One-pot Selective Conversion of Biomass-derived Furfural into Cyclopentanone/Cyclopentanol over TiO2 Supported Bimetallic Ni-M (M = Co, Fe) Catalysts

One-pot selective conversion of biomass-derived furfural (FFald) into cyclopentanone (CPO) or cyclopentanol (CPL) using bimetallic nickel-based supported on TiO2 (denoted as Ni-M(3.0)/TiO2; M = Co and Fe; 3.0 is Ni/M molar ratio) have been investigated. Catalysts were synthesized via a hydrothermal method at 150 °C for 24 h, followed by H2 reduction at 450 °C for 1.5 h. X-ray Diffraction (XRD) analysis showed that the formation of Ni-Co alloy phase at 2θ = 44.2° for Ni-Co(3.0)/TiO2 and Ni-Fe alloy at 2θ = 44.1° for Ni-Fe(3.0)/TiO2. The amount of acid sites was measured by using ammoniatemperature programmed desorption (NH3-TPD). Ni-Co(3.0)/TiO2 has three NH3 desorption peaks at 180 °C, 353 °C, and 569 °C with acid site amounts of 1.30 μmol.g-1, 1.0 μmol.g-1, and 2.0 μmol.g-1, respectively. On the other hand, Ni-Fe(3.0)/TiO2 consisted of NH3 desorption peaks at 214 °C and 626 °C with acid site amounts of 3.3 μmol.g-1 and 2.0 μmol.g-1, respectively. Both Ni-Co(3.0)/TiO2 and NiFe(3.0)/TiO2 catalysts were found to be active for the selective hydrogenation of FFald to furfuryl alcohol (FFalc) at low temperature of 110 °C, H2 3.0 MPa, 3 h with FFalc selectivity of 81.1% and 82.9%, respectively. High yields of CPO (27.2%) and CPL (41.0%) were achieved upon Ni-Fe(3.0)/TiO2 when the reaction temperature was increased to 170 °C, 3.0 MPa of H2, and a reaction time of 6 h. The yield of CPO+CPL on the reused catalyst decreased slightly after the second reaction run, but the activity was maintained for at least three consecutive runs. Copyright © 2020 BCREC Group. All rights reserved

Recently, cyclopentanone (CPO) and cyclopentanol (CPL), versatile chemical intermediates that containing five-membered alicyclic rings, can be obtained via combined-step of hydrogenation and rearrangement of biomassderived furfural in aqueous media using carbon supported platinum or palladium-based catalysts [7][8][9]. CPO and CPL can be utilized as precursor of medicines, rubber, fuel energy, and materials CPO is also used in the fragrance and perfume industry as there are the major ingredients of jasmine family [10][11][12]. Traditionally, the synthesis of CPO involves catalytic vapor-phase cyclization of 1,6-hexandiols or ester of adipic acid with yields of 53% and 22%, respectively [13][14].
Several previous works on the synthesis of CPO from FFald have been extensively investigated, the highest yield of CPO and CPL together (81%) was achieved over 5% Pt/C under very high pressure of H2 (8 MPa) and temperature of 160 °C [7]. By using similar Pt/NC-BS catalyst, Liu et al. reported that maximum yield of CPO (76%) was obtained from FFald at 150 °C and 3.0 MPa H2 [15]. Date et al. reported that efficient reductive rearrangement of furfural using Pd/SiO2 catalyst resulted cyclopentanone with 87% yield (at 98% conversion of FFald) at 165 °C and 5 MPa H2 [16]. However, precious metals, such as Au, Ru, Pd, and Pt were utilized in these catalyst systems. Therefore, alternative economical and eco-friendly heterogeneous catalysts that would ensure the preferred hydrogenation and rearrangement of furanic ring into CPO/CPL are highly desired.
Catalysts based on Ni, which is also a Ptgroup metal, would be good candidates because of the similarity of their catalytic behavior to that of Pt, and such catalysts have been widely used for numerous chemical reactions both in the laboratory and in industry [17]. A few reports have shown that bimetallic Ni-based exhibited a unique catalytic performance for the hydrogenation and rearrangement of furfural to cyclopentanone/cyclopentanol, such as: bimetallic Ni-Fe/SBA-15 catalyst in a methanol/water solvent [18]. Yang et al. reported the use of bimetallic Ni-Cu in the conversion of furfural to cyclopentanone and claimed that furfuryl alcohol, 4-hydroxy-2-cyclopentenone and 2cyclopentenone were verified as three key intermediates and rearrangement of the furan ring was independent of hydrogenation, starting from furfuryl alcohol rather than furfural [19]. In our previous reports, the synthetic pro-cedure for bimetallic Ni-based with second metals of Sn, In, Co, and Fe applied for chemoselective hydrogenation of biomassderived furfural, levulinic acid, and dodecanoic acid, have developed. Most recently, the bulk bimetallic Ni-Sn alloy for hydrogenation and rearrangement of FFald in H2O or H2O/ethanol solvents and provided CPO and CPL as minor products was applied [20][21]. Therefore, in the present report, an extended investigation on the catalytic conversion of FFald and FFalc using bimetallic Ni-Co(3.0) and Ni-Fe(3.0) supported on TiO2 (denoted as Ni-Co(3.0)/TiO2 and Ni-Fe(3.0)/TiO2 whereas 3.0 is Ni/Co or Ni/Fe molar ratio based on the feeding ratio) is described. Both Ni-Co(3.0)/TiO2 and Ni-Fe(3.0)/TiO2 catalysts were synthesized via hydrothermal method according to previous reports [22][23].

Synthesis of Ni/TiO2
A typical procedure of the synthesis of TiO2 supported Ni catalyst is described as follows [22]. NiCl2.6H2O (0.4392 gram) was dissolved in deionised water and TiO2 (1 g) was added at room temperature; the temperature was subsequently raised to 50 °C 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 150 °C 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 450 °C for 1.5 h. A similar procedure was also applied for the syn-thesis of supported Co/TiO2 and Fe/TiO2 catalysts.

Synthesis of Ni-Co(3.0)/TiO2 and Ni-Fe(3.0)/ TiO2
A typical procedure of the synthesis of TiO2 supported Ni-Co (3.0 is feeding ratio) alloy catalyst is described as follows [22]: NiCl2.6H2O (0.4408 gram) was dissolved in deionized water (denoted as solution A), and CoCl2.6H2O (0.1488 gram) was dissolved in deionized water (denoted as solution B) at room temperature. Solutions A, B, and TiO2 (1 g) were mixed at room temperature; the temperature was subsequently raised to 50 °C 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 150 °C 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 450 °C for 1.5 h. A similar procedure was also applied for the synthesis of Ni-Fe(3.0) alloy supported on TiO2 catalysts.

Catalyst Characterization
XRD measurements were recorded on a Mac Science M18XHF instrument using monochromatic Cu-Kα radiation ( = 0.15418 nm). The XRD was operated at 40 kV and 200 mA with a step width of 0.02° and a scan speed of 4° min -1 (1 = 0.154057 nm, 2 = 0.154433 nm). ICP measurements were performed on an SPS 1800H plasma spectrometer of Seiko Instruments Inc. The BET surface area (SBET) and pore volume (Vp) were measured using N2 physisorption at 77 K on a Belsorp Max (BEL Ja-pan). 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 [24]. The ammonia-temperature programmed-desorption (NH3-TPD) was carried out on a Belsorp Max (BEL Japan). The samples were degassed at elevated temperature of 100-200 °C for 2 h to remove physisorbed gases prior to the measurement. The temperature was then kept at 200 °C for 2 h while flushed with He gas. NH3 gas (balanced NH3, 80% and He, 20%) was introduced at 100 °C for 30 min, then evacuated by helium gas to remove the physisorbed also for 30 min. Finally, temperature programmed desorption was carried out at temperature of 100-800 °C and the desorbed NH3 was monitored by TCD.

Catalytic Reaction
The catalyst (50 mg), furfural (1.1 mmol), and ethanol/H2O (1.5 mL/2.0 mL) as the solvent were placed into a glass reaction tube, which fit inside a stainless steel reactor. After H2 was introduced into the reactor at an initial H2 pressure of 3.0 MPa at room temperature, the temperature of the reactor was increased to 110-170 °C. After 3 h, the conversion of furfural (FFald) and the yields of furfuryl alcohol (FFalc) and tetrahydrofurfuryl alcohol (THFalc) were determined using GC analysis. The used Ni-Co(3.0)/TiO2 catalyst was easily separated using either simple centrifugation (4000 rpm for 10 min) or filtration, then finally dried overnight under vacuum at room temperature prior to re-usability testing.

Product Analysis
GC analysis of the reactant (FFald) and products (FFalc, THFalc, 2-MeTF, cyclopentanone, and cyclopentanol) was performed on a Shimadzu GC-8A with a flame ionization detector equipped with a silicone OV-101 packed column (length (m) = 3.0; inner diameter (mm) = 2.0; methyl-silicone from Sigma-Aldrich Co. Ltd.). Gas chromatography-mass spectrometry (GC-MS) was performed on a Shimadzu GC-17B equipped with a thermal conductivity detector and an RT-DEXsm capillary column. 1 H and 13 C NMR spectra were obtained on a JNM-AL400 spectrometer at 400 MHz; the samples for NMR analysis were dissolved in chloroform-Scheme 1. Possible reaction pathways for the hydrogenation/hydrogenolysis of furfural. d1 with TMS as the internal standard. The products were confirmed by a comparison of their GC retention time, mass, 1H and 13 C NMR spectra with those of authentic samples.
The conversion of reactant, yield, and selectivity of the products were calculated according to the following equations: where F0 is the introduced mol reactant (furfural, FFald) (mol), Ft is the remaining mol reactant (mol), Pi is mol product (mol), and Ptotal is total mol products (mol), which are all obtained from GC analysis using an internal standard technique.
In the alcoholic solvents, (e.g. ethanol and 2propanol), FFald was converted completely to FFalc and THFalc (entries 1, 2). The main products were FFalc and THFalc, only a small amount of CPL was obtained (3.2%) in 2propanol (entry 2). In alcohols, the hydrogenation of C=O and C=C occurred simultaneously to produce FFalc and THFalc, respectively. Only a small amount of hydrogenolyzed product was obtained. Interestingly, a remarkable difference was observed in H2O, whereas the conversion of FFald was 86.8% and the products were distributed into FFalc, THFalc, CPO, and CPL with yields of 40.1%, 20.6%, 5.1%, and 15.0%, respectively (entry 3). Using ethanol/H2O mixture solvent, a high yield of CPL (25.8%) was obtained with 84.9% FFald conversion under the same reaction conditions (entry 4). Meanwhile, in 2-propanol/H2O solvent resulted 4.0% CPL at 87.2% conversion of FFald (entry 5). These results confirmed that the solvent significantly inhibited the C=C hydrogenation as reported previously [19,21,28]. Therefore, it can be concluded that ethanol/H2O is suitable solvent for the catalytic conversion of FFald to CPL or CPO in presence of bimetallic catalysts.    [16,30]. In fact, the selectivity of THFalc was remained unchanged at relatively higher reaction temperature or longer reaction time.

Effect of reaction temperature
To understand the effect of temperature in the hydrogenation of FFald over bimetallic Ni-Co(3.0)/TiO2 catalyst, the reactions were carried out at temperature range of 110-170 °C and the results are shown in Figure 3. In the case of Ni-Co(3.0)/TiO2 catalyst, the increase of reaction temperature to 170 °C resulted an increase of FFald conversion up to 100%. The increase of reaction temperature enhanced the C=C bond hydrogenation of FFald or FFalc as indicated by the increase of THFalc selectivity (Figure 3a). When the reaction time was prolonged to 6 h at 170 °C, the amount of CPO and CPL mixtures remarkably increased up to 36.1% selectivity ( Table 3, entry 9). It was reported that the C-O bond hydrogenolysis of furfural or furfuryl alcohol over Ni-based catalysts preferentially occurred at relatively higher reaction temperature and required longer reaction time [31].
Over Ni-Fe(3.0)/TiO2 catalyst, FFald conversion gradually increased as the reaction temperature was increased and 100 % FFald conversion was achieved at 170 °C at a reaction time of 3 h (Figure 3b). While the selectivity of THFalc increased as the increase of reaction temperature, the amount of FFalc decreased gradually and then totally disappeared at reaction temperature of 170 °C indicating that over hydrogenation of C=C and C=O bonds occurred at higher temperature. In addition, the higher temperature not only enhanced C=C hydro-   Table 1 dan 2. genation but also facilitated the C-O hydrogenolysis resulting hydrogenolyzed products, e.g. 2-methylfuran and 2-methyltetrahydrofuran instead of 1,2-pentanediol and 1,5-pentanediol (Figure 3b) [32][33].

Kinetic profiles
The reaction profiles of the catalytic conversion of FFald over supported Ni-Fe(3.0)/TiO2 catalyst were performed and the plot is shown in Figure 4. At the early reaction time (0.5-2.0 h), the conversion of FFald gradually increased from 36.4% to 64.7% and the main product was FFalc (the highest yield is 46% after reaction time of 2 h). After reaction time was prolonged to 3 h, the conversion of FFald also increased, meanwhile the yield of FFalc slightly decreased. On the other hand, yields of THFalc, CPO, and CPL increased smoothly. These re-sults indicated that, the first step reaction would be the hydrogenation of C=O bond of FFald to produce FFalc. During the reaction, FFalc is converted via Piancatelli rearrangement into CPO whereas the 4-hydroxy-2cyclopentenone (4-HCP) is the intermediate product [10,34]. However, once the THFalc was formed, the formation of CPO or CPL will be constant as indicated in Figure 4 due to the activation and hydrogenolysis of hydrofuran ring of THFalc required a specific active site, such as strong acid site or multicenter metal catalysts [5,35].
The formation of FFalc was the first step then rearrangement of FFalc structure occurred on the acidic surface of bimetallic Ni-Sn alloy catalysts [20][21]. Therefore, similar approach would be applicable to the catalytic conversion of FFald over bimetallic Ni-Fe(3.0)/TiO2 catalyst.

Effect of initial H2 pressure
The effect of initial H2 pressure on conversion and selectivity in hydrogenation of FFald was evaluated over Ni-Fe(3.0)/TiO2 catalyst and the results are shown in Figure 5. The conversion of FFald gradually increased as the initial pressure of H2 was increased to reach a maximum conversion (100%) at 4.0 MPa. At initial pressure of H2  converted to 38% FFalc, 7% THFalc, and 25% CPO in yields. Conversion of FFald slightly increased to 78% and the products were distributed to FFalc, THFalc, CPO, and CPL. The yield of CPL significantly increased to 27% as result of further hydrogenation of CPO as indicated by decreasing yield of CPO at initial H2 pressure of 3.0 MPa. Further increasing the initial H2 pressure to 4.0 MPa, the products distributed to 30% FFalc, 20% THFalc, 20% CPO, 21% CPL, and hydrogenolized product of pentanediol and methyl furan with yields of 11%. Therefore, the optimized initial pressure of H2 for one-pot conversion of FFald to CPO/CPL over Ni-Fe(3.0)/TiO2 catalyst was 3.0 MPa.

Catalyst re-usability test
A reusability test was performed on the Ni-Fe(3.0)/TiO2 catalyst, and the results are summarized in Table 5. The used Ni-Fe(3.0)/TiO2 was easily separated by either simple centrifugation or filtration after the reaction. The recovered catalyst was reactivated by H2 at 450 °C for 1.5 h prior to used in the next reaction run. The yield of CPO+CPL on the reused catalyst decreased slightly after the second reaction run, but the activity was maintained for at least three consecutive runs.

Conclusions
The synthesis, characterization, and catalytic properties of bimetallic Ni-Co(3.0) and Ni-Fe(3.0) supported on TiO2 catalysts was described. XRD analyses revealed that the formation bimetallic Ni-Fe(111) alloy at 2θ = 44.06° in Ni-Fe(3.0)/TiO2 easily observed after reduction with H2 at 450 °C for 1.5 h. On the other hand, Ni-Co(3.0)/TiO2 did not provide the formation of Ni-Co alloy phase or hardly to distinguish the overlapped peaks of Ni(111) and Co(111). Monometallic Co/TiO2 exhibited higher selectivity towards FFalc than that of other monometallic catalysts. Bimetallic Ni-Co(3.0)/TiO2 and Ni-Fe(3.0)/TiO2 catalysts also demonstrated high selectivity toward FFalc at relatively lower reaction temperature. At the higher reaction temperature, the hydrogenation C=C bond and the arrangement of furan ring produced THFalc and CPO/CPL as the main products. The highest yields of CPO and CPL were 27.2% and 41.0%, respectively over Ni-Fe(3.0)/TiO2 at 170⁰C for 6 h.