Water-Medium Barbier Reaction over a Mesoporous Pd(II

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Water-Medium Barbier Reaction over a Mesoporous Pd(II) Organometallic Catalyst Immobilized on the Ethyl-Bridged PMOs Hexing Li,* Mingwen Xiong, Fang Zhang, Jianlin Huang, and Wei Chai Department of Chemistry, Shanghai Normal UniVersity, 100 Guilin Road, Shanghai, China, 200234 ReceiVed: September 21, 2007; In Final Form: January 12, 2008

A mesoporous Pd(II) organometallic catalyst is synthesized by coordinating the Pd(II) with the amine-ligand anchored on ethyl-bridged PMOs. During Barbier reaction in water as an environmentally friendly medium, the as-prepared Pd(II)-PMOs (Et) exhibits matchable catalytic activity and selectivity with the corresponding homogeneous Pd(II) catalyst and could be used repetitively for more than 5 times, which could reduce the cost and even diminish the environmental pollution from heavy metallic ions, showing a good potential in industrial applications. On one hand, the excellent catalytic performance could be attributed to the high surface area and ordered mesporous structure of the PMOs support, which ensures the higher dispersion of Pd(II) active sites and also facilitates the diffusion of reactant molecules. On the other hand, the ethyl fragments embedded in the pore walls could enlarge mesopores and also enhance surface hydrophobility of the PMOs support, which further promotes the diffusion and adsorption of organic molecules, especially in aqueous medium, leading to higher activity and selectivity.

Introduction Most organic reactions are carried out in isotropic organic solvents. The use of large quantities of organic solvents for reaction and product isolation purposes eventually adds to environmental problems because volatile organic compounds are the principal cause of industrial pollution. Developing new approaches for conducting organic synthesis in environmentally friendly media represents an important trend in green chemistry.1-3 Water is the most innocuous substance on Earth and, therefore, the safest solvent possible. Numerous biochemical organic reactions in the life system have successfully occurred in aqueous medium with the help of enzymes,4 implying that the catalysts play crucial roles in realizing water-medium organic reactions.5-7 Barbier reactions have been widely used to acquire biologically active molecules such as macrolides, polyether and antibiotics.8 Due to solubility limit, water-medium Barbier reaction has seldom studied which is mainly focused on homogeneous organometallic catalysts.9,10 Though highly active and selective, the homogeneous catalysts are difficult for separation from the reaction system and thus, could not be used repetitively, leading to enhanced cost and even the environmental pollution from heavy metallic ions. Heterogeneous catalysts could overcome the above disadvantages but they usually exhibit much lower activity and selectivity than the corresponding homogeneous catalysts due to the poor distribution of active sites, the steric hindrance and the alternation of the chemical environment.11-13 The periodic mesoporous organosilicas (PMOs) with large surface area14-16 provide a promising way for designing immobilized homogeneous catalysts with high efficiency owing to the uniform distribution of active sites.17 Meanwhile, the highly ordered mesopores and the enhanced surface hydrophobicity resulting from the organic fragments embedded in the pore walls could facilitate the diffusion and adsorption of organic molecules, especially in * To whom correspondence should be addressed. Fax: +86-21 64322272. E-mail: [email protected].

Figure 1. FTIR spectra of the NH2-PMOs(Et) and the Pd(II)-PMOs (Et) samples.

aqueous medium.18,19 In this work, we report a novel Pd(II) organometallic catalyst immobilized on a ethyl-bridged PMOs, which exhibits matchable activity and selectivity with the corresponding Pd(II) homogeneous catalyst and could be used repetitively, showing a good potential in industrial applications. Experimental Section A. Catalyst Preparation. The PMOs with ethyl fragments embedded in the channel walls and NH2CH2CH2CH2-ligands anchored on the surface, denoted as NH2-PMOs(Et), is synthesized by self-assembly assisted co-condensation of 1,2bis-(triethoxysilyl) ethane (BTE) and 3-aminopropyltrimethoxysilane (APTS). In a typical synthesis, 2.6 g P123 ([EO20-PO70EO20] Mavg ) 5800) and 7.2 g NaCl are dissolved in 80 mL 0.50 M HCl aqueous solution at 313 K, followed by adding

10.1021/jp0776053 CCC: $40.75 © 2008 American Chemical Society Published on Web 03/29/2008

Water-Medium Barbier Reaction

Figure 2.

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Si MAS NMR and 13C CP MAS NMR spectra of the NH2-PMOs(Et) and the Pd(II)-PMOs (Et) samples.

4.0 mL BTE and 0.45 mL APTES dropwise. The solution is kept stirring at 353 K for 24 h and then kept static at 353 K for another 24 h. The solid product is filtered, washed thoroughly with distilled water, dried at 353 K for 12 h in vacuum, followed by extracting in ethanol at 353 K for 24 h to remove surfactant and other organic residues. The Pd(II)-PMOs (Et) (see Scheme 1) is prepared according to following procedures. A 1.0-g portion of the as-prepared NH2-PMOs(Et) is added into 10 mL toluene solution containing desired amount of Pd(PPh3)2Cl2. After being stirred at 303 K for 12 h, the solid is filtered and washed thoroughly with dichloromethane, followed by refluxing in dichloromethane for 5 h to remove physisorbed Pd(II) species. Finally, the sample is dried at 353 K for 10 h under a vacuum condition. For comparison, both the MCM-41 and the SBA-15 functionalized with the NH2CH2CH2CH2-group are prepared according to the procedures described in our previous papers.20,21 The Pd(II) organometallic catalysts immobilized on these two supports are prepared in the same way as that used for preparing Pd(II)-PMOs (Et), which are denoted as Pd(II)-MCM-41 and Pd(II)-SBA-15, respectively. B. Catalyst Characterization. Small-angle X-ray diffraction (XRD) patterns are conducted on a Rigaku D/maxr B diffractometer with Cu Ka. N2 adsorption-desorption isotherms are measured at 77 K on a Quantachrome NOVA 4000e analyzer, from which the specific surface area (SBET), the pore size distribution curve, the pore volume (VP) and the average pore diameter (DP) are calculated based on the adsorption-branch in the N2 sorption isotherms by using the multiple-point Brunauer-

SCHEME 1: A Schemetical Diagram of the Pd(II)-PMOs (Et)

Emmett-Teller (BET) method and the Barrett-Joyner-Halenda (BJH) model, respectively. Surface morphologies and porous structures are observed through Transmission electron microscopy (TEM, JEOL JEM2011). FT-IR and Solid NMR spectra are recorded on a Nicolet Magna 550 IR spectrometer and a Bruker DRX-400 NMR spectrometer, respectively. The

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Figure 5. TEM morphologies of (a) the NH2-PMOs(Et) and (b) the Pd(II)-PMOs (Et) samples.

Figure 3. N2 adsorption-desorption isotherms of the NH2-PMOs(Et) and the Pd(II)-PMOs (Et) samples.

Figure 6. XPS spectra of the Pd(PPh3)2Cl2 and the Pd(II)-PMOs (Et) samples.

and a FID detector in which the column temperature is kept at 373 K and N2 is used as carrier gas. The reproducibility is checked by repeating each result at least three times and is found to be within (5%. In order to determine the catalyst durability, the heterogeneous catalyst is allowed to settle down after each run of reactions and the clear supernatant liquid is decanted slowly. The residual solid catalyst is washed thoroughly with distilled water and diethyl ether, followed by drying at 353 K for 8 h under vacuum condition. Then, the catalyst is re-used with fresh charge of water and reactants for subsequent recycle runs under the same reaction conditions. The content of Pd(II) species leached off from the heterogeneous catalyst is determined by ICP analysis. Results and Discussion

Figure 4. Small-angle XRD patterns of the PMO(Et), SBA-15, NH2PMOs(Et) and the Pd(II)-PMOs (Et) samples.

surface electronic states are analyzed by X-ray photoelectron spectroscopy (XPS, Perkin-Elmer PHI 5000C ESCA). All of the binding energy values are calibrated by using C1S ) 284.6 eV as a reference. The Pd(II)-loading is determined by inductively coupled plasma optical emission spectrometer (ICPOES, Varian VISTA-MPX). C. Activity Test. In a typical run of activity test, 0.75 mmol benzaldehyde, 2.0 mmol allyl bromide, 10 mL distilled water, a catalyst containing 0.054 mmol Pd(II), and 20 mmol SnCl2 as a reducing agent are mixed in a 25 mL round-bottomed flask. After reaction at 323 K for 12 h under vigorous stirring, the products are extracted with diethyl ether, followed by analysis on a gas chromatograph (GC, Agilent 1790) equipped with a JWDB-5, 95% dimethyl 1-(5%)-diphenylpolysiloxane column

A. Structural Characteristics. The FTIR spectra (Figure 1) demonstrate the presence of surface Si-O-H groups in both the NH2-PMOs(Et) and the Pd(II)-PMOs (Et) samples, corresponding to absorbance bands at 3500 cm-1 (νO-H), 1474 cm-1 (δO-H), 600 cm-1 (ωO-H), 1000-1300 cm-1 (νas, Si-O), and 456 cm-1 (δSi-O), respectively.22,23 The absorbance peak at 1640 cm-1 indicative of the νas(N-H) vibration24 confirms that the NH2CH2CH2CH2-group has been successfully anchored onto the PMOs support. The peak at 3300 cm-1 corresponding to νN-H vibration is difficult to distinguish since it is overlapped by the broad absorbance peak indicative of surface OH at 3500 cm-1 (νO-H). In comparison with the NH2-PMOs(Et), the Pd(II)-PMOs (Et) displays additional absorbance peaks around 690∼740 cm-1 and 1440 cm-1, which could be attributed to the vibrations of δC-H and νC-C in the benzene ring connecting with the phosphorus (PPh3-).19 As the Pd(II) species adsorbed on the PMOs have been completely removed by extraction in dichloromethane, the presence of the PPh3 suggests that the Pd(II) organometallic catalyst has been successfully immobilized

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Figure 7. N2 adsorption-desorption isotherms of Pd(II)-MCM-41 and Pd(II)-SBA-15 samples. The insets are the BJH pore size distribution curves.

TABLE 1: Structural Parameters and Catalytic Behaviors of Pd(II)-PMOs (Et) Series Catalystsa catalyst

loading (wt %Pd)

SBET (m2/ g)

DP (nm)

VP (mL/ g)

T (K)

time (h)

conversion (%)

selectivity (%)

yield (%)

NH2-PM Os(Et) Pd(II)-PM Os (Et)-1 Pd(II)-PM Os (Et)-2 Pd(II)-PM Os (Et)-3 Pd(II)-PM Os (Et)-4 Pd(II)-PM Os (Et)-5 Pd(II)-PM Os (Et)-4 Pd(II)-PM Os (Et)-4 Pd(II)-PM Os (Et)-4 Pd(II)-PM Os (Et)-4

0 0.017 0.033 0.050 0.058 0.076 0.058 0.058 0.058 0.058

623 538 497 461 448 402 448 448 448 448

7.6 6.5 5.9 5.5 5.3 5.1 5.3 5.3 5.3 5.3

0.79 0.63 0.47 0.46 0.44 0.39 0.44 0.44 0.44 0.44

323 323 323 323 323 323 303 343 323 50

12 12 12 12 12 12 12 12 9 15

/ 67 75 83 88 68 61 71 81 87

/ 90 91 93 94 90 92 91 94 93

/ 60 68 77 83 61 56 65 76 81

a Reaction conditions: a catalyst containing 0.054 mmol Pd(II), 0.75 mmol benzaldehyde, 2.0 mmol allyl bromide, 10 mL H2O, 20 mmol SnCl2. The conversion is calculated by benzaldehyde.

TABLE 2: Structural Parameters and Catalytic Performance of Different Pd-based Catalystsa catalyst Pd(PPh3)2Cl2 Pd(II)-PMOs (Et)-4 Pd(II)-SBA-15 Pd(II)-MCM-41 a

loading (wt %Pd) 0.058 0.058 0.058

SBET (m2/ g) 448 551 976

DP (nm) 5.3 3.9 1.7

VP (mL/ g)

conversion (%)

selectivity (%)

yield (%)

0.44 0.49 0.26

96 88 76 74

94 94 94 92

90 83 71 68

Reaction conditions are given in Table 1.

SCHEME 2: Barbier Reaction between Benzaldehyde and Allyl Bromide in Water Medium

by replacing one PPh3-ligand in Pd(PPh3)2Cl2 with the NH2CH2CH2CH2-ligand originally anchored on the PMOs support.25 As shown in Figure 2, the 29Si MAS NMR spectra of both the NH2-PMOs(Et) and the Pd(II)-PMOs (Et) samples display two peaks downfield corresponding to T3 (δ ) -64 ppm) and T2 (δ ) -59 ppm), where Tm ) RSi(OSi)m-(OH)3-m, m ) 1 ∼ 3. However, no Qn peaks are observed, where Qn ) Si(OSi)n-(OH)4-n, n ) 2 ∼ 4. These results demonstrate that all the Si species are covalently bonded with carbon atoms.26 According to the 13C CP MAS NMR spectra, both the NH2PMOs(Et) and the Pd(II)-PMOs (Et) samples display a strong peak around 7.3 ppm indicative of the C atoms in the ethyl group embedded in the pore walls (-Si-CH2CH2-Si-) and three weak signals around 20, 42, and 58 ppm, corresponding to three carbon atoms in the NH2CH2CH2CH2-group.24 Besides these NMR peaks, the Pd(II)-PMOs (Et) displays an additional signal at 129 ppm characteristic of the C atoms in the benzene ring connecting with the phosphorus (PPh3),27 which further

confirms the above conclusion that the Pd(II) organometallic catalyst has been successfully immobilized via replacing one PPh3-ligand in Pd(PPh3)2Cl2 by the NH2CH2CH2CH2-ligand originally anchored on the PMOs support. Figure 3 reveals that both the NH2-PMOs(Et) and the Pd(II)-PMOs (Et) samples display typical IV type N2 adsorptiondesorption isotherms with a H2 hysteresis loop indicative of the mesoporous structure. The initial step of adsorption isotherms represents the adsorption in mesopores to form multilayers of adsorbed nitrogen on the pore surface.28 The attached pore size distribution curves demonstrate that both two samples contain uniform mesopores with a narrow pore size range. The Pd(II)PMOs (Et) exhibits smaller pore diameter owing to the immobilization of the Pd(II) organometallic catalyst in the pore channels. On the basis of the N2 adsorption-desorption isotherms, some structural parameters are listed in Table 1. The SBET, VP, and DP decrease with the increase of Pd(II)-loading,

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Figure 8. TEM images of (a) Pd(II)-MCM-41 and (b) Pd(II)-SBA15.

which further confirms the immobilization of the Pd(II) organometallic catalyst into the pore channels. The small-angle XRD patterns (Figure 4) confirm that all the PMO(Et), SBA-15, NH2-PMOs(Et), and Pd(II)-PMOs (Et) samples exhibit a strong peak around 2θ ) 0.8°-1.0° indicative of (100) diffraction and two weak peaks around 1.7° and 2.3° characteristic of (110) and (200) diffractions, indicating that both two samples contain long-range ordered hexagonal mesoporous structure.29 Such ordered mesoporous structure still retained after immobilization of the Pd(II) organometallic catalyst onto the PMOs support. However, the peak intensity decreases with the increase of the Pd(II)-loading on the PMOs support, indicating the lower ordering degree of the mesoporous structure. Meanwhile, the principal peak position shifts to higher 2θ values with the increase of the Pd(II)-loading, indicating a decrease in basal spacing (d100).30 As shown in Figure 5, the TEM images further confirm that both the NH2-PMOs(Et) and the Pd(II)-PMOs (Et) samples exhibit hexagonally mesoporous structure with highly ordering degree. The Pd(II) species are homogeneous distributed on the outer surface and in the pore channels since no Pd species have been identified by either the wide-angle XRD pattern or the HRTEM images. As shown in Figure 6, the XPS spectra demonstrate that all the Pd species in the Pd(II)-PMOs (Et) sample are present in +2 state, corresponding to the binding energy (BE) of 337.9 eV in Pd3d5/2 level.31 In comparison with the BE of the Pd(II) in Pd(PPh3)2Cl2, the BE of the Pd(II) in Pd(II)-PMOs (Et) shifts positively by 0.6 eV, indicating that the PPh3-ligand could donate more electrons to Pd(II) than the NH2CH2CH2CH2ligand, taking into account that one PPh3-ligand is replaced by a NH2CH2CH2CH2-ligand to obtain the immobilized Pd(II)PMOs (Et). B. Catalytic Performance. The water-medium Barbier reaction between benzaldehyde (1) and allyl bromide (2) is used as a probe to evaluate the catalytic performance. The reaction could be simply described in Scheme 2.10,11 Besides the target product 1-phenyl-3-buten-1-ol (3), the major side product is determined as 1,5-hexadiene formed through the coupling reaction of the allylic bromide. Trace of other side products including benzyl alcohol and pinacol are also identified, suggesting that the Barbier reaction follows a single-electrontransfer (SET) process involving the formation of a radical anion intermediate.10,11 As shown in Table 1, the Pd(II)-loading exhibits very strong effect on the activity but only little influence on the selectivity. The activity first increases with the Pd(II)loading up to 0.058 wt %. It is not clear why the Pd(II)-PMOs (Et) with very low Pd(II)-loading exhibits poor activity, taking account that the total Pd(II) amount is fixed at 0.054 mmol by adjusting the amount of the catalyst added into the reaction system. A possible reason is that the activity of the Pd(II) coordinated with the PPh3-ligand is more active than the Pd(II) coordinated with the NH2CH2CH2CH2-ligand since the PPh3-

Figure 9. N2 adsorption-desorption of (a) the fresh Pd(II)-PMOs (Et)-4 and (b) the Pd(II)-PMOs (Et)-4 after being used repetitively for 5 times. The insets are the low-angle XRD patterns.

ligand donates more electrons to Pd(II), as confirmed by the XPS spectra (Figure 6). The higher electron density on the Pd(II) might facilitate Barbier reaction since the reaction proceeds through a single-electron-transfer (SET) process involving the formation of free radicals,10,11 leading to the enhanced activity. At very low Pd(II)-loading, the number NH2CH2CH2CH2ligands on the PMOs support is greatly excessive in comparison with the number of PPh3-ligands, which could replace two PPh3ligands in the Pd(PPh3)2Cl2, leading to the lower activity. However, very high Pd(II)-loading is also harmful for the activity due to both the poor distribution of Pd(II) active sites and the blockage of the pore channels (see the structural parameters in Table 1). The optimum Pd(II)-loading is determined as 0.058%, corresponding to Pd(II)-PMOs (Et)-4. Besides the Pa(II)-loading, the effects of reaction temperature and reaction time on the catalytic efficiency have also been investigated. As shown in Table 1, from the influence of the reaction time on the activity, one could conclude that the Barbier reaction reaches completion after 12 h under present conditions. At lower reaction temperature (303 K), the Pd(II)-PMOs (Et)-4 shows lower 1 conversion due to incomplete reaction. The lower 1 conversion obtained at higher reaction temperature (343 K) could be attributed to the reversibility of Barbier reaction since the reaction is exothermic. In addition, the higher reaction temperature also induces a partial damage of the ordered mesoporous structure since mesoporous materials usually exhibit poor hydrothermal stability. Table 2 shows the catalytic properties of different Pd(II)based catalysts. The Pd(II)-PMOs (Et)-4 exhibits much higher activity than either the Pd(II)-MCM-41 or the Pd(II)-SBA-15. These three catalysts display the same content of Pd(II) active sites with almost the identical chemical environment. Meanwhile, according to the N2 adsorption-desorption isotherms (Figure 7) and the TEM images (Figure 8), both the Pd(II)MCM-41 and the Pd(II)-SBA-15 also display highly ordered mesoporous structures. Both the Pd(II)-MCM-41 and the Pd(II)-SBA-15 exhibit even higher SBET than the Pd(II)-PMOs (Et)4. Thus, one could conclude that the ethyl fragments embedded in the pore walls of the PMOs play a key role in promoting the activity. On one hand, the ethyl fragments could enlarge the pore channels (see DP values in Table 2) which facilitates diffusion of reactant molecules, leading to the easy adsorption and reaction on the Pd(II) active sites. On the other hand, the ethyl fragments could enhance the surface hydrophobility which is also favorable for the diffusion and adsorption of organic molecules in the pore channels, especially in water medium.

Water-Medium Barbier Reaction TABLE 3: Recycle Test of the Pd(II)-PMOs (Et)-4 in Water-Medium Barbier Reactiona recycle no. Pd content conversion (%) selectivity (%) yield (%) (mmol) 1 0.054 88 94 83 2 0.054 86 96 83 3 0.053 83 93 77 4 0.051 78 91 71 5 0.049 73 92 67 a

Reaction conditions are given in Table 1.

Meanwhile, the strong surface hydrophobility could prevent the entrance of water molecules into the pore channels and thus, could enhance the hydrothermal stability of the mesoporous structure.32 In comparison with the Pd(PPh3)2Cl2 homogeneous catalyst, the Pd(II)-PMOs (Et)-4 exhibits nearly the same selectivity and only a slightly lower activity which, could possibly be attributed to the replacement of the PPh3-ligand by the NH2CH2CH2CH2-ligand, as discussed above. To make sure whether the heterogeneous Pd(II) organometallic catalyst immobilized on the PMOs support or the dissolved homogeneous Pd(II) species is the real catalyst responsible for the present Barbier reaction, the following procedure is carried out, as proposed by Sheldon et al.33 After reaction for 6 h in which the 1 conversion exceeds 45%, the reaction mixture is filtered to remove the solid catalyst and then the mother liquor is allowed to react for another 12 h at the same reaction conditions. No significant change in the 1 conversion is observed, indicating that the active phase is not the dissolved Pd(II) organometallic complexleached from Pd(II)-PMOs (Et). Therefore, it is reasonable to conclude that the present catalysis is really heterogeneous in nature. The durability of the Pd(II)-PMOs (Et)-4 in water-medium Barbier reaction is also examined by recycling the solid catalyst at the end of each run of reactions. As shown in Table 3, the yield decreases by less than 20% after being used repetitively for 5 times, showing a relatively good durability. Table 3 also demonstrates that the leaching of the Pd(II) active sites during the reaction in aqueous solution is not significant. However, both the N2 adsorption-desorption isotherms and the attached XRD patterns (Figure 9) demonstrate that the ordered mesoporous structure has been partially destroyed after being used five times. Accordingly, the damage of the ordered mesoporous structure is perhaps the main reason responsible for the deactivation of the Pd(II)-PMOs (Et)-4 catalyst during the recycling uses. Conclusions The present work supplies a facile approach for preparing immobilized Pd(II) organometallic catalyst with ordered mesoporous structure by coordinating the Pd(II) with the amineligand originally anchored on a PMOs support containing ethyl fragments in the pore walls. Such Pd(II)-PMOs (Et) catalysts exhibit similar activity and selectivity to the corresponding Pd(II) homogeneous catalyst in the water-medium Barbier reaction and could be used repetitively. On the basis of the present method, other immobilized organometallic catalysts with high efficiency could be designed and their catalytic properties could

J. Phys. Chem. C, Vol. 112, No. 16, 2008 6371 be further improved by adjusting the organic fragments embedded in the pore walls and the ligands anchored on the supports, which offers opportunities for industrial applications of watermedium clean organic reactions. Acknowledgment. This work was supported by the 973 Preprogram (2005CCA01100), the National Natural Science Foundation of China (20377031), the Shanghai Science and Technology Committee (06JC14060), and the Shanghai Education Committee (T0402). References and Notes (1) Li, C. J.; Chen, L. Chem. Soc. ReV. 2006, 35, 68. (2) Shaughnessy, K. H. Eur. J. Org. Chem. 2006, 67, 1827. (3) Joh, F. Acc. Chem. Res. 2002, 35, 738. (4) Creighton, T. E. Proteins: Structure and Molecular Properties; Freeman: New York, 1993. (5) Li, H. X.; Zhang, F.; Wan, Y.; Lu, Y. F. J. Phys. Chem. B 2006, 110, 22942. (6) Li, H. X.; Zhang, F.; Yin, H.; Wan, Y.; Lu, Y. F. Green Chem. 2007, 9, 500. (7) Li, H. X.; Chai, W.; Zhang, F.; Chen, J. Green Chem. 2007, DOI: 10.1039/b706360a. (8) Roush, W. R. ComprehensiVe Organic Synthesis; Pergamon Press: New York, 1991. (9) Narayan, S.; Muldoon, J.; Finn, M. G.; Fokin, V. V.; Kolb, H. C.; Sharpless, K. B. Angew. Chem., Int. Ed. 2005, 44, 3275. (10) Li, C. J. Chem. ReV. 1993, 93, 2023. (11) Chan, T. H.; Li, C. J.; Lee, M. C.; Wei, Z. Y. Can. J. Chem. 1994, 72, 1181. (12) Tanko, J. M.; Blackert, J. F. Science 1994, 263, 203. (13) Lu, Z.; Lindner, E.; Mayer, H. A. Chem. ReV. 2002, 102, 3543. (14) Tan, X. H.; Shen, B.; Deng, W.; Zhao, H.; Guo, Q. X. J. Org. Lett. 2003, 5, 1833. (15) Shimazu, S.; Baba, N.; Ichikuni, N.; Uematsu, T. J. Mol. Catal. A 2002, 182, 343. (16) Zhou, J.; Zhou, R.; Mo, L.; Zhao, S.; Zheng, X. J. Mol. Catal. 2002, 178, 289. (17) Wan, Y.; Zhang, D.; Feng, C.; Chen, J.; Li, H. X. Chem. An. Asian J. 2007, 2, 875. (18) Li, H. X.; Chen, J.; Wan, Y.; Zhang, F.; Lu, Y. F. Green Chem. 2007, 9, 273. (19) Wan, Y.; Chen, J.; Zhang, D. Q.; Li, H. X. J. Mol. Catal. A 2006, 258, 89. (20) Chen, J.; Wan, Y.; Li, H. X. Chin. J. Catal. 2006, 27, 339. (21) Wan, Y.; Zhang, F.; Lu, Y. F.; Li, H. X. J. Mol. Catal. A 2007, 267, 165. (22) Huang, H.; Yang, R.; Munson, C. J. Ind. Eng. Chem. Res. 2003, 42, 2427. (23) Burleigh, M.; Michael, A.; Markowitz, M.; Gaber, B. J. Phys. Chem. B 2001, 105, 9935. (24) Chong, A. S. M.; Zhao, X. S. J. Phys. Chem. B 2003, 107, 12650. (25) Joseph, T.; Deshpande, S.; Halligudi, S.; Ernst, S.; Hartmann, M. J. Mol. Catal. 2003, 206, 13. (26) Choi, J. Y.; Kim, C. H.; Kim, D. K. J. Am. Ceram. Soc. 1998, 81, 1184. (27) Inagaki, S.; Guan, S.; Fukushima, Y.; Ohsuna, T.; Terasaki, O. J. Am. Chem. Soc. 1999, 121, 9611. (28) Beck, J.; Vartuli, J.; Roth, W. J. Am. Chem. Soc. 1992, 114, 10834. (29) Matos, J. R.; Kruk, M.; Mercuri, L. P.; Jaroniec, M.; Asefa, T.; Coombs, N.; Ozin, G. A.; Kamiyama, T.; Terasaki, J. Chem. Mater. 2002, 14, 1903. (30) Gergg, S. J.; Sing, K. S. Adsorption, Surface Area and Porosity; Elsevier: New York, 1982. (31) Thomas, A. C. Photoelectron and Auger Spectroscopy; Plenum: New York, 1975. (32) Zhang, W. H.; Lu, X. B.; Xiu, J. H. AdV. Funct. Mater. 2004, 14, 544. (33) Sheldon, R. A.; Wallau, M. I.; Arends, W. C. E.; Schuchardt, U. Acc. Chem. Res. 1998, 31, 485.