Metal-organic frameworks derived bimetallic Cu-Co catalyst for efficient and selective hydrogenation of biomass-derived furfural to furfuryl alcohol

Molecular Catalysis 436 (2017) 128–137

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Metal-organic frameworks derived bimetallic Cu-Co catalyst for efficient and selective hydrogenation of biomass-derived furfural to furfuryl alcohol Yuan Wang, Yanan Miao, Shuai Li, Lijing Gao, Guomin Xiao ∗ School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, PR China

a r t i c l e

i n f o

Article history: Received 3 December 2016 Received in revised form 16 March 2017 Accepted 17 April 2017 Keywords: Metal organic frameworks CuCo/C catalyst Furfural Furfuryl alcohol Hydrogenation

a b s t r a c t In this study we report an efficient CuCo bimetallic catalyst with highly dispersed CuCo-based mixed metal/metal oxide on porous carbon matrix for the hydrogenation of furfural to furfuryl alcohol. The catalyst is derived from Co-doped Cu-BTC metal-organic frameworks (MOFs) by thermolysis in nitrogen atmosphere at temperatures ranging from 773 K to 1073 K. The best catalytic performance is achieved when cobalt to copper molar ratio is 0.4 and the precursor is calcined at 873 K. The characterization of prepared catalytic materials is done by multi-techniques including XRD, XPD, SEM, TEM, TG and BET analysis. The sum of their results reveal that the doping of cobalt helps the dispersion of nanoparticles of copper and cobalt and thermolysis temperature has an impact on both the particle size and the chemical states. The resulting catalyst CuCo0.4 /C-873 shows 98.7% furfural conversion and 97.7% furfuryl alcohol selectivity in ethanol solvent with good stability which is better than most reported catalysts. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Biomass is an ideal alternative to fossil fuels as a feedstock of fine chemicals and fuels due to its abundance and sustainability [1]. The application of biomass-derived chemicals and energy can alleviate the energy crisis efficiently. Furfural, as one of major biomass platform chemicals, can be primarily produced from acid-catalyzed hydrolysis of hemicellulose and then converted into diverse chemicals and fuels [2,3]. And about 60% of the furfural produced all around the world is converted to furfuryl alcohol every year [4]. Furfuryl alcohol is an important industrial chemical and has a wide range of applications in polymer industries and fine chemical. It is widely used in the production of thermostatic resins, fiber glass, polymer concrete and liquid resins for galvanic bath tub [5,6]. Besides, it is also employed as a chemical intermediate for the synthesis of lysine, vitamin C, lubricants, dispersing agents, binders and adhesives [7]. In industry, copper chromite catalysts have been used in hydrogenation of furfural to furfuryl alcohol for decades [8]. The main drawbacks of copper chromite catalysts are the toxic nature of chromium oxides and their moderate activity in conversion of furfural [9]. Thus, many research interests have been drawn to design

∗ Corresponding author. E-mail address: [email protected] (G. Xiao). http://dx.doi.org/10.1016/j.mcat.2017.04.018 2468-8231/© 2017 Elsevier B.V. All rights reserved.

environmentally friendly catalysts for conversion of furfural to furfuryl alcohol with high activity and selectivity. For example, S. Sitthisa et al. [10] reported that furfuryl alcohol is the main product over Cu/SiO2 and furan is the dominant product with catalyst Pd/SiO2 . In addition, ring opening products (butanal, butanol and butane) are dominating over catalyst Ni/SiO2 . Hydrogenation of furfural to furfuryl alcohol can take place both in the liquid and gas phase. In comparison with the liquid phase process, a high reaction temperature is always employed to vaporize the furfural in gas phase hydrogenation, which means a higher consumption of energy. Thus, liquid phase process is preferable in consideration of safety and energy efficiency. Besides, liquid phase chemistry is possibly preferred for compatibility with the upstream production [11,12]. In addition, some researchers [13–15] have reported the liquid phase hydrogenation of furfural through catalytic transfer hydrogenation by using secondary alcohols as hydrogen donors. In the catalytic transfer hydrogenation, furfural concentration in reaction solvent is pretty low such as 1 wt% [15], 2 wt% [14] and 2.5 wt% [13], because adequate hydrogen donor should be provided to ensure high furfural conversion. While the furfural concentration can reach 34 wt% in process that molecular H2 as hydrogen source [16]. Thus, the catalytic transfer hydrogenation always suffers from low production efficiency. Another advantage of using H2 molecular as hydrogen source is the cheap price of hydrogen due to the increased production of shale gas.

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The liquid phase furfural hydrogenation with H2 molecular as hydrogen source has been studied using various metal-based catalysts including Pt, Pd, Co, Cu and Ni [17–20]. Among all tested catalysts, Cu has been the most frequently employed metal, because it can selectively hydrogenate the C O bond while leaving the furan ring untouched [9]. Besides, a second metal is often added to improve the selectivity and/or the activity of monometallic catalysts because the catalytic performance can be modified by structure and electronic effects. K. Fulajtarova et al. [18] reported 98.7% furfuryl alcohol selectivity and total furfural conversion over catalyst 5% Pd-5% Cu/MgO but only 59.6% furfuryl alcohol selectivity and 92.1% conversion over 5% Pd/MgO in aqueous hydrogenation of furfural to furfuryl alcohol, illustrating the promoting effect of copper. S. Huang et al. [21] even achieved 100% yield of furfuryl alcohol over hollow-core Pt-Cu catalyst at 423 K, 2 MPa H2 and 10 h. From the consideration of high price of precious metal, some non-noble metal modified Cu-based catalysts also have been explored. S. A. Khromova et al. [19] reported bimetallic Ni-Cu catalysts for the liquid phase selective hydrogenation of furfural to furfuryl alcohol with selectivity of 90% at high reaction temperature (523 K). S. Srivastava et al. [22] optimized different operating parameters on the conversion of furfural to furfuryl alcohol over Cu-Co/SBA-15 and 96.7% yield of furfuryl alcohol was achieved at optimized conditions (403 K, 3 MPa H2 , 3 h, 1 g catalyst and furfural to solvent ratio 1:7). Although many active catalysts have been developed for liquid phase hydrogenation of furfural to furfuryl alcohol, there is no catalyst that can replace copper chromite catalysts for industrial application due to either severe deactivation or less activity/selectivity. Within the present work, CuCo/C nanocomposites are fabricated by one-step thermal decomposition of Co-doped coppercontaining metal-organic frameworks (MOFs) (Cu-BTC, BTC = 1,3,5benzene tricarboxylate) and applied in hydrogenation of furfural to furfuryl alcohol in liquid phase. As a new class of porous materials, MOFs have recently been considered as ideal sacrificial templates for the preparation of various carbon-based metal and metal oxides nanoparticles because of their excellent properties such as ordered structures, high surface area and tunable functionality [23,24]. The ordered porous structure and long Cu-Cu dimer distance in CuBTC enable the uniform dispersion of cobalt ions in the channel of Cu-BTC [25]. While the confinement effect of MOFs ensures high dispersion of the metal/metal oxides nanoparticles throughout porous carbon which derives from the organic ligands of MOFs and the carbon matrix can stabilize the metal/metal oxide particles and prevent them from aggregation [26]. The prepared CuCo/C catalysts show excellent catalytic performance in selective hydrogenation of furfural to furfuryl alcohol with good stability. The characterization and catalytic behavior of CuCo/C were investigated in detail. To the best of our knowledge, these novel CuCo/C hybrid nanocomposites used as the catalysts for hydrogenation of furfural to furfuryl alcohol have not been reported so far.

2. Experimental section 2.1. Catalysts preparation All of the chemicals used were of analytical grade and used without further purification. Cu-BTC MOFs were synthesized by the method according to the literature with some minor revision [27]. In a typical synthesis, the methanol solution of copper nitrate was prepared by dissolving 5.46 g cupric nitrate trihydrate in 1000 ml methanol, and the methanol solution of benzene-1,3,5-tricarboxylic acid was prepared by dissolving 3.492 g benzene-1,3,5-tricarboxylic acid in 1000 ml methanol. Then, the obtained two solutions were mixed by pouring the cop-

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per nitrate solution into the tricarboxylic acid solution and kept at room temperature for 2 h until MOFs precipitated finished. The precipitation was retrieved by filtration and washed with methanol to remove the possible residues, dried under vacuum and kept for further use. The successful synthesis of octahedral Cu-BTC was confirmed by scanning electron microscopy (SEM), FT-IR spectrum and Powder X-ray diffraction (XRD) (Fig. S1). CuCo/C bimetallic catalysts were obtained by thermal decomposition of the Cu-BTC precursor impregnated with Co2+ ions. Typically, a predetermined amount of cobalt nitrate hexahydrate was dissolved in 50 mL ethanol. Then, 5 g Cu-BTC was added to the cobalt nitrate solution. The mixture was stirred for 24 h and then dried by rotary evaporator at 323 K for 0.5 h. The precursor was labeled as Cu-BTC/CN-m, where m represents the molar ratio of Co to Cu (m = 0.1, 0.2, 0.3, 0.4 and 0.5). The resulting solid was calcined at varying temperatures for 2 h under N2 flow (80 mL/min). The as-synthesized catalysts were denoted as CuCox/C-y, where x denotes the molar ratio of Co to Cu (x = 0.1, 0.2, 0.3, 0.4 and 0.5) and y denotes the calcination temperature (y = 773 K, 873 K, 973 K and 1073 K). 2.2. Characterization of catalysts Powder X-ray diffraction (XRD) was analyzed on an Ultima IV X-ray diffractometer with Cu K␣ radiation, operating at 40 kV and 30 mA. The average crystallite size of metallic copper and cobalt was calculated by Scherrer equation: D = K␭/(Bcos␪). Fourier transformed infrared (FT-IR) patterns were collected on a TENSOR27 PMA 50 spectrophotometer using KBr disks. Thermal gravimetric analysis (TG) was measured with a TA SDT-Q600 thermo gravimetric analyzer. The samples were heated from 323 K to 1073 K at a heating rate of 20 K/min under N2 atmosphere. N2 adsorptiondesorption isotherm of the samples was performed at 77 K, using a Beishide 3H-2000PS1 instrument. Samples degassing was carried out at 473 K prior to acquisition of the adsorption isotherm. The surface area was calculated using the multipoint BrunauerEmmett-Teller (BET) method. The pore volume and pore size were calculated from desorption branches of the isotherms with the Barrett-Joyner-Halenda (BJH) method. The morphology of the catalysts was investigated with an S-4800II FESEM scanning electron microscopy (SEM) with an accelerating voltage of 15.0 kV. Transmission electron microscopy (TEM) images were obtained on a Tecnai G2 20 high resolution transmission electron microscopy operated at 200.0 kV. X-ray photoelectron spectroscopy (XPS) was performed on an ESCALAB-250 spectrometer with a monochromatic Al K␣ (1486.6 eV) source. The obtained binding energies were calibrated using the C1 s peak (284.6 eV) as the reference. 2.3. Evaluation of catalytic performances Catalytic tests were performed in a 100 mL stainless steel autoclave equipped with an electromagnetic driven stirrer at a stirring speed of 900 rpm. The calculated quantities of furfural, catalyst and solvent were charged into the autoclave which was purged with H2 three times and then pressurized with H2 to the desired pressure. The reaction temperature was raised to the desired value and agitation was started. Reaction time zero was defined when the reaction temperature reached the desired one. The experiment was rapidly stopped with an ice-bath after reaction for 1 h and the liquid phase was separated from the catalyst by centrifugation. The total mass of the reaction solution was 60 g, while the weight percentage of furfural was 5 wt%, 10 wt%, 15 wt%, 20 wt% and 25 wt%. The catalyst dosage ranged from 0.1 g to 0.9 g. The solvents were deionized water, ethanol (AR, ≥ 95%), methanol (AR, ≥ 99.5%), toluene (AR, ≥ 99.5%) and hexane (AR, ≥ 97.0%). They were obtained from commercial supplier and used as provided. The initial hydrogen

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pressure ranged from 1 MPa to 5 MPa and the reaction temperature ranged from 383 K to 423 K. The reaction products were analyzed by an Ouhua GC 9160 gas chromatograph (GC) equipped with a flame ionization detector (FID) and a capillary column (SE-54, 30 m × 0.32 mm × 0.5 ␮m). The chromatographic conditions were as follows: the vaporization temperature was 573 K and the detector temperature was 553 K. The column temperature was kept at 323 K for 2 min firstly. Then the temperature was raised to 453 K with a ramp rate of 5 K/min and finally kept at 453 K for 5 min. The reproducibility of the results was examined, and the error in all experiments was <2 %. The mass balance of carbon which was all about 96% if not specified was calculated by the sum of unreacted furfural and reaction products in comparison with fed furfural. Furfural conversion and product selectivity were calculated and defined as follows:



Convention of furfural =

1−

 Selectivity of products =

mole of furfural after reaction mole of furfural introduced

mole of each product mole of furfural consumed



× 100%,

 × 100%.

3. Results and discussion 3.1. Synthesis and characterization As shown in Fig. 1, Cu-BTC crystal with smooth surface and uniform octahedral structure (Fig. S1D) was first synthesized and employed as a precursor to obtain Cu-BTC/CN. The molar ratio of Co to Cu was controlled between 0.1 and 0.5, because in this range the octahedral structure could be well preserved after pyrolysis which is the key to the porous and high metallic dispersion of the obtained samples [28]. After impregnated with cobalt nitrate, the Co-doped Cu-BTC still keeps the octahedron morphology as illustrated by SEM pictures (Fig. S2). The XRD patterns of Cu-BTC/CN precursors with different Co to Cu molar ratio (Fig. S3) have the same characteristic peaks as Cu-BTC, which also indicates the well preserved structure of the obtained Cu-BTC/CN precursors. However, the peak intensity diminishes with the increase amount of cobalt nitrate which might be caused by the coverage of Cu hydroxide on the surface of CuBTC. During the impregnation process, protons generated from the hydrolysis of Co2+ ions can gradually etch Cu-BTC templates and releases Cu2+ ions. Then the Cu2+ ions coordinate with OH− firstly on the surface of Cu-BTC due to the different solubility of products of Co2+ and Cu2+ . The hydrolysis of Co2+ ions may result from the insolubility property of Co hydroxide in ethanol and water introduced by the addition of cobalt nitrate hexahydrate. However, the amount of hydrolyzed Co2+ ions is pretty small due to the low content of water, thus the structure of Cu-BTC can be well preserved during the process. A similar process has been reported by Y. Zhang et al. and Z. Jiang et al. [29,30]. This phenomenon is also evidenced by the slightly wrinkled surface of Cu-BTC/CN as shown in Fig. S2D. The as-prepared Cu-BTC/CN was further transformed into CuCo/C by thermolysis in N2 . As displayed in Fig. S4, the TG curve of Cu-BTC clearly has three apparent stages of mass loss. The initial weight loss of 28% below 473 K is due to the loss of solvent molecules in Cu-BTC. Another obvious weight loss of 30% at 573–673 K is due to the decomposition of Cu-BTC. During this transformation, the organic part of Cu-BTC was converted into amorphous carbon because of the carbonization reaction in inert atmosphere. Meanwhile, the metal ions can be reduced into metal by carbon under high temperature conditions. Thus, a great deal of weight has lost in this progress. When the temperature reaches 773 K, the change of the TG curve is accompanied by the evaporation of CO2 and CO. Meanwhile, the addition of Co(NO3 )2 ·6H2 O leads to an additional weight loss at about 553 K which is the

Table 1 Physicochemical characterization of prepared CuCo/C catalysts. Catalyst

SBET (cm2 /g)

Vp (cm3 /g)

dp (nm)

Cu/C-873 CuCo0.1 /C-873 CuCo0.2 /C-873 CuCo0.3 /C-873 CuCo0.4 /C-873 CuCo0.5 /C-873 CuCo0.4 /C-773 CuCo0.4 /C-973 CuCo0.4 /C-1073

93.6 74.3 72.6 70.3 68.8 66.9 91.5 23.1 17.4

0.168 0.188 0.185 0.183 0.182 0.179 0.176 0.074 0.061

10.13 9.63 9.54 9.26 8.98 8.75 8.29 11.64 14.39

dM (nm) dCu

dCo

28.2 10.5 10.8 9.9 10.4 10.6 9.3 39.8 54.6

– – 8.9 10.2 10.1 9.7 8.5 37.7 51.3

decomposition temperature of cobalt nitrate. Besides that, the total mass loss for Cu-BTC/CN-0.4 is higher than Cu-BTC as shown in TG patterns. The main reason is that cobalt nitrate has a higher mass loss Than Cu-BTC in the process. Another reason is that more carbon has been consumed to reduce the added cobalt ions into cobalt metal. In the selected-area electron diffraction (SAED) pattern of CuCo0.4 /C-873 (Fig. S5), only the diffraction rings of Cu and Co can be found, indicating that the carbon obtained is amorphous and lacks long-range order. Though almost complete decomposition of Cu-BTC/CN could be achieved at around 773 K, we have studied the impact of thermolysis temperature in the range from 773 K to 1073 K because the oxidation states of metal ions in MOFs may vary with the decomposition temperature [31], which would influence their catalytic performance. The nature of the products is firstly investigated by powder XRD. Fig. 2 shows the XRD patterns of synthesized samples with different Co to Cu molar ratio and calcination temperature. All samples display characteristic peaks (2␪ = 43.5◦ , 50.6◦ and 74.2◦ ) ascribed to cubic Cu phase (JCPDS no.04-0836) derived from the regular arranged metal nodes. With the doping of cobalt, the characteristic peaks located at 2␪ = 44.5◦ and 51.8◦ corresponding to cubic cobalt phase (JCPDS no.15-0806) are also observed in Cu-BTC/CN derived samples, suggesting a separate Cu and Co phase without the formation of a notable degree of solid solution. The width of peaks ascribed to Cu increases obviously in CuCo/C-873 samples compared to that of Cu/C-673, which indicates that the addition of cobalt ions helps the dispersion of Cu in the final product. In other words, Cu has better dispersion and smaller particle size with the doping of cobalt. This may be due to the partial substitution of cobalt for copper on the framework of Cu-BTC and/or the cobalt ions in the channel of Cu-BTC could prevent aggregation of copper ions in thermolysis process. As the percentage of cobalt increases, the peak intensity of Co increases gradually and that of Cu decreases steadily (Fig. 2A), which is in accordance with the change of metal content in the samples. In addition, with the increased thermolysis temperature, both diffraction peaks of Cu and Co become sharp gradually (Fig. 2B) and only a weak peak of Co (111) plane can be detected for CuCo0.4 /C-773. This indicates that the aggregation of metal ions during the thermolysis process is inevitable even with the prevention effect of organic ligands. The crystallite size of metallic copper and cobalt calculated by Scherrer equation (Table 1, column 5, 6) confirms the dispersed effect of Co and the aggregation of metal ions at high calcination temperature. It can also be found that the crystallite size of metallic copper and cobalt for all CuCo/C-873 samples is almost the same (about 10 nm), which indicates that the dispersed effect of Co has no relationship with the amount of Co. The chemical composition of CuCo0.4 /C-873 which has relatively better catalytic performance (this will be discussed later) was studied by XPS measurement. XPS spectrum (Fig. 3A) reveals the presence of copper, cobalt, carbon and oxygen. Cu 2p3/2 (Fig. 3B) peak is at the binding energy of 932.3 eV with no satellite. This

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Fig. 1. Schematic illustration of the synthetic strategy of CuCo/C catalyst.

Fig. 2. XRD patterns of CuCo/C catalysts with different Co to Cu molar ratio (A) and different calcination temperature (B).

result combined with the Cu LMM spectrum (Fig. 3C) indicates that copper species are in the zero-valent state. [32] Co 2p3/2 peak shows complex patterns (Fig. 3D) and the deconvolution of it leads to the identification of three patterns: zero-valent metal cobalt signal (778 eV), cobalt oxides signals (779.1 eV, 781.2 eV) and the shakeup satellite signal (786.3 eV) [33] which indicates the existence of cobalt oxides. CuCo0.5 /C-873 was tested to investigate the impact of Cu to Co molar ratios on chemical states. The results show that copper exists in zero-valent and cobalt species display two states in CuCo0.5 /C-873: metallic cobalt and cobalt oxides. The only difference is that zero-valent cobalt species have a lower atomic concentration compared to that of CuCo0.4 /C-873. The chemical

composition of CuCo0.4 /C-773 was also tested to study the effect of thermolysis temperature (Fig. 3). The XPS spectrum of it reveals that copper species have zero-valent and divalent states, while oxidation state (bivalent cobalt and trivalent cobalt) is the dominated state in the cobalt species. It can be seen that total reduction of copper ions in Cu-BTC/CN precursors could only be achieved at temperatures above 873 K in an inert atmosphere, while the percentage of metallic cobalt increases with thermolysis temperature. B. Chen et al. have reported that complete reduction of cobalt ions cannot be achieved even at a high temperature of 1073 K [34]. The morphology of CuCo0.4 /C at different thermolysis temperatures was characterized by SEM. As shown in Fig. 4, the octahedral

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Fig. 3. XPS spectra for catalysts CuCo0.4 /C-773, CuCo0.4 /C-873 and CuCo0.5 /C-873: full range XPS spectrums (A), Cu 2p3/2 spectra (B), Cu LMM spectra (C) and Co 2p3/2 spectra (D).

Fig. 4. SEM images of CuCo0.4 /C-773 (A-B), CuCo0.4 /C-873 (C-D), CuCo0.4 /C-973 (E-F) and CuCo0.4 /C-1073 (G-H).

structure is not kept very well during the thermolysis process due to shrinkage. At low thermolysis temperatures (773 K and 873 K), three-dimensional structure can still be observed and the surface of obtained samples wrinkle obviously. What is more, on the surface of CuCo0.4 /C-873 lamellar structure was formed, suggesting the slight collapse of the template. Further increase in thermolysis

temperature (973 K) leads to evident collapse into small particles (Fig. 4E–F). With the increase of calcination temperature to 1073 K, the aggregation of collapsed particles is apparent and the sample has a more rounded appearance (Fig. 4G–H). The TEM experiments were also performed to characterize the micro-structure of CuCo/C. TEM images of Cu/C-873 (Fig. 5A–B) and CuCo0.4 /C-873 (Fig. 5D–E)

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Fig. 5. TEM images of Cu/C-873 (A-B), CuCo0.4 /C-873 (D-E) and CuCo0.4 /C-1073 (G-H) and Particle size distributions of Cu/C-873 (C) and CuCo0.4 /C-873 (F).

show uniform dispersion of copper and cobalt particles which are embedded in the carbon matrix. The particle size distribution pictures of them (Fig. 5C, F) show that CuCo0.4 /C-873 has a smaller mean particle size (9 nm) compared with CuCo0.4 /C-873 (46 nm). The particle sizes are consistent well with the data calculated from XRD patterns. While at high thermolysis temperature (1073 K), no metal/metal oxide particles can be found and they have sintered into pieces as indicated by the dark part in Fig. 5H. These findings are in accordance with the result of XRD that Cu has better dispersion with the doping of cobalt ions in Cu-BTC/CN precursors and high thermolysis temperature leads to increased copper and cobalt particle size. The BET surface area, pore volume and average pore diameter of these prepared samples are listed in Table 1 (column 2–4). The surface area of the catalyst decreases obviously from 93.6 m2 /g for Cu/C-873 to 74.3 m2 /g for CuCo0.1 /C-873 accompanied by the decrease of average pore diameter, indicating that doped cobalt species exist in the channel of Cu-BTC precursor [35]. By further increasing the amount of Co, the surface area, pore volume and average diameter suffer from a little decrease. On the other side, the impact of thermolysis temperature on the structure of

samples is huge. CuCo0.4 /C-773 has a relatively high surface area compared to that of CuCo0.4 /C-873, which also indicates the slight destruction of the structure as revealed by SEM. At the same time, CuCo0.4 /C-973 and CuCo0.4 /C-1073 undergo a sharp decrease in surface area (23.1 m2 /g and 17.4 m2 /g, respectively) and an increase in average pore diameter (11.61 nm and 14.39 nm, respectively), which demonstrates the collapse of the structure at high calcination temperature. N2 adsorption isotherms (Fig. S6) of all CuCo/C-873 materials exhibit type-IV shape, which is the characteristic type for mesoporous materials implying easy transport of furfural and products in the pores. 3.2. Hydrogenation of furfural with various CuCo/C catalysts The prepared CuCo/C catalysts were investigated for the reaction of furfural (FFA) to furfuryl alcohol (FOL) in ethanol. The catalytic activity and selectivity for FFA hydrogenation on different catalysts are listed in Table 2. For all tested catalysts except CuCo0.4 /C-1073, the predominant product is FOL and the main byproduct is 2-furaldehyde diethyl acetal (2-FA) which is a product between furfural and alcohol solvent. The acetalization between

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Fig. 6. Possible products during furfural hydrogenation.

Table 2 Catalytic performance of CuCo/C catalysts for the hydrogenation of FFA.a Entry

1 2 3 4 5 6 7 8 9

Catalyst

Cu/C-873 CuCo0.1 /C-873 CuCo0.2 /C-873 CuCo0.3 /C-873 CuCo0.4 /C-873 CuCo0.5 /C-873 CuCo0.4 /C-773 CuCo0.4 /C-973 CuCo0.4 /C-1073

Conversion (%)

46.6 55.9 71.6 89.6 98.7 99.0 91.8 73.4 22.9

Selectivity (%) FOL

2-FAb

81.3 95.2 97.2 97.2 97.7 97.3 96.4 92.5 1.3

16.3 2.5 1.8 1.7 1.3 1.5 1.8 5.2 97.3

a Reaction conditions: 0.7 g catalyst; 15 wt% furfural in ethanol solution; 413 K; 3 MPa. b FFA: furfural; FOL: furfuryl alcohol; 2-FA: 2-furaldehyde diethyl acetal.

furfural and ethanol can proceed in the absence of catalyst [36,37], thus high selectivity to 2-FA would be obtained when the catalyst is less active. The reaction pathway for the formation of 2-FA is shown in Fig. S7. The first step is the formation of a protonated intermediate (1) via protonation of the carbonyl group of furfural by H+ ion sourced from ethanol. Then, ethanol reacts with this intermediate forming the hemiacetal (2). A proton is removed from the hemiacetal which subsequently re-protonates and dehydrates to form another intermediate (3). Afterwards, this intermediate reacts with another ethanol molecule forming another intermediate (4) which is deprotonated to form 2-FA. Some minor components such as 2methylfuran and tetrahydrofurfuryl alcohol are also identified for all catalysts except CuCo0.4 /C-1073, as shown in Fig. 6. It can be seen from Table 2 (Entry 1–6) that FOL selectivity improves a lot with the addition of cobalt (81.3% for Cu/C-873 and 95.1% for CuCo0.1 /C-873, respectively) and the main by-product for Cu/C-873 is 2-FA accounting for 16% of the total product. At the same time, the selectivity of FOL keeps at a high level around 96% in all CuCo/C-873 catalysts. By further increasing the molar ratio of cobalt to copper from 0.1 to 0.4, the conversion of FFA increases steadily from 55.9% to 98.7%. Both CuCo0.4 /C-873 and CuCo0.5 /C873 show high FOL yield. The increased furfuryl alcohol selectivity may result from the better dispersion of metal particles and tuned structure of active sites by the doping of cobalt. The hydrogenation of furfural to furfuryl alcohol requires the selective activation of C O bond of FFA and the dissociation of hydrogen. For catalyst Cu/C-873, metal copper acts as an active site for both the activation

of C O bond and the dissociation of hydrogen molecule. And the relatively large copper particle size is less active in hydrogen activation [13,38]. With the doping of cobalt, a better dispersion and smaller particle size of copper and cobalt particles can be achieved in the bimetallic CuCo/C-873 catalysts which will contribute to more active sites for the reaction to take place as revealed by XRD and TEM characterization [39]. Besides, the cobalt oxide (CoOx ) can interact with the C O group strongly due to the presence of cationic species [40]. Thus, more activated C O group can be prepared for hydrogenation as more CoOx sites are available in catalysts with high content of cobalt. In addition, metallic cobalt is more active in hydrogen dissociation than copper. Though single metallic cobalt may active both the C O bond and the furan ring, the synergistic effect of copper and cobalt may prevent the activation of furan ring. Therefore, bimetal CuCo/C-873 catalysts have a better catalytic performance than Cu/C-873. As for different calcination temperatures from 773 K to 1073 K with the same copper to cobalt molar ratio (Table 2, Entry 5, 7–9), CuCo0.4 /C-873 has the best catalytic performance regarding conversion and FOL yield. The existence of CuO in catalyst CuCo0.4 /C-773 may be responsible for its relatively low conversion. The inactive role of CuO in hydrogenation of furfural to furfuryl alcohol has been found over catalysts Cu/SBA-15 and Cu-MgOCr2 O3 [38,41]. At higher thermolysis temperatures, the conversion of FFA drops dramatically which is most likely due to the increased particle size. And the high degree of aggregation of copper and cobalt particles in CuCo0.4 /C-1073 has caused a severe drop in hydrogenation activity as only trace amounts of FOL could be detected in CuCo0.4 /C-1073 reaction system. Based on the above experimental results, a possible reaction mechanism of the chemoselective hydrogenation process catalyzed by CuCo/C catalyst in ethanol is proposed. The reduction of furfural to furfuryl alcohol would proceed in several consecutive steps. Firstly, the furfural substrate is adsorbed on the surface of CuCo/C catalyst and interacts with Cu and CoOx via the C O bond. Meanwhile, hydrogen molecule is adsorbed and decomposed into hydrogen atoms on Cu and Co surfaces. In the second, the C O bond would be hydrogenated selectively to form C OH by the attack of the activated hydrogen atoms. 3.3. Hydrogenation of furfural over catalyst CuCo0.4 /C-873 The effect of catalyst amount, furfural concentration, initial hydrogen pressure and reaction temperature on the conversion

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Fig. 7. (A) Effect of catalyst amount (15 wt% furfural in solution; 413 K; 3 MPa), (B) furfural concentration (0.7 g catalyst; 413 K; 3 MPa), (C) initial hydrogenation pressure (0.7 g catalyst; 15 wt% furfural in solution; 413 K), and (D) reaction temperature (0.7 g catalyst; 15 wt% furfural in solution; 3 MPa) on conversion of furfural and selectivity toward furfuryl alcohol.

of furfural (FFA) and selectivity toward furfuryl alcohol (FOL) was studied using ethanol as a solvent over catalyst CuCo0.4 /C-873, and the results are shown in Fig. 7A–D, respectively. The conversion of FFA increases from 24.9% to 98.7% as the catalyst amount increases from 0.1 to 0.7 g (Fig. 7A), which is due to a proportional increase in the active site of the catalyst. However, about 98% FOL selectivity is obtained at all catalyst loadings. Highest conversion of furfural to furfural alcohol is observed with 0.7 and 0.9 g of CuCo0.4 /C-873 loading. The reaction in the case of 0.9 g catalyst loading is found to be faster compared to a loading of 0.7 g; though, no much difference in the total conversion of furfural to furfuryl alcohol is observed. In contrast, the conversion of furfural decreases from 98.7% to 61% as the weight percentage of FFA in ethanol increases from 15 wt% to 30 wt% without affecting selectivity of furfuryl alcohol (about 98%) (Fig. 7B). And the reaction is very fast at 10 wt%. This is justified by the fact that the available active sites for FFA hydrogenation gradually decrease with the increase of FFA concentration. And the decrease in FFA conversion can be attributed to the fact that there are no additional active sites to active the carbonyl functionality in FFA at higher FFA concentration. As shown in Fig. 7C, the conversion of FFA increases when the hydrogen pressure increases from 1 MPa to 3 MPa, and then keeps around 99% with the further increase of hydrogen pressure. However, the selectivity of FOL suffers from a slight decrease at high hydrogen pressure (4 MPa and 5 MPa). The increased solubility of hydrogen in the reaction solution at high hydrogen

Table 3 Effect of solvent on furfural hydrogenation over CuCo0.4 /C-873.a Entry

Solvent

Solvent Polarity

FFA Conversion (%)

FOL Selectivity (%)

1 2 3 4 5

Water Ethanol Methanol Toluene Hexane

10.2 4.3 6.6 2.4 0.06

40.6 98.7 99.6 80.7 47.5

62.3 97.7 97.0 98.6 93.9

a

Reaction conditions: 0.7 g catalyst; 15 wt% furfural in solution; 413 K; 3 MPa.

pressure should be responsible for the increased conversion. However, higher hydrogen pressure also promotes side reactions, which leads to a decreased FOL selectivity. The effect of reaction temperature is studied in the range of 383 K to 423 K. It can be seen from Fig. 7D that the increase of activity by higher reaction temperature is quite obvious while FOL selectivity is not affected by reaction temperature. Catalytic properties of CuCo0.4 /C-873 catalyst for hydrogenation of furfural to furfuryl alcohol were also investigated in solvents other than ethanol (Table 3). Solvent properties such as polarity can affect solvent-reactant interactions and consequently, hydrogenation activity [18]. As can be seen from Table 3, the catalytic activity is strongly influenced by solvent. The best catalytic activity is obtained in less polar solvents, ethanol and methanol. In the

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most polar solvent, water, a low FFA conversion and FOL selectivity is observed (40.6% and 62.3%, respectively) and cyclopentanone is detected as a by-product with selectivity of 23.4%. The low solubility of furfural in water may be responsible for the low conversion. Besides, water also acts as a nucleophile to facilitate the furan ring rearrangement to cyclopentanone [42]. When the reaction takes place in weakly polar solvents, toluene and hexane, it exhibits poor FFA conversion (80.7% and 47.5%, respectively). Similar observations are also reported by A. B. Merlo et al. [43] that hydrogenation of furfural in n-heptane and toluene suffers from decreased activity compared with other alcoholic solvents. The strong adsorption of furfural in the solvent might account for the loss of activity. The different solvent-furfural interactions may also be responsible for varied selectivity to furfuryl alcohol. The selective hydrogen of furfural to furfuryl alcohol requires the preferred activation of C O bond on furfural by the active sites. Whereas the interaction between solvent and reactant may impact the activation of C O bond, which would further effect the hydrogenation of C O bond and lead to different selectivity to furfuryl alcohol with different solvents. However, this theory needs to be further investigated. These results indicate that hydrogenation of furfural to furfuryl alcohol with CuCo0.4 /C-873 catalyst take place more easily in medium polar solvent. To better compare with previous works from literature, the catalytic performance of CuCo0.4 /C-873 in the hydrogenation of furfural to furfuryl alcohol in this work has been compared with the representative results of the recently reported works as shown in Table S1. In comparison, to the best of our knowledge, our results are excellent, which are even better than some noble metal catalysts. It is important to investigate the reusability of the catalyst in heterogeneous reactions from the cost point of view. Therefore, stability tests were performed using 0.5 g CuCo0.4 /C-873 catalyst, 15 wt% furfural in ethanol solution at 413 K, 3 MPa. After each reaction, the catalyst was recovered by centrifugation, followed by washing with ethanol several times and drying at 323 K. Then, the catalyst was used again, completing four cycles of reaction. It can be seen from Fig. 8 that the conversion of furfural keeps around 78% while the selectivity to FOL undergoes a slight decrease from 98% to 90% after these four recycling runs, indicating the relatively good stability of prepared catalyst. Then, the used CuCo0.4 /C-873 catalyst was characterized to verify the reasons for the little loss of FOL yield. As shown in Fig. 9A, the used catalyst CuCo0.4 /C-873 has the same characteristic peaks ascribed to metallic copper and cobalt as the fresh one. However, the surface of spent catalyst may

Fig. 8. Recycling study of catalyst CuCo0.4 /C-873 in furfural hydrogenation (reaction conditions: 0.7 g catalyst; 15 wt% furfural in ethanol solution; 413 K; 3 MPa).

be covered by coke after the consecutive runs, which is indicated by the additional 6% weight loss in the range of 523 K to 773 K of used catalyst (Fig. 9B). The formed coke on catalyst surface should be responsible for the selectivity loss as it may cover some particular active sites [44].

4. Conclusion In summary, CuCo bimetal catalysts embedded in carbon matrix with highly dispersed metal/metal oxide sites can be obtained via the controlled calcination of Co-doped Cu-BTC precursor. By varying the molar ratio of cobalt to copper and calcination temperature, CuCo0.4 /C-873 shows best catalytic performance with 98.7% furfural conversion and 97.7% furfuryl alcohol selectivity in medium polar solvent at 413 K and 3 MPa hydrogen pressure, which is among the top catalytic activity based on reported ones. Its good catalytic performance mainly ascribes to the small particle size (about 9 nm) and the synergistic effect of copper and cobalt. The relatively high surface area and porosity also make a contribution to the high conversion and selectivity. CuCo0.4 /C-873 also shows good stability in recycling test. The present study proposed a novel idea to prepared carbon embedded metal catalysts with high dispersion and stability.

Fig. 9. XRD patterns (A) and TG curves (B) of fresh CuCo0.4 /C-873 catalyst (a) and CuCo0.4 /C-873 used for four times (b).

Y. Wang et al. / Molecular Catalysis 436 (2017) 128–137

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