Selective Hydrogenation of Biomass-derived Furfural over Supported Ni 3 Sn 2 Alloy : Role of Supports

A highly active and selective hydrogenation of biomass-derived furfural into furfuryl alcohol was achieved using supported single phase Ni3Sn2 alloy catalysts. Various supports such as active carbon (AC), -Al2O3, Al(OH)3, ZnO, TiO2, ZrO2, MgO, Li-TN, and SiO2 have been employed in order to understand the role of the support on the formation of Ni3Sn2 alloy phase and its catalytic performance. Supported Ni3Sn2 alloy catalysts were synthesised via a simple hydrothermal treatment of the mixture of aqueous solution of nickel chloride hexahydrate and ethanol solution of tin(II) chloride dihydrate in presence of ethylene glycol at 423 K for 24 h followed by H2 treatment at 673 K for 1.5 h, then characterised by using ICP-AES, XRD, H2and N2-adsorption. XRD profiles of samples showed that the Ni3Sn2 alloy phases are readily formed during hydrothermal processes and become clearly observed at 2θ = 43-44o after H2 treatment. The presence of Ni3Sn2 alloy species that dispersed on the supports is believed to play a key role in highly active and selective hydrogenation of biomass-derived furfural towards furfuryl alcohol. Ni3Sn2 on TiO2 and ZnO supports exhibited much lower reaction temperature to achieved >99% yield of furfuryl alcohol product compared with other supports. The effects of loading amount of Ni-Sn, reaction conditions (temperature and time profile) on the activity and selectivity towards the desired product are systematically discussed. Copyright © 2016 BCREC GROUP. All rights reserved


Introduction
Selective hydrogenation of furfural (FFald) to furfuryl alcohol (FFalc) is great industrial interest since it widely use in various applica-tions [1].Industrially, furfuryl alcohol was produced by liquid hydrogenation of furfural at the high temperature and H2 pressure by using copper-chromite (Cu-Cr) catalysts which exhibits moderate in activity and selectivity.The main drawbacks of this catalyst system are toxicity and unrecyclable due to generated Cr2O3 and severe leaching of the metal into product * Corresponding Author.E-mail: rodiansono@unlam.ac.id (R. Rodiansono) [2,3].Therefore, several attempts have been reported in order to replace Cu-Cr catalysts or to develop a new metallic catalyst system which have more efficient catalytic process and less severe of environmental problem.Among developed metal catalysts, nickelbased catalyst with metal co-promotor or modified supports has been studied intensively due to its high activity for hydrogenation both of C=C and C=O.Several metal co-promotors were applied to nickel such as Cu [4,5], Fe, Ce [6,7], and Sn [8] in order to improve its chemoselectivity towards C=O rather than to C=C.In this advantage, system based on Ni modified with Fe, Ce or heteropolyacids have been proved to be successful, reaching 98% selectivity to FFalc at almost total conversion [4][5][6][7][8].However, in some cases these modified nickel catalysts cannot reuse [4] and also showed moderate in activity or selectivity [8].Recently, Merlo et al. reported that tin modified of Pt/SiO2 catalyst showed 96% selectivity to furfuryl alcohol and required 6 h to reach a complete reaction [9].Moreover, the employing of noble metal catalyst is less favorable in economical advantageous.Therefore, the design less expensive the active and selective catalyst system for production furfuryl alcohol is an issue of interest, which still presents great challenges.
We have reported the preparation of Ni-Sn alloy catalysts supported on various inorganic compounds such as Al2O3, aluminium hydroxide (AlOH), active carbon (AC), SiO2, and TiO2.The supported Ni-Sn alloy catalysts were prepared via the hydrothermal treatment of a solution that contained Ni and Sn species at 423 K for 24 h followed by H2 treatment at 573-873 K for 90 min.We found that the catalyst that consisted of Ni-Sn alloy (as Ni3Sn2 species) dispersed on TiO2 allowed a remarkable reduction of the reaction temperature to 383 K. Our previous results have shown that the high conversion of FFald and the high selectivity of FFalc was achieved over Ni-Sn(1.5)/TiO2catalyst that can be attributed to the relatively high dispersion of Ni-Sn alloy on TiO2 giving rise to active sites with a significantly higher catalytic activity [10].The formation of Ni3Sn2 or Ni3Sn alloy phases after H2 treatment of Ni-Sn alloy catalysts supported on aluminium hydroxide are believed to play a key role in the enhancement of the chemoselectivity [11,12].
In the present report, we have extended our study to the investigation of catalytic behaviour of Ni3Sn2 alloy catalysts supported on various inorganic compounds, such as: Al2O3, alumin-ium hydroxide (AlOH), active carbon (AC), ZnO, SiO2, Li-taeniolite (Li-TN), MgO, ZrO2, and TiO2.The supported Ni-Sn alloy catalysts were prepared with similar way to our previous report [10].The catalytic performance of the synthesised catalysts was evaluated selective hydrogenation of biomass-derived furfural (FFald) to furfuryl alcohol (FFalc) and tetrahydrofurfuryl alcohol (THFalc).

Catalyst preparation
A typical procedure of the synthesis of g-Al2O3 supported Ni-Sn (1.5 feeding ratio) alloy catalyst is described as follows [10].NiCl2.6H2O(7.2 mmol) was dissolved in deionised water (denoted as solution A), and SnCl2.2H2O(4.8 mmol) was dissolved in ethanol/2-methoxy ethanol (2:1) (denoted as solution B) at room temperature.Solutions A, B, and -Al2O3 (1 g) were mixed at room temperature; the temperature was subsequently raised to 323 K and the mixture was stirred for 12 h.The pH of the mixture was adjusted to 12 through the dropwise addition of an aqueous solution of NaOH (3.1 M).The mixture was then placed into a sealed-Teflon autoclave for the hydrothermal reaction at 423 K for 24 h.The resulting black precipitate was filtered, washed with distilled water, and then dried under vacuum overnight.Prior to the catalytic reaction, the obtained black powder was treated under hydrogen at 673 K for 90 min.A similar procedure was also applied for the synthesis of supported Ni-Sn(1.5)alloy on SiO2, ZrO2, ZnO, MgO, and Li-TN.

Characterisations
Gas chromatography (GC) analyses were performed on a Shimadzu GC-8A equipped with a flame ionisation detector and with Thermon 3000 and Silicone OV-101 packing.GC-MS was performed on a Shimadzu GC-17B equipped with a thermal conductivity detector and with an RT-βDEXsm capillary column. 1 Hand 13 C NMR spectra were obtained on a JNM-AL400 spectrometer at 400 MHz; samples for NMR were dissolved in chloroform-d1 with TMS as an internal standard.Products were confirmed by the comparison of their GC retention time, mass, 1 H and 13 C NMR spectra with those of authentic samples.XRD measurements were recorded on a Mac Science M18XHF instrument using monochromatic CuKα radiation (λ= 0.15418 nm).The XRD was operated at 40 kV and 200 mA with a step width of 0.02 o and a scan speed of 4 o min -1 (α1 = 0.154057 nm, α2 = 0.154433 nm).ICP measurements were performed on an SPS 1800H plasma spectrometer of Seiko Instruments Inc. (Ni: 221.7162 nm and Sn: 189.898 nm).
The BET surface area (SBET) and pore volume (Vp) were measured using N2 physisorption at 77 K on a Belsorp Max (BEL Japan).The samples were degassed at 473 K for 2 h to remove physisorbed gases prior to the measurement.The amount of nitrogen adsorbed onto the samples was used to calculate the BET surface area via the BET equation.The pore volume was estimated to be the liquid volume of nitrogen at a relative pressure of approximately 0.995 according to the Barrett-Joyner-Halenda (BJH) approach based on desorption data [13].SEM images of the synthesised catalysts were taken on a JEOL JSM-610SEM after the samples were coated using a JEOL JTC-1600 autofine coater.
The H2 uptake was determined through irreversible H2 chemisorption.After the catalyst was heated at 393 K under vacuum for 30 min, it was treated at 673 K under H2 for 30 min.The catalysts were subsequently cooled to room temperature under vacuum for 30 min.The H2 measurement was conducted at 273 K, and H2 uptake was calculated according to the method described in the literature [14]

Typical procedure for hydrogenation of furfural
Catalyst (0.05 g), FFald (1.1 mmol), and iso-PrOH (3 mL) as solvent were placed into a glass reaction tube, which fitted inside a stainless steel reactor.After H2 was introduced into the reactor with an initial H2 pressure of 3.0 MPa at room temperature, the temperature of the reactor was increased to 383 K.After 75 min, the conversion of FFald and the yield of FFalc were determined via GC analysis.The Ni3Sn2/-Al2O3 catalyst was easily separated using either simple centrifugation or filtration.The solvent was removed in vacuo, and the residue was purified via silica-gel column chromatography.

Catalyst characterisation
Ten types of supports (carbon (AC), -Al2O3, Al(OH)3, ZnO, ZrO2, MgO, Li-TN, TiO2, and SiO2) were employed for the preparation of the supported Ni3Sn2 alloy catalysts and the physicochemical properties of the supported Ni3Sn2 alloy catalysts have been reported previously [10].Among of the selected supports, we highlighted the XRD patterns of synthesised supported Ni3Sn2 alloy catalysts on general support (-Al2O3) and strong metal support interaction (SMSI) supports (TiO2) as a function of loading amount of Ni3Sn2.In the case of Ni3Sn2/-Al2O3, the XRD patterns revealed that Ni3Sn2, a major alloy phase was formed on the Al2O3 and the diffraction peak of Ni3Sn2 alloy phase at 2θ=43-44 o become intensified with the increase of the loading amount of Ni-Sn (Figure 1a-d).The crystallite sizes of Ni3Sn2(101) in Ni3Sn2/-Al2O3 increased as the increase of loading amount whereas 6 nm, 9 nm, 14 nm, and 16 nm, respectively.
We also are intentionally evaluated the formation of Ni3Sn2 alloy phase on another supports rather than -Al2O3 and TiO2 as comparison and the XRD patterns of various supported Ni3Sn2 are showed in Figure 3. Unlike on the g-Al2O3 and TiO2 supports, Ni3Sn2(101) alloy phase was formed and overlapped with the diffraction peaks of the supports.In the case of Ni3Sn2/SiO2, the diffraction peak of Ni3Sn2 alloy phases are broadened and hardly observed that indicating a high dispersion of Ni-Sn alloy species on the support (Figure 3a).On ZrO2, ZnO, MgO, and Li-TN supports, the diffraction peaks at 2θ of 43.28 and 44.28 o which characteristic peak for Ni3Sn2(102) and Ni3Sn2(110) alloy
The catalytic reaction results of each the synthesized supported Ni3Sn2 alloy in the hydrogenation of biomass-derived furfural into furfuryl alcohol are summarized in Table 1.For catalysts supported on Al2O3, AlOH, SiO2, and AC, relatively high FFald conversions and yields of FFalc were obtained (Table 1, entries 1-4).For the Ni3Sn2/-Al2O3 catalyst, FFald conversion was 85% with a FFalc yield of 84% (Table 1, entry 1), whereas the Ni3Sn2/AlOH, Ni3Sn2/AC, and Ni3Sn2/SiO2 catalysts produced FFalc yields of 67%, 72%, and 62%, respectively (Table 1, entries 2-4).A remarkably high FFald conversion (>99%) and FFalc selectivity (100%) were obtained when Ni3Sn2/TiO2 was used under the same conditions (Table 1, entry 5).The high conversion of FFald and the high selectivity of FFalc over Ni3Sn2/TiO2 can be attributed to the relatively high dispersion of Ni-Sn alloy on TiO2 giving rise to active sites with a significantly higher catalytic activity.Alternatively, the high conversion and selectivity may be a result of the strong interactions between the active metals and TiO2 generating significant interactions between C=O groups and Ni-TiOx sites and leading to high selectivity to unsaturated alcohols [15].As comparison, Kijenski et al. have reported that Pt catalysts supported on TiO2 gave higher selectivity to FFalc in the hydrogenation of FFald than did Pt supported on SiO2, ZrO2 or MgO [16].Our results also showed that Ni3Sn2 alloy catalysts supported on ZrO2, MgO, and Li-TN only gave 32%, 0%, and 20% yield of FFalc, respectively (Table 1, entries 7, 8, and 9).In the case of Ni3Sn2 supported on ZnO, the conversion of FFald was 89% with selectivity of FFalc and THFalc of 99% and 1%, respectively (entry 6).Recently, Corma et al. studied the chemoselectivity of Ni supported on TiO2 in the hydrogenation of substituted nitro aromatics [17].Moreover, the monometallic R-Ni/AlOH catalyst converted FFald to give >99% THFalc, which indicates that R-Ni/AlOH hydrogenated both C=C and C=O of FFald (entry 10), whereas

Bulletin of Chemical Reaction Engineering & Catalysis, 11(1), 2016, 4
Copyright © 2016, BCREC, ISSN 1978-2993   Sn/AlOH was not active for the hydrogenation of FFald under the same conditions (entry 11).These results suggest that the addition of tin to form Ni-Sn allsoy retards the C=C hydrogenation activity of nickel.Swift et al. have reported that the formation of a Ni-Sn alloy by the addition of tin to a Ni/SiO2 catalyst remarkably changed the reactivity of Ni/SiO2 because of the change in the electron density of nickel metal [18].Delbecq et al. indicated that the C=O hydrogenation selectivity in the of α,β-unsaturated aldehydes could be enhanced by the formation of a Pt-Sn alloy because of the higher affinity of the alloy towards C=O rather than towards C=C bonds, as noted previously [19].Resasco et al. have reported that the selective hydrogenation of C=O versus C=C in α,β-unsaturated aldehydes by Pd-Cu alloy supported on silica was caused by the preferential η 2 -coordination of C=O to Pd [20].In addition, the catalytic reaction over supports (carbon (AC), -Al2O3, Al(OH)3, ZnO, ZrO2, MgO, Li-TN, TiO2, and SiO2) was also carried out under the same reaction condi-     [22].Since the crystallite size or dispersion of Ni-Sn alloy could affect the length of induction period, Ni3Sn2/TiO2 showed a high activity at lower temperature (Figure 4) without an induction period (Figure 5).Notably, the supported Ni3Sn2/TiO2, demonstrated a conversion of FFald 1.5 times greater than that of the Ni3Sn2/-Al2O3 and Ni3Sn2/ZnO catalysts.The effect of loading amount of Ni3Sn2 (1.5-16 mmol g -1 ) on supports of -Al2O3 and TiO2 are also evaluated and the results are shown in Figure 6A and 6B, respectively.On Ni3Sn2/-Al2O3 system, the conversion of FFald gradually increased from loading amount of 1.5 to 6.0 mmol g -1 then decreased at loading amount of 8.0-16.0mmol g -1 .The highest conversion of FFald (74%) was achieved when loading amount of 6.0 mmol g -1 and gave 70% yield of FFalc.On the other hand, the highest conversion of FFald (>99%) was achieved over 6.0 mmol g -1 Ni3Sn2/TiO2 catalyst with yield and selectivity towards FFalc of >99% at the same reaction conditions.These results revealed that supported Ni3Sn2/TiO2 catalysts are superior to the supported Ni3Sn2/-Al2O3 and other that can be attributed due to high dispersion of Ni3Sn2 alloy species as well as the induction period did not occur on the Ni3Sn2/TiO2 catalyst system [10,21,22].
A reusability test was performed on the Ni3Sn2/-Al2O3 and Ni3Sn2/ZnO catalysts, and the results are summarized in Table 2.In the case of reusability test of Ni3Sn2/TiO2 have been reported elsewhere [10].The used Ni3Sn2/-Al2O3 and Ni3Sn2/ZnO catalysts were easily separated by either simple centrifugation or filtration after the reaction.The activity of the catalyst decreased while the high selectivity was maintained for at least six consecutive runs.The amount of Ni and Sn that leached into the reaction solution was 0.38% and 1.2% after four runs, respectively.

Conclusion
Catalytic performance of various supported Ni3Sn2 alloy catalysts have been evaluated in hydrogenation of biomass-derived furfural into furfuryl alcohol.Supported Ni3Sn2 alloy on TiO2 and ZnO showed the higher yield and selectivity towards furfuryl alcohol product compared to the activity of other supports.Supported Ni3Sn2/-Al2O3 was also found to be reusable without any significant loss of selectivity even after six consecutive reactions run.
This work was partially supported by JSPS-DGHE through Joint Bilateral Research Project FY 2014-2017 and DGHE-KLN project f tract.056/UN8.2/PL/2015.RD would like to thank to Husni Wahyu Wijaya for kindly help in XRD measurements of supported of Ni3Sn2/-Al2O3 catalyst series and other supports.

Table 1 .
Comparison of catalytic activity of various supported Ni3Sn2 alloy catalysts for selective hydrogenation of biomass-derived furfural