Synthesis of Furfuryl Alcohol from Furfural: a Comparison Between Batch and Continuous Flow Reactors

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Communication Synthesis of Furfuryl Alcohol from Furfural: A Comparison between Batch and Continuous Flow Reactors

Maïté Audemar 1, Yantao Wang 2,3, Deyang Zhao 2,4,Sébastien Royer 5 , François Jérôme 1, Christophe Len 2,4 and Karine De Oliveira Vigier 1,* 1 Institut de Chimie des Milieux et Matériaux de Poitiers (IC2MP), UMR CNRS 7285, Université de Poitiers, 1 rue Marcel Doré, CEDEX 9, 86073 Poitiers, France; [email protected] (M.A.); [email protected] (F.J.) 2 Centre de recherche Royallieu, Sorbonne universités, Université de Technologie de Compiègne, CS 60 319, F-60203 Compiègne CEDEX, France; [email protected] (Y.W.); [email protected] (D.Z.); [email protected] (C.L.) 3 School of Resources Environmental & Chemical Engineering, Nanchang university, Nanchang 330031, China 4 Institute of Chemistry for Life and Health Sciences, ChimieParisTech, PSL Research University, CNRS, 11 rue Pierre et Marie Curie, F-75005 Paris, France 5 UMR 8181 - UCCS - Unité de Catalyse et de Chimie du Solide, Univ. Lille, CNRS, ENSCL, Centrale Lille, Univ. Artois, F-59000 Lille, France; [email protected] * Correspondence: [email protected]

Received: 2 December 2019; Accepted: 19 February 2020; Published: 24 February 2020

Abstract: Furfural is a platform molecule obtained from hemicellulose. Among the products that can be produced from furfural, furfuryl alcohol is one of the most extensively studied. It is synthesized at an industrial scale in the presence of CuCr catalyst, but this process suffers from an environmental negative impact. Here, we demonstrate that a non-noble metal catalyst (Co/SiO2) was active (100% conversion of furfural) and selective (100% selectivity to furfuryl alcohol) in the hydrogenation of furfural to furfuryl alcohol at 150 ◦C under 20 bar of hydrogen. This catalyst was recyclable up to 3 cycles, and then the activity decreased. Thus, a comparison between batch and continuous flow reactors shows that changing the reactor type helps to increase the stability of the catalyst and the space-time yield. This study shows that using a continuous flow reactor can be a solution to the catalyst suffering from a lack of stability in the batch process.

Keywords: continuous ﬂow; batch reactor; furfural; furfuryl alcohol; hydrogenation

1. Introduction Motor fuel components and fine chemicals can be produced from non-edible plant-based feedstocks using catalytic processes. Among all available starting materials, furfural is one of the most promising compounds, as it is a platform molecule for the synthesis of a high number of chemicals for a wide range of applications [1–4]. Furfural production is based on acid hydrolysis of hemicellulose [5]. One interesting reaction from furfural is the hydrogenation reaction, which is the most significant process in the furfural conversion. The hydrogenation of furfural leads to the production of valuable chemicals, such as furfuryl alcohol (FOL), 2-methylfuran (2-MF), tetrahydrofurfuryl alcohol (THFA), and so on. Currently, around 50% of furfural production is employed for the synthesis of furfuryl alcohol (FA), which can be used for resins, flavors, as components of motor fuels (alkyl levulinates), and in the pharmaceutical industry (ranitidine), biochemistry, and so on. During the hydrogenation of furfural to FOL many side reactions can occur, such as the formation of THFA, 2-methylfurane, and so on (Scheme1). Precise control of the selectivity of the reaction by using an appropriate and stable catalyst is in high demand.

Energies 2020, 13, 1002; doi:10.3390/en13041002 www.mdpi.com/journal/energies Energies 2020, 13, x FOR PEER REVIEW 2 of 11

biochemistry, and so on. During the hydrogenation of furfural to FOL many side reactions can occur, Energiessuch2020 as, 13 , the 1002 formation of THFA, 2-methylfurane, and so on (Scheme 1). Precise control of the2 of 10 selectivity of the reaction by using an appropriate and stable catalyst is in high demand.

OH O O O OH H2 H2 H2 - H2O H2 H2 H2 - H O O O OH 2 O O O H2 H2 H2 HO OH

- CO O O H2 H2 HO

SchemeScheme 1. 1.Hydrogenation Hydrogenation of of furfural. furfural.

Copper-chromiumCopper-chromium (CuCr) (CuCr) alloy alloy is the the catalyst catalyst used used on an on industrial an industrial scale toscale produce to produceFOL, with FOL, a high yield (98%) [5]. This catalyst has some drawbacks, such as, for instance, the presence of with a high yield (98%) [5]. This catalyst has some drawbacks, such as, for instance, the presence chromium, which can contaminate FOL and hamper its use in pharmaceutical industry. Moreover, of chromium, which can contaminate FOL and hamper its use in pharmaceutical industry. chromium-containing catalysts can be deactivated due to shielding of copper by chromium [6]. Many Moreover,studies chromium-containing have been devoted to the catalysts replacement can be of deactivated this catalyst due by catalysts to shielding based of on copper noble bymetals chromium such [6]. Manyas studies Pt and havePd [7– been11], leading devoted to toan theincrease replacement in the process of this cost. catalyst Furthermore, by catalysts FOL selectivity based on nobleis lower metals suchin as the Pt andpresence Pd [of7– these11], leading catalysts to than an in increase the presence in the of process chromium cost.–copper Furthermore, systems. To FOL increase selectivity the is lowerselectivity in the presence to FOL, ofthe these addition catalysts of metals than such in theas Cu presence to Pd based of chromium–copper catalysts results in the systems. improvement To increase the selectivityof the selectivity to FOL, to the FOL addition (98% of FOL of metals was obtained) such as Cu[12]. to Non Pd- basednoble metals catalysts catalysts results were in the also improvement studied of thein selectivity the selective to FOLhydrogenation (98% of FOL of furfural was obtained) to FOL, such [12]. as Non-noble supported metals Ni, Cu, catalysts Fe, Mo, Zn, were and also so studiedon in the[13 selective–18]. Various hydrogenation methods were of used furfural for the to synthesis FOL, such of catalysts, as supported additives, Ni, process Cu, Fe, conditions, Mo, Zn, and and so various solvents in the case of a liquid phase process [19–21]. Several drawbacks are present using on [13–18]. Various methods were used for the synthesis of catalysts, additives, process conditions, this system: deactivation due to the sintering of active species; poisoning of the catalyst surface by and various solvents in the case of a liquid phase process [19–21]. Several drawbacks are present using coke formation; low selectivity of FOL; and high temperature and pressure. Up to now, several this system:studies of deactivation selective hydrogenation due to the sinteringof furfural of to active FOL have species; been poisoningperformed ofin the liquid catalyst phase surface in by coke formation;batch reactors low using selectivity different of solvents, FOL; and but high verytemperature little attention and has pressure.been paid Upto the to process now, several in a flow studies of selectivesystem hydrogenation[22–26]. To this aim, of furfural hydrogenation to FOL of have furfural been to performed FOL was studied in the liquidin the presence phase in of batch Co/SiO reactors2 usingcatalyst different in solvents, batch and but in very continuous little attention flow reactors. has been We paid demonstrate to the process here that, in a despite flow system the high [22 –26]. To thisselectivity aim, hydrogenation and activity of of the furfural Co/SiO to2 catalyst FOL was in the studied hydrogenation in the presence of furfural of Co in/ SiOthe 2batchcatalyst reactor, in batch and inthe continuous reaction performed flow reactors. in a continuous We demonstrate flow reactor here led that,to a higher despite space the time high yield selectivity (STY = quantity and activity of FOL produced per unit volume unit per time unit). STY was three times higher when the of the Co/SiO2 catalyst in the hydrogenation of furfural in the batch reactor, the reaction performed hydrogenation of furfural was performed in a continuous flow reactor than in a batch process. The in a continuous flow reactor led to a higher space time yield (STY = quantity of FOL produced per selectivity was slightly lower in a continuous flow reactor than in a bath reactor, but the activity was unit volume unit per time unit). STY was three times higher when the hydrogenation of furfural was similar. The catalyst was more stable in the flow reactor than in the batch reactor. performed in a continuous flow reactor than in a batch process. The selectivity was slightly lower in a continuous2. Materials flow and reactor Methods than in a bath reactor, but the activity was similar. The catalyst was more stable in the flow reactor than in the batch reactor. Catalyst preparation: The Co3O4/SiO2 material, with a metal loading of 10 wt.% was prepared using incipient wetness impregnation method as described previously [27] using Co(NO3)2·and 2. Materials and Methods aerosil silica (380). The dry solid was calcined at 500 °C for 6 h to obtain the Co2O3/SiO2 sample. −1 CatalystCo2O3/SiO preparation:2 (around 100 mg) The was Co3 reducedO4/SiO2 undermaterial, hydrogen with flow a metal (3 L·h loading) at 500 of °C 10 for wt.% 10 h. was prepared Catalyst characterizations: Co/SiO2 catalyst was characterized by ICP-OES, XRD analysis, N2- using incipient wetness impregnation method as described previously [27] using Co(NO3)2 and aerosil physisorption, transmission electronic microscopy (TEM), thermal analysis. A Perkin Elmer Optima· silica (380). The dry solid was calcined at 500 ◦C for 6 h to obtain the Co2O3/SiO2 sample. Co2O3/SiO2 2000 DV instrument was used for ICP analysis. The catalysts1 were first dissolved in a mixture of HF (around 100 mg) was reduced under hydrogen flow (3 L h− ) at 500 ◦C for 10 h. and HCl under microwaves for digestion before analysis.· X-Ray Diffraction (XRD) analysis is Catalyst characterizations: Co/SiO2 catalyst was characterized by ICP-OES, XRD analysis, performed using a Bruker Empyrean with a Co cathode. N2-physisorption experiments were N2-physisorption,obtained on an Autosorb transmission 1-MP electronicinstrument, microscopy at 77 K. The catalysts (TEM), thermalwere treated analysis. under vacuum A Perkin à 350 Elmer Optima°C for 2000 3 h DV and instrument the surface area, was the used pore for size ICP as analysis. well as the The pore catalysts volume were determined first dissolved as described in a mixture of HFpreviously and HCl [27]. under Transmission microwaves Electron for digestion Microscopy before (TEM analysis.) experiments X-Ray were Diff ractionperformed (XRD) on a analysisJEOL is performed using a Bruker Empyrean with a Co cathode. N2-physisorption experiments were obtained on an Autosorb 1-MP instrument, at 77 K. The catalysts were treated under vacuum à 350 ◦C for 3 h and the surface area, the pore size as well as the pore volume were determined as described previously [27]. Transmission Electron Microscopy (TEM) experiments were performed on a JEOL 2100 UHR (Ultra High Resolution) instrument operated at 200 kV, equipped with a LaB6 source and a Gatan ultra scan Energies 2020, 13, 1002 3 of 10 camera. Thermogravimetric analysis (TGA) was performed using a TA instrument (SDTQ 600) under air flow of 100 mL min 1 from 25 C to 800 C. · − ◦ ◦ General procedure for the hydrogenation of furfural in a batch reactor: in a typical experiment, 1 g of furfural was added to 9 g of ethanol and 50 mg of catalyst was added in a batch reactor (75 mL). The hydrogen pressure was fixed to the desired one. Then, the temperature was increased up to the Energies 2020, 13, x FOR PEER REVIEW 3 of 11 desired reaction temperature, i.e., 150 ◦C. At the end of the reaction, the reactor was cooled down to room temperature,2100 UHR (Ul andtra liquidHigh Resolution) phase was instrument analysed. operated at 200 kV, equipped with a LaB6 source and Generala Gatan procedure ultra scan for camera. the hydrogenation Thermogravimetric of analysis furfural (TGA) in flow was reactor:performed The using experiments a TA instrument were carried out in H-Cube(SDTQ ProTM 600) under Flow air Reactorflow of 100 ThalesNanoTM, mL·min−1 from 25 °C Hungary, to 800 °C. connected to an HPLC pump to supply General procedure for the hydrogenation of furfural in a batch reactor: in a typical experiment, a continuous feed of 10 wt.% furfural in ethanol. A 70 mm catalyst cartridge (0.88 mL empty volume) 1 g of furfural was added to 9 g of ethanol and 50 mg of catalyst was added in a batch reactor (75 catalyst wasmL). packed The hydrogen with 260pressure mg catalystwas fixed byto the applying desired one. vacuum Then, the suction temperature at the was bottom increased ofthe up cartridge. The total flowto the throughdesired reaction volume temperature, (including i.e., feed,150 °C. reactor, At the end and of productthe reaction, sections) the reactor was was 6 cooled mL. First, pure ethanol wasdown pumped to room temperature, through the and system, liquid phase and was then analysed. the feed was changed to the furfural-ethanol General procedure for the hydrogenation of furfural in flow reactor: The experiments were mixture. The flow was continued until the desired temperature and hydrodynamic pressurization carried out in H-Cube ProTM Flow Reactor ThalesNanoTM, Hungary, connected to an HPLC pump (20–60 bar) of the reactor module were reached. Depending on the flow rate used (0.2–0.5 mL min 1), to supply a continuous feed of 10 wt.% furfural in ethanol. A 70 mm catalyst cartridge (0.88 mL empty · − the reactionvolume) time catalyst was set was (20–50 packed min) with before260 mg collectingcatalyst by applying the first vacuum sample suction (time at zero).the bottom The of samples the were collected aftercartridge. regular The total time flow intervals. through volume (including feed, reactor, and product sections) was 6 mL. First, pure ethanol was pumped through the system, and then the feed was changed to the furfural- Analytical methods: yields to furfuryl alcohol and conversion of furfural were determined by ethanol mixture. The flow was continued until the desired temperature and hydrodynamic external calibrationpressurization at (20 25–60◦ bar)C using of the reactor HPLC module equipped were reached. with aDepending nucleosil on 100–5the flow C18 rate used column (0.2– (250 mm and diameter0.5 mL of min 4.6−1), mm), the reaction a Shimadzu time was set LC-20AT (20–50 min) pump, before collecting and a Shimadzu the first sample RID-10 (time zero). A detector The using samples were collected after regular time intervals. 1 acetonitrile/water (10:90) as the mobile phase (0.6 mL min− ). Analytical methods: yields to furfuryl alcohol and· conversion of furfural were determined by Continuous flow results were detected on a gas chromatograph (HP, 14009 Arcade, New York, external calibration at 25 °C using HPLC equipped with a nucleosil 100–5 C18 column (250 mm and United States)diameter coupled of 4.6 mm), with a a Shimadzu Flame Ionization LC-20AT pump, Detector and a(FID) Shimadzu detector RID-10 equipped A detector with using a Supelco 2-8047-U capillaryacetonitrile/water column (10:90) (15 mas the0.25 mobile mm phase i.d. (0.6 and mL·min 0.25 µ−1).m film thickness, Alltech Part No.31163-01). Continuous flow results× were detected on a gas1 chromatograph (HP, 14009 Arcade, New York, The flow rate of the carrier gas (H2) was 1 mL min− . The injector temperature was 250 ◦C and the oven United States) coupled with a Flame Ionization· Detector (FID) detector equipped with a Supelco 2- started at 70 C for 1 min. The temperature was then increased up to 250 C at a rate of 20 C min 1, 8047◦-U capillary column (15 m × 0.25 mm i.d. and 0.25 μm film thickness, Alltech◦ Part No.31163-01). ◦ · − and then maintainedThe flow rate atof the 250 carrier◦C for gas 10 (H min.2) was 1 mL min−1. The injector temperature was 250 °C and the oven started at 70 °C for 1 min. The temperature was then increased up to 250 °C at a rate of 20 °C 2.1. Catalystmin Charcaterizations−1, and then maintained at 250 °C for 10 min.

The catalyst2.1. Catalyst was C preparedharcaterizations using the Incipient Wetness Impregnation (IWI) method. After calcination of the solid atThe 500 catalyst◦C under was air, prepared oxide using precursor, the Incipient Co3O 4 Wetness/SiO2, is Impregnation obtained, confirmed (IWI) method. by After XRD analysis (Figure1), withcalcination the presence of the solid of peaksat 500 °C corresponding under air, oxide to precursor, the awaited Co3 positionO4/SiO2, is (PDF obtained, file 01-1152).confirmed Consideringby the width ofXRD the analysis reflections, (Figure the 1), cobaltwith the crystal presence size of ispeaks relatively corresponding low (< 10to the nm). awaited This position result confirms (PDF file that when using this impregnation01-1152). Considering method, the width high of dispersion the reflections, of the the cobaltcobalt crystal oxide size phase is relatively can be achieved.low (<10 nm). The loading This result confirms that when using this impregnation method, high dispersion of the cobalt oxide of cobalt wasphase evaluated can be achieved. by ICP-OES The loading and of was cobalt of was 9 wt.%. evaluated by ICP-OES and was of 9 wt.%

Figure 1. X-Ray Diﬀraction analysis of non reduced Co/SiO2 catalyst (vertical bars, Co3O4 reference PDF ﬁle 01-1152). Energies 2020, 13, x FOR PEER REVIEW 4 of 11 Energies 2020, 13, 1002 4 of 10

Figure 1. X-Ray Diffraction analysis of non reduced Co/SiO2 catalyst (vertical bars, Co3O4 reference PDF file 01-1152). The catalyst was reduced under H2 ﬂow at 500 ◦C for 10 h prior to catalytic test leading to a large surface area and large pore diameter, suitable for liquid phase hydrogenation reaction. Moreover, The catalyst was reduced under H2 flow at 500 °C for 10 h prior to catalytic test leading to a large the limitedsurface area evolution and large of thepore textural diameter, properties suitable for indicates liquid phase adequate hydrogenation stabilities reaction. of the support Moreover, (Table the 1). limited evolution of the textural properties indicates adequate stabilities of the support (Table 1). Table 1. Main characteristics of the Co3O4/SiO2 and reduced Co/SiO2 materials.

Table 1. Main characteristics of the Co3O4/SiO2 and reduced Co/SiO2 materials. Characteristics Co3O4/SiO2 Co/SiO2 Characteristics Co3O4/SiO2 Co/SiO[a] 2 XRD phase Poorly crystalized Co3O4 n.d. XRD phase Poorly crystalized Co3O4 n.d.[a] [b] [a] Aggregates 10 to > 100 nm [b]D / nm [a] n.d. Dpart./ nmpart. n.d. AggregatesCrystals <1020 to nm > 100 nm [c] 2 1 SBET/ m g− 185Crystals 169 < 20 nm [c] 2[c] −1 3 ·1 SBET/ V mp/ ·g m g− 185 0.71 0.63169 [c] · Vp/[c] mD3·gp−1/ nm 0.71 15.0 14.70.63 [c] [a] n.d.:Dp/ notnm determined; [b] mean particle15.0 size obtained by TEM image observation;14.7 [c] surface area (SBET), pore[a] volumen.d.: not (V determined;p) and pore diameter[b] mean (Dparticlep) issued size from obtained N2 physisorption by TEM image at 77K. observation; [c] surface area (SBET),

pore volume (Vp) and pore diameter (Dp) issued from N2 physisorption at 77K. Figure2 shows that the particle sizes and localization throughout the support porosity are not homogeneousFigure and 2 shows aggregates that the of particle cobalt sizes nanoparticles and localization are observed throughout throughout the support the surfaceporosity of are the not silica homogeneous and aggregates of cobalt nanoparticles are observed throughout the surface of the silica with a size of 20–100 nm. Energy dispersive x-ray spectroscopy (EDS) showed that dark mark are with a size of 20–100 nm. Energy dispersive x-ray spectroscopy (EDS) showed that dark mark are cobalt particle and hexagonal cobalt phase was observed using electron diﬀraction. cobalt particle and hexagonal cobalt phase was observed using electron diffraction.

(a) (b) (c)

Figure 2. Transmission electronic microscopy (TEM) images recorded for reduced Co/SiO2 catalyst Figure 2. Transmission electronic microscopy (TEM) images recorded for reduced Co/SiO2 catalyst (a), energy(a), dispersiveenergy dispersive x-ray spectroscopyx-ray spectroscopy (EDS) (EDS) (b) and (b) and electron electron diﬀ diffractionraction analysis analysis (c ).(c).

2.2. Hydrogenation2.2. Hydrogenation of Furfuralof Furfural in in Batch Batch Reactor Reactor. TheThe hydrogenation hydrogenation of furfuralof furfural was was performed performed in in a a batch batch reactor reactor starting starting fromfrom 11 gg ofof furfural in in 9 g of ethanol9 g of ethanol in the presencein the presence of 50 mgof 50 of mg catalyst of catalyst (Table (Table2). 2).

Table 2. Hydrogenation of furfural in a batch reactor 1. FOL; furfuryl alcohol, STY; space time yield. Table 2. Hydrogenation of furfural in a batch reactor 1. FOL; furfuryl alcohol, STY; space time yield. Reaction Temperature Conversion Selectivity STY PH2 (bar) Reaction PH2 Temperature Conversion Selectivity STY −1 −1 Time (h) (°C) (%) to FOL (%) 1(g·L1 ·h ) Time (h) (bar) (◦C) (%) to FOL (%) (g L h ) entry 1 1 20 150 100 100 · − · −13.2 entryentry 2 11.5 1 20 150150 100100 10065 13.2 5.9 entryentry 3 21 1.5 2040 150150 100100 6585 5.9 11.4 entry 3 1 40 150 100 85 11.4 entry 4 1 20 180 100 80 10.8 entry 4 1 20 180 100 80 10.8 1 Furfural = 1 g, ethanol = 9 g, Co/SiO2 = 50 mg 1 Furfural = 1 g, ethanol = 9 g, Co/SiO2 = 50 mg We were pleased to see that furfural conversion reached 100% at 150 °C after 1h of reaction, with aWe FOL were yield pleased of 100% to (Table see that 2, entry furfural 1). By conversion prolonging reached the reaction 100% time, at 150 the◦C yield after of 1h FOL of reaction,decreased with a FOLfrom yield 100% of to 100% 65% (Tabledue to 2further, entry hydrogenation 1). By prolonging of FOL the to THFA reaction (Table time, 2, entry the yield 2). This of FOLresult decreased shows fromthat 100% Co/SiO to 65%2 was due active to further and selective hydrogenation in the ofhydrogenation FOL to THFA of (Table furfural2, entry to FOL. 2). ThisAn increase result shows in that Co/SiO2 was active and selective in the hydrogenation of furfural to FOL. An increase in hydrogen pressure led to a decrease in FOL yield due to the further hydrogenation of FOL to THFA that can be favored by a higher solubility of hydrogen than at lower pressure of hydrogen (Table2, entry 3). Energies 2020, 13, x FOR PEER REVIEW 5 of 11 Energies 2020, 13, 1002 5 of 10 hydrogen pressure led to a decrease in FOL yield due to the further hydrogenation of FOL to THFA that can be favored by a higher solubility of hydrogen than at lower pressure of hydrogen (Table 2, In both conditions, FOL was further hydrogenated to THFA. A similar trend was observed with an entry 3). In both conditions, FOL was further hydrogenated to THFA. A similar trend was observed increase of the temperature from 150 to 180 C. With the increase of the temperature, THFA was also with an increase of the temperature from 150◦ to 180 °C. With the increase of the temperature, THFA observed as a by-product and the selectivity of FOL decreased on the beneﬁt of the formation of THFA. was also observed as a by-product and the selectivity of FOL decreased on the benefit of the formation Based on these results, it is of prime importance to control the kinetic of the reaction in order to prevent of THFA. Based on these results, it is of prime importance to control the kinetic of the reaction in further hydrogenation of FOL to THFA. order to prevent further hydrogenation of FOL to THFA. The recyclability of the catalyst was then studied under the optimum conditions (150 C, 20 bar of The recyclability of the catalyst was then studied under the optimum conditions (150◦ °C, 20 bar hydrogen, 1 h of reaction). This is a key parameter, as in a batch liquid phase reactor catalysts used of hydrogen, 1 h of reaction). This is a key parameter, as in a batch liquid phase reactor catalysts used in the literature suﬀer from leaching and from deposition of furanic derivatives on the catalytic sites, in the literature suffer from leaching and from deposition of furanic derivatives on the catalytic sites, preventing the reuse of the catalyst. The recycling of the catalyst was performed by recovering the preventing the reuse of the catalyst. The recycling of the catalyst was performed by recovering the solid, owing to its magnetic properties. It was then reused without any treatment. The amount of solid, owing to its magnetic properties. It was then reused without any treatment. The amount of furfural used was always 1g for each cycle in 9 g of ethanol. Four cycles were performed under 20 bar furfural used was always 1g for each cycle in 9 g of ethanol. Four cycles were performed under 20 of hydrogen for 1 h of reaction at 150 C (Figure3). bar of hydrogen for 1 h of reaction at ◦150 °C (Figure 3).

100

60 % 40

0 1 2 3 4 Run conversion FOL selectivity

2 Figure 3. RecyclingRecycling of of Co/SiO Co/SiO2. .1 1g gof of furfural, furfural, 9 9g gof of ethanol, ethanol, T T= =150150 °C,◦ C,20 20bar bar of ofhydrogen, hydrogen, 1 h 1 of h ofreaction. reaction.

FOL selectivity and conversion of furfural slightly decreased after the third cycle, which can be ascribed to the work up process.process. However, after the fourth cycle, the conversion of furfural dropped from 100% to 69% and the selectivity to FOL also decreased (88%).(88%). This could be due to the poisoning of cobaltcobalt inin the the solution, solution, as as this this is alreadyis already mentioned mentioned in the in literature,the literature, or to theor to leaching the leaching of cobalt, of cobalt, [24,26] leading[24,26] leading to the formation to the formation of by-products of by-products such as THFA such andas THFA other unidentifiedand other unidentified by-products. by To-products. confirm theseTo confirm hypotheses, these hypotheses, TGA analysis TGA of analysis the spent of catalystthe spent were catalyst performed were performed and compared and compared to the TGA to analysisthe TGA ofanalysis the fresh of catalystthe fresh (Figure catalyst4). (Figure 4). It was shown that 18% of the weight was lost during the TGA analysis of the spent catalyst, catalyst, whereas only 7% 7% was was lost lost during during the the fresh fresh catalyst catalyst analysis. analysis. This This increase increase in weight in weight loss losscould could be due be dueto the to deposition the deposition of carbon of carbon species species on the on catalys the catalystt due dueto the to thesorption sorption of furanic of furanic compounds. compounds. To Toconfirm confirm this this hypothesis, hypothesis, TEM TEM analysis analysis of the of the spent spent catalyst catalyst was was performed performed (Figure (Figure 5).5 ).

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FigureFigure 4. 4. TGATGA analysis analysis of of the the reduced reduced Co/SiO Co/SiO22 catalystcatalyst before before reaction reaction and and after after reaction reaction under under batch batch andandFigure flow ﬂow 4. conditions conditions.TGA analysis. of the reduced Co/SiO2 catalyst before reaction and after reaction under batch and flow conditions.

Figure 5. TEM images recorded for reduced Co/SiO2 catalyst after reaction under batch reaction and Figure 5. TEM images recorded for reduced Co/SiO2 catalyst after reaction under batch reaction and EDSFigure analysis. 5. TEM images recorded for reduced Co/SiO2 catalyst after reaction under batch reaction and EDS analysis. It was clearly shown using EDS that carbon was deposited on the catalyst surface, leading to less accessibilityIt was clearly of furfural shown to usingactive EDS sites that and carbon a decrease was deposited of the selectivity on the catalyst to FOL. surface, Several leading zones ofto less the catalystaccessibility contained of furfural carbon to deposited. active sites In and order a decreaseto prevent of this the selectivitydeactivation, to hydrogenation FOL. Several zones of furfural of the wascatalyst performed contained in the carbon continuous deposited. flow In reactor. order to prevent this deactivation, hydrogenation of furfural was performed in the continuous flowflow reactor.reactor. 2.3. Continuous Flow Reactor 2.3. Continuous Flow Reactor The hydrogenation of furfural was performed while keeping the concentration of furfural used for batchThe experiments hydrogenation (5 g of of furfural furfural was in 45 performed g of ethanol) while and keeping the catalyst the concentration amount of 260 of furfuralmg to fill used the catalystfor batch cartridge. experiments In the (5 first g of set furfural of experiments, in 45 g of ethanol)the flow and rate the of the catalyst alcoholic amount solution of 260 of mg furfural to fill fill was the studied.catalyst cartridge.To this aim, In the the firstflow first set rate of was experiments, increased thefrom flow flow 0.2 rate to 0.5 of mL·minthe alcoholic −1 (Figure solution 6). of furfural was 1 studied. To this aim, the flowflow rate was increased fromfrom 0.20.2 toto 0.50.5 mLmL·minmin−−1 (Figure(Figure 66).). ·

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Conversion (%) Conversion 60 20 selectivityFOL (%) 50 20 10 0.2 mL/min 0,3 mL/min 0.5 mL/min 40 40 0.2 mL/min 0,3 mL/min 0.5 mL/min 0 30 0

Conversion (%) Conversion 0 50 100 150 200 250 300 FOL selectivityFOL (%) 0 50 100 150 200 250 300 20 20 0.2 mL/minTime on0,3 stream mL/min (min)0.5 mL/min Time on stream (min) 10 0.2 mL/min 0,3 mL/min 0.5 mL/min 0 0 0 50 100 150 200 250 300 0 50 100 150 200 250 300 (a) (b) Time on stream (min) Time on stream (min) Figure 6. Effect of the alcoholic furfural solution (5 g of furfural in 45 g of ethanol) flow rate. 150 °C, (a) (b) 20 bar of hydrogen in the presence of 260 mg of Co/SiO2. (a) conversion vs. Time On Stream (TOS). (bFigure) FOL 6.selectivityEffect of vs the. TOS. alcoholic furfural solution (5 g of furfural in 45 g of ethanol) flow rate. 150 C, Figure 6. Effect of the alcoholic furfural solution (5 g of furfural in 45 g of ethanol) flow rate. 150 °C,◦ 20 bar of hydrogen in the presence of 260 mg of Co/SiO2.(a) conversion vs. Time On Stream (TOS). When20 bar ofthe hydrogen flow rate in ofthe the presence furfural of solution260 mg of increased Co/SiO2. ( afrom) conversion 0.2 to 0.3 vs. mL·minTime On−1 Stream, similar (TOS trend). in (b) FOL selectivity vs. TOS. the conversion(b) FOL selectivity of furfural vs. TOS. was obtained, a drop of the conversion from 94% to 50% being observed in 1 the functionWhen the of flowTOS rate(Time of On the furfuralStream) solutionfrom 0 to increased 180 min. from The 0.2selectivity to 0.3 mL wasmin maintained− , similar trendat around in the When the flow rate of the furfural solution increased from 0.2 to 0.3 mL·min· −1, similar trend in 97%conversion for 0.3 of mL·min furfural−1 waswhereas obtained, under a drop 0.2 ofmL·min the conversion−1 of furfural from solution, 94% to 50% the being selectivity observed to in FOL the the conversion of furfural was obtained, a drop of the conversion from 94% to 50% being observed in decreasedfunction of from TOS 97 (Time to 74%, On which Stream) is fromdue to 0 tothe 180 increase min. The of residence selectivity time was that maintained favored the at around formation 97% the function of TOS1 (Time On Stream) from 0 1to 180 min. The selectivity was maintained at around offor THFA. 0.3 mL min− whereas under 0.2 mL min− of furfural solution, the selectivity to FOL decreased 97% for 0.3· mL·min−1 whereas under 0.2· mL·min−1 of furfural solution, the selectivity to FOL fromA 97 further to 74%, increase which isof duethe furfural to the increase solution of flow residence rate up time to that0.5 mL·min favored−1 the led formation to a significant of THFA. drop decreased from 97 to 74%, which is due to the increase of residence time that1 favored the formation of furfuralA further conversion increase from of the 90% furfural to 40% solution after flow 110 min rate upof reaction. to 0.5 mL Themin FOL− led selectivity to a significant was kept drop of THFA. · constant.of furfural Based conversion on these fromresults, 90% it was to 40% decided after to 110 keep min the of flow reaction. rate at 0.3 The mL·min FOL selectivity−1 for the following was kept A further increase of the furfural solution flow rate up to 0.5 mL·min−1 led to a1 significant drop experiments.constant. Based The on effect these of results, the pressure it was decidedof hydrogen to keep was the then flow studied rate at 0.3from mL 20min to− 60for bar the (Figure following 7). of furfural conversion from 90% to 40% after 110 min of reaction. The FOL· selectivity was kept Increasingexperiments. the pressure The effect of of hydrogen the pressure led to of an hydrogen increase wasof the then conversion studied of from furfural 20 to from 60 bar 92 (Figure to 100%.7). constant. Based on these results, it was decided to keep the flow rate at 0.3 mL·min−1 for the following ItIncreasing was interesting the pressure to see that of hydrogen under 60 led bar to of an hydrogen increase ofthe the conversion conversion was of always furfural 100% from when 92 to 100%.TOS experiments. The effect of the pressure of hydrogen was then studied from 20 to 60 bar (Figure 7). increasedIt was interesting up to 180 to min, see whereas that under at lower 60 bar pressure of hydrogen a slight the decrease conversion of the was conversion always 100% was whenobserved. TOS Increasing the pressure of hydrogen led to an increase of the conversion of furfural from 92 to 100%. Concerningincreased up the to selectivity, 180 min, whereas it was atkept lower constant pressure independently a slight decrease of the of thehydrogen conversion pressure, was observed. but the It was interesting to see that under 60 bar of hydrogen the conversion was always 100% when TOS selectivityConcerning to FOL the selectivity,was lower under it was 60 kept bar constantof hydrogen independently due to the formation of the hydrogen of THFA pressure, as a by-product. but the increased up to 180 min, whereas at lower pressure a slight decrease of the conversion was observed. Thisselectivity can be toex FOLplained was by lower a higher under solubility 60 bar of of hydrogen hydrogen due due to to the its formation pressure, ofas THFApreviously as a by-product. shown in Concerning the selectivity, it was kept constant independently of the hydrogen pressure, but the batchThis canreactions. be explained by a higher solubility of hydrogen due to its pressure, as previously shown in selectivity to FOL was lower under 60 bar of hydrogen due to the formation of THFA as a by-product. batch reactions. This can be explained by a higher solubility of hydrogen due to its pressure, as previously shown in batch100 reactions. 100

80 80

10060 10060

4080 4080

20 20

Conversion (%) Conversion 60 60 FOL selectivityFOL (%) 400 400 0 30 60 90 120 150 180 0 30 60 90 120 150 180 20 Time on stream(min) 20

Conversion (%) Conversion Time on stream (min) 20 bar 40 bar 60 bar selectivityFOL (%) 20 bar 40 bar 60 bar 0 0 0 30 60 90 120 150 180 0 30 60 90 120 150 180 Time on stream(min) (a) Time(b) on stream (min) 20 bar 40 bar 60 bar 20 bar 40 bar 60 bar Figure 7. Effect of the hydrogen pressure. 5 g of furfural in 45 g of ethanol at 0.3 mL.min 1 flow rate. Figure 7. Effect of the hydrogen pressure. 5 g of furfural in 45 g of ethanol at 0.3 mL.min-−1 flow rate. 150 ◦C in the presence(a) of 260 mg of Co/SiO2.(a) conversion vs. TOS. (b) FOL(b selectivity) vs. TOS. 150 °C in the presence of 260 mg of Co/SiO2. (a) conversion vs. TOS. (b) FOL selectivity vs. TOS. 3. Discussion-Conclusion . -1 3. DiscussionFigure 7. -EffectConclusion of the hydrogen pressure. 5 g of furfural in 45 g of ethanol at 0.3 mL min flow rate. 150Our °C results in the presence suggest of that 260 hydrogenationmg of Co/SiO2. (a of) conversion furfural tovs. FOLTOS. in(b) aFOL batch selectivity reactor vs. in TOS. the presence of Co/SiO2 catalyst is efficient at 150 ◦C under 20 bar of hydrogen using a solution of furfural of 3. Discussion-Conclusion

Energies 2020, 13, x FOR PEER REVIEW 8 of 11 Energies 2020, 13, 1002 8 of 10 Our results suggest that hydrogenation of furfural to FOL in a batch reactor in the presence of Co/SiO2 catalyst is efficient at 150 °C under 20 bar of hydrogen using a solution of furfural of 10 wt.% 10in ethanol. wt.% in ethanol.However, However, the stability the of stability the catalyst of the is catalyst not optimal, is not as optimal, shown asby shownthe catalyst by the recycling. catalyst recycling.TGA and MET TGA analysis and MET showed analysis that showed carbon that was carbon adsorbed was adsorbedon the catalyst on the surf catalystace due surface to the due sorption to the sorptionof furanic of molecules furanic molecules in batch inreactions, batch reactions, as has already as has already been mentioned been mentioned in the inliterature the literature [28]. [The28]. Therecyclability recyclability of the of catalyst the catalyst was wasthus thus hampered hampered by this by thiscoke coke formation formation on the on catalyst the catalyst surface. surface. The hydrogenation ofof furfuralfurfural to to FOL FOL in in a a continuous continuous flow flow reactor reactor can can aff affordord a higha high conversion conversion of furfuralof furfural with with a selectivitya selectivity higher higher than than 90% 90% under under 60 60 bar bar of of hydrogen hydrogen at at 150 150◦ °C.C. ThisThis meansmeans thatthat the catalyst waswas not not poisoned poisoned when when a hydrogena hydrogen pressure pressure of 60 of bar 60 was bar used,was butused, the but selectivity the selectivity was slightly was lowerslightly due lower to further due to hydrogenation further hydrogenation to THFA. Fromto THFA. an industrial From an point industrial of view, point it can of beview, interesting it can be to seeinteresting the space to time see yield the space of the time reaction yield (Table of the3). Thereaction space (Table time yield3). The (STY) space was time calculated yield and (STY) it was was highercalculated if the and hydrogenation it was higher of if furfuralthe hydrogenation was performed of furfura in a continuousl was performed flow reactor in a continuous than in a batch flow reactor. Thus, under similar conditions of pressure and temperature, the STY was 13.2 g L 1 h 1 for reactor than in a batch reactor. Thus, under similar conditions of pressure and temperature,· − the· − STY the batch reaction versus 16.6 g L 1 h 1 for the continuous flow reaction. By increasing the hydrogen was 13.2 g·L−1·h −1 for the batch· reaction− · − versus 16.6 g·L−1·h −1 for the continuous flow reaction. By pressure, the STY increased in the continuous flow reaction from 16.6 to 30.6 g L 1 h 1. These results increasing the hydrogen pressure, the STY increased in the continuous flow reaction· − · −from 16.6 to 30.6 showg·L−1·h that−1. These using results a same show catalyst, that theusing hydrogenation a same catalyst, of furfural the hydrogenation to FOL was moreof furfural performant to FOL in was the continuousmore performant flow reactor in the continuous that in the batch flow reactor.reactor that in the batch reactor.

Table 3. Hydrogenation of furfural: effect of the reactor 1. Table 3. Hydrogenation of furfural: effect of the reactor 1. 1 1 Reactor PH2 (bar) Conversion (%) Selectivity to FOL (%) STY (g L− h− ) Reactor PH2 (bar) Conversion (%) Selectivity to FOL (%) STY (g·L· −1··h −1) 2 Batch 2 2020 100 100 100 100 13.2 13.2 2020 52 52 50 50 16.6 16.6 3 Flow 3 4040 81 81 77 77 26.6 26.6 60 100 92 30.6 60 100 92 30.6 1 2 1 furfural in ethanol 10 wt.%, Co/SiO2 = 5 wt.% of furfural solution, 1502 ◦C. reaction time 3= 1 h. furfural3 in ethanol 10wt.%, Co/SiO2 = 5 wt.% of furfural solution, 150°C.1 reaction time = 1 h. time on time on stream = 3 h; flow rate of alcoholic solution of furfural = 0.3 mL min− stream = 3 h; flow rate of alcoholic solution of furfural = 0.3 mL·min−1· In order to go deep in the comparison of these two processes, it was interesting to compare the In order to go deep in the comparison of these two processes, it was interesting to compare the stability of the catalyst. In the batch reactor, the catalyst was recyclable up to 3 times and lost its activity, stability of the catalyst. In the batch reactor, the catalyst was recyclable up to 3 times and lost its but the selectivity was kept constant. This was due to an adsorption of the furanic molecules on the activity, but the selectivity was kept constant. This was due to an adsorption of the furanic molecules catalyst surface. For the continuous flow reactor, TGA analysis was performed on the spent catalyst on the catalyst surface. For the continuous flow reactor, TGA analysis was performed on the spent and only a slight difference in the weight loss was observed between the fresh and the spent catalyst catalyst and only a slight difference in the weight loss was observed between the fresh and the spent (Figure4). The specific surface area was similar before and after the reaction. ICP analysis of spent catalyst (Figure 4). The specific surface area was similar before and after the reaction. ICP analysis of catalyst showed that there was no leaching of cobalt (9 wt.% before and after reaction). TEM showed spent catalyst showed that there was no leaching of cobalt (9 wt.% before and after reaction). TEM that the particle size did not change after the reaction, and that no carbon was formed on the catalyst showed that the particle size did not change after the reaction, and that no carbon was formed on the surface (Figure8). catalyst surface (Figure 8).

(a) (b)

Figure 8. TEM images recordedrecorded for for Co Co/SiO/SiO2 catalyst2 catalyst before before (a) ( anda) and after after reaction reaction (b) under (b) under continuous continuous flow. flow. The diﬀerence between the two processes is the sorption of furanic compounds on the catalyst. In batchThe processesdifference the between catalysts the were two inprocesses contact withis the the sorption reactant of andfuranic the productscompounds formed, on the which catalyst. led toIn abatch higher processes contact the time catalysts between were the furanicin contact molecules with the andreactant the catalysts and the products than in the formed, continuous which ﬂow led

Energies 2020, 13, 1002 9 of 10 process. With these results, it is clearly demonstrated that the stability of the catalyst is higher under continuous flow processes than under batch processes. In the continuous flow reactor, it was interesting to see that by increasing the hydrogen pressure it was possible to maintain the activity of the catalyst for at least 3 h and that the selectivity was kept constant above 90%. This could be due to an increase in the hydrogen solubility, which was also observed during the hydrogenation in the batch reactor, since FOL produced was then hydrogenated to THFA with an increase of the hydrogen pressure from 20 to 40 bar. In conclusion, this study shows that using a continuous flow reactor can be a solution when a catalyst suffers from poisoning during batch processes due to sorption of molecules (reactants or products). The optimization of the reaction conditions which are not the same in both processes has to be performed in order to reach similar conversion of furfural and selectivity to FOL.

Author Contributions: Investigation, M.A., D.Z. and Y.W., F.J.; supervision, S.R., C.L., K.D.O.V.; writing—K.D.O.V. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Poitou-Charentes region for the funding of the PhD grant of Maïté Audemar. ANR agency for the funding of FurCab Project ANR-15-CE07-0016. Région Nouvelle Aquitaine for the funding of this project through the FR CNRS INCREASE 3707, the chaire TECHNOGREEN and FEDER, the University of Poitiers and the CNRS for their financial support. China Scholarship Council (CSC). Acknowledgments: The authors are grateful for all the financial support for this study. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

References

1. Espro, C.; Gumina, B.; Szumelda, T.; Paone, E.; Mauriello, F. Catalytic Transfer Hydrogenolysis as an eﬀective tool for the reductive upgrading of cellulose, hemicellulose, lignin, and their derived molecules. Catalysts 2018, 8, 313. [CrossRef] 2. Mariscal, R.; Maireles-Torres, P.; Ojeda, M.; Sádaba, I.; López Granados, M. Furfural: A renewable and versatile platform molecule for the synthesis of chemicals and fuels. Energy Environ. Sci. 2016, 9, 1144–1189. [CrossRef] 3. Bonacci, S.; Nardi, M.; Costanzo, P.; De Nino, A.; Di Gioia, M.; Oliverio, M.; Procopio, A. Montmorillonite K10-Catalyzed Solvent-Free Conversion of Furfural into Cyclopentenones. Catalysts 2019, 9, 301. [CrossRef] 4. Smirnov, A.; Selishcheva, S.; Yakovlev, V. Acetalization Catalysts for Synthesis of Valuable Oxygenated Fuel Additives from Glycerol. Catalysts 2018, 8, 595. [CrossRef] 5. Morozov, E. Furfural Production, 2nd ed.; Forest Industry: Moscow, Russia, 1988; pp. 32–56. 6. Liu, D.; Zemlyanov, D.; Wu, T.; Lobo-Lapidus, R.J.; Dumesic, J.A.; Miller, J.T.; Marshall, C.L. Deactivation mechanistic studies of copper chromite catalyst for selective hydrogenation of 2-furfuraldehyde. J. Catal. 2013, 299, 336–345. [CrossRef] 7. Musci, J.J.; Merlo, A.B.; Casella, M.L. Aqueous phase hydrogenation of furfural using carbon-supported Ru and RuSn catalysts. Catal. Today 2017, 296, 43–50. [CrossRef] 8. Vorotnikov, V.; Mpourmpakis, G.; Vlachos, D.G. DFT study of furfural conversion to furan, furfuryl alcohol, and 2-methylfuran on Pd(111). ACS Catal. 2012, 2, 2496–2504. [CrossRef] 9. Vaidya, P.D.; Mahajani, V.V. Kinetics of Liquid-Phase Hydrogenation of Furfuraldehyde to Furfuryl Alcohol over a Pt / C Catalyst. Ind. Eng. Chem. Resour. 2003, 42, 3881–3885. [CrossRef]

10. Zhao, Y. Facile synthesis of Pd nanoparticles on SiO2 for hydrogenation of biomass-derived furfural. Environ. Chem. Lett. 2014, 12, 185–190. [CrossRef] 11. Mironenko, R.M.; Belskaya, O.B.; Gulyaeva, T.I.; Nizovskii, A.I.; Kalinkin, A.V.; Bukhtiyarov, V.I.; Lavrenov, A.V.; Likholobov, V.A. Eﬀect of the nature of carbon support on the formation of active sites in Pd/C and Ru/C catalysts for hydrogenation of furfural. Catal. Today 2015, 249, 145–152. [CrossRef] 12. Fulajtárova, K.; Soták, T.; Hronec, M.; Vávra, I.; Dobroˇcka, E.; Omastová, M. Aqueous phase hydrogenation of furfural to furfuryl alcohol over Pd–Cu catalysts. Appl. Catal. A 2015, 502, 78–85. [CrossRef] Energies 2020, 13, 1002 10 of 10

13. Wu, J.; Shen, Y.; Liu, C.; Wang, H.; Geng, C.; Zhang, Z. Vapor phase hydrogenation of furfural to furfuryl

alcohol over environmentally friendly Cu-Ca/SiO2 catalyst. Catal. Commun. 2005, 6, 633–637. [CrossRef] 14. Villaverde, M.M.; Bertero, N.M.; Garetto, T.F.; Marchi, A.J. Selective liquid-phase hydrogenation of furfural to furfuryl alcohol over Cu-based catalysts. Catal. Today 2013, 213, 87–92. [CrossRef] 15. Khromova, S.A.; Bykova, M.V.; Bulavchenko, O.A.; Ermakov, D.Y.; Saraev, A.A.; Kaichev, V.V.; Venderbosch, R.H.; Yakovlev, V.A. Furfural Hydrogenation to Furfuryl Alcohol over Bimetallic Ni–Cu Sol–Gel Catalyst: A Model Reaction for Conversion of Oxygenates in Pyrolysis Liquids. Top. Catal. 2016, 59, 1413–1423. [CrossRef] 16. Sitthisa, S.; Sooknoi, T.; Ma, Y.; Balbuena, P.B.; Resasco, D.E. Kinetics and mechanism of hydrogenation of

furfural on Cu/SiO2 catalysts. J. Catal. 2011, 277, 1–13. [CrossRef] 17. Smirnov, A.A.; Shilov, I.N.; Alekseeva, M.V.; Selishcheva, S.A.; Yakovlev, V.A. Study of the Composition Effect of Molybdenum-Modified Nickel–Copper Catalysts on Their Activity and Selectivity in the Hydrogenation of Furfural to Different Valuable Chemicals. Catal. Ind. 2018, 10, 228–236. [CrossRef] 18. Audemar, M.; Ciotonea, C.; De Oliveira Vigier, K.; Royer, S.; Ungureanu, A.; Dragoi, B.; Dumitriu, E.; Jérôme, F. Selective Hydrogenation of Furfural to Furfuryl Alcohol in the Presence of a Recyclable Cobalt/SBA-15 Catalyst. ChemSusChem 2015, 8, 1885–1891. [CrossRef] 19. Villaverde, M.M.; Garetto, T.F.; Marchi, A.J. Liquid-phase transfer hydrogenation of furfural to furfuryl alcohol on Cu–Mg–Al catalysts. Catal. Commun. 2015, 58, 6–10. [CrossRef] 20. Srivastava, S.; Mohanty, P.; Parikh, J.K.; Dalai, A.K.; Amritphale, S.S.; Khare, A.K. Cr-free Co–Cu/SBA-15 catalysts for hydrogenation of biomass-derived α-, β-unsaturated aldehyde to alcohol. Chin. J. Catal. 2015, 36, 933–942. [CrossRef]

21. Wei, S.; Cui, H.; Wang, J.; Zhuo, S.; Yi, W.; Wang, L.; Li, Z. Preparation and activity evaluation of NiMoB/-Al2O3 catalyst by liquid-phase furfural hydrogenation. Particuology 2011, 9, 69–74. [CrossRef] 22. Garcia-Olmo, J.; Yepez, A.; Balu, A.M.; Prinsen, P.; Garcia, A.; Mazière, A.; Len, C.; Luque, R. Activity of continuous flow synthesized Pd-based nanocatalysts in the flow hydroconversion of furfural. Tetrahedron 2017, 73, 5599–5604. [CrossRef] 23. Wang, Y.; Prinsen, P.; Triantafyllidis, K.S.; Karakoulia, S.A.; Yepez, A.; Len, C.; Luque, R. Batch versus continuous flow performance of supported mono- and bimetallic nickel catalysts for catalytic transfer hydrogenation of furfural in isopropanol. ChemCatChem. 2018, 10, 3459–3468. [CrossRef] 24. Wang, Y.; Prinsen, P.; Triantafyllidis, K.S.; Karakoulia, S.A.; Trikalitis, P.N.; Yepez, A.; Len, C.; Luque, R. Comparative study of supported monometallic catalysts in the liquid-phase hydrogenation of furfural: Batch and continuous flow. ACS Sustainable Chem. Eng. 2018, 6, 9831–9844. [CrossRef] 25. Selishcheva, S.A.; Smirnov, A.A.; Fedorov, A.V.; Ermakov, D.Y.; Gulyaeva, Y.K.; Yakovlev, V.A. Production of Furfuryl Alcohol in the Presence of Copper-Containing Catalysts in the Selective Hydrogenation of Furfural. Catal. Ind. 2019, 11, 216–223. [CrossRef] 26. Wang, Y.; Deyang, P.; Rodriguez-Padrón, D.; Len, C. Recent Advances in Catalytic Hydrogenation of Furfural. Catalysts 2019, 9, 796. [CrossRef] 27. Audemar, M.; Ramdani, W.; Junhui, T.; Ifrim, A.R.; Ungureanu, A.; Jérôme, F.; Royer, S.; De Oliveira Vigier, K.

Selective hydrogenation of xylose to xylitol over Co/SiO2 catalysts. ChemCatChem 2020.[CrossRef] 28. Taylor, M.J.; Jiang, L.; Reichert, J.; Papageorgiou, A.C.; Beaumont, S.K.; Wilson, K.; Lee, A.F.; Barth, J.V.; Kyriakou, G. Catalytic Hydrogenation and Hydrodeoxygenation of Furfural over Pt(111): A Model System for the Rational Design and Operation of Practical Biomass Conversion Catalysts. J. Phys. Chem. C Nanomater. Interfaces 2017, 121, 8490–8497. [CrossRef]

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