Au@Cu(II)-MOF: Highly Efficient Bifunctional Heterogeneous Catalyst

Jun 20, 2016 - College of Chemistry, Chemical Engineering and Materials Science, Collaborative Innovation Center of Functionalized Probes for Chemical...
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Au@Cu(II)-MOF: Highly Efficient Bifunctional Heterogeneous Catalyst for Successive Oxidation−Condensation Reactions Jing-Si Wang, Fa-Zheng Jin, Hui-Chao Ma, Xiao-Bo Li, Ming-Yang Liu, Jing-Lan Kan, Gong-Jun Chen,* 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 composite Au@Cu(II)-MOF catalyst has been synthesized via solution impregnation and full characterized by HRTEM, SEM-EDS, XRD, gas adsorption−desorption, XPS, and ICP analysis. It has been shown here that the Cu(II)-framework can be a useful platform to stabilize and support gold nanoparticles (Au NPs). The obtained Au@ Cu(II)-MOF exhibits a bifunctional catalytic behavior and is able to promote selective aerobic benzyl alcohol oxidation−Knoevenagel condensation in a stepwise way.

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INTRODUCTION Heterogeneous catalysis has received more and more attention in the past few decades.1 The fabrication of a composite type of heterogeneous catalysts is one of the practical approaches to access solid catalysts for heterogeneous catalysis.2 For design and synthesis of composite catalysts, various porous materials, such as zeolites, mesoporous aluminosilicates, metal oxides, carbon, and polymers, have been chosen and used as supports to upload active catalytic metal species, especially the high catalytic performance Au nanoparticles (Au NPs).3 As we know, Au NPs have been recognized as very active and effective green catalysts and are widely used in promoting many types of organic reactions such as oxidation of alcohols, reduction of nitro-aromatic compounds, water−gas shift reaction, and so on.4 As a unique class of porous materials, metal−organic frameworks (MOFs) are promising platforms for creating a composite heterogeneous catalyst system.5 Owing to their diverse chemical compositions, high surface areas, tunable pore size, and internal microenvironments, MOFs allow the implementation of catalytic properties by embedding metal NPs.6 This strategy is widely accepted and becomes a significant tool for expanding MOF-supported application in catalysis. On the other hand, MOFs themselves are inherent heterogeneous catalysts due to their inorganic−organic hybrid composition and polymeric nature. Therefore, the incorporation of MOF-supports with the metal NPs would lead to enhanced or even novel catalytic properties compared to their pristine counterparts.7 As a continuation of our work with the NPs-loaded MOFs materials,8 we report herein the fabrication of Au NPs © XXXX American Chemical Society

embedded composite catalyst Au@Cu(II)-MOF (1) via solution impregnation. The obtained 1 exhibits a bifunctional heterogeneous catalytic nature and, furthermore, catalyzes benzyl alcohol oxidiation and Knoevenagel condensation reactions in a successive manner.

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EXPERIMENTAL SECTION

Materials and Chemicals. The reagents and solvents employed were commercially available and used without further purification. Cu(II)-MOF (Cu3L6) was prepared by the combination of a pyridyl substituted diketonate ligand (HL) and Cu(OAc)2 in a CH2Cl2/ MeOH mixed solvent system, which was reported by us previously.8a,9 Instrumentation. The powder diffractometer (XRD) patterns were collected by a D8 ADVANCE X-ray with Cu Kα radiation (λ = 1.5405 Å). The total surface areas of the catalysts were measured by the BET (Brunauer−Emmer−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 InterpidII XSP and NU AttoM. HRTEM (high resolution transmission electron microscopy) analysis was performed on a JEOL 2100 electron microscope at an operating voltage of 200 kV. Scanning electron microscopy (SEM) images and energy dispersive X-ray (EDX) spectroscopy were taken on a SUB010 scanning electron microscope with acceleration voltage of 20 kV. Gas chromatography (GC) analysis was performed on an Agilent 7890B GC. Electron spin resonance (ESR) spectra were obtained from a Bruker A300-10/12/S-LC. XPS spectra were obtained from PHI Versaprobe II. UV−vis absorption spectra were recorded at room temperature in quartz cells of 1 cm path length using a TU-1800 SPC spectrophotometer. Received: April 14, 2016

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DOI: 10.1021/acs.inorgchem.6b00925 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Schematic Representation of Synthesis of 1 from Cu(II)-MOF and HAuCl4

Synthesis of 1. Cu(II)-MOF (15 mg, 0.003 mmol) was added to a CH3OH (10 mL) solution of chloroauric acid (20 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 solid was mixed with NaBH4 (50 mg, 1.3 mmol) in water (4 mL), and the mixture was stirred for an additional 5 h to afford 1 as a dark brown solid. The obtained crystalline solids were washed with CH3CN and EtOH and dried in air. Inductively coupled plasma (ICP) measurement indicated that the encapsulated amount of Au NPs in 1 is 9.38 wt %. Reduction of 4-Nitrophenol (4-NP) to 4-Aminophenyl (4AP). Freshly prepared NaBH4 aqueous solution (2 M, 30 equiv) was added to an aqueous solution (3 mL) of 4-NP (0.35g, 2.5 mmol) with 1 (5.4 mg, 0.1 mol %) at room temperature. The absorbance spectra were used to monitor (wavelength range 280−600 nm) the reaction progress. Benzyl Alcohol Oxidation Reaction Procedure. The catalytic activity of 1 for the aerobic oxidation of benzyl alcohol to benzaldehyde was tested in air. A mixture of benzyl alcohols (20 μL, 0.20 mmol), 1 (13 mg, Au 3 mol %), and toluene (3 mL) was stirred at 110 °C for 15 h in air (monitored by GC) to afford the corresponding aldehydes. Benzyl Alcohol Oxidation−Knoevenagel Reaction Procedure. A mixture of benzyl alcohols (30 μL, 0.30 mmol) in the presence of 1 (19 mg, 3 mol %) in toluene (3 mL) was heated at 110 °C for 15 h in air. After cooling down to the room temperature, a MeOH solution (2 mL) of malononitrile (13.4 μL, 0.34 mmol) was added to the above system. The mixture was stirred for an additional 7 h at room temperature in air to afford the corresponding 2benzylidenemalononitriles (monitored by GC). Catalyst Regeneration. Material 1 was recovered by centrifugation, then washed with ethanol (3.0 mL) and dichloromethane (3.0 mL) (3 times), and dried at 90 °C in vacuum for the next run under the same reaction conditions. Leaching Test. The solid catalyst Au@Cu(II)-MOF (1) was separated from the hot solution right after reaction for 6 h. The reaction was continued with the filtrate in the absence of 1 for additional 14 h. No further increase in either the conversion of benzyl alcohol or selectivity of aldehyde was detected, which confirms the catalytically active sites for this oxidation reaction located on 1.

the reduction of Au(III)@Cu(II)-MOF with NaBH4 in aqueous solution along with a distinct color change (from green-yellow to dark brown, Scheme 1). The uploaded amount of Au, as determined by inductively coupled plasma (ICP) measurement, is up to ca. 9.38 wt %. The oxidation state of the encapsulated Au after reduction was determined by X-ray photoelectron spectroscopy (XPS). The observation of the Au 4f7/2 and 4f5/2 peaks at 83.86 and 87.46 eV in XPS of 1 demonstrated the reduction from Au(III) to Au(0) (Supporting Information, Figure S1).9,10 In addition, the ESR spectra show that no valence state change of the Cu(II) in 1 was observed after the treatment by NaBH4 in aqueous solution under the reaction conditions (Supporting Information, Figure S2). High resolution transmission electron microscopy (HRTEM) was used to investigate the dispersion and size distribution of the Au NPs in 1. As shown in Figure 1, nearly

Figure 1. Left: HRTEM image of 1. Right: XRPD patterns of Cu(II)MOF, Au(III)@MOF, and 1.

monodisperse Au NPs with approximately spherical shape were uniformly distributed in 1, and the average particle size was ca. 2 nm. The atomic lattice fringes with spacing of 0.23 nm corresponding to Au (111) planes were observed.11 The SEMEDX elemental mapping of 1 was also measured. Figure 2 shows the elemental maps of the Au and Cu images, respectively, indicating that Au NPs were highly dispersed in

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RESULTS AND DISCUSSION Synthesis and Characterization of 1. Cu(II)-MOF was prepared by our previously reported method. It possesses a 3fold interpenetrating NbO-type of network with nanosized hexagonal channels.8a,9 The opposite Cu···Cu distance in the channel is 22.9330(3) Å. Material 1 was prepared via solution impregnation (Scheme 1). The combination of HAuCl4 and Cu(II)-MOF in CH3OH at room temperature (1 h) yields Au(III)@Cu(II)-MOF as green-yellow crystalline solid (Scheme 1). The Au NPs-embedded 1 was synthesized by

Figure 2. SEM image and SEM-EDX elemental mapping of 1. B

DOI: 10.1021/acs.inorgchem.6b00925 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

the Au NPs are well-embedded in the MOF matrix without observable aggregation, and crystallinity of the MOF was maintained after the reaction (Supporting Information, Figure S4). Next, we find that 1 exhibits similar reactivity with excellent yields toward the reduction of 2-nitrophenol (2-NP) and 3-nitrophenol (3-NP) under the same conditions (Supporting Information, Figures S5 and S6). The peaks at 400 (for 2-NP) and 420 (for 3-NP) nm disappeared; meanwhile, the new peaks at 279 and 285 nm attributed to 2-aminophenol (2-AP) and 3-aminophenol (3-AP) appeared, and no more changes in the UV−vis spectra have been observed in 30 min, indicating the reactions were finished. On the basis of the above observation, 1 can be considered as a highly effective Au NP-based heterogeneous catalyst. Oxidation of Benzyl Alcohol in Air. Reaction Conditions and Leaching Test. It is well-known that the selective catalytic oxidation of benzyl alcohols to benzaldehydes is one of the most important organic transformations in modern synthetic chemistry and in the industry, so the development of high performance and stable heterogeneous catalysts are very significant.13 The selective aerobic oxidation of benzyl alcohol was chosen as a model reaction to evaluate the catalytic activity of 1 and optimize the reaction conditions. Reactions were carried out in air (with 1, 3 mol % Au) and base-free conditions. As shown in Table 1, weak polar toluene (entries 3−5, Supporting Information, Figures S9−S15) instead of water

Cu(II)-MOF matrix. Notably, the XRPD pattern of 1 is in good agreement with that of Cu(II)-MOF and 1 (Figure 1), which indicates that the structural integrity of the Cu(II)-MOF is well-preserved. In addition, the permanent porosity of 1 was determined by measuring carbon dioxide gas (CO2) adsorption at 195 K (Supporting Information, Figure S3). Compared to that of Cu(II)-MOF, the surface area of 1 decreased from 1129.24 to 70.27 m2/g. The adsorption change was clearly caused by the Au 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 Cu(II)-MOF and Au@Cu(II)-MOF (1), respectively (Supporting Information, Figure S3). Catalytic Activity Test of 1. The catalytic activity of the encapsulated Au NPs in 1 was preliminarily examined by the reduction reaction of 4-NP. The catalytic reduction reaction of 4-NP is not only an effective approach for degradation toxic organic pollutant, but also a model reaction for evaluation of Au NPs-loaded catalysts.12 The reaction was carried out in the presence of a hydrogen source, NaBH4. Initially, the 4-NP solution has a light yellow color, which turned dark yellow upon the addition of NaBH4 because of the generation of 4nitrophenolate. Notably, with the use of 1 (0.1 mol %), the yellow color gradually faded away and completely vanished in about 30 min, indicating that the reduction of 4-NP to 4-AP was finished. Besides naked-eye detectable color change, the reaction progress was monitored by UV−vis spectra. As indicated in Figure 3, the adsorption of 4-NP at 400 nm

Table 1. Optimization of the Benzyl Alcohol Oxidation Reaction Conditionsa

entry

T (°C)

solvent

t (h)

conversion (%)

benzaldehyde (%)

1 2 3 4 5

95 70 60 90 110

H2O CH3CN toluene toluene toluene

20 20 20 20 20

8 12 37 63 98

>99 >99 >99 >99 >99

a Reaction conditions: air, benzyl alcohol (0.20 mmol), 1 (13 mg, 3 mol % Au), solvent (3 mL).

(entry 1, Supporting Information, Figure S7) and acetonitrile (entry 2, Supporting Information, Figure S8) is an ideal solvent for this aerobic selective oxidation. The influence of temperature on the catalytic performance of the present reaction system was also investigated in toluene. With the increase of the reaction temperature from 60 to 110 °C, the conversion of benzaldehyde was significantly enhanced (Supporting Information, Figures S9−S15). The excellent conversion (98%) and selectivity (>99%) were achieved when the reaction was carried out at 110 °C. The total number (TON) and turnover frequency (TOF) for oxidation of benzyl alcohol are 32.67 and 2.18 h−1, respectively, under the investigated conditions. It is noteworthy that the Au NPs free Cu(II)-MOF showed almost no conversion (only 3%) of benzyl alcohol (Supporting Information, Figure S16), demonstrating the requirement of the Au NPs to perform this aerobic oxidation. The relationship between conversion and reaction time is shown in Figure 4. The initial conversion of benzaldehyde is continuously increased, and the maximum yield (98%) was observed at 15 h (Supporting Information, Figure S15). In

Figure 3. Representative time-dependent UV−vis adsorption spectra for the reduction of 4-NP over 1 in aqueous solution at room temperature. The color change of 4-NP reduction with 1 and NaBH4 in aqueous media.

rapidly decreased with a concomitant increase in the peak at 300 nm, and no change in UV−vis spectra was observed in 30 min. On the other hand, the isosbestic point between the two peaks further confirmed a direct conversion of 4-NP to 4-AP under the reaction conditions. Since the concentration of NaBH4 is much higher than those of 4-NP and 1, and it is reasonable to assume that the concentration of NaBH4 remains constant during the reaction. The TON and TOF for the reduction of 4-NP reaction are 1000 and 2000 h−1. After the reaction, the catalyst can be readily recovered by centrifugation. The HRTEM analysis and XRPD patterns demonstrated that C

DOI: 10.1021/acs.inorgchem.6b00925 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

can be a beneficial complement for the MOFs to heterogeneous catalysts for the selective aerobic benzyl alcohol oxidation. Recyclability Studies. The recyclability of 1 was examined. After each catalytic run, the solid catalyst was easily collected by centrifugation, washed with acetonitrile, dried at 90 °C, and reused in the next run under the same conditions. Yields of between 98% and 94% were obtained within the five recycling runs with the excellent selectivity (>99%) (Figure 5a and Supporting Information, Figures S17−S20). The XRD patterns of 1 and that after reused for five catalytic cycles indicated that the structural integrity of the Cu(II)-MOF was well-preserved, indicating the Cu(II)-MOF herein is an ideal support for uploading Au NPs (Figure 5b). The diffraction at 38.2° related to Au NPs gradually appeared with the increase of catalytic runs which might be caused by intensity decrease of the MOF peaks after reaction. On the other hand, HRTEM image of used 1 only shows slight aggregation of Au NPs (Figure 5c), which does not significantly affect the activity of the catalyst within five catalytic cycles. This result demonstrates that the Cu(II)MOF can effectively prevent the uploaded Au NPs from sintering and aggregation in the MOF matrix even at higher reaction temperature. In addition, the elemental maps of the Au and Cu images confirm Au NPs highly dispersed in Cu(II)MOF matrix (Figure 5d, and Supporting Information, Figure S21) and the ICP measurement indicated that the Au content in 1 is basically unchanged (9.24%) even after five catalytic runs. After that, we investigated the catalytic activity of 1 for the selective aerobic oxidation of a series of other substituted

Figure 4. Reaction time examination and leaching test for benzyl alcohol reaction based on 1. Reaction conditions: air, 1 (3 mol % Au), benzyl alcohol (0.21 mmol), and toluene (3 mL). The solid catalyst was filtrated from the reaction solution after 6 h, whereas the filtrate was transferred to a new vial and reaction was carried out under the same conditions for an additional 14 h.

order to gain insight into the heterogeneous nature of 1, the hot leaching test was carried out. As indicated in Figure 4, no further reaction took place without 1 after ignition of the oxidation reaction at 6 h. This finding demonstrated that 1 exhibits a typical heterogeneous catalyst nature. Compared to most other reported Au NPs-loaded catalysts, 1 herein exhibits better conversion and selectivity for the selective aerobic benzyl alcohol oxidation. As shown in Table 2, most reported benzyl alcohol oxidations are carried out in O2, sometimes with higher pressure, instead of in air. In addition, Au@Cu(II)-MOF herein shows a good reusability compared to many other Au catalysts loaded on inorganic or inorganic− organic composite supports. So Au@Cu(II)-MOF (1) herein

Table 2. Summary of the Reported Benzyl Alcohol to Benzaldehydes Oxidation Reactions Using Au NPs as the Heterogeneous Catalysts entry

catalyst

condition

run

conv (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Au NPs-rGO Au/MgO Au/HT-3 Au (45 nm)/TiO2 IMAuH Au/Mn-CeO2 Au/CuOco Au/MgO Au/Mg-500 Au/NaY magnetitite−silica−gold Au/Fe3O4@SiO2 Au/SiO2@Al2O3 Au/MIL-101 Au@ZIF-8 Au/ZnO@MOF-5 Au/PMAMIL-101 Au@UiO-66 Au@UiO-66-NH2 Au/HT Au/NiAl-LDH-F-36 Au/TiO2 Au101/TiO2−O2 Au/MnO2 Au/SiO2 Au/HT Au−C Au@Cu(II)-MOF

O2, 130 °C O2, 120 °C visible light O2, 120 °C O2, 80 °C O2, 90 °C O2, 80 °C 10 atm, O2, 100 °C 10 atm, O2, 110 °C 1.0 MPa, O2, 80 °C O2, 140 °C 6 atm, O2, 100 °C solvent-free aerobic oxidation 308 K, air O2, 80 °C O2, 80 °C air flow, 80 °C O2 flow, 80 °C O2, 100 °C 393 K O2, 100 °C TBHP, 94 °C 5 bar O2, 80 °C O2 (0.2 MPa), 120 °C steady-state liquid phase O2 (3 mL/min), 393 K water, 60 °C toluene, 110 °C, air

1 4 3 1 1 3 2 1 1 4 3 4 1 6 1 1 4 9 5 5 3 1 2 3 1 1 1 5

75 55.5 72.93 42 42.9 43.22 85.7 15 78 94.4 49.0 84.3 21.90 99 81 74 60 64 94 89 53 50 96 8.8 11.3 99 99 >95

D

selectivity (%) 95.0 96.78 100 95 99 >99 100 66 97.1 95 72.32 99 98 100 100 99 99 73 98.5 43.4 99 99 >99

ref. 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 this work

DOI: 10.1021/acs.inorgchem.6b00925 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 5. Recycling test. (a) The conversion and selectivity obtained at 15 h in repeated runs of the benzyl alcohol aerobic oxidation promoted by 1 (air, 110 °C, 3 mol % Au). (b) Corresponding XRPD patterns. (c) HRTEM image after five catalytic runs. (d) SEM image and SEM-EDX elemental mapping of 1 after five catalytic runs.

temperature; the GC analysis indicated that the condensation reaction was finished in 7 h to afford 2-benzylidenemalononitrile as the expected product with the excellent conversion (98%) and selectivity (99%) (Supporting Information, Figure S28). On the basis of this result and the above observation, 1 could be a bifunctional catalyst to promote benzyl alcohol oxidation and the following Knoevenagel condensation in due succession. When a toluene solution (2 mL) of benzyl alcohol (0.3 mmol) in the presence of 1 (3 mol %) was heated at 110 °C for 15 h, a MeOH solution (2 mL) of malononitrile (0.34 mmol) was added to the above system after cooling down to room temperature. The mixture was stirred for an additional 7 h at room temperature to afford benzylidenemalononitrile with excellent conversion (98%) and selectivity (99%). The TON and TOF for the oxidation−Knoevenagel reaction are 32.67 and 1.48 h−1 (Supporting Information, Figure S29). Again, the diffractions (2θ value at 38.2°) of Au NP species were detected from powder XRD patterns after oxidation−Knoevenagel reaction with the decrease of crystallinity of the Cu(II)-MOF support after three runs (Figure 6a, Supporting Information, Figures S30−S31). HRTEM analysis revealed that the mean diameter of the Au NPs slightly increases (