Metal−Organic Framework Supported Gold Nanoparticles as a Highly

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Metal-Organic Framework Supported Gold Nanoparticles as a Highly Active Heterogeneous Catalyst for Aerobic Oxidation of Alcohols Hongli Liu,† Yaling Liu,‡ Yingwei Li,*,† Zhiyong Tang,*,‡ and Huanfeng Jiang† School of Chemistry and Chemical Engineering, South China UniVersity of Technology, Guangzhou 510640, China, and National Center for Nanoscience and Technology, Beijing 100190, China ReceiVed: June 19, 2010

The use of metal-organic frameworks (MOFs), an emerging class of porous materials, as supports for gold nanoparticles (NPs) may bring new opportunities in the development of highly active heterogeneous gold catalysts for a variety of catalytic reactions. Although currently a few examples of MOF-supported nanoparticulate Au have been reported, the preparation of an active Au NPs deposited MOF catalyst in solution is still a challenging research target. In this study, we report a highly efficient heterogeneous gold catalyst, which was deposited on a zeolite-type MOF (MIL-101) by a simple colloidal method with polyvinylpyrrolidone (PVP) as protecting agent using HAuCl4 as the Au precursor. The resulting Au/MIL-101 (CD/PVP) catalyst exhibited extremely high catalytic activities in liquid-phase aerobic oxidation of a wide range of alcohols, which could even efficiently catalyze the oxidation under ambient conditions in the absence of water or base. Moreover, the catalyst was easily recoverable and could be reused several times without leaching of metals or debasing of activity. The high dispersion of Au NPs and the electron donation effects of aryl rings to the Au NPs within the large cages of the MIL-101 support are suggested to be the main reasons for the observed high activities of the Au/MIL-101 (CD/PVP) catalyst in aerobic oxidation of alcohols under base-free conditions. 1. Introduction The selective oxidation of alcohols is one of the most important transformations in organic chemistry.1 Many oxidizing reagents have been traditionally used in order to accomplish this transformation. However, these reagents usually require stoichiometric amounts of metal oxidants, which are expensive and have serious toxicity issues associated with them.2 With ever-increasing environmental concerns, there is substantial interest in the development of heterogeneous catalysts that use molecular oxygen as the oxidant.3 Recently, the preparation of gold nanoparticles (NPs) with different sizes and shapes has attracted tremendous attention due to their wide range of applications particularly in catalysis.4 Although some excellent supported gold catalysts for the aerobic oxidation of alcohols have been reported, in most cases they have been applied at temperatures above 100 °C (under solvent-free conditions)5 or in the presence of a large excess of base addtives.6 Oxidation reactions performed under mild conditions (preferably at room temperature) in appropriate solvents are necessary for alcohols with high melting points or low stability at high temperatures.6d Moreover, most of the catalysts can be applied only to a very limited range of substrates under base-free conditions.5 Therefore, the development of reusable catalysts that exhibit a wide range of substrate tolerance under mild and base-free conditions for aerobic oxidation of alcohols still remains a major challenge in both organic synthesis and green chemistry. Metal-organic frameworks (MOFs) have emerged as a particular class of multifunctional materials due to their high surface area, thermally robust structure, and chemical tunability.7 The spatial construction of metal ions and organic linkers leads to the rationally designed networks with nanosized channels and * To whom correspondence should be addressed. E-mail: [email protected]. † South China University of Technology. ‡ National Center for Nanoscience and Technology.

pores, similar to that found in zeolites. Due to these properties MOFs could be promising materials for applications in heterogeneous catalysis. However, over the past decade, research works have been mostly focused on preparing new MOFs and studying their applications in gas adsorption and separation.8 So far, reports of catalytic studies on MOFs, especially on MOF supported noble metal NPs, have been relatively limited.9,10 Although currently a few examples of MOF supported nanoparticulate Au catalysts have been reported,10 all the active catalysts were prepared by chemical vapor deposition (CVD) or solid grinding (SG) method, employing expensive organogold complexes as Au precursors.10 To date, the preparation of an active Au NPs deposited MOF catalyst by a liquid-phase synthesis method is still a challenging research target. Herein, we report a highly active heterogeneous Au catalyst, which was deposited on a zeolite-type MOF by a simple colloid method with polyvinylpyrrolidone (PVP) as protecting agent using HAuCl4 as gold precursor. The supported gold catalyst has been shown to be highly efficient for liquid-phase aerobic alcohol oxidation in the absence of base under mild conditions. To the best of our knowledge, this work represents the first example of an active MOF supported Au catalyst in heterogeneous catalysis by using a solution-based synthesis strategy for the deposition of gold NPs. We also mention the interesting aspects such as substrate scope and recycling of the catalyst as well as the size effect of the formed Au NPs on the catalytic activities. The MIL-101 framework (Cr3(F,OH)O[(O2C)-C6H4-(CO2)]3), one of the representative MOFs, was used as a support in this study. We have chosen MIL-101 because it holds a large surface area (Brunauer-Emmett-Teller (BET), ca. 3000 m2 g-1), large pore size (ca. 30 Å), and good chemical resistance to water and organic solvents, which are desirable for depositing small metal NPs by using a solution-based preparation method.9g,h,11

10.1021/jp105666f  2010 American Chemical Society Published on Web 07/21/2010

MOFs as Supports for Gold NPs 2. Experimental Methods 2.1. Catalyst Preparation. Synthesis of MIL-101. The MIL101 support was prepared from hydrothermal reaction of Cr(NO3)3 · 9H2O (2.007 g, 5 mmol), HF (48 wt %, 5 mmol), terephthalic acid (0.823 g, 5 mmol), and 24 mL of deionized water at 220 °C for 8 h.11 The mixture was first cooled to 150 °C in 1 h and then slowly to room temperature in 12 h. To eliminate most of the carboxylic acid, the mixture was filtered first using a large-pore fritted glass filter (n°2, Schott). The water and the MIL-101 powder passed through the filter while the free acid stayed inside the glass filter. Then, the MIL-101 powder was separated from the solution using a small-pore paper filter (1°, Whatman). The powder was washed thoroughly with deionized water and ethanol and then soaked in ethanol (95% EtOH with 5% water) at 80 °C for 24 h. The solid was finally dried overnight at 150 °C under vacuum. Synthesis of Au/MIL-101 by Colloidal Deposition with PVP as Protecting Agent (CD/PVP). The Au colloids stabilized by PVP were synthesized according to the reported method with slight modifications.6c An aqueous solution of HAuCl4 (197 mg/ L) was prepared; PVP (K-30 kDa) was added as a protecting agent (PVP monomer/metal ) 80:1 molar ratio). The obtained solution was placed in an ice bath and vigorously stirred for 1 h. A freshly prepared aqueous solution of NaBH4 (0.1 M, NaBH4/metal ) 1:1 (w/w)) was added under vigorous stirring to obtain a dark brown colloidal dispersion. After the Au:PVP colloids were formed, they were immediately deposited onto the MIL-101 support to prepare the Au/MIL-101 catalyst. In a typical catalyst preparation, the preformed Au:PVP hydrosol was subjected to sonication (180 W) for 20 s, and during the sonication an aqueous solution of MIL-101 (4 mL of H2O/(g of support)) was added. The suspension was stirred at 0 °C for 4 h, followed by washing thoroughly with deionized water. The sample was dried under vacuum at 100 °C for 2 h and then heated at 200 °C in H2 for 2 h to obtain Au/MIL-101 by CD/PVP. Synthesis of Au/MIL-101 by Impregnation (IMP). An appropriate concentration of HAuCl4 aqueous solution was added dropwise to the activated MIL-101 (typically 2 mL/(g of support)). The sample was aged at room temperature for 1 h and then was washed twice with an aqueous ammonia solution (30 mL, 1 M) and twice with deionized water (30 mL).12 The solid was centrifuged between each washing. The as-synthesized sample was dried under vacuum at 100 °C for 2 h and then was treated in a stream of H2 at 200 °C for 2 h to yield Au/ MIL-101 (IMP). Synthesis of Au/MIL-101 by Deposition-Precipitation with Sodium Hydroxide (DPSH). In a typical synthesis, an aqueous solution of HAuCl4 (3 × 10-4 M) was heated to 80 °C. The pH was adjusted to 8 by dropwise addition of NaOH (1 M), then MIL-101 was dispersed in the solution, and the pH was readjusted to 8 with NaOH.13 The suspension was vigorously stirred at 80 °C for 2 h, followed by washing thoroughly with deionized water. The sample was dried under vacuum at 100 °C for 2 h and then calcined at 200 °C in H2 for 2 h. Synthesis of Au/MIL-101 by Colloidal Deposition with Glucose as Protecting Agent (CD/glucose). An aqueous solution of HAuCl4 (3 × 10-4 M) was prepared with glucose (0.2744 g) as a protecting agent (glucose/metal ) 50:1 molar ratio). The mixture was vigorously stirred for 10 min at room temperature. A freshly prepared solution of NaBH4 (0.1 M, NaBH4/metal ) 1:1 (w/w)) was added. The color of the reaction mixture immediately turned from pale yellow to pink and then to dark brown, indicating the formation of small Au nanoparticles.14

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Figure 1. Powder XRD patterns of MIL-101 samples: (a) MIL-101, as-synthesized; (b) Au/MIL-101 (CD/PVP), before reaction; (c) Au/ MIL-101 (CD/PVP), after six runs of catalysis; (d) Au/MIL-101 (CD/ glucose); (e) Au/MIL-101 (DPSH); (f) Au/MIL-101 (IMP).

The resulting brown sol was stirred for 1 h under N2 atmosphere. Then the sol was contacted with MIL-101 for 4 h, followed by washing thoroughly with deionized water. The sample was dried under vacuum at 100 °C for 2 h and then calcined at 200 °C in H2 for 2 h. 2.2. Characterization. Powder X-ray diffraction (XRD) patterns of the samples were obtained by a Rigaku diffractormeter (D/MAX-IIIA, 3 kW) using Cu KR radiation (40 kV, 30 mA, λ ) 0.1543 nm). The BET surface area measurements were performed with N2 adsorption/desorption isotherms at 77 K on a Micromeritics ASAP 2010 instrument. Before the analysis, the samples were evacuated at 150 °C for 12 h. The size and morphology of Au/MIL-101 were investigated by using a transmission electron microscope (TEM, FEI Tecnai G2 20 S-TWIN) with EDS analysis operated at 200 kV. The gold contents of the samples were determined quantitatively by atomic absorption spectroscopy (AAS) on a HITACHI Z-2300 instrument. 2.3. Catalytic Reactions. Typically, alcohol (1 mmol) and supported gold catalyst (1 mol % Au) were added to 10 mL of toluene. The reaction mixture was stirred at 80 °C under 1 atm of O2 (O2 bubbling rate, 30 mL min-1). After the reaction, the catalyst particles were removed from the solution by filtration. The products were quantified by GC-MS analysis (Shimadzu GCMS-QP5050A equipped with a 0.25 mm ×30 m DB-WAX capillary column). The typical GC-MS analysis program was as follows: initial column temperature 100 °C, hold 2 min, to 280 °C at 15 °C/min, and hold for 5 min. To study the leaching of Au during the reaction, after reaction, the mixture was hot-filtrated under vacuum. The solid was washed with toluene, and the liquid phase was analyzed by AAS. For the recyclability tests, the reactions were performed under the same reaction conditions as described above, except using the recovered catalyst. Each time, the catalyst was isolated from the reaction solution at the end of the catalytic reaction, washed with ethanol, and heated at 150 °C. The dried catalyst was then reused in a next run. 3. Results and Discussion 3.1. Characterization of Supported Gold Catalysts. The powder XRD pattern of as-synthesized MIL-101 matches well with the already published XRD patterns (Figure 1).11 After the loading of Au and H2 reduction, there is no apparent loss of

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TABLE 1: Characterization Results of the Au/MIL-101 Catalysts sample MIL-101 Au/MIL-101 Au/MIL-101 Au/MIL-101 Au/MIL-101 Au/MIL-101 a

(CD/PVP) (CD/glucose) (DPSH) (IMP) (CD/PVP)a

SBET (m2/g) Vpore (cm3/g) 2973 2470 2550 2582 2310 2365

1.48 1.22 1.27 1.29 1.15 1.18

Au loading (wt %) 0.48 0.50 0.49 0.52 0.47

Au/MIL-101 (CD/PVP) catalyst after six runs of catalysis.

Figure 3. TEM images (a, b) of Au/MIL-101 (CD/glucose) and corresponding size distribution of Au NPs (c).

Figure 2. TEM images (a-c) of Au/MIL-101 (CD/PVP) and corresponding size distribution of Au NPs (d). The inset of b is the EDX pattern.

crystallinity in XRD patterns (Figure 1), indicating that the framework of MIL-101 is mostly maintained. Moreover, no diffractions were detected for gold NPs, which indicate gold loadings are too low or Au NPs are too small in all samples. The gold content for all catalysts is about 0.5 wt %, measured by AAS analysis (Table 1). The specific surface areas and pore volumes of the Au/MIL101 samples were determined by the N2 physisorption measurement (Table 1). The appreciable decreases in surface areas and pore volumes indicate that the cavities of MIL-101 are occupied by highly dispersed Au NPs or/and blocked by Au NPs that are located at the surface, as in the case of metal NPs loaded to MOF-5, ZIF-8, and zeolitic materials.10b,15 Figure 2 shows the typical TEM images of the Au/MIL-101 catalyst prepared by the CD/PVP method. It can be seen that small Au NPs were highly dispersed on the Au/MIL-101 (CD/ PVP) with mean diameters of 2.3 ( 1.1 nm (as estimated by size distribution), which are in good agreement with the cage diameters of MIL-101. The EDX analysis confirms the presence of gold (inset Figure 2b) for the Au/MIL-101 sample. To determine whether the gold NPs are incorporated within the pores of the MIL-101 support, we carried out a high-resolution TEM (HRTEM) measurement as shown in Figure 2c. HRTEM has been reported to be an effective technique to directly image the metal particles within the frameworks of MOFs, although the images are difficult to obtain because of the charging of the

sample under the electron beam especially for small particles less than 3 nm.15a,16 For small metal NPs embedded within the frameworks, charges tend to accumulate and images become distorted.15a,16 Our results are consistent with the literature reports. The HRTEM image (Figure 2c) showed that small gold particles (99% selectivity for acetophenone (entry 15). Notably, the Au/MIL101 (CD/PVP) catalyst was capable of selectively oxidizing 1-phenylethanol to acetophenone even when the reaction was carried out using air instead of pure O2, although a longer reaction time was required to achieve a complete conversion (entry 16). 3.3. Comparison of Catalytic Performance on Au/MIL101 Catalysts Prepared by Different Methods. To compare the potential abilities of the Au/MIL-101 catalysts, the catalytic performance of the Au/MIL-101 catalysts prepared by different methods was tested for the aerobic oxidation of benzyl alcohol under the same conditions. Turnover frequencies (TOF) for the oxidation of benzyl alcohol for the initial 15 min of reaction were measured, and the results are shown in Figure 6. It can be seen that Au/MIL-101 (CD/PVP) is highly active for the oxidation of benzyl alcohol in the absence of base with a TOF of 258 h-1. In contrast, under base-free conditions the Au/MIL101 catalysts prepared by CD/glucose, DPSH, and IMP were almost inactive. Moreover, the parent MIL-101 did not catalyze the alcohol oxidation reactions (Supporting Information Table S1). The size-dependent catalytic activity of gold for oxidation reactions has recently been studied. It has been observed that the catalytic activity was a function of the gold particle size and that smaller gold particles usually exhibited higher catalytic activities than larger ones toward oxidation reactions, such as CO, and alcohol oxidations.21 Our experimental results are in agreement with these findings. The mean diameters of Au NPs prepared by different methods increased in the order CD/PVP (ca. 2.3 nm) < CD/glucose (ca. 6.4 nm) < DPSH (ca. 8.9 nm) ≈ IMP (ca. 9.8 nm). The benzyl alcohol oxidation activities of the gold catalysts decreased as follows: CD/PVP > CD/glucose > DPSH ≈ IMP. This indicates that the oxidation activities of the Au/MIL-101 catalysts are dependent on the particle sizes of gold NPs. The remarkably high catalytic activity of the Au/ MIL-101 (CD/PVP) as compared to the other three gold catalysts (CD/glucose, DPSH, and IMP) may be ascribed to the higher metal dispersion on the MIL-101 support. It has to be pointed out that the only product detected for all catalysts was benzaldehyde under the reaction conditions and the selectivity is independent of gold particle size. To further confirm the remarkable difference in activity of the supported gold catalysts, the catalytic activities for the Au/ MIL-101 (CD/glucose, DPSH, and IMP) were also investigated on the aerobic oxidation of a variety of alcohols under the same conditions as in Table 2 for the Au/MIL-101 (CD/PVP) catalyst. Under base-free conditions, Au/MIL-101 prepared by CD/ glucose, by DPSH, or by IMP method was hardly active in the oxidation of various alcohols including benzylic, allylic, and aliphatic alcohols (Supporting Information Tables S2-S4). The

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TABLE 2: Aerobic Oxidation of Various Alcohols Catalyzed by Au/MIL-101 Prepared by CD/PVPa

a Alcohol (1 mmol), catalyst (Au 1 mol %), toluene as solvent (10 mL), 80 °C, p ) 1 atm, O2 bubbling rate (30 mL min-1). b 1.5 mol % Au. c 1 wt % Au/Ga3Al3O9 (1.5 mol % Au) prepared following the procedure described in ref 20. d 2 mol % Au. e Alcohol (0.5 mmol), catalyst (Au 3 mol %), toluene (10 mL), ambient temperature, p ) 1 atm, O2 bubbling rate (30 mL min-1). f Air was used as the oxidant instead of pure O2.

Figure 6. Turnover frequencies for the oxidation of benzyl alcohol measured at t ) 0.25 h, given as the ratio of moles of benzyl aldehyde per mole of Au per hour (reaction conditions as in Table 2).

results demonstrate that CD/PVP is much superior to the other three liquid-phase preparation methods in the synthesis of a highly active heterogeneous gold catalyst using a MOF support. Recently, Haruta and co-workers deposited Au clusters on several MOFs (such as MOF-5 and Al-MIL53) by solid grinding (SG) and investigated their catalytic activities for the liquidphase aerobic oxidation of benzyl alcohol and 1-phenylethanol in methanol.10c MOF-5 supported Au NPs (0.5 wt % Au with a mean diameter of 4.8 ( 2.2 nm) exhibited the highest catalytic activity among all the Au catalysts by SG, affording about 79% yield of acetophenone in the oxidation of 1-phenylethanol in

the presence of base. Under base-free condition, the Au/MOF-5 catalyst gave ca. 31% yield of corresponding aldehyde product at about 69% conversion in the oxidation of benzyl alcohol. Our Au/MIL-101 (CD/PVP) catalyst exhibited higher catalytic activity and selectivity under similar conditions. This is probably due to the high dispersion of Au NPs in combination with a beneficial synergetic effect of the MIL-101 support and the nature of the carrier. As is well-known, the support may play an important role, either direct or indirect, in determining the activity and selectivity of gold.19,21c,22 MOF-5 has a large surface area similar to MIL-101, but it is unstable upon water adsorption and is observed to decompose even in ambient air.23 In contrast, MIL-101 exhibits good chemical resistances to water and organic solvents.11 Additionally, the larger pore size of MIL101 (ca. 30 Å) as compared to the MOFs used for the SG method (99

third fourth

99 >99

fifth sixth

98 >99

a Alcohol (1 mmol), catalyst (Au 1 mol %), toluene (10 mL), 80 °C, O2 bubbling rate (30 mL min-1).

a (a) Au NPs within the framework (C, gray; O, red; Cr, blue); (b) Au NPs at the surface of the crystals.

SCHEME 2: TOF for the Aerobic Oxidation of (()1-Phenylethanol for the Initial 0.5 h of Reaction (Ratio of Moles of Acetophenone per Mole of Au per Hour)a

a Reaction conditions: (()1-phenylethanol (35 mmol), Au (4 × 10-4 mol %).

surrounded by numerous terephthalate ligands, which construct the networks with chromium cations (Scheme 1a). The electron donation effects of the aryl rings to the Au NPs surfaces through the π-bond interactions may facilitate the formation of anionic Au. It is well-known that the anionic Au clusters are favorable for the activation of O2 on the Au surfaces.24 Therefore, the oxygen molecule may be adsorbed onto the surface of the Auδsite, probably in a superoxo-like form (step 1).10c,24 Subsequently, the cooperative actions of O2δ- with a neighboring Au atom facilitate the rupture of the O-H bond of the adsorbed alcohol molecule to form the gold-alcoholate and -hydroperoxy intermediates (OOH) (step 2). Finally, the metal alcoholate species undergoes a β-hydride elimination to give the corresponding carbonyl compounds,10c along with the formation of O2 and H2O. And the initial metallic site is recovered (step 3). For the Au/MIL-101 prepared by CD/glucose, DPSH, and IMP, the TEM images (Figures 3-5) indicate that most (over 95%) of the Au NPs (>3 nm) should be loaded on the surfaces of the MIL-101 crystals. As reported previously, large metal particles (>3 nm) tend to be present at the external surface of the frameworks.15a,16 Thus, the electronic interactions between the gold NPs and the support are relatively weak as compared to the Au/MIL-101 by CD/PVP.21d,25 The surfaces of Au NPs

Figure 7. TEM images (a, b) of Au/MIL-101 (CD/PVP) after catalytic reaction and corresponding size distribution of Au NPs (c). Inset b is the EDX pattern.

on MIL-101 are not negatively charged to adsorb oxygen and thus are not sufficiently active to form the gold-alcoholate (Scheme 1b). Therefore, these catalysts exhibited extremely low activities toward alcohol oxidations in the absence of base (Supporting Information Tables S2-S4). However, the alcoholate species can be preformed if a base is added with an alcohol.26 Then, oxygen could adsorb onto the Auδ-sites.24 Therefore, the catalytic activities of the Au/MIL-101 (CD/ glucose, DPSH, and IMP) were significantly improved with the addition of base (Supporting Information Tables S5-S7). 3.5. Recycling of the Au/MIL-101 (CD-PVP) Catalyst. The recyclability of the Au/MIL-101 (CD/PVP) catalyst was examined in the aerobic oxidation of 4-methoxy benzyl alcohol. After the aerobic oxidation reaction, the catalyst was separated from the reaction mixture by centrifugation, thoroughly washed with ethanol, and then reused as catalyst for the next run under the same conditions. The catalytic results indicate that no significant loss of efficiency for the aerobic oxidation was observed between the first and sixth runs (Table 3). The crystalline structure of the MIL-101 host matrix mostly remained unchanged after six runs of catalysis (Figure 1). TEM images reveal that the Au NPs were well-dispersed (Figure 7) with an average diameter of 2.2 ( 1.2 nm, which is very similar as that of the catalyst before reaction (2.3 ( 1.1 nm). The results indicate that no sintering of Au NPs occurred during the reaction. AAS analysis of the filtrate after the removal of the catalyst after reaction confirmed that the Au content was below the detection limit, indicating the leaching of Au into the solvent was negligible during the reaction. Moreover, the catalyst was

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removed from the solution after ca. 15% conversion at the reaction temperature. The isolated solution did not exhibit any further reactivity under similar reaction conditions. These studies indicate that the present alcohol oxidation proceeds on the Au NPs located on the MIL-101. 3.6. Aerobic Alcohol Oxidation under Solvent- and BaseFree Conditions. Under solvent- and base-free conditions, the Au/MIL-101 (CD/PVP) catalyst is also highly active for the aerobic oxidation of alcohols. Recently, Cao and co-workers reported a very high TOF of 25000 h-1 for the conversion of 1-phenylethanol (at 160 °C) over Au/Ga3Al3O9, which represents the most active Au catalysts in the current literature.20 Under the same conditions, our Au/MIL-101 (CD/PVP) catalyst afforded an exceptionally high TOF of 29300 h-1, with greater than 99% selectivity for the desired product (Scheme 2), thus showing a great potential for practical applications. 4. Conclusions In summary, a zeolite-type MOF was employed as a support for preparing nanoparticulate Au catalysts by liquid-phase synthesis methods, for the first time, which were used as catalysts in the liquid-phase aerobic alcohol oxidation. The CD/ PVP method with PVP as protecting agent using HAuCl4 as an Au precursor is demonstrated to be the most efficient method to deposit fine Au NPs onto the MOF support. The resulting Au/MIL-101 (CD/PVP) catalyst exhibited extremely high catalytic activity in liquid-phase aerobic oxidation of a variety of alcohols under base-free conditions. It can even efficiently catalyze the oxidation of 1-phenylethanol under ambient conditions in the absence of water or base. Moreover, the catalyst was highly stabilized against metal agglomeration and leaching, maintaining the high activities during a number of recycles. Although the exact mechanism for the oxidation of alcohols over the Au/MIL-101 is still unclear, the high catalytic performance may be attributed to the high dispersion of Au NPs as well as the electron donation effects of aryl rings to the Au NPs in the large cages of the support. The success in the use of a solution-based synthesis strategy might bring new opportunities in the development of highly active gold catalysts by employing rapidly growing MOFs as supports. In addition, the method needs neither organic solvents nor organogold precursors that are expensive, highly reactive, and air-sensitive. Work is underway to investigate the catalytic mechanism and expand MIL-101 to other MOFs for Au NPs and their catalysis in other oxidation processes. Acknowledgment. This work was supported by NSF of China (Grants 20803024 and 20936001), the program for New Century Excellent Talents in Universities (Grant NCET-080203), Doctoral Fund of Ministry of Education of China (Grant 200805611045), and the Fundamental Research Funds for the Central Universities (Grant 2009ZZ0023). We thank Prof. Ralph T. Yang of the University of Michigan for his valuable suggestions. Supporting Information Available: Tables of oxidation reaction results of parent MIL-101, and Au/MIL-101 catalysts (CD/glucose, DPSH, and IMP) with or without the addition of base. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Sheldon, R. A.; Kochi, J. K. Metal-Catalyzed Oxidation of Organic Compounds; Academic Press: New York, 1981.

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