J. Phys. Chem. 1996, 100, 19595-19602
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Synthesis and Spectroscopic Characterization of Vanadosilicate Mesoporous MCM-41 Molecular Sieves Zhaohua Luan,† Jie Xu,† Heyong He,‡ Jacek Klinowski,‡ and Larry Kevan*,† Department of Chemistry, UniVersity of Houston, Houston, Texas 77204-5641, and Department of Chemistry, UniVersity of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K. ReceiVed: August 5, 1996; In Final Form: September 24, 1996X
A series of mesoporous vanadosilicate VMCM-41 molecular sieves with variable Si/V ratios have been hydrothermally synthesized at pH 11 using 2-propanol as a phase stabilizer. The chemical environment of the vanadium centers in VMCM-41 is investigated by powder X-ray diffraction (XRD), electron probe microanalysis (EPMA), framework FTIR, diffuse reflectance UV-visible spectroscopy (UV-vis), electron spin resonance (ESR), and 29Si and 51V NMR. XRD, EPMA, and ESR show that the solid products have the MCM-41 structure and contain only atomically dispersed vanadium consistent with framework vanadium in VMCM-41. UV-vis, ESR, and 51V NMR reveal that most of the vanadium exists as tetrahedral V5+ ions in as-synthesized samples, but some square pyramidal VO2+ ions simultaneously occur. Both tetrahedral V5+ and square pyramidal VO2+ centers occur at two different sites inside and on the surface of the hexagonal tubular walls of MCM-41. Upon calcination and hydration, only vanadium species on the wall surfaces can be completely oxidized to tetrahedral V5+ and transformed to square pyramidal and then distorted octahedral V5+ species by additional coordination to water molecules. Vanadium species located inside the walls remain stable upon calcination and hydration. Upon thermal or CO reduction, also only the V5+ species on the wall surfaces of VMCM-41 are reversibly reduced to VO2+ species or lower valences.
Introduction One major goal of present research in chemistry is to develop ecologically friendly and technologically simple processes. This is applicable to the oxidation of organic compounds where effective but toxic stoichiometric oxidants are used.1,2 Toward this goal various transition-metal-substituted molecular sieves have been developed and shown to effectively oxidize a variety of organic compounds with “clean” and economical oxygen donors such as 30% aqueous hydrogen peroxide.1-4 One successful example is titanium-substituted silicalite.5,6 Vanadium-substituted molecular sieves have also attracted catalytic interest, including vanadium-substituted silicalite7-9 and aluminophosphate molecular sieves.10,11 However, most vanadium-modified molecular sieves are microporous solids with channels less than 10 Å which restrict the oxidation of large organic molecules. The hexagonal tubular silica material, MCM-41,12,13 possesses uniform mesopore channels varying from about 15 to about 100 Å, which allow faster diffusion of large organic molecules than smaller channel microporous molecular sieves. This opens an opportunity for the preparation of vanadium-containing mesoporous molecular sieve materials. Several research groups have attempted this goal. Abe et al.14 and Luca et al.15 synthesized nonsilica-based hexagonal mesoporous materials consisting of vanadium oxide and phosphorus oxide. Morey et al.16 grafted vanadium oxide centers onto surface silanol sites of mesoporous materials by reacting cubic MCM-48 with dry hexane solutions of vanadium alkoxide. Despite these achievements, the vanadium centers in these materials are thermally unstable, and their catalytic applications seem limited. So far, the direct hydrothermal synthesis of vanadium-substituted MCM-41, VMCM41, was only achieved by Reddy et al.17 Their sample contained †
University of Houston. University of Cambridge. X Abstract published in AdVance ACS Abstracts, December 1, 1996. ‡
S0022-3654(96)02353-2 CCC: $12.00
atomically dispersed vanadium centers in tetrahedral positions and had catalytic potential for the selective oxidation of cyclododecane and 1-naphthol by dilute hydrogen peroxide. However, their characterization of the coordination of vanadium was only by 51V NMR.17 A more complete characterization of the coordination and location of vanadium ions in VMCM-41 is achieved in this work. More complete characterization of the local environment of the vanadium in VMCM-41 requires a combination of spectroscopic techniques because vanadium can change its oxidation state and coordination geometry on thermal and other treatments used in catalytic applications. In this work a systematic study of VMCM-41 by powder X-ray diffraction (XRD), electron probe microanalysis (EPMA), framework FTIR, diffuse reflectance UV-visible spectroscopy (UV-vis), electron spin resonance (ESR), and 29Si and 51V NMR is given. The results show that various vanadium species in different oxidation states and with different coordinations occur in MCM-41 and that reversible transformations occur thermally and under various redox conditions. Experimental Section Synthesis. Fumed silica (Sigma) and sodium silicate solution (27 wt % silica, Fluka) were used as silicon sources, and vanadyl sulfate trihydrate (Aldrich) was used as the vanadium source. A solution of cetyltrimethylammonium hydroxide was prepared by batch exchange of a 25 wt % aqueous solution of C16H33(CH3)3NCl (Aldrich) using an IRA-420(OH) ionexchange resin (Aldrich). A 25 wt % aqueous solution of tetramethylammonium hydroxide and 2-propanol were obtained from Aldrich and EM Science, respectively. VMCM-41 was synthesized following a modified method for purely siliceous MCM-41.18 A typical synthesis procedure is as follows. A 10 g 25 wt % solution of tetramethylammonium hydroxide (TMAOH) is combined with 5.9 g of sodium silicate dispersed in 50 g of water with stirring; then 34.2 g of © 1996 American Chemical Society
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TABLE 1: Color and Elemental Composition of VMCM-41 Si/V ratio
color of VMCM-41
sample
as-synthesized
calcined
as-synthesized
calcined
rehydrated
VMCM-41-(10) VMCM-41-(20) VMCM-41-(40) VMCM-41-(80)
13 30 89 304
15 39
green green light green white
white white white white
orange orange orange light orange
C16H33(CH3)3NCl/OH (CTACl/OH) solution and 4.52 g of silica are added and stirred for 2 h. At the same time, an amount of vanadyl sulfate trihydrate for the desired Si/V ratio is dissolved in a mixture of 10 g of water and 2 g of 2-propanol and then slowly added to the silica gel with stirring. The pH of the gel mixture is carefully adjusted to 11 by addition of aqueous NaOH solution or dilute sulfuric acid. Finally, the gel mixture is homogenized for 1 h at room temperature by stirring and then transferred into a Teflon bottle and heated at 95 °C for 72 h. The molar composition of the final gel mixture was SiO2:0.27 CTACl/OH:0.13 Na2O:0.26 TMAOH: 60H2O: (0-0.05) V2O5. The solid products are filtered, repeatedly washed with deionized water, and dried in air. These samples are designated as VMCM-41-(X), where X is the Si/V ratio in the gel, or MCM-41 for purely siliceous MCM-41. Calcination is performed in air at 550 °C for 24 h to remove the organic material. For thermal reduction, samples are evacuated and heated at increasing temperature. CO reduction is performed on dehydrated samples under 50 Torr of CO at 120 °C. Characterization. The elemental composition of the resultant solid products was analyzed by EMPA using a Jeol JXA8600 spectrometer. The results are given as Si/V ratios in Table 1. The Na/Si ratio is 0.001. Powder XRD patterns were collected before and after calcination using a Philips 1840 powder diffractometer with Cu KR radiation (40 kV, 25 mA) at 0.025° step size and 1 s step time over the range 1.5° < 2θ < 15°. The samples were prepared as thin layers on aluminum slides. FTIR measurements were performed on a Nicolet 740 FTIR spectrometer using the KBr self-supported pellet technique. The pellets contained about 1% of finely powdered sample and were pressed at 4 t/cm2. The diffuse reflectance UV-vis spectra were measured with a Perkin-Elmer 330 spectrophotometer equipped with a 60 mm Hitachi integrating sphere accessory. Powder samples were loaded in a quartz cell with Suprasil windows, and spectra were collected in the 200-1000 nm wavelength range against a quartz standard. For ESR measurements, samples were loaded into 3 mm o.d. by 2 mm i.d. Suprasil quartz tubes. ESR spectra were recorded at X-band at either 293 or 77 K on a Bruker ESP 300 spectrometer. The magnetic field was calibrated with a Varian E-500 gaussmeter. The microwave frequency was measured by a Hewlett-Packard HP 5342A frequency counter. Solid-state NMR spectra were recorded at 9.4 T using a Chemagnetics CMX-400 spectrometer. 29Si MAS spectra were measured at 79.45 MHz with 60° pulses and 600 s recycle delays using zirconia rotors 7.5 mm in diameter spun at 3 kHz. 51V static and MAS spectra were acquired at 105.1 MHz with very short (7 T) the second-order quadrupolar effect of the 51V nucleus is suppressed, and the static 51V NMR spectra are dominated by the chemical shift anisotropy which is diagnostic of the coordination geometry.16,34-36 A distorted octahedral V5+ species typically shows a nearly axially symmetric spectrum with the most intense chemical shift at δ⊥ in the range of -200 to -500 ppm relative to VOCl3, while a distorted tetrahedral V5+ species typically shows a rhombic anisotropic spectrum with the most intense chemical shift at δ⊥ in the range of -500 to -800 ppm36 relative to VOCl3. The large chemical shift anisotropy of V5+ is not averaged out completely by MAS, and the spectra of V5+ compounds are then dominated by a set of spinning sidebands separated by the rotation frequency with relative intensities approximating the static spectrum or a somewhat narrowed static spectrum. Our 51V NMR spectra were recorded at 9.4 T for both static and MAS conditions. Since the NMR spectral features are not much affected by the overall vanadium content, only the spectra with the highest signal-to-noise ratio from VMCM41-(10) are presented in Figures 9 and 10. The static spectrum of as-synthesized VMCM-41 is characterized by one fairly narrow 51V NMR resonance line at -559 ppm (Figure 9a). In addition, a narrow band at -1740 nm is usually observed which is due to 23Na. When the 51V NMR spectrum of the assynthesized VMCM-41 sample is recorded with MAS, a welldefined spinning sideband pattern centered at -622 ppm is observed (Figure 10a). The relatively narrower envelope of the resonance line at -622 ppm in the MAS spectrum indicates that the chemical shift anisotropy of V5+ is somewhat averaged by MAS. After calcination to decompose the surfactant and immediate examination by NMR after cooling, the resonance line at -559
19600 J. Phys. Chem., Vol. 100, No. 50, 1996
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Figure 10. 51V MAS NMR spectra of VMCM-41-(10) (a) assynthesized, (b) freshly calcined, and (c) calcined and hydrated. The isotropic lines are indicated by ppm values; the other lines are spinning sidebands.
ppm observed in a static spectrum of VMCM-41 becomes much broader, and the maximum occurs at -680 ppm (Figure 9b). The corresponding MAS spectrum reveals a strong spinning sideband pattern centered at -639 ppm which extends over the frequency range of the static spectrum (Figure 10b). The large line width in the static spectrum and the corresponding strong spinning sidebands in the MAS spectrum indicate that the V5+ species in calcined VMCM-41 has more chemical shift anisotropy than in as-synthesized VMCM-41 and is in a more distorted tetrahedral environment. Upon rehydration of calcined VMCM-41 in moist air the center of the anisotropic spectrum remains at -680 ppm consistent with tetrahedral V5+. In addition, an apparent line near -410 ppm seems to exist which can be tentatively assigned to an axially symmetric chemical shift value δ⊥ of a distorted octahedral V5+ species.36 This suggests additional coordination of water molecules to some of the V5+ ions and is consistent with the UV-vis observations discussed above. In the MAS spectrum (Figure 10c) the profiles of the spinning sidebands of V5+ are similar to the static pattern (Figure 9c). Both the static pattern and the strong spinning sidebands indicate significant chemical shift anisotropy and somewhat distorted environments for the V5+ species. The absence of a 51V NMR line near -300 ppm, which is characteristic of octahedral V5+ coordination in V2O5, indicates that the VMCM-41 materials are free of V2O5. The NMR spectrum of calcined VMCM-41 is similar to that for calcined VS-12,37 which shows a peak at -625 ppm. However, the one previous report17 of NMR for VMCM-41 with a low vanadium content of Si/V ) 60 gives a very much narrower line with components at -506 and -527 ppm for both static and MAS spectra in contrast to our results in Figures 9 and 10.
In the past few years various vanadium-substituted zeolitic materials such as vanadosilicalites VS-1 and VS-27-9 and vanadoaluminophosphates VAPO-5 and VAPO-1110,11 were studied because of their applications in selective oxidation reactions. Several spectroscopic techniques including UV-vis, ESR, and 51V NMR were employed to characterize the vanadium centers in these materials. Two overlapped, intense UV absorption bands are detected in the spectra of VS-132 and VAPO-5.10 Maxima at 290 and 340 nm were ascribed to colorless tetrahedrally coordinated V5+ ions as monomers and as one-dimensional chains, respectively.32 Only the 290 nm band occurs when the V5+ ions are located in a relatively symmetric environment of four nearly equivalent oxygen ligands.38 The 340 mn band was interpreted as due to chains of vanadia tetrahedra in the framework.32 Assuming this assignment to be valid, the 340 nm band should correspond to a higher vanadium concentrations and should increase relatively with increasing vanadium content. In fact, the opposite is observed in Figure 4, which does not support this earlier assignment.32 Also, the observed spectral changes upon calcination in air seem inconsistent with this assignment. ESR spectra show two axially symmetric signals I and II with different g|| values in both VS-19,32 and VAPO-5.10,11 These two signals were ascribed to two kinds of VO2+ species in square pyramidal environments. But no more specific assignments were made for signals I and II. 51V NMR spectra in vanadium-containing molecular sieve materials vary depending on the specific materials. In a high magnetic field (>7 T) a broad line (200-1000 ppm line width at half-height) with maxima in the range of -500 to -800 ppm vs VOCl3 was identified in VS-1,32,38 VS-12,37 and several supported vanadium oxide catalysts16,34-36 and was ascribed to a V5+ species in a distorted tetrahedral environment. In contrast, 51V NMR spectra in VZSM-48,39 VS-2,19,40 and VMCM-41 with Si/V ) 6017 show a narrow resonance line (20-100 ppm line width at half-height) centered in the range -500 to -580 ppm. MCM-41 synthesized with cetyltrimethylammonium chloride as a structure director has uniform one-dimensional channels of ∼30 Å diameter with ∼12.5 Å thick tubular walls.41 The walls do not contain significant micropores accessible by molecules larger than N2 since the BET surface area measured by N2 isotherms is totally accounted for by the 30 Å channels.21,41 27Al MAS NMR of AlMCM-41 showed that the 27Al has a tetrahedral environment which is apparently distorted by a wide range of Si-O-Si bond angles as supported by 29Si MAS NMR.18 Vanadium ions substituted into the MCM-41 framework should also have a tetrahedral environment. In addition, 29Si MAS NMR suggests that about 20% of the silicons are silanols in calcined siliceous MCM-41.21,22 The silanol positions are mostly located on the wall surfaces and may serve as a possible locations for vanadium ion incorporation into VMCM41. Based on this description of the MCM-41 structure along with our UV-vis, ESR, and 51V NMR results as summarized in Table 2, a clear picture concerning vanadium ion coordinations and locations in VMCM-41 can be described as follows. In as-synthesized VMCM-41 samples, most of the incorporated vanadium exists as V5+ ions in two different tetrahedral environments, one inside the hexagonal tubular walls and the other on the surface of the hexagonal tubular walls. The wall location enables those V5+ ions to achieve higher coordination numbers than four and their coordination state can be changed by exposure to other coordinating ligands such as water molecules. This is supported by the observation of a relatively symmetric 51V NMR line for tetrahedrally coordinated V5+ ions
Vanadosilicate Mesoporous MCM-41 Molecular Sieves TABLE 2: Spectroscopic Parameters of Vanadium Species in VMCM-41 spectrum UV-vis
parameters 275 nm 340 nm
ESR
NMR
I (g| ) 1.945, A| ) 191 G) II (g| ) 1.936, A| ) 189 G) -560 to -680 ppm
assignment tetrahedral V5+ inside the walls tetrahedral V5+ on the wall surfaces square pyramidal VO2+ inside the walls square pyramidal VO2+ on the wall surfaces tetrahedral V5+ at both sites
near -560 ppm (static) and -620 ppm (MAS) and two UV absorption bands of tetrahedrally coordinated V5+ ions at 275 and 340 nm. These two UV absorption bands can be interpreted as due to two different types of V5+ ions as supported by their differential intensities with vanadium concentration (Figure 4). The 340 nm band is assigned to a V5+ species on a channel wall. Exposure to moist air causes color and spectral changes (Figure 5) consistent with increasing coordination from water to form square pyramidal and then octahedral geometries. The other V5+ species, corresponding to the 275 nm band, is assigned to a location inside the hexagonal tubular walls where it is more stable to thermal or chemical treatment. This assignment is consistent with the UV-vis result previously reported16 which showed that cubic silica MCM-48 material with surface grafted vanadium centers gives only a 340 nm band. No ESR signal from tetrahedrally coordinated V4+ ions is observed which usually exhibits quite different parameters and is detectable only at or below 77 K.42 However, two ESR signals I and II of vanadyl VO2+ species with square pyramidal coordination are observed. They exhibit differential stability upon thermal or chemical treatment and are thus assigned to different locations inside (signal I) and on the surface (signal II) of the tubular walls of MCM-41. Upon calcination in air and without subsequent exposure to moist air, only the square pyramidal vanadyl VO2+ ions on the wall surfaces (signal II) are oxidized to tetrahedral V5+ ions. This process is supported by a color change from green to white and by the disappearance of ESR signal II. The other vanadyl VO2+ species (ESR signal I) remains stable within the walls on calcination. After exposure of calcined VMCM-41 samples to moist air, the tetrahedrally coordinated V5+ ions located on the wall surface transform from distorted tetrahedral (340 nm band) to square pyramidal (415 nm band) and then to distorted octahedral coordination (450 nm band) as they are coordinated by more water molecules. This transformation corresponds to the color changes from white to yellow and then orange. The other V5+ species inside the walls are stable toward additional water coordination. When calcined VMCM-41 samples are subjected to thermal and CO reduction in vacuum, only the V5+ ions on the wall surfaces are reduced to VO2+ ions. The other V5+ and VO2+ species inside the walls are stable and ESR signal I of VO2+ remains observable. This indicates that in VMCM-41 only those vanadium centers on the wall surfaces are able to undergo redox cycles at high temperature. Conclusions Vanadium-substituted mesoporous VMCM-41 molecular sieves that contain atomically dispersed vanadium centers have been synthesized. UV-vis, ESR, and 51V NMR reveal that vanadium occurs in VMCM-41 simultaneously in two oxidation states and coordinations, namely, two tetrahedral V5+ species and two square pyramidal VO2+ species. The two
J. Phys. Chem., Vol. 100, No. 50, 1996 19601 different types of vanadium species suggest that vanadium is incorporated into two different framework sites, one inside the wall and the other on the wall surface of the MCM-41 structure. Only those vanadium ions located on the wall surface can achieve higher coordination than four and can carry out reversible redox cycles. Acknowledgment. This research was supported by the Robert A. Welch Foundation, the University of Houston Energy Laboratory, and the National Science Foundation. We thank M. Narayana for discussions of the NMR spectra. References and Notes (1) Clerici, M. G. In Heterogeneous Catalysis and Fine Chemicals III; Guisnet, M., Barbier, J., Barrault, J., Bouchoule, C., Duprez, D., Perot, G., Montassier, C., Eds.; Studies in Surface Science and Catalysis, Vol. 78; Elsevier: Amsterdam, 1993; pp 21-33. (2) Corma, A.; Iglesias, M.; Sa´nchez, F. J. Chem. Soc., Chem. Commun. 1995, 1635. (3) Camblor, M. A.; Corma, A.; Martı´nez, A.; Pe´rez-Pariente, J.; Primo, J. In Heterogeneous Catalysis and Fine Chemicals III; Guisnet, M., Barbier, J., Barrault, J., Bouchoule, C., Duprez, D., Perot, G.; Montassier, C., Eds.; Studies in Surface Science and Catalysis, Vol. 78; Elsevier: Amsterdam, 1993; pp 393-399. (4) Tuel, A. Zeolites 1995, 15, 236. (5) Taramasso, M.; Perego, G., Notari, B. U.S. Patent 4 410 501, 1983. (6) Notari, B. Catal. Today 1993, 18, 163. (7) Kornatowski, J.; Wichterlova´, B.; Jirkovsky, J.; Lo¨ffler, E.; Pilz, W. J. Chem. Soc., Faraday Trans. 1996, 92, 1067. (8) Hari Prasad Rao, P. R.; Kumar, R.; Ramaswamy, A. V.; Ratnasamy, P. Zeolites 1993, 13, 663. (9) Rigutto, M. S.; Van Bekkum, H. Appl. Catal. 1991, 68, L1. (10) Weckhuysen, B. M.; Vannijvel, I. P.; Schoonheydt, R. A. Zeolites 1995, 15, 482. (11) Bellussi, G.; Rigutto, M. S. In AdVanced Zeolite Science and Applications; Jansen, J. C., Sto¨cker, M., Karge, H. G., Weitkamp, J., Eds.; Studies in Surface Science and Catalysis, Vol. 85; Elsevier: Amsterdam, 1994; pp 177-213. (12) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (13) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, 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. (14) Abe, T.; Taguchi, A.; Iwamoto, M. Chem. Mater. 1995, 7, 1429. (15) Luca, V.; MacLachlan, D. J.; Hook, J. M.; Withers, R. Chem. Mater. 1995, 7, 2220. (16) Morey, M.; Davidson, A.; Eckert, H.; Stucky, G. Chem. Mater. 1996, 8, 486. (17) Reddy, K. M.; Moudrakovski, I.; Sayari, A. J. Chem. Soc., Chem. Commun. 1994, 1059. (18) Luan, Z.; Cheng, C.-F.; Zhou, W.; Klinowski, J. J. Phys. Chem. 1995, 99, 1018. (19) Chang, T. -H.; Leu, F. -C. Zeolites 1995, 15, 496. (20) Feuston, B. P.; Higgins, J. B. J. Phys. Chem. 1994, 98, 4459. (21) Chen, C-Y.; Li, H-X.; Davis, M. E. Microporous Mater. 1993, 2, 17. (22) Kolodziejski, W.; Corma, A.; Navarro, M.-T.; Perez-Pariente, J. Solid State NMR 1993, 2, 253. (23) Sohn, J. R.; Decanio, S. J.; Lunsford, J. H. Zeolites 1986, 6, 225. (24) Corma, A.; Navarro, M. T.; Pariente, J. P. J. Chem. Soc., Chem. Commun. 1994, 147. (25) Thangaraj, A.; Kumer, R.; Ratnasamy, P. Appl. Catal. 1990, 57, L1. (26) Corma, A.; Navarro, M. T.; Pe´rez-Pariente, J.; Sa´nchez, F. In Zeolite and Related Microporous Materials: State of the Art 1994; Weitkamp, J., Karge, H. G.; Pfeifer, H., Ho¨lderich, W., Eds.; Studies in Surface Science and Catalysis, Vol. 84; Elsevier: Amsterdam, 1994; pp 69-75. (27) Lo´pez, A.; Tuilier, M. H.; Guth, J. L.; Delmotte, L.; Popa, J. M. J. Solid State Chem. 1993, 102, 480. (28) Lischke, G.; Hanke, W.; Jerschkewitz, H. G.; O ¨ hlmann, G. J. Catal. 1985, 91, 54. (29) Schrami-Marth, M.; Wokaun, A.; Pohl, M.; Krauss, H. -L. J. Chem. Soc., Faraday Trans. 1991, 87, 2635. (30) Eon, J. G.; Olier, R.; Volta, J. C. J. Catal. 1994, 145, 318. (31) Dutoit, D. C. M.; Schneider, M.; Fabrizioli, P.; Baiker, A. Chem. Mater. 1996, 8, 734. (32) Kornatowski, J.; Wichterlova´, B.; Rozwadowski, M.; Baur, W. H. In Zeolite and Related Microporous Materials: State of Art 1994;
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