# Development of bimetallic Ni-Cu/SiO2 catalysts for liquid phase selective hydrogenation of furfural to furfuryl alcohol

Pacharaporn Weerachawanasak, Pacharaporn Krawmanee, Weerachat Inkamhaeng, Francisco José Cadete Santos Aires, Tawan Sooknoi, Joongjai Panpranot

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Pacharaporn Weerachawanasak, Pacharaporn Krawmanee, Weerachat Inkamhaeng, Francisco José Cadete Santos Aires, Tawan Sooknoi, et al.. Development of bimetallic Ni-Cu/SiO2 catalysts for liquid phase selective hydrogenation of furfural to furfuryl alcohol. Catalysis Communications, 2021, 149, pp.106221. 10.1016/j.catcom.2020.106221. hal-03438819

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# Development of bimetallic Ni-Cu/SiO₂ catalysts for liquid phase selective hydrogenation of furfural to furfuryl alcohol

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Patcharaporn Weerachawanasak<sup>a,b,*</sup>, Pacharaporn Krawmanee<sup>a</sup>, Weerachat Inkamhaeng<sup>a</sup>, Francisco J. Cadete Santos Aires<sup>c,d</sup>, Tawan Sooknoi<sup>a,b</sup>, Joongjai Panpranot<sup>e</sup>

## ARTICLE INFO

### Keywords:

Furfural

Furfuryl alcohol

Selective hydrogenation

Bimetallic catalyst

## ABSTRACT

Bimetallic Ni-Cu/SiO₂ catalysts with different Cu loading (2–5 wt%) were developed for liquid phase selective hydrogenation of furfural to furfuryl alcohol. Among these, bimetallic 2%Ni-X%Cu/SiO₂ (X = 2, 5) catalysts exhibited better catalytic performances than monometallic 2%Ni/SiO₂ and 2%Cu/SiO₂. Moreover, the bimetallic 2%Ni-5%Cu/SiO₂ catalyst showed the best catalytic performance with 94% of furfural conversion and 64% of furfuryl alcohol selectivity. The synergetic effect of NiCu alloy particles that are present on bimetallic Ni-Cu/SiO₂ catalysts change the adsorption configuration of furfural on the catalyst surface resulting in high catalytic performance in liquid phase selective hydrogenation of furfural to furfuryl alcohol.

## 1. Introduction

Furfural is a promising biomass-derived platform molecule that is a key intermediate for a wide variety of chemicals [1–3]. Since the presence of carbonyl group and conjugated carbon double bonds in furfural structure provides high versatility for synthesis of other high valuable compounds by means of a large variety of reactions such as decarbonylating, hydrogenolysis, ring opening, and hydrogenation [2,4]. Among these reactions, the selective hydrogenation of furfural to furfuryl alcohol is the most popular application. Furfuryl alcohol is an intermediate for production of polymers, fibers, rubber, resin, lubricants, farm chemicals, vitamin C, lysine, and the commercially fine chemicals [1,5–7]. However, hydrogenation of furfural in the presence of H₂ and metallic catalysts can lead to complex reactions (Fig. S.1) [8]. Many undesired products can be produced via additional side reactions if suitable catalysts are not used. The commercial catalysts for this reaction are copper chromite catalysts [9,10]. Since the toxicity from the latter due to chromium leaching was evidenced, environmentally benign catalysts with high catalytic performance have been continuously

developed. Noble metal catalysts such as Pd/SiO₂ [11], Pt/SiO₂ [8,12], Pt/TiO₂ [12] and Ru/C [13] have been reported to be active and selective in the liquid phase hydrogenation of furfural to furfuryl alcohol. However, using these catalysts tend to render high cost of furfuryl alcohol production. Therefore, non-noble metal catalysts, especially Ni and Cu, are good candidates for low-cost production. Some Ni and Cu catalysts have been employed in furfural hydrogenation such as Ni/CNT [3], Ni-Al₂O₃ [4], Ni/Hydrotalcite [14], Cu/SiO₂ [15], Cu/LDH [16]. Ni-based catalysts usually favour hydrogenation of furfural whereas Cu-based catalysts often show low catalytic reaction rate with moderate to high selectivity of furfuryl alcohol. To improve the activity and selectivity of furfural hydrogenation, bimetallic Ni—Cu catalyst with high metal dispersion is an alternative route. Therefore, this work aims to synthesize efficient bimetallic Ni-Cu/SiO₂ catalysts for liquid phase selective hydrogenation of furfural to furfuryl alcohol by strong electrostatic adsorption combined with impregnation method. The characteristics and catalytic properties of bimetallic Ni-Cu/SiO₂ catalysts have been investigated and comparatively studied with the monometallic Ni/SiO₂ and Cu/SiO₂ catalysts.

<footnote><sup>a</sup> Department of Chemistry, Faculty of Science, King Mongkut's Institute of Technology Ladkrabang, Chalongkrung Road, Ladkrabang, Bangkok 10520, Thailand</footnote>

<footnote><sup>b</sup> Catalytic Chemistry Research Unit, Faculty of Science, King Mongkut's Institute of Technology Ladkrabang, Chalongkrung Road, Ladkrabang, Bangkok 10520, Thailand</footnote>

<footnote><sup>c</sup> Université de Lyon, Université Claude Bernard Lyon 1, CNRS - UMR 5256, IRCELYON 2, Avenue Albert Einstein, 69626 Villeurbanne Cedex, France</footnote>

<footnote><sup>d</sup> Laboratory for Catalytic Research, National Research Tomsk State University, 36 Lenin Avenue, Tomsk 634050, Russian Federation</footnote>

<footnote><sup>e</sup> Center of Excellence on Catalysis and Catalytic Reaction Engineering, Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok 10330, Thailand</footnote>

<footnote>* Corresponding author at: Department of Chemistry, Faculty of Science, King Mongkut's Institute of Technology Ladkrabang, Chalongkrung Road, Ladkrabang, Bangkok 10520, Thailand.</footnote>

<footnote>E-mail address: patcharaporn.we@kmitl.ac.th (P. Weerachawanasak).</footnote>
## 2. Experimental

### 2.1. Catalysts preparation

#### 2.1.1. Preparation of monometallic 2%Ni/SiO₂ catalyst by strong electrostatic adsorption method

The 2 wt% of Ni/SiO₂ catalyst was prepared by strong electrostatic adsorption method. Typically, 1400 ppm of hexamine nickel (II) chloride solution was prepared in volumetric flask 250 mL. The pH of the solution was immediately adjusted to 12 by adding 30% NH₄OH. Afterwards, 100 mL of solution was pipetted and mixed with 0.5 g of SiO₂ support in the Erlenmeyer flask. The slurry solution was continuously shaken to adsorb Ni complex on the SiO₂ support for 1 h. The solution was filtrated by vacuum filtration and the resulting powder was washed with 30 mL of mother liquor. The resulting powder or Ni/SiO₂ catalyst was dried in the oven at 110 °C overnight and then calcined in air (60 mL/min) at 450 °C with a heating rate of 2 °C/min for 3 h. The catalyst was denoted as 2%Ni/SiO₂ catalyst.

#### 2.1.2. Preparation of monometallic 2%Cu/SiO₂ catalyst by impregnation method

The 2 wt% of Cu/SiO₂ catalyst was prepared by impregnation method. Approximately, 0.2328 g of copper (II) nitrate trihydrate was dissolved in 20 mL of deionized water. The aqueous solution of copper (II) nitrate trihydrate was slowly impregnated onto 3.00 g of SiO₂ support. The impregnated catalyst was left to stand at room temperature for 6 h. To assure an adequate distribution of the metal complex. After that, the catalyst was dried in the oven at 110 °C overnight and then calcined in air at 450 °C with a heating rate of 10 °C/min for 3 h. The catalyst was denoted as 2%Cu/SiO₂ catalyst.

#### 2.1.3. Preparation of bimetallic Ni-Cu/SiO₂ catalysts by strong electrostatic adsorption combined with impregnation method

Briefly, monometallic 2%Ni/SiO₂ catalyst was firstly prepared by strong electrostatic adsorption method. Then the desired amount of copper (II) nitrate trihydrate solution at 2 and 5 wt% was impregnated on 2%Ni/SiO₂ catalysts. The resulting powder was left to stand at room temperature for 6 h. The catalysts were dried in the oven at 110 °C overnight and then calcined in air at 450 °C with a heating rate of 10 °C/min for 3 h. The catalysts were denoted as 2%Ni-2%Cu/SiO₂ and 2%Ni-5%Cu/SiO₂ catalysts.

### 2.2. Catalysts characterization

Structural information was obtained by X-ray diffraction (XRD); the XRD patterns of the catalysts were measured by an X-ray diffractometer (SIEMENS XRD D5000) in the range of 2θ from 20° to 80° using Cu Kα radiation with a Ni filter at 40 kV and 20 mA. The temperature programmed reduction (H₂-TPR) was performed to evaluate the reduction temperature of catalyst. 100 mg of catalyst was placed into a quartz tube reactor, which was inside a temperature-regulated furnace. Prior to the H₂-TPR, each sample was heated to its activation temperature in air zero (30 mL/min). After that, 10%H₂/Ar with a heating rate of 10 °C/min was applied for TPR analysis. The XPS analysis was performed using a Kratos Axis Ultra DLD spectrometer with a monochromatic Al Kα X-ray source (75–150 W) and analyzer pass energies of 160 eV (for survey scans) or 40 eV (for detail scans). Since it is difficult (and certainly unreliable) to measure the Fermi energy level of the analyzer with our samples and due to probable charge effects often arising with insulator supports as silica, we minimize the imprecision on the BE measurements by taking an internal energy reference either based on the material of support of the catalysts [17,18] or on the adventitious contaminant carbon [19,20]; in this study we have chosen the first solution and reference all the peaks a characteristic peak of the support, the Si 2p of silicon oxide at a BE of 103.5 eV [18]. Prior to the measurements a Shirley background subtraction and peak fitting to theoretical

Gaussian-Lorentzian functions were performed. The morphology and metal crystallite sizes of catalysts were assessed by transmission electron microscopy (TEM) using a JEOL JEM 2010 TEM operated at 200 kV and equipped with a LaB₆ thermoionic electron gun, a high resolution pole-piece (UHR: 0.196 nm point resolution and 0.140 nm information limit) and an energy dispersive X-ray (EDX) spectrometer (Pentafet from Oxford Insts.) capable of assessing the chemical composition on large regions (decondensed probes over several µm²) as well as on small groups of particles (3–5) and individual particles with sizes above 3–4 nm (condensed probes down to 10 nm²) [17–21]. Despite being able to use very small probes we decided to limit ourselves to probes with a minimum of 25 nm in diameter generally over 3–5 nanoparticles. In this way we limit damaging of the nanoparticles while still having enough signal to make a consistent analysis; the use of smaller probes would require far too long exposure to the beam and much higher probability of damaging the nanoparticles. The specific surface areas, pore volumes, and average pore diameters were determined by N₂ physisorption using an Autosorb (Quantachrome) instrument. The surface area was analyzed employing the Brunauer-Emmet-Teller (BET) method. The average pore size was calculated using the BJH desorption method. The chemical composition was performed by X-ray fluorescence spectroscopy (WD-XRF).

### 2.3. Liquid phase selective hydrogenation of furfural to furfuryl alcohol

The liquid phase hydrogenation of furfural was carried out in a 100 mL stainless steel autoclave reactor. Prior to the test, the catalysts were reduced at 450 °C in flowing H₂ gas at 50 mL/min for 2 h; then 0.2 g of catalyst was loaded in an autoclave reactor. Furfural was mixed with methanol in a volumetric flask and then introduced in the reactor. The reactor was purged with N₂ gas (3 bar) three times to remove air. After that, reaction was carried out at 100 °C and 20 bar of H₂ with a stirring speed of 900 rpm using a magnetic stirrer for 2 h. After reaction, the reactor was cooled down to room temperature in an ice bath and then carefully depressurized. The liquid product was separated from the catalyst and analyzed by gas chromatography with flame ionization detector.

## 3. Results and discussion

### 3.1. Catalysts characterization

Structural characterization of catalysts after reduction at 450 °C was performed by XRD (result shown in Fig. S.2). The characteristic peaks of metallic Ni fcc structure (space group 225: *Fm*̇-3*m*; metallic Ni⁰ species) at 2θ = 44.5°, 51.6°, and 76.4° were not observed on 2%Ni/SiO₂ catalyst indicating a high degree of Ni dispersion while the characteristic peaks at 2θ = 43.3°, 50.4°, 74.1°, assigned to Cu fcc structure (space group 225: *Fm*̇-3*m*; metallic Cu⁰ species), was clearly observed on 2%Cu/SiO₂ catalyst [22,23]. In case of bimetallic Ni-Cu/SiO₂ catalysts, peaks at 2θ = 43.6°, 50.7° and 74.5° were found. These peaks significantly shifted to intermediate angles (and thus lattice parameters) between Ni and Cu, indicating the formation of NiCu alloy particles [3,4,24].

The reduction behavior of monometallic 2%Ni/SiO₂, 2%Cu/SiO₂ and bimetallic Ni-Cu/SiO₂ catalysts were evaluated by H₂-TPR which result is shown in Fig. 1. Since SiO₂ is a non-reducible oxide, the present of reduction peaks can be attributed to the reduction process of different Ni and Cu species within the catalysts. For monometallic 2%Ni/SiO₂ catalyst, a main broad hydrogen consumption peak in the range of 280–640 °C with a maximum at 475 °C was observed. This peak was attributed to the reduction of NiO to metallic Ni⁰ species. As for the monometallic 2%Cu/SiO₂, two reduction peaks appeared in the range of 200–360 °C with two center peaks at 250 °C and 320 °C suggesting the existence of several different Cu species [16,25]. The former peak was associated with the reduction of highly disperse CuOₓ nanoparticles, while the latter peak was representative of CuOₓ species having strong interaction with the support. In the case of bimetallic 2%Ni-2%Cu/SiO₂
<figure><img src="image_4.png" /><figcaption>**Fig. 1.** H₂-TPR profile of (a) 2%Ni/SiO₂, (b) 2%Cu/SiO₂, (c) 2%Ni-2%Cu/SiO₂, and (d) 2%Ni-5%Cu/SiO₂ catalysts.</figcaption></figure>

catalyst, two hydrogen consumption peaks were observed. The first peak located in the range of 160–280 °C with a center peak around 200 °C and the second peak located in the range of 400–680 °C with a center peak around 565 °C. The former reduction peak was assigned to the reduction process of Cu²⁺ to Cu⁰ and the latter broad reduction peak was characteristic of the reduction of Ni species [26]. Similarly, for 2%Ni-5%Cu/SiO₂ catalyst two reduction peaks were obtained; the first peak centered around 225 °C and the second peak centered around 565 °C. The first reduction peak of Cu species in both bimetallic Ni-Cu/SiO₂ catalysts obviously migrated to lower reduction temperature when compared with the monometallic 2%Cu/SiO₂ catalyst. On the other hand, the second reduction peak of Ni species migrated to higher reduction temperature when compared with monometallic 2%Ni/SiO₂ catalyst. These phenomena indicate the synergetic interaction of Ni and Cu significantly change in both of bimetallic Ni-Cu/SiO₂ catalysts and thus the formation of NiCu alloy species in our catalysts.

To understand the synergetic interaction between Ni and Cu for bimetallic Ni-Cu/SiO₂ catalysts, the electronic state of Ni and Cu species on the catalysts were investigated by XPS. Prior to analysis, all catalysts were reduced in situ in gaseous H₂ at 450 °C for 1 h and the results are presented in Fig. 2. For Ni 2p₃/₂, two types of Ni species that are metallic Ni⁰ and NiO (located at BE 852.6 and 856.0 eV, respectively) were observed for monometallic 2%Ni/SiO₂ and bimetallic Ni-Cu/SiO₂ catalysts [3,27]. However, the BE of Ni 2p₃/₂ for both of bimetallic Ni-Cu/SiO₂ catalysts were significantly shifted to lower BE at 852.6 eV, which attributed to more metallic Ni⁰ species. Moreover, the BE of Cu 2p₃/₂ was found to be 931.4 for monometallic 2%Cu/SiO₂ and 932.1 eV for both of bimetallic Ni-Cu/SiO₂ catalysts. Both peaks were assigned to metallic Cu⁰ [28]. However, BE of Cu 2p₃/₂ for bimetallic Ni-Cu/SiO₂ catalysts were shifted to higher indicate that the electronic transfers between Cu and Ni. These results were consistence with XRD and H₂-TPR profiles that confirm the formation of NiCu alloy species in both of bimetallic Ni-Cu/SiO₂ catalysts.

Highly dispersed of Ni and Cu with the average particle size of 2.7 nm and 2.9 nm were found on monometallic 2%Ni/SiO₂ and 2%Cu/SiO₂ catalysts, respectively (Fig. S.3); for the latter a few additional large size of Cu particles (100–200 nm) could also be detected. Fig. 3 shows the TEM images illustrating the morphology of bimetallic 2%Ni-2%Cu/SiO₂ and 2%Ni-5%Cu/SiO₂ catalysts and corresponding particle size distributions. In the case of these bimetallic catalysts, broader particle size distribution in the range of 1–20 nm were found with an average particle size of NiCu alloy particles around 5.6 nm and 8.2 nm on 2%Ni-2%Cu/SiO₂ and 2%Ni-5%Cu/SiO₂, respectively. Increasing of Cu loading in bimetallic Ni-Cu/SiO₂ catalysts significantly increase the NiCu alloy

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /><figcaption>**Fig. 2.** XPS spectra of (a) Ni 2p and (b) Cu 2p for monometallic 2%Ni/SiO₂, 2%Cu/SiO₂ and bimetallic 2%Ni-2%Cu/SiO₂, 2%Ni-5%Cu/SiO₂ catalysts.</figcaption></figure>

particle size. EDX measurement of the composition of individual and (mostly) small groups (3–5) of nanoparticles was undertaken to further assess the bimetallic character of the nanoparticles. In Fig. S.4 we present the measured Cu content (Ni content is then the complementary value to 100%) for both bimetallic catalysts (2%Ni-2%Cu/SiO₂, 2%Ni-5%Cu/SiO₂). From the XRF chemical composition measurements we would expect catalyst 2%Ni-2%Cu/SiO₂, which is actually 2.6 wt%Ni and 1.7 wt%Cu, to have a nominal atomic composition of 62 at%Ni and 38 at%Cu; in the same way for catalyst 2%Ni-5%Cu/SiO₂, which is actually 2.6 wt%Ni and 4.8 wt%Cu the nominal atomic composition is expected to be 37 at%Ni and 63 at%Cu. These values are represented by black lines as a reference to the eye in the two corresponding graphs of Fig. S.4. We can clearly see that the values of the EDX measurements correspond to a small spread around slightly higher values than the expected ones: we have indeed 41 ± 8 at% and 65 ± 10 at% for 2%Ni-2%Cu/SiO₂ and for 2%Ni-5%Cu/SiO₂, respectively. The nanoparticles are clearly bimetallic with Ni and Cu in close interaction. These results, at very local level, are consistent with the global level (large scale compared to EDX) XRD and XPS results.

Table 1 summarizes the surface area, pore volume, pore size diameter, and metal compositions of catalysts. The surface area of all catalysts was around 300 m²/g. The pore volume of monometallic 2%Ni/SiO₂ (0.96 cm³/g) is higher than 2%Cu/SiO₂ (0.77 cm³/g) even though the amount of metal loading is similar, indicating that Ni is better
<figure><img src="image_7.png" /><figcaption>Fig. 3. TEM images and the corresponding particle size distribution of bimetallic (a) 2%Ni-2%Cu/SiO₂, and (b) 2%Ni-5%Cu/SiO₂ catalysts.</figcaption></figure>

**Table 1**

Surface area, pore volume, pore size diameter, and metal compositions of monometallic 2%Ni/SiO₂, 2%Cu/SiO₂ and bimetallic Ni-Cu/SiO₂ catalysts.

<table><thead><tr><th rowspan="2">Catalysts</th><th rowspan="2">Surface Area (m²/g)</th><th rowspan="2">Pore Volume (cm³/g)</th><th rowspan="2">Pore size diameter (Å)</th><th colspan="2">Amount of metal <sup>a</sup> (wt%)</th></tr><tr><th>Ni</th><th>Cu</th></tr></thead><tbody><tr><td>2%Ni/SiO<sub>2</sub></td><td>304</td><td>0.96</td><td>123.5</td><td>2.6</td><td>–</td></tr><tr><td>2%Ni-2%Cu/SiO<sub>2</sub></td><td>305</td><td>0.84</td><td>123.5</td><td>2.6</td><td>1.7</td></tr><tr><td>2%Ni-5%Cu/SiO<sub>2</sub></td><td>308</td><td>0.79</td><td>123.6</td><td>2.6</td><td>4.8</td></tr><tr><td>2%Cu/SiO<sub>2</sub></td><td>303</td><td>0.77</td><td>123.5</td><td>–</td><td>1.6</td></tr></tbody></table>

dispersed than Cu. For the bimetallic Ni-Cu/SiO₂ catalysts, increasing of Cu loading significantly decreased the pore volume (0.84 cm³/g for 2% Ni-2%Cu/SiO₂ and 0.79 cm³/g for 2%Ni-5%Cu/SiO₂, respectively). The

pore size diameter of all catalysts did not significantly change, meaning that the loading mainly occurred on the external surface of SiO₂. Furthermore, the elemental compositions of the catalysts, evaluated by XRF, show that the actual Ni loading was slightly higher than expected and around 2.6 wt% for all catalysts, while the Cu loading in each sample is slightly lower than the desired amount.

## 3.2. Catalytic test

Table 2 summarizes the catalytic activities of monometallic 2%Ni/SiO₂, 2%Cu/SiO₂ and bimetallic Ni-Cu/SiO₂ catalysts in the liquid phase selective hydrogenation of furfural to furfuryl alcohol. To understand the catalytic behavior of Ni-Cu/SiO₂, various amounts of furfural in feed (0.604 and 2.415 mmol) were used. With an amount of 0.604 mmol of furfural, conversion of furfural decreases, while the selectivity to furfuryl alcohol increases, in the order: 2%Ni/SiO₂ (conversion 100%; selectivity 35%) > 2%Ni-5%Cu/SiO₂ (conversion 94%, selectivity 64%) > 2%Ni-2%Cu/SiO₂ (conversion 86%, selectivity 65%) > 2%Cu/SiO₂

**Table 2**

Catalytic activities of monometallic 2%Ni/SiO₂, 2%Cu/SiO₂, and bimetallic Ni-Cu/SiO₂ catalysts in the liquid phase selective hydrogenation of furfural to furfuryl alcohol a.

<table><thead><tr><th rowspan="2">Catalysts</th><th rowspan="2">Conversion (%)</th><th rowspan="2">Yield of furfuryl alcohol (%)</th><th colspan="4">Selectivity (%)</th></tr><tr><th>Furfuryl alcohol</th><th>Tetrahydrofurfuryl alcohol</th><th>2-Furaldehyde dimethyl acetal</th><th>other</th></tr></thead><tbody><tr><td colspan="7"><strong>• Furfural 0.604 mmol</strong></td></tr><tr><td>2%Ni/SiO<sub>2</sub></td><td>100</td><td>35</td><td>35</td><td>62</td><td>3</td><td>0</td></tr><tr><td>2%Ni-2%Cu/SiO<sub>2</sub></td><td>86</td><td>56</td><td>65</td><td>22</td><td>10</td><td>3</td></tr><tr><td>2%Ni-5%Cu/SiO<sub>2</sub></td><td>94</td><td>60</td><td>64</td><td>24</td><td>11</td><td>2</td></tr><tr><td>2%Cu/SiO<sub>2</sub></td><td>60</td><td>56</td><td>94</td><td>0</td><td>5</td><td>1</td></tr><tr><td colspan="7"><strong>• Furfural 2.415 mmol</strong></td></tr><tr><td>2%Ni/SiO<sub>2</sub></td><td>32</td><td>25</td><td>78</td><td>10</td><td>3</td><td>9</td></tr><tr><td>2%Ni-2%Cu/SiO<sub>2</sub></td><td>58</td><td>27</td><td>47</td><td>6</td><td>45</td><td>2</td></tr><tr><td>2%Ni-5%Cu/SiO<sub>2</sub></td><td>63</td><td>43</td><td>68</td><td>4</td><td>27</td><td>1</td></tr><tr><td>2%Cu/SiO<sub>2</sub></td><td>14</td><td>7</td><td>47</td><td>0</td><td>53</td><td>0</td></tr></tbody></table>

<footnote>a Determined by the XRF.</footnote>

<footnote>a Conditions: methanol 10 mL, catalyst 0.2 g, H₂ 20 bar, 100 °C, 900 rpm, 2 h.</footnote>
(conversion 60%, selectivity 94%), respectively. Most of the byproducts in this case are, in one hand, tetrahydrofurfuryl alcohol which due to the hydrogenation of furfuryl alcohol, and in the other hand, 2-furaldehyde dimethyl acetal, which is due to the acetalization of furfural and the solvent. Based on these results, both bimetallic Ni-Cu/SiO₂ catalysts exhibit better catalytic performances (combined activity/selectivity) than the monometallic 2%Ni/SiO₂ and 2%Cu/SiO₂ catalysts. The formation of NiCu alloy nanoparticles (evidenced by XRD, EDX and XPS), leading to a synergetic effect between Ni and Cu, suggested by the electronic modification of Ni and Cu observed in XPS, may change the adsorption configuration of furfural on bimetallic Ni-Cu/SiO₂ catalysts. The NiCu alloy nanoparticles can reduce the adsorption of furfural via η²(C, O)-aldehyde configuration, which is typically observed on monometallic Ni. In the other hand, it can enhance the adsorption of furfural via η¹(O)-aldehyde configuration which usually occurs on monometallic Cu sites [29]. Increasing the furfural amount to 2.415 mmol, furfural conversion decrease in the order: 2%Ni-5%Cu/SiO₂ (63%) > 2%Ni-2% Cu/SiO₂ (58%) > 2%Ni/SiO₂ (32%) > 2%Cu/SiO₂ (14%) whereas the selectivity of furfuryl alcohol decrease in the order: 2%Ni/SiO₂ (78%) > 2%Ni-5%Cu/SiO₂ (68%) > 2%Ni-2%Cu/SiO₂ (47%) > 2%Cu/SiO₂ (47%), respectively. In this case, both bimetallic Ni-Cu/SiO₂ catalysts show better catalytic performance (combined activity/selectivity) in liquid phase selective hydrogenation of furfural to furfuryl alcohol than the monometallic catalysts. The highest yield of furfuryl alcohol (60% and 43%) is obtained with 2%Ni-5%Cu/SiO₂ catalyst in both conditions. Increasing the furfural amount in the feed (0.604 to 2.415 mmol) influences the competitive side reactions and ultimately the obtained byproducts; acetalization between furfural and methanol producing 2-furaldehyde dimethyl acetal increases with the amount of furfural in the feed. Therefore, the existence of synergetic effects of NiCu alloy nanoparticles and the use of suitable feed ratio are significantly important to improve the catalytic performance of liquid-phase selective hydrogenation of furfural to furfuryl alcohol.

## 4. Conclusions

Bimetallic Ni-Cu/SiO₂ catalysts were successfully prepared by strong electrostatic adsorption combined with impregnation method. The bimetallic 2%Ni-5%Cu/SiO₂ catalyst exhibits the best catalytic performance in liquid-phase selective hydrogenation of furfural to furfuryl alcohol (94% of furfural conversion, 64% of furfuryl alcohol selectivity) with the highest yield of furfuryl alcohol (60%). The synergetic effect of NiCu alloy nanoparticles in bimetallic Ni-Cu/SiO₂ catalysts is beneficial to change the adsorption configuration of furfural on the catalyst surface and promotes a good catalytic performance.

### CRediT authorship contribution statement

**Patcharaporn Weerachawanasak:** Conceptualization, Methodology, Investigation, Writing - original draft, Writing - review & editing; **Pacharaporn Krawmanee:** Investigation, Visualization; Weerachat Inkamhaeng Investigation, Visualization; **Francisco J. Cadete Santos Aires:** Investigation, Visualization, Writing - review & editing; **Tawan Sooknoi:** Resources, Supervision; **Joongjai Panpranot:** Funding acquisition, Resources, Supervision.

### Declaration of Competing Interest

None.

### Acknowledgments

Financial supports by the Office of the Higher Education Commission and the Thailand Research Fund for Patcharaporn Weerachawanasak (Grant number: MRG6180230) and Faculty of Science, King Mongkut's Institute of Technology Ladkrabang, Thailand (Grant number: 2563-02-

05-24) and the National Research Council of Thailand are gratefully acknowledged.

### Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.catcom.2020.106221.

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