ring, sinusoidal channels
5.3 x 5.6 5.1 x 5.5
Mordenite
12-ring 8-ring
6.5 x 7.0 2.6 x 5.7
Table 3 Major products formed from cyclic precursors on ZSM5 and Mordenite
Precursor HZSM5
5-, 6-, 7-member ring
Major Product "+ll )
C7H12 H-Mordenite
R 5-, 6-, 7-member ring C6HIo, C7H12 R = H or CH3
accommodating products that are too large to fit in the 10-ring channels of ZSM5. Reaction of cyclic olefins on HZSM5 below 340 K is limited to unimolecular rearrangment. For example, a family of seven different compounds with 5-, 6- and 7-member rings and the empirical formula C7H12 all reacted to give 1,2dimethylcyclopentene (Figure 4a). 28 Such transformations - hydrogen and alkyl shifts, ring contraction, ring expansion - are well-known processes involving Bronsted acid catalysis. 43'~ Using the carbenium ion formalism, one such transformation is shown in Scheme 1, which depicts the intermediacy of a protonated cyclopropane ring. A question currently debated is whether free carbenium ions exist on zeolite catalysts. The question really is how acidic are zeolite catalysts, are they super acids? Evidence suggests that most reactive
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Figure 4. a) Radiolysis/EPR result after reaction of 1,5-dimethylcyclopentene on HZSM5 for 16 h at 318 K. b) Radiolysis/EPR result after reaction of cyclohexene on H-Mordenite for 16 h at 295 K. The spectrum is assigned to the bicyclo[5.5.0] dodecene radical cation. intermediates are framework-bonded species, 45"48 but zeolite catalysis can nevertheless involve transition states with enough ionic character such that rules governing carbenium ion reactions are obeyed to some degree within the geometry contraints imposed by the zeolite. Scheme 1" Carbenium ion mechanism of ring contraction and methyl side-chain generation
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Bimolecular reaction products were more prevalent when cyclic olefins were reacted on H-Mordenite. 2s These products (Table 3, Figure 4) reflected dimerization and ring expansion processes. The exception, 1,2dimethylcyclopentene, suggests that factors other than size exclusion are also important. This molecule was stable, even up to 318 K (16 h), whereas another C7 compound (1-methylcyclohexene) was converted to the more typical bicyclic dimer product. Another exception, cyclohexene, showed greater resistance to ring contraction on HZSM5 than methylcyclohexenes or cycloheptene. The carbenium ion-type mechanism does not disfavor the ring contraction in the case of cyclohexene, and the thermodynamic benefit of going from a secondary carbenium ion to a tertiary carbenium ion (methylcyclopentyl) should be a strong driving force for ring contraction. The greater .acidity of secondary carbenium ions, such as cyclohexyl, compared to tertiary ions, such as methylcyclohexyl, could partly underlie the stability of cyclohexene on HZSM5. However, that suggests that cycloheptene should also be slow to undergo ring contraction, which was not the case. Furthermore, H-mordenite, a comparable acid to HZSM5, is able to activate the conversion of cyclohexene to the dimer product at equally low temperatures. 6 H/D E X C H A N G E Certain observations of reaction selectivity, such as those described in the preceding paragraphs, suggest that the carbenium ion paradigm - that is, the direct analogy to reactions of olefins in superacid solutions - is a simplistic model for Bronsted acid catalysis in zeolites. The carbenium ion formalism can account for most of the products, but glosses over the specific characteristics of zeolite catalysis, such as volume constraints, relative acidity and possible bifunctional nature of zeolite catalysts owing to the proximity of basic oxygen atoms adjacent to the acid site. An experimental test for the intermediacy of carbenium ions in zeolite catalysis is isotope exchange. One such experiment from our laboratory involved the oligomerization of acetylene-d 2 on HZSM5 at room temperature. 23 The benzene product of the B ronsted acid-catalyzed trimerization of acetylene was detected as benzene radical cation. The EPR spectrum showed that the oligomerization of acetylene proceeds without exchange with zeolitic protons. While it is conceivable for H/D exchange to occur without the formation of a free ion, the lack of H/D exchange seems strong negative evidence for the intermediacy of free carbenium ions. This example fits with the growing experimental and theoretical evidence that zeolites are not superacids and that
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intermediates involved in many catalytic reactions retain some degree of covalent bonding to the lattice. Scrambling of the isotope label on benzene did not occur under the conditions used to study acetylene trimerization, but we have shown by radiolysis/EPR that benzene does undergo H/D exchange on HZSM5 at higher temperatures. 25 Beck et al. determined the activation energy (14.4 kcal/mol) for this reaction by N1VIR. 49 This fundamental reaction is itself an important test of theoretical calculations of zeolite acidity and reaction mechanisms. An interesting and remaining puzzle regarding H/D exchange in benzene on HZSM5 is the observation by radiolysis/EPR that the exchange is very nonstatistical; some of the benzene undergoes extensive exchange and some of the benzene exchanges very little. 25 This observation could not be attributed to a fraction of the benzene residing on the external surface of the zeolite. 7
CONCLUSION
Radiation chemical methods, when coupled with appropriate detection techniques, can be used to study many different aspects of solid heterogeneous catalysis. Ionizing radiation can be used to generate reactive intermediates, to change the oxidation state of metal ions or clusters, to create reactive defects in the solid lattice and to spin label reaction products. When radiolysis and EPR are used together, products of catalysis can be identified in situ and at low temperatures with the excellent structural specificity and sensitivity inherent in the EPR method. As always, exceptions to the rule can offer greater insights into chemical mechanisms. Subtle structure/reactivity dependences revealed by radiolysis/EPR experiments carded out under very mild conditions provide interesting examples to theorists in their attempts to learn more about the intimate details of the steric factors and specific interactions of reactant molecules with the catalytically active site.
Acknowledgements We acknowlege the generosity of Chemie Uetikon of Switzerland for the gift of the zeolite materials used in most of our research.
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395
Zeolite catalysis studies by radiation chemical methods D. W. Werst and A. D. Trifunac
Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439
1 INTRODUCTION Ionizing radiation has found applications in the study of heterogeneous catalysis primarily as a means of producing unstable species and reactive intermediates, many of them paramagnefic, whose study can elucidate fundamental issues of structure, mechanisms, transport and reactivity. Fast detection techniques are needed to observe short-lived species and measure their kinetics. However, solid catalysts and microporous solids, by virtue of specific surface-adsorbate interactions, geometric constraints and strong ionic character, possess the ability to stabilize highly reactive species. Thus radiolysis has been used to produce a variety of paramagnefic species that can be characterized by non-time-resolved methods, and in particular, electron paramagnetic resonance (EPR) spectroscopy, at ambient and sub-ambient temperatures. Intermediates in high-temperature processes have been stabilized at low temperature after ~/irradiation of metal oxides and zeolites. Important early examples were oxygen anions, O-, 0 2- and O3-.~-3 Some of their reactions with small molecules were also elucidated by EPR. Metal cluster ions have also been produced by radiolysis and stabilized in zeolites. Examples include alkali metal cation clusters 3'4 in faujasites and silver cation clusters 5-8 in zeolite A and in silicoaluminophosphate molecular sieves. Detailed information was obtained from EPR studies about their structure, thermal stability and formation of adducts. Radical cations can be stabilized inside zeolites and on some amorphous metal oxide surfaces. This was recognized early by the EPR observation of radical cations generated spontaneously upon exposure of certain solid catalysts to easily oxidized species such as aromatic hydrocarbons and certain olefms. 9-~3 These observations reveal the presence of electron acceptor sites and, whether or not "]'Work at Argonne performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Science, US-DOE under contract number W-31-109-ENG-38.
396
the radical cations are actual intermediates in catalysis, their detection can shed light on reaction mechanisms. Similarly, radiolytically produced radical cations can be stabilized in zeolites and related materials. This possibility was exploited by spectroscopists to study the EPR of radical cations and some neutral radicals even before the development of inert matrices such as rare gases and freons for radical cation stabilization. ~4'15 Recently, work in our laboratory has developed the use of inert zeolites as microreactors to control radical cation reactions and to study radiation chemistry in heterogeneous s y s t e m s . 16-26 In the case of active catalysts, radiolysis can potentially produce radical cations of products as well as starting material. 23'27'28 Thus, like the spontaneous oxidation process described above, radiolysis combined with EPR permits a method of post-reaction analysis of products by in situ spectroscopy. It is not practical to cover every topic in this review in which radiation chemical techniques have contributed to the understanding of catalyst function or catalytic reactions. With this introduction as a cursory guide to relevant topics, we move on to a discussion of the radiolyfic spin labeling technique for analyzing products of catalytic reactions in zeolites, which has been the main thrust of experiments directed at fundamental aspects of catalysis in our laboratory. 2 SPIN L A B E L I N G BY IONIZATION Radiolytic spin labeling of molecules adsorbed in zeolites occurs by ionization to form radical cations and by formation of H-adduct radicals by H atom addition. 2628 Ionization of adsorbed molecules is a two-step process, equations (1) and (2). Because the adsorbate loading used in experiments is low (typically one percent or less by weight), energy is absorbed by the matrix and not directly by the adsorbate. Holes (Z "§ created in the zeolite lattice migrate to adsorbate (A) by charge transfer. Stabilization of radical cations is made possible at low temperature by sequestration in the zeolite pores and by trapping of electrons by the matrix. Y Z
~
Z "§ +
Z "+
A
~
+
e-trappe d
Z
+
(1)
A "§
(2)
397
HZSM5
NaZSM5
a)
~
1 -i ~1 i I-I
I '! I
Figure I. Radiolysis/EPR results for a) 1,3,5-cycloheptatriene and b) 2,3-dimethyl2-pentene on HZSM5 and NaZSM5. The radical cations of catalytic products, toluene and tetramethylethylene, were observed only on the acidic zeolite and not on NaZSM5.
It sounds simple, but as mass spectroscopists know, ionization as an analysis technique creates very reactive species. It is not enough to be able to detect ions; one must also have knowledge of how the ions react. Fortunately, many radical cations are quite stable in selected zeolites at low temperature. For example, the excellent stability of radical cations of simple aromatic hydrocarbons and olefins in MFI zeolites has contributed to the successful application of the radiolytic spin labeling technique to catalytic processes of considerable importance to the fuels and petrochemical industries hydrocarbon transformations in zeolite Z S M 5 . 23'27'28 How is it possible to distinguish radical cation reactions from genuine transformations catalyzed by the zeolite, and what types of processes tend to intrude on the identification of products of catalysis? To answer the first question, we rely on the .ion-exchangeability of zeolites. Zeolites are crystalline aluminosilicates, natural or man-made, that can adopt a remarkable range of channel-type and cage-type lattice architectures, depending on the
398
connectivity of ring structures built from (SiO4) 4" and (A104) s tetrahedra. 29 Extraframework cations are present in stoichiometric amounts to charge balance the negative lattice. Ion exchange is an important method of varying the acid/base character of zeolites and of incorporating other metal centers (for example, transition metal ions) with catalytic function. In this chapter we limit our discussion to the proton- and Na§ zeolites. The former derive catalytic activity from bridging hydroxyl groups ( --- A10(H)Si =--), that are strong Bronsted acid sites and the latter are inert adsorbents. The availability of both active and inactive, isomorphous zeolites provides a direct control experiment for the radiolytic spin labeling technique because the radical cation reactions can be elucidated on the inactive zeolite. This is best explained by example and two cases are illustrated in Figure 1. In the first example, radiolysis of an HZSM5 sample exposed at room temperature to 1,3,5-cycloheptatriene gave rise to the EPR spectrum of the toluene radical cation, and in the second, the tetramethylethylene radical cation was observed on HZSM5 loaded with 2,3-dimethyl-2-pentene. Alongside each result on the active catalyst the result for the same hydrocarbon loaded on the inactive NaZSM5 is shown. In each case, the EPR spectrum on the inactive zeolite shows only the radical cation of the starting material. Clearly, the transformations observed on HZSM5, ring contraction and cracking, are caused by acid catalysis since the radical cations do not undergo these reactions. When the parent radical cation of a catalysis product is not stabilized, more knowledge is needed to interpret the results of catalysis. For example, adsorption of 1,3- or 1,4-cyclohexadiene on H-Beta gave, upon radiolysis, the EPR spectrum of benzene radical cation (Figure 2a). Control experiments for both olefins on Na-Beta also gave benzene radical cation and no cyclohexadiene radical cation. Proof of catalytic dehydrogenation of cyclohexadiene to give benzene, involving intermolecular hydrogen transfer, an important step in aromatization on zeolites, was revealed by changes in the EPR spectrum upon annealing the samples. In the H-Beta experiment, the benzene radical cation decayed and was replaced by benzene dimer radical cation upon raising the temperature above 120 K (Figure 2b). The exact same behavior was observed on Na-Beta loaded with benzene. In Na-Beta loaded with cyclohexadiene, the benzene dimer radical cation did not appear upon sample annealing - there is no neutral benzene to react with benzene radical cations. From this behavior it can be deduced that extensive conversion of olefin to benzene occurs on H-Beta (catalytically), whereas on Na-Beta only those molecules that are ionized undergo H2 elimination. When the primary species (products of catalysis) must be deduced from knowledge of the radical cation chemistry, interpretation becomes more
399
a)
b)
Figure 2. Radiolysis/EPR result after reaction of 1,3-cyclohexadiene on H-Beta. a) Radiolysis at 77 K gave the benzene radical cation, a = 4.5 G. b) Annealing the sample above approximately 120 K caused the transformation of benzene radical cations to benzene dimer radical cations, a = 2.2 G.
challenging. Not in every case, however, will the radical cation reaction mimic the catalytic reaction as in the preceding example. A last caveat regards quantitative estimates of relative product yields. Since ionization is used as the detection method, then charge migration can skew the representation of relative product yields in the EPR spectrum. That is, hole transfer among different products favors stabilization of radical cations of those species with the lowest ionization potential. We have investigated hole transfer in some detail in mixtures, both in zeolites and in frozen solutions. 24 The degree of bias can be estimated in this way, but the occurrence of hole transfer ultimately makes radiolysis a somewhat selective labeling technique. 3
RADIOLYSIS/EPR METHOD
The advantages of the radiolysis/EPR method for studying mechanisms of zeolite catalysis are due to the sensitivity and structural specificity of EPR, surpassing that of other in situ spectroscopies, such as FTIR and NMR, and the ability to identify products at low temperature. It is often the case that at high temperatures needed to evolve products from the zeolite for ex situ analysis, a
400
complex sequence of reactions has already occurred. Therefore, elucidation of the elementary reactions of the sequence necessitates in situ analysis at low temperatures. In addition, product selectivity can be more pronounced for reactions carded out under mild conditions. Product studies under these conditions allow deeper insights into the subtle details of zeolite-reactant interactions. The radiolysis/EPR experiment can be succinctly described as follows. The zeolite powder is first activated to approximately 450~ under vacuum for several hours in a 4 mm o.d. suprasil tube. Under such activation conditions, no Lewis acid sites are created to give rise to spontaneous oxidation of adsorbed species. No EPR signals are observed in loaded zeolites prior to radiolysis. The adsorbate is quantitatively transferred via a glass vacuum manifold to the tube containing zeolite and the tube is sealed. The sealed tube is equilibrated at some temperature Teq for a time t,q. At the end of the reaction period, transfer of the sample tube to a liquid nitrogen storage dewar quenches any further chemistry. Radiolysis is carded out at 77 K and EPR spectra are collected between 4 K and room temperature. Catalytic transformations can be arrested after formation of a single p~oduct or the evolution to many products can be dissected into a sequential development of different intermediates and products by systematically varying Teq and teq. Taking snapshots of the material composition on the zeolite as it incrementally changes greatly aids the interpretation of multic0mponent EPR spectra. Increasing multiplicity of products will eventually defeat any chance of deconvolving the EPR spectrum and one must resort to alternative (ex situ) techniques. However, in the study of isobutene reactions on HZSM5 (vide infra), five different species, including starting material, were identified. 27 4
R E A C T I O N S OF A C Y C L I C OLEFINS ON ZSM5
The investigation of reactions of isobutene and related acyclic olefins on HZSM5 provides a good basis for comparison of the radiolysis/EPR technique and other in situ spectoscopies, such as FTIR, 3~ NMR 32'33 and EPR without radiolysis. 344~ Results obtained by the different techniques are in general agreement, but advantages of the radiolysis/EPR method can be noted. A chief advantage was the ability to distinguish and identify structurally similar products. The loss of isobutene starting material and the build-up of Ca and C6 products, reflecting dimerization, isomerization and cracking processes, were all evident in radiolysis/EPR experiments on HZSM5 at room temperature and
401
I
c)
f-Figure 3. Radiolysis/EPR results after reaction of isobutene on HZSM5 a) for 16 h at 295 K and b) for 16 h at 338 K. c) Radiolysis/EPR result after reaction of propene on HZSM5 for 16 h at 338 K.
below. 27 Above r o o m temperature, products larger than Cs began to form. By appropiate variation of T,q and t,q, the sequence of appearance of products could be partially deduced, adding important chemical insight into the identity of the products.
1 "§
2 .+
Figure 3 shows just two "EPR snapshots" of the evolving material composition on the catalyst loaded with isobutene. The EPR spectrum observed after 16 h equilibration at 295 K consisted of signals from two radical cations with structures 1.+ (broad lines, a = 17 (3) and 2"* (sharp lines,
402 Table 1 Acyclic olefins included in radiolysis/EPRstudies of reactions on HZSM5
\
a = 17 G). There was no evidence of any remaining monomeric starting material; by this stage, it has been consumed in dimerization reactions followed by isomerization and cracking. The radiolysis/EPR study of isobutene was supported by experiments on C3C s olefins on both HZSM5 and NaZSM5 (Table 1). This allowed screening for possible radical cation reactions (minimal for these compounds), and the survey of related compounds aided spectroscopic assignment and tested the catalytic reaction steps from different starting points. For example, 2 was also formed from C7 and Ca feed molecules, corroborating the conclusion that 2 can be formed from isobutene by cracking the dimer as opposed to addition of C4 and C2 units. After reaction at higher temperatures (approximately 338 K), the resulting EPR spectrum evolved frofn that in Figure 3a into that in Figure 3b. The hyperfine structure indicates a radical cation with structure analogous to 1.§ and the sequence of formation of this product suggests that it is a higher molecular weight species. It is clearly a polymeric radical cation, or combination of radical cations, with structure 3"*. Discrimination of different length polymers, n >_ 4, from the EPR data was not possible.
~(CH2)nCH3 3"+
n>4
403
A general observation and one that clearly points out the advantage of following catalytic reactions under very mild conditions, was that the radiolysis/EPR result for samples equilibrated for an hour or more at 338 K began to be indistinguishable regardless of feed molecule (for all cases studied, C3-C8). For comparison, the result for propene on HZSM5, equilibrated under the same conditions as the sample in Figure 3b, is shown in Figure 3c. After extensive reaction, the hydrocarbon composition on the catalyst essentially loses memory of the starting material. Experiments under such reaction conditions do not lead to an understanding of the elementary reactions steps, reaction sequence or differences that can account for reaction selectivity. Our conclusions are in general agreement with previous N I ~ R 32'33 and FTIR 3~ studies of olefin reactions on HZSM5. In particular, isobutene isomerization was observed as low as 143 K by NMR. The average size range of Cg-CI2 was reported for reaction up to 300 K, but no evidence of cracking was cited under these conditions. Discrimination of C6, C 8 and C12 species by NMR or FTIR is considerably less certain than by EPR. While chemical shift and vibrational spectra allow the estimation of relative amounts of aliphatic and olefinic protons, vinyl vs. methylene C-H bonds, etc., assignments of unique chemical structures, especially in mixtures, from such data is usually not possible. Finally, radiolysis/EPR results forced the reinterpretation of literature spanning more than 25 years on EPR studies of olefin reactions on zeolites in the presence of Lewis acid s i t e s . 3441 Spontaneous oxidation of olefins and products of their Bronsted acid-catalyzed reactions on dehydroxylated (to form Lewis acid sites) ZSM5 and Mordenite (vide infra) provides an alternative method of spin labeling, and the technique gives results mostly consistent with those from radiolysis/EPR. However, in the spontaneous oxidation method, the limited degree of control over the ionization process and progress of the catalysis has led to erroneous conclusions (mainly in the earlier work). Interpretation of the radiolysis/EPR experiments is greatly aided by the ability to methodically vary the relative contribution of different products to the EPR spectrum. 5
SHAPE SELECTIVITY-
ZSM5 AND MORDENITE
One of the paramount advantages of microporous reactors is the influence of geometry/spatial constraints on the reactions of guest species. This gives rise to shape selectivity which is vividly displayed in the products of cyclic olefin reactions on zeolites ZSM5 and Mordenite. 28 The largest contrast is that Mordenite, with its system of 12-ring channels (Table 2), is capable of
404 Table 2 Diameters of elliptical channels in ZSM5 and Mordenite [ref. 42]
Diameter (A) ZSM5
10-ring, straight channels 10-ring, sinusoidal channels
5.3 x 5.6 5.1 x 5.5
Mordenite
12-ring 8-ring
6.5 x 7.0 2.6 x 5.7
Table 3 Major products formed from cyclic precursors on ZSM5 and Mordenite
Precursor HZSM5
5-, 6-, 7-member ring
Major Product "+ll )
C7H12 H-Mordenite
R 5-, 6-, 7-member ring C6HIo, C7H12 R = H or CH3
accommodating products that are too large to fit in the 10-ring channels of ZSM5. Reaction of cyclic olefins on HZSM5 below 340 K is limited to unimolecular rearrangment. For example, a family of seven different compounds with 5-, 6- and 7-member rings and the empirical formula C7H12 all reacted to give 1,2dimethylcyclopentene (Figure 4a). 28 Such transformations - hydrogen and alkyl shifts, ring contraction, ring expansion - are well-known processes involving Bronsted acid catalysis. 43'~ Using the carbenium ion formalism, one such transformation is shown in Scheme 1, which depicts the intermediacy of a protonated cyclopropane ring. A question currently debated is whether free carbenium ions exist on zeolite catalysts. The question really is how acidic are zeolite catalysts, are they super acids? Evidence suggests that most reactive
405
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J_ .
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. . . . . . . .
I
,' 50G] b)
Figure 4. a) Radiolysis/EPR result after reaction of 1,5-dimethylcyclopentene on HZSM5 for 16 h at 318 K. b) Radiolysis/EPR result after reaction of cyclohexene on H-Mordenite for 16 h at 295 K. The spectrum is assigned to the bicyclo[5.5.0] dodecene radical cation. intermediates are framework-bonded species, 45"48 but zeolite catalysis can nevertheless involve transition states with enough ionic character such that rules governing carbenium ion reactions are obeyed to some degree within the geometry contraints imposed by the zeolite. Scheme 1" Carbenium ion mechanism of ring contraction and methyl side-chain generation
406
Bimolecular reaction products were more prevalent when cyclic olefins were reacted on H-Mordenite. 2s These products (Table 3, Figure 4) reflected dimerization and ring expansion processes. The exception, 1,2dimethylcyclopentene, suggests that factors other than size exclusion are also important. This molecule was stable, even up to 318 K (16 h), whereas another C7 compound (1-methylcyclohexene) was converted to the more typical bicyclic dimer product. Another exception, cyclohexene, showed greater resistance to ring contraction on HZSM5 than methylcyclohexenes or cycloheptene. The carbenium ion-type mechanism does not disfavor the ring contraction in the case of cyclohexene, and the thermodynamic benefit of going from a secondary carbenium ion to a tertiary carbenium ion (methylcyclopentyl) should be a strong driving force for ring contraction. The greater .acidity of secondary carbenium ions, such as cyclohexyl, compared to tertiary ions, such as methylcyclohexyl, could partly underlie the stability of cyclohexene on HZSM5. However, that suggests that cycloheptene should also be slow to undergo ring contraction, which was not the case. Furthermore, H-mordenite, a comparable acid to HZSM5, is able to activate the conversion of cyclohexene to the dimer product at equally low temperatures. 6 H/D E X C H A N G E Certain observations of reaction selectivity, such as those described in the preceding paragraphs, suggest that the carbenium ion paradigm - that is, the direct analogy to reactions of olefins in superacid solutions - is a simplistic model for Bronsted acid catalysis in zeolites. The carbenium ion formalism can account for most of the products, but glosses over the specific characteristics of zeolite catalysis, such as volume constraints, relative acidity and possible bifunctional nature of zeolite catalysts owing to the proximity of basic oxygen atoms adjacent to the acid site. An experimental test for the intermediacy of carbenium ions in zeolite catalysis is isotope exchange. One such experiment from our laboratory involved the oligomerization of acetylene-d 2 on HZSM5 at room temperature. 23 The benzene product of the B ronsted acid-catalyzed trimerization of acetylene was detected as benzene radical cation. The EPR spectrum showed that the oligomerization of acetylene proceeds without exchange with zeolitic protons. While it is conceivable for H/D exchange to occur without the formation of a free ion, the lack of H/D exchange seems strong negative evidence for the intermediacy of free carbenium ions. This example fits with the growing experimental and theoretical evidence that zeolites are not superacids and that
407
intermediates involved in many catalytic reactions retain some degree of covalent bonding to the lattice. Scrambling of the isotope label on benzene did not occur under the conditions used to study acetylene trimerization, but we have shown by radiolysis/EPR that benzene does undergo H/D exchange on HZSM5 at higher temperatures. 25 Beck et al. determined the activation energy (14.4 kcal/mol) for this reaction by N1VIR. 49 This fundamental reaction is itself an important test of theoretical calculations of zeolite acidity and reaction mechanisms. An interesting and remaining puzzle regarding H/D exchange in benzene on HZSM5 is the observation by radiolysis/EPR that the exchange is very nonstatistical; some of the benzene undergoes extensive exchange and some of the benzene exchanges very little. 25 This observation could not be attributed to a fraction of the benzene residing on the external surface of the zeolite. 7
CONCLUSION
Radiation chemical methods, when coupled with appropriate detection techniques, can be used to study many different aspects of solid heterogeneous catalysis. Ionizing radiation can be used to generate reactive intermediates, to change the oxidation state of metal ions or clusters, to create reactive defects in the solid lattice and to spin label reaction products. When radiolysis and EPR are used together, products of catalysis can be identified in situ and at low temperatures with the excellent structural specificity and sensitivity inherent in the EPR method. As always, exceptions to the rule can offer greater insights into chemical mechanisms. Subtle structure/reactivity dependences revealed by radiolysis/EPR experiments carded out under very mild conditions provide interesting examples to theorists in their attempts to learn more about the intimate details of the steric factors and specific interactions of reactant molecules with the catalytically active site.
Acknowledgements We acknowlege the generosity of Chemie Uetikon of Switzerland for the gift of the zeolite materials used in most of our research.
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