9906
J. Phys. Chem. 1996, 100, 9906-9910
Ethylene Dimerization and Butene Isomerization in Nickel-Containing MCM-41 and AlMCM-41 Mesoporous Molecular Sieves: An Electron Spin Resonance and Gas Chromatography Study Martin Hartmann, Andreas Po1 ppl, and Larry Kevan* Department of Chemistry, UniVersity of Houston, Houston, Texas 77204-5641 ReceiVed: January 23, 1996; In Final Form: April 4, 1996X
Nickel-containing mesoporous materials MCM-41 and AlMCM-41 have been prepared by synthesis and ion exchange. These materials are catalytically active for ethylene dimerization and 1-butene isomerization. The catalytic activity for the dimerization reaction is shown to be due to nickel(I) species, while the 1-butene dimerization is dominated by the acidic properties of the MCM-41 materials. The ethylene dimerization activity and product distribution are different in nickel-exchanged Ni-MCM-41 and Ni-AlMCM-41 in contrast to Ni/MCM-41, where Ni(II) was introduced into the synthesis mixture. The activity for ethylene dimerization decreases in the order Ni/MCM-41 > Ni-AlMCM-41 > Ni-MCM-41, which reflects the concentration of the catalytically active nickel(I) species. The product distribution is dominated by the acidity of the MCM-41 materials, which drives the isomerization of the primary reaction product 1-butene into cis- and trans-2butene. Electron spin resonance studies show that an isolated Ni(I) species can be stabilized, which converts to a Ni(I)-C2D4 complex after ethylene adsorption. After subsequent heating to 343 K two new species are observed, which can be ascribed to Ni(I) product complexes.
Introduction The new crystalline mesoporous materials M41S, first reported in 1992, are attracting increasing attention.1 Among the different members of the M41S type of materials, the socalled MCM-41 family, which exhibits a hexagonal array of uniform mesopores, has been the focus of most studies. An exciting property of these materials is the possibility to control the internal diameter of the mesopores between 2 and 10 nm by the chain length of the micellar surfactant template.2,3 A number of important applications of MCM-41 materials both in catalysis4 and as advanced materials5 have already been identified. The catalytic properties of molecular sieves can be controlled by the incorporation of transition metal ion species leading to specifically tailored catalytic application of these materials. Isomorphous substitution of a fraction of the framework silicon by Al3+ leads to a significant increase in the exchange capacity for transition metal cations.6,7 Isomorphous substitution is often accompanied by the formation of Bro¨nsted acid sites. Depending on the nature and number of trivalent framework cations, such as Al3+, B3+, or Fe3+, both the density and the strength of the acid sites may be varied.8 Transition metal ions like Ni(I) or Pd(I) can be active sites in catalytic reactions such as ethylene and propylene dimerization as well as acetylene cyclomerization.9-11 However, application of transition metal ion species such as Ni(I) in microor mesoporous materials requires information about the formation of the active species and a detailed characterization of the metal ion environment. Additionally, the acidity of MCM-41 material has to be investigated since it has a significant impact on the overall catalytic performance of a bifunctional catalyst. In this study, we ion-exchanged Ni(II) ions by liquid state reaction into pure siliceous (MCM-41) and into aluminumcontaining (AlMCM-41) materials. The Si/Al ratio of the latter sample was 16. For comparison, a sample was prepared by adding NiCl2 to the synthesis mixture of MCM-41. As a test reaction, ethylene dimerization is used, which is catalyzed by X
Abstract published in AdVance ACS Abstracts, May 15, 1996.
S0022-3654(96)00218-3 CCC: $12.00
Ni(I) ions.12 To evaluate the acidity of the different MCM-41 materials, the acid-catalyzed dimerization of 1-butene is explored. The reactions are monitored by gas chromatography (GC), electron spin resonance (ESR), and electron spin echo modulation (ESEM) to detect different reaction products and their interaction with the catalytically active Ni(I) center. Experimental Section Sample Preparation. MCM-41 and AlMCM-41 were synthesized by hydrothermal synthesis using the procedures described in previous publications.6,13 Liquid state ion exchange was performed by adding 40 mL of 1 mM NiCl2 solution to 0.25 g of MCM-41 or AlMCM-41. The resulting mixture was stirred for 18 h at room temperature. Finally, the samples were washed with deionized water and filtered. These samples are designated as Ni-MCM-41 and Ni-AlMCM-41. In both ionexchange procedures the amount of Ni(II) added initially to the MCM-41 or AlMCM-41 materials corresponded to 1.5 × 10-4 mol/g. In another preparation method, NiCl2 was introduced into the synthesis mixture of MCM-41. The synthesis was performed as follows: 2.2 g of NaOH was dissolved in 25 mL of H2O. Subsequently 14.5 mL of 25 wt % cetyltrimethylammonium chloride (CTAC) solution was added slowly and the resulting mixture was stirred for 30 min. Then 0.206 g of NiCl2 was added and stirred another 30 min. Finally, 20.5 mL of triethyl orthosilicate (TEOS) was added to the solution. The obtained gel was stirred for 1 h, transferred into a Teflon bottle, and kept at room temperature for 72 h. After the reaction the product was filtered and washed thoroughly with deionized water and ethanol. Powder X-ray diffraction patterns taken before and after calcination confirm that the hexagonal MCM-41 phase was formed in this sample. The XRD spectra showed an intense peak at 2θ ) 2.17°, indicating the existence of the hexagonal phase with a spacing of about d100 ) 40 Å.6 After calcination the intense peak is shifted to 2θ ) 2.5° corresponding to d100 ) 35 Å. Such a shift is frequently observed after calcination.14 © 1996 American Chemical Society
Nickel Containing Mesoporous Molecular Sieves
J. Phys. Chem., Vol. 100, No. 23, 1996 9907
TABLE 1: Ethylene Dimerization on Various MCM-41 Materials after 24 h at 343 K catalyst
wt % conversion
wt % selectivity
MCM-41 Ni-MCM-41 Ni/MCM-41 AlMCM-41 Ni-AlMCM-41
0.3 1.7 5.0 0.4 4.5
>99 >98 >99 >99 >99
This sample was named Ni/MCM-41 where the Ni site is unknown and is probably not in the framework. Sample Treatment and Measurements. Prior to the catalytic reaction the samples were dehydrated at 723 K in vacuum [p < 10-4 (hPa)] for 18 h. Ni(I) is formed during this activation process from Ni(II) by water or hydroxyl group desorption.15 The catalytic ethylene dimerization was performed inside a glass reactor of total internal volume of 51 cm3 using 100 mg of sample material. Ethylene (250 hPa) was adsorbed on the activated sample at 343 K and 1200 hPa of helium was also introduced. The gas phase over the samples was analyzed periodically by withdrawing an aliquot into a Varian Model 3300 gas chromatograph equipped with a thermal conductivity detector. A 6 ft column of 0.085 in. i.d. packed with 0.19 wt % picric acid supported on 80/100 mesh graphic-GC support was used, and all runs were conducted isothermally at 308 K. For the butene isomerization reaction 500 hPa of 1-butene was adsorbed on the samples. The reaction was also performed at 343 K. All ESR spectra were recorded with a Bruker ESP-300 X-band spectrometer at 77 K. The magnetic field was calibrated with a Varian E-500 gaussmeter. The microwave frequency was measured by a Hewlett Packard HP 5342A frequency counter. ESEM spectra were measured at 4 K with a Bruker ESP 380 pulsed ESR spectrometer. Three pulse echoes were measured by using a (π/2-t-π/2-T-π/2) pulse sequence as a function of time T to obtain the time domain spectrum. The deuterium modulations were analyzed by a spherical approximation for powder samples in terms of N nuclei at distance R with an isotropic hyperfine coupling Aiso. The best fit simulation of an ESEM signal is found by varying the parameters until the sum of the squared residuals is minimized.16 Ethylene, 1-butene, and cis/trans-2-butene were obtained from Union Carbide Linde Division while ethylene-d4 (99% D) was obtained from Cambridge Isotope Laboratories. All gases were purified before use by repeated freeze-pump-thaw cycles. Results Catalytic Activity of Ni-Containing MCM-41 Materials. The parent materials MCM-41 and AlMCM-41 are found to be essentially inactive for ethylene dimerization under the conditions applied. However, nickel-containing MCM-41 materials are catalytically active for this reaction (Table 1). Analysis of the gas phase after 24 h at 70 °C shows a total turnover of 5.0 wt % in Ni/MCM-41, 4.7 wt % in Ni-AlMCM-41, and 1.7 wt % in Ni-MCM-41. In all cases the selectivity for the formation of n-butenes is close to 100%. After an initial period, 1-butene, cis-2-butene, and trans-2butene can be detected by gas chromatography. The distribution of the n-butenes in the gas-phase changes with the reaction time (Figure 1a-c). In Ni-AlMCM-41 after a 24 h reaction period the thermal equilibrium distribution of the n-butenes (∼10 % 1-butene, 30% cis-2-butene, and 60% trans-2-butene) is obtained, but in Ni/MCM-41 and Ni-MCM-41 the relative amount of 1-butene in the gas phase is much higher (34 and 23%, respectively).
Figure 1. Ethylene dimerization products vs reaction time on (a) NiMCM-41, (b) Ni/MCM-41 and (c) Ni-AlMCM-41 (b, 1-butene; 2, cis-2-butene; 9, trans-2-butene). Reaction conditions: catalyst 100 mg, T ) 343 K, p(ethylene) ) 250 hPa.
Figure 2. 1-Butene isomerization products vs reaction time on NiAlMCM-41 (b, 1-butene; 2, cis-2-butene; 9, trans-2-butene). Reaction conditions: catalyst 100 mg, T ) 343 K, p(1-butene) ) 400 hPa.
TABLE 2: 1-Butene Isomerization on Various MCM-41 Materials after 24 h at 343 K catalyst
wt % conversion
cis/trans ratio
MCM-41 AlMCM-41 Ni/MCM-41 Ni-MCM-41 Ni-AlMCM-41
5.97 15.78 15.67 12.53 21.52
0.81 0.71 0.72 0.78 0.53
To understand these differences we have also studied the isomerization of 1-butene in all our samples at a reaction temperature of 70 °C. An example of the time dependence of this reaction is given in Figure 2. All catalysts were found to be active for isomerization of 1-butene to cis- and trans-2-butene (Table 2). The isomerization activity increases in the order MCM-41 < Ni-MCM-41 < Ni/MCM-41 < AlMCM-41 < NiAlMCM-41. The cis/trans ratio also decreases in this order. ESR and ESEM. Prior to any treatment the nickelcontaining samples do not show any ESR signals at 77 K. Thus, the nickel species exist in the form of Ni(II). Ni(I) can be generated and stabilized in Ni-AlMCM-41 by thermal or hydrogen reduction and by γ-irradiation.13 The ESR spectrum of Ni-AlMCM-41 after dehydration at 723 K is shown in Figure 3a. An isolated Ni(I) species A can be detected with g⊥ )
9908 J. Phys. Chem., Vol. 100, No. 23, 1996
Hartmann et al.
Figure 3. ESR spectra at 77 K in Ni-AlMCM-41 after (a) dehydration at 723 K and (b) ethylene adsorption and subsequent heating to 343 K for 2 h.
Figure 5. Experimental (s) and simulated (- - -) three pulse ESEM spectra at 4.2 K showing 2H modulation in Ni-AlMCM-41 after (a) ethylene adsorption (g ) 1.92) and (b) 2 h of reaction (g ) 2.05). The simulation parameters are in Table 3.
TABLE 3: 2H ESEM Parameters in Ni-AlMCM-41 after Different Sample Treatments catalysta
shell
N
R/nm
Aiso/MHz
Ni-AlMCM-41 D + ethylene Ni-AlMCM-41 G + ethylene Ni-AlMCM-41 D after reaction Ni-AlMCM-41 G after reaction
1 1 1 2 1 2
4 4 2 6 2 6
0.37 0.35 0.37 0.44 0.36 0.45
0.10 0.15 0.12 0.05 0.13 0.03
a
Figure 4. ESR spectra at 77 K of Ni-AlMCM-41 after (a) γ-irradiation at 77 K and subsequent annealing at RT for 21 h, (b) after ethylene adsorption onto (a) and (c) after heating of (b) to 343 K for 2 h.
2.102. The parallel part of this species cannot be seen because of a very broad underlying species C (g ≈ 2.2), which can be ascribed to superferromagnetic Ni(0) clusters.17 A signal at g ∼ 2.0 is attributed to O2- formed by water decomposition or a lattice defect as has also been observed in SAPO materials.18,19 After ethylene adsorption and subsequent heating to 343 K, several new species can be seen (Figure 3 b), but the signals are distorted by superposition with species C. To identify these species, γ-irradiation was used to generate Ni(I), since it does not produce a large amount of superferromagnetic Ni(0) clusters. After γ-irradiation, an isolated Ni(I) species A (g| ) 2.521 and g⊥ ) 2.105) can be detected (Figure 4a). A Ni(I)C2D4 complex (species E) with g1 ) 2.677, g2 ) 2.464, and g3 ) 1.916 is obtained directly after ethylene adsorption at room temperature (Figure 4b). Further heating to 343 K leads to the development of two new species B1 (g⊥ ) 2.023 and g| ) 2.051) and B2 (giso ) 2.07) (Figure 4c), which are also obtained after n-butene adsorption. ESEM analysis at g ) 1.92 of the Ni(I)-C2D4 complex shows the interaction of four deuteriums with a Ni-D distance of 0.35 nm indicating a typical π-bonding interaction between Ni(I) and one ethylene molecule (Figure 5a).
D ) dehydrated sample; G ) γ - irradiated sample.
The analysis of the deuterium modulation of species B1 and B2 shows the interaction of eight deuteriums with one nickel(I) center indicating butene formation (Figure 5b). The 2H ESEM fit parameters are summarized in Table 3. Discussion Ni(I) ions formed by reduction of Ni(II) can be stabilized in zeolites20 and SAPO materials.18,19 It has been shown that Ni(I) ions are catalytically active for ethylene dimerization in these materials.12,21 An isolated Ni(I) species A can also be stabilized in Ni-AlMCM-41 after thermal or hydrogen treatment or by γ - irradiation. Stabilization of Ni(I) in pure siliceous MCM-41 seems to be less efficient.13 Species A was found to decrease significantly upon the adsorption of ethylene accompanied by the formation of species E with a rhombic g tensor indicating a lower symmetry for this Ni(I) species E. As in zeolites and silicoaluminophosphates, species E can be ascribed to a Ni(I)-C2D4 complex. ESEM analysis indicates the formation of a π-bonding interaction between Ni(I) and ethylene with a Ni(I)-D distance of 0.36 ( 0.01 nm. After the sample is heated to 343 K, species E slowly disappears and two new species B1 and B2 are seen. These species can also be formed after n-butene adsorption on these samples. ESEM analysis shows an interaction with eight deuteriums. Due to the fact that there are three butene isomers with different
Nickel Containing Mesoporous Molecular Sieves geometries, the complex geometry of the Ni(I)-C2D8 complex is difficult to determine. Assuming a π-bonding interaction two (cis/trans-2-butene) or three (isobutene) deuteriums should be closer to Ni(I) than the other six or five deuteriums. This is somewhat reflected by the ESEM results, which show two closer nuclei with a Ni(I)-D distance of 0.36 nm and six more distant deuteriums with a Ni(I)-D distance of 0.45 nm. Similar results were also found in SAPO-5 and SAPO-11.21 We have shown in an earlier publication21 that in materials with smaller channels like SAPO-11 (0.63 by 0.39 nm) and SAPO-5 (0.73 nm) Ni-C4D8 complexes with a large g-factor anisotropy are obtained. The g-factor anisotropy decreases with increasing channel diameter.21 Therefore, we expect a lower anisotropy in this MCM-41 system. In fact, we obtained an isotropic species and an axially symmetric species. These species were also obtained in NiH-SAPO-8,22 a molecular sieve with large straight channels of 0.87 by 0.79 nm diameter. This is also consistent with results in X and Y zeolites, which have cages of 1.2 nm diameter. In NiCa-Y zeolites there are three different species with axially symmetric (g| ) 2.109 and g⊥ ) 2.023; g| ) 1.965 and g⊥ ) 2.620) and isotropic (giso ) 2.048) parameters.12 Surprisingly, none of these Ni(I) species are found in MCM-41 materials or in silicoaluminophosphates. It has been reported that ethylene is initially dimerized to 1-butene over various transition metal ion exchanged zeolites.23 In a subsequent reaction step 1-butene is isomerized to an equilibrium composition of n-butenes with predominant trans2-butene (cis/trans ratio ≈ 0.5). In zeolites and SAPO materials it has been shown that monovalent transition metal ions with a d9 configuration like Ni(I) and Pd(I) are the catalytically active sites for ethylene dimerization.12,24,25 The subsequent isomerization to cis- and trans-2-butene is catalyzed by the acid sites of the MCM-41 materials. Ethylene dimerization occurs in all nickel-containing MCM-41 materials after dehydration at 723 K, which produces Ni(I) by reduction of Ni(II) by desorbing water or hydroxyl groups.15 The activity for the formation of C4 olefins decreases in the order Ni/MCM-41 > Ni-AlMCM41 > Ni-MCM-41, which reflects also the order of initial Ni(II) concentration after ion exchange or synthesis. The differences between Ni-AlMCM-41 and Ni-MCM-41 are most likely due to different ion-exchange sites leading to a higher Ni(I) concentration in AlMCM-41 which is confirmed by our ESR results. In MCM-41 Ni(I) replaces silanol protons, whereas the introduction of alumina into the walls of AlMCM-41 produces a net negative charge, which is initially balanced by sodium ions. The different strengths and behaviors of these ionexchange positions were recently also shown in coppercontaining MCM-41 materials.6 The high turnover in Ni/MCM41 is somewhat surprising but is consistent with its higher initial nickel concentration. The isomerization of 1-butene to cis-2-butene and trans-2butene is catalyzed by acid sites of the molecular sieve.26 In Ni-AlMCM-41 the thermal equilibrium distribution of n-butenes is detected after ∼24 h, but in pure siliceous Ni-MCM-41 and Ni/MCM-41 the amount of 1-butene in the gas-phase is much higher (Figure 1, a and c). The thermal equilibrium distribution is not even reached after 96 h of reaction. In NiCa-X the equilibrium distribution is reached very rapidly (∼30 min),12 which shows that X-zeolite has higher acidity compared to the MCM-41 materials. Preliminary, recently published results comparing the acidity of AlMCM-41 and MCM-41 show that AlMCM-41 is slightly more acidic than MCM-41.28 So far no detailed investigation of the acidity of MCM-41 and AlMCM-41 has been published, but our catalytic ethylene dimerization results show that the
J. Phys. Chem., Vol. 100, No. 23, 1996 9909 incorporation of aluminum into the walls of MCM-41 seems to enhance the acidity. To test the acidity of the MCM-41 materials under the conditions present during ethylene dimerization, the isomerization of 1-butene was used as a test reaction. MCM-41 and Ni-MCM-41 are the materials with the lowest 1-butene turnover and the highest cis/trans ratio indicating the lowest acidity for these materials. The increase in conversion in nickel-containing MCM-41 is probably due to the formation of acid sites during the reduction of Ni(II) to Ni(I) or Ni(0).29 The conversion of 1-butene is higher in AlMCM-41 than in MCM-41 showing that the incorporation of aluminum into the walls of MCM increases the acidity. This is also reflected in the decrease of the cis/trans ratio. By ion exchange of Ni(II) into AlMCM-41 the material again becomes more acidic, as reflected by the increased turnover and a cis/trans ratio of 0.53 which is close to the thermal equilibrium value of 0.45. Ni/ MCM-41 seems to be comparable to AlMCM-41 in acidity, indicating that Ni(II) might be incorporated into the walls of the MCM-41 molecular sieve and therefore lead to a material with enhanced acidity. All samples deactivate after a longer dimerization reaction time, most likely due to the reduction of the catalytically active Ni(I) to Ni(0) by ethylene or butene.21 Coke formation could not be detected by ESR spectroscopy, which in this case should show a strong isotropic signal at g ) 2.0 after a longer reaction time.30 Due to space limitations in zeolites and SAPO materials n-butenes are the major dimerization products. In the larger MCM-41 and AlMCM-41 materials the formation of larger olefins could be more likely. However, the formation of hexenes was not detected in the MCM-41 materials under investigation. This is in contrast to previous results in the Ni(I)/SiO2 system,11 where the formation of hexenes is obtained after a 24 h reaction period. One possibly relevant difference between MCM-41 and silica is that the pore sizes are regular in MCM-41 but have a significant distribution in silica encompasing some much larger pores that could contribute to hexene formation. Conclusions Ethylene dimerization occurs at 343 K in nickel-containing MCM-41 and AlMCM-41 materials after dehydration at 723 K, but there are significant differences in the product distribution. In ion-exchanged Ni-MCM-41 and in synthesized Ni/ MCM-41 a higher relative concentration of 1-butene is detected than in ion-exchanged Ni-AlMCM-41, where the thermal equilibrium distribution of n-butenes is detected, suggesting that the acidity of Ni-AlMCM-41 is higher. This assumption was confirmed by investigating 1-butene isomerization, an acidcatalyzed reaction. The turnover was maximal in Ni-AlMCM41, showing that this is the material with the highest acidity. The catalytic activity for the formation of n-butenes increases with the incorporation of aluminum into the walls of the MCM-41 structure. This is probably due to better stabilization of the catalytically active Ni(I) in AlMCM-41. This is supported by the ESR data, where significant amounts of Ni(I) are only detected in the aluminum containing form. After adsorption of ethylene a Ni(I)-C2D4 complex is obtained, which is converted into a Ni(I)-C4D8 complex after the reaction. The formation of these complexes is confirmed by ESEM analysis. Acknowledgment. This research was supported by the National Science Foundation and the Robert A. Welch Foundation.
9910 J. Phys. Chem., Vol. 100, No. 23, 1996 References and Notes (1) Casci, J. L. In AdVanced Zeolite Science and Application Studies; Jansen, J. C., Sto¨cker, M., Karge, H. G., Weitkamp, J., Eds.; Studies in Surface Science and Catalysis, Vol. 85; Elsevier: Amsterdam, 1994; pp 329-356. (2) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (3) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmit, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (4) Corma, A.; Martinez, A.; Martinez-Soria, V.; Monton, J. B. J. Catal. 1995, 153, 25. (5) Wu, C. G.; Bein, T. Science 1994, 266, 1013. (6) Po¨ppl, A.; Hartmann, M.; Kevan, L. J. Phys. Chem. 1995, 99, 17251. (7) Kim, J. M.; Kwak, J. H.; Jun, S.; Ryoo, R. J. Phys. Chem. 1995, 99, 16742. (8) Chen, C. Y.; Li, H. X.; Davis, M. E. Microporous Mater. 1993, 2, 17. (9) Kazansky, V. B.; Elev, I. V.; Shelimov, B. N. J. Mol. Catal. 1983, 21, 265. (10) Moller, B. W.; Kemball, C.; Leach, H. F. J. Chem. Soc., Faraday Trans. 1 1983, 79, 453. (11) Bonneviot, L.; Olivier, D.; Che, M. J. Mol. Catal. 1983, 21, 415. (12) Ghosh, A. K.; Kevan, L. J. Phys. Chem. 1990, 94, 3117. (13) Hartmann, M.; Po¨ppl, A.; Kevan, L. J. Phys. Chem. 1995, 99, 17494. (14) Luan, Z.; Cheng, C. F.; Zhou, W.; Klinowski, J. J. Phys. Chem. 1995, 99, 1018. (15) Azuma, N.; Kevan, L. J. Phys. Chem. 1995, 99, 5083.
Hartmann et al. (16) Kevan, L. In Time Domain Electron Spin Resonance; Kevan, L., Schwartz, R. N., Eds.; Wiley: New York, 1979; Chapter 8. (17) Che, M.; Richard, M.; Olivier, D. J. Chem. Soc., Faraday Trans. 1 1980, 76, 1526. (18) Azuma, N.; Hartmann, M.; Kevan, L. J. Phys. Chem. 1995, 99. 6670. (19) Hartmann, M.; Azuma, N.; Kevan, L.; J. Phys. Chem. 1995, 99. 10988. (20) Michalik, J.; Narayana, M.; Kevan, L. J. Phys. Chem. 1984, 88, 5236. (21) Hartmann, M.; Kevan, L. J. Chem. Soc., Faraday Trans. 1996, 92, 0000. (22) Hartmann, M.; Kevan, L. In Proceedings of the 11th International Zeolite Conference, Seoul, 1996; submitted for publication. (23) Yoshima, T.; Ushida, Y.; Ebisowa, M.; Hara, N. J. Catal. 1975, 36, 320. (24) Hartmann, M.; Kevan, L. J. Phys. Chem. 1996, 100, 4606. (25) Ghosh, A. K.; Kevan, L. J. Am. Chem. Soc. 1988, 110, 8044. (26) Kazansky, V. B. In AdVanced Zeolite Science and Application Studies; Jansen, J. C., Sto¨cker, M., Karge, H. G., Weitkamp, J., Eds.; Studies in Surface Science and Catalysis, Vol. 85; Elsevier: Amsterdam, 1994; p 251. (27) Borade, R. B.; Clearfield, A. Presented at the 209th ACS National Meeting, Anaheim, CA, April 1995. (28) On Trong, D.; Joshi, P. N.; Lemay, G.; Kaliaguine, S. In Zeolites: A Refined Tool for Designing Catalytic Sites; Bonneviot, L., Kaliaguine, S., Eds.; Studies in Surface Science and Catalysis, Vol. 97; Elsevier: Amsterdam, 1995; pp 543-549. (29) Coughlan, B.; Keane, M. A. J. Catal. 1992, 136, 170. (30) Kevan, L.; Yu, J.-S. Res. Chem. Intermed. 1991, 15, 67.
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