Visible-Light Induced High-Yielding Benzyl Alcohol-to-Benzaldehyde

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Visible-Light Induced High-Yielding Benzyl Alcohol-to-Benzaldehyde Transformation over Mesoporous Crystalline TiO2: A Self-Adjustable Photo-oxidation System with Controllable Hole-Generation Renhong Li,† Hisayoshi Kobayashi,*,‡ Junfang Guo,† and Jie Fan*,† †

Key Lab of Applied Chemistry of Zhejiang Province, Department of Chemistry, Zhejiang University, Hangzhou, Zhejiang Province 310027, People's Republic of China ‡ Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto, Japan ABSTRACT: High-yielding benzyl alcohol-to-benzaldehyde (BA-to-BAD) transformation has been achieved over mesoporous crystalline TiO2 at mild conditions under visible-light irradiation with/without molecular oxygen. DFT calculations show that the antibonding π molecular orbitals (MOs) of BA can hybridize with the O2p atomic orbitals (AOs) of TiO2, leading to the appearance of new energy levels located in the band gap where holes can be generated under visible-light irradiation. However, the oxidative product, BAD, is not stable when adsorbed on TiO2 surface. This fact is supposed to prevent BAD from overoxidation by ceasing the hole formation. As a result, the hole generation becomes controllable in this type of photoreaction system, and a so-labeled “self-adjustable photo-oxidation system” (denoted as SAPS) can be established. In addition, we also found that lattice oxygen (OL) and Ti atoms within TiO2 frameworks serve as the efficient reservoir for hydrogen and electrons in anaerobic reaction.

1. INTRODUCTION Selective oxidation of alcohols to aldehydes, in particular benzyl alcohol (BA) to benzaldehyde (BAD), is one of the most common organic transformations and is of fundamental importance for laboratory and commercial processes.1 As this reaction has been widely studied, it is an excellent benchmark for the study of possible new selective oxidation technologies. The desired selective oxidation product (BAD) is used chiefly in the synthesis of other organic compounds, ranging from pharmaceuticals to plastic additives, and is also highly valuable for the perfume industry. Currently, BAD is produced through stoichiometric oxidation by toxic and hazardous dichromate and permanganate salts in the laboratory, or by liquid phase chlorination and oxidation of toluene in industrial processes. In the wake of increasing concern about the environment, the BA-to-BAD transformation should preferably be accomplished via a highly selective and “green” oxidation process that is able to use molecular oxygen or other nontoxic compounds as the oxidant.2 In practice, the oxidation of alcohols can be performed in the liquid or gas phase, depending mainly on the thermal stability and volatility of the reagents and products. They both require relatively high reaction temperature/pressure and cannot be applied to a very wide range of substrates and types of oxidation transformation. A challenge is thus to develop an approach that is able to oxidize BA or other organic molecules under very mild conditions (room temperature, atmospheric pressure, and r 2011 American Chemical Society

neutral pH).3 Recently, photocatalysis using TiO2 materials has been proposed to address this problem.4
9 Zhao et al. reported that highly selective aerobic oxidation of alcohols can be achieved in a novel visible-light responsive coupled photoreaction system.5 In their system, dye-sensitized TiO2 was used as the visible-light absorbent. The high selectivity toward aldehydes owes to the nitroxyl radicals and TEMPO (2,2,6,6-tetramethylpiperidinyloxyl). More interestingly, Higashimoto et al. found that BA-to-BAD transformation proceeded effectively on naked TiO2 with high selectivity under visible-light in the presence of molecular oxygen.7
9 They suggested that the surface complex formed by adsorption of BA on TiO2 led to the visible-light absorption. The photoexcited holes are not capable of producing BAD without the assistance of oxygen molecules. However, in our previous work, we have shown that anaerobic oxidation of BA occurred on the surface of TiO2, and notably the resulting reconstructed surface is visiblelight responsive.10 In addition, it has also been reported that anaerobic photocatalytic oxidation of alcohols proceeds in TiO2based systems.11,12 As a matter of fact, we currently do not have a fundamental understanding of the origin of the exceptionally high-yield in visible-light induced BA-to-BAD transformation over TiO2 surface. Received: July 29, 2011 Revised: October 12, 2011 Published: October 17, 2011 23408

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Figure 2. Parallel experiments for the transformation of BA (0.2 M in acetonitrile) over different photocatalysts in air under visible-light (λ > 400 nm); the reaction time was 5 h.

Figure 1. (a) Barret
Joyner
Halenda (BJH) pore size distribution plots and N2 adsorption
desorption isotherm (inset) of mTiO2, (b) TEM image of mTiO2 (inset is HRTEM image), and (c) Wide-angle XRD pattern of mTiO2 (inset is small-angle XRD pattern).

Here, we show that the BA-to-BAD transformation can be achieved at exceptionally high yield (∼99%) upon visible-light irradiation over mesoporous TiO2 with/without molecular oxygen. The origin of the high yields is subsequently clarified by DFT calculations and experimental observations.

2. EXPERIMENTAL SECTION 2.1. Materials and Catalyst Preparation. Mesoporous TiO2 photocatalysts are prepared by the AcHE process.13,14 In a typical synthesis, 10 mmol of Ti(OBu)4, 40 mmol HOAc, 12 mmol HCl (or HNO3), and 1.6 g of F127 (EO96PO70EO96, MW = 12 000 g/mol) were dissolved in 30 mL ethanol. The mixture was stirred vigorously for 1 h and transferred into a Petri dish (diameter 125 mm). The ethanol was evaporated at 40 °C with a relative humidity of ∼40%. After 12 h, a transparent membrane was formed, and it was transferred into a 65 °C oven and aged for 24 h. As-synthesized mesostructured hybrids were calcined at 450 °C in air for 5 h (ramp rate 2 °C/min) in order to obtain

mesoporous membranes. N-TiO2 (TS-S4230) was obtained from Sumitomo Chemical (Japan). P25 TiO2 powders were supplied by Degussa Co. Ltd. Other commercial chemicals and photocatalysts were of the highest available grade and were used without further purification. 2.2. Analytical Methods. Nitrogen sorption analysis was carried out at 77 K using a Micrometrics TriStar 3000 system. Transmission electron microscopy (TEM) images were taken using a JEOL 2010 electron microscope operating at 200 keV. The samples for TEM were prepared by dispersing the final powders in ethanol; the dispersion was then dropped on carbon
copper grids. Low-angle XRD and wide-angle XRD patterns were recorded on a Bruker D8 diffractometer using CuKα radiation. XPS measurements were performed in a VG Scientific ESCALAB Mark II spectrometer equipped with two ultrahigh vacuum (UHV) chambers. All binding energies were referenced to the C1s peak at 284.8 eV of the surface adventitious carbon. 2.3. DFT Calculation. In order to get electronic interaction between BA or BAD and TiO2 surface, the plane wave based density functional calculations were carried out with Castep software.15,16 TiO2 surface was modeled by the (001) face of anatase, and consists of three Ti and six O layers including protruding O atoms on both sides. The O atoms in the bottom side were terminated by adding two hydrogen atoms, and the resulting atomic composition, Ti48O112H32 was grouped into [(TiO2)12(OH2)4]4. The lattice constants, a = b = 15.104 Å and α = β = γ = 90° were adopted based on the anatase crystalline data. As to the c direction, the lattice constant was set to 30 Å including the vacuum region. These lattice constants were fixed, and only the atomic coordinates were optimized. The Perdew, Burke, and Ernzerhof (PBE) functional17,18 was used together with the ultra soft core potentials.19 The basis set cutoff energy was set to 340 eV. The electron configurations of atoms are Ti: 3s2, 3p6, 4s2, 3d2, O: 2s2, 2p4, C: 2s2, 2p2, and H: 1s1. The numbers of occupied bands are 661 and 660 for Ti48O112H32
C6H5CH2OH and Ti48O112H32
C6H5CHO, respectively. 2.4. Photoinduced Reaction. The experiments were carried out in a photochemical reactor fitted with an immersed mercury lamp. An optical filter was adopted to provide visible-light with λ > 400 nm. The 0.1 mmol alcohols were added into a suspension of photocatalysts in different solvents (8 mg/mL) for 23409

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Figure 3. Unit cell structures for (a) Ti48O112H32
C6H5CH2OH and (b) Ti48O112H32
C6H5CHO.

Figure 4. Band dispersion and projected density of states (PDOS) for (a) Ti48O112H32
C6H5CH2OH and (b) Ti48O112H32
C6H5CHO.

photocatalytic oxidation. Anaerobic and aerobic reactions were carried out in a 50 mL quartz tube containing 5 mL of the above suspensions by bubbling with pure N2 and O2, respectively, for at least 30 min before visible-light irradiation. The process performance was followed by measuring the values of alcohol and product concentration by GC-FID and calculating the substrate conversion and the selectivity toward aldehyde (toluene was used as the internal standard). GC-TCD was employed for evaluating the percentage of mineralization, that is, the amount of CO2, and also for evaluating the amount of H2.

3. RESULTS AND DISCUSSION 3.1. Characterization Results. Figure 1a shows the pore size distributions and N2 adsorption
desorption isotherms (inset) of mTiO2. The isotherm is type IV with a H1 hysteresis loop, which is characteristic of mesoporous materials. The pore size distribution of the sample is broad, and the average size is determined to be 10.1 nm. The surface area and pore volume of mTiO2 are 85 m2/g and 0.13 cm3/g, respectively. The photocatalysts possess one diffraction peak in the low-angle

region, indicating the presence of partial mesoscopic periodicity (Figure 1a (inset)). It is known that wormhole-like mesostructures often display a single peak in low-angle XRD,20 which are apparent from TEM observations (Figure 1b). They are less regular in diameter and randomly packing in a three-dimensional manner. The high-resolution TEM (HRTEM) image (Figure 1b (inset)) clearly shows the crystal lattice of anatase phase. Wideangle XRD analysis (Figure 1c) reveals that the channel walls of mTiO2 consist of nanosized anatase crystals (JCPDS, No. 21
1272), which is consistent with the HRTEM image. The average crystalline size calculated from the broadening of the (101) XRD peak of anatase phase is 11.9 nm. 3.2. Photo-oxidation Using Various Photocatalysts. In control experiments, we found no obvious transformation of BA in the absence of photocatalysts under visible-light, and no reaction occurred in the dark with bubbling O2, as expected. Since the solvent/TiO2 system is not visible-light responsive, no apparent oxidation product of the solvent molecules (e.g., acetonitrille) is detected even after 20 h photoreaction. Figure 2 presents the results of BA oxidation under the same conditions over multiple photocatalysts. Interestingly, visible-light responsive photocatalysts, such 23410

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Figure 5. Projected density of states (PDOS) for (a) Ti atoms, (b) O atoms, and (c) C6H5CH2OH part in Ti48O112H32
C6H5CH2OH, and (d) for Ti atoms, (e) O atoms, and (f) C6H5CHO part in Ti48O112H32
C6H5CHO.

as WO3 and N-TiO2, performed much worse than that of mTiO2 photocatalysts that could only adsorb UV light by itself. It is noted that the product selectivity of N-TiO2 is rather low (∼80%) even the conversion of BA is only 4.5%, while the selectivity of mTiO2 maintains around 99% when the conversion exceeds 90%. The poor performances of visible-light photocatalysts possibly stem from their weak interactions with organic adsorbates, and the overoxidation of BAD by their excessive valence band holes. In particular, with respect to N-TiO2, visible-light induced hole generation is caused by the fixed impurity level originated from the nitrogen atom,21 thus the selective oxidation product—BAD is prone to be further oxidized into carboxylic acids or CO2 by excessive holes with high oxidative potential. In addition to the band structure and band gap position, other factors, such as the activity of surface atoms (or sticking probability), and quantum yield (probability of photon absorption), the degree of charge separation, and electron and hole migration capability are also important issues for the performance of photocatalysts. Some of these factors are unfortunately beyond the normal DFT calculations. In general, the photocatalytic activity of TiO2 mainly relates to the oxidative ability of photogenerated holes in its valence band.22 The hole transfers from the valence band to the molecules adsorbed on the TiO2 surface, which subsequently oxidizes the molecules into a final product (aldehydes, carboxylic acids, or CO2). It is worth noting that the alcohol-to-aldehyde

transformation is dominated by a two-electron-transfer mechanism.5 No more than two holes are required for the BA-to-BAD transformation via photocatalysis, which is much different from water oxidation,23 and certain organic pollutants decomposition.24 For selective alcohol oxidation, if electron
hole pairs generate incessantly upon photoexcitation (e.g., under UV irradiation), the partial oxidized product (BAD) will be inevitably overoxidized into carboxylic acid, and even CO2, leading to a poor partial oxidation selectivity.4,5,25,26 Therefore, to achieve high product selectivity for BA-to-BAD transformation over TiO2, the control of electron
hole pair generation process is of vital importance. To understand the high selectivity in visible-light induced BAto-BAD oxidation over TiO2, we carried DFT calculations to elucidate the possible electronic interaction between BA (or BAD) and TiO2 surface and the origin of the visible-light response. Figure 3 shows the unit cells as models of adsorption systems. Both molecules were adsorbed with a flat conformation (i.e., almost parallel to the surface plane). The adsorption energy was estimated to be 37 kJ mol
1 for C6H5CH2OH with respect to an isolated molecule and a free surface. For C6H5CHO, the structure was optimized but the stabilization was not obtained (negative adsorption energy). Since the evaluation of adsorption energies is rather delicate, we re-estimated the energy with another reference where the molecule is included in the cell, and floated at the midpoint of vacuum region. Under these circumstances, BA and 23411

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Figure 6. Electron density contour maps of orbitals at the VBM and CBM regions for Ti48O112H32
C6H5CH2OH where the LUMO is constructed from the Ti3d orbitals, and the HOMO forms density lobes in the benzene ring as well as the surface O atoms.

BAD are slightly stabilized 11 and 4 kJ mol
1, respectively. Thus, in both estimations, the interaction of BAD-to-TiO2 was weaker than that of BA-to-TiO2. These results suggest that the BAD molecules are likely to bounce off the TiO2 surface once BA-to-BAD transformation is completed, which is consistent with the interpretation by Higashimoto et al., even qualitatively.7 Figure 4 shows the band dispersion and the projected density of states (PDOS) of all atoms for Ti48O112H32
C6H5CH2OH and Ti48O112H32
C6H5CHO. The occupied bands are Ti3s, Ti3p, O2s, and O2p in the increasing order of the energy. For the O2s and O2p bands, small peaks in the lower energy side are due to BA and BAD molecules. The conduction band minimum (CBM) are constructed by the Ti3d orbitals (the conduction band is shown partly). Figure 5 shows the atom decomposed PDOS for Ti48 O112H32
C6H5CH2OH and Ti48O112H32
C6H5CHO. The Ti3d bands (Figure 4a,d) are almost the same between the two adsorption systems. Several differences can be found for the O atoms and the adsorbed molecules. An important difference appears at the valence band maximum (VBM). Compared to Ti48O112H32
C6H5CH2OH, the PDOS for Ti48O112H32
C6H5CHO is shifted to the lower energy side (Figure 4b,e), and the peak belonging to the O2p orbitals is depressed at the VBM. There is also a difference in the PDOS for the adsorbed molecule. The peak just below the VBM is depressed for Ti48O112H32
C6H5CHO. These differences are explained again in terms of the orbital density contour maps. Figures 6 and 7 show the electron density contour plots of orbitals at the VBM and CBM regions for Ti48O112H32
C6H5CH2OH and Ti48O112H32
C6H5CHO, respectively. In Figure 6, the orbital #659 is a typical valence band composed of the O2p orbital alone. However, in the next HOMO #660 and the HOMO #661, the electron densities are delocalized between BA and the surface O atoms. It implies that the electrons in the occupied orbital of BA could also be photoexcited to CBM of TiO2 together with the electrons located on VBM of TiO2. On the other hand, for the orbitals from #658 to #661 in the Ti48O112H32
C6H5CHO system shown in Figure 7, there are no mixing between the BAD part and TiO2 part.

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Figure 7. Electron density contour maps of orbitals at the VBM and CBM regions for Ti48O112H32
C6H5CHO where the LUMO is also constructed from the Ti3d orbitals, and the HOMO consists of BAD moiety only.

On the basis of the DFT calculations, we can conclude that the antibonding π molecular orbitals (MOs) of BA can hybridize with the O2p atomic orbitals (AOs) of TiO2, at least at the HOMO and the next HOMO, such hybridization not only results in the generation of new energy levels in the band gap, thus making TiO2 responsive to visible-light, but also enables the oxidation of BA to BAD by photogenerated holes in the delocalized VBM. More importantly, the DFT calculations indicate that there is no orbital mixing between BAD molecules and TiO2, even for the HOMO. As a consequence, the modified band gap of TiO2 returns to its original one as soon as BA transforms into BAD. The weaker interaction between BAD and TiO2, as compared to that between BA and TiO2, suggests that the BAD molecules are likely to bounce off the TiO2 surface once BA-to-BAD transformation is completed. This behavior may prevent BAD from overoxidation by ceasing the hole formation. As a result, the hole generation becomes controllable in this type of visible-light photoreaction system, and a so-labeled “selfadjustable photo-oxidation system” (denoted as SAPS) can be established based on the interaction between BA/BAD and TiO2, as illustrated in Scheme 1. Considering the nature of hole initiated photo-oxidation, we argue that selective oxidation of alcohols can proceed over mTiO2 in the absence of molecular oxygen.27 Indeed, a wide range of alcohols were oxidized smoothly to afford the corresponding aldehydes quantitatively under N2 (Table 1). Although this type of reaction progressed more slowly than that in aerobic condition, the desired product aldehydes were also obtained in high yields only by increasing the reaction times (Table 1, entries 7
8). In the anaerobic reaction, a small amount of H2 gas was detected as gas phase products during the initial period of photoreaction, suggesting BAD (400 nm); CHX, HX and MeCN represent cyclohexane, hexane and acetonitrile, respectively.

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Table 2. Aerobic Oxidation of Alcohols over mTiO2 and P25 under Visible-Lighta

a

Reaction conditions are identical to Table 1 with the exception that 0.1 MPa O2 was used; mTi and EA represent mTiO2 and ethyl acetate, respectively.

mTiO2 could be regarded as the “real” photocatalyst, where oxygen molecules act as the oxidant for BA-to-BAD transformation. Aerobic BA oxidations over mTiO2 were also carried out for comparison. Detailed experimental results of aerobic oxidation of alcohols in different solvents are summarized in Table 2. It is worth noting that all kinds of alcohols can be converted into their corresponding aldehydes with very high selectivity (>98%).

Among a variety of alcohols tested, those with methoxy or methyl substituent are particularly efficient, and notably nitrogencontaining compound can also be oxidized with high efficiency (Table 2, entry 11). Whereas polar solvents (e.g., MeCN) inhibit the reaction to a certain extent (Table 2, entries 1
6), smooth aerobic oxidation takes place in nonpolar solvents such as hexane (Table 2, entries 7
12). These experiments demonstrated that the solvent effect is an important factor to the success of the 23414

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The Journal of Physical Chemistry C reaction, which can be partially ascribed to the competitive adsorption of polar solvent molecules on the TiO2, which blocks the adsorption sites for BA and subsequent photoinduced oxidation. Mesoporous TiO2 has always been considered as a highly photocatalytically active photocatalyst because it has a high surface-to-volume ratio and offers more active sites for carrying out catalytic reactions, making them outperform other photocatalysts.29
35 Our experiments showed that the mesochannels of mTiO2 serve as nanoreactor providing optimal reaction microenvironments for BA-to-BAD transformation. As shown in Table 2, when polar solvent (acetonitrile) was used, the conversion rate of mTiO2 is similar to that of commercial P25 TiO2 (Table 2, entries 1
4, and entries 13
16). However, when nonpolar solvent, such as hexane was used, the conversion rate of mTiO2 is much faster than that of P25 (Table 2, entries 7 and 10, and entries 17
18). For example, it only took 20 h to completely transfer BA into BAD using mTiO2 and hexane (Table 2, entry 12), while only 68% conversion was obtained within the same reaction time when P25 and hexane were used (Table 2, entry 17). Owing to the small difference in the surface areas of mTiO2 and P25 (85 vs. 50 m2 3 g
1), the much better performance accomplished by using mTiO2 together with nonpolar solvent most likely resulting from the pore confinement effect of mesochannels, in which several weak interactions, such as van der Waals forces, hydrogen bonding, and physical adsorption, probably involved in facilitating the adsorption and sequentially oxidation of alcohols. In the light of the theoretical and experimental studies, it is believed that visible-light excited holes on valence band primarily account for the selective oxidation of BA, so that we propose mechanistic pathways as follows. New energy levels appear in the band gap of mTiO2 by adsorbing BA molecules, and the consequent modified band gap can be photoexcited by visible-light. When electron
hole pairs are generated in the new energy levels, the electrons transfer to the conduction band and the holes to the adsorbed BA.22 The BA molecule then turns into an oxidized state, and subsequently transforms into BAD by releasing H atoms as well as an electron due to current doubling effect.36 With the assistance of molecular O2 (for aerobic oxidation), or mTiO2 itself (for anaerobic oxidation), the hydrogen atoms and electrons can be effectively removed by O2, or be trapped by OL and Ti atoms, resulting in the formation of hydroxyls and Ti3+ species on TiO2 surface, respectively. Importantly, the mesostucture of mTiO2 makes the oxidative processes much feasible because of the pore confinement effect facilitating hole transfer. Although in this work we cannot quantify how many holes will be generated in BA adsorbed mTiO2 system under visible-light, the controllable electron
hole pairs are sufficient for the oxidation of adsorbed BA into BAD, because high-yields of BAD were both found for anaerobic and aerobic reactions. It should be noted that theoretical investigations on the solvent effects and the influence of mesoporous structures are, of course, important subjects. However, the DFT calculations with the plane wave basis sets seem to be less sensitive to small changes in potential energy surfaces unless the number of the plane wave is extremely increased. In addition, our recent work indicates that the electronic structure of the TiO2 layer seems less sensitive to the adsorption of solvent molecules due to rather wide band gap.37

4. CONCLUSIONS In summary, a DFT method showed that the antibonding π MOs of BA can hybridize with the O2p AOs of TiO2, leading to

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the generation of new energy levels located in the band gap. Therefore the absorption edge of pristine TiO2 can be extended into visible-light region by simply absorbing BA molecules. Importantly, a controllable electron
hole pair generation was realized according to the different electronic interactions of BA/ BAD with mTiO2. The controllable hole generation is not only responsible for BA oxidation, but also suppresses the overoxidation of BAD to undesirable products. Thus high-yielding BA to BAD transformation is successfully achieved over mTiO2 under visible-light irradiation. Aerobic and anaerobic processes were both applied to oxidize BA using the SAPS. In aerobic conditions, molecular oxygen acts as the hydrogen and electron acceptors, whereas OL and Ti atoms within TiO2 frameworks serve as the efficient reservoir for hydrogen and electrons under anaerobic conditions, suggesting that TiO2 itself is a stoichiometric oxidant in this oxidation reaction when there is an absence of oxygen molecules. In comparison with P25 TiO2, an interesting pore confinement effect has been observed for mTiO2 in nonpolar solvents (e.g., hexane), which dramatically accelerated the conversion rate of BA-to-BAD transformation. By applying the basic concepts proposed here, other commonly occurring organic syntheses may also be achieved in an economically and environmentally friendly way.

’ AUTHOR INFORMATION Corresponding Author

*Tel: (+86) 571 87952338; Fax: (+86) 571 87952338; E-mail: [email protected] (J.F.). E-mail: [email protected] (H.K.).

’ ACKNOWLEDGMENT We are thankful for financial support from the National Science Foundation of China (20873122 and J0830413), the Science & Technology Department of Zhejiang Province (2008C11125), and the Fundamental Research Funds for the Central Universities (2010QNA3005). ’ REFERENCES (1) Kroschwitz, J. I. Kirk Othmer Encyclopeida of Chemical Technology, 4th ed.; Wiley-Interscience Publications: New York, 1992; Vol. 4. (2) Mallat, T.; Baiker, A. Chem. Rev. 2004, 104, 3037. (3) Centi, G.; Misono, M. Catal. Today 1998, 41, 287. (4) Yurdakal, S.; Palmisano, G.; Loddo, V.; Augugliaro, V.; Palmisano, L. J. Am. Chem. Soc. 2008, 130, 1568. (5) Zhang, M. A.; Chen, C. C.; Ma, W. H.; Zhao, J. C. Angew. Chem., Int. Ed. 2008, 47, 9730. (6) Zhang, M.; Wang, Q.; Chen, C. C.; Zang, L.; Ma, W. H.; Zhao, J. C. Angew. Chem., Int. Ed. 2009, 48, 6081. (7) Higashimoto, S.; Kitao, N.; Yoshida, N.; Sakura, T.; Azuma, M.; Ohue, H.; Sakata, Y. J. Catal. 2009, 266, 279. (8) Higashimoto, S.; Okada, K.; Morisugi, T.; Azuma, M.; Ohue, H.; Kim, T. H.; Matsuoka, M.; Anpo, M. Top. Catal. 2010, 53, 578. (9) Higashimoto, S.; Suetsugu, N.; Azuma, M.; Ohue, H.; Sakata, Y. J. Catal. 2010, 274, 76. (10) Li, R. H.; Kobayashi, H.; Guo, J. F.; Fan, J. Chem. Commun. 2011, 47, 8584. (11) Yang, X. Y.; Salzmann, C.; Shi, H. H.; Wang, H. Z.; Green, M. L. H.; Xiao, T. C. J. Phys. Chem. A 2008, 112, 10784. (12) Kim, K. S.; Barteau, M. A. Surf. Sci. 1989, 223, 13. (13) Fan, J.; Boettcher, S. W.; Stucky, G. D. Chem. Mater. 2006, 18, 6391. (14) Fan, J.; Dai, Y. H.; Li, Y. L.; Zheng, N. F.; Guo, J. F.; Yan, X. Q.; Stucky, G. D. J. Am. Chem. Soc. 2009, 131, 15568. 23415

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