A versatile bi-metallic copper–cobalt catalyst for liquid phase hydrogenation of furfural to 2-methylfuran

Sanjay Srivastava, G. C. Jadeja and Jigisha Parikh*
Department of Chemical Engineering, Sardar Vallabhbhai National Institute of Technology, Surat-395007, Gujarat, India. E-mail: jk_parikh@yahoo.co.in; jkp@ched.svnit.ac.in

Received 29th July 2015 , Accepted 16th December 2015

First published on 21st December 2015


Abstract

Liquid phase hydrogenation of furfural (FFR) to 2-methyl furan (2-MF) was examined using noble metal free, Cr-free, bi-metallic Cu–Co catalysts. Three supported bi-metallic catalysts (Cu–Co/SiO2, Cu–Co/H-ZSM-5, and Cu–Co/γ-Al2O3) with various Cu/Co molar ratios (x/y = 1, 2, and 4) with fixed Cu loading (x = 10 wt%) were prepared by the impregnation method. The physico-chemical properties of various catalysts were studied by using XRD, N2-sorption, SEM, TEM, TPR, TPD, XANES/EXAFS and CHNS methods. The results confirmed the formation of spinel CuCo2O4 oxides, and much higher dispersion of Cu on acidic supports such as H-ZSM-5 and γ-Al2O3. However, the absence of a spinel CuCo2O4 oxide was observed in Cu–Co/SiO2 via XANES/EXAFS results. XRD and TEM results revealed the formation of bigger Cu particles in Cu–Co/SiO2. In the catalytic activity studies, Cu–Co/γ-Al2O3 catalyzed the hydrogenation of furfural with 98.8% conversion, resulting in maximal selectivity of 2-MF due to the presence of maximal Cu-CoOx sites. The H-ZSM-5 supported catalyst had marginally less 2-MF selectivity, whereas the silica supported catalyst exhibited maximum selectivity towards furfuryl alcohol (FOL) because of the large copper particles. H2-TPR and EXAFS results revealed that the incorporation of cobalt metal improves the reducibility of Cu-catalysts, thus improving the catalytic activity. Bi-metallic Cu–Co/γ-Al2O3 catalysts displayed higher activity as compared to their monometallic counterpart, and Cu–Co/γ-Al2O3 (x/y = 1) exhibited the best catalytic performance with 78% selectivity to 2-MF at 220 °C and 4 MPa.


Introduction

Nowadays, 85% of the world's energy demands rely on fossil derived energy and fuels.1 Due to the ultimate diminution of crude oil reserves together with increasing anthropogenic emissions of greenhouse gases, climate change, and ever increasing prices of crude petroleum, the search for renewable resources has attracted worldwide attention leading to the development of new energy alternatives for possible substitution of fossil reserves.2,3 The conversion of lignocellulosic biomass (only carbon containing renewable feedstock) into bio-oil is of special interest because of its wide availability.4 However, direct use of bio-oil is restricted as a fuel due to its adverse properties such as low calorific value, deficient volatility, acidity, and instability, which were not compatible to the I. C. engine.5 The undesirable properties of bio-oil are attributed to the presence of different class of oxygenates such as aldehydes, acids, and ketones. These oxygenates often condense and polymerize during storage owing to their high reactivity.5,6 The removal of oxygen is thus mandatory to convert bio-oil into a liquid fuel.7

Furfural (FFR) is one of the innumerable oxygenated compounds which is found in bio-oil, containing both C[double bond, length as m-dash]C and C[double bond, length as m-dash]O bonds. The C[double bond, length as m-dash]C stays within one 5 member ring and 6 electrons stabilized system that makes the C[double bond, length as m-dash]C bond very stable.8,9 Therefore, hydrogenation of C[double bond, length as m-dash]O bond was often catalyzed, resulting in furfuryl alcohol (FOL) as a primary product, and further hydrogenolysis of FOL enables 2-methyl furan (2-MF) (see Scheme 1). As compared with hydrocarbons, 2-MF contains the oxygen atom having better combustion performance and the higher research octane number (RON = 103) than that of gasoline (RON = 96.8).10 Owing to this admirable property, 2-methyl furan has been recently used as a gasoline blend in standard vehicles.11 In other applications, 2-MF is used as perfume intermediates, chloroquine lateral chains in medical intermediates, and as a raw material for the production of crysanthemate pesticides.11,12


image file: c5ra15048e-s1.tif
Scheme 1 Hydrogenation of furfural to 2-methylfuran.

Hydrogenation of furfural to 2-methylfuran can take place both in the vapour and in the liquid phase due to its high vapour pressure.13 The vapour phase conversion of FFR to 2-MF has been widely studied over RANEY®-Cu, Cu/SiO2, Cu/SBA-15, Cu/MgO, Cu–Mn–Si, Cu–Zn–Al, Ni–Fe/SiO2 at a high temperature range (250–270 °C).13–19 However, deactivation of supported Cu-catalysts is a key issue in vapour phase hydrogenation, although they could be regenerated via calcinations above 500 °C.12 In addition, vapour phase hydrogenation of furfural produces not only the 2-MF but also a wide range of by products, including furan, tetrahydrofuran (THF), and even C-4 products.18

In contrast, liquid phase hydrogenation is most likely preferred for compatibility with the upstream production of furfural.13 The liquid phase hydrogenation of furfural to 2-methyl furan has been performed either on noble metals such as Ru, and Pt13,20–22 or Cu–Cr, and Cu–Fe catalysts.10–12 Yan et al.10,11 have reported the liquid phase synthesis of 2-MF via furfural using Cu–Cr, and Cu–Fe catalysts, wherein a reasonable yield of 51% was reported over a Cu–Fe catalyst at 220 °C and 90 bar. However, such high temperature and pressure can account for the extra energy cost. In addition, owing to high toxicity and environmental concern for Cu–Cr alloys, its use has been restricted as industrial catalyst.11,23 Recently, Paraskevi et al.13 have reported the moderate yield of 2-MF (61%) over a Ru/C catalyst at temperature of 220 °C. The research done so far in this area manifests the incompatibility of most of the catalysts for industrial applications owing to certain disadvantages, such as severe environmental concerns, the low selectivity, the high cost, and ruthless deactivation. Thus, an efficient, cost effective, environmentally benign and a stable catalyst for FFR hydrogenation to 2-MF is the need of the hour. Recently, a highly stable bimetallic Cu–Co catalyst has shown good versatility as it can be used in many reactions including FT-synthesis and other hydrogenation reactions.24–28 The major advantage of this bi-metallic catalyst system is the presence of metallic copper and partially reduced cobalt species.29–32 Furthermore, the strong interaction between copper and cobalt may result in the formation of mixed oxide phases.33 Xu et al.34 have reported that early transition metal oxides such as CoOx, ReOx, and MnOx etc., can break the C–O bond. Therefore, the combination of Cu metal and CoOx can be effective in the production of 2-MF via hydrogenation/hydrogenolysis process using bi-metallic Cu–Co system. In our previous work, we have reported the higher conversion of furfural to furfuryl alcohol over SBA-15 supported Cu–Co catalysts.27,28

Consequently, in the present study noble metal free, and Cr-free bi-metallic Cu–Co catalysts with various Cu/Co molar ratios (x/y = 1, 2, and 4) over different supports such as SiO2, H-ZSM-5, and γ-Al2O3 were prepared. Further, the ability of synthesized catalysts was explored towards production of 2-MF via liquid phase hydrogenation of FFR. Subsequently, the effect of temperature (140–220 °C) was studied over Cu–Co/γ-Al2O3 (x/y = 1).

Experimental

Material

Metal precursors such as cobalt nitrate (Co (NO3)2·6H2O), copper nitrate (Cu (NO3)2·6H2O); Chemicals such as furfural, furfuryl alcohol, 2-methyl furan, 2-methyltetrahydrofuran, 2-pentanone, and 2-pentanol (all GC grade) were purchased from Sigma-Aldrich, Mumbai, India. Supports such as SiO2, H-ZSM-5 (Si/Al = 25), and γ-Al2O3 having purity (99%) were purchased from the local vendor.

Catalysts synthesis and characterization

The bi-metallic Cu–Co catalysts with various Cu/Co molar ratios (x/y = 1, 2, and 4) over different supports such as SiO2, H-ZSM-5, and γ-Al2O3 were synthesized by impregnation method using salt of respective metals. The prepared catalysts were characterized by X-ray diffraction (XRD), N2-adsorption desorption, field emission-scanning electron microscopy analysis (FE-SEM), transmission electron microscopy analysis (TEM), ammonia-temperature-programmed desorption (NH3-TPD), H2-temperature programmed reduction (TPR), X-ray absorption spectroscopy (XANES)/(EXAFS), and CHNS elemental analysis. The detailed procedures regarding the synthesis and characterization of catalysts are provided in the ESI.

Catalytic activity study

Catalytic activity studies were performed in a 100 mL autoclave reactor using 2-propanol as a solvent. Briefly, in a typical experiment, 1 g of catalyst was reduced at 280 ± 2 °C by passing H2 at pressure of 1 MPa for 3 h. The reactor was then cooled down to room temperature and flushed with N2. Furfural (3.72 mL) was initially added into 2-propanol solvent (20 mL) and charged into the reactor. The catalytic activity of prepared catalysts was investigated at 140–220 °C under hydrogen pressure of 4 MPa with stirring speed at 1000 rpm, thereby ensuring the absence of gas–liquid, liquid–solid, and intra-particle mass transfer limitations.35 The reproducibility of the experimental analysis was ensured by repeating each run twice; the reproducibility was within acceptable limits. Product samples were analysed using a Sigma GC system with FID detector. The carbowax capillary column with dimension of 30 m × 0.25 mm × 0.5 μm was used. In a typical procedure, the initial oven temperature was held at 80 °C for 4 min and increased up to 220 °C with the ramp of 20 °C min−1 and hold for 4 min. The product suspension (1 μL) in 2-propanol was injected into the capillary column by using the split ratio of 20[thin space (1/6-em)]:[thin space (1/6-em)]1, where nitrogen was used as a carrier gas. The catalytic parameters such as conversion, and selectivity were calculated by using the following formulae:
 
image file: c5ra15048e-t1.tif(i)
 
image file: c5ra15048e-t2.tif(ii)
C = FFR conversion, S = Selectivity, X = FFR moles.

Result and discussion

Catalysts characterization

Fig. 1 shows XRD spectra of bi-metallic Cu–Co (x/y = 1) catalysts supported on SiO2, H-ZSM-5, and γ-Al2O3. As observed, the bimetallic catalysts contain either segregated CuO, or spinel oxides of Co3O4/CuCo2O4. Owing to a similar cubic framework and approximately similar unit cell parameters (aCo3O4 = 8.177 and aCuCo2O4 = 8.122 Å), Co3O4 and CuCo2O4 were not easily distinguishable from their XRD patterns,33 which were further confirmed using XANES/EXAFS analysis. Cu–Co/SiO2 (x/y = 1) catalyst showed sharp diffraction patterns at 2θ = 35.5°, 38.7° which are assigned to the planes (111) and (111) of monoclinic copper oxide (JCPDS, 80-1917, 45-0937),16,26,36 and few diffraction patterns pertaining to the Co3O4 at 2θ = 37°, 44°, and 56° (JCPDS, 43-1003).26,33,37 Sharp diffraction peaks indicate the presence of large metallic crystallite sizes in Cu–Co/SiO2. In SiO2 supported catalyst, the size of the CuO crystallite was found to be in the range of 35 to 40 nm, as calculated by the Scherrer formula. On the other hand, Cu–Co (x/y = 1) catalysts supported on acidic carriers, i.e. H-ZSM-5 and γ-Al2O3, showed mixed reflections of CuO, Co3O4, and spinel oxides of Co3O4/CuCo2O4.26,33
image file: c5ra15048e-f1.tif
Fig. 1 XRD patterns for supported Cu–Co (x/y = 1) catalysts [(o) CuCoO4, (#) Co3O4 (Δ) CuO].

The smaller size metal particles were found in the case of both H-ZSM-5 and γ-Al2O3 supported catalysts. The size of metal particles lies in the range of 15–20 nm as calculated by the Scherrer formula. These observations are in well accordance with those previously reported.31,32 The difference in the phases obtained with different supports may be explained on the basis of a distinct metal –metal and metal–support interactions. It is worth noting that all the oxide supports used in the present study have weak interaction with copper and cobalt,38 and the pattern of interaction follows as SiO2 > H-ZSM-5 > γ-Al2O3. In addition, both copper and cobalt metals are known to possess strong interaction between them.33 Accordingly, the relatively stronger interaction of copper and cobalt with SiO2 resulted in segregated phases of CuO and Co3O4, having bigger sizes, whereas the weaker interaction of copper and cobalt oxides with acidic supports resulted in the formation of CuCo2O4 phase. These observations are well matched with previous reports,33 wherein the presence of bi-metallic Cu–Co phases were confirmed at even low Cu content (5%). Fig. 1S depicts the XRD patterns of Cu–Co/γ-Al2O3 with various Cu/Co ratios (x/y = 1, 2, and 4). An increase in the intensity of the peaks was observed when Cu/Co ratio decreased from 4 to 1. This increase in intensity is due to an agglomeration of the metal particles which was further justified by the BET surface area and SEM micrographs. In addition, few peaks pertaining to the Co3O4 appeared with an increase in Co metal which suggests the preferential localization of cobalt oxide particles in alumina pores, attributed to the high loading of Co.33,37

The textural properties of supports and synthesized catalysts Cu–Co (x/y = 1) over different supports such as SiO2, H-ZSM-5, and γ-Al2O3 are summarized in Table 1 It was observed that silica supported catalyst shows the large decrease in the % surface area of parent material as compared to acidic supports H-ZSM-5 and γ-Al2O3. Decline in the surface area can be attributed to the nature of supports as silica had a stronger interaction with both metals used in the present work.30

Table 1 Textural and structural characteristics of various supports
Catalysts/supports Cu/Co molar ratio (x/y) EDX (x[thin space (1/6-em)]:[thin space (1/6-em)]y) wt% Surface area (m2 g−1) Pore volume (cm3 g−1) Pore size (Å) Avg. particle size (nm)
SiO2     179.4 0.34 42  
H-ZSM-5     392.5 0.46 64  
γ-Al2O3     166.1 0.54 66  
Cu–Co/SiO2 1 9.2[thin space (1/6-em)]:[thin space (1/6-em)]8.4 102.6 0.17 28 35
Cu–Co/H-ZSM-5 1 10.6[thin space (1/6-em)]:[thin space (1/6-em)]10.2 239.9 0.34 58 15
Cu–Co/γ-Al2O3 1 10.4[thin space (1/6-em)]:[thin space (1/6-em)]9.9 113.5 0.32 46 20


Effects of Cu/Co ratios on textural and structural characteristics of Cu–Co/γ-Al2O3 are presented in Table S1. The gradual decrease in surface area, pore volume, and pore size were observed with decreasing Cu/Co ratio. This may be attributed to the agglomeration of metal particles within the pores of γ-Al2O3 due to high loading of metal particles.

The morphology of Cu–Co (x/y = 1) catalysts in the bulk and on surface of SiO2, H-ZSM-5, and γ-Al2O3, are presented in Fig. 2. The distinct morphology of all the three catalysts appeared with a little agglomeration of metal particles over different supports. Uniform distribution having small metal crystallites was observed in Cu–Co/γ-Al2O3 (x/y = 1). Similar dispersion having large metal crystallites was observed in Cu–Co/SiO2 (x/y = 1). However, non-uniform distribution having un-even sizes of metal crystallites appeared in Cu–Co/H-ZSM-5. The shape and the size of the fresh Cu–Co/γ-Al2O3 (x/y = 1, 2, and 4) catalysts are presented in Fig. S3. An average crystallites size was increased with increasing doping of the second metal (Co) as revealed by SEM data. The size of crystallites in Cu–Co/γ-Al2O3 (x/y = 4) lies between 10–15 μm, in Cu–Co/γ-Al2O3 (x/y = 2) lies between 20–25 μm, and in Cu–Co/γ-Al2O3 (x/y = 1), it lies between 20–30 μm. Furthermore, samples were examined using energy dispersive X-ray microanalysis (EDX) to quantify the metal loadings (in wt%) and the results were compared with actual metal loadings deposited during the preparation (refer. Tables 1 & S1). As compared with the actual metal loadings of the samples, EDX results are well supported within the limit of acceptable error.


image file: c5ra15048e-f2.tif
Fig. 2 Scanning electron micrographs of Cu–Co (x/y = 1) over (a) SiO2, (b) H-ZSM-5, (c) γ-Al2O3.

The TEM micrographs of freshly reduced Cu–Co (x/y = 1) over different supports, and used Cu–Co (x/y = 1)/γ-Al2O3 (2nd cycle) without calcinations are presented in Fig. 3. As observed, metal–support interaction plays key role in stabilizing the morphology/ordering of the Cu–Co nanoparticles. Ordered large metal nanoparticles were observed in SiO2 supported catalysts owing to stronger metal–support interaction between silica and metal particles. However, over H-ZSM-5, the Cu–Co nanoparticles were found to be non-uniformly distributed. Whereas, Cu–Co/γ-Al2O3 showed well dispersed and ordered small nanoparticles having uniform spherical shape as evident from the pictorial view. Furthermore, the size of copper–cobalt nano-particles was different to a certain extent, as Cu–Co/H-ZSM-5 contains mixed sizes of particles (15–25 nm). However, Cu–Co/γ-Al2O3 showed mostly uniform size about 20 nm. This morphology/ordering of the Cu–Co nanoparticles over both H-ZSM-5 and γ-Al2O3 catalysts can be explained on the basis of a combination of porous structure and metal–support interaction. The more uniform distributions of copper–cobalt particles over γ-Al2O3 support may be ascribed to weaker interaction of uniform porous alumina to both copper–cobalt species amongst all three supports used. On the contrary, H-ZSM-5 has both Si and Al metals in it. So, there is a possibility of slightly stronger interaction, due to combined effect of Si and Al with copper and cobalt oxides. This may lead to deviation in the metal distribution on the catalyst support surface. Furthermore, there are different morphologies of Al in zeolites (such as tetragonal and octahedral). Hence, when these Al particles interact with copper and cobalt oxides, the particle size of copper and cobalt oxides may change to certain extent.39


image file: c5ra15048e-f3.tif
Fig. 3 Transmission electron micrographs of Cu–Co (x/y = 1) over (a) SiO2, (b) H-ZSM-5, (c) γ-Al2O3, (d) used Cu–Co (x/y = 1)/γ-Al2O3 (2nd cycle).

The surface acidity of bi-metallic Cu–Co (x/y = 1) catalysts supported on SiO2, H-ZSM-5, and γ-Al2O3 was determined by NH3-TPD experiments (Fig. 4, Table S2). Generally, the strength of acid sites which was determined by the NH3 desorption temperature can be classified as weak (<250 °C), medium (250–400 °C) and strong (>400 °C) acidic sites. The overall acidity in various catalysts can be calculated from the relative peak area of NH3-desorption curves.37 As expected, reduced Cu–Co/SiO2 (x/y = 1) showed insufficient weak acid sites, as SiO2 had scarcely the acidic property. Cu–Co/γ-Al2O3 (x/y = 1) showed three peaks (145 °C, 335 °C, and 555 °C) which can be ascribed to NH3 desorption from weak, medium and strong acid sites. While, Cu–Co/H-ZSM-5 (x/y = 1) showed two major peaks (〈230–260〉 °C, and 〈400–450〉 °C) which can be ascribed to NH3 desorption due to the presence of medium and strong acid sites (refer. Fig. 3, and Table S2). It was observed that an acidity of both γ-Al2O3 and H-ZSM-5 is improved (0.28 to 0.49 and 0.66 to 1.08) by the doping of Cu and Co. This may be attributed to the formation of CoOx and/or mixed Cu–Co phases over acidic supports.


image file: c5ra15048e-f4.tif
Fig. 4 Ammonia-TPD profiles of Cu–Co (x/y = 1) supported on SiO2, H-ZSM-5, and γ-Al2O3.

The surface acidity of Cu–Co/γ-Al2O3 (x/y = 1, 2, and 4) catalysts changed with Cu/Co ratio (see Fig. S4 and Table S3). However, total acidity was not at all influenced by the effect of Cu/Co ratio. It was observed that weak and moderate acid sites increased slightly with the decrease in a Cu/Co ratio (from 4 to 1), while for strong acidic sites, a reverse trend was observed and maximum strong acid sites were found at a Cu/Co (x/y = 4) ratio. The strong acidic sites present in both the catalysts suggests the strong attraction of Cu to OH of alumina.38 In addition, few copper species (after reduction) may be interacting with γ-Al2O3 owing to the high electron affinity of aluminium. This electron affinity of aluminium would seize electrons from metal copper. The transfer of electron density from metal copper to aluminium would increase the electropositive (or the oxophilic) nature of copper and the further improvement of the oxophilic nature would contribute to hydrogenolysis of the saturated C–O band.32,33

To investigate the reducibility and structural evolutions of bi-metallic Cu–Co catalysts, H2-TPR experiments were employed. Generally, when metal species interact with carriers by creating new surface compounds or altering their chemical states, their reduction temperature would change. Accordingly, the interaction of each metal component with supports would have significant influence on the chemical environment of metals. A weak interaction would favour the reduction of metal oxide to metal.38 On the contrary, due to the presence of a strong interaction between metal species and supports, metal particles would face difficulty in getting reduced, and would result in low valence metal or incomplete reduction.37,38

Fig. 5 shows the reducibility of Cu–Co (x/y = 1) catalysts supported on SiO2, H-ZSM-5, and γ-Al2O3. It was observed that the nature of the support had a significant effect on the reducibility of bi-metallic catalysts. The differences in the reduction temperature revealed that SiO2 (peak centred 305–345 °C) support had stronger interaction with both metals than γ-Al2O3 (peak centred 273 °C) and H-ZSM-5 (peak centred 242 °C). Cu–Co/SiO2 (x/y = 1) exhibited the highest reduction temperature as a result of the largest CuO crystallites and poor porous structure.


image file: c5ra15048e-f5.tif
Fig. 5 TPR pattern for Cu–Co (Cu/Co = 1) supported on SiO2, H-ZSM-5, and γ-Al2O3.

Concomitantly, TPR patterns for other two catalysts (Cu–Co/H-ZSM-5, and Cu–Co/γ-Al2O3) were different and reduced at low temperature ascribed to the reduction of mixed copper–cobalt oxides and smaller copper particles. These observations were well supported by XRD, and TEM results. The occurrence of multiple reductions peaks indicated the two step reduction of copper, and partial reduction of CuCo2O4/Co3O4. The reduction of copper occurs in two steps such as Cu2+ → Cu+ at low temperature, and Cu+ → Cu° at high temperature.16,35 Furthermore, amongst all three supports, weaker interaction between alumina and copper–cobalt oxide phases enhances the reducibility of both the copper oxides and mixed CuCo2O4, and thus increasing Cu–CoOx sites in Cu–Co/γ-Al2O3.

Fig. 6 displays the TPR patterns of bi-metallic Cu–Co/γ-Al2O3 catalysts with a various Cu/Co ratio (x/y = 1, 2, and 4), and comparison with their monometallic counterparts. The H2-TPR profile of monometallic 10 wt% Co/γ-Al2O3 displayed two hydrogen consumption peaks (at 504 and 657 °C). This can be attributed to the two-step reduction of Co3O4 to Co(0), which takes place via the intermediate CoO.40,41 The reduction of smaller CoO particles to Co(0) is often difficult and requires higher temperatures which can be attributed to the high-temperature broad TPR peak. The monometallic 10 wt% Cu/γ-Al2O3 also showed two peaks (at 193 and 249 °C), which probably corresponds to the reduction of CuO; Cu2+ → Cu+ at low temperature, and Cu+ → Cu° at high temperature. In contrast, much different H2-TPR patterns were observed for bimetallic Co–Cu/γ-Al2O3 catalysts with various Cu/Co ratio (x/y = 1, 2, and 4) due to strong interaction between Cu and Co species. It was believed that H2–TPR proceeds through the reduction of both segregated (CuO) and mixed oxides. These observations are in good agreement with literature available for bimetallic Cu–Co catalysts.31


image file: c5ra15048e-f6.tif
Fig. 6 Comparison of H2-TPR patterns of bi-metallic Cu–Co/γ-Al2O3 with various (Cu/Co) to monometallic Cu and Co supported on γ-Al2O3.

Furthermore, H2-TPR profiles changed significantly with Cu/Co ratio. This behaviour of the catalysts may be attributed to the step wise reduction of Cu and its level of interaction with Co. In the Cu–Co (x/y = 1), a broad peak was found with two lower shoulders appearing in between 321 to 354 °C, whereas in case of Cu–Co (x/y = 2), both the peaks started shifting to the slightly higher side. Cu–Co (x/y = 4) revealed three distinct peaks, which might be due to highly dispersed CuO at moderate temperature, and the formation of small Cu–Co clusters at higher temperature above 400 to 440 °C that can be observed distinctly (Fig. 6).

XAS has provided extra information about copper and cobalt phases and their dispersion in prepared catalysts. The XANES and derivative XANES spectra of fresh calcined and reduced Cu–Co (X/Y = 1) catalysts over various supports, and Cu–Co/γ-Al2O3 (X/Y = 1, 2, 4) were compared with Cu-foil, CuO references (Fig. 7 and S5). Copper oxide (Cu2+) is likely to have the threshold binding at higher energy than metallic copper Cu(0) which can be observed in Fig. 7. The XANES results for calcined catalysts are indicating the presence of Cu2+ in the bulk of CuO which is characterized by the edge position located at near 8990 eV, corresponding to the maximum of the first derivative (Fig. 7 and S5). The XANES spectrum of CuO has displayed two characteristic features of Cu2+ (3d9) compounds42 with a pre-edge absorption at 8979 eV, and shoulder at 8986 eV that determines 1s → 4p ‘shake-down’ transition in copper. Cu–Co/SiO2 (X/Y = 1) has resemblance to the CuO characteristic XANES spectrum indicating the presence of Cu2+. However, relative to crystalline CuO, the calcined Cu–Co (X/Y = 1) supported on H-ZSM-5, γ-Al2O3, and Cu–Co/γ-Al2O3 (X/Y = 1, 2, 4) have an additional deep white line (first peak after the absorption edge), indicating higher 4p σ density of state.43 The EXAFS Fourier transform modulus for fresh Cu–Co (X/Y = 1) calcined catalysts over various supports and Cu–Co/γ-Al2O3 (X/Y = 1, 2, 4) are displayed in Fig. S6.


image file: c5ra15048e-f7.tif
Fig. 7 X-ray absorption near spectroscopy (XANES) spectra of Cu–Co (X/Y = 1) over various supports (a) Fresh calcined 450 °C, (b) reduced at 280 °C, and Cu–Co/γ-Al2O3 with various Cu/Co ratios (X/Y = 1, 2, and 4) (c) fresh calcined 450 °C, (d) reduced at 280 °C.

Cu–Co (X/Y = 1) supported on H-ZSM-5, and γ-Al2O3 displayed a peak in EXAFS Fourier transform modulus at 4.90 Å (refer. Fig. S6) indicating the presence of mixed oxide CuCo2O4 which can be attributed to the distance between Cu2+ and Co3+ in the mixed oxide.33 However, related peak was almost absent in the Cu–Co/SiO2 (X/Y = 1) catalyst which almost resembles the CuO Fourier transform modulus. These results validate those obtained by XRD analysis. The XANES and derivative XANES results of reduced catalysts indicate the presence of metallic Cu, and have the similar absorption energy pattern as that of the copper edge of copper foil (8979 eV). The quantity of CuO, and Cu(0) can be estimated from the XANES spectrum by Linear Combination Fitting in the Athena program.44,45 Cu foil and CuO is used as a reference material to fit the data of reduced catalysts samples. The LCF results are summarized in the Table 2. As expected, the reduction of CuO to Cu metal is more in Cu–Co/γ-Al2O3 (X/Y = 1) instead of Cu–Co/SiO2 and Cu–Co/H-ZSM-5 (X/Y = 1) catalysts. This is due to preferential localization of Cu particles in the uniform porous alumina. Furthermore, incorporation of cobalt metal improved the reducibility of Cu–Co/γ-Al2O3 catalysts, thus improving the catalytic activity. Nevertheless, we have studied Co–K edge XANES of fresh and reduced Cu–Co/γ-Al2O3 (X/Y = 1) so as to determine the reduction of Co3O4 (Fig. S7). The fresh catalyst showed similar spectra which is characterized by Co3O4 spectrum. However, XANES spectra of reduced Cu–Co/γ-Al2O3 (X/Y = 1) was slightly shifted towards lower energy level, and its pre-edge appeared probably in between Co-foil and Co3O4. Thereby, Co–K edge, XANES spectra indicate the presence of Co3O4 in fresh calcined catalysts and the partial reduction of Co3O4 to CoO in reduced catalyst.

Table 2 Cu–K edge LCF data of reduced catalysts
Catalysts X/Y % CuO % Cu(0)
Cu–Co/SiO2 1 54 46
Cu–Co/H-ZSM-5 1 35 65
Cu–Co/γ-Al2O3 1 21 79
Cu–Co/γ-Al2O3 2 38 62
Cu–Co/γ-Al2O3 4 48 52


Catalytic activity

Effect of support on the catalytic activity. In order to investigate the effect of support, Cu–Co (x/y = 1) catalysts supported on SiO2, H-ZSM-5, and γ-Al2O3 were examined for the liquid phase hydrogenation of furfural to 2-MF at 200 °C, and 4.0 MPa (Fig. 8). The results revealed 84% conversion of furfural over Cu–Co/SiO2, and almost 99% conversion over Cu–Co/γ-Al2O3 and Cu–Co/H-ZSM-5 catalysts respectively. As expected, Cu–Co/SiO2 (x/y = 1) catalyst showed lower activity than other two catalysts (i.e., Cu–Co/γ-Al2O3, and Cu–Co/H-ZSM-5). Notably, weak metal–support interactions facilitate the dispersion of Cu, and thus the hydrogenation of furfural which was well supported by XRD, TEM, XANES, and H2-TPR results. However, the selectivity towards 2-MF to a greater extent depends on the type of support, Co loading and/or both. For e.g., minimum 2-MF and/or maximal FOL selectivity was obtained over Cu–Co/SiO2 due to the presence of large Cu particles. The highest selectivity towards 2-MF was obtained over Cu–Co/γ-Al2O3 due to the presence of an additional number of Cu–CoOx which seems to be a key reason for enhanced MF formation, wherein, both the metallic copper and partially reduced CoOx have acted as active sites, favouring the conversion of intermediate FOL to 2-MF. As reported, Cu preferentially reduced the C[double bond, length as m-dash]O bond but the reduction of C–O was difficult over Cu catalysts. Xu et al.34 have reported that CoOx can effectively break the C–O bond and lead to a high selectivity to pentanediol from furfural. Hence, it is proposed that the combination of CuO and CuCo2O4 effectively break the C–O bond of intermediate FOL during reaction via –CH2–OH hydrogenolysis which resulted in selective production of MF.35
image file: c5ra15048e-f8.tif
Fig. 8 Catalytic activity of bi-metallic Cu–Co (x/y = 1) over different supports for hydrogenation of furfural (reaction conditions: T = 200 °C, P = 4.0 MPa, furfural concentration = 2.25 M, time = 4 h).

Furthermore, even though bi-metallic catalysts (Cu–Co/γ-Al2O3, and Cu–Co/H-ZSM-5) have resemblance in their catalytic structures as evident from XRD and XANES/EXAFS, Cu–Co/γ-Al2O3 is found to be more selective towards MF (Fig. 8). The increasing number of mixed Cu–CoOx species in Cu–Co/γ-Al2O3, having ordered metal particles with uniform porosity which responsible for more selective production of MF. However, the combined results from XANES/EXAFS, TEM, and H2-TPR studies revealed the presence of copper–cobalt nanoparticles with wide ranging shape and size and having comparatively less Cu–CoOx. In addition, these bi-metallic nanoparticles are un-evenly distributed on micro/meso porous H-ZSM-5. Thus, Cu–Co/H-ZSM-5 showed relatively less hydrogenolysis activity in comparison with Cu–Co/γ-Al2O3. The order of the selectivity towards 2-MF for this reaction is as follows: Cu–Co/γ-Al2O3 > Cu–Co/H-ZSM-5 > Cu–Co/SiO2.

To further investigate the reaction pathways and address the enhanced hydrogenolysis activity, we examined the liquid phase hydrogenation of furfuryl alcohol over the same catalysts under same reaction conditions (Fig. 9). As expected, hydrogenolysis activity of Cu–Co/SiO2 catalyst was much lower than the other two catalysts. Only 68% FOL got converted over Cu–Co/SiO2 with 42% selectivity towards MF. Interestingly, THFOL was observed to be the next selective product over Cu–Co/SiO2, which might be due to the saturation of the furan ring over metallic Cu. However, both Cu–Co/H-ZSM-5 and Cu–Co/γ-Al2O3 displayed slightly different trends than that observed with furfural hydrogenation. On comparative basis, Cu–Co/γ-Al2O3 exhibited higher hydrogenolysis activity (92% conversion of FOL) with 72% selectivity towards MF. In addition, the selectivity of pentanol (POL), derived from C2–O2 hydrogenolysis of 2-methylfuran,46 also increased significantly over Cu–Co/γ-Al2O3. Cu–Co/H-ZSM-5 showed 84% conversion of FOL with 52% selectivity towards MF. Notably, Cu–Co/H-ZSM-5 produced higher amount of THMF and THFOL and other hydrogenated products than Cu–Co/γ-Al2O3. These findings strengthen the results obtained in the case of one-step hydrogenation of furfural to MF.


image file: c5ra15048e-f9.tif
Fig. 9 Catalytic activity of bi-metallic Cu–Co (x/y = 1) over different supports for hydrogenation of furfuryl alcohol (reaction conditions: T = 200 °C, P = 4.0 MPa, FOL concentration = 2.25 M, time = 4 h).
Effect of Cu/Co molar ratio (x/y) on the catalytic activity. In order to depict the role of Cu/Co ratio on the catalytic activity in the liquid-phase hydrogenation of furfural, bi-metallic Cu–Co/γ-Al2O3 with (x/y = 1, 2, and 4) were examined at 200 °C and 4.0 MPa. The results were compared with monometallic Cu/γ-Al2O3 (10 wt%) (Table 3). As observed, Cu/γ-Al2O3 (10 wt%) displayed low conversion of furfural of nearly 92.6%, with moderate selectivity of 2-MF (44%). In contrast, by introducing cobalt as a co-metal, the activity of the resulting bi-metallic catalysts was increased (Table 3). The hydrogenation on monometallic Cu/γ-Al2O3 is expected to be less due to the much lower affinity of the surface for H2 dissociation. Increase in the activity of bi-metallic Co–Cu catalysts may be attributed to a hydrogen spill-over effect whereby the aldehyde hydrogenation reaction takes place. Incorporation of Co as a co-metal helps to form mixed Cu–Co oxides due to strong interaction between Cu and Co as evident from XRD and XANES analyses. Thereby, H2 consumption by both CuO and CoOx may have contributed proportionately to their available weight proportion to the reduction process as a result of spill-over effects, thus improving the activity of bi-metallic system.47 Furthermore, the selectivity of 2-MF increased with a decrease in the Cu/Co ratio (x/y = 4 to 1), and Cu–Co/γ-Al2O3 (x/y = 1) displayed the highest selectivity towards 2-MF (62%). However, the effect of Cu/Co ratio was insignificant for the conversion of FFR. The increased selectivity of 2-MF can be attributed to the increase in the number of CoOx phases, copper metal, and weak acid sites, as Cu–Co/Al2O3 (X/Y = 1) shows the maximum number of Co3O4/CuCo2O4 phases, copper metal, and the weak acidity (refer Fig. S1 & Table S3) which are accountable for the increased rate of hydrogenolysis, and the resultant maximum selectivity of MF.34,48
Table 3 Effect of Cu/Co (x/y) on the catalytic activity of bi-metallic Cu–Co catalysts supported on γ-Al2O3a
Catalysts Loading % XFFR % S
x/y Cu (wt%) 2-MF THMF FOL OTH
a Reaction conditions: T = 200 °C, P = 4.0 MPa, furfural concentration = 2.25 M, time = 4 h.
Cu/γ-Al2O3   10 92.6 44.6 0.2 46.4 8.8
Cu–Co/γ-Al2O3 1 10 98.8 62.1 1.7 25.6 10.6
Cu–Co/γ-Al2O3 2 10 99.0 58.4 1.2 22.2 18.2
Cu–Co/γ-Al2O3 4 10 99.0 54.2 0.8 20.8 24.2


Effect of temperature on the activity of Cu–Co/γ-Al2O3 (x/y = 1). Control of the reaction temperature on the catalytic activity for Cu–Co/γ-Al2O3 (x/y = 1) is shown in Fig. 10. It is interesting to see that the reaction temperature plays an important role in the product distribution. It was observed that FOL is the main product at low temperature (140 °C), and selectivity towards 2-MF increased with an increase in reaction temperature from 140 to 220 °C. The furfural conversion increased from 94.6% (140 °C) to 100% (220 °C).
image file: c5ra15048e-f10.tif
Fig. 10 Effect of temperature on the selectivity of 2-MF over Cu–Co/γ-Al2O3 (x/y = 1) via furfural hydrogenation (reaction conditions: P = 4.0 MPa, furfural concentration = 2.25 M, time = 4 h).

The selectivity of desired 2-MF was found to be 78% with 100% conversion of furfural. The selectivity of 2-MF at a lower temperature (<200 °C) was very low, due to the presence of intermediate furfuryl alcohol. It is worth noting that higher temperature promotes the C–O hydrogenolysis for the cleavage of the carbonyl group due to the higher activation of the C–O bond.19 Therefore, the rise of temperature is much more important for the –CH2–OH hydrogenolysis of the intermediate FOL. However, beyond certain temperature, further increase in temperature will increase the rate of hydrogenolysis and form other hydrogenolysis products such as 2-pentanone, 1-pentanol, and 2-pentanol. In order to examine the conversion of intermediate FOL to 2-MF, the effect of temperature was also studied for hydrogenation of furfuryl alcohol (FOL) to 2-MF over the temperature range from 140 to 220 °C. The results revealed the similar trend for the selectivity towards 2-MF as observed with FFR hydrogenation. The selectivity of 2-MF was increased with increasing temperature and reached the maximum of 84% (Fig. 11). These observations suggest that hydrogenation of FFR to FOL is a metal catalyzed reaction which favours at lower temperature whereas, hydrogenolysis of FOL to 2-MF occurs at a high temperature which is be catalyzed by mixed copper–cobalt species.


image file: c5ra15048e-f11.tif
Fig. 11 Effect of temperature on the selectivity of 2-MF over Cu–Co/γ-Al2O3 (x/y = 1) via FOL hydrogenolysis (reaction conditions: P = 4.0 MPa, FOL concentration = 2.25 M, time = 4 h).
Catalysts reusability for liquid phase hydrogenation of furfural. Cu–Co/γ-Al2O3 (x/y = 1) catalyst that exhibited the best catalytic efficiency was recycled three times to evaluate its stability. Owing to possible deposits (organic reactant/products and/or possible carbon deposits) on the catalyst surface during the catalytic reactions, catalyst was dried in an inert atmosphere at 100 °C followed by calcinations at 500 °C to remove potentially adsorbed or deposited organic/carbon species. The used catalyst without calcinations (2nd Cycle, dried at 100 °C) is characterized using TEM, XRD, and CHNS analysis to determine the possible deposits of organic template and/or carbon on the active sites (refer Fig. 3d, S2, and Table S4). The XRD pattern of used catalyst resembles the characteristic features of γ-Al2O3. Furthermore, as compared with Cu–Co/γ-Al2O3 (x/y = 1) catalyst, no peak corresponding to Cu, Co, and/or their mixed phases were observed (Fig. S2). This can be attributed to the possible deposition of carbon over active sites. However, the absence of carbon peaks in XRD pattern indicates the uniform distribution of carbon. The XRD result of used catalyst is well supported by TEM and CHNS analyses, which have confirmed the presence of carbon (Fig. 3d, Table S4). Furthermore, TEM results demonstrate that not only carbon deposited on the active sites but also the size of particles slightly increased after 2nd cycle which may be the reason for a drop in the activity of catalysts In order to explore the recyclability of Cu–Co/Al2O3 (X/Y = 1) catalysts, the used catalyst was separated from a reaction mixture and dried at 100 °C for overnight in the oven. The hydrogenation of furfural was then performed over the used catalyst with and without calcination (Table 4).
Table 4 Reusability of Cu–Co/γ-Al2O3 (x/y = 1) catalyst toward hydrogenation of furfural to 2-methylfurana
Catalysts XFFR (%) S2-MF (%)
a XFFR = conversion of furfural, S2-MF = selectivity of 2-methylfuran reaction conditions: T = 220 °C, P = 4.0 MPa, furfural concentration = 2.25 M, time = 4 h.b No calcination.c Calcination.
Cu–Co/γ-Al2O3 (x/y = 1) fresh 100 78.0
Cu–Co/γ-Al2O3 (x/y = 1)b 1st cycle 88.6 59.5
Cu–Co/γ-Al2O3 (x/y = 1)b 2nd cycle 72.2 40.6
Cu–Co/γ-Al2O3 (x/y = 1)c 1st cycle 99.2 77.2
Cu–Co/γ-Al2O3 (x/y = 1)c 2nd cycle 94.2 76.8
Cu–Co/γ-Al2O3 (x/y = 1)c 3rd cycle 93.6 76.4


A significant decrease in conversion of furfural was observed after the catalyst was reused twice (from 100 to 72%) without calcinations. However, a slight decrease in the conversion of FFR (nearly 7% up to 3rd cycle) was observed over calcined catalysts owing to re-dispersion of active sites by coke burn off during calcinations. Thus, it could be concluded that catalyst had lost its activity due to coke formation during the hydrogenation of furfural, covering active sites of catalysts.

Conclusion

Three supported bi-metallic catalysts (Cu–Co/SiO2, Cu–Co/γ-Al2O3, and Cu–Co/H-ZSM-5) with various Cu/Co molar ratios (x/y) were prepared and examined for the one-step liquid phase hydrogenation of furfural to 2-MF. The inherent properties of carriers, and the strong interaction between Cu and Co have significant influence on the catalytic performance of bi-metallic Cu–Co catalysts towards the production of 2-MF. Cu–Co catalysts supported over acidic carrier are found to be much more selective towards MF owing to the formation of spinel CuCo2O4 oxides, and much higher dispersion of Cu on acidic supports such as H-ZSM-5, and γ-Al2O3. On the other hand, Cu–Co/SiO2 found to be less active and selective towards MF due to the presence of large Cu particles. Amongst three catalysts, Cu–Co/γ-Al2O3 (x/y = 1) exhibited the best catalytic performance. The conversion of furfural and the selectivity towards 2-MF were as high as 100% and 78%, respectively, at 220 °C and 4.0 MPa. Systematic characterization techniques revealed that Cu–Co/γ-Al2O3 (x/y = 1) catalyst had more Cu–CoOx phases which are accountable for hydrogenation/hydrogenolysis of furfural to MF. Moreover, present study explored the role of support acidity and metal–support interactions, which greatly influences particle size distribution/dispersion and reducibility of active sites, which in turn provided in depth insight for the rational design of a robust bi-metallic catalyst system for the hydrogenation and/or hydrogenolysis reactions. Further studies are being focused on optimization of process parameters, kinetics and mechanism of this reaction so as to provide basis for the reactor design and scale up studies.

Acknowledgements

We gratefully acknowledge Dr S. N. Jha for help with XANES experiments which were performed using beam line-8 at the Raja Ramanna Centre for Advanced Technology, Indore, India. We also acknowledge SAIF, IIT Mumbai for TEM analysis.

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra15048e

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