Insight into Photoactive Sites for the Ethylene Oxidation on

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Insight into Photoactive Sites for the Ethylene Oxidation on Commercial HZSM-5 Zeolites with Iron Impurities by UV Raman, X-ray Absorption Fine Structure, and Electron Paramagnetic Resonance Spectroscopies Guiyang Yan, Jinlin Long, Xuxu Wang,* Zhaohui Li, Xinchen Wang, Yiming Xu, and Xianzhi Fu* Research Institute of Photocatalysis, Fuzhou UniVersity, Fuzhou 350002, China ReceiVed: September 26, 2006; In Final Form: January 31, 2007

This study investigated the chemical nature of impurity iron species and photoactive sites in the commercially available HZSM-5 zeolites with different Si/Al ratios. The samples were characterized by X-ray fluorescence spectroscopy, atomic absorption spectroscopy, X-ray absorption near-edge structure spectroscopy, extended X-ray absorption fine structure spectroscopy, electron paramagnetic resonance spectroscopy, UV Raman spectroscopy, and photocatalytic degradation of ethylene. The results revealed that the photocatalytic activity of the HZSM-5 originated from the isolated tetrahedrally coordinated iron-oxo species present in the zeolite framework. The formation of such active isolated iron-oxo species was strongly influenced by the chemical compositions (e.g., Si/Al ratio, and iron content) and the preparation methods of zeolite. This could explain the difference in photoactivity of the different types of HZSM-5 and FeZSM-5 zeolites. Possible mechanisms of the photocatalysis on the zeolites were also discussed.

1. Introduction Zeolites are crystalline microporous aluminosilicates consisting of tetrahedral SiO4 and AlO4 as basic building units. Traditional applications of zeolites involve catalysis, adsorption, and separation. Recently, they have also been found to be excellent catalysts for photooxidation and photosynthesis reactions. For instance, Kato et al.1 reported that H-mordenite and HZSM-5 zeolites showed catalytic activity for the nonoxidative coupling of methane to C2H6 under UV irradiation. Anpo et al.2 found that zeolites modified with metal ions (Cu+, Ag+, and Pr3+) and transition metal oxides (TiO2, V2O5, Mo2O3, and Cr2O3) exhibited high photocatalytic activities for various reactions, including the decomposition of NOx and the reduction of CO2. Sun et al.3 observed that toluene could be oxidized to benzaldehyde by O2 on zeolite Y under visible light irradiation. Panov et al.4 demonstrated that toluene and p-xylene could be selectively oxidized to benzaldehyde and p-tolualdehyde by O2 on several cation-exchanged zeolites (such as NaY, BaX, and BaY) under visible light irradiation. Unlike semiconductor photocatalysts (e.g., TiO2, ZnO, SrTiO3), aluminosilicate zeolites are insulators with a band gap of about 7 eV5-7 and cannot be excited, not even by UV light, during photochemical reactions. Therefore, the mechanism of the photocatalytic process occurring in zeolites is intriguing, but it has not been unambiguously understood. Kato et al. suggested that the highly isolated Al-O units were the photoactive sites of H-form zeolites.1 Anpo et al. proposed that charge-transfer pairs [Me(n-1)+-O-]* formed upon light excitation led to the formation of photocatalytically active species.2 The photooxidation of toluene and p-xylene over cationexchanged Y zeolites probably resulted from the photogenerated charged-transfer complex stabilized within zeolite channels.3,4 * Corresponding author. Tel.: +86 591 83779251. Fax: +86 591 83738608. E-mail address: [email protected] (X. Wang), xzfu@ fzu.edu.cn (X. Fu).

In our previous work, we also found that commercial HZSM-5 zeolite was highly active for photocatalytic oxidation of some organic compounds, and impurity iron in the zeolite was responsible for the photocatalytic reactions.8 This work aims to find a correlation between the photocatalytic activity and the chemical nature, particularly coordination chemistry, of Fe species in the HZSM-5. The results revealed that the photocatalytic activity of the iron-containing zeolites is derived from the isolated iron-oxo species presented in the zeolite framework. 2. Experimental Section 2.1. Catalysts Synthesis. The NaZSM-5 zeolites were purchased from Nankai University, China, and Zeolyst International Ltd., USA. The HZSM-5 samples prepared from the two types of commercial NaZSM-5 were denoted as HZSM5-NUx and HZSM-5-ZLx, respectively. The number x stands for Si/Al ratio of the samples. FeZSM-5 zeolites were prepared by hydrothermal and ion exchange methods and were denoted as FeZSM-5-HT-n and FeZSM-5-EX-n, respectively. The number n is the serial number of the samples with different iron content. FeZSM-5-HT-n samples were synthesized by using a conventional procedure.9 Tetrapropylammonium hydroxide (TPAOH) was used as template, and water glass (modulus 3.2.), Al (NO3)3‚9H2O, and Fe(NO3)3‚9H2O were used as the sources of silicon, aluminum, and iron, respectively. The hydrothermal synthesis was performed in a Teflon-lined stainless steel autoclave at 180 °C for 72 h without agitation. The resulting solid was separated by filtration, washed with distilled water, and dried at 110 °C for 2 h. These as-synthesized samples were calcined in air at 550 °C for 10 h to remove the template and then converted into the H-form by ion exchange with NH4NO3 (0.1 M) and subsequent calcination in air at 550 °C for 5 h. The FeZSM-5-EX-n samples were prepared by ion exchange of the HZSM-5-NU50 mentioned above with a FeCl3 solution.

10.1021/jp066314b CCC: $37.00 © 2007 American Chemical Society Published on Web 03/09/2007

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TABLE 1: Chemical Composition and Photocatalytic Activity of the Zeolites samples

sample no.

Si/Al ratio

XRF (wt %) Al

AAS (wt %) Fe

ethylene conv (%)

commercially available HZSM-5

HZSM-5-NU25 HZSM-5-NU38 HZSM-5-NU50 HZSM-5-ZL60 HZSM-5-ZL100 HZSM-5-ZL160 FeZSM-5-HT-1 FeZSM-5-HT-2 FeZSM-5-HT-3 FeZSM-5-HT-4 FeZSM-5-HT-5 FeZSM-5-EX-1 FeZSM-5-EX-2 FeZSM-5-EX-3 FeZSM-5-EX-4 FeZSM-5-EX-5 FeSiO4-100 FeSiO4-78 FeSiO4-39

25 38 50 60 100 160 36 36 36 36 36 50 50 50 50 50 ∝ ∝ ∝

2.3 1.7 1.2 2.2 1.1 0.7 1.3 1.2 1.5 1.2 1.1 1.2 1.2 1.1 1.2 1.2 0 0 0

0.043 0.050 0.057 0.022 0.024 0.017 0.82 0.37 0.21 0.15 0.060 0.21 0.44 0.60 0.81 1.0 0.9 1.1 2.0

20.2 29.6 51.9 10.9 13.8 15.8 5.35 13.3 20.9 29.0 37.9 37.9 46.8 36.8 36.3 43.4 HZSM-5-NU38 > HZSM-5-NU50 > FeSiO4-100. This can be explained by the difference in the extent of hybridization of Fe p with Fe d states and hence difference in the number, nature, and symmetry of the nearest neighbors.18,20 The difference between the HZSM5-NUx samples and R-Fe2O3 or FeSiO4-100 can also be seen in the edge shape and the postedge region (B, C, D, and E). k3-weighted EXAFS Fourier transformation spectra of R-Fe2O3 and HZSM-5-NUx samples are shown in Figure 5. The feature at ca. 1.6 Å is attributed to Fe-O backscattering in the first coordination sphere. Two peaks at 2.4-3.4 Å observed for R-Fe2O3 originate from two different kinds of Fe-O-Fe,21 and the wide scattering at about 4 Å is assignable to large oxide particles.17 In contrast to R-Fe2O3, all HZSM-5-NUx samples show one major peak at ca. 1.6 Å. This peak becomes stronger with increasing Fe content, and its centroid shifts appreciably toward longer radial distance, in the order HZSM-5-NU50 > HZSM-5-NU38 > HZSM-5-NU25. This reflects the difference in the iron coordination number of the first coordination sphere in three samples. Some RSF peaks at 2-6 Å may include contributions from Fe-Si, Fe-Fe, and Fe-Al back scatter-

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Figure 7. EPR spectra of the HZSM-5-NUx samples. Figure 5. Fourier transformations of k3-weighted Fe K-edge EXAFS spectra (uncorrected) of the HZSM-5-NUx samples and the conference materials (k ) 2.6-13.6 Å-1 for R-Fe2O3; k ) 2.6-12.0 Å-1 for HZSM-5-NUx samples).

TABLE 2: Fitting Results for the EXAFS Spectra of HZSM-5-NUx and Reference Samples (Coordination Number (CN), Shell Radius (R), and Debye-Waller Factors (Σ, Å2) samples R-Fe2O3

HZSM-5-NU25 HZSM-5-NU38 HZSM-5-NU50 FeZSM-5-HT-1 HZSM-5-EX-2 FeSiO4-100

Figure 6. Experimental (solid line) and fitted (hollow dotted line) Fourier transform EXAFS spectra of the HZSM-5-NUx sample. The inset is experimental and fitted K3-weighted scattering functions.

ings,19 but it is very difficult to distinguish between these backscatterers and noise because of the extremely low content of iron in HZSM-5-NUx samples. The best fitting results of the Fourier-filtered EXAFS spectra are given in Table 2. As for the HZSM-5-NU50 sample, the presence of the majority of Fe3+ in the isolated state can be inferred, since the average coordination number (4.5) is closely similar to that (4.0) for the FeSiO4-100 with MFI structure. But, interestingly, the Fe-O distance (2.03) of the HZSM-5-NU50 is far larger than that (1.85) of the FeSiO4-100. This suggests that they belong to two different types of framework irons in a tetrahedral configuration. One of the most reasonable explains to the results is that Fe of these two samples is located at the different T positions in the framework, since the existence of Al atoms in the zeolite framework has an important effect on the geometric structure of tetrahedral Fe. Furthermore, it is interesting to note that the coordination numbers of iron in the HZSM-5-NUx samples increase with the increasing Al content, while the bond distance of Fe-O is almost the same. The higher coordination number of iron in the HZSM-5-NU38 and NU25 may arise from much aggregated Fe species. The discrepancy among the Fe states of the HZSM-5-NUx samples might result

shell

CN (10%)

R (Å) (0.01 Å)

Σ2 (Å2) (10%)

E0 shift (10%)

Fe-O Fe-O Fe-Fe Fe-Fe Fe-Fe Fe-O Fe-O Fe-O Fe-O Fe-O Fe-O Fe-O

3.0 3.0 3.0 3.0 6.0 6.6 5.1 4.5 4.0 4.0 5.1 4.0

1.95 2.10 2.95 3.35 3.68 2.04 2.06 2.03 1.88 2.03 2.00 1.85

0.0024 0.0043 0.0039 0.0041 0.0081 0.014 0.0069 0.0041 0.0066 0.0063 0.0099 0.0062

17.9 10.3 11.0 6.3 10.5 15.0 17.5 16.5 12.7 1.3 10.8 14.9

from different Si/Al ratio and high-temperature pretreatment. Upon calcination, some iron ions escape from the inside of the framework to the outside due to their low stability in the zeolite framework,22 especially when the Si/Al ratio of zeolite is low.20 When iron cations migrate to the surface, they capture some hydroxyls or water molecules, leading to a higher coordination number around Fe and a shorter Fe-O distance. For the HZSM5-NU50 with low Al content, all of the iron ions are in the framework and tetrahedrally coordinated with SiO4 units or hydroxyls even at a higher Fe content. For the HZSM-5-NU38 and HZSM-5-NU25 with higher Al content, iron ions can exist partly outside the framework even at a lower Fe content. Unfortunately, the current EXAFS characterizations cannot distinguish among these iron species. The characterizations given here remain incomplete for the identification of the states of all iron ions, and some questions remains unanswered, including how many iron ions are the isolated species tetrahedrally coordinated to the zeolite framework oxygen. As a result, it is difficult to establish a relationship of the photocatalytic activity with the amount of tetrahedrally coordinated iron species inside the HZSM-5 zeolite framework. But this characterization, in combination with the above activity results, finds that the change in structure of iron species is closely correlated with the photocatalytic activity, namely, the photocatalytic activity increases with the decrease in the coordination numbers of iron in the HZSM-5-NUx samples. This suggests that the highly isolated iron-oxo species in tetrahedral coordination is the photoactive site for the ethylene oxidation on these commercial HZSM-5 zeolites. 3.5. Electron Paramagnetic Resonance (EPR) Studies. Figure 7 shows the EPR spectra of HZSM-5-NUx samples at 77 K. A sharp increase in intensity of EPR lines is observed

5200 J. Phys. Chem. C, Vol. 111, No. 13, 2007 with the increase in the iron content from 0.043% (HZSM-5NU25) to 0.057% (HZSM-5-NU50). The tremendous difference in the signal intensities illuminates that a great deal of iron may be nonparamagnetic oxide clusters in the HZSM-5-NU25 and NU38 samples. All the samples show several typical signals at g ) 2.0, 4.3, 5.6, and 7.0. It is interesting to note that a significant difference in the EPR signals at g ) 2.1-2.6 is observed for three HZSM-5-NUx samples. These signals are strong and broad for both HZSM-5-NU25 and HZSM-5-NU38, whereas for the HZSM-5-NU50 the signals are absent. The signals at g ) 2.1-2.6 are definitely assigned to the iron ions in the interstitial oxide or hydroxide phases according to the literature.23,24 This indicates that such iron species are formed in the HZSM-5-NU25 and HZSM-5-NU38 at a certain level but not in the HZSM-5-NU50. This result is consistent with those obtained from EXAFS (section 3.3) that some interstitial iron oxide or hydroxide phases coexist with tetrahedrally coordinated iron species in the HZSM-5-NU25 and HZSM-5NU38 samples. The assignment to these two signals at g ) 2.0 and 4.3 has been controversial,17,19,21 but a typical understanding, the signal at g ) 4.3 is assignable to the isolated iron species in tetrahedral coordination,24,25 is accepted extensively. The signal at g ) 2.0 is either attributed to isolated extraframework ferric at cationic positions or indicative of framework iron.18,23,26-28 For these three HZSM-5-NUx samples, the change in intensity of the signal g ) 2.0 is irregular, indicating that the signal at g ) 2.0 is not responsible for the photocatalytic activity, whereas the intensity of the signal at g ) 4.3 increases with increasing Fe content, indicating that the amount of the isolated iron species in tetrahedral coordination increases in the order HZSM-5-NU50 > HZSM-5-NU38 > HZSM-5-NU25. In addition, the strong signal of g ) 3.4 observed for the HZSM-5-NU50 sample, originating from a symmetry ligand field,29-32 also may indicate that an iron species is inside the zeolite framework. This, in combination with the photocatalytic activity, supports strongly the conclusion from the XAFS results that the highly isolated iron-oxo species in tetrahedral coordination is the photoactive site of the ZSM-5 zeolite. 4. Discussion Because of its promising activities in numerous reactions, FeZSM-5 has been well characterized over the past decade and different forms of iron species have been identified.33-38 Compared to FeZSM-5, HZSM-5 samples contain very low content of iron, and the chemical state of the iron ion has been less studied in the literature. According to the above results, in combination with the literature, it was concluded that three types of iron species exist in the commercial HZSM-5, as depicted in Figure 8. Type i is isolated iron ions with a tetrahedral coordination configuration. Type ii is located at ion-exchangeable sites, whereas type iii is located outside the framework of zeolite. The content of different iron species in the zeolites depends not only on the total content of iron in the zeolites but also on Si/Al ratio. In the HZSM-5-NU50, types i and ii of Fe species are predominant. For the HZSM-5-NU25 and NU38, besides the isolated iron species, a few interstitial iron oxides and hydroxides also exist. According to the UV Raman spectra and the EXAFS results, the photocatalytic activity of HZSM-5 is strongly dependent on the type of isolated tetrahedral coordinated iron species in the zeolite framework. On the basis of the above analysis, it is easy to explain the difference in the photocatalytic activity of three HZSM-5-NU samples by the amount of the isolated iron species. The

Yan et al.

Figure 8. Possible local structure of iron species in the HZSM-5.

difference between the photocatalytic activity of the FeZSM-5 and the HZSM-5 can also be explained by the above models (Figure 8). The iron ions introduced to the zeolites by the ion exchange method have no effect on the photocatalytic behavior of the zeolites. The introduced Fe species can combine to aluminum sites in the framework via oxygen bridges to give additional iron species like types ii and iii, without increasing the number of the photocatalytic site like type i. It suggests that both types ii and iii of iron species are not responsible for the photocatalytic reaction. For the hydrothermally synthesized FeZSM-5-HT-n samples with higher iron content than the commercial HZSM-5 zeolites, unexpectedly, the photocatalytic activity decreases with increasing iron content. When parts a and c of Figure 1 are compared together, it is estimated that the maximum photocatalytic activity over the iron-containing ZSM-5 occurs at ca. 0.05-0.06 wt % of iron content. Above this limit, the states of iron species become complicated upon thermal treatment. Iron species in the FeZSM-5-HT-n samples exist in types i and ii states. The majority of iron ions incorporated inside the zeolite framework can migrate easily into extraframework position upon calcination to form the type ii of iron species, or to connect with the originally isolated iron species by oxygen bridges, leading to destruction of photoactive sites. This is a possible reason why the introduced iron species by the hydrothermal method does not enhance the photocatalytic activity. However, it seems hard to understand why the Al-free Fe silicates with high amount tetrahedral Fe exhibit a negligible photoactivity for the oxidation of ethylene according to the above analysis. Among these Fe-containing MFI samples, one should distinguish FeZSM-5 from Fe silicate, the former contains Si, Al, and Fe atoms in T positions, while the latter has only Si and Fe atoms.39 The geometric structure of tetrahedral iron in the FeZSM-5 zeolites and the commercial HZSM-5 with impurity iron is tremendously different from that in the Fe silicates, as demonstrated by the EXAFS results. The Fe-O distance (2.03) of FeO4 in the HZSM-5-NU50 is far larger than that (1.85) in the Fe silicate with a Si/Fe ratio of 100 (FeSiO4-100). Two types of tetrahedral Fe can exist in the FeZSM-5-HT-1, one of which is consistent with that in the HZSM-5-NU-50 sample, and the

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Figure 9. Suggested mechanism for photocatalytic oxidation of ethylene on HZSM-5.

other is in quite good agreement with that in the FeSiO4-100 sample. It suggested that the tetrahedral Fe with a Fe-O distance of 2.03 Å in the zeolite framework is responsible for the photocatalytic reaction, while the tetrahedral Fe with a Fe-O distance of 1.88 Å is photocatalytically inactive. This could well explain the tremendous difference in the photoactivity between the Al-free Fe silicates and the HZSM-5-NU-50 as well as the FeZSM-5-HT-1. The observation that the Al-free Fe silicate sample with high amount tetrahedral Fe presented low photocatalytic activity implies that the presence of Al atoms inside zeolite framework is necessary for the photocatalysis. This might lead some authors to propose that AlO4 species are photoactive sites for the photoinduced nonoxidative coupling of methane. Although no direct correlation between the photocatalytic activity and the Si/Al ratio or the Al content can be found, the role of Al species, herein, needing to be investigated further, seems also to be pivotal for understanding the photocatalytic activity of zeolite. Thus, it was concluded that the photocatalytic activity of HZSM-5 is indeed strongly dependent on Fe in specific framework geometry. The isolated tFe-O-Si-OAlt species in the zeolite framework can be a photoactive species. It is considered that photoinduced reaction of organic compounds on a semiconductor, such as TiO2, is initiated by the electron-hole pairs formed by UV light excitation. Fast interfacial charge-carrier transfer to reactants can be crucial for a photocatalytic reaction. Analogously, photocatalysis on the ZSM-5 having photocatalytically active framework is also a dynamic process. The Fe-O of isolated [FeO4] units may be excited under 254 nm light irradiation to produce an excitation state of [Fe2+-O-]*, as shown by UV Raman (Figure 2) and diffuse reflectance spectroscopy spectra.8 As special electronhole pair states, the [Fe2+-O-]* may activate both O2 and ethylene molecules, leading to the oxidation of ethylene to CO2 and H2O (Figure 9). This mechanism, based on the suggestion by Anpo et al.,2 can be well applied to explain the photocatalytic behavior on metal-contained zeolites. But, the generation of [Fe2+-O-]* inside the framework of zeolite under the light irradiation does not necessary give a high conversion of ethylene. To maintain the effective separation of the electronhole pairs outside the framework of zeolite is essential, where the active C2H4 is located. Our previous work40 showed that the isolated TiO4 species existing on the surface of MCM-41 are photocatalytically active, while the TiO4 units inside the

framework can be not pivotal for photocatalysis. Therefore, the isolated [FeO4] units on surface of the zeolite can be responsible for the photocatalysis. Some studies attributed the photocatalysis of zeolite to the large electrostatic field of the cavities, which could lower the energy needed to activate dioxygen and hydrocarbon.35,41-44 Our study supported the mechanism based on the surface metal sites, especially emphasizing a pivotal role of the iron ion. 4. Conclusions The photocatalytic activity of ZSM-5 was strongly influenced by the chemical states of iron ions and the preparation methods of the zeolite. The commercial HZSM-5 zeolites were highly photocatalytically active for the oxidation of ethylene to H2O and CO2 under 254 nm UV light irradiation, and the activity increased with increasing impurity iron content. The added iron species by the ion exchange and hydrothermal methods did not enhance the photocatalytic activity of the zeolite. The ionexchanged FeZSM-5 presented a stable activity irrespective of Fe content, whereas the hydrothermally synthesized FeZSM-5 showed a decreasing activity with increasing Fe content. The Al-free Fe silicate samples exhibit almost negligible photocatalytic activity for the ethylene oxidation. Although the iron ions in the commercial HZSM-5 zeolites existed in the oxidation state of +3, their coordination chemistry was different and depended on the Si/Al ratio of the zeolites. For the HZSM-5-NU50 with a high Si/Al ratio of 50, the iron ions existed as the isolated species tetrahedrally coordinated to the framework oxygen. In the case of HZSM-5-NU25 and HZSM-5-NU38 with a low Si/Al ratio, some additional small interstitial oxide and hydroxide clusters coexisted with the isolated iron species. The photocatalytically active sites of the HZSM-5 and FeZSM-5 zeolites were the isolated tetrahedral coordinated iron species in the zeolite framework. The iron oxide and hydroxide clusters dispersed on the pore surface of the zeolite were not photocatalytically active. The well-defined tetrahedral coordinated iron sites in the Al free Fe silicate were photocatalytically inert. The suggested active site model could well explain the difference in the photocatalytic behavior among the samples. Acknowledgment. This work was supported by the National Foundation of Natural Science of China (Nos. 20373011,

5202 J. Phys. Chem. C, Vol. 111, No. 13, 2007 20573020, 20537010, 20673020), the National Key Basic Research Special Foundation (2004CCA07100), and the Fujian Province Foundation of Science (No. 2003F004). The authors thank Professor Yaning Xie, Beijing Photon Factory for the XAFS experiment, Professor Can Li and Dr. Jing Zhang, State Key Laboratory of Catalysis in Dalian Institute of Chemical Physics, Chinese Academy of Science, for UV Raman spectroscopy determinations, Professor Jincai Zhao, Laboratory of Photochemistry in the Institute of Chemistry, Chinese Academic of Science, for EPR analysis. Supporting Information Available: UV-vis reflection spectra of HZSM-5 and UV Raman spectra of commercial HZSM-5 samples. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Kato, Y.; Yoshida, H.; Satsuma, A.; Hattori, T. Microporous Mesoporous Mater. 2002, 51, 223. (2) Matsuoka, M.; Anpo, M. J. Photochem. Photobiol., C 2003, 3, 225. (3) Sun, H.; Blatter, F.; Frei, H. J. Am. Chem. Soc. 1994, 116, 7951. (4) Panov, A. G.; Larsen, R. G.; Totah, N. I.; Larsen, S. C.; Grassian, V. H. J. Phys. Chem. B 2000, 104, 5706. (5) Zheng, N.; Bu, X.; Wang, B.; Feng, P. Science 2002, 298, 2366. (6) Palmqvist, A. E. C.; Iversen, B. B.; Zanghellini, E.; Behm, M.; Stucky, G. D. Angew. Chem., Int. Ed. 2004, 43, 700. (7) Calzaferri, G.; Leiggener, C.; Glaus, S.; Schu¨rch, D.; Kuge, K. Chem. Soc. ReV. 2003, 32, 29. (8) Yan, G.; Wang, X.; Fu, X.; Li, D. Catal. Today 2004, 93-95, 851. (9) Argauer, R. J.; Kensington, Md.; Landolt, G. R.; Audubon, N. J. US Patent 3,702,886, Nov 14, 1972. (10) Pirutko, L. V.; Chernyavsky, V. S.; Uriarte, A. K.; Panov, G. I. Appl. Catal., A 2002, 227, 143. (11) Bordiga, S.; Buzzoni, R.; Geobaldo, F.; Lamberti, C.; Giamello, E.; Zecchina, A.; Leofanti, G.; Petrini, G.; Tozzola, G.; Vlaic, G. J. Catal. 1996, 158, 486. (12) Inui, T.; Nagata, H.; Takeguchi, T.; Iwamoto, S.; Matsuda, H.; Inoue, M. J. Catal. 1993, 139, 482. (13) Li, C. J. Catal. 2003, 216, 203. (14) Bordiga, S.; Damin, A.; Bonino, F.; Ricchiardi, G.; Zecchina, A.; Tagliapietrad, R.; Lamberti, C. Phys. Chem. Chem. Phys. 2003, 54, 390. (15) Ricchiardi, G.; Damin, A.; Bordiga, S.; Lamberti, C.; Spano`, G.; Rivetti, F.; Zecchina, A. J. Am. Chem. Soc. 2001, 123, 11409. (16) Li, C.; Xiong, G.; Xin, Q.; Liu, J.; Ying, P.; Feng, Z.; Li, J.; Yang, W.; Wang, Y.; Wang, G.; Liu, X.; Lin, M.; Wang, X.; Min, E. Angew. Chem., Int. Ed. 1999, 38, 2220. (17) Heinrich, F.; Schmidt, C.; Lo¨ffler, E.; Menzel, M.; Gru¨nert, W. J. Catal. 2002, 212, 157.

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