energies 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 verycontinuous little attention flow reactors. has been We paiddemonstrate to the process here that, in adespite 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.