Pd@Cu(II)-MOF-Catalyzed Aerobic Oxidation of Benzylic Alcohols in

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Pd@Cu(II)-MOF-Catalyzed Aerobic Oxidation of Benzylic Alcohols in Air with High Conversion and Selectivity Gong-Jun Chen,* Jing-Si Wang, Fa-Zheng Jin, Ming-Yang Liu, Chao-Wei Zhao, Yan-An Li, and Yu-Bin Dong* College of Chemistry, Chemical Engineering and Materials Science, Collaborative Innovation Center of Functionalized Probes for Chemical Imaging in Universities of Shandong, Key Laboratory of Molecular and Nano Probes, Ministry of Education, Shandong Normal University, Jinan 250014, P. R. China S Supporting Information *

ABSTRACT: A new 3D porous Cu(II)-MOF (1) was synthesized based on a ditopic pyridyl substituted diketonate ligand and Cu(OAc)2 in solution, and it features a 3D NbO motif which is determined by the X-ray crystallography. Furthermore, the Pd NPs-loaded hybrid material Pd@Cu(II)MOF (2) was prepared based on 1 via solution impregnation, and its structure was confirmed by HRTEM, SEM, XRPD, gas adsorption−desorption, and ICP measurement. 2 exhibits excellent catalytic activity (conversion, 93% to >99%) and selectivity (>99% to benzaldehydes) for various benzyl alcohol substrates (benzyl alcohol and its derivatives with electronwithdrawing and electron-donating groups) oxidation reactions in air. In addition, 2 is a typical heterogeneous catalyst, which was confirmed by hot solution leaching experiment, and it can be recycled at least six times without significant loss of its catalytic activity and selectivity.

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INTRODUCTION Selective catalytic oxidation of benzyl alcohols to the corresponding benzaldehydes is one of the most important issues in modern synthetic chemistry and chemical industry. Hitherto, a great deal of research effort has been devoted to finding novel and efficient catalysts for this type of reaction.1−3 Over the past decades, nanoparticles (NPs) have been intensively pursued in a wide range of important catalytic processes owing to their large surface area-to-volume ratios. For example, Cu,4−6 Ru,7 Pd,8−11 Au,12−16 and Au−Pd alloy NPs17−22 were demonstrated to be the efficient species for alcohol oxidation. Among these NPs, Pd NPs have attracted more attention owing to their excellent catalytic performance.23,24 Although Pd NPs show good catalytic behaviors for many useful reactions under reaction conditions, Pd NPs are prone to aggregation and form Pd black because of their high surface energy.25 Therefore, the immobilization of Pd NPs in porous supports is regarded as one of the most practical ways to address this problem. So far, various inorganic materials, such as CeO226−28 SiO2,29−34 ZrO2,35 and porous carbon,36−40 and organic materials, such as polystyrene,41 amphiphilic resin,42 and copolymers,43 have been utilized as supports to incorporate Pd NPs. However, the inorganic−organic hybrid materials as a heterogeneous supporting matrix for metal NPs have not yet attracted much attention. Metal−organic frameworks (MOFs) are a new class of inorganic−organic hybrid materials composed of inorganic © XXXX American Chemical Society

metal nodes and organic linkers. The structurally ordered and molecularly tunable features make them an ideal scaffold to integrate various functional moieties. Their incorporation between distinct individual doped species will endow such composite materials with great potential applications, especially in the catalytic field. High surface areas, controllable pore sizes, and tunable pore environments of MOFs would facilitate the MOF supports to entrap various NPs.44−50 In addition, the crystalline porous structure together with heteroatom donors on MOFs would effectively limit the aggregation and migration of small active catalytic NPs in the solid state, consequently, making the NPs involved NPs@MOFs catalysts to be highly active and reusable.51,52 In this contribution, we report the synthesis and structure of a new porous Cu(II)-MOF (1) and its Pd NPs-embedded composite Pd@Cu(II)-MOF (2) via solution impregnation (Scheme 1). Notably, 2 can be a highly active catalyst to promote the aerobic oxidation of various benzylic alcohols in air with high conversion rates (93% to >99%) and chemoselectivity (aldehydes, >99%).53 Moreover, the catalyst can be easily recovered and reused due to its heterogeneous catalytic nature. Received: December 25, 2015

A

DOI: 10.1021/acs.inorgchem.5b02973 Inorg. Chem. XXXX, XXX, XXX−XXX

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an operating voltage of 200 kV. Scanning electron microscopy (SEM) analysis was performed on a Gemini Zeiss Supra 55. Gas chromatography (GC) analysis was performed on an Agilent 7890B GC. X-ray Crystallography. Diffraction data for the complex were collected at 293(2) K, with a Bruker Smart 1000 CCD diffractometer using Mo−Kα radiation (λ = 0.71073 Å) with the ω-2θ scan technique. An empirical absorption correction was applied to raw intensities.55 The structure was solved by direct methods (SHELX-97) and refined with full-matrix least-squares technique on F2 using the SHELX-97.56 The hydrogen atoms were added theoretically, and riding on the concerned atoms and refined with fixed thermal factors. The details of crystallographic data and structure refinement parameters are summarized in Table S1. The selected bonds lengths and angles for 1 are summarized in Table S2. CCDC 1443366 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_requeat/ cif. Synthesis of Cu(II)-MOF (1). A solution of Cu(OAc)2 (2.4 mg, 0.01 mmol) in MeOH (1 mL) was layered onto a solution of HL (4.8 mg, 0.02 mmol) in CH2Cl2 (2 mL). The solutions were left for about 3 days at room temperature, and compound 1 was obtained as bright green crystals (Scheme 1). Yield, 75%. IR (KBr pellet, cm−1): 3068(w), 1607 (s), 1569 (s), 1536 (s), 1458 (m), 1299 (m), 1250 (m), 1193 (s), 1145 (s), 1073 (m), 787 (s), 701 (s). Elemental analysis (%) calcd for C96H54Cu3F18N6O18 (desolvated): C, 54.59, H, 2.58, N, 3.98; Found: C, 54.48, H 2.71, N 3.96. Synthesis of Pd@Cu(II)-MOF (2). 1 (100 mg, 0.02 mmol) was added to a CH3CN (10 mL) solution of palladium nitrate (90 mg, 0.2 mmol). The mixture was stirred for 1 h at room temperature. The resulting solid was isolated by centrifugation and washed with CH2Cl2. The obtained green-yellow crystalline solids (Figure S1) were mixed with NaBH4 (50 mg, 1.3 mmol) in water (4 mL), and the mixture was stirred for an additional 15 h to afford 2 as a dark brown solid (Scheme 1). The obtained crystalline solids were washed with water and EtOH and dried in air. ICP measurement indicated that the encapsulated amount of Pd NPs in 2 is up to 5.1% (mass fraction). The General Catalytic Reaction Procedure. The catalytic activity of 2 for the aerobic oxidation of benzyl alcohol to benzaldehyde was tested in air. A mixture of benzyl alcohols (25 μL, 0.21 mmol), 2 (20 mg, 5% Pd), and xylene (3.0 mL) was stirred at 130 °C for 25 h in the air (monitored by gas chromatography (GC)) to afford the corresponding aldehydes. The conversion and selectivity were determined by GC. 2 was recovered by centrifugation. After it was washed with ethanol (3.0 mL × 3) and dichloromethane (3.0 mL

Scheme 1. Synthesis of Cu(II)-MOF (1) and Pd@Cu(II)MOF (2)a

a

The photographs of 1 and 2 are inserted.

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EXPERIMENTAL SECTION

Materials and Instrumentation. The reagents and solvents employed were commercially available and used without further purification. Ligand (HL = 4,4,4-trifluoro-1-(4-(pyridin-4-yl)phenyl)butane-1, 3-dione) was synthesized according to our reported procedure.54 Elemental analyses for C, H, and N were obtained on a PerkinElmer analyzer model 240. The powder diffractometer (XRD) patterns were collected by a D8 ADVANCEX-ray with Cu Kα radiation (λ = 1.5405 Å). Electron spin resonance (ESR) spectra were obtained from a Bruker A300-10/12/S-LC. Infrared (IR) samples were prepared as KBr pellets, and spectra were obtained in the 400−4000 cm−1 range using a PerkinElmer 1600 FTIR spectrometer. The total surface areas of the catalysts were measured by the BET (Brunauer− Emmett−Teller) method using carbon dioxide adsorption at 195 K. This was done by the Micromeritics ASAP 2000 sorption/desorption analyzer. ICP-LC was performed on an IRIS Interpid (II) XSP and NU AttoM. HRTEM (high-resolution transmission electron microscopy) analysis was performed on a JEOL 2100 electron microscope at

Figure 1. (a) Simulated and measured XRPD patterns of 1. (b) Coordination sphere of Cu(II) in 1. (c) Single cubic unit of 1. (d) 3-fold interpenetrating NbO-net of 1 with 1D channel down the crystallographic c axis. B

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Figure 2. Left: SEM and elemental maps of Cu and Pd images of 2. Simulated and measured XRPD patterns of 2. Right: HRTEM image of 2.

Figure 3. Left: CO2 adsorption isotherms for Cu(II)-MOF (1) and Pd@Cu(II)-MOF (2) at 195 K. Right: The pore widths of 1 and 2 are centered at 1.3 and 0.9 nm, respectively. × 3) and dried at 90 °C in vacuum, 2 was used in the next run under the same reaction conditions. Leaching Test. The solid catalyst Pd@Cu(II)-MOF (2) was separated from the hot solution right after reaction for 5 h. The reaction was continued with the filtrate in the absence of 2 for an additional 15 h. No further increase in either the conversion of benzyl alcohol or the selectivity of aldehyde was detected, which confirms that the catalytically active sites for this oxidation reaction were located on 2.

molecules can be removed at 110 °C based on the TGA trace (Figures S2 and S3). The desolvated sample of 1 is stable up to ca. 200 °C (demonstrated by XRPD, Figure S4). The high thermal stable feature of 1 would make it suitable to be a catalyst support for uploading active NPs to promote organic reactions at relative high temperature. Palladium-embedded Pd@Cu(II)-MOF (2) was prepared by the impregnation with the Pd(NO3)2 in CH3CN for 5 h, yielding Pd(II)@Cu(II)-MOF (Scheme 1). The Pd NPs-loaded Pd@Cu(II)-MOF (2) was obtained by the reduction of Pd(II) @Cu(II)-MOF with NaBH4 in aqueous solution. The uploaded amount of Pd, as determined by inductively coupled plasma (ICP) measurement, is up to 5.1 wt %. Presumably, the generated Pd NPs might be stabilized by the multitudinous F atoms in the framework. The energy-dispersive X-ray spectrum (EDS) measurement (Figure 2) shows that the uploaded palladium homogeneously distributes in 2. In addition, the structure of Cu(II)-MOF (1) remained intact after the Pd uploading and no changes in the XRPD patterns were detected, which supports that Cu(II)-MOF (1) was chemically stable during the palladium reducing process (Figure 2). In addition, the ESR spectra show that no valence state change of the Cu(II) in Pd@Cu(II)-MOF (2) was observed after the treatment by NaBH4 in aqueous solution under the reaction conditions (Supporting Information, Figure S5). To get a further insight into the structure of 2, highresolution transmission electron microscopy (HRTEM) was used to investigate the dispersion and size distribution of the Pd NPs in 2. HRTEM analysis revealed that the Pd NPs were crystalline and highly dispersed with an average particle size of ca. 2 nm (Figure 2). The lattice fringes had an interplanar spacing of 0.24 nm, corresponding to 1/3 (4 2 2) fringes of face-centered cubic (fcc) Pd.57

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RESULTS AND DISCUSSION Synthesis and Structural Analysis of Cu(II)-MOF (1) and Pd@Cu(II)-MOF (2). The combination of HL with Cu(OAc)2 in a CH2Cl2/MeOH mixed solvent system to afford 1 ([Cu3(L)6]) as bright green crystals in 75% yield. Simulated and measured XRPD patterns match each other very well, indicating that 1 was obtained in pure phase (Figure 1a). Compound 1 crystallizes in the hexagonal space group R3.̅ Each Cu(II) center in 1 adopts a 4 + 2 octahedral coordination sphere (Figure 1b), in which two coplanar chelating β-diketone units form the square (Cu−O distances of 2.107(3)−2.124(3) Å), and the axial positions are occupied by two pyridyl Ndonors (Cu−N distance of 2.016(3) Å). As shown in Figure 1c, octahedral Cu(II) centers are linked together by L to a 4connected 3D NbO network. The shortest opposite Cu(II)··· Cu(II) distance in a single cubic unit is 26.6139(4) Å. Notably, 1 is 3-fold interpenetrating, and the three sets of NbO frameworks are displaced by ca. (1/3, 1/3, 1/3) relative to each other so that the large hexagonal channels appear along the crystallographic c axis (Figure 1d). The opposite Cu···Cu distance in the channel is 22.9330(3) Å. All the fluorine atoms on the framework face toward the center of the channels, and the opposite F···F distance is 14.4423(3) Å. In addition, solvent molecules are located in 1, and the encapsulated solvent C

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Inorganic Chemistry BET Surface Area Measurement of 1 and 2. In order to investigate the BET surface area of Cu(II)-MOF (1) and Pd@ Cu(II)-MOF (2), gas adsorption−desorption experiment was performed on 1 and 2. A N2 adsorption isotherm was first measured, but it exhibits negligible uptake by the framework even at low temperature. Such a phenomenon has been observed for some MOFs.58 However, CO2 adsorption of 1 and 2 at 195 K revealed a typical type IV mode, showing pore condensation with pronounced adsorption−desorption hysteresis (Figure 3). It indicates the existence of mesopores in 1; meanwhile, 2 shows no adsorption−desorption hysteresis, indicating that 2 is microporous. The surface areas of 1 and 2 are 1129.24 and 373.24 m2/g, respectively. The different adsorption capacities and behaviors of 1 and 2 are clearly caused by the Pd NPs doping. The mesopore size distribution curve, calculated from Barrett−Joyner−Halenda analysis, shows a narrow pore diameter distribution at ca. 1.3 nm and ca. 0.9 nm for 1 and 2, respectively (Figure 3), which is well consistent with the single-crystal analysis of 1. Thus, most of the Pd NPs might not be located inside the MOF cavity, as they are larger than the pore size. Therefore, these Pd NPs are presumably sandwiched in the crystallite matrix of Cu(II)-MOF (1) and partly stabilized by F atoms on the framework. Oxidation of Benzyl Alcohol in Air. Reaction Conditions. The aerobic oxidation reactions (with 2, 5% Pd) of the benzylic alcohols with different substituted groups in xylene were carried out in air (monitored by GC). We found that the temperature, solvent system, and reaction time are the key parameters for this Pd@Cu(II)-MOF-catalyzed oxidation reaction. The optimized reaction conditions are obtained based on the benzyl alcohol oxidation in atmospheric air (Table 1). The reaction temperature was first investigated for

xylene is the best solvent for this oxidation reaction in atmospheric air. As shown in Table 1 (entries 5 and 6), the reaction conversion and selectivity (to benzaldehyde) in toluene (110 °C, 25 h, Figure S12) are 88% and 91%, respectively. Meanwhile, the corresponding conversion and selectivity in CH3CN (70 °C, 25 h, Figure S13) are 15% and 95%, respectively. Besides benzaldehyde, benzoic acid and an unidentified species were also detected in entries 5 and 6 by GC. The conversion and the selectivity of benzyl alcohol to benzaldehyde in xylene at different reaction times are shown in Figure 4. The initial conversion of benzaldehyde is continu-

Figure 4. Reaction time examination (black line) and leaching test (red line) for benzyl alcohol reaction catalyzed by 2. Reaction conditions: air, Pd@Cu(II)-MOF (2) (5% Pd), benzyl alcohol (0.21 mmol), xylene (3 mL). The solid catalyst was filtrated from the reaction solution after 5 h, whereas the filtrate was transferred to a new vial and reaction was carried out under the same conditions for an additional 15 h.

ously increased, and the maximum yield (95%) appeared at 25 h (Figures S14−S19). No more obvious changes in the conversions can be detected after 25 h; the TON and TOF are 19 and 0.76 h−1. In order to gain insight into the heterogeneous nature of 2, the hot leaching test was carried out. As indicated in Figure 4, no further reaction took place without 2 after ignition of the oxidation reaction at 5 h (Figures S20 and S21). This finding demonstrated that no leaching of the catalytically active sites occurs and that 2 exhibits a typical heterogeneous catalyst nature (Figure 4). In addition, the catalytic activities of Cu(II)-MOF (1) and treating it with NaBH4 without Pd NPs as the catalysts for this oxidation reaction were also examined under the same reaction conditions; however, the benzyl alcohol conversions are only up to 16% and 15%, respectively (Figures S22 and S23). Recyclability Studies. Reusability of catalysts is very important for industrial applications. The recyclability of 2 was tested. The Pd@Cu(II)-MOF (2) was recycled seven times for benzyl alcohol oxidation (Figure 5). After each run, the solid catalyst was easily collected by centrifugation, washed with ethanol/dichloromethane, dried at 90 °C, and reused in the next run under the same conditions. Interestingly, the reaction conversion increased from 95% at the first run to >99% at the 4−6 runs (Figures S24−S29). Although we distance ourselves from any type of explanation and say forthright that we do not know why the conversion enhanced after three catalytic cycles, it is possible because of some smaller solid catalyst particles formed and dispersed more uniformly in the MOF matrix during the reactions (Figure S30). The size and dispersion of the Pd NPs in 2 did not show any detectable aggregation after six runs, and the atomic lattice fringes (0.24 nm) corresponding

Table 1. Oxidation of Benzyl Alcohol in Different Solvent Systems at Different Temperaturesa

entry

T (°C)

solvent

t (h)

conv. (%)

select (%)

1 2 3 4 5 6

25 70 110 130 110 70

xylene xylene xylene xylene toluene CH3CN

25 25 25 25 25 25

0 15 88 95 88 15

0 >99 >99 >99 91 95

a

Reaction conditions: air, benzyl alcohol (0.21 mmol), Pd@Cu(II)MOF (2) (5% Pd), solvent (3 mL).

the impact on the efficiency of oxidation reactions in air. As shown in Table 1, upon increase of the reaction temperature from 25 to 130 °C in xylene (entries 1−4), the conversion of benzyl alcohol dramatically enhanced. Notably, >99% selectivity toward benzaldehyde was maintained during the temperature increasing process. It should be noted that an excellent yield (95%) was achieved when the reaction was carried out at 130 °C (Table 1, entry 4). However, the chemoselectivity to benzaldehyde is still >99% at this temperature, which was confirmed by GC analysis (Figures S6−S11). It, therefore, appears that 130 °C is the optimum temperature for this benzaldehyde oxidation reaction. In addition, we also investigated the effect of the solvent system and found that D

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Figure 5. Recycling catalytic test. Left: conversion and selectivity obtained at 25 h in repeated runs of the benzyl alcohol oxidation catalyzed by 2 (air, 130 °C, benzyl alcohol (0.21 mmol), 2 (5% Pd), xylene (3.0 mL)). The average conversion and chemoselectivity based on six catalytic cycles are >97% and >99%, respectively. Right: corresponding XRPD patterns (a−g corresponding to 1−7 runs).

Table 2. Summary of the Reported Benzyl Alcohol Oxidation Reactions Using Pd NPs as the Heterogeneous Catalysts catalyst

cond.

run

conv. (%)

Pd/NaX zeolite Pd/hybrid SiO2 Pd/A20E10 Pd/Al2O3 Pd/GC Pd/SBA-16 Pd-pol CM-CeO2-Pd-1.0 Pd/CB Pd/AC ARP-Pd Pd/PEG Pd@U-E15 Pd cluster colloid PdHAP-0

toluene/100 °C/O2 (3 mL/min) CO2/O2 (18 MPa)/O2 (8 vol %)/80 °C toluene/water/80 °C/air (5 bar) solvent free/120 °C/O2 (50 mL/min) solvent free/110 °C/O2 (20 mL/min) water/50 °C/air or O2 water (K2CO3)/100 °C/air (1 atom) solvent free/O2 353 K/O2 water/60 °C/O2 (1.5 atm) water/100 °C/O2 (1 atm) solvent free/80 °C/O2/scCO2 water/K2CO3/90 °C/O2 (1 atm) water/32 °C/air bubbling/(pH 3.5) trifluorotoluene/90 °C/O2 (1 atm) water/110 °C/O2 (1 atm) water/K2CO3/80 °C/O2 toluene/K2CO3/85−90 °C/air xylene/130 °C/air (atmospheric pressure)

1 1 1 1 1 11 6 1 1 1 1 1 1 10 1 1 1 4 6

66 99.5 85.6 80.1 72.5 >99 93−99 82.1 99 50 97 83.1 90 86−93 >99 >99 48 86−100 95 to >99

Pd@hmC MNP−Pd Pd@Cu(II)-MOF

to 1/3(422) of face-centered cubic (fcc) Pd were also clearly observed (Figure S27). The conversion, however, decreased to 85% (Figure S31) at the seventh run, indicating that 2 began to be deactivated. HRTEM analysis revealed that some of the Pd NPs in 2 began to aggregate (Figure S32), which could be the reason for the low catalysis efficiency of 2 after six runs. Such an observation was further confirmed by the elemental maps of Cu and Pd in 2 after six runs (Figure S33). In addition, ICP measurement indicated that the amount of Pd NPs in 2 has no significant changes after six runs; the amount of Pd species, however, dropped to 3.85% after the seventh cycle, which could be an additional reason for the catalyst inactivation (Table S3). The XRD patterns of 2 and that after being reused for six cycles indicated that the structural integrity of 2 was well preserved. Therefore, Cu(II)-MOF can be an ideal support to upload Pd NPs, furthermore, effectively preventing their sintering and aggregation in the solid state even at a higher reaction temperature (Figure 5). It has been reported that the oxidative dehydrogenation of alcohols to the corresponding aldehydes happened on all

select (%) 97 89.8 97 94.3 98.3 >99 >99 62.9 95 >99 99.8 90 100 99 90 37 90−93 >99

ref. 32 33 61 62 36 63 64 26 37 38 40 65 66 41 67 68 69 this work

exposed metallic palladium faces; therein, the decarbonylation often occurred and the undesired products preferentially formed on hollow sites of Pd particle faces.31,59 However, in our experiments, almost no decarbonylating products were detected during reaction, so we proposed that the sites on facecentered cubic Pd are the primary sites for benzyl alcohol oxidation.60,67 The distance of the H(Cα-H)−H(hydroxy) on benzyl alcohol is about 0.21 nm, which is close to the Pd−Pd bonding distance of 0.24 nm found in 2. This might effectively enhance the interactions between Pd NPs and reaction substrates, consequently, leading to a high oxidation catalytic efficiency. Compared to various reported Pd NPs-loaded heterogeneous catalysts (Table 2), 2 herein exhibits good catalytic activity and selectivity for benzyl alcohol oxidation in the air. Therefore, it can be a valuable complement for the MOFs to heterogeneous aromatic carbinols oxidation composite catalysts. Oxidation of Other Substituted Benzylic Alcohols. The scope of the oxidation catalytic system was explored by performing the oxidation reactions of various substituted benzylic alcohols. Table 3 summarizes the results of these E

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Table 3. Oxidation Reactions of Benzyl Alcohols with Different Substituted Groups Catalyzed by 2 in Aira

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for financial support from NSFC (Grant Nos. 21475078, 21271120, and 21301109), the 973 Program (Grant Nos. 2012CB821705 and 2013CB933800), the Taishan Scholar’s Construction Project, the Reward Fund for Outstanding Young and Middle Aged Scientists of Shandong Province (no. BS2012CL032), and the Jinan Science and Technology Bureau (OUT_06623).

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(1) Backvall, J. E. Modern Oxidation Reactions, 1st ed.; Wiley-VCH: Weinheim, Germany, 2004. (2) Fernandez, M. I.; Tojo, G. Oxidation of Alcohols to Aldehydes and Ketones. In A Guide to Current Common Practice; Springer: New York, 2006. (3) Long, R.; Huang, H.; Li, Y. P.; Song, L.; Xiong, Y. J. Adv. Mater. 2015, 27, 7025−7042. (4) Hu, Z.; Kerton, F. M. Appl. Catal., A 2012, 413−414, 332−339. (5) Ryland, B. L.; McCann, S. D.; Brunold, T. C.; Stahl, S. S. J. Am. Chem. Soc. 2014, 136, 12166−12173. (6) Li, L. C.; Matsuda, R. I.; Sato, T.; Kanoo, H. P.; Jeon, H. J.; Foo, M. L.; Wakamiya, A.; Murata, Y.; Kitagawa, S.; Tanaka, I. J. Am. Chem. Soc. 2014, 136, 7543−7546. (7) Sheldon, R. A.; Arends, I. W. C. E.; ten Brink, G.-J.; Dijksman, A. Acc. Chem. Res. 2002, 35, 774−781. (8) Tonucci, L.; Nicastro, M.; d'Alessandro, N.; Bressan, M.; D’Ambrosio, P.; Morvillo, A. Green Chem. 2009, 11, 816−820. (9) Edwards, J. K.; Pritchard, J.; Lu, L.; Piccinini, M.; Shaw, G.; Carley, A. F.; Morgan, D. J.; Kiely, C. J.; Hutchings, G. J. Angew. Chem., Int. Ed. 2014, 53, 2381−2384. (10) Klotter, F.; Studer, A. Angew. Chem., Int. Ed. 2014, 53, 2473− 2476. (11) Guo, Z.; Liu, B.; Zhang, Q. H.; Deng, W. P.; Wang, Y.; Yang, Y. H. Chem. Soc. Rev. 2014, 43, 3480−3524. (12) Miyamura, H.; Matsubara, R.; Miyazaki, Y.; Kobayashi, S. Angew. Chem., Int. Ed. 2007, 46, 4151−4154. (13) Shang, C.; Liu, Z. P. J. Am. Chem. Soc. 2011, 133, 9938−9947. (14) Asao, N.; Hatakeyama, N.; Menggenbateer, T.; Minato, E. I.; Hara, M.; Kim, Y.; Yamamoto, Y.; Chen, M.; Zhang, W.; Inoue, A.; Ito, E. Chem. Commun. 2012, 48, 4540−4542. (15) Tsukamoto, D.; Shiraishi, Y.; Sugano, Y.; Ichikawa, S.; Tanaka, S.; Hirai, T. J. Am. Chem. Soc. 2012, 134, 6309−6315. (16) Liu, H. L.; Liu, Y. L.; Li, Y. W.; Tang, Z. Y.; Jiang, H. F. J. Phys. Chem. C 2010, 114, 13362−13369. (17) Yang, X.; Huang, C.; Fu, Z. Y.; Song, H. Y.; Liao, S. J.; Su, Y. L.; Du, L.; Li, X. J. Appl. Catal., B 2013, 140−141, 419−425. (18) Cui, W. J.; Xiao, Q.; Sarina, S.; Ao, W. L.; Xie, M. X.; Zhu, H. Y.; Bao, Z. Catal. Today 2014, 235, 152−156. (19) Cao, E. H.; Sankar, M.; Nowicka, E.; He, Q.; Morad, M.; Miedziak, P. J.; Taylor, S. H.; Knight, D. W.; Bethell, D.; Kiely, C. J.; Gavriilidis, A.; Hutchings, G. J. Catal. Today 2013, 203, 146−152. (20) Wang, H. W.; Wang, C. L.; Yan, H.; Yi, H.; Lu, J. L. J. Catal. 2015, 324, 59−68. (21) Wang, J. C.; Kondrat, S. A.; Wang, Y. Y.; Brett, G. L.; Giles, C.; Bartley, J. K.; Lu, L.; Liu, Q.; Kiely, C. J.; Hutchings, G. J. ACS Catal. 2015, 5, 3575−3587. (22) Sun, D. H.; Zhang, G. L.; Jiang, X. D.; Huang, J. L.; Jing, X. L.; Zheng, Y. M.; He, J.; Li, Q. B. J. Mater. Chem. A 2014, 2, 1767−1773. (23) Dhakshinamoorthy, A.; Alvaro, M.; Garcia, H. ACS Catal. 2011, 1, 48−53. (24) Ding, S. Y.; Gao, J.; Wang, Q.; Zhang, Y.; Song, W. G.; Su, C. Y.; Wang, W. J. Am. Chem. Soc. 2011, 133, 19816−19822. (25) Iwasawa, T.; Tokunaga, I. M.; Obora, Y.; Tsuji, Y. J. Am. Chem. Soc. 2004, 126, 6554−6555.

Reaction conditions: air, 130 °C, benzylic alcohols (0.21 mmol), 2 (5% Pd), xylene (3.0 mL). Reaction conversion and selectivity are determined based on GC analysis (Figures S34−S39).

a

oxidation reactions. We found that all benzylic alcohols with both electron-withdrawing (−F, −NO2) and electron-donating groups (−OMe, −Me) at either para- or meta-position gave excellent conversions (93−99%). In addition, the reaction selectivity toward benzaldehydes is basically 100% in all cases. Therefore, Pd@Cu(II)-MOF is indeed a good heterogeneous catalyst for aromatic carbinols oxidation.

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CONCLUSIONS A new porous Cu(II)-MOF (1) with a NbO network was synthesized. Pd NPs-loaded Pd@Cu(II)-MOF (2) was prepared based on 1 by impregnation and reduction reaction in solution. The Cu(II)-MOF with fluorine atoms can effectively stabilize the palladium nanoparticles in the crystals. More importantly, the obtained Pd NPs-embedded Pd@ Cu(II)-MOF can be a highly active heterogeneous catalyst for the oxidation of aromatic carbinols to the corresponding benzaldehydes in the air with high conversion rates and almost 100% selectivity. The catalyst can be recovered and reused for at least six runs without a significant loss in its activity and selectivity. Further work on exploring new catalytic reactions with such NPs@MOF catalytic systems is currently underway.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02973. Single-crystal data, TGA traces, XRPD patterns, photographs for Pd(II)@Cu(II)-MOF, ICP measurement, HRTEM images, EDS measurement, and GC analysis (PDF) Crystallographic data for 1 (CIF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. F

DOI: 10.1021/acs.inorgchem.5b02973 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.5b02973 Inorg. Chem. XXXX, XXX, XXX−XXX