Ceria Promoted Cu-Ni/SiO2 Catalyst for Selective Hydrodeoxygenation of Vanillin.

A ceria (CeO2) promoted Cu-Ni bimetallic catalyst supported on SiO2 (Cu-Ni/CeO2-SiO2) was prepared and evaluated for catalytic hydrodeoxygenation (HDO) of vanillin. Silica supported monometallic Cu and Ni catalysts and bimetallic Cu-Ni catalyst (Cu/SiO2, Ni/SiO2, and Cu-Ni/SiO2), without a ceria promoter, were also synthesized and tested for the same application. The highest conversion of vanillin was achieved with the Cu-Ni/CeO2-SiO2 catalyst. Vanillyl alcohol was the sole product in the initial 2 h, followed by the formation of 2-methoxy-4-methylphenol, which was observed. Characterization of the synthesized catalysts revealed the presence of overlapping crystalline phases of CuO, NiO, and CeO2 on the Cu-Ni/CeO2-SiO2 surface. We extended our study to find out the results of using CeO2 as the support of the Cu-Ni bimetallic catalyst (Cu-Ni/CeO2). Partial incorporation of Cu and Ni cations into the ceria lattice took place, leading to the decrease of specific surface area and a concomitant compromise in the conversion. In the case of the Cu-Ni/CeO2-SiO2 catalyst, the higher conversion was accredited to the facile formation of Cu+ active centers by the synergistic interaction between Ce+4/Ce+3 and Cu+2/Cu+ redox couples and the incorporation of oxygen vacancies on the catalyst surface.

Introduction

Lignocellulose biomass derived bio-oil is recognized as a green sustainable energy resource against the limited fossil fuel stock. The lignin fraction of the lignocellulose biomass has a heterogeneous composition and can be potentially utilized for the production of biofuel and other fine chemicals.[1,2] However, the lignin-derived pyrolysis oil is high in oxygen content, which significantly compromises the quality of bio-oil by imparting some undesirable properties, like difficulty in mixing with conventional fuels, less volatility, high viscosity, corrosive properties, and unstable nature during long-time storage.[3−5] Deoxygenation of lignin-derived pyrolysis oil is a challenging task due to its complex structure compared to cellulose-derived pyrolysis oil. Several approaches have been introduced for the reduction of the oxygen content of the bio-oil.[6,7] Catalytic hydrodeoxygenation (HDO) is considered to be one of the most important and feasible strategies due to its high conversion efficiency and less energy penalty.[8] A considerable research focus could be found in the literature on the catalytic HDO using different lignocellulosic model compounds.[9−11]

Vanillin is an important chemical obtained by depolymerization of lignin, presently being carried out on an industrial scale.[12] The major products formed during HDO of vanillin are vanillyl alcohol and 2-methoxy-4-methylphenol. Under H2 gas pressure, the formyl group of vanillin reduces to the corresponding alcohol producing vanillyl alcohol. Subsequent hydrogenolysis of the C–O σ bond of the −CH2OH group in the vanillyl alcohol produces 2-methoxy-4-methylphenol.[4] The occurrence of the whole process on a heterogeneous catalyst surface requires two types of active sites, one for activation of the gas phase molecular hydrogen and the other for the C–O bond activation and rupture. Verma and Kishore[13] have studied the mechanistic aspects of the catalytic conversion of 2-hydroxybenzaldehyde over the Pd(111) surface through DFT. Deoxygenation reaction of 2-hydroxybenzaldehyde via alternative routes involving cleavage of the −OH group and the formyl group was considered. Cleavage of the formyl group through the formation of a phenol intermediate was energetically more feasible than the cleavage of the −OH group. Literature reports also reveal a large control of the catalyst surface on the product selectivity.[14] For example, the repulsion between the Cu surface and the aromatic ring of furfural leads to a vertical η1 (−O) orientation of furfural over the Cu surface via the formyl O. This results in preferential hydrogenation of the formyl group, forming furfuryl alcohol as the major product. On the contrary, Ni catalyzed HDO reaction of furfural leads to decarbonylation to furan and the ring opening to C4 products. Banerjee and Mushrif[15] showed an energetically more favorable flat η2 (C–O) orientation of the formyl group and parallel alignment of the furan ring over the Ni(111) surface in a DFT study of furfural HDO.

The reported catalysts active for the HDO of lignin model compounds include Mo-based sulfides, base metals, noble metals, supported noble metals, metal phosphides, carbides, and bifunctional catalysts.[16−20] An anion vacancy may serve as the active site for interaction of the O atom with the catalyst surface.[21] In Mo-based sulfide catalysts, a sulfur vacancy acts as the active site for interaction with the O atom lone pair. The catalytic activity increases with the surface acidity, though catalyst poisoning is a problem.[14] With the base metal catalysts, undesirable side reactions are commonly observed.[22,23] Noble metal nanoparticles are excellent catalysts for hydrogenolysis, though the cost is high.[24−29] Ni is a cheaper substitute of noble metals for efficient dissociative chemisorption of gas phase H2, but Ni also promotes the aromatic ring hydrogenation.[4,30] However, Ni-based bimetallic catalysts show a better performance.[31] Heeres et al.[32] investigated the catalytic hydrotreatment of fast pyrolysis oil of lignocellulose biomass employing a δ-Al2O3 supported Cu-Ni catalyst. Seemala et al.[33,34] have also reported recently the application of Cu-Ni bimetallic catalysts for HDO of furfural. Cu-Ni bimetallic catalysts are also known for several other catalytic applications, like selective hydrogenation of aldehydes,[35] ethanol steam reforming,[36,37] water–gas-shift reaction,[38] hydrogen production via steam reforming of glycerol,[39] and hydrochlorination reaction.[40] The Cu-Ni bimetallic catalyst exhibits several advantages over the monometallic catalysts like suppression of unwanted side reactions, coke formation, sintering, deactivation of the catalyst, and so on.[38,39,41]

In the present study, we have explored the promotional effect of CeO2 in the HDO activity of the SiO2 supported Cu-Ni bimetallic catalyst employing vanillin as a model lignocellulosic compound. The reaction has been carried out in aqueous medium, the obvious green solvent, under H2 gas pressure. As is known, the anion vacancy acts as the active site for adsorbing phenol/ aldehyde; hence, the prepared catalyst has been pretreated under a H2 gas atmosphere to ensure higher density of O vacancies on the catalyst surface. Ceria is a well-known catalyst or catalyst promoter in numerous industrial redox processes, three-way catalysis, and so on. The easy interconversion between +4 and +3 oxidation states of cerium makes CeO2 a perfectly suitable candidate for redox catalysts. These facts provoked our interest to take up the present investigation. In this study, the CeO2 promoted and SiO2 supported Cu-Ni bimetallic catalyst (Cu-Ni/CeO2-SiO2) and CeO2 supported Cu-Ni bimetallic (Cu-Ni/CeO2) catalyst were prepared by a deposition–coprecipitation method. SiO2 supported monometallic Cu and Ni catalysts and bimetallic Cu-Ni catalyst were also prepared (Cu/SiO2, Ni/SiO2, and Cu-Ni/SiO2, respectively) for comparison. All the prepared catalysts were thoroughly characterized by different techniques and subjected to hydrodeoxygenation of vanillin.

Results and Discussion

Characterization Studies

The X-ray diffraction patterns of the bimetallic catalysts Cu-Ni/SiO2, Cu-Ni/CeO2, and Cu-Ni/CeO2-SiO2 are furnished in Figure . The NiO, CuO, and CeO2 XRD profiles are also provided for reference purposes. The diffraction pattern of the Cu-Ni/SiO2 sample clearly showed the peaks related to the fcc NiO (111), (200), and (220) planes at 2θ values of 37.2, 43.2, and 62.7°, respectively (JCPDS card no. 47-1049), and the CuO (002), (111), and (022) planes at 2θ values of 35.5, 38.5, and 49.2°, respectively (JCPDS card no. 80-1916). No characteristic peaks for SiO2 were found due to its amorphous nature. The broad nature of concerned peaks indicates good dispersion of NiO and CuO over the amorphous SiO2 support. The Cu-Ni/CeO2 sample, on the other hand, showed only the peaks related to the reflection from the (111), (200), (220), (311), and (222) planes of cubic fluorite CeO2 (JCPDS card no. 81-0792). The peaks pertaining to CuO were not observed. At around 43°, a low intensity peak can be seen due to the NiO (200) plane. Few unidentified low intense peaks are visible in the pattern, which could be indicative of alloy formation. This signified a higher efficiency of the SiO2 support in stabilizing the CuO and NiO phases than CeO2 support. The Cu-Ni/CeO2-SiO2 XRD pattern displayed characteristic peaks pertaining to the reflection from the CeO2 (111), (200), (220), (311), and (222) planes, NiO (111), (200), and (220) planes, and CuO (002), (111), and (022) planes. This indicated existence of well dispersed cubic CeO2, NiO, and CuO phases on the amorphous SiO2 support. No evidence of the alloy formation was apparent.

Figure 1

X-ray diffraction patterns of CeO2 promoted SiO2 supported Cu-Ni bimetallic catalyst (Cu-Ni/CeO2-SiO2), CeO2 supported Cu-Ni bimetallic catalyst (Cu-Ni/CeO2), SiO2 supported Cu-Ni bimetallic catalyst (Cu-Ni/SiO2), pure CeO2, NiO, and CuO.

The Raman spectra of bimetallic catalysts are presented in Figure . The Cu-Ni/SiO2 showed a high intense broad hump between 385 and 587 cm–1 (approx.) with a maximum at around 501 cm–1. Another low intense broader hump appeared in between 955 and 1102 cm–1 with a maximum around 1040 cm–1. The broadness and the asymmetry on both sides of the first peak indicate merging of peaks. According to the literature, the Raman spectrum of cubic NiO shows several bands above 400 cm–1.[42] The first four bands in the lower wavelength region have vibrational origin and correspond to one-phonon (1P) TO and LO modes (at ∼570 cm–1), two-phonon (2P) 2TO modes (at ∼730 cm–1), TO + LO (at ∼906 cm–1), and 2LO (at ∼1090 cm–1) modes. A strong band at ∼1490 cm–1 appears due to two-magnon (2M) scattering. Monoclinic CuO with a C26 space group, on the other hand, exhibits only three Raman active modes (Ag + 2Bg) at around ∼282, 330, and 616 cm–1.[43] By comparing with the literature reports, the first Raman peak between 385 and 587 cm–1 in the present study has been assigned to Bg Raman mode of CuO and one-phonon light scattering of NiO. The hump at the higher wavelength is ascribed to 2LO vibrational mode of NiO. The Cu-Ni/CeO2 catalyst shows only a sharp peak at 440 cm–1. This peak is ascribed to the F2g vibrational mode of fcc CeO2. In accordance with the XRD pattern, the Raman spectrum of Cu-Ni/CeO2 exhibited only a Raman peak related to CeO2 F2g mode. A low intense broad and asymmetric hump can be observed around 550 to 600 cm–1, which can be ascribed to the presence of oxygen vacancies in CeO2. The Cu-Ni/CeO2-SiO2 catalyst shows a very broad hump extending from 356 to 590 cm–1 with two maxima at around 432 and 512 cm–1. The first maximum could be ascribed to the fcc lattice of CeO2, and the second maximum would arise due to the presence of CuO and NiO, as in the Cu-Ni/SiO2 sample, and the oxygen vacancy defects of CeO2. Another low intense broad hump in the spectrum with a maximum around 1051 cm–1 was correlated with the NiO 2LO vibrational mode. Interestingly, the Raman spectra of all the catalysts corroborated very well with their respective XRD patterns.

Figure 2

Raman spectra of CeO2 promoted SiO2 supported Cu-Ni bimetallic catalyst (Cu-Ni/CeO2-SiO2), CeO2 supported Cu-Ni bimetallic catalyst (Cu-Ni/CeO2), and SiO2 supported Cu-Ni bimetallic catalyst (Cu-Ni/SiO2).

The influence of different supports on the surface areas of the materials was investigated by N2-sorption studies, and the corresponding results are shown in Figure . According to the IUPAC classification, all the samples exhibited a type-IV isotherm pattern with obvious hysteresis loops, showing that the materials were essentially mesoporous in nature. However, the hysteresis loops of the samples were distinctly different. It can be seen that the hysteresis loop for Cu-Ni/SiO2 is an H2-type and H1-type for the Cu-Ni/CeO2 and Cu-Ni/CeO2-SiO2 catalysts. According to the literature, H1-type is characteristic of mesoporous materials having uniform nearly cylindrical channels, whereas H2-type is frequently associated with solids consisting of “ink-bottle” shaped pores.[44] Further, the position in terms of relative pressure and size/volume of the N2 hysteresis loop of each sample is also different. It can be clearly seen from the figure that the larger hysteresis loop was observed in the isotherm for Cu-Ni/SiO2 at P/Po of 0.58 to 0.88 while Cu-Ni/CeO2 and Cu-Ni/CeO2-SiO2 have narrow hysteresis loops at almost equal P/Po of 0.71 to 0.95, which is comparable to the BET surface area. In addition, as shown in Figure , the displacement of the hysteresis loops in Cu-Ni/CeO2 and Cu-Ni/CeO2-SiO2 toward the higher P/Po might be an indication of the porosity’s evolution. The specific surface areas of Cu-Ni/SiO2, Cu-Ni/CeO2, and Cu-Ni/CeO2-SiO2 catalysts are estimated by the BET method, and the corresponding values were 209, 79, and 142 m2/g, respectively. The variation in surface areas of the samples can be due to the different interaction of Ni-Cu species with the used supports. Among all the prepared catalysts, the Cu-Ni/SiO2 catalyst exhibited the highest surface area of 209 m2/g. The high surface area of Cu-Ni/SiO2 could be mainly attributed to the use of colloidal silica and the high dispersion of more active Ni-Cu species over its surface.[45] Nonetheless, all our prepared catalysts exhibit satisfactory surface areas with mesoporous nature.

Figure 3

N2 adsorption-desorption isotherms of CeO2 promoted SiO2 supported Cu-Ni bimetallic catalyst (Cu-Ni/CeO2-SiO2), CeO2 supported Cu-Ni bimetallic catalyst (Cu-Ni/CeO2), and SiO2 supported Cu-Ni bimetallic catalyst (Cu-Ni/SiO2).

Figure a,b and Figure S1 represent the TEM, HRTEM, and EDX images of the Cu-Ni/CeO2-SiO2 catalyst. The TEM image showed almost spherical shaped particles. In the HRTEM image, overlapping phases of NiO, CuO, and CeO2 were observed. The appearance of the NiO (200), CuO (002), and CeO2 (111) planes with d-spacing values of 2.09, 0.25, and 0.301 nm, respectively, was visible. The corresponding XRD pattern also showed peaks related to reflection from those planes. The EDX pattern showed the actual presence of all the elements including Cu, Ni, Ce, Si, and O. The surface morphology of the Cu-Ni/CeO2-SiO2, Cu-Ni/SiO2, and Cu-Ni/CeO2 catalysts was studied by FESEM and is presented in Figure and Figures S2 and S3. The mesoporous nature of the samples was revealed from these images. Nearly spherical morphology was observed in line with the TEM analysis. The particles of the Cu-Ni/CeO2-SiO2 catalyst had an average diameter of 25 nm.

Figure 4

(a) TEM and (b) HRTEM of CeO2 promoted SiO2 supported Cu-Ni bimetallic catalyst (Cu-Ni/CeO2-SiO2).

Figure 5

FESEM image of CeO2 promoted SiO2 supported Cu-Ni bimetallic catalyst (Cu-Ni/CeO2-SiO2).

The oxidation state of the active metal site has critical control over its catalytic performance.[46,47] The best performing Cu-Ni/CeO2-SiO2 catalyst was subjected to XPS analysis in the energy window of (a) Cu 2p, (b) Ni 2p, (c) Ce 3d, and (d) O 1s, which are represented in Figure a–d. As shown in Figure a, the Cu 2p3/2 and Cu 2p1/2 peaks for Cu+ appeared at 933.3 and 953.2 eV, respectively, with the splitting energy of 19.9 eV, characteristic of Cu+. The Cu2+ peaks of 2p3/2 and 2p1/2 were located at 935.2 and 955.5 eV, respectively. Hence, the spectrum indicated the presence of both +1 and +2 oxidation states of Cu.[48] The Ni 2p spectrum (Figure b) showed the presence of Ni 2p3/2 and Ni 2p1/2 peaks along with satellite peaks. The Ni 2p3/2 and Ni 2p1/2 peaks were deconvoluted into two peaks each at binding energies of 854.8 and 856.8 eV and 872.2 and 873.7 eV, respectively. The spectrum showed characteristic features of Ni+2.[49]Figure c represents the Ce 3d XPS spectrum. The complex spectrum was assigned to two sets of spin-orbital multiplets, corresponding to the 3d3/2 and 3d5/2, labeled u and v, respectively. The u and v peaks were deconvoluted into 10 peaks. The peaks denoted as v, v″, v‴, u, u″, and u‴ were attributed to the Ce+4 and v′, v0, u′, and u0 to the Ce3+ contribution.[50] The O 1s core level spectra illustrated in Figure d were broad and asymmetric in nature and deconvoluted into two peaks. The peak observed around 530.2 eV was attributed to lattice oxygen (Oα), while the non-lattice oxygen was assigned to the peaks observed at around 531.9 eV, which included surface adsorbed oxygen (Oβ).[51]

Figure 6

Deconvoluted (a) Cu 2p, (b) Ni 2p, (c) Ce 3d, and (d) O 1s core level XPS spectra of CeO2 promoted SiO2 supported Cu-Ni bimetallic catalyst (Cu-Ni/CeO2-SiO2).

Catalytic Activity

The HDO of vanillin in aqueous solution was investigated as a model catalytic reaction for biofuel upgradation. Two main products, that is, 2-methoxy-4-methylphenol (MC) and vanillyl alcohol (VA), were obtained during the HDO of vanillin under mild conditions, as shown in Scheme .

Scheme 1

Hydrodeoxygenation of Vanillin

We have investigated the catalytic performance of monometallic Ni/SiO2 and Cu/SiO2 and bimetallic Cu-Ni/SiO2, Cu-Ni/CeO2, and Cu-Ni/CeO2-SiO2 catalysts for hydrodeoxygenation of vanillin under the optimized reaction conditions of 25 bar H2 pressure and 160 °C temperature. The percentage conversions of vanillin after 12 h of reaction are presented in Figure . The monometallic catalysts, Ni/SiO2 and Cu/SiO2, exhibited much lower conversion compared to the bimetallic catalysts. On the Ni/SiO2 catalyst, facile dissociation of H2 takes place, but preferential coordination with the aromatic ring also happens resulting in poor conversion. On the other hand, Cu is known to selectively interact with the aldehyde group, but exhibits a very low rate of H2 dissociation, resulting in lower conversion over the Cu/SiO2 catalyst. On the supported Cu-Ni bimetallic catalysts, dissociation of molecular H2 readily takes place at the catalytically active Ni sites and spillover of atomic H occurs on the Cu sites.[33,34] Consequently, facile hydrodeoxygenation of vanillin takes place exploiting the inherent catalytic properties of Cu. The influence of varying Cu/Ni (2:1, 1:2, 1:1) ratios in the presence of the CeO2 promoter (20 wt %) was also investigated. The best conversion was achieved with the Cu-Ni/CeO2-SiO2 catalyst with a 1:1 ratio of Cu and Ni. This indicated a better synergism between Cu and Ni when present in equal proportions.

Figure 7

Conversion (%) of vanillin in hydrodeoxygenation reaction over SiO2 supported Ni (Ni/SiO2), SiO2 supported Cu (Cu/SiO2), CeO2 promoted SiO2 supported Cu-Ni bimetallic catalyst (Cu-Ni/CeO2-SiO2), CeO2 supported Cu-Ni bimetallic catalyst (Cu-Ni/CeO2), and SiO2 supported Cu-Ni bimetallic catalyst (Cu-Ni/SiO2).

The conversion of vanillin and selectivity of the products on the best performing Cu-Ni/CeO2-SiO2 catalyst were checked with time and are presented in Figure . Samples were withdrawn every 2 h and analyzed. In the initial 2 h of the reaction, VA was the sole product. After that, the selectivity of MC started increasing gradually with a steady decrease in VA selectivity. In the initial stages of the reaction, vanillin was converted to VA at a moderate rate, and more than 75% conversion was achieved within 8 h. When the reaction time was prolonged up to 12 h, the conversion percentage enhanced to almost 96%. The reusability of the Cu-Ni/CeO2-SiO2 catalyst was verified for 10 consecutive cycles and is presented in Figure S4. During the initial six cycles, a nominal loss of conversion was noticed, which could be due to the loss of catalyst during the recovery process after each cycle. After the sixth cycle, a considerable decrease of the activity was noticed. In order to check the stability of the Cu-Ni/CeO2-SiO2 catalyst on longer duration, the reaction was continued for 25 h at 160 °C and 25 bar H2 gas pressure. Subsequent gas chromatographic (GC) analysis confirmed the formation of a number of over-hydrogenated products with decreased concentrations of VA and MC, as shown in Figure S5. The leaching test was also performed with hot filtration (experimental details provided in page S9 of the Supporting Information, Figure S6), which confirmed the heterogeneous nature of the Cu-Ni/CeO2-SiO2 catalyst. No detectable leaching of the catalyst was found in the AAS analysis.

Figure 8

Vanillin conversion (%) and product selectivity against reaction time in hydrodeoxygenation reaction over CeO2 promoted SiO2 supported Cu-Ni bimetallic (Cu-Ni/CeO2-SiO2) catalyst.

The influence of varying reaction temperature and pressure was also checked over the Cu-Ni/CeO2-SiO2 catalyst and is shown in Figure a,b, respectively. The influence of reaction temperature was investigated within the 100–160 °C temperature range under 25 bar pressure for 12 h. At 100 °C, the conversion was less than 50%. With rising temperature, the conversion started increasing gradually and reached a steady state around 150 °C. At a lower temperature range, the selectivity of VA was much higher than that of MC. Around 115 °C temperature, the selectivities of VA and MC were equal. With a further increase in temperature, the VA selectivity showed a sharp decrease with a gradual rise in the MC selectivity. Beyond 120 °C, over-hydrogenated products started forming. The influence of pressure was studied by varying the H2 pressure from 10 to 40 bar at 150 °C for 12 h. Figure b apparently shows that, with increasing pressure, the conversion gradually increases and reaches a steady state at around 25 bar. The selectivity of MC also gradually increased with pressure and reached a maximum at 25 bar H2 pressure. On further increase of pressure, the selectivity of MC sharply decreased with a concomitant increase in the selectivity of other products.

Figure 9

Vanillin conversion (%) and product selectivity (%) against (a) pressure (bar) and (b) temperature (°C) in the hydrodeoxygenation reaction over CeO2 promoted SiO2 supported Cu-Ni bimetallic (Cu-Ni/CeO2-SiO2) catalyst.

Promotional Role of CeO2

The vanillin conversion and the product selectivity were studied after 12 h of reaction over the bimetallic catalysts, namely, Cu-Ni/CeO2-SiO2, Cu-Ni/SiO2, and Cu-Ni/CeO2, under the optimized conditions (Figure ). The vanillin conversion was higher over the Cu-Ni/CeO2-SiO2 catalyst than that of Cu-Ni/SiO2, despite lower surface area. This was a clear indication of the promotional role of CeO2 in the catalytic HDO of vanillin over the Cu-Ni/CeO2-SiO2 catalyst. In the literature, the CeO2 supported CuO catalyst (CuO/CeO2) has been deeply investigated for several catalytic applications. The CO oxidation bears special importance among them.[52−54] Liu and Flytzanistephanopoulos[55] proposed the formation of Cu+ species in CuO/CeO2 due to the interaction of Cu clusters with CeO2. Jia et al.[56] also demonstrated facile reduction of Cu2+ to Cu+ in the CuO/CeO2 due to the Ce4+/Ce3+ redox process. The Cu2+ in contact with the CeO2 gains electrons from Ce3+ → Ce4+ oxidation giving rise to Cu+. Another recent study[57] related to the catalytic CO2 hydrogenation application of the Cu-Ni/CeO2 nanotube has also confirmed that the Cu metal strongly interacts with CeO2, resulting in the electron transfer between Ce4+ and Cu facilitating the formation of Cu+. The Cu+ enhances the adsorption of CO by forming Cu+-carbonyl species.[58,59] Interestingly, Tan et al.[57] have also demonstrated that the higher oxygen vacancies are formed due to the strong interaction between Cu-Ni alloy and CeO2 and participate in the activation of CO2. The formation of CuO/CeO2 interfacial sites plays a vital role in the creation of such synergistic redox properties.[60] In the present study, the promotional effect of CeO2 can be attributed to similar synergism between the Ce4+/Ce3+ and Cu2+/Cu+ redox couples. Existence of Cu+ was evident from the XPS study. The Ce4+/Ce3+ redox process facilitated the reversible formation of Cu2+ ↔ Cu+, which assisted the adsorption of the reactant molecule through the formyl group and desorption of the product molecule from the catalyst surface. Based on this, a reaction scheme has been proposed as shown in Figure . Moreover, the catalysts were pretreated in H2 flow, which ensured the abundance of O vacancies on the catalyst surface. The O vacant sites might also play an important role in the activation of the carbonyl/hydroxyl groups of vanillin/VA.[21] Interestingly, in the Cu-Ni/CeO2 catalyzed reaction, over-hydrogenation of the product MC was arrested. The use of CeO2 as support provides ample oxygen vacancies, which might stop the over-hydrogenation of the product MC. However, the conversion percentage was much lower than that of the Cu-Ni/CeO2-SiO2 catalyst, which can be reasonably justified from the low surface area of the catalyst.

Figure 10

Conversion (%) of vanillin and product selectivity in the hydrodeoxygenation reaction over CeO2 promoted SiO2 supported Cu-Ni bimetallic catalyst (Cu-Ni/CeO2-SiO2), CeO2 supported Cu-Ni bimetallic catalyst (Cu-Ni/CeO2), and SiO2 supported Cu-Ni bimetallic catalyst (Cu-Ni/SiO2).

Figure 11

Plausible mechanism of the hydrodeoxygenation reaction over CeO2 promoted and SiO2 supported Cu-Ni bimetallic catalyst (Cu-Ni/CeO2-SiO2).

Conclusions

The present study was focused on the promotional effect of CeO2 on the catalytic hydrodeoxygenation of vanillin over the Cu-Ni bimetallic catalyst supported on SiO2. Bimetallic catalysts Cu-Ni/CeO2-SiO2, Cu-Ni/SiO2, and Cu-Ni/CeO2 were prepared, characterized, and tested for their catalytic activity. SiO2 supported monometallic Cu and Ni catalysts were also prepared and evaluated for their catalytic activity. All the bimetallic catalysts exhibited better activity than the monometallic catalysts. The SiO2 supported Cu-Ni bimetallic catalyst showed significantly improved catalytic activity on the use of the CeO2 promoter. The XRD results and Raman spectra of the Cu-Ni/CeO2-SiO2 catalyst revealed the presence of crystalline phases of NiO, CuO, and CeO2 over the amorphous SiO2 support. Meanwhile, in the CeO2 supported Cu-Ni bimetallic catalyst, the Cu and Ni ions were partially incorporated into the CeO2 lattice. Traces of alloy formation were also observed. TEM/HRTEM, EDX, FESEM, and XPS studies were carried out on the best performing Cu-Ni/CeO2-SiO2 catalyst. Surface morphology showed the presence of nearly spherical shaped nanoparticles. The HRTEM image displayed the overlapping phases of CuO, NiO, and CeO2. XPS analysis revealed the presence of +1 and +2 oxidation states of Cu, +3 and +4 oxidation states of Ce, and +2 oxidation state of Ni over the catalyst surface. The Ni+2 served as the active site for dissociative adsorption of H2, and the Cu+ ions served as active centers for hydrogenation of the formyl group. The Ce+4 → Ce+3 processes facilitated the formation of Cu+ species. The reaction pathway followed the initial reduction of the formyl group yielding VA, as can be understood from the sole formation of VA in the first 2 h of the reaction, followed by deoxygenation of the −CH2OH group of VA forming MC. The better catalytic activity of the Cu-Ni/CeO2-SiO2 was accredited to the synergism between the Cu+/Cu+2 and Ce+3/Ce+4 redox couples and the incorporation of oxygen vacancies on the catalyst surface by CeO2. A plausible reaction mechanism has been proposed based on the synergism between the Cu+/Cu2+ and Ce3+/Ce4+ redox cycles. The observations made in the present study indicate a promotional role of CeO2 in the catalytic hydrodeoxygenation of vanillin on the SiO2 supported Cu-Ni bimetallic catalyst. Nevertheless, further investigations should be followed for a deeper understanding of the promoting effect of CeO2.

Experimental Section

Catalyst Preparation

Monometallic catalysts Cu/SiO2 (10 wt % Cu) and Ni/SiO2 (10 wt % Ni) and bimetallic catalysts Cu-Ni/SiO2 (10 wt % each), Cu-Ni/CeO2 (10 wt % each), and Cu-Ni/CeO2-SiO2 (10 wt % each and 20 wt % CeO2, optimized) were prepared by the conventional deposition–coprecipitation method (details provided in the Supporting Information, page S2), calcined at 500 °C, and subsequently treated in the H2 gas flow in the temperature range of 500–550 °C.

Characterization

The prepared materials were characterized employing X-ray diffraction (XRD), Raman spectroscopy, N2 adsorption-desorption, transmission electron microscopy (TEM)/high-resolution transmission electron microscopy (HRTEM), field emission scanning electron microscopy (FESEM), energy dispersive X-ray (EDX), X-ray photoelectron spectroscopy (XPS), and atomic absorption spectroscopy (AAS) techniques. The details of the catalyst characterizations are provided in page S3 of the Supporting Information.

Catalytic Activity

The catalytic HDO of vanillin was performed in a sealed autoclave. In a Teflon lined 50 mL stainless steel autoclave, 20 mg of the catalysts was dispersed in 10 mL of DI water. Subsequently, 228 mg of vanillin was added to it. Then the autoclave was sealed, and the reactor was thoroughly purged with N2, followed by H2. Next, the reactor was pressurized with H2 to 25 bar and heated to 150 °C under magnetic stirring at 1000 rpm. Cold water was used to cool down the autoclave quickly after the reaction. The organic compounds present in the reaction mixture were extracted in ethyl acetate. The aqueous phase was centrifuged to recover the catalyst, which was washed with water and ethanol several times. The reaction solutions were analyzed by a gas chromatograph (Shimadzu 2010) equipped with a flame ionization detector using a capillary column (diameter: 0.25 mm, length: 30 m) in the presence of 1,4-dichlorobenzene as the internal standard. The products were also identified by GC–MS (Shimadzu, GCMS-QP2010S) analysis.