Maïté Audemar

Yantao Wang

Deyang Zhao

Sébastien Royer
email: sebastien.royer@univ-lille.fr

François Jérôme

Christophe Len
email: christophe.len@chimieparistech.psl.eu

Karine De Oliveira Vigier

Synthesis of furfuryl alcohol from furfural: a comparison between batch and continuous flow reactors

Keywords: Continuous flow, batch reactor, furfural, furfuryl alcohol, hydrogenation

Furfural is a platform molecule obtained from hemicellulose. Among the products that can be produced from furfural, furfuryl alcohol is one of the most 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 reactor shows that changing the reactor type helps to increase the stability of the catalyst and the space-time yield. This study shows that continuous flow reactor can be a solution when a catalyst suffered from a lack of stability in batch process.

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][START_REF] Mariscal | A renewable and versatile platform molecule for the synthesis of chemicals and fuels[END_REF][START_REF] Bonacci | Montmorillonite K10-Catalyzed Solvent-Free Conversion of Furfural into Cyclopentenones[END_REF][START_REF] Smirnov | Acetalization Catalysts for Synthesis of Valuable Oxygenated Fuel Additives from Glycerol[END_REF]. Furfural production is based on acid hydrolysis of hemicellulose [START_REF] Morozov | Furfural Production[END_REF]. 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%) [START_REF] Morozov | Furfural Production[END_REF]. 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 [START_REF] Liu | Deactivation mechanistic studies of copper chromite catalyst for selective hydrogenation of 2-furfuraldehyde[END_REF]. Many researches were devoted to the replacement of this catalyst by catalysts based on noble metals such as Pt and Pd [START_REF] Musci | Aqueous phase hydrogenation of furfural using carbon-supported Ru and RuSn catalysts[END_REF][START_REF] Vorotnikov | DFT study of furfural conversion to furan, furfuryl alcohol, and 2-methylfuran on Pd(111)[END_REF][START_REF] Vaidya | Kinetics of Liquid-Phase Hydrogenation of Furfuraldehyde to Furfuryl Alcohol over a Pt / C Catalyst[END_REF][START_REF] Zhao | Facile synthesis of Pd nanoparticles on SiO2 for hydrogenation of biomass-derived furfural[END_REF][START_REF] Mironenko | Effect of the nature of carbon support on the formation of active sites in Pd/C and Ru/C catalysts for hydrogenation of furfural[END_REF], 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) [START_REF] Fulajtárova | Aqueous phase hydrogenation of furfural to furfuryl alcohol over Pd-Cu catalysts[END_REF]. Non-noble metals catalysts were also studied in the selective hydrogenation of furfural to FOL such as supported Ni, Cu, Fe, Mo, Zn, etc. [START_REF] Wu | Vapor phase hydrogenation of furfural to furfuryl alcohol over environmentally friendly Cu-Ca/SiO2 catalyst[END_REF][START_REF] Villaverde | Selective liquid-phase hydrogenation of furfural to furfuryl alcohol over Cu-based catalysts[END_REF][START_REF] Khromova | Furfural Hydrogenation to Furfuryl Alcohol over Bimetallic Ni-Cu Sol-Gel Catalyst: A Model Reaction for Conversion of Oxygenates in Pyrolysis Liquids[END_REF][START_REF] Sitthisa | Kinetics and mechanism of hydrogenation of furfural on Cu/SiO2 catalysts[END_REF][START_REF] Smirnov | 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[END_REF][START_REF] Audemar | Selective Hydrogenation of Furfural to Furfuryl Alcohol in the Presence of a Recyclable Cobalt/SBA-15 Catalyst[END_REF].

Various methods were used for the synthesis of catalysts, additives, process conditions, and various solvents in the case of a liquid phase process [START_REF] Villaverde | Liquid-phase transfer hydrogenation of furfural to furfuryl alcohol on Cu-Mg-Al catalysts[END_REF][START_REF] Srivastava | Cr-free Co-Cu/SBA-15 catalysts for hydrogenation of biomass-derived α-, β-unsaturated aldehyde to alcohol[END_REF][START_REF] Wei | Preparation and activity evaluation of NiMoB/-Al2O3 catalyst by liquid-phase furfural hydrogenation[END_REF]. 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 [START_REF] Garcia-Olmo | Activity of continuous flow synthesized Pd-based nanocatalysts in the flow hydroconversion of furfural[END_REF][START_REF] Wang | Batch vs continuous flow performance of supported mono-and bimetallic nickel catalysts for catalytic transfer hydrogenation of furfural in isopropanol[END_REF][START_REF] Wang | Comparative study of supported monometallic catalysts in the liquid-phase hydrogenation of furfural : batch and continuous flow[END_REF][START_REF] Selishcheva | Production of Furfuryl Alcohol in the Presence of Copper-Containing Catalysts in the Selective Hydrogenation of Furfural[END_REF][START_REF] Wang | Recent Advances in Catalytic Hydrogenation of Furfural[END_REF]. 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.

Materials and Methods

Catalyst preparation: The Co3O4/SiO2 material, with a metal loading of 10 wt.% is prepared using incipient wetness impregnation method as described previously [START_REF] Audemar | Selective hydrogenation of xylose to xylitol over Co/SiO2 catalysts[END_REF] using Co(NO3)2•and aerosil silica (380). The dry solid is calcined at 500 °C for 6 h to obtain the Co2O3/SiO2 sample. Co2O3/SiO2

(around 100 mg) is reduced under hydrogen flow (3 L.h -1 ) at 500 °C for 10 h.

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 [START_REF] Audemar | Selective hydrogenation of xylose to xylitol over Co/SiO2 catalysts[END_REF]. 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 [START_REF] Wang | Comparative study of supported monometallic catalysts in the liquid-phase hydrogenation of furfural : batch and continuous flow[END_REF][START_REF] Wang | Recent Advances in Catalytic Hydrogenation of Furfural[END_REF] 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 [START_REF] Taylor | Catalytic Hydrogenation and Hydrodeoxygenation of Furfural over Pt(111): A Model System for the Rational Design and Operation of Practical Biomass Conversion Catalysts[END_REF]. 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 -1 for batch reaction versus 16.6 g.L -1 .h -1 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 -1 . 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 -

 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 1 .

 1 Figure 1. XRD analysis of non reduced Co/SiO2 catalyst (vertical bars, Co3O4 reference PDF file 01-1152).

Figure 2 3 Figure 2 .

 232 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 3 .

 3 Figure 3. Recycling of Co/SiO2. 1g of furfural, 9g of ethanol, T = 150°C, 20 bar of hydrogen, 1h of reaction.

Figure 4 . 3 EDS4Figure 5 .

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

Figure 6 .

 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 .

 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.

  2 reaction time = 1h.3 time on stream = 3h; flow rate of alcoholic solution of furfural = 0.3 mL.min-1

Table 1 .

 1 Main characteristics of the Co3O4/SiO2 and reduced Co/SiO2 materials

		Co3O4/SiO2	Co/SiO2
	XRD phase	Poorly crystalized Co3O4	n.d. [a]
	Dpart./b nm	n. d .[a]	Aggregates 10 to >100 nm
			Crystals <20 nm
	SBET/c m 2 .g -1	185	169
	Vp/ cm 3 .g -1	0.71	0.63
	Dp/ nm	15.0	14.7

1

[a] n.d.: not determined; [b] mean particle size obtained by TEM image observation; [c] surface area (SBET), pore volume (Vp) and pore diameter (Dp) issued from N2 physisorption at 77K.

Table 2 .

 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

		Reaction time (h)	PH2 (bar)	Temperature (°C)	Conversion (%)	Selectivity to FOL (%)	STY (g.L -1 .h -1 )
	entry 1	1	20	150	100	100	13.2
	entry 2	1,5	20	150	100	65	5.9
	entry 3	1	40	150	100	85	11,4
	entry 4	1	20	180	100	80	10,8
			1 Furfural = 1g, ethanol = 9g, Co/SiO2 = 50 mg
	We were pleased to see that furfural conversion reached 100% at 150°C after 1h of reaction, with
	a FOL yield of 100% (

Table 3 .

 3 Hydrogenation of furfural: effect of the reactor 1 .

	Reactor	PH2 (bar)	Conversion (%)

Selectivity to FOL (%) STY (g.L -1 .h -1 )

  furfural in ethanol 10wt.%,, Co/SiO2 = 5 wt% of furfural solution, 150°C.

	Batch 2	20	100	100	13.2
		20	52	50	16.6
	Flow 3	40	81	77	26.6
		60	100	92	30.6

1

Acknowledgments:

In this section you can acknowledge any support given which is not covered by the author contribution or funding sections. This may include administrative and technical support, or donations in kind (e.g., materials used for experiments).

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).

The difference of the two processes is the sorption of furanic compounds on the catalyst. In batch processes the catalysts is in contact with the reactant and the products formed which led to a higher contact time between the furanic molecules and the catalysts than in continuous flow 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 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 3h and that the selectivity was kept constant above 90%. This can be due to an increase of the hydrogen solubility which is also observed during the hydrogenation in batch reactor, since FOL produced is then hydrogenated to THFA with an increase of the hydrogen pressure from 20 to 40 bar.

In conclusion, this study shows that continuous flow reactor can be a solution when a catalyst suffers from a 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.

Supplementary Materials:

The following are available online at www.mdpi.com/xxx/s1, Figure S1: title, Table S1: title, Video S1: title.

Author


Maïté Audemar

Wahiba Ramdani

Tang Junhui

Raluca Andreea

Adrian Ifrim

François Ungureanu

Sébastien Jerome

Karine Royer

Oliveira Vigier

Selective Hydrogenation of Xylose to Xylitol over Co/SiO 2 Catalysts

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Selective hydrogenation of xylose to xylitol over Co/SiO2 catalysts

Maïté Audemar, [a] Ifrim Andreea Raluca, [a] Tang Junhui, [a] Adrian Ungureanu, [b] François Jérôme, [a] Sébastien Royer, [c] * and Karine de Oliveira Vigier* [a] Dedication ((optional))

Abstract: Xylose can be selectively converted to xylitol in water, with an optimized yield of 98%, in the presence of a simple silica supported metallic cobalt -Co/SiO2 -catalyst. This catalyst displays initial outstanding catalytic properties in proper solvent, the best one being pure water. Recyclability studies show a moderate deactivation of the catalyst, while selectivity to xylitol remains almost unchanged after 4 cycles, confirming the promising character of this catalyst formulation for the xylitol production process.

Xylose is a product issued from the hydrolysis of beechwood hemicelluloses. Different routes exists for the valorisation of xylose, amongst the selective hydrogenation to xylitol (scheme 1). Xylitol is an extensively used molecule in food, cosmetics, and pharmaceutical industry [1][START_REF]Reference 2: Example for Books[END_REF][3][4]. Xylitol has then been identified as one of the top-twelve value added chemicals that can be obtained from biomass [5]. As an example, xylitol is a valuable water soluble sweetener due to its atypical properties amongst, compared to sucrose, a higher sweetness strength and a lower energy capacity. Today, xylitol is widely incorporated in daily consumed products (chewing gums, chocolates, toothpastes, etc.) [6]. With an estimated market of 340 M$, xylitol is becoming the most popular "natural" sweeteners [7]. Industrial production of xylitol by selective catalytic hydrogenation appears as an economically viable approach. Xylitol can be produced by catalytic hydrogenation in the presence of RANEY® nickel. Reaction is performed in liquid phase, under high hydrogen pressure and temperature in batch-type reactors [8]. Selectivity above 90%, at high conversion degree can be obtained at 80-130°C under 40-70 bar of H2 (5 wt.% catalyst of the xylose mass present at 40-60 wt.% in water) in the presence of RANEY® nickel. Despite the high selectivity and activity reached using this catalyst, with economic advantages related to the use of noble metal free catalysts for the reaction, the intrinsic properties of the nickel, amongst toxicity and limited stability,[8-9] makes the research of efficient catalyst substitutes. Indeed, the use of xylitol for food applications imply the absence of any trace of nickel in final product, nickel being recognized as a carcinogen compound. Most of recent studies were devoted to the study of supported noble metals, such as Ru, Rh and Pd, in monometallic or bimetallic catalytic systems, with the possible adding of dopants as Sn [8][9][10][11][12][13][14][15] Ru appeared as the most efficient. Hernandez-Mejia et al. [16] reported that xylose can be selectively converted to xylitol in the presence of rutile phase TiO2 supported Ru (1 wt.%), despite the low surface area displayed by the rutile support. Authors then reported xylose yields higher than 90%, for a reaction performed in water at <140°C under 20 bar of H2 and at a catalyst to xylose ratio (wt./wt.) of 1. Compared with rutile, better dispersion of Ru over anatase does not allowed to reach comparable activity, and conversion was not selective to xylitol. As recently reviewed by Delgado Arcano et al., nickel and ruthenium catalysts then remains the most widely studied phases for the xylose to xylitol reaction, while titania is an adapted support [15] Replacement of noble metal in catalytic formulation is however preferable due to limited resources, fluctuating costs, and consumption by high technology markets. Alternative catalysts, based on earth abundant transition metals such as cobalt, and in a lower extent copper, displaying lower toxicity than nickel are presenting satisfying activity and stability in water under neutral to basic conditions for some hydrogenation reactions [17]. Then, for some hydrogenation of biomass derived molecules, cobalt formulations are considered as a viable formulations for hydrogenation reactions. Metallic cobalt indeed present interesting activity for the C=O bonds, as already observed for the selective conversion of cinnamaldehyde to cinnamyl alcohol, [18][19][20] furfural to furfuryl alcohol, [17,21] HMF hydroconversion,[17,22] etc. With this study, we demonstrated that, using a non noble metal catalyst based on cobalt supported on a commercially Aerosil® 380 silica, high yield to xylitol are obtained when reaction conditions are optimized, with a catalyst that can be reused at least three time before notable deactivation. Silica supported cobalt oxide is prepared by Incipient Wetness Impregnation (IWI), starting from hydrated cobalt nitrate precursor, a classical and industrialized approach. Oxide precursor, Co3O4/SiO2, is obtained by calcination of the solid at 500°C under air. Formation of Co3O4 phase is confirmed by XRD analysis (Figure 1(a)), with the presence of visible broad and poorly intense reflections corresponding to the awaited position (ICDD file n°42-1467). Considering the width of the reflections, awaited crystal domain size of the cobalt phase is relatively low (<10 nm). This results confirms that the impregnation used for the catalyst preparation is efficient to afford a high dispersion of the cobalt oxide phase. However, in view of the catalyst application, the catalyst is first reduced under H2 flow at 500°C for 10h. Main characteristics of the reduced catalyst (Co/SiO2) are presented in Table 1. Solid retains satisfying textural properties, mainly large surface area and large pore diameter, compatible for liquid phase catalytic reaction. In addition, the limited evolution of the textural properties indicate adequate stabilities of the support (Table 1). Dpart./ [b] nm n. d. [a] Aggregates 10 to >100 nm Crystals <20 nm SBET/c m 2 g -1 185 169

Vp/ cm 3 g -1 0.71 0.63 Dp/ nm 15.0 14.7

[a] n.d.: not determined; [b] mean particle size obtained by TEM image observation; c, surface area (SBET), pore volume (Vp) and pore diameter (Dp) issued from N2 physisorption at 77K.

Main Text Paragraphs----with Indentation.

Experimental Section

Experimental Details.

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Entry for the

Scheme 1 .

 1 Scheme 1. Hydrogenation of xylose to xylitol.

Figure 1 .

 1 Figure 1. XRD analysis of Co/SiO2 catalyst before and after reaction (vertical bars, reference ICDD 42-1467).

Table 1 .

 1 Main characteristic of the Co2O3/SiO2 and reduced Co/SiO2 materials.

		Co3O4/SiO2	Co/SiO2
	XRD phase	Poorly crystalized	n.d. [a]
		Co3O4

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Shi Jiang

Changru Ma

Eric Muller

Marc Pera-Titus

François Jerôme

Karine De Oliveira Vigier

Selective synthesis of THF-derived amines from biomass-derived carbonyl compounds

Keywords: Biomass, Furfural, Aldol Condensation, Amination, Palladium

Selective transformation of biomass or biomass-based feedstocks into value-added amines is highly desirable and constitutes one of the most important challenges in Catalysis. As of today, only few amination studies have been reported on bio-based substrates, targeting mainly the reductive amination of small platform molecules issued directly from biomass. Here we present a simple and highly efficient system using NH3 or amines as a nitrogen source and molecular hydrogen as reducing agent for the transformation of furan-derived ketones into THF-derived amines over Pd/Al2O3 (up to 98% yield). Detailed analysis of the reaction system provided insight into the reaction mechanism. To further realize the production of THF-derived amines, a one-pot two-step strategy combining C-C and C-N condensation reactions was attempted. A high yield (85%) towards 5-methyl-1-(tetrahydrofuran-2-yl)hexan-3-amine could be successfully achieved starting directly from furfural.

Introduction

Amines represent a valuable class of compounds of wide interest in the chemical industry due to their nucleophilic properties, conferring them high reactivity. [START_REF] Lawrence | Amines: Synthesis, Properties and Applications[END_REF][START_REF] Koutinas | Introduction to Chemicals from Biomass[END_REF][START_REF] Gandini | Progress of Polymers from Renewable Resources: Furans, Vegetable Oils, and Polysaccharides[END_REF][START_REF] Delidovich | Alternative Monomers Based on Lignocellulose and Their Use for Polymer Production[END_REF][START_REF] Willcox | A General Catalytic β-C-H Carbonylation of Aliphatic Amines to β-Lactams[END_REF][START_REF] Jagadeesh | MOF-derived Cobalt Nanoparticles Catalyze a General Synthesis of Amines[END_REF][START_REF] Pelckmans | Bio-Based Amines through Sustainable Heterogeneous Catalysis[END_REF] Among the different types of amines, primary amines constitute valuable fine and bulk chemicals, which serve as versatile feedstocks and key intermediates for the synthesis of advanced chemicals, life science molecules and polymers. [START_REF] Froidevaux | Biobased Amines: From Synthesis to Polymers; Present and Future[END_REF][START_REF] Talwar | Primary Amines by Transfer Hydrogenative Reductive Amination of Ketones by Using Cyclometalated Ir III Catalysts[END_REF][START_REF] Senthamarai | Simple ruthenium-catalyzed reductive amination enables the synthesis of a broad range of primary amines[END_REF][START_REF] Tan | Asymmetric Synthesis of Chiral Primary Amines by Ruthenium Catalyzed Direct Reductive Amination of Alkyl Aryl Ketones with Ammonium Salts and Molecular H2[END_REF] Industrially relevant aliphatic and aromatic amines, as well as aminoalcohols, are currently manufactured from fossil resources. [START_REF] Roose | Aliphatic, Ullmann's Encyclopedia of Industrial Chemistry[END_REF] In contrast, transformation of biomass into valuable nitrogen-containing chemicals is still very limited owing to the lack of efficient amination methods for biomass-derived molecules. [START_REF] Pelckmans | Bio-Based Amines through Sustainable Heterogeneous Catalysis[END_REF][START_REF] Froidevaux | Biobased Amines: From Synthesis to Polymers; Present and Future[END_REF][START_REF] Deng | Catalytic amino acid production from biomass-derived intermediates[END_REF][START_REF] Li | N-formyl-stabilizing quasi-catalytic species afford rapid and selective solvent-free amination of biomass-derived feedstocks[END_REF] So far, the conversion of carbohydrates into valuable nitrogen compounds is a good opportunity and starts to be an important goal of research programs. [START_REF] Yan | Don't Waste Seafood Waste[END_REF][START_REF] Brun | Expanding the Biomass Derived Chemical Space[END_REF][START_REF] Imm | Improved Ruthenium-Catalyzed Amination of Alcohols with Ammonia: Synthesis of Diamines and Amino Esters[END_REF][START_REF] Chatterjee | Reductive Amination of Furfural to Furfurylamine Using Aqueous Ammonia Solution and Molecular Hydrogen: an Environmentally Friendly Approach[END_REF][START_REF] Pelckmans | Low-Temperature Reductive Aminolysis of Carbohydrates to Diamines and Aminoalcohols by Heterogeneous Catalysis[END_REF] The promotion of catalytic system for the selective and sustainable synthesis of amines from available and inexpensive starting materials using green reagents and mild reaction condition is highly demanded and challenging. [START_REF] Pelckmans | Low-Temperature Reductive Aminolysis of Carbohydrates to Diamines and Aminoalcohols by Heterogeneous Catalysis[END_REF][START_REF] Liang | Production of Primary Amines by Reductive Amination of BiomassDerived Aldehydes/Ketones[END_REF][START_REF] Niemeier | Is Water a Suitable Solvent for the Catalytic Amination of Alcohols? Green chem[END_REF][START_REF] Knaus | Amine Dehydrogenases: Efficient Biocatalysts for the Reductive Amination of Carbonyl Compounds[END_REF] As a rule, four mechanisms can be exploited for amine alkylation: (1) nucleophilic substitution of alkyl halides with amines; [START_REF] Roose | Aliphatic, Ullmann's Encyclopedia of Industrial Chemistry[END_REF] (2) N-alkylation of amines with alcohols under catalytic conditions; [START_REF] Yan | Iron Catalysed Direct Alkylation of Amines with Alcohols[END_REF][START_REF] Elangovan | Efficient and Selective N-Alkylation of Amines with Alcohols Catalysed by Manganese Pincer Complexes[END_REF] (3) reductive amination of carbonyl compounds with amines. [START_REF] Gomez | The Reductive Amination of Aldehydes and Ketones and the Hydrogenation of Nitriles Mechanistic Aspects and Selectivity Control[END_REF] (4) hydroamination of olefins with amino source. [START_REF] Utusunomiya | Rhodium-Catalyzed Anti-Markovniko Hydroamination of Vinylarenes[END_REF] The first method is not recommended from a green synthesis point of view, since a massive amount of useless inorganic salts is generated. Alkylation of amines with alcohols normally requires harsh conditions, especially high temperature or well-designed catalyst, as well as in some cases the use of additives.

Hydroamination of olefins is acknowledged as a typical way for synthesis of aliphatic amines, combining alkenes and simple N-H functional groups in a direct and atom-economical fashion.

At the same time it is one of 10 challenges for catalysis and limited examples are reported, which promotes the potential to create a significant synthetic benefit. Direct reductive amination, involving the reaction of carbonyl compounds, represents one of the most versatile, and waste-free process to access a variety of amines. [START_REF] Jagadeesh | MOF-derived Cobalt Nanoparticles Catalyze a General Synthesis of Amines[END_REF][START_REF] Senthamarai | Simple ruthenium-catalyzed reductive amination enables the synthesis of a broad range of primary amines[END_REF][START_REF] Liang | Production of Primary Amines by Reductive Amination of BiomassDerived Aldehydes/Ketones[END_REF] However, overalkylation, reduction to the corresponding alcohols and poor catalyst stability are common drawbacks in catalytic processes for the direct synthesis of amines, especially in the presence of NH3. [START_REF] Ogo | pH-Dependent Chemoselective Synthesis of α-Amino Acids. Reductive Amination of α-Keto Acido Acids with Ammonia Catalyzed by Acid-Stable Iridium Hydride Complexes in Water[END_REF][START_REF] Maya | Direct Reductive Amination of Aldehydes via Environmentally Benign Bentonite-Gold Nanohybrid Catalysis[END_REF][START_REF] Gallardo-Donaire | Direct Synthesis of Primary Amines via Ruthenium-Catalysed Amination of Ketones with Ammonia and Hydrogen[END_REF][START_REF] Gross | Synthesis of Primary Amines: First Homogeneously Catalyzed Reductive Amination with Ammonia[END_REF][START_REF] Yuan | Reductive Amination of Furanic Aldehydes in Aqueous Solution over Versatile NiyAlOx Catalysts[END_REF][START_REF] Deng | Low-Temperature Reductive Amination of Carbonyl Compounds over Ru Deposited on Nb2O5•nH2O[END_REF][START_REF] Manzoli | Microwave-Assisted Reductive Amination with Aqueous Ammonia: Sustainable Pathway Using Recyclable Magnetic Nickel-Based Nanocatalyst[END_REF][START_REF] Murugesan | Reusable Nickel Nanoparticles-catalyzed Reductive Amination for Selective Synthesis of Primary Amines[END_REF][START_REF] Zhang | Nitrogen-doped carbon supported nickel nanoparticles: a robust catalyst to bridge the hydrogenation of nitriles and the reductive amination of carbonyl compounds for the synthesis of primary amines[END_REF] Hence, from an atomeconomy and green standpoint, there is an urgent need to develop a selective and efficient system for producing bio-based primary amines, which is the prime task of this investigation.

Recently, we reported an efficient method for the synthesis of furfural from highly concentrated xylose in biphasic media, where furfural was extracted by methyl isobutyl ketone (MIBK). [START_REF] Jiang | Unveiling the Role of Choline Chloride in Furfural Synthesis from Highly Concentrated Feeds of Xylose[END_REF] As a top chemical produced from biomass, furfural is furanic derivative that is available in large scale (>200 kT/year), making it an attractive raw material for the production of valueadded chemicals. [START_REF] Mariscal | Furfural: A Renewable and Versatile Platform Molecule for the Synthesis of Chemicals and Fuels[END_REF][START_REF] Li | Furfural: A Promising Platform Compound for Sustainable Production of C4 and C5 Chemicals[END_REF] For instance, furfural can be selectively converted into furfurylamine by reductive amination with NH3 in presence of H2, [START_REF] Komanoya | Electronic Effect of Ruthenium Nanoparticles on Efficient Reductive Amination of Carbonyl Compounds[END_REF][START_REF] Chandra | A High Performance Catalyst of Shape-Specific Ruthenium Nanoparticles for Production of Primary Amines by Reductive Amination of Carbonyl Compounds[END_REF] which is used in the synthesis of pesticides, herbicides and pharmaceuticals (for diuretic furosemide synthesis). Besides, furan-based compounds are highly versatile and key derivatives used in the manufacture of a wide range of chemicals, and can serve as precursors for the synthesis of jet fuels substitutes or additives such as alkanes. [START_REF] Huber | Production of Liquid Alkanes by Aqueous-Phase Processing of Biomass-Derived Carbohydrates[END_REF][START_REF] Sutton | The Hydrodeoxygenation of Bioderived Furans into Alkanes[END_REF][START_REF] Julis | Synthesis of 1-Octanol and 1,1-Dioctyl Ether from Biomass-Derived Platform Chemicals[END_REF][START_REF] Xia | Energy-Efficient Production of 1-Octanol from Biomass-Derived Furfural-Acetone in Water[END_REF][START_REF] Shao | Pd/Nb2O5/SiO2 Catalyst for the Direct Hydrodeoxygenation of Biomass-Related Compounds to Liquid Alkanes under Mild Conditions[END_REF][START_REF] Li | Ultrafinely Dispersed Pd Nanoparticles on a CN@MgO Hybrid as a Bifunctional Catalyst for Upgrading Bioderived Compounds[END_REF][START_REF] Luska | Enhancing the Catalytic Properties of Ruthenium Nanoparticle-SILP Catalysts by Dilution with Iron[END_REF][START_REF] Xu | Synthesis of Diesel and Jet Fuel Range Alkanes with Furfural and Angelica Lactone[END_REF][START_REF] Xu | Synthesis of Renewable C8-C10 Alkanes with Angelica Lactone and Furfural from Carbohydrates[END_REF][START_REF] Rojas-Buzo | Hf-Based Metal-Organic Frameworks as Acid-Base Catalysts for the Transformation of Biomass-Derived Furanic Compounds into Chemicals[END_REF][START_REF] Yang | Solvent-Free Synthesis of C10 and C11 Branched Alkanes from Furfural and Methyl Isobutyl Ketone[END_REF][START_REF] Lee | CO2-Catalysed aldol condensation of 5-hydroxymethylfurfural and acetone to a jet fuel precursor[END_REF] Herein, we present a simple and efficient catalytic system combining C-C and C-N condensation reactions for producing 5-methyl-1-(tetrahydrofuran-2-yl)hexan-3-amine, and extensively a library of THF-derived biobased amines with variable molecular complexity and diversity, starting directly from furfural (Scheme 1). In particular, Pd/Al2O3 was found as an active catalyst for the highly selective formation of THF-derived primary amines using NH3 and H2 in an efficient one-step process starting from bio-derived ketone (methylisobutylketone, MIBK which is obtained from aceton a bulk chemical). This approach was successfully extended to the selective synthesis of a new family of THF-derived amines. The presence of the THF moiety is of prime importance for solvent properties and the manufacture of polymers. [START_REF] He | Evaluation of furfurylamines as curing agents for epoxy resins[END_REF][START_REF] Zhu | Design of Thermoresponsive Polymers with Aqueous LCST, UCST, or Both: Modification of a Reactive Poly(2-vinyl-4,4dimethylazlactone) Scaffold[END_REF][START_REF] Liu | Switchable synthesis of furfurylamine and tetrahydrofurfurylamine from furfuryl alcohol over RANEY® nickel[END_REF] Scheme 1. Synthesis of THF-derived amines starting directly from furfural.

Results and Discussion

Catalyst Characterization. X-ray diffraction of all noble metal catalysts were performed and are shown in Figure 1. The XRD patterns only contain features of the supports (Al2O3/C), implying the absence of large metal particles. The textural properties of the catalysts are listed in Table 1. Catalytic Testing. On the guidance of a previous study, [START_REF] Jiang | Direct catalytic conversion of furfural to furan-derived amines[END_REF] a bio-based carbonyl-conjugated vinylfuran (1a) was selected as model platform, which can be selectively synthesized by the aldol condensation reaction of furfural with methyl isobutyl ketone (MIBK). The reductive amination of 1a with NH3 to the primary amine using H2 was chosen as benchmark reaction. Due to the polyfunctionnality of 1a (C=C, C=O bonds and furanic cycle), various products can be in principle generated, including furanic ketone (2a), THF-derived ketone (3a), THF-derived amine (4a), or furanic amine (5a). Targeting the selective synthesis of amine products and more specifically 4a, a series of commercially available noble metal catalysts based on Ru, Pt, Rh and Pd (5 wt% metal) supported over alumina (Al2O3) or active carbon (C) were screened, and the results are shown in Table 2.

In a first set of experiments, 0.2 g of 1a (10 wt% in EtOH) was reacted in the presence of 10 mg of the alumina-supported Ru, Pt, Rh and Pd catalysts at 120 °C for 12 h with 5 bar NH3 and 20 bar H2. Except for Pd/Al2O3, a complex mixture of 2a-5a products was obtained. In particular, Ru and Pt favored the formation of the furanic amine 5a with ~50% yield, but the latter exhibited a lower carbon balance which is probably due to condensation reaction. Rh afforded the formation of all the products with a yield of 8%, 10%, 46% and 33% of products 2a, 3a, 4a and 5a, respectively. In the presence of Pd/Al2O3, the yield of product 4a was around 90% with a carbon mass balance over 95%. We further investigated the effect of the support on the catalytic properties. Pd/C afforded 83% and 5% yield of products 4a and 3a, respectively, with a carbon mass balance of 88%. When the Pd/Al2O3 catalyst loading was decreased to 5 mg, a drop of the 4a yield (from 92% to 71%) was observed along with an increase of the 3a yield from 7% to 23%.

In light of these results, we further optimized the process conditions for the Pd/Al2O3 catalyst. Effect of the Reaction Temperature. To attain high selectivity to product 4a at mild conditions and also to understand the reaction mechanism, the effect of the reaction temperature on the catalytic properties was first investigated (Figure 2a). No effect was observed on the 1a conversion, but the product distribution differed substantially. Indeed, a decrease of the temperature from 120 °C to 80 °C led to a drastic decrease of the yield of THF-derived amine 4a from 90% to 33% to the benefit of the furanic ketone (2a) and furanic amine (5a). A further decrease of the temperature down to 50 °C resulted in few amine products at the expense of a mixture the hydrogenated products 2a and 3a with a yield of 44% and 39%, respectively, at full 1a conversion. These results suggest that a temperature below 80 °C is insufficient for generating the amine products with good selectivity within 12 h, which might be attributed to a faster hydrogenation of the ketone compared to the intermediate imine. In light of these results, a reaction temperature of 120 °C was selected to ensure the formation of THF-derived amine 4a. Effect of the Hydrogen Pressure. H2 as the reducing agent plays an important role in the reaction (1 mol of reagent 1a could consume 4 mol of H2, Scheme 2). Thus, we investigated the effect of the H2 pressure keeping the other parameters constant (Figure 2b). When the H2 pressure was decreased from 20 bar to 10 bar, the yield of the THF-derived amine 4a decreased from 92% to 67% to the benefit of 2a, 3a and 5a with a yield of 5%, 13% and 4%, respectively. In contrast, an increase of the H2 pressure from 20 bar to 40 bar did not result in an appreciable change of the catalytic performance (94% yield of 4a and 4% yield of 3a). These results point out two possible pathways for generating the THF-derived primary amine (Scheme 2): (i) hydrogenation of the furanic amine, or (ii) reductive amination of the THF-derived ketone. One can mention that the formation of the Schiff base is likely to occur in the presence of amine and ketone in organic solvent. However, due to the electronic hindrance of the ketone or amine groups, we did not detect its existence in our work.

Scheme 2. Possible reaction pathways for generating THF-derived amines: (i) reductive amination of THF-derived ketone; and (ii) hydrogenation of the furanic amine.

To sort out the contribution of both pathways, DFT calculations at the CCSD(T)/cc-pVTZ level were conducted to unveil the thermodynamic stability of the different products (Figure 3, see calculation details in the SI). In the presence of H2, carbonyl-conjugated vinylfuran (1a) was first converted into the furanic ketone (2a) with a free energy of reaction [ΔG o (1a→2a)] of -13.8 kcal mol -1 (120 °C, in ethanol). In the presence of NH3, a narrowly positive free energy of reaction compared to 2a [ΔG o (2a→2b) = +8.3 kcal mol -1 ] pushed the formation of the furan imine 2b, followed by the hydrogenation to the furanic amine 5a [ΔG o (2b→5a) = -8.6 kcal mol -1 ]. Finally, by deep hydrogenation, the THF-derived amine can be generated with a further stabilization [ΔG o (2a→4a) = -14.9 kcal mol -1 ] compared to 2a. In the meantime, in the presence of H2, a thermodynamically more favorable reaction can occur driven by the hydrogenation of the furan ring [ΔG o (2a→3a) = -13.7 kcal mol -1 compared to 2a], resulting in the formation of the THFderived ketone (3a). Then the THF-derived amine can be accessed by the reductive amination of 3a. In light of these results, from a thermodynamic perspective, pathway (ii) appears to be more feasible for generating THF-derived amines. Here we only calculated the free energies of the reactions, not the barriers. Thus the reaction kinetics was not considered in the calculations. Reaction Kinetics. We further measured the kinetics of the formation of the different products over Pd/Al2O3 by varying the reaction time from 1 min to 18 h (Figure 4). The results show a complete transformation of the product mixture into 4a from the very beginning of the reaction.

Indeed, after 1 min, 50% of reactant 1a was converted into 2a and 3a with 28% and 8% yield, respectively, with little formation of the amine and imine products. This observation suggests a faster hydrogenation rate of the vinyl group after 1 min of reaction (2a, 0.315 mmol min -1 ) compared to the furan ring (3a, 0.090 mmol min -1 ). After 2 min, 1a was fully converted into a mixture of products 2a and 3a, whereas when the reaction was prolonged to 20 min, the target product 4a started to be generated, reaching a maximum yield of 98% after 18 h. These results point out that, on a kinetic perspective, pathway (ii) is favored for the formation of the THF-derived amine 4a from the carbonyl-conjugated vinylfuran 1a over Pd/Al2O3 (Scheme 2). First, the hydrogenation of 1a gives the furanic ketone 2a, followed by the deep hydrogenation to the THFderived ketone 3a. The subsequent reversible reaction of 3a and NH3 leads to the formation of the imine intermediate with concomitant H2O formation. Meanwhile, 4a reaches a maximum as the reaction proceeds. In light of these results, Pd/Al2O3 appears as a very active and chemo-selective catalyst for promoting the transformation of 1a into 4a without the formation of undesirable byproducts. Catalyst Recyclability. The recyclability of the Pd/Al2O3 catalyst was evaluated in the reductive amination of 1a over a series of consecutive runs (Figure 5). After each run, the catalyst was recovered by centrifugation and reused without further reactivation. After the 2 nd run, the yield of 4a decreased slightly from 97% to 91%, while product 3a appeared with 5% yield. After the 3 rd and 4 th runs, the product distribution fully changed, since the yield of 4a decreased to <50% at the expense of 5a. One can mention that the recycled Pd/Al2O3 catalyst behave like Ru, Pt and Rh/Al2O3 catalysts but with the formation of less by-products. After the 4 th run, the catalyst was recovered by filtration and washed with ethanol three times, dried overnight, calcined at 500 °C for 2 h and reduced under H2. The refreshed catalyst exhibited a yield of product 4a of 78% along with 13% yield of 5a. Interestingly, no Pd loss was detected during the reaction, as inferred from ICP measurements in the reaction solution after the 4 th run. These results point out that Pd leaching is not at the origin of the observed catalyst deactivation. To explain this change in the selectivity, the spent catalyst after the 4 th run was subjected to characterization using different techniques. Characterization of Recycled Pd/Al2O3. Figure S3 plots the N2 adsorption/desorption curves at 77 K of the fresh and recycled Pd/Al2O3 catalyst after the 4 th run, whereas Table 3 lists the textural properties. Although, both samples exhibited the presence of mesopores, the spent sample showed a slight decline of the specific surface area, pore volume and average pore size. This observation might be linked to the modification of the active sites leading to a different pathway driven by a change of the Pd particle, as well as by pore blockage due to coking or due to the adsorption of oligomers. TG analysis was then performed on the spent catalyst (Figure 6). The thermal profile of the fresh Pd/Al2O3 exhibited three main regions i) at around 100 °C (water desorption; (ii) at about 200 °C (decomposition or vaporization of small organic molecules); and (iii) in the range of 250-600 °C (possible dehydration of the alumina support). The TG curve of the recycled Pd/Al2O3 under air exhibited a quite different thermal profile. For the spent catalyst, three regions were observed in the temperature range 250-600 °C reflecting the possible formation of carbon deposits.

Besides, the total weight loss increased from 9.7 wt% for the fresh catalyst to 12.3 wt% for the recycled catalyst. Thus the catalyst can be regenerated by calcination at 500 °C. To further understand the nature of adsorbed species on the recycled Pd/Al2O3, the catalyst was analyzed by FT-IR. Figure 7 shows the FT-IR spectra of the fresh, spent and regenerated Pd/Al2O3. Two characteristic bands centered at 1157 cm -1 and 1230 cm -1 belonging to C-O bonds in the furan ring were preserved after regeneration. [START_REF] Tian | A Thermally Remendable Epoxy Resin[END_REF] Furthermore, by calcination at 500 °C for 1 h, the carbon deposits could be almost completely removed.

To achieve more information on the catalyst surface, the recycled Pd/Al2O3 was analyzed by XPS after 4 th run (Figure 8) and it has two doublets with BE (Pd 3d5/2) 335.1 and 336.7 eV attributed to metallic Pd 0 and Pd 2 + (PdO), respectively. [START_REF] Wang | Highly Selective Hydrogenation of Phenol and Derivatives over a Pd@Carbon Nitride Catalyst in Aqueous Media[END_REF][START_REF] Ainembabazi | Acceptorless Amine Dehydrogenation and Transamination Using Pd-Doped Hydrotalcites[END_REF] Both of the states coexisting on the surface almost had no shift in comparison to that of fresh Pd/Al2O3 (335.1 and 336.9 eV). At the same time most of the Pd (82 atom%) still kept in metallic form, suggesting that the apparent change of selectivity is not attributed to Pd oxidation. Finally TEM was used to measure the size distribution and average size of Pd nanoparticles in the Pd/Al2O3 catalyst before and after reaction (Figure 9). The recycled catalyst exhibits an obvious increase of the average size of the Pd nanoparticles from 2.3 nm to 4.7 nm. In the meantime, a clear agglomeration of Pd nanoparticles is observed (Figure S4). Moreover, EDS analysis on the fresh and spent catalysts (Figure S5 and S6) confirmed the formation of carbon (2) formation of carbon deposits on Pd. It seems that in the presence of used catalyst, the reaction pathway changed and the major route is reaction pathway (i) (Scheme 2). Substrate Scope. The scope of the reaction was further extended to the reductive amination of a series of furan-derived ketones over Pd/Al2O3 (Table 4). The detailed reaction conditions are summarized in the SI. For all substrates, the corresponding amines were efficiently obtained with 91-98% yield. These results clearly show that Pd/Al2O3 can enable the efficient and selective synthesis of a large variety various THF-derived amines from the corresponding biomass-derived carbonyl vinylfurans. One pot two step reaction. It is highly desired to design intensified reactions in a single reactor avoiding the separation and purification of intermediates. In this view, first the aldol condensation reaction was optimized using Amberlyst-26 (A26) as catalyst to achieve high yield towards the carbonyl-conjugated vinylfuran 1a at an equimolar MIBK/furfural ratio in ethanol. When the reaction was conducted at 80 °C with an A26/furfural weight ratio of 40 wt%, a maximum yield of 4a of 90% was achieved after 6 h with 99% furfural conversion (Figure S8). Furthermore, when the MIBK/furfural molar ratio increased to 1.2, 96% yield of 1a was achieved after 6 h at 80 °C. The one-reactor catalytic tests were further carried out using a physical mixture of A26 and Pd/Al2O3 (Table 5). A yield of 36% to product 4a was obtained together with a complex mixture of hydrogenated products and amines (entry 2). A much higher yield (60%) of 4a was obtained when Pd/Al2O3 was added during the reductive amination step (entry 3). The reaction conditions of the second step were further optimized to increase further the yield of product 4a (entries 4-6).

A yield of product 4a up to 75% with 10% yield of product 3a could be achieved by increasing the reaction time from 20 h to 40 h, the H2 pressure from 20 bar to 40 bar, or the Pd/Al2O3 amount from 5 wt% to 10 wt%. By increasing the MIBK/furfural molar ratio in the first step to 1.2 to promote the formation of the aldolization product 1a, a maximum yield of 85% of product 4a could be achieved in the presence of 10 wt% Pd/Al2O3 (entry 7).

Conclusion

In summary, a new family of THF-derived amines was selectively synthesized with ~98% yield starting from bio-derived ketones using NH3 or amines as a nitrogen source, molecular hydrogen as a reducing agent and Pd/Al2O3 as catalyst. This system efficiently catalyzed the hydrogenation of the unsaturated C=C bond and the reductive amination of the carbonyl group at mild reaction conditions in a single reactor. Although no Pd leaching from the catalyst was evidenced during the reaction, a change of the catalyst selectivity was observed after the 3 rd catalytic run mostly due to the Pd sintering and the deposit of carbon species. Besides, furfural could be converted into functionalized THF-amine in a one-pot two steps process without any purification between the two steps and with a yield up to 85%. Thus, our present work opens a new avenue for producing biobased amines with high potential for the synthesis of biosolvents or biosurfactants. Besides, this study provides a toolbox that enables the sustainable conversion of furfural into target THF-derived amines. The deposition of Pd over a multifunctional supports and the design of a continuous flow processes are current lines of investigation to optimize the performance of the one-reactor process.

ASSOCIATED CONTENT Supporting Information. A listing of the contents of each file supplied as Supporting Information should be included. For instructions on what should be included in the Supporting Information as well as how to prepare this material for publications, refer to the journal's

Figure 1 .

 1 Figure 1. XRD patterns of all noble metal catalysts (peak indication: *=γ-Al2O3)

Figure 2 .

 2 Figure 2. a) Effect of temperature; b) Effect of H2 pressure on the catalytic performance of Pd/Al2O3 in the reductive amination of 1a. Reaction Conditions: a) Pd/Al2O3 (10 mg, 0.42 mol%), 1a (0.2 g), EtOH (2 g), NH3 (5 bar), H2 (20 bar), 12 h; b) Pd/Al2O3 (10 mg), 1a (0.2 g), EtOH (2 g), NH3 (5 bar), 120 °C, 12 h. The 1a conversion was over 99 % in all the tests.

Figure 3 .

 3 Figure 3. Free energy diagrams for the reductive amination of 1a and related reactions; Values in parentheses are the computed free energies referred to 1a.

  CCSD(T)/cc-pVTZ//B3LYP/def2-TZVP COSMO (in ethanol)

Figure 4 .

 4 Figure 4. Time-evolution of the reaction products in the reductive amination of 1a over Pd/Al2O3. Reaction conditions: Pd/Al2O3 (10 mg), 1a (0.2 g), EtOH (2 g), NH3 (5 bar), H2 (20 bar), 120 °C.

Figure 5 .

 5 Figure 5. Product distribution in consecutive catalytic runs in the reductive amination of 1a. Reaction conditions: Pd/Al2O3 (20 mg), 1a (0.4 g), EtOH (4 g), NH3 (5 bar), H2 (20 bar), 120 °C, 20 h.

Figure 6 .

 6 Figure 6. TG profiles of Pd/Al2O3 under air: (a) fresh and (b) recycled catalyst after the 4 th run.

Figure 7 .

 7 Figure 7. FT-IR spectra of (a) fresh, (b) recycled and (c) regenerated Pd/Al2O3.

Figure 8 .

 8 Figure 8. XPS Pd 3d spectra of fresh Pd/Al2O3 and recycled Pd/Al2O3.

Figure 9 .

 9 Figure 9. TEM micrographs of the catalyst and distribution of the Pd nanoparticle size. (a) Fresh Pd/Al2O3; (b) Recycled Pd/Al2O3.

Table 1 .

 1 Physical-chemical properties of all noble metal catalysts.

	Catalyst	SBET (m 2 /g)	Pore size (nm)	Pore volume (cm 3 /g)
	Ru/Al2O3	138	19.1	0.74
	Pt/Al2O3	166	15.6	0.81
	Rh/Al2O3	152	10.3	0.44
	Pd/Al2O3	158	14.4	0.57
	Pd/C	1016	4.8	0.53
	The highest surface area is obtained for the carbon support (1016 m 2 /g) and the other four alumina-
	supported catalysts show quite similar surface area (between 138 and 166 m 2 /g). One can mention
	that the pore size is on the opposite, the lowest for Pd supported on carbon (4.6 nm) and for the
	other catalysts it in in the range of 10.3 to 19.1 nm. The pore volume of all the catalysts are
	comprise between 0.44 to 0.81 cm 3 /g.
	XPS analysis was performed in order to identify the probable species of noble metals on
	the catalysts surfaces. The identification of the probable metal species is based on XPS patterns
	obtained from the National Institute of Standards and Technology (NIST). To correct binding
	energies (BEs), the Al 2p peak with BE 74.4 eV for Al-containing samples and the adventitious
	carbon C 1s peak with BE 284.8 eV are used as reference. Figure S1 presents the XPS spectra of
	selected Ru, Pt, Rh and Pd catalysts and binding energies of Ru 3d5/2, Pt 4d5/2, Rh 3d5/2 and Pd
	3d5/2 for the analyzed solids. It is clearly noted that all noble metal species contained oxidized
	species due to passivation under air due to experimental work up. However, reduced species are
	the main species for all the catalysts. Hence, Pt, Rh, Pd catalysts could be sufficiently reduced into
	metallic species at room temperature whereas H2-temperature-programmed reduction (Figure S2)
	reveals that Ru on Al2O3 can be reduced into metallic Ru at the reaction temperature 90 °C. These

Table 2 .

 2 Reductive amination of 1a over supported metal catalysts a

	Entry Catalyst	2a	3a	Yield / %	4a	5a	Carbon balance / %
	1	Ru/Al2O3	19	-		9	60	88
	2	Pt/Al2O3	5	-		14	43	62
	3	Rh/Al2O3	8	10		46	33	97
	4	Pd/Al2O3	-	7		92	-	99
	5	Pd/C	-	5		83	-	88
	6 b	Pd/Al2O3	-	23		71	-	94

a Reaction conditions: 1a (0.2 g), catalyst (0.01 g), EtOH (2 g), NH3 (5 bar), H2 (20 bar), 120 °C, 12 h. b Pd/Al2O3 (5 mg). The 1a conversion was over 99% for all catalysts

Table 3 .

 3 Textural properties of the Pd/Al2O3 catalysts.

	Pd/Al2O3	SBET (m 2 /g)	Pore volume (cm 3 /g)	Pore size (nm)
	Fresh	158	0.57	14.4
	Recycled	147	0.51	13.9

  deposits during the reaction. Overall, catalyst deactivation during the reductive amination of 1a occurred mainly by two different pathways: (1) sintering/agglomeration of Pd nanoparticles, and

			Fresh Pd/Al2O3
			Recycled Pd/Al2O3
			Recycled Pd/Al2O3 (calcinated)
					c
					a
					1230
					b		C-O	1157
	Fresh Pd/Al 2 O 3		Pd 0 3d 5/2		Recycled Pd/Al	2 O	3	Pd 0	3d 5/2
			335.1					335.1
	Pd 3d XP intensity		Pd 2+ 3d 5/2 336.9	intensity XP Pd 3d			Pd 2+ 336.7 3d	5/2
	345	340	335	330	345		340	335	330
		Binding Energy (eV)			Binding Energy (eV)

Table 4 .

 4 THF-derived amines produced from furan-derived ketones over Pd/Al2O3.

	Entry	Substrate	Product	Yield / %
	1			94
	2			96
	3			96
	4			98
	5			97
	6			91
	7			92

Table 5 .

 5 Catalytic conversion of furfural to THF-derived amine (4a) with Amberlyst-26 and

	Pd/Al2O3 solid catalysts in a one-reactor process. a
	Entry Substrate	Catalyst (wt%) A-26 Pd/Al2O3 1 st Time (h) 2 nd	P H2 (bar)	2a	Yield (%) 3a 4a	5a
	1 b	1a	-	5	-	20	20	n.d.	n.d.	98	n.d.
	2 c	Furfural	40	5	6	20	20	10	29	36	6
	3	Furfural	40	5	6	20	20	n.d.	23	60	n.d.
	4	Furfural	40	5	6	40	20	n.d.	11	74	n.d.
	5	Furfural	40	5	6	20	40	n.d.	10	75	n.d.
	6	Furfural	40	10	6	20	20	n.d.	11	74	n.d.
	7 d	Furfural	40	10	6	20	20	n.d.	9	85	n.d.

a Furfural conversion was 100% for all the reactions. (1 st ) All runs were carried out at 80 ºC in 2 g EtOH with 108 mg furfural while keeping an equimolar MIBK/furfural ratio and an Amberlyst-26/furfural weight ratio of 40 wt%, (2 nd ) the reductive amination reactions were all following the first step of aldol condensation, the mass ratio of Pd/Al2O3theoretical 1a 5 wt%, 5 bar NH3, H2. b Comparing Reaction: Pd/Al2O3 (10 mg), 1a (0.2 g), EtOH (2 g), NH3 (5 bar), H2 (20 bar), 120 °C, 20h. c Pd/Al2O3 was added with Amberlyst-26 at the beginning of the reaction.

d

The MIBK/furfural molar ratio was 1.2/1.

Instructions for Authors.

Corresponding Authors *marc.pera-titus-ext@solvay.com *karine.vigier@univ-poitiers.fr

Author Contributions

Funding Sources

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.

Notes

Any additional relevant notes should be placed here.