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

: Furfural is a platform molecule obtained from hemicellulose. Among the products that 19 can be produced from furfural, furfuryl alcohol is one of the most studied. It is synthesized at an 20 industrial scale in the presence of CuCr catalyst but this process suffers from an environmental 21 negative impact. Here, we demonstrate that a non-noble metal catalyst (Co/SiO 2 ) was active (100% 22 conversion of furfural) and selective (100% selectivity to furfuryl alcohol) in the hydrogenation of 23 furfural to furfuryl alcohol at 150°C under 20 bar of hydrogen. This catalyst was recyclable up to 3 24 cycles and then the activity decreased. Thus, a comparison between batch and continuous flow 25 reactor shows that changing the reactor type helps to increase the stability of the catalyst and the 26 space-time yield. This study shows that continuous flow reactor can be a solution when a catalyst 27 suffered from a lack of stability in batch process. 28


Introduction
Motor fuels components and fine chemicals can be produced from non-edible plant-based feedstocks using catalytic processes.Among all the 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][2][3][4].Furfural production is based on acid hydrolysis of hemicellulose [5].One interesting reaction from furfural is hydrogenation reaction that 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), etc .Currently, around 50% of the 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, etc.During the hydrogenation of furfural to FOL many side reactions can occur such as the formation of THFA, 2methylfurane, etc (Scheme 1).The precise control of the selectivity of the reaction by using an appropriate and stable catalyst is highly demanded.Copper-chromium (CuCr) alloy is the catalyst used on industrial scale to produce FOL with a high yield (98%) [5].This catalyst has some drawbacks such as the presence of chromium which can contaminate FOL and hampers its use in pharmaceutical industry for instance.Moreover chromiumcontaining catalysts can be deactiveted due to shielding of copper by chromium [6].Many researches were devoted to the replacement of this catalyst by catalysts based on noble metals such as Pt and Pd [7][8][9][10][11], leading to an increase of the process cost.Furthermore, FOL selectivity is lower in the presence of these catalysts than in the presence of chromium-copper systems.To increase the selectivity to FOL, the addition of metals such as Cu to Pd based catalysts results in the improvement of the selectivity to FOL (98% of FOL was obtained) [12].Non-noble metals catalysts were also studied in the selective hydrogenation of furfural to FOL such as supported Ni, Cu, Fe, Mo, Zn, etc. [13][14][15][16][17][18].
Various methods were used for the synthesis of catalysts, additives, process conditions, and various solvents in the case of a liquid phase process [19][20][21].Several drawbacks are present using this system: deactivation due to the sintering of active species; poisoning of the catalyst surface by coke formation; low selectivity of FOL; high temperature and pressure.Up to now, several studies of selective hydrogenation of furfural to FOL are performed in the liquid phase in batch reactors using different solvents, but very little attention is paid to the process in a flow system [22][23][24][25][26].To this aim, hydrogenation of furfural to FOL was studied in the presence of Co/SiO2 catalyst in batch and in continuous flow reactors.We demonstrate here that despite the high selectivity and activity of the Co/SiO2 catalyst in the hydrogenation of furfural in batch reactor, the reaction performed in a continuous flow reactor led to a higher space time yield (STY=quantity of FOL produced per unit volume unit per time unit).STY was three times higher when the hydrogenation of furfural is performed in a continuous flow reactor than in a batch process.The selectivity was slightly lower in continuous flow reactor than in bath reactor but the activity was similar.The catalyst was more stable under flow reactor than in batch reactor.
Catalyst characterizations: Co/SiO2 catalyst was characterized by ICP-OES, XRD analysis, N2physisorption, Transmission Electronic Microscopy, Thermal analysis.Perkin Elmer Optima 2000 DV instrument is used for ICP analysis.The catalysts was first dissolved in a mixture of HF and HCl under micro-wave for digestion before analysis.XRD analysis is performed using a Bruker Empyrean with a Co cathode.N2-physisorption experiments were obtained on an Autosorb 1-MP instrument, at 77K.The catalysts are treated under vacuum à 350 °C for 3 h and the surface area, the pore size as well as the pore volume are determined as described previously [27].TEM experiments are performed on a JEOL 2100 UHR instrument operated at 200 kV, equipped with a LaB6 source and a Gatan ultra scan camera.Thermal analysis are 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 is added to 9 g of ethanol and 50 mg of catalyst is added in a batch reactor (75 mL).The hydrogen pressure is fixed to the desired one.Then, the temperature is increased up to the desired reaction temperature i.e. 150 °C.At the end of the reaction, the reactor is cooled down to room temperature, and liquid phase is analysed.

General procedure for the hydrogenation of furfural in flow reactor:
The experiments were carried out in H-Cube ProTM Flow Reactor ThalesNanoTM, Hungary, connected to a HPLC pump to supply a continuous feed of 10 wt% furfural in ethanol.A 70 mm catalyst cartridge (0.88 mL empty volume) catalyst was packed with 260 mg catalyst by applying vacuum suction at the bottom of the cartridge.
The total flow 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-ethanol mixture.The flow was continued until the desired temperature and hydrodynamic pressurization

Catalyst charcaterizations
The catalyst was prepared using Incipient Wetness Impregnation (IWI) methodAfter calcination of the solid at 500°C under air, oxide precursor, Co3O4/SiO2, is obtained, confirmed by XRD analysis (Figure 1(a)), with the presence of peaks corresponding to the awaited position (PDF file 01-1152).
Considering the width of the reflections, the cobalt crystal size is relatively low (<10 nm).This result confirms that using this impregnation method, high dispersion of the cobalt oxide phase was achieved.The loading of cobalt was evaluated by ICP-OES and was of 9 wt.% The catalyst is reduced under H2 flow at 500°C for 10h prior to catalytic test leading to a large surface area and large pore diameter, suitable for liquid phase hydrogenation reaction.Moreover, the limited evolution of the textural properties indicates adequate stabilities of the support (Table 1).(2) and electron diffraction analysis (3).
The hydrogenation of furfural was performed in a batch reactor starting from 1g of furfural in 9g of ethanol in the presence of 50 mg of catalyst (Table 2).that can be favored by a higher solubility of hydrogen than at lower pressure of hydrogen (Table 2, entry 3).In both conditions, FOL is further hydrogenated to THFA.A similar trend was observed with an increase of the temperature from 150 to 180°C.With the increase of the temperature, THFA was also observed as a by-product and the selectivity of FOL decreased on the benefit of the formation of THFA.Based on these results, it is of prime importance to control the kinetic of the reaction in 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 hydrogen, 1h f reaction).This is a key parameter as in batch liquid phase reactor, catalysts used 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 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 9g of ethanol.Four cycles were performed under 20 bar of hydrogen for 1h of reaction at 150°C (Figure 3).FOL selectivity and conversion of furfural slightly decreased after the third cycle, which can be ascribed to the work up process.However, after the fourth cycle the conversion of furfural dropped from 100 to 69% and the selectivity to FOL decreased also (88%).This can be due to the poisoning of cobalt in the solution as it is already mentioned in the literature or to the leaching of cobalt [24,26] leading to the formation of by-products such as THFA and other unidentified by-products.To confirm these hypotheses, TGA analysis of the spent catalyst were performed and compared to the TGA analysis of the fresh catalyst (Figure 4).
It was shown that 18% of weight was lost during the TGA analysis of the spent catalyst whereas only 7% was lost during the fresh catalyst analysis.This increase of the weight loss can be due to the deposition of carbon species on the catalyst due to the sorption of furanic compounds.To confirm this hypothesis MET analysis of the spent catalyst was performed (Figure 5).It was clearly shown by EDS that carbon was deposited on the catalyst surface leading to less accessibility of furfural to active sites and a decrease of the selectivity to FOL.Several zones of the catalyst contain carbon deposit.In order to prevent this deactivation, hydrogenation of furfural was performed in continuous flow reactor.

Continuous flow reactor
The hydrogenation of furfural was performed while keeping the concentration of furfural used for batch experiments (5g of furfural in 45 g of ethanol) and the catalyst amount of 260 mg to fill the catalyst cartridge.In a first set of experiments, the flow rate of the alcoholic solution of furfural was studied.To this aim, the flow rate was increased from 0.2 to 0.5 mL.min -1 (Figure 6).When the flow rate of the furfural solution increased from 0.2 to 0.3 mL min -1 , similar trend in the conversion of furfural was obtained, a drop of the conversion from 94% to 50% being observed in the function of TOS (Time On Stream) from 0 to 180 min.The selectivity was maintained at around 97% for 0.3 mL.min -1 whereas under 0.2 mL.min -1 of furfural solution, the selectivity to FOL decreased from 97 to 74%, which is due to the increase of residence time that favored the formation of THFA.A further increase of the furfural solution flow rate up to 0.5 mL.min -1 led to a significant drop of furfural conversion from 90% to 40% after 110 min of reaction.The FOL selectivity was kept constant.Based on these results, it was decided to keep the flow rate of 0.3 mL min -1 for the following experiments.The effect of the pressure of hydrogen was then studied from 20 to 60 bar (Figure 7).
Increasing the pressure of hydrogen led to an increase of the conversion of furfural from 92 to 100%.
It was interesting to see that under 60 bar of hydrogen the conversion was always 100% when TOS increased up to 180 min whereas at lower pressure a slight decrease of the conversion was observed.
Concerning the selectivity, it was kept constant independently of the hydrogen pressure but the selectivity to FOL was lower under 60 bar of hydrogen, due to the formation of THFA as a byproducts.This can be explained by a higher solubility of hydrogen due to its pressure as previously shown in batch reactions.

Discussion-Conclusion
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.% in ethanol.However, the stability of the catalyst is not optimal as it is shown by the catalyst recycling.TGA and MET analysis showed that carbon was adsorbed on the catalyst surface due to the sorption of furanic molecules in batch reactions as it is already mentioned in the literature [28].The recyclability of the catalyst was thus hampered by this coke formation on the catalyst surface.
The hydrogenation of furfural to FOL in a continuous flow reactor can afford a high conversion of furfural with a selectivity higher than 90% under 60 bar of hydrogen at 150°C.It means that the catalyst was not poisoned when a hydrogen pressure of 60 bar was used but the selectivity was slightly lower due to further hydrogenation to THFA.At an industrial point of view, it can be interesting to see the space time yield of the reaction (Table 2).The space time yield (STY) was calculated and it was higher if the hydrogenation of furfural was performed in a continuous flow reactor than in a batch reactor.Thus, under similar conditions of pressure and temperature, the STY was 13.2 g.L -1 .h - for batch reaction versus 16.6 g.L -1 .h - for continuous flow reaction.By increasing the hydrogen pressure, the STY increased in continuous flow reaction from 16.6 to 30.6 g.L -1 .h - .These results show that using a same catalyst, the hydrogenation of furfural to FOL is more performant in continuous flow reactor that in batch reactor.In order to go deeply in this comparison from these two processes, it was interesting to compare the stability of the catalyst.In batch reactor, the catalyst was recyclable up to 3 times and lost its activity but the selectivity was kept constant.This was due to an adsorption of the furanic molecules on the catalyst surface.For the continuous flow reactor, TGA analysis was performed on the spent catalyst and only a slight difference in the weight loss was observed between the fresh and the spent catalyst (Figure 4).The specific surface area was similar before and after reaction.ICP analysis of spent catalyst showed that there was no leaching of cobalt (9 wt.% before and after reaction).TEM showed that the particle size did not change after reaction and that no carbon was formed on the catalyst surface (Figure 8).

( 20 -
60 bar) of the reactor module were reached.Depending on the flow rate used (0.2-0.5 mL min −1 ), the reaction time was set (20-50 min) before collecting the first sample (time zero).The samples were collected after regular time intervals.Analytical methods : yields to furfuryl alcohol and conversion of furfural are determined by external calibration at 25°C by HPLC equipped with a nucleosil 100-5 C18 column (250 mm and diameter of 4.6 mm), a Shimadzu LC-20AT pump, and a Shimadzu RID-10 A detector using acetonitrile/water (10:90) as the mobile phase (0.6 mLmin -1 ).Continuous flow results were detected on a gas chromatograph (HP, 14009 Arcade, New York, United States) coupled with a FID detector equipped with a Supelco 2-8047-U capillary column (15 m x0.25 mm i.d. and 0.25 μm film thickness, Alltech Part No.31163-01).The flow rate of the carrier gas (H2) was 1 mL min −1 .The injector temperature was 250 °C and the oven started at 70 °C for 1 min, and the temperature was increased up to 250 °C at a rate of 20 °C min −1 , and the temperature was then maintained at 250°C for 10 min.

Figure 2 3 Figure 2 .
Figure2shows that the particle sizes and localization throughout the support porosity are not homogeneous and aggregates of NPs of cobalt 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 cobalt particle and hexagonal cobalt phase was observed using electron diffraction. 1 2 3 Figure 2. TEM images recorded for reduced Co/SiO2 catalyst (1), Energy Dispersive X-Ray Spectroscopy (EDS)

Figure 4 . 3 EDS4Figure 5 .
Figure 4. TGA analysis of the reduced Co/SiO2 catalyst before reaction and after reaction under batch and flow conditions.

Figure 6 .
Figure 6.Effect of the alcoholic furfural solution (5g of furfural in 45g of ethanol) flow rate.150 °C, 20 bar of hydrogen in the presence of 260 mg of Co/SiO2.(A) conversion vs. TOS.(B) FOL selectivity vs. TOS.

Figure 7 .
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 of 260 mg of Co/SiO2.(A) conversion vs. TOS.(B) FOL selectivity vs. TOS.

Table 2 .
Hydrogenation of furfural in a batch reactor1.Table2, entry 1).By prolonging the reaction time, the yield of FOL decreased from 100% to 65% due to further hydrogenation of FOL to THFA (Table2, entry 2).This result shows that Co/SiO2 was active and selective in the hydrogenation of furfural to FOL.An increase of hydrogen pressure led to a decrease of FOL yield due to the further hydrogenation of FOL to THFA

Table 3 .
Hydrogenation of furfural: effect of the reactor 1 .