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Research Article Cite This: ACS Sustainable Chem. Eng. 2019, 7, 11485−11492

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Cu/Cu2O‑MC (MC = Mesoporous Carbon) for Highly Efficient Hydrogenation of Furfural to Furfuryl Alcohol under Visible Light Min Zhang and Zhaohui Li* Research Institute of Photocatalysis, State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, No. 2, Xueyuan Road, New District, Fuzhou 350116, P. R. China

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S Supporting Information *

ABSTRACT: Cu/Cu2O-MC (MC = mesoporous carbon), in which small sized metallic Cu and Cu2O nanoparticles were deposited on graphitized mesoporous carbon, has been successfully obtained via a pyrolysis at 1100 °C of a Cu/mesostructured polymer framework precursor obtained via an ion-exchange process between Cu2+ in [Cu(NH3)4]2+ and H+ in polymeric gel. Due to a synergistic effect played by Cu2O and plasmonic metallic Cu, the as-obtained Cu/Cu2OMC showed superior performance for selective transfer hydrogenation of furfural (FAL) to produce furfuryl alcohol (FOL) under visible light, with 94.3% of FAL converted and a yield of 90.9% to FOL after being irradiated for 14 h. The TOF for the production of FOL was determined to be 4.65 h−1 in 14 h, comparable to those previously reported over Cu-based catalysts for transfer hydrogenation of FAL to produce FOL carried out under high temperature. This study provides a green, sustainable, and cost-effective method for highly efficient transformation of FAL to produce FOL. This study also highlights the great potential of using Cu-based catalysts for light initiated transfer hydrogenation reactions. KEYWORDS: transfer hydrogenation, visible light, copper-based catalyst, furfural, furfuryl alcohol

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INTRODUCTION The production of high-value-added chemicals from biomass is of great interest since the dependence on petroleum-based chemicals can be reduced by replacing fossil feedstocks with renewable ones.1−3 Furfural (FAL), a furan derivative primarily obtained by acid-catalyzed dehydration of biomass-derived hemicellulose, is considered to be a promising platform molecule, which can be converted to a variety of high-valueadded chemicals and fuels.4 Particularly, selective hydrogenation of FAL to furfuryl alcohol (FOL) shows great potential for industrial applications, since FOL is an important intermediate to drugs as well as a precursor in the synthesis of polymers, resins and adhesives.5−7 However, due to the presence of different functionalities including furan ring, CC as well as CO bonds in FAL, selective hydrogenation of only its CO bond is a great challenge. Byproducts are often produced by hydrogenolysis of the side chain −CHO to −CH3 or hydrogenation of the furan ring and its opening, which results in low yield to the desired unsaturated alcohol and increases the cost for product purification.8 The conventional processes for hydrogenation of FAL include both the use of pressurized molecular hydrogen and the organic hydrogen sources via a transfer hydrogenation process.9−11 As compared with hydrogenation using pressured molecular hydrogen, transfer hydrogenation which involves the using of different organic hydrogen sources is safer and easier to handle. In particular, the alcohols, the most widely used © 2019 American Chemical Society

hydrogen source in transfer hydrogenation reactions, are cheap, environmentally friendly, and easy to be removed from the reaction systems, which makes the transfer hydrogenation to be especially appealing and applicable.12 Noble metal nanoparticles like Pd/SiO2, Ru/C, Pt/C, etc. have been extensively investigated for the transfer hydrogenation reactions, including the hydrogenation of FAL. Although appreciable conversion of FAL over these supported noble metal nanoparticles is usually observed, their selectivity to the desirable FOL is relatively poor.13,14 Theoretical calculations on metal nanoparticles catalyzed hydrogenation of furanic compounds indicate that their catalytic performance is highly dependent on the adsorption modes of these furanic compounds on the metal surfaces, which are related to the nature of the metals.15 Unlike that over Pd, Pt and other noble metals, the binding energy of the furan ring of FAL on Cu (111) surface is relative low.16−18 Actually, DFT calculations reveal that FAL bound to the surface of Cu with the carbonyl group via the O lone pair, with the rest of the molecule swifts away from the surface due to the existence of a net repulsion between C of carbonyl group and Cu. Such a η1(O)-adsorption mode over Cu surface has been proposed to be responsible for the high selectivity to FOL over Received: March 5, 2019 Revised: May 3, 2019 Published: May 29, 2019 11485

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ACS Sustainable Chemistry & Engineering Cu-based catalysts.19,20 Therefore, although the conversion of FAL over copper-based catalysts is relatively lower, the selectivity to FOL is high as compared with those observed over the noble metal-based catalysts.21−24 Most of the already reported metal-based transfer hydrogenation reactions occur in high temperature (80−150 °C) that requires additional heating. Recently, with the aim of developing sustainable chemical processes, the use of light, in particular visible light, as the driving force for chemical reactions has attracted extensive current research attentions.25−30 Light initiated transfer hydrogenation reactions based on both homogeneous and heterogeneous photocatalysts has also been reported.31−40 For example, transfer hydrogenation of carbonyl compounds under visible light has been realized over a series of hybrid photocatalytic systems containing CdS as the photosensitizer and Ir/Ru-based complexes as the hydrogenation catalysts.32,33 Supported plasmonic metal nanoparticles like Au, Pt, and Cu on a variety of supports including SiC, CeO2, TiO2, ordered mesoporous carbons (OMCs), and graphene have also been reported to be efficient photocatalysts for transfer hydrogenation of a series of organic compounds, like cinnamaldehyde and FAL, to produce their corresponding alcohols under visible light.34−38 Although Cu-based catalysts are less studied for light initiated reactions, several plasmonic metallic Cu containing catalysts like CuOMCs and Cu/graphene nanocomposites have been successfully applied in hydrogenation of dimethyl oxalate as well as the coupling reactions of aromatic nitro compounds to produce corresponding azoxy or azo compounds under visible light irradiation,39,40 in which a transfer hydrogenation process is also involved. In addition to plasmonic Cu nanoparticles, cuprous oxide (Cu2O), with a narrow direct band gap of 2.17 eV well responsive in the visible light region, is one of the most promising photocatalysts for solar energy conversion.41,42 Considering that Cu and its metal oxide are inexpensive, abundant, and environmental friendly, it is therefore very interesting to investigate photocatalytic transfer hydrogenation of FAL to produce FOL over Cu-based catalysts although their application on this reaction has never been reported previously. In this manuscript, we reported the preparation of Cu/ Cu2O-MC (MC = mesoporous carbon), in which small sized Cu and Cu2O nanoparticles are deposited on graphitized mesoporous carbon, by a pyrolysis at 1100 °C of a Cu/ mesostructured polymer framework precursor obtained via an ion-exchange process between Cu2+ in [Cu(NH3)4]2+ and H+ in the polymeric gel. The as-obtained Cu/Cu2O-MC showed superior performance for selective transfer hydrogenation of FAL to produce FOL under visible light, due to a synergistic effect played by semiconducting Cu2O and plasmonic metallic Cu. An optimum TOF to the formation of FOL over Cu/ Cu2O-MC under visible light was determined to be 4.65 h−1, which is comparable to those previously reported Cu-based heterogeneous catalysts for selective transfer hydrogenation of FAL to produce FOL carried out under high temperature. This study not only provides a green, sustainable, and cost-effective method for the conversion of FAL to produce FOL but also highlights the great potential of Cu-based materials for the light-induced transfer hydrogenation.

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copolymer PEO−PPO-PEO following a previously reported method.43 In a typical process, 2.31 g of 2,4-dihydroxybenzoic acid, 0.70 g of hexamethylenetetramine, 0.45 g of ethylenediamine, and 2.62 g of Pluronic P123 were dissolved in 60 mL of H2O. The light yellow transparent solution was transferred into a 100 mL Teflon-lined stainless steel autoclave, sealed, and heated at 130 °C for 4 h. After cooling, the pink solid polymer was mashed and washed with deionized water for several times and was dried at 50 °C overnight (1.92 g). Cu/polymeric gel precursor was obtained via an ion-exchange process between [Cu(NH3)4]2+ and the as-obtained polymeric gel.44 The polymeric gel was dispersed in 40 mL of a solution containing ammonium hydroxide solution (28.0−30.0%, 8 mL) and Cu(NO3)2· 3H2O (0.278 g, 1.15 mmol). The resultant brown suspension was stirred at 50 °C for 6 h. The product was isolated by filtering and washed for several times with deionized water and was dried at 50 °C under vacuum for 8 h (denoted as Polymer-Cu). The as-obtained Polymer-Cu was calcinated in N2 atmosphere at different temperature (900, 1100, and 1200 °C) for 1 h to obtain different MC supported Cu-based catalysts. The black Cu-based catalysts obtained by calcinating polymer-Cu at different temperature were denoted as Polymer-Cu-T, in which T refers to the calcinations temperature. Characterizations. Thermogravimetry (TG) and different scanning calorimetry (DSC) were carried out on an automatic synchronous thermal analysis (NETZSCH STA449F5) under N2 atmosphere with 10 °C/min heating rate. X-ray diffraction (XRD) patterns were collected on a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation (λ = 0.15406 nm). Raman spectra were obtained on an invia-Reflex micro-Raman spectroscopy system (Renishaw Co.), with 532 nm line of an Ar ion laser at room temperature. X-ray photoelectron spectroscopy (XPS) studies were carried out on an ESCALAB 250 (ThermoScientific) using an Al Kα monochromated (150 W, 20 eV pass energy, 500 μm spot size). Fourier transform infrared (FT-IR) spectra were recorded in transmittance mode with a resolution of 4 cm−1 over a Nicolet Nexus 670 FT-IR spectrometer. The transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) images were obtained using a JEOL model JEM 2010 EX instrument at an accelerating voltage of 200 kV. The amount of metal was determined by an inductively coupled plasmaoptical emission spectrometer (ICP-OES) on Optima 8000 (PerkinElmer). BET surface area was carried out on an ASAP 2020 M apparatus (Micromeritics Instrument Corp., USA). The samples were degassed in vacuum at 150 °C for 10 h and then measured at −196 °C. UV− visible diffuse reflectance spectra (UV−vis DRS) of the powders were obtained by a UV−vis spectrophotometer (Varian Cary 500) with BaSO4 used as a reflectance standard. The morphology of the samples was examined by field emission scanning electron microscopy (SEM) (JSM-6700F). Catalytic Reactions. The photocatalytic hydrogenation of FAL was carried out in a multichannel photochemical reaction system (Beijing Perfect Light, PCX50B; Supporting Information, Figure S1). In a typical catalytic process, 0.05 mmol of FAL and 4 mL of hydrogen source were added to a Schlenk tube containing the catalyst (10 mg) and the base. The reaction system was irradiated with a white light LED (65 mW/cm2) at room temperature under N2 atmosphere. After removal of the catalyst, the solution was analyzed by a gas chromatograph (Shimadzu GC-2014) equipped with an HP-5 capillary column.

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RESULTS AND DISCUSSION Polymer-Cu gel precursor was obtained via an ion-exchange between Cu2+ released slowly from a solution containing [Cu(NH3)4]2+ and H+ in the polymeric gel. High temperature pyrolysis of the as-obtained Polymer-Cu led to the formation of different mesoporous carbon (MC) supported Cu-based catalysts based on the calcinations temperature. A similar strategy was previously reported in the preparation of MC

EXPERIMENTAL SECTION

Syntheses. Polymeric gel was prepared via self-assembly of phenol, amide, and amino resin in the presence of a triblock 11486

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ACS Sustainable Chemistry & Engineering supported small-sized Co3O4 nanocomposite.43 The introduction of Cu component into the polymeric gel via ion exchange ensures a homogeneous distribution of Cu component in the polymeric matrix, which upon pyrolysis leads to the formation of carbon-supported well distributed small sized metallic Cu or its oxide nanoparticles depending on the calcinating temperature. To provide guidance for the pyrolysis, TG-DSC was carried out on the as-obtained Polymer-Cu gel (Supporting Information, Figure S2). The TG-DSC results show that the formation of Cu2O begins at 770 °C. But a further increase of the calcinating temperature to 960 °C leads to the reduction of Cu2O to form metallic Cu. All Cu2O is transformed to metallic Cu when temperature is above 1200 °C. In addition, the formation of graphitized carbon occurs above 900 °C. Therefore, to obtain graphitized carbon supported both Cu and Cu2O nanocomposite, a pyrolysis of the as-obtained Polymer-Cu was carried out at 1100 °C. The XRD of the as-obtained Polymer-Cu-1100 shows peaks at 29.6°, 36.5°, 42.4°, 52.6°, 61.6°, 65.7°, 69.8°, 73.7°, 77.6°, and 85.2°, which can be assigned to (110), (111), (200), (211), (220), (221), (310), (311), (222), and (321) of Cu2O (JCPDS-74-1230), as well as peaks at 43.4°, 50.6°, 74.3°, and 90.2°, attributable to (111), (200), (311), and (222) of cubic Cu (JCPDS-85-1326; Figure 1a). The morphology of the as-

the XPS spectra of Polymer-Cu-1100 in the C 1s region shows a major peak at 284.8 eV assignable to CC in graphitized carbon, accompanied by two minor peaks at 285.5 and 286.2 eV corresponding to C−N and C−O (Figure 3a).47,48 This is consistent with the removal of the oxygenated groups in the Polymer-Cu during the calcinations process, as evidenced from the significantly decrease of the intensity of the peaks at 3600− 3600, 1473, and 1107 cm−1 attributable to the O−H/N−H, C−OH, C−O (epoxy) in the FT-IR spectrum of Polymer-Cu1100 compared with that of Polymer-Cu (Supporting Information, Figure S5).45 The presence of both metallic Cu and Cu2O was also evidenced in the XPS spectrum. The XPS spectrum in Cu 2p region shows peaks of Cu 2p1/2 at 952.6 eV and Cu 2p3/2 at 932.9 eV, which confirms the presence of either Cu0 or Cu+ or both (Figure 3b). The Cu LMM Auger spectra shows binding energies at 570.0 eV, attributable to Cu+, while that of 568.6 eV, assignable to Cu0, suggesting the existence of both Cu+ and Cu0 (Figure 3c). The N2 sorption/ desorption isotherm of Polymer-Cu-1100 shows a type-IV curve, a characteristic for the mesoporous materials, with its BET specific surface area determined to be 243.4 m2/g (Supporting Information, Figure S6). The amount of Cu in Polymer-Cu-1100 was calculated to be 0.87 wt % based on the ICP-OES result, while the ratio of Cu2O to Cu was determined to be ca. 3.5 based on the XPS result. All these characterizations indicate that graphitized MC supported Cu/Cu2O has been obtained (Polymer-Cu-1100 hereafter denoted as Cu/ Cu2O-MC). The UV−vis DRS spectrum of the as-prepared Cu/Cu2O-MC shows a broad intense absorption in 400−800 nm region, due to the strong absorption induced by the mesoporous carbon in this region (Supporting Information, Figure S7). The visible light initiated photocatalytic transfer hydrogenation of FAL over Cu/Cu2O-MC was investigated in different alcohols (methanol, propanol, and isopropanol) acting both as solvent and hydrogen source, with 1 equiv of sodium borate used as the base. It was found that 42.8% of FAL was converted after irradiated for 16 h, with a yield of 41.9% to FOL when methanol was used (Table 1, entry 1). The use of propanol led to even lower performance, with a conversion of FAL to be 30.9% and a yield of 30.3% to FOL (Table 1, entry 2). Isopropanol was found to be the optimum hydrogen source for FAL hydrogenation, with the highest FAL conversion (81.2%) and the best yield to FOL (80.0%) obtained under otherwise similar condition (Table 1, entry 3). Since isopropanol is the optimum hydrogen source, the following reactions were carried out in the presence of the isopropanol. On the contrary, almost negligible product was detected either in absence of Cu/Cu2O-MC or without light (Table 1, entries 4 and 5). This indicated that the transfer hydrogenation of FAL to produce FOL is indeed initiated by photocatalysis over Cu/Cu2O-MC and the alkaline condition is essential for this conversion since no hydrogenation of FAL occurred in absence of sodium borate (Table 1, entry 6). An extension of the reaction time to 24 h led to only slightly increase of the conversion of FAL to 90.3%, with a yield of 89.1% to FOL (Table 1, entry 7). Cycling use showed that the performance of the catalyst significantly decreased (Supporting Information, Figure S8a). Since the catalyst shows almost unchanged XRD patterns after the reaction (Supporting Information, Figure S8b), we proposed that the lowering of the catalytic activity for transfer hydrogenation of FAL over Cu/Cu2O-MC should not be due to the instability of the

Figure 1. XRD patterns of the as-obtained (a) Polymer-Cu-1100, (b) Polymer-Cu-1200, and (c) Polymer-Cu-900.

obtained Polymer-Cu-1100 was shown in the SEM image (Supporting Information, Figure S3). The TEM image shows small nanoparticles of ca. 1 nm are deposited on the carbon matrix with mesopores of approximately 15 nm (Figure 2a,b). HRTEM shows characteristic lattice fringes of 0.174 nm, matching that of (211) crystallographic planes of Cu2O as well as lattice fringes of 0.181 and 0.204 nm, which can be ascribed to (200) and (111) crystallographic planes of Cu (Figure 2c,d). Lattice fringes of 0.340 nm assignable to the interplanar distance of the graphitic structure are also observed, suggesting that graphitized carbon has also been formed by pyrolysis in this temperature (Figure 2c). The Raman spectrm of PolymerCu-1100 shows intense D band at 1330−1340 cm−1 and G band at 1595−1600 cm−1, with its ID/IG ratio determined to be 0.963, another confirmation of the formation of graphitized carbon (Supporting Information, Figure S4a).45,46 Moreover, 11487

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Figure 2. Polymer-Cu-1100 (a) TEM image; (b) enlarged TEM image (inset shows the size distribution of Cu- based nanoparticles); (c and d) HRTEM images.

Figure 3. XPS spectra of Polymer-Cu-1100 (a) in C 1s region, (b) in Cu 2p region, and (c) Cu LMM Auger spectra.

also investigated for the transfer hydrogenation of FAL in otherwise similar conditions. A weak alkaline condition when 2.88 equiv of NaHCO3 was used gave a low FAL conversion of 32.3% under visible light for 16 h, although with a high selectivity of 97.8% to FOL (Table 1, entry 8). On the contrary, an almost complete conversion of FAL was realized in the presence of a strong base KOH, but the selectivity to FOL was extremely low (11.3%) due to the occurrence of the side aldol condensation and acetal reaction, which have been reported to occur in a strong alkaline medium (Table 1, entry 9). A compromise was achieved when 2.88 equiv of K2CO3 was used, which showed a conversion of FAL to be 97.8% and a selectivity of 72.5% to FOL (Table 1, entry 10). The decrease of the amount of K2CO3 used led to improved selectivity to FOL without obviously sacrificing the conversion of FAL. An optimum performance by showing a conversion of FAL to be 96.1% and a selectivity of 94.0% to FOL was achieved when 1.44 equiv of K2CO3 was used (Table 1, entry 11). A further increase of the amount of the catalyst used to 15

catalyst. Instead, the deactivation of Cu/Cu2O-MC should be induced by the deposition of tris(1-methylethyl)ester borate on its surface, which was detected as a byproduct in the reaction system based on the GC-MS result and was believed to be generated via the alcoholysis of the sodium borate. However, the deposition of tris(1-methylethyl)ester borate on the surface of the catalyst did not lead to the permanent damage on the catalytic active sites. The surface deposited tris(1-methylethyl)ester borate can be removed from the surface of the catalyst by washing with alcohol and acetone for several times until no tris(1-methylethyl)ester borate was detected in the filtrate. The used catalyst after washing still showed a conversion of FAL to be 78.5% and a yield of 77.4% to FOL, which is almost similar to those observed over the fresh catalyst, with a conversion ratio of 81.2% and a yield of 80% to FOL (Supporting Information, Figure S9). Since the alkaline condition is essential for the conversion of FAL and sodium borate is not a suitable base for this reaction, a variety of bases, including NaHCO3, K2CO3 and KOH, were 11488

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ACS Sustainable Chemistry & Engineering Table 1. Light-Induced Selective Hydrogenation of FAL to form FOL under Different Conditions

entry

catalyst

hydrogen source

base (the molar ratio of base/FAL)

time (h)

FAL conv. (%)

FOL yield (%)

1 2 3 4 5b 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Cu/Cu2O-MC Cu/Cu2O-MC Cu/Cu2O-MC a Cu/Cu2O-MC Cu/Cu2O-MC Cu/Cu2O-MC Cu/Cu2O-MC Cu/Cu2O-MC Cu/Cu2O-MC Cu/Cu2O-MC Cu/Cu2O-MCe Cu/Cu2O-MC Polymer-Cu-900 Polymer-Cu-1200 Polymer-Cu-1100-air Polymer-Cu-1100−1200 Cu/Cu2O Cu/Cu2O-RGO mixed catalystf

methanol propanol isopropanol isopropanol isopropanol isopropanol isopropanol isopropanol isopropanol isopropanol isopropanol isopropanol isopropanol isopropanol isopropanol isopropanol isopropanol isopropanol isopropanol isopropanol

sodium borate (1.04) sodium borate (1.04) sodium borate (1.04) sodium borate (1.04) sodium borate (1.04) c sodium borate (1.04) NaHCO3 (2.88) KOH (2.88) K2CO3 (2.88) K2CO3 (1.44) K2CO3 (1.44) K2CO3 (1.44) K2CO3 (1.44) K2CO3 (1.44) K2CO3 (1.44) K2CO3 (1.44) K2CO3 (1.44) K2CO3 (1.44) K2CO3 (1.44)

16 16 16 16 16 16 24 16 16 16 16 16 14 14 14 14 14 14 14 14

42.8 30.9 81.2 d 2.1 3.9 90.3 32.3 100 97.8 96.1 97.8 94.3 64.4 37.5 58.5 42.3 36.3 67.7 57.3

41.9 30.3 80.0 d 0.5 d 89.1 31.6 11.3 70.9 90.3 92.5 90.9 61.9 36.2 55.9 40.6 34.9 65.3 55.1

a

Without catalyst. bWithout light irradiation. cWithout base. dNegligible products were detected. e15 mg of catalyst of Cu/Cu2O-MC. fA physical mixture of Polymer-Cu-900 and Polymer-Cu-1200 with a ratio of 3.5. Reaction conditions: furfural (0.05 mmol), isopropanol (4 mL), catalyst (10 mg), base, visible light irradiation, N2.

TOF of 1.83 h−1 to FOL. This value is comparable to those previously reported over Cu-based catalysts for transfer hydrogenation of FAL to produce FOL as shown in Table S1 (Supporting Information), most of which were carried out in temperature above 100 °C. Cu/Cu2O-MC is also very stable during the light initiated transfer hydrogenation of FAL to produce FOL. The performance after 3 cycling runs did not show obvious change (Figure 5). In addition, the XRD patterns, the TEM and the XPS spectrum of the catalyst after used are similar to those of the fresh one (Supporting Information, Figures S10−S12). To elucidate the roles of metallic Cu and Cu2O played in the light initiated transfer hydrogenation of FAL to form FOL over Cu/Cu2O-MC, MC supported bare metallic Cu, and bare Cu2O were also prepared. Based on the TG/DSC results, MC supported bare Cu2O and MC supported bare metallic Cu were obtained by calcinating the Polymer-Cu at 1200 °C

mg only led to a slightly increase of the conversion of FAL to 98.7% (Table 1, entry 12). Therefore, an optimum catalyst amount was fixed at 10 mg in our studies. The time-dependent conversion of FAL and the formation of FOL over Cu/Cu2O-MC in the presence of isopropanol with 1.44 equiv of K2CO3 showed that the conversion of FAL increased with the irradiation time. An optimum performance was achieved at 14 h, which showed a conversion of FAL to be 94.3% and a yield of 90.9% to FOL (Table 1, entry 13). An extension of the reaction time to 16 h led to a slight increase of the conversion of FAL to be 96.1%, with almost unchanged yield of 90.3% to FOL (Figure 4). An optimum TOF for the production of FOL was determined to be 4.65 h−1 in 14 h. Such a performance is superior to those reported for hydrogenation based on H2, for example, hydrogenation of FAL over Cu/AC−SO3H in 105 °C and 0.4 MPa H2 showed a

Figure 4. Time-dependent conversion of FAL and the yield to FOL using K2CO3 as the base over light irradiated Polymer-Cu-1100 (Cu/ Cu2O-MC).

Figure 5. Cycling of Polymer-Cu-1100 (Cu/Cu2O-MC) for hydrogenation of FAL. 11489

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Scheme 1. Proposed Mechanism for Light Initiated Selective Hydrogenation of FAL to Produce FOL over MC Supported CuBased Catalysts

(denoted as Polymer-Cu-1200) and at 900 °C (denoted as Polymer-Cu-900), respectively. The XRD patterns of the asobtained Polymer-Cu-1200 and Polymer-Cu-900 show characteristic diffraction peaks well indexed to Cubic Cu (JCPD85-1326) and Cu2O (JCPDS-74-1230), respectively (Figure 1b,c). The Raman spectra of both Polymer-Cu-900 and Polymer-Cu-1200 also show typical D band at 1330−1340 cm−1 and G band at 1595−1600 cm−1, with the ID/IG ratio observed over Polymer-Cu-900 and Polymer-Cu-1200 to be 0.925 and 1.032, respectively, indicating that the degree of the graphitization as well as the defects in the MC in these two catalysts is not quite different as compared to Polymer- Cu1100 (Supporting Information, Figure S4b,c).The light initiated transfer hydrogenation of FAL over Polymer-Cu-900 and Polymer-Cu-1200 under otherwise similar condition was also investigated. It was found that after irradiated for 14 h over Polymer-Cu-900, 64.4% of FAL was converted, with a yield of 61.9% to the desirable FOL (Table 1, entry 14). Although Polymer-Cu-1200 was also active for the reaction, the conversion of FAL over Polymer-Cu-1200 was even lower, only 37.5% of FAL was converted, with a yield of 36.2% to FOL (Table 1, entry 15). These results indicated that although MC supported bare Cu2O and MC supported bare metallic Cu is also active for the light initiated transfer hydrogenation, they show inferior performance as compared with MC supported both Cu2O and metallic Cu (Cu/Cu2O-MC). The different activity observed over Cu/Cu2O-MC, Polymer-Cu-900 (Cu2O-MC) and Polymer-Cu-1200 (Cu-MC) is really induced by their different composition instead of the different morphology and size of the nanoparticles since the activities over bare Cu2O supported on mesoporous carbon (Supporting Information, Figure S13a) and bare Cu supported on mesoporous carbon (Supporting Information, Figure S13b) obtained from the as-obtained Polymer-Cu-1100 match well with those of Polymer-Cu-900 and Polymer-Cu-1200, respectively (Table 1 Entry 16 and 17). The obvious higher activity observed over Cu/Cu2O-MC as compared with that of Polymer-Cu-900 and Polymer-Cu-1200 indicates that there is a synergistic effect between Cu and Cu2O in Polymer-Cu1100. In addition, MC plays an important role in this reaction since both Cu/Cu2O and Cu/Cu2O-RGO (XRD shown in Supporting Information, Figure S14a,b) showed inferior photocatalytic activity for the hydrogenation of FAL as compared with Cu/Cu2O-MC, with 36.3% and 67.7% of FAL converted over Cu/Cu2O and Cu/Cu2O-RGO respec-

tively under otherwise similar condition (Table 1, entries 18 and 19). Based on the above observations and previous studies on plasmonic metal and semiconductor-based transfer hydrogenation, the selective transfer hydrogenation of FAL to form FOL over these Cu-based photocatalysts can be proposed as following. For MC supported bare Cu2O (Polymer-Cu-900), the light irradiation on Cu2O lead to the formation of the photogenerated electrons and holes.49 The photogenerated electrons can transfer to the MC support to form electron-rich MC, which is beneficial for the adsorption of the CO bond in the side chain of FAL. The isopropanol is adsorbed on the positively charged Cu2O, which react with the left photogenerated holes, leading to the dissociation of protons from the isopropanol to form acetone. The released protons from isopropanol then transfer to MC adsorbed CO bond via a concerted process involving a six-member ring TS, leading to the formation of FOL.50 (Scheme 1). For MC supported bare Cu nanoparticles (Polymer-Cu-1200), the direct excitation of plasmonic Cu nanoparticles can lead to the formation of “hot” electrons as already demonstrated in plasmonic Au nanoparticles,34,40,51 which can transfer to MC support. Via a similar mechanism as that proposed above for MC supported bare Cu2O, light initiated transfer hydrogenation of FAL to produce FOL also occurs over MC supported plasmonic Cu nanoparticles, although with an inferior performance, indicating that the direct excitation of Cu2O is more efficient than that of plasmonic Cu nanoparticles. For MC supported both Cu2O and metallic Cu (Cu/Cu2O-MC), both Cu2O and plasmonic Cu can be excited when irradiated. In addition, metallic Cu can act as a mediator to promote the electron transfer from excited Cu2O to MC, due to the lower work function of Cu (4.65 eV vs vacuum level) as compared with the conduction band of Cu2O (4.22 eV vs vacuum level).52 Therefore, the presence of both Cu2O and metallic Cu on MC can play synergistically on the light inititated transfer hydrogenation of FAL to produce FOL over Cu/Cu2O-MC. On the contrary, a physical mixture of Polymer-Cu-900 (Cu2O-MC) and Polymer-Cu-1200 (Cu-MC) showed a much lower activity, with only 57.3% of FAL was converted and a yield of 55.1% to FOL (Table 1 entry 20). 11490

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CONCLUSION In summary, Cu/Cu2O-MC, in which small sized metallic Cu and Cu2O nanoparticles were deposited on graphitized mesoporous carbon, has been successfully obtained via a pyrolysis of a Cu/mesostructured polymer framework precursor at 1100 °C. The as-obtained at 1100 °C showed superior performance for selective transfer hydrogenation of FAL to produce FOL under visible light due to a synergistic effect of Cu2O and plasmonic metallic Cu. This study provides a green, sustainable and cost-effective method for highly efficient transformation of FAL to produce FOL. This study also highlights the great potential of using Cu-based catalysts for light initiated transfer hydrogenation reactions.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.9b01305.

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TG-DSC, Raman spectra, FT-IR spectra, N2 adsorption/ desorption isotherm, UV−visible DRS of Polymer-Cu1100 and other characterizations. (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel (Fax): 86-59122865855. ORCID

Zhaohui Li: 0000-0002-3532-4393 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSFC (21872031, U1705251). Z.L. thanks the Award Program for Minjiang Scholar Professorship for financial support.

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