C catalyst for furfural hydrogenation at low temperatures

Molecular Catalysis 480 (2020) 110639

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A highly selective and efficient Pd/Ni/Ni(OH)2/C catalyst for furfural hydrogenation at low temperatures

T

Luna Ruana, Huan Zhanga, Man Zhoub, Lihua Zhua,d,⁎, An Peia, Jiexiang Wangc, Kai Yanga, Chuanqun Zhanga, Suqun Xiaoa, Bing Hui Chend a

College of Chemistry and Chemical Engineering, Faculty of Materials Metallurgy and Chemistry, Jiangxi University of Science and Technology, Ganzhou 341000, Jiangxi, China b School of Pharmaceutical Sciences, Gannan Medical University, Ganzhou 341000, Jiangxi, China c Fine Chemical Industry Research Institute, School of Chemistry, Sun Yat-Sen University, Guangzhou 510275, China d Department of Chemical and Biochemical Engineering, National Engineering Laboratory for Green Productions of Alcohols-Ethers-Esters, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Pd/Ni/Ni(OH)2/C bimetallic nanocatalyst Nanostructure Furfural hydrogenation Furfuryl alcohol Quite low temperatures

Hydrogenation of furfural (FF) produces a train of products such as furfuryl alcohol (FFA), tetrahydrofurfuryl alcohol (THFFA) and 2-methylfuran (2-MF). The Pd/Ni/Ni(OH)2/C nanocatalyst was successfully prepared under mild conditions by hydrazine hydrate reduction and galvanic replacement methods. Pd/Ni/Ni(OH)2/C had much higher conversion of furfural and selectivity toward furfuryl alcohol for the selective hydrogenation of furfural than the monometallic catalysts (eg. Pd/C or Ni/C) due to its unique nanostructure of palladium islandon-Ni/Ni(OH)2 nanoparticles and thus the synergy effect between Pd, Ni and Ni(OH)2 related species. The proposed mechanism of the synergistic effect was also provided. Pd/Ni/Ni(OH)2/C showed high selectivity (90.0% or 92.4%) to furfuryl alcohol at quite low reaction temperatures (5 °C or 10 °C), and had good stability. We used various characterization techniques (XRD, HRTEM, STEM-EDS elemental mapping and line-scanning, XPS, HS-LEIS) to compare the nanostructural differences between the monometallic and bimetallic catalysts as well as to explain the possible reasons for the superior performance of Pd/Ni/Ni(OH)2/C to corresponding monometallic catalysts.

1. Introduction Nowadays, non-renewable resources such as coal, oil and natural gas et al. are decreasing, so it is urgent to develop renewable resources. Biomass, as a renewable resource, can be used to alleviate the energy problems. For example, cellulose is the world's largest biomass that can be converted to biofuels and chemical raw materials (e.g. furfural) through biological or chemical pathways [1,2]. Furfural can be obtained by hydrolysis and dehydration of cellulose [3]. Furfural is an important industrial chemical [4–6]. The selective hydrogenation of furfural is beneficial to the utilization of biomass resources, but it also produces a series of by-products [7–13]. Therefore, improving the selectivity is particularly important in furfural hydrogenation reaction. Different products can be obtained by using the monometallic catalysts (Pd, Pt, Ru, Rh, Ir) with different supports. By comparing the conversion and selectivity of the catalysts of Pd, Cu and

Ni supported on silica in furfural hydrogenation reaction, it can be found that furfural is converted to furan over the Pd/SiO2 catalyst [14]. Luo et al. [15] used Pt/C as catalyst to study the effect of reaction pressure on the selective hydrogenation of furfural, and they found that the main product was furan at low pressure, while with the increase of pressure, the selectivity to FFA and 2-MF increased. By adjusting the structures of the catalyst supports and the ruthenium precursor, good catalytic performance of Ru/RGO (RGO-reduced graphene oxide) could be achieved [16]. Generally, the stability, selectivity and activity of the bimetallic nanocatalysts, because of the synergistic effect between different metals, are much better than those of the corresponding monometallic catalysts. And the bimetallic catalysts are extensively applied in heterogeneous catalysis [17–19]. At present, Pd, Pt, Ru, Rh-based [20–25] and bimetallic catalysts (such as the Cu-Ni bimetallic catalyst) are often reported for furfural hydrogenation reaction [26–30]. And the most

⁎ Corresponding author at: College of Chemistry and Chemical Engineering, Faculty of Materials Metallurgy and Chemistry, Jiangxi University of Science and Technology, Ganzhou 341000, Jiangxi, China. E-mail address: [email protected] (L. Zhu).

https://doi.org/10.1016/j.mcat.2019.110639 Received 25 August 2019; Received in revised form 16 September 2019; Accepted 18 September 2019 2468-8231/ © 2019 Elsevier B.V. All rights reserved.

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widely used bimetallic catalysts in the furfural selective hydrogenation reaction are the noble metal doped with transition metal. For instance, the hydrogenation products of the Pd-based catalysts for this reaction can be controlled by doping with transition metal (Cu, Co, Ni, Fe). The Pd-Cu/C catalyst, which was prepared in Hronec’s Group using the improved electroless copper plating method, contained Pd0 and Cu+, and it facilitated the hydrogenation rearrangement of furfural to cyclopentanone [31]. The MWNT in the Pd-Ni/MWNT catalyst (MWNTmodified multi-walled carbon nanotubes) had strong acidic active sites, which could improve the catalytic selectivity to tetrahydrofurfuryl alcohol [32]. The Pd-Fe/SiO2 catalyst had Pd-Fe nano-alloy, and the electron synergistic effect between Pd and Fe improved its catalytic performance [33]. Besides doping with metal, the performance of the catalyst could also be improved by changing the support [34–36]. The above catalysts showed high conversion and selectivity to certain product, but the reaction temperature was relatively high. In our research, we try to obtain the catalyst with excellent catalytic performance and high selectivity to furfuryl alcohol at relatively low reaction temperature. Herein, we prepared the Pd/Ni/Ni(OH)2/C catalyst by simple chemical reduction and galvanic replacement approaches. This work also investigated the stability, selectivity and activity of the Pd/Ni/Ni (OH)2/C catalyst for furfural hydrogenation at different temperatures. It was found that Pd/Ni/Ni(OH)2/C had high selectivity to furfuryl alcohol, superior activity and stability at quite low reaction temperatures (5 °C or 10 °C). Characterization techniques such as TEM, XRD, HAADFSTEM, HRTEM, XPS and HS-LEIS were used to demonstrate the Pd-onNi/Ni(OH)2 nanostructures in the Pd/Ni/Ni(OH)2/C catalysts.

Fig. 1. XRD patterns of the catalysts: (a) Ni/C, (b) Pd/C and (c) Pd/Ni/Ni (OH)2/C.

as the X-ray source (λ = 0.15406 nm, 40 kV, 30 mA) and with a scanning range of 2θ = 10°–90°. The morphology, particle size and nanostructures of the catalysts can be determined by using transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM). The highangle annular dark field scanning transmission electron microscopy (HAADF-STEM), STEM-EDS elemental mapping and line-scanning analysis further proved the surface morphology, nanostructures and elemental distribution of the catalysts. X-ray photoelectron spectroscopy (XPS) spectra of the samples were obtained by using PHI Quantum 2000 Scanning ESCA Microprobe with monochromatic Al Ka radiation. High-sensitivity low-energy ion scattering (HS-LEIS) tests were conducted to analyze the elemental composition of the catalyst surface. And the surface elements contained in the catalyst were measured by sputtering with 4He+ ions (3 keV kinetic energy) and 20Ne+ ions (5 keV kinetic energy), with a 145° scattering angle.

2. Experimental section 2.1. Materials The reagents (nickel chloride hexahydrate-NiCl2·6H2O, hydrazine hydrate-N2H4·H2O, sodium hydroxide-NaOH, ethanol-C2H5OH, hexaneC6H14, tetraammine dichloropalladium(II) monohydrate(NH4)2PdCl4·H2O, furfural-C5H4O2, furfuryl alcohol-C5H6O2, tetrahydrofurfuryl alcohol-C5H10O2) used in this experiment were provided by Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Carbon black (C, BLACK PEARLS 2000 LOT-1366221) was obtained from Cabot Corporation.

2.4. Catalyzing furfural hydrogenation The catalytic behaviors of the catalysts were evaluated by performing furfural hydrogenation reaction in a fast-opening high pressure reactor (Parr 4848). 0.05 g of the Pd/Ni/Ni(OH)2/C catalyst and 10 mL of furfural ethanol solution (concentration-40 mL furfural in 960 mL ethanol) were mixed and charged into the autoclave. After the reactor was installed, it was purged with H2 for 2 min, and then hydrogen gas was injected until the pressure in the autoclave reached 5.0 MPa. After the reaction was completed, the catalyst was separated from the reaction solution by centrifugation, and then the conversion and selectivity of the catalyst were determined by gas chromatography (Agilent GC 7820A) equipped with flame ionization detector. In this work, FF conversion and the selectivity of the main productFFA were calculated by the following equation:

2.2. Catalyst synthesis Synthesis of the Ni/C and Pd/C samples: Typically, Ni/C was prepared at room temperature via hydrazine hydrate reducing method. The detailed preparation procedures of Ni/C were basically similar to our previous reported literatures [37,38]. Impregnation method was adopted for the preparation of the Pd/C catalyst. Preparation of the Pd/Ni/Ni(OH)2/C nanocatalyst: Pd/Ni/Ni(OH)2/ C was prepared by galvanic replacement method [39–42]. Firstly, a precalculated volume of aqueous (NH4)2PdCl4H2O·H2O solution (concentration-0.033 mol/L) was diluted with deionized water. 0.3 g of Ni/ C was added to the above solution and magnetically stirred for 6 h. Then, the above solution was filtered and washed for several times to obtain a black solid, which was dried at 60 °C for 6 h under vacuum. The content of Pd in Pd/C was 3.0 wt%, and the content of Ni in Ni/C was 6.7 wt%. The catalyst with 2.1 wt% Ni and 3.0 wt% Pd loadings was designated as Pd/Ni/Ni(OH)2/C. The content of Ni and Pd element was determined by inductively coupled plasma atomic mass spectrometry (Agilent ICP-MS 4500-300).

X=n

conversion of FF/n total addition of FF

S = (m

FFA × 96.08)/(m conversion of FF × 98.04)

The turnover frequency (TOF), which can be used to indicate catalytic performance, represents the amount of active sites required to convert each mole of furfural per unit time. TOF was calculated as: TOF = n

conversion of FF/(n total palladium

× t × D)

where n conversion of furfural is the moles of consumed furfural, n total palladium, t and D represent the total moles of palladium, reaction time and dispersion, respectively. The dispersion can be determined by the following equation [43,44]:

2.3. Characterization The X-ray diffraction (XRD) patterns of the synthesized catalysts were obtained from Rigaku Ultima IV X-ray diffractometer with Cu Kα 2

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Fig. 2. (a) TEM image and (b) Pd/Ni/Ni(OH)2 size distribution of Pd/Ni/Ni(OH)2/C.

Fig. 3. (a, b) HRTEM images and (c) selected area electron diffraction patterns of the Pd/Ni/Ni(OH)2/C catalyst.

3. Results and discussions

D = (6n s M) / (ρNdp ) 6 × the number of metal atoms per unit area × mass of metal atom = metal density × Avogado constant × average size of the metal particles

−2

3.1. Catalyst characterization results The crystallite phase of the catalyst can be detected by XRD. The XRD patterns of the Ni/C, Pd/C and Pd/Ni/Ni(OH)2/C samples are shown in Fig. 1. In Fig. 1a, the diffraction peaks at ∼19.2°, 33.1°, 38.1°,

where ns = 9.9 × 1018 m , M = 106.42 g/mol, ρ = 12.023 g/cm , N = 6.023 × 1023/mol, dp = 7.83 × 10−9 m. 3

3

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Fig. 4. SEM images of the catalysts: (a) Pd/C, (c) Pd/Ni/Ni(OH)2/C; SEM-EDS spectra of the catalysts: (b) Pd/C, (d) Pd/Ni/Ni(OH)2/C.

To further confirm the existence of the metal elements in the particles in the catalysts, SEM-EDS analysis was performed on the Pd/C and Pd/Ni/Ni(OH)2/C catalysts. In the EDS spectra of Pd/C, Pd element exists. It proves that Pd is highly dispersed in the Pd/C catalyst. As it can be seen from Fig. 4d, Pd and Ni elements co-exist in the Pd/Ni/Ni (OH)2/C catalyst. After Pd replaces Ni via galvanic replacement reaction method, Ni is still present and highly dispersed in the catalyst. In order to intuitively observe the surface element distribution of the catalyst, HAADF-STEM test was performed on Pd/Ni/Ni(OH)2/C. A small amount of brighter particles can be attributed to the metal-containing nanoparticles on carbon without agglomeration (Fig. 5a), which coincides with the TEM results. EDS spectrum and elemental mapping analysis indicated that the Pd element was well supported on the support in the Pd/Ni/Ni(OH)2/C catalyst. In Fig. 5b–d, the presence of Pd and Ni indicates that both Pd and Ni are supported on carbon. And Ni is uniformly loaded on carbon with small particles. Moreover, it is very obviously observed that nickel and palladium co-exist in the same nanoparticle, but can not overlap each other. For the sake of visually understanding the nanostructure and metal loading of the catalyst, STEM-EDS elemental line-scanning test was performed. The small amount of brighter particles in Fig. 6a is possibly metallic nanoparticles supported on carbon, and no obvious agglomeration can be seen in the sample. Fig. 6b shows the STEM-EDS elemental line-scanning of a single Pd/Ni/Ni(OH)2 nanoparticle along the red arrow in Fig. 6a. It is easy to find that the elemental intensity of Pd exceeds that of Ni. More importantly, the highest intensity region of Pd can not overlap that of Ni in the nanoparticle, indicating that Pd-on-Ni/ Ni(OH)2 nanostructure formed in Pd/Ni/Ni(OH)2/C instead of PdNi alloy.

52.1°, 59.1° and 62.7° are indexed to Ni(OH)2(001), Ni(OH)2(100), Ni (OH)2(101), Ni(OH)2(102), Ni(OH)2(110) and Ni(OH)2(111) planes (JCPDS card No. 04-0117), respectively [45]. In addition, a strong peak can be observed at 2θ = 44.4°, attributed to Ni(111) plane [46] (JCPDS card No. 04-0850). The diffraction peaks of Pd crystalline phase can not be found in Fig. 1b, which indicated that Pd was highly dispersed on carbon with relatively small size, not within the XRD detection range [47] (Fig. 1b). The diffraction peaks at ∼40.1° and 46.7° corresponded to Pd(111) and Pd(200) planes [48], respectively (JCPDS card No. 652867). Since palladium atoms substituted nickel atoms and nickel was highly dispersed, no diffraction peak of Ni crystalline phase is observed in Fig. 1c. In addition, the reduction process promoted the dispersion of Ni(OH)2 into tiny nanoclusters (less than 1 nm). Ni(OH)2, which interacts with Pd on the outer surface of the catalyst, improves the catalytic performance of the catalyst. Fig. 2 shows the TEM image of the Pd/Ni/Ni(OH)2/C catalyst and its particle size distribution. The Pd/Ni/Ni(OH)2 particles are uniformly distributed, with an average size of about 7.83 nm and relatively narrow size range. From the HRTEM micrograph of the Pd/Ni/Ni (OH)2/C catalyst, the lattice spacing observed at 0.224 nm is the same as that of the Pd(111) planes (Fig. 3a, b). Fig. 3c displays the selected area electron diffraction patterns of the Pd/Ni/Ni(OH)2/C catalyst, where Pd(111) planes (0.224 nm), Pd(200) planes (0.194 nm), Pd(220) planes (0.137 nm), Pd(311) planes (0.117 nm) and NiOH(101) planes (0.230 nm) were found. It proves that Pd and Ni(OH)2 particles co-exist in the catalyst, which is coincident with the results of XRD. Combined with the above characterization results, a conclusion can be drawn that palladium is probably present on the surface with tiny islands supported on nickel/nickel hydroxide nanoparticles (Pd-on-Ni/Ni(OH)2).

4

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Fig. 5. (a) HAADF-STEM image, (b) EDS spectrum and STEM-EDS elemental mapping of the selected region in the red square of Pd/Ni/Ni(OH)2/C, (c) Ni (red) and (d) Pd (yellow). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 6. (a) HAADF-STEM image of Pd/Ni/Ni(OH)2/C and (b) STEM-EDS elemental line-scanning of a single Pd/Ni/Ni(OH)2 nanoparticle along a red arrow. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Fig. 7. Pd 3d XPS spectra of the samples: (b) Pd/C and (d) Pd/Ni/Ni(OH)2/C; Ni 2p XPS spectra of the catalysts: (a) Ni/C and (c) Pd/Ni/Ni(OH)2/C.

Fig. 8. (A) 3 keV 4He+ and (B) 5 keV

20

Ne+ HS-LEIS spectra of the samples: (a) Ni/C, (b) Pd/C and (c) Pd/Ni/Ni(OH)2/C.

Table 1 Catalytic performance of the catalysts for furfural selective hydrogenation.a Entry

1 2 3 4 5 6 7 8 9 a b c d

Catalyst (0.05 g)

Ni/C Pd/C Pd/Ni/Ni(OH)2/C Pd/Ni/Ni(OH)2/C Pd/Ni/Ni(OH)2/C Pd/Ni/Ni(OH)2/C Pd/Ni/Ni(OH)2/C Pd/Ni/Ni(OH)2/C Pd/Ni/Ni(OH)2/C

tb (h)

1 1 1 1 1 1 1 1 2

Tc (°C)

/ 100 100 60 40 25 10 5 10

TOFd (h−1)

Conversion (%)

0 1788 1830 1830 1830 1830 1494 1338 936

0 97.6 100 100 100 100 81.5 73.1 100

Selectivity (%) FFA

THFFA

2-MF

others

0 0 10.00 27.2 32.6 40.6 92.4 90.0 90.7

0 10.8 34.1 28.5 25.6 22.3 2.4 3.8 1.9

0 3.4 1.0 0.6 0.4 0.4 0 0 0

0 85.8 54.9 43.7 41.4 36.7 5.2 6.2 7.4

Reaction conditions: 10 mL ethanol furfural solution (concentration: 40 mL furfural +960 mL ethanol), reaction pressure (5.0 MPa). Reaction time. Reaction temperature. Turnover frequency. 6

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Fig. 9. Possible reaction model of furfural on the catalyst surface.

Fig. 10. Recycle experiments of the Pd/Ni/Ni(OH)2/C catalyst for furfural selective hydrogenation. Reaction conditions: 10 mL ethanol furfural solution (concentration: 40 mL furfural in 960 mL ethanol), Pd/Ni/Ni(OH)2/C (0.05 g), reaction temperature (10 °C), reaction time (2 h), reaction pressure (5.0 MPa).

XPS characterization of the Ni/C, Pd/C monometallic and Pd/Ni/Ni (OH)2/C bimetallic nanocatalysts was carried out to analyze the elemental ratio, different atomic chemical states and related species composition on the surface of different catalysts. As shown in Fig. 7a, c, there are four binding energies in Ni 2p3/2 of 852.7 eV, 853.9 eV, 855.4 eV, and 857.1 eV, corresponding to Ni(0), NiO, Ni(OH)2, and NiOOH specie, respectively [49,50]. By calculating the peak area, we can find that Ni element is mostly present with the oxidation state (NiO, NiOOH, Ni(OH)2) on the surface of the catalyst. In Fig. 7b, d, the XPS spectrum of Pd 3d5/2 line is mainly divided into two peaks, attributed to Pdn+ (337.5 eV) and Pd(0) (335.6 eV) species, respectively [51,52]. By comparing the two peaks area, we can know that the amount of Pd(0) on the Pd/Ni/Ni(OH)2/C catalyst surface is much smaller than the amount of Pdn+. It can be thus inferred that Pd in the Pd/Ni/Ni(OH)2/C catalyst exists mainly in the form of oxidation state, and only a small amount of zero-valent Pd exists. However, in Fig. 7b, the peak area of Pd(0) is much larger than that of Pdn+, implying that Pd is mostly present in the Pd(0) state in the Pd/C catalyst. The probable reason for the big difference in the surface Pd species is that the particles with Pd related species are with higher dispersion in the Pd/Ni/Ni(OH)2/C bimetallic catalyst than in the Pd/C monometallic catalyst, making it more easily oxidized in air. The excellent performance of the Pd/Ni/Ni (OH)2/C catalyst for furfural hydrogenation could be attributed to the synergistic effect between Pd, Ni and Ni(OH)2, and the high dispersion of palladium. The presence of Ni(OH)2 and NiO would also enhance the selectivity to furfuryl alcohol. The Pd and Ni in various valence states are relatively stable in the catalyst under mild reaction conditions (5 °C or 10 °C), so the Pd/Ni/Ni(OH)2/C catalyst has high stability in the reaction, which is very consistent with the following recycle experimental data. HS-LEIS can be used to determine the elemental composition on the outer surface of the catalyst. Fig. 8 shows the 3 keV 4He+ and 5 keV 20 Ne+ HS-LEIS spectra of the catalysts (Pd/C, Ni/C, Pd/Ni/Ni(OH)2/C). It is very obvious that both the Ni/C and Pd/C catalysts contain carbon and oxygen atoms but different metal atoms (Ni or Pd). The Ni/C catalyst contains Ni metal atom, while the Pd/C catalyst contains Pd metal atom. Besides carbon and oxygen atoms, palladium and nickel atoms are also present on the outer surface of Pd/Ni/Ni(OH)2/C. And the amount of Pd atoms on the outmost surface of Pd/Ni/Ni(OH)2/C is slightly larger than that of Ni atoms due to most nickel atoms being

occupied by Pd atoms. 3.2. Catalytic performance of the catalysts Table 1 displays the catalytic behaviors of the monometallic (Ni/C, Pd/C) and bimetallic catalysts (Pd/Ni/Ni(OH)2/C) for furfural selective hydrogenation. In this reaction, the stability, selectivity and activity of the Pd/Ni/Ni(OH)2/C catalyst at different reaction temperatures were also investigated. The Ni/C catalyst had no catalytic activity in furfural hydrogenation reaction, even though at high temperature (Table 1, entry 1). Under the action of the Pd/C catalyst, almost no furfuryl alcohol was produced at 100 °C. The Pd/C catalyst provided 10.8% selectivity to tetrahydrofurfuryl alcohol and 3.4% selectivity to 2-methylfuran with TOF of 1788 h−1 (Table 1, entry 2). This shows that the Pd/C catalyst can promote the catalytic hydrogenation of furfural. It was obvious that the Pd/Ni/Ni(OH)2/C catalyst gave 100% conversion of furfural at the reaction temperature from room temperature (∼25 °C) to 100 °C, and its TOF was 1830 h−1 (Table 1, entry 3). When the reaction temperature increased (from 10 °C to 100 °C), the selectivity and yield to furfuryl alcohol gradually decreased (selectivity from 92.4% to 10%) (Table 1, entries 3–7), but the selectivity and yield to the product-tetrahydrofurfuryl alcohol gradually increased (selectivity from 3.8% to 34.1%). At high reaction temperatures, the furfuryl alcohol was probably hydrogenated to form tetrahydrofurfuryl alcohol. Under mild reaction conditions (10 °C, 5.0 MPa, 2 h), the conversion of furfural was 100% (TOF = 936 h−1), and the selectivity toward furfuryl alcohol was as high as 90.7%. However, the conversion of furfural and the selectivity to furfuryl alcohol decreased with the reaction temperature declining to 5 °C, which may be due to the fact that it is not easy to adsorb hydrogen on the surface of the catalyst at very low temperature. But the furfuryl alcohol was further hydrogenated to tetrahydrofurfuryl alcohol over Pd/Ni/Ni(OH)2/C. The selectivity and yield to tetrahydrofurfuryl alcohol was both 34.1% at the reaction temperature of 100 °C. Obviously, the performance of the Pd/ Ni/Ni(OH)2/C catalyst in the selective hydrogenation of furfural to furfuryl alcohol was significantly better than the monometallic catalysts (Pd/C, Ni/C). It was probably due to the synergistic effect among Pd, Ni

Table 2 Literature reports on the catalytic hydrogenation of furfural. Ref.

Catalyst

Solvent

Ta (°C)

Pb (MPa)

TOFc (h−1)

Conversion (%)

FFA Selectivity (%)

[51] [52] [53]

Fe-L1/C-800 Pt/BC Pd/C

2-butanol toluene water

160 210 40

/ 10 3

/ 432 /

91.6 60.8 85.4

83.0 79.2 76.1

a b c

Reaction temperature. Reaction pressure. Turnover frequency. 7

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and Ni(OH)2 related species, and the unique nanostructure of Pd-on-Ni/ Ni(OH)2 in the Pd/Ni/Ni(OH)2/C catalyst. Fig. 9 shows the synergistic effect which is present in the catalyst for furfural selective hydrogenation. Because the size of palladium islands is too small and thanks to the outstanding capacity of activating hydrogen, hydrogen is preferentially adsorbed and activated at the Pd active sites not Ni or Ni (OH)2, forming activated hydrogen species (H*). Furfural is activated by the electrophilic adsorption between carbonyl group and Ni(OH)2. The activated H* species are transferred to the Ni(OH)2 surface via the spillover effect of Ni sites, reacting with the activated furfural to produce furfuryl alcohol [53–57]. The conversion of furfural (< 92%) and selectivity to furfuryl alcohol (< 90%) of various catalysts under different reaction conditions are given in Table 2. It can be found that the Pd/Ni/Ni(OH)2/C catalyst shows relatively good activity, selectivity and stability under mild reaction conditions compared to other catalysts reported in the literatures [58–60].

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3.3. Stability of the Pd/Ni/Ni(OH)2/C catalyst for furfural selective hydrogenation The reusability of the Pd/Ni/Ni(OH)2/C catalyst was tested in the hydrogenation of furfural under 5.0 MPa of H2 at 10 °C for 2 h. The catalytic hydrogenation performance of the catalyst re-used for five times was shown in Fig. 10. TOF of the Pd/Ni/Ni(OH)2/C catalyst for five hydrogenations reaction was maintained at 1830 h−1. The conversion of furfural remained unchanged. After it was re-used for five times, the yield to furfuryl alcohol of the Pd/Ni/Ni(OH)2/C catalyst slightly changed, maybe due to the loss of catalyst during separation process, which can be ignored. Therefore, the Pd/Ni/Ni(OH)2/C catalyst was relatively stable in hydrogenation reaction and had huge potential in industrial application. 4. Conclusions In this work, Pd/Ni/Ni(OH)2/C was prepared by simple chemical reduction and galvanic replacement approaches. Due to the nano-synergy effect between Pd, Ni and Ni(OH)2 related species, novel nanostructure of Pd-on-Ni/Ni(OH)2 and higher Pd dispersion in the Pd/ Ni/Ni(OH)2/C catalyst, the catalytic performance of the bimetallic catalyst (Pd/Ni/Ni(OH)2/C) was superior to that of the monometallic catalysts (Ni/C, Pd/C). By comparing the stability, selectivity and activity of the Pd/Ni/Ni(OH)2/C catalyst for furfural selective hydrogenation at different temperatures, it was found to have 100% conversion to furfural and high selectivity (90.7%) toward furfuryl alcohol under quite mild reaction conditions (10 °C, 2 h, 5.0 MPa). And the catalytic performance of the Pd/Ni/Ni(OH)2/C catalyst did not decrease significantly after being reused. Acknowledgments The funding supports of this research by the National Natural Science Foundation of China (Grant No. 21763011), Natural Science Foundation of Jiangxi Province for distinguished young scholars (Grant No. 20192BCB23015), Natural Science Foundation of Jiangxi Province of China (Grant No. 20171ACB21041), China Postdoctoral Science Foundation (Grant No. 2018M642597), Postdoctoral Science Foundation of Jiangxi Province of China, Program of Qingjiang Excellent Young Talents, Jiangxi University of Science and Technology (JXUSTQJYX2017006) are gratefully acknowledged. References [1] A.J. Ragauskas, C.K. Williams, B.H. Davison, G. Britovsek, J. Cairney, C.A. Eckert, W.J. Frederick, J.P. Hallett, D.J. Leak, C.L. Liotta, J.R. Mielenz, R. Murphy, R. Templer, T. Tschaplinski, The path forward for biofuels and biomaterials, Science 311 (2006) 484–489.

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