Jordan Mangue

Clément Gondre

Jacques Pécaut

Carole Duboc

Stéphane Ménage

Stéphane Torelli
email: stephane.torelli@cea.fr

Controlled O 2 reduction at a mixed-valent (II,I) Cu 2 S core

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Inspection of Oxygen Reduction Reactions (ORR) using a mixed-valent Cu 2 S complex as pre-catalyst revealed a tuneable H 2 O 2 vs. H 2 O production under mild conditions by controlling the amount of sacrificial reducer. The fully reduced bisCuI state is the main active species in solution, with fast kinetics. This new catalytic system is robust for H 2 O 2 production with several cycles achieved and opens up perspectives for integration into devices.

With the increase in world population and the shrinking of unsustainable fossil fuels that we depend on, there is a crucial need in exploring carbon-free alternatives to ensure a safe and sustainable future. In this line, the so-called Oxygen Reduction Reactions (ORR) are important processes in fuel cells technology [1] for achieving a hydrogen-based society. [2] However this cathodic event remains the limiting step regarding the efficiency of a complete device. [3] ORR find its essence in Nature with biological respiration [4] including laccases that catalyse the 4e -/4H + reduction of O 2 into H 2 O. [5] The other important product formed upon O 2 reduction is hydrogen peroxide (H 2 O 2 ) via a 2e -/2H + process. H 2 O 2 is important for living organisms where it is for instance biosynthesized by the immune system to kill microbes, [6] as signal molecule under oxidative stress conditions [7] or used for metabolic purposes by copper metalloenzymes such as Galactose Oxidase.[5b] It is a staple in the industry with a ranking in the top 100 most important reactants with more than 3 million tons produced per year [START_REF] Myers | The most 100 Most Important Chemical Compounds[END_REF] and widely used in pharmaceuticals, cosmetics and electronics. [START_REF] Campos-Martin | [END_REF] H 2 O 2 has recently emerged as a potent latent energy carrier through its O-O bond (Gibbs free energy of formation of G 0 f = -120 kJ mol -1 from H 2 and O 2 ) and is thus a suitable candidate for energy storage for fuel cells technology. [10] However, given a production that mainly relies on the energy-consuming and precarious anthraquinone process, [11] new eco-friendly methodologies for controlled H 2 O 2 production from O 2 reduction have yet to be discovered.

The conception of efficient catalysts for homogeneous ORR based on noble but also nonnoble metal ions such as Fe, Co or Mn has stimulated intensive research. [12] In these cases, the electron source for the catalytic activity comes from sacrificial reducers such as metallocenes or via electrocatalysis. With respect to homogenous Cu-based catalysts, mono-,[13] di-[14] and trinuclear [15] copper complexes have been studied and relevant activities reported. Interestingly, the presence of Lewis acid (Sc 3+ ) was shown to induce selective catalysis towards two-electron O 2 reduction. [16] Under heterogeneous conditions, immobilized Cu complexes are particularly efficient for O 2 reduction into H 2 O. [17] With the aim of targeting ORR with original dicopper systems, we report here the activity of our previously described mixed-valent (MV) copper complex 1 [18] possessing a N 6 Cu2S environment (Figure 1). We evidenced a tuneable chemical H 2 O 2 vs. H 2 O selectivity in acetonitrile using controlled amounts of ferrocene derivatives as electron sources and in the presence of an organic acid. This study represents, to the best of our knowledge, the first example for selective ORR involving a Cu 2 S core under homogeneous and mild conditions. Catalytic ORR by 1 (0.05 mM, final concentration) were evaluated at room temperature (298 K) in air-saturated MeCN solutions. 2,6-lutidinium tetrafluoroborate (LutHBF 4 , 400 molar equiv.) was used as innocent proton donor (weak coordinating ability of both the conjugate base and the BF4counter-anion) and sacrificial electrons (10 to 100 molar equiv.) were provided by the means of ferrocene (Fc, E 1/2 Fc +/0 = 0 V vs. Fc +/0 ), dimethylferrocene (Me 2 Fc, E 1/2 Me 2 Fc +/0 = -0.10 V vs. Fc +/0 ), octamethylferrocene (Me 8 Fc, E 1/2 Me8Fc +/0 = -0.42 V vs. Fc+/0) or decamethylferrocene (Me 10 Fc, E 1/2 Me 10 Fc +/0 = -0.49 V vs. Fc +/0 ). Fc and Me 2 Fc are not compatible with the in situ reduction of any redox form of 1 (E 1 1/2 = -0.44 V vs. Fc+/0, E = 0.07 V, Cu 2 I,II → Cu 2 I,I; E 2 1/2 = -0.30 V vs. Fc+/0, E = 0.08 V, Cu 2 I,II → Cu 2 II,II), [18] whereas Me8Fc and Me10Fc are suitable for these processes. Monitoring the reaction by UV-Visible (UV-vis) spectrophotometry unambiguously shows the formation of Fc + , Me 2 Fc + , Me 8 Fc + or Me 10 Fc + at 614 nm ( = 410 M -1 cm -1 ), 650 nm ( = 290 M -1 cm -1 ), 750 nm ( = 390 M -1 cm -1 ) and 778 nm ( = 495 M -1 cm -1 ), respectively (Figs. S1-S8 and Table S1, ESI) and attests for O 2 reduction in all cases. Interestingly, only 2 molar eq. of Fc+ and 30 molar eq. of Me 2 Fc + (Figs. S3 andS4) are detected when going up to 100 molar eq., whereas the maximum of possible turnover numbers (TONs) is reached with Me 8 Fc and Me 10 Fc (denoted Me8-10Fc when compared in the following) regardless the excess (Fig. 2 and Figs S5-S8, ESI). A key result is the change in selectivity, H 2 O 2 vs. H 2 O, obtained by varying the amount of Me 8- 10 Fc (Table 1): almost exclusive H 2 O 2 formation occurs with 10 molar eq., whereas going to 100 molar eq. mainly leads to H 2 O production (entries 1 and 6, Table 1). The selectivity is moderately affected by either the kinetics or the difference in the reducing abilities of Me 8-10 Fc (E 1/2 = 0.07 V). As control experiments, no O 2 or H 2 O 2 reduction by Me 8-10 Fc occur under the same conditions (and reaction times) in the absence of 1 (Fig. S10,ESI). This strongly supports a H 2 O formation process originating from a catalysed two-electron reduction of a coordinated (hydro)peroxide at high Me8-10Fc concentrations. The inactivity of low-valent [Cu(Tol) 2 ](OTf) (OTf = trifluoromethanesulfonate anion and Tol = toluene) finally reinforces the remarkable effect of the S/N coordination spheres in 1 on the reactivity and excludes solvated CuI ions as activators.

Kinetically speaking, the fact that the reactions with Fc and Me 2 Fc are relatively slow and not complete compared to those with Me8-10Fc (Table 1 andTable S2, ESI) suggests that 1 or its protonated form 1 H (see below for the behaviour of 1 with LutHBF 4 ) could initiate the reaction. However, its rather modest efficiency indicates that MV states cannot be considered as the most active forms. It also demonstrates that the redox potentials of the oxidized species generated along the reaction course are not (or partially) thermodynamically compatible with the reducing abilities of Fc and Me 2 Fc to reach total consumption of the electron source.

Consequently, from now on, only the case of Me 8-10 Fc is discussed since quantitative and fast conversions were obtained. For 10 and 20 molar eq., the kinetic traces (Figs. S5 and S7, ESI) display pseudo-first-order profiles and comparable reaction times. Starting with 40 molar eq. and above, the reaction times are significantly shorter with Me 10 Fc compared to Me 8 Fc, for which successive steps are identifiable (Figs. S5-S8, ESI). The overall comparison of the kinetics traces clearly indicates the involvement of more active species when using Me 10 Fc compared to Me 8 Fc. It is worth noting that the kobs values are inversely proportional or quasi-independent with respect to [Me 8 Fc] (to a certain extent) or [Me 10 Fc], respectively (Fig. S11,ESI). This suggests that the gradual accumulation of H 2 O results in competitive reaction pathways for Me 8 Fc (with a global steady state starting from 60 molar eq.) that are not present for Me 10 Fc (its consumption not being the rate-determining step). This new catalytic system has proven to be robust for H 2 O 2 production, especially in the most favourable conditions. For instance, at least four consecutive cycles were achieved with successive additions of 10 molar eq. Me 8 Fc and its quantitative consumption after each injection (Fig. S12). An overall selectivity of 85% in H 2 O 2 , similar to that observed for a single run, was determined. This result demonstrates that H 2 O 2 accumulation neither (i) affects the selectivity/efficiency nor (ii) poisons the catalyst.

The behaviour of 1 in the presence of Me 8-10 Fc and LutHBF 4 prior exposure to air was then investigated in order to get insights into the nature of putative relevant copper species involved during the O 2 reduction process. For solubility reasons, as the concentrations required to perform such experiments are different from those used for catalysis, only one condition (i.e. 10 molar eq. Me 8-10 Fc and 400 molar eq. LutBF 4 ) was tested. Since identical results were obtained whatever the nature of the electron source, only the data obtained with Me 8 Fc are discussed. Under strict anaerobic conditions, the UV-vis/NIR spectrum of 1 is modified upon addition of LutHBF 4 and / or Me 8 Fc (Fig. S13,ESI). The presence of protons leads to the formation of a new 1H species with conservation of a dinuclear delocalized MV state (absorbance in the NIR region). Table 1. ORR experiments performed with 1 at room temperature using Me 8-10 Fc and LutHBF 4 as electron and proton sources. (a) (a) See the ESI for further experimental details.

As expected, Me 8 Fc in excess can reduce 1 to form 1 red . Addition of both the electron and proton sources results in the formation Me 8 Fc + , the loss of the MV signature of 1 and the generation of the reduced/protonated form 1 red H . In this case, the spectrum of the final mixture can be adjusted by combining 10% of remaining 1 H , 0.9 molar eq. Me 8 Fc + and 9.1 molar eq. Me 8 Fc. 1 red H should weakly contribute to the final spectrum due to the presence of d 10 Cu I ions. In the meantime, the EPR spectra show modifications of the hyperfine when 1 is converted into 1 H , followed by the decrease of the signal intensity consecutive to the reduction into 1 red H (10% remaining intensity, Fig. S14,ESI). An identical spectrum was obtained by adding a mixture of protons and electrons to a solution of 1. Finally, the low-valent state was trapped by ESI-MS using similar experimental conditions (Fig. S15, ESI). All these experiments provide solid evidences for a mono-electronic reduction/protonation process of 1 into 1 red H via 1 H . Independently, exposition of a solution of 1 redH to air lead to full Me 8 Fc consumption and the H 2 O 2 /H 2 O ratio is in line with that reported in Table 1 (entry 1). These results clearly indicate that low-valent 1 red H is highly reactive and a key actor during catalysis.

O 2 oxidation of 1 has also to be considered as part of the catalytic event. The corresponding 1 ox form was generated by exposing a MeCN solution of 1 to air. Its crystal structure shows the presence of a doubled-bridged dicopper(II) unit involving the S atom from the ligand and an additional hydroxyl anion (Fig. S16,ESI). 1 ox has a significantly different UV-vis/NIR spectrum compared to its precursor with the loss of the NIR band (Fig. S17,ESI). The extinction of the EPR signal (> 90%, Fig. S18, ESI) agrees with a strong antiferromagnetic coupling between the two metal ions leading to a S = 0 ground state. Such magnetic behaviour was already observed with a similar scaffold [20] or in the case of a phenolate spacer with identical Me(N,N-bis(methylpyridyl))amine pendant arms.[21] The ESI-MS spectrum of 1 ox (Fig. S19, ESI) shows two prominent peaks at m/z = 344.3 (m/z = 0.5) and 839.3 (m/z = 1.0) consistent with the solved solid-state structure. Catalytic experiments with 1 ox , 10 molar eq. Me 8 Fc and 400 molar eq. LutHBF 4 lead to quantitative conversion within a reaction time scale close to that of 1 (k obs = 1.17 s -1 , Fig. S20 ) and reduction (Fig. S21, ESI) under anaerobic conditions. Such species could thus be part of the reactivity.

These results suggest that the H 2 O 2 /H 2 O production cycle mainly relies on 1r ed H and allow the proposition of a reaction scheme (Scheme 1). 1 red H is generated by reduction/protonation of 1 or alternatively by air oxidation/protonation/reduction of 1 via 1 ox H (Fig. S22, ESI). O 2 activation at 1redH followed by protonation lead to the key O 2 -adduct(s) from which the selectivity can be explained by the inevitable competition between its protonation and/or reduction. Taking into account the composition of the mixture at the end of the reaction (presence of O 2 , LutHBF 4 and Me 8 Fc + ), 1oxH is certainly the final fate of the catalyst, as observed by ESI-MS (Fig. S23,ESI). This proposition is also reinforced by the ability of performing several cycles, necessarily through an oxidized species, with no loss in selectivity and efficiency. Nevertheless, 1oxH cannot be considered as a predominantly active species during catalysis at that point, since H2O would be the main product under any experimental condition.

To conclude, we demonstrate here that copper/sulphur assemblies such as 1 are very efficient for ORR at room temperature with fast kinetics. By controlling the amount of Me 8-10 Fc, a significant and tuneable H 2 O 2 vs. H 2 O selectivity can be achieved. H 2 O accumulation results from peroxide ligation and reduction at an active species since H 2 O 2 is not reduced in situ in the absence of 1. The reductive power of the reaction mixture is a factor that helps explaining the selectivity. With rather "low excess" Me 8-10 Fc, the two-electron reduction into H 2 O 2 is favoured. When the concentration of the electron source increases, a competition between H 2 O 2 release and its subsequent reduction sets up and becomes gradually predominant. Independently, a similar study with 1ox will be of great interest to compare its reactivity with a related phenolate-bridged dicopper II complex, whose ability for ORR was reported in 2012.[14c] The impact of the thiophenolate vs phenolate moiety could be appreciated (at first approximations 1ox seems faster) and correlated to electronic properties. In a different light, even if H 2 O 2 is quite an aggressive molecule in solution, the observation that several cycles can be performed with successive Me8Fc injections attests for the robustness of the system. These results are encouraging for further applications in heterogeneous catalysis upon grafting air stable 1ox whether onto an inert surface with the presence of an external electron source or on electroactive materials for electrocatalysis. Finally, a global reaction sequence for the activity of 1 and its derivatives is proposed and is now under dissection with complementary kinetic experiments and the characterization of the pivotal O 2 -adduct(s).

Fig. 1

 1 Fig. 1 Chemical structure of 1 and targeted ORR.

Fig. 2

 2 Fig. 2 Representatives UV-vis spectra for Me8Fc+ accumulation during ORR mediated by 1 in air-saturated MeCN at 298 K using 1 /Me 8 Fc/LutHBF 4 at 1/20/400; inset: variation of the Abs750nm as a function of time (the green dotted line indicates the theoretical Amax for total Me8Fc conversion); the black arrow indicates the injection of 1. Quantitative Me8-10Fc+ formation allowed the use of the H 2 O 2 -specific TiO-tpyp procedure (Fig. S9, ESI) in order to discriminate between H 2 O 2 and H 2 O production.[19] This method is extremely precise compared to iodine titration. H 2 O production was calculated considering the amount of Me 8-10 Fc+ not involved in H 2 O 2 formation (see ESI for more details).

  in ESI). H 2 O 2 titration indicates a selectivity similar to that of 1 (88/12 for H 2 O 2 /H 2 O). The activity of 1 ox is in line with its facile protonation (to 1 ox H

This work was supported by the Labex ARCANE and the CBH-EUR-GS (ANR-17-EURE-0003)

fund via the CNRS, CEA and the Grenoble-Alpes University.

Conflicts of interest

There are no conflicts to declare.

Notes and references


Dan Wu

Songwei Zhang

Willinton Y Hernández

Walid Baaziz

Ovidiu Ersen

Maya Marinova

Andrei Y Khodakov
email: andrei.khodakov@univ-lille1.fr

Vitaly V Ordomsky
email: vitaly.ordomsky@univ-lille.fr

Dual Metal-Acid Pd-Br Catalyst for Selective Hydrodeoxygenation of 5-Hydroxymethylfurfural (HMF) to 2, 5-Dimethylfuran at Ambient Temperature

ABSTRACT.

Supported metal catalysts have found broad applications in heterogeneous catalysis.

In the conventional bifunctional catalyst, the active metal sites are associated with the metal nanoparticles, while the acid sites are usually localized over the oxide support.

Herein, we report a novel type of supported metal bifunctional catalyst, which combined the advantages of the promotion and bifunctionality. The catalyst was designed by the pretreatment of supported palladium catalysts with bromobenzene compounds. The promotion with bromine creates Brønsted acid sites, which are localized directly on the surface of metal nanoparticles. An intimacy between metal and acid functions in this bifunctional catalyst generates unique catalytic properties in hydrodeoxygenation of 5-hydroxymethylfurfural to dimethylfuran, occurring with the yield up to 96 % at ambient temperature under 5 bar of H 2 . The catalyst exhibits stable catalytic performance. KEYWORDS. Heterogeneous catalysis; Bifunctional catalysis; Biomass; Hydrodeoxygenation; Dimethylfuran.

Introduction

Chemical reactions over heterogeneous catalysts usually involves multiple steps which may occur over different active sites. A rational combination of different active sites in one catalytic process for chemoselective transformation is highly desired [START_REF] Wheeldon | Substrate Channelling as An Approach to Cascade Reactions[END_REF][START_REF] Lohr | Thermodynamic Strategies for C-O Bond Formation and Cleavage via Tandem Catalysis[END_REF][START_REF] Wu | Tandem Catalysis: Transforming Alcohols to Alkenes by Oxidative Dehydroxymethylation[END_REF] .

While noble metal catalyst is active for hydrogenation, acid catalysts are effective for isomerization, dehydration and condensation [START_REF] Liu | Selective Phenol Hydrogenation to Cyclohexanone over A Dual Supported Pd-Lewis Acid Catalyst[END_REF][START_REF] Konwar | SO 3 H-Containing Functional Carbon Materials: Synthesis, Structure, and Acid Catalysis[END_REF] . Bifunctional metal-acid catalysts containing both metal and acid sites have been developed and widely applied for numerous important processes including conversion of biomass-derived platform molecules [START_REF] Cheng | Bifunctional Catalysts for One-Step Conversion of Syngas into Aromatics with Excellent Selectivity and Stability[END_REF][START_REF] Chen | How Catalysts and Experimental Conditions Determine the Selective Hydroconversion of Furfural and 5-Hydroxymethylfurfural[END_REF][START_REF] Xu | Recent Catalytic Routes for The Preparation and The Upgrading of Biomass Derived Furfural and 5-Hydroxymethylfurfural[END_REF][START_REF] Pang | Transition Metal Carbide Catalysts for Biomass Conversion: A Review[END_REF] . 5-hydroxymethylfurfural (HMF), which is produced from the abundant lignocellulosic biomass, has attracted intensive attention because of its conversion to value-added chemicals such as 2,5-bis(hydroxymethyl)furan (BHMF), 2,5-bis(hydroxymethyl)tetrahydrofuran (DHMTHF) and 2,5-dimethylfuran (DMF) [START_REF] Ordomsky | Glucose Dehydration to 5-Hydroxymethylfurfural over Phosphate Catalysts[END_REF][START_REF] Wang | Constructing Co@N-doped Graphene Shell Catalyst via Mott-Schottky Effect for Selective Hydrogenation of 5-Hydroxylmethylfurfural[END_REF][START_REF] Ordomsky | The Effect of Solvent Addition on Fructose Dehydration to 5-Hydroxymethylfurfural in Biphasic System over Zeolites[END_REF][START_REF] Sudarsanam | Advances in Porous and Nanoscale Catalysts for Viable Biomass Conversion[END_REF][START_REF] Roman-Leshkov | Production of Dimethylfuran for Liquid Fuels from Biomass-derived Carbohydrates[END_REF] . Among them, DMF has been proposed as a promising renewable transportation fuel due to its high boiling point, high energy density, high octane number and low water solubility [START_REF] Roman-Leshkov | Production of Dimethylfuran for Liquid Fuels from Biomass-derived Carbohydrates[END_REF] . Besides, DMF can be converted to p-xylene (PX) via Diels-Alder reactions, which is an important raw material in the production of polyethylene terephthalate (PET) [START_REF] Tao | Toward an Integrated Conversion of 5-Hydroxymethylfurfural and Ethylene for the Production of Renewable p-Xylene[END_REF][16][START_REF] Stadler | Catalytic Approaches to Monomers for Polymers Based on Renewables[END_REF] .

There are two groups of catalysts which have been proposed for conversion of HMF to DMF: 1) Pd, Pt, Ru, Ir, Co, Ni and Cu-based monometallic or bimetallic catalysts, which perform hydrogenolysis of C-O bond at the temperatures in the range of 150~250 o C. [START_REF] Roman-Leshkov | Production of Dimethylfuran for Liquid Fuels from Biomass-derived Carbohydrates[END_REF][START_REF] Wang | Hydrogenolysis of 5-Hydroxymethylfurfural to 2,5-Dimethylfuran over Supported Pt-Co Bimetallic Catalysts under Mild Conditions[END_REF][START_REF] Jae | Production of Dimethylfuran from Hydroxymethylfurfural Through Catalytic Transfer Hydrogenation with Ruthenium Supported on Carbon[END_REF][START_REF] Endot | Fast Catalytic Hydrogenation of 2,5-Hydroxymethylfurfural to 2,5-Dimethylfuran with Ruthenium on Carbon Nanotubes[END_REF][START_REF] Jin | Metal-Acid Interfaces Enveloped in Zeolite Crystals for Cascade Biomass Hydrodeoxygenation[END_REF][START_REF] Chen | Hydroconversion of 5-Hydroxymethylfurfural to 2,5-Dimethylfuran and 2,5-Dimethyltetrahydrofuran over Non-Promoted Ni/SBA-15[END_REF][START_REF] Chen | Carbon-Coated Cu-Co Bimetallic Nanoparticles as Selective and Recyclable Catalysts for Production of Biofuel 2,5-Dimethylfuran[END_REF] and 2) bifunctional metal-acid catalysts for hydrodeoxygenation through protonation of the hydroxyl groups and dehydration at milder reaction conditions (60~130 o C) [START_REF] Mitra | Pd/C-Catalyzed Reactions of HMF: Decarbonylation, Hydrogenation, and Hydrogenolysis[END_REF][START_REF] Nishimura | Selective Hydrogenation of Biomass-Derived 5-Hydroxymethylfurfural (HMF) to 2,5-Dimethylfuran (DMF) under Atmospheric Hydrogen Pressure over Carbon Supported PdAu Bimetallic Catalyst[END_REF][START_REF] Chatterjee | Hydrogenation of 5-Hydroxymethylfurfural in Supercritical Carbon Dioxide-Water: A Tunable Approach to Dimethylfuran Selectivity[END_REF][START_REF] Thananatthanachon | Efficient Production of The Liquid Fuel 2,5-Dimethylfuran from Fructose using Formic Acid as A Reagent[END_REF][START_REF] Zu | Efficient Production of The Liquid Fuel 2,5-Dimethylfuran from 5-Hydroxymethylfurfural over Ru/Co 3 O 4 Catalyst[END_REF][START_REF] Gawade | Biobased Green Process: Selective Hydrogenation of 5-Hydroxymethylfurfural to 2,5-Dimethyl Furan under Mild Conditions Using Pd-Cs 2.5 H 0.5 PW 12 O 40 /K-10 Clay[END_REF][START_REF] Kong | Ni Nanoparticles Inlaid Nickel Phyllosilicate as a Metal-Acid Bifunctional Catalyst for Low-Temperature Hydrogenolysis Reactions[END_REF][START_REF] Guo | Selective Hydrogenolysis of 5-Hydroxymethylfurfural to Produce Biofuel 2, 5-Dimethylfuran over Ni/ZSM-5 Catalysts[END_REF][START_REF] Han | Selective Hydrogenolysis of 5-Hydroxymethylfurfural to 2,5-Dimethylfuran Catalyzed by Ordered Mesoporous Alumina Supported Nickel-Molybdenum Sulfide Catalysts[END_REF][START_REF] Saha | Zinc-Assisted Hydrodeoxygenation of Biomass-Derived 5-Hydroxymethylfurfural to 2,5-Dimethylfuran[END_REF] . The disadvantage of the first route is that the high reaction temperature induces polymerization of highly reactive HMF to humins [START_REF] Shen | Evolution Process and Controlled Synthesis of Humins with 5-Hydroxymethylfurfural (HMF) as Model Molecule[END_REF] . The best performance has been achieved so far over metal-based catalysts in the presence of homogeneous acids, however, this strategy induces environmental problems related to separation, recycling of acid solutions and corrosion of equipment [START_REF] Nishimura | Selective Hydrogenation of Biomass-Derived 5-Hydroxymethylfurfural (HMF) to 2,5-Dimethylfuran (DMF) under Atmospheric Hydrogen Pressure over Carbon Supported PdAu Bimetallic Catalyst[END_REF] .

Traditional heterogeneous bifunctional metal catalysts with acidic supports have been reported for the production of DMF from HMF at mild conditions [START_REF] Zu | Efficient Production of The Liquid Fuel 2,5-Dimethylfuran from 5-Hydroxymethylfurfural over Ru/Co 3 O 4 Catalyst[END_REF][START_REF] Gawade | Biobased Green Process: Selective Hydrogenation of 5-Hydroxymethylfurfural to 2,5-Dimethyl Furan under Mild Conditions Using Pd-Cs 2.5 H 0.5 PW 12 O 40 /K-10 Clay[END_REF][START_REF] Kong | Ni Nanoparticles Inlaid Nickel Phyllosilicate as a Metal-Acid Bifunctional Catalyst for Low-Temperature Hydrogenolysis Reactions[END_REF][START_REF] Guo | Selective Hydrogenolysis of 5-Hydroxymethylfurfural to Produce Biofuel 2, 5-Dimethylfuran over Ni/ZSM-5 Catalysts[END_REF][START_REF] Han | Selective Hydrogenolysis of 5-Hydroxymethylfurfural to 2,5-Dimethylfuran Catalyzed by Ordered Mesoporous Alumina Supported Nickel-Molybdenum Sulfide Catalysts[END_REF][START_REF] Saha | Zinc-Assisted Hydrodeoxygenation of Biomass-Derived 5-Hydroxymethylfurfural to 2,5-Dimethylfuran[END_REF] . Zu et al. have developed a heterogeneous Ru/Co 3 O 4 catalyst for hydrodeoxygenation of HMF to DMF and unveiled that Ru was responsible for hydrogenation of carbonyl groups to the hydroxyl groups, while CoO x species played an important role in the hydrogenolysis of hydroxyl groups to methyl groups [START_REF] Zu | Efficient Production of The Liquid Fuel 2,5-Dimethylfuran from 5-Hydroxymethylfurfural over Ru/Co 3 O 4 Catalyst[END_REF] . A 93.4% yield was achieved at the temperature of 130 o C. Bifunctional Pd-Cs 2.5 H 0.5 PW 12 O 40 /K10 clay has been found as an efficient catalyst for HMF deoxygenation [START_REF] Gawade | Biobased Green Process: Selective Hydrogenation of 5-Hydroxymethylfurfural to 2,5-Dimethyl Furan under Mild Conditions Using Pd-Cs 2.5 H 0.5 PW 12 O 40 /K-10 Clay[END_REF] . With the rapid hydrogenolysis rate facilitated by the acidic support, a yield up to 80 % could be obtained at 90 o C. Note that in the conventional catalysts, metal and acid functions are spatially separated. Metal sites are localized over metal nanoparticles, while acid sites are related to the zeolite phase. It has been shown that the relative proximity of metal sites in the Pt/Al 2 O 3 catalysts modified by phosphonic acid could significantly improve the selectivity and activity towards hydrodeoxygenation (HDO) of alcohols [START_REF] Zhang | Control of Interfacial Acid-metal Catalysis with Organic Monolayers[END_REF] . Traditional bifunctional metal-acid catalysts such as metals supported over zeolites, cannot therefore provide a high level of intimacy between metal and acid sites for deoxygenation and result in undesirable hydrogenation and polymerization reactions [START_REF] Kong | Ni Nanoparticles Inlaid Nickel Phyllosilicate as a Metal-Acid Bifunctional Catalyst for Low-Temperature Hydrogenolysis Reactions[END_REF][START_REF] Guo | Selective Hydrogenolysis of 5-Hydroxymethylfurfural to Produce Biofuel 2, 5-Dimethylfuran over Ni/ZSM-5 Catalysts[END_REF][START_REF] Han | Selective Hydrogenolysis of 5-Hydroxymethylfurfural to 2,5-Dimethylfuran Catalyzed by Ordered Mesoporous Alumina Supported Nickel-Molybdenum Sulfide Catalysts[END_REF] .

Combining metal and acid properties in a close proximity over the surface of metal nanoparticles could provide a breakthrough in the development of highly efficient processes for deoxygenation reactions [START_REF] Wu | In Situ Generation of Brønsted Acidity in the Pd-I Bifunctional Catalysts for Selective Reductive Etherification of Carbonyl Compounds under Mild Conditions[END_REF] . These sites would allow at the same time hydrogenation or carbonyl group, protonation, dehydration and hydride addition as the final step (Scheme 1). There are very few papers about modification of nanoparticles of noble metals with halogens. Precious metal catalysts covered by bromide salts have been reported for the selective production of cyclohexanone from aromatic ethers [START_REF] Meng | Synthesis of Ketones from Biomass-derived Feedstock[END_REF] . The Au-Pd catalysts on MgO and C supports for the direct synthesis of hydrogen peroxide were pretreated with aqueous bromide solution [START_REF] Ntainjua | The Effect of Bromide Pretreatment on the Performance of Supported Au-Pd Catalysts for the Direct Synthesis of Hydrogen Peroxide[END_REF] .

The bromide pretreatment significantly enhanced H 2 O 2 productivity over the MgO-based catalysts. The promotional effect has been correlated to selective poisoning of sites responsible for H 2 O 2 hydrogenation and decomposition. Our approach here relies on the deposition of electronegative Br atoms over Pd surface by bromobenzene hydrogenolysis, which induces the generation of strong Brønsted acidity by hydrogen dissociation over Pd. These dual sites enable extremely selective hydrodeoxygenation of HMF to DMF at ambient temperature which has been achieved so far.

Experiment section

Materials

Pd/Al 2 O 3 and Pd/C (5 wt. % metal loading) were purchased from Johnson Matthey Chemicals Company. Pd/SiO 2 (5 wt.%) was a home-made catalyst prepared by incipient wetness impregnation. Pd black, bromobenzene, iodobenzene, chlorobenzene, and ammonium bromide (NH 4 Br) were purchased from Alfa-Aesar Materials Company. Tetrahydrofuran (THF), 5-hydroxymethylfurfural (HMF), 1-bromohexadecane, pyridine, elemental Br 2 , HBr, Pd 0 (PPh 3 ) 4 , benzyl alcohol, dimethylfuran (DMF), 3-phenylropanal, and biphenyl have been supplied by Sigma-Aldrich Company. All chemicals were of analytical grade and used as received without further purification.

Catalytic reactions

The hydrogenation reaction has been performed in a 25 ml stainless-steel autoclave equipped with a magnetic stirrer, pressure gauge and an automatic temperature controller. Typically, for the hydrodeoxygenation of HMF, 5 g of THF, 0.1 g of HMF and 50 mg of catalyst with or without 10 µl of bromobenzene were added into the reactor. Afterward, the reactor was sealed and pressurized by 5 or 20 bar of hydrogen, followed by heating up to the target temperature with continuous magnetic stirring. After the reaction, the reactor was cooled down, the solution was separated by filtration and the products were identified by GC-MS (Agilent Technologies 5977A MSD with Agilent Technologies 7890B GC system equipped with HP-5 capillary column) and quantified by GC (Agilent Technologies 7820A, equipped with HP-5 capillary column and flame ionization detector) with biphenyl as the internal standard.

The conversion of HMF, selectivity and yield to corresponding products were defined as follows:

Conversion (%) =
Selectivity to the product p (%) = Yield (%) = Conversion Selectivity and refers to the final, initial number of the moles of HMF, respectively.

is number of the moles of converted HMF to product p. Biphenyl was used as an internal standard for quantification.

Kinetic reaction rate constants (k) of benzyl alcohol hydrodeoxygenation to toluene were calculated by linearization of the reaction rate by the first-order reaction law as follows:

ln (A) = -kt + ln A 0 ,
where A is the concentration of benzyl alcohol (mol/L), k is rate constant (min -1 ), t is time (min). A 0 is the initial concentration of benzyl alcohol. A plot of ln (A) vs time gives the rate constant from the slope of the linear regression equation.

Activation energy E a was calculated as follows:

,

where k 1 and k 2 represent the kinetic reaction rate constants measured at the T 1 and T 2 temperatures, respectively.

Catalyst preparation

To

Catalyst characterization

For transmission electron microscopy (TEM) analysis, the scanning mode were carried out on a JEOL 2100 FEG S/TEM microscope operated at 200 kV and equipped with a spherical aberration probe corrector. Before analysis, the samples were dispersed in ethanol and deposited on a holey carbon coated TEM grid. In STEM, the images were recorded using a high-angular annular dark field (HAADF)

detector with inner and outer diameters of about 73 and 194 mrad. Energy Dispersive X-ray Spectroscopy (EDS) analyses allowing for the elemental mapping of Pd and Br, were performed in STEM using a JEOL Silicon Drift Detector (DrySD60GV, sensor size 60 mm 2 ) with a solid angle of approximately 0.6 srad.

X-ray fluorescence (XRF) analysis was performed with the use of an energy dispersive micro-X Ray Fluorescence spectrometer M4 TORNADO (Bruker). This instrument is equipped with 2 anodes a Rhodium X-ray tube 50 kV/600 mA (30 W) and a Tungsten X-Ray tube 50 kV/700 mA (35 W). For sample characterization, the X-rays Rhodium with a polycapillary lens enabling excitation of an area of 200 μm was used. The detector used was a Silicon-Drift-Detector Si(Li) with <145 eV resolution at 100000 cps (Mn Kα) and cooled with a Peltier cooling (253°K). The measurement was done under vacuum (20 mbar). The elements, that can be measured by this instrument unit range from sodium (Na) to uranium (U). Quantitative analysis was done using fundamental parameter (FP) (standardless). As elements are present in stoichiometric compounds, its formula was used for quantification of the weight percent of each element.

X-ray photoelectron spectroscopy (XPS) analysis has been performed in a

ThermoFischer ESCALAB 250Xi photoelectron spectrometer using monochromated X-ray irradiation Al Kα (1486.7 eV) and 180° double-focusing hemispherical analyzer with a six-channel detector. The binding energy (BE) of the photoemission spectra was calibrated to Al 2p peak with BE 74.5 eV for Al-containing samples and to adventitious carbon C 1s peak with BE 284.8 eV for others.

The CO pulse adsorption was performed on AutoChem II 2920 apparatus from Micromeritics. 80 mg of sample was loaded in a quartz reactor and then the samples were firstly reduced in a flow of 5% H 2 /Ar flow (60 ml/min) with heating rate 10 ºC/min at 60 ºC for 0.5 h then cooling down to 45 ºC. The catalyst has been treated by CO pulses in a He flow till full saturation.

Fourier transform infrared (FTIR) spectra were recorded using a Thermo Fisher Scientific Nicolet 6700 FTIR (32 scans at a resolution of 4 cm -1 ) equipped with a mercury cadmium telluride (MCT) detector. Pyridine and CO-FTIR experiments were performed in a vacuum cell (less than 10 -5 torr). The catalyst samples for analysis were pressed in a 40~50 mg/cm 2 (D=13 mm) self-supporting discs. Before the analysis, all the samples were reduced at 60 ºC for 1 h with subsequent vacuum treatment for 3 h. CO adsorption has been performed by the addition of CO doses in the cell at room temperature till full saturation. Pyridine (Py) adsorption has been performed in the same way by the addition of Py doses. For some experiments, 500 torr of hydrogen have been introduced in the cell after Py adsorption.

Results and discussions

HMF deoxygenation over the Pd-Br catalyst at 60°C

Hydrogenation of HMF has been conducted at 60 o C under 20 bar of H 2 pressure for 1 h both over promoted and unpromoted Pd catalysts. First of all, hydrogenation of HMF was performed over the non-promoted 5 wt% Pd/Al 2 O 3 catalyst. In agreement with previous reports, the main products were BHMTHF and 1,2,6-hexanetriol (HT), respectively, representing the strong hydrogenation and hydrogenolysis abilities of Pd (Table 1, Entry 1) [START_REF] Chen | Selective Hydrogenation of Biomass-Based 5-Hydroxymethylfurfural over Catalyst of Palladium Immobilized on Amine-Functionalized Metal-Organic Frameworks[END_REF] . The main reaction products are given in BrBen totally changes the selectivity patterns during HMF hydrogenation over both supported and non-supported Pd catalysts. The effect seems to be not related to the catalytic support.

To clarify the effect of Br on the catalytic system, we have performed the pre-treatment of Pd catalysts by BrBen and hydrogen without HMF in the reactor.

After the pretreatment, the Br-Pd/Al 2 O 3 , Br-Pd/C, Br-Pd/SiO 2 catalysts and Br-Pd black have been separated from the liquid phase, thoroughly washed and tested in the conversion of HMF (Table 1, Entry 12-15). Interestingly, although the pre-treated catalyst showed a lower activity at the same reaction conditions in comparison to the non-modified catalyst in the presence of BrBen, the selectivity has totally changed.

The main products of the reaction over the non-promoted Pd catalysts were BHMTHF and HT. Over the pre-modified Pd-Br catalysts, the product distribution has changed to BHMF, MFA and DMF. It is worth noting that BHMF and MFA can be considered as intermediates for hydrodeoxygenation of HMF to DMF [START_REF] Goyal | Studies of Synergy Between Metal-Support Interfaces and Selective Hydrogenation of HMF to DMF in Water[END_REF] . They exhibit high selectivity over Pd/Al 2 O 3 at lower conversion in the presence of BrBen (Table 1, As shown in Table S2, SI, both catalytic systems show almost no activity for HMF conversion.

Entry 3-4). To investigate the mechanism of promotion of

HMF deoxygenation at ambient temperature

HMF is a complex biomass-based molecule, which is not stable during thermochemical reactions due to fast condensation reactions [START_REF] Chimentão | Selective Hydrodeoxygenation of Biomass Derived 5-Hydroxymethylfurfural over Silica Supported Iridium Catalysts[END_REF][START_REF] Wang | Recent Advances in Catalytic Conversion of Biomass to 5-Hydroxymethylfurfural and 2, 5-Dimethylfuran[END_REF] . Thus, it would be highly desirable to perform hydrodeoxygenation at ambient reaction conditions where condensation does not take place.

Other promoting agents

Other bromine sources such as 1-bromohexadecane, ammonium bromide, elemental Br 2 , and HBr have been tested for this reaction (Table 1

, Entry 19-22).

With addition of 1-bromohexadecane and ammonium bromide, the resulting catalysts, however, demonstrated the catalytic performance similar to the non-promoted Pd catalyst. This fact is due to the insignificant hydrogenolysis of these bromine compounds over the Pd surface. The absence of the products of decomposition of 1-bromohexadecane during these pretreatments was further confirmed by the GC-MS analysis (Figure S3, SI). Oxidative dissociation of elemental Br 2 on the Pd surface has been reported [START_REF] Tysoe | Surface Chemistry of The Metal-halogen Interface: Bromine Chemisorption and Dibromide Formation on Palladium (111)[END_REF] , however, in the presence of H 2 , elemental Br 2 cannot efficiently modify the Pd surface and demonstrates the product distribution similar to the initial Pd catalyst. It is interesting to note that the addition of HBr leads to the synthesis of DMF as the main product. However, in comparison to the promotion effect of BrBen, HBr leads to lower conversion (81%) and a significant amount of side-products.

These results are in agreement with the literature which showed that side-reactions such as acid-catalyzed condensation could happen in the presence of homogeneous acids [START_REF] Mitra | Pd/C-Catalyzed Reactions of HMF: Decarbonylation, Hydrogenation, and Hydrogenolysis[END_REF] . The decrease of activity could be explained by the competitive adsorption of The effect of other halogens such as Cl and I has been also studied. The HMF conversion over Pd/Al 2 O 3 was investigated in the presence of ClBen and IBen (Table 1, Entry 23-24). The presence of iodine resulted in the complete deactivation of the catalyst. Our earlier studies have confirmed that the treatment of Pd by organic iodine leads to full coverage of Pd surface by I [START_REF] Wu | In Situ Generation of Brønsted Acidity in the Pd-I Bifunctional Catalysts for Selective Reductive Etherification of Carbonyl Compounds under Mild Conditions[END_REF] . The addition of ClBen leads to low deoxygenation activity and noticeable hydrogenation of furan-rings. Thus, in the row of halogens, the activity of the catalysts in HMF hydrogenation increases in row These results suggest the low stability of Cl on Pd surface in the reaction.

To provide more insights into the reaction pathway, the kinetic study of HMF hydrogenation over Br-Pd/Al 2 O 3 has been performed at ambient temperature (Figure 1). As expected, the conversion of HMF increases with the increase in the reaction time. The selectivity to BHMF is close to 100% at the initial reaction time. This suggests that BHMF is the primary product of the reaction over Br-Pd/Al S6, SI). The TOF numbers of the primary products are given in Table S3, SI. The Pd-Br catalyst shows high TOF numbers for the BHMF formation (579.5 h -1 ). However, the parent Pd catalyst exhibits a higher BHMTHF formation rate (46.5 h -1 ). The parent Pd catalyst is totally inactive in hydrodeoxygenation during the whole reaction in comparison with the Br modified catalyst. Thus, the role of Br can be assigned both to suppression of the ring hydrogenation pathway and activation of the hydrodeoxygenation route.

To confirm the recyclability of Pd-Br catalyst, the hydrodeoxygenation of HMF

Characterization of the Pd-Br catalysts

The Pd-Br catalysts have been studied further to identify the nature of the interaction of Br with the catalyst and nature of active sites for HMF hydrodeoxygenation.

First of all, the location of Br atoms was determined by TEM-EDS analysis. Due Br-Pd/Al 2 O 3 , which could be ascribed to the presence of a higher fraction of defect sites over Pd nanoparticles on the C support. [START_REF] Ayyoub | A Comparative Study of Solvent-Free and Highly Efficient Pinene Hydrogenation over Pd on Carbon, Alumina, and Silica Supports[END_REF] FTIR spectroscopy of CO adsorption has been used to identify the localization of Br. A set of CO absorption bands have been observed on non-promoted Pd/Al 2 O 3 .

The band at 2084 cm -1 with a shoulder at 2057 cm -1 is due to CO linearly adsorbed on defect sites of Pd nanoparticles presence at edges and corners. The intensive band at 1941 cm -1 is related to CO adsorbed on the Pd nanoparticles planes with bridge-bonds (Figure 4) [START_REF] Campisi | Selectivity Control in Palladium-Catalyzed Alcohol Oxidation through Selective Blocking of Active Sites[END_REF] . The modification of Pd by Br leads to the almost complete disappearance of the peak at 2084 cm -1 with an obvious decrease in the intensity of the peaks attributed to CO adsorption over planes. Well-resolved peaks at 1973 cm -1 and 1934 cm -1 over Br-Pd/Al 2 O 3 could be assigned to CO adsorbed on Pd (100) and

(111) facets, respectively [START_REF] Campisi | Selectivity Control in Palladium-Catalyzed Alcohol Oxidation through Selective Blocking of Active Sites[END_REF] . The less adsorption of CO on (100) facets and the defect sites could be explained by simplified hydrogenolysis of BrBen and deposition of Br over these low-coordination sites of Pd. Moreover, a small peak with a higher frequency at 2140 cm -1 is visible and assigned to CO bonded to Pd-Br sites, where electronic deficient Pd heightens the vibrational frequency of adsorbed CO [START_REF] Ball | AgPd and CuPd Catalysts for Selective Hydrogenation of Acetylene[END_REF][START_REF] Nieminen | Intrinsic Metal Size Effect on Adsorption of Organic Molecules on Platinum[END_REF] .

The 5a, has two doublets with BE (Pd 3d 5/2 ) at 335.3 eV and 336.6 eV attributed to metallic Pd 0 and Pd 2+ (PdO), respectively [START_REF] Bhogeswararao | Catalytic Conversion of Furfural to Industrial Chemicals over Supported Pt and Pd Catalysts[END_REF] .

Both of these states coexist on the surface with the dominant metallic state.

Interestingly, a shift (0.3 eV) of Pd 3d peaks to higher BE for the Br-Pd/Al 2 O 3 catalysts has been observed after modification by Br. It can be attributed to the interaction of metallic Pd with Br adatoms with the electron withdrawal from Pd to electrophilic Br [START_REF] Komanoya | Electronic Effect of Ruthenium Nanoparticles on Efficient Reductive Amination of Carbonyl Compounds[END_REF] . Signal of oxidized Pd 2+ in PdO can be observed before and after modification by Br due to the partial oxidation of surface Pd atoms. Notably, an additional peak appears at much higher binding energy at 338.0 eV in the Pd 3d XPS spectrum after modification of the Br-Pd/Al 2 O 3 catalyst. PdBr 2 has a signal of Pd 2+

with binding energy at 337.8 eV. The peak at higher binding energy at 338.0 eV could be assigned to the presence of PdBr 2 species adsorbed over the surface of metal nanoparticles.

For the Br-Pd/Al 2 O 3 catalyst, the Br 3d 5/2 peak is located at 68.6 eV, which is lower than 69.1 eV for Br -in PdBr 2 . The red-shift (0. (Figure 6). In the absence of hydrogen, the spectra of Py adsorption are very similar.

Both non-promoted Pd/Al 2 O 3 and Br-Pd/Al 2 O 3 exhibit a peak at 1449 cm -1 attributed to the Lewis acid sites, which are probably present on the Al 2 O 3 support [START_REF] An | Acid-Promoter-Free Ethylene Methoxycarbonylation over Ru-Clusters/Ceria: The Catalysis of Interfacial Lewis Acid-Base Pair[END_REF] . Notably, there is no peak at 1540 cm -1 for both catalysts, which is related to the adsorption of pyridine over the Brønsted acid sites. In-situ pretreatment of Br-Pd/Al 2 O 3 in H 2 at 60 o C at atmospheric pressure with subsequent adsorption of Py after hydrogen evacuation, leads to the appearance of a well-resolved peak at 1538 cm -1 indicating the generation of Brønsted acidity in the presence of hydrogen. It is worth mentioning that if hydrogen were still in the cell, Py has been hydrogenated leading to the complex spectrum (Figure S9, SI). Thus, the exposure of the Br-Pd/Al 2 O 3 catalyst to hydrogen generates Brønsted acidity. This acidity is stable and does not disappear after the removal of hydrogen. The phenomenon is different from our previous data reported on the Pd catalyst treated by iodine, where the Brønsted acidity vanished after removing of H 2 [START_REF] Wu | In Situ Generation of Brønsted Acidity in the Pd-I Bifunctional Catalysts for Selective Reductive Etherification of Carbonyl Compounds under Mild Conditions[END_REF] . This difference can be attributed to the higher However, the addition of H 2 in the FTIR cell results in the appearance of the Brønsted acidity. These results confirm the generation of Brønsted acidity by the dissociation of H 2 to form Pd-Br-(H + ).

Model reactions

It is interesting to note that the presence of Br on the surface of Pd results in suppression of the hydrogenation of the aromatic ring in HMF. Considering that the hydrogenation of aromatic-rings is a moderately structure sensitive reaction [START_REF] Pushkarev | Hydrogenation of Benzene and Toluene over Size-controlled Pt/SBA-15 Catalysts: Elucidation of The Pt Particle Size Effect on Reaction Kinetics[END_REF] , the suppression of aromatic ring hydrogenation can be assigned to the electronic effect of Br over the Pd surface leading to a decrease of the electronic density over Pd with low homolytic hydrogenation activity. The strong electronic effect on aromatic rings hydrogenation has been confirmed in the literature [START_REF] Cui | Highly Selective Hydrogenation of Arenes Using Nanostructured Ruthenium Catalysts Modified with A Carbon-nitrogen Matrix[END_REF][START_REF] Jiang | Selective Hydrogenation of Aromatic Carboxylic Acids over Basic N-doped Mesoporous Carbon Supported Palladium Catalysts[END_REF] . In contrast, this catalyst provides high activity for heterolytic dissociation of H 2 towards hydride and proton on the Pd-Br sites. This results in the appearance of the Brønsted acidity and may favor the hydrogenation of polar functional groups [START_REF] Mitsui | Dissociative Hydrogen Adsorption on Palladium Requires Aggregates of Three or More Vacancies[END_REF][START_REF] Doudin | Understanding Heterolytic H 2 Cleavage and Water-Assisted Hydrogen Spillover on Fe 3 O 4 (001)-Supported Single Palladium Atoms[END_REF] . To support these assumptions,

Discussion

Hydrogenation of HMF to DMF has been widely studied by different groups (Table 3). Almost for all catalysts the catalytic route for DMF synthesis passes through 3 steps: 1) Conversion of HMF to BHMF or methylfurfural (MFF), 2)

hydrogenation of BHMF or MFF to MFA and finally 3) deoxygenation of MFA to DMF [START_REF] Aellig | Combined 1,4-Butanediol Lactonization and Transfer Hydrogenation/Hydrogenolysis of Furfural-Derivatives under Continuous Flow Conditions[END_REF] . The scheme proposed during kinetic study of hydrodeoxygenation of HMF over Pd-Br catalyst in agreement with the literature reported by Alexis T. Bell et al.

(Table 1) [START_REF] Chidambaram | A Two-Step Approach for the Catalytic Conversion of Glucose to 2,5-Dimethylfuran in Ionic Liquids[END_REF] .

However, this route is usually accompanied by different side reactions, which could be subdivided into two groups: hydrogenation of aromatic ring instead of the C=O and C-OH bonds and oligomerization of HMF at high temperature. Generally, the hydrogenation of the C=C bonds is thermodynamically preferably than hydrogenation of C=O. However, the C=C bond in furan ring is stabilized by aromatic conjugation, which makes the situation different [START_REF] Volkov | Mild Deoxygenation of Aromatic Ketones and Aldehydes over Pd/C Using Polymethylhydrosiloxane as the Reducing Agent[END_REF] . It results in the formation of different by-products like MTHFA, BHMTF etc. The by-products formation can also lead to catalyst deactivation. Indeed, even heating of HMF to 100 o C in the presence of water leads to generation of humins [START_REF] Shen | Evolution Process and Controlled Synthesis of Humins with 5-Hydroxymethylfurfural (HMF) as Model Molecule[END_REF] .

The catalytic performance of Br-Pd/Al 2 O 3 has been compared with the most efficient catalytic systems in the literature. Table 3 shows two different types of catalysts that have been reported for the hydrogenation HMF to DMF. Metallic catalysts without acid promotions need high temperature for DMF production from HMF. For example, Román-Leshkov et al. integrated a CuRu/C catalyst in the process of production of DMF from fructose [START_REF] Roman-Leshkov | Production of Dimethylfuran for Liquid Fuels from Biomass-derived Carbohydrates[END_REF] . A yield up to 71% in the liquid phase hydrogenolysis of HMF was obtained at 220 o C and 6.8 bar H 2 pressure. The Pt-Co bimetallic on multi-walled carbon nanotubes (MWCNTs) catalyst needs a high temperature of up to 160 o C for this reaction (>90 %) [START_REF] Wang | Hydrogenolysis of 5-Hydroxymethylfurfural to 2,5-Dimethylfuran over Supported Pt-Co Bimetallic Catalysts under Mild Conditions[END_REF] .

While pure metallic catalysts need high temperature for DMF production and often exhibit fast deactivation, the usage of metal-acid bifunctional catalytic systems could significantly promote the deoxygenation rate of HMF to DMF (Table 3, Entry 8-14) [START_REF] Mitra | Pd/C-Catalyzed Reactions of HMF: Decarbonylation, Hydrogenation, and Hydrogenolysis[END_REF][START_REF] Nishimura | Selective Hydrogenation of Biomass-Derived 5-Hydroxymethylfurfural (HMF) to 2,5-Dimethylfuran (DMF) under Atmospheric Hydrogen Pressure over Carbon Supported PdAu Bimetallic Catalyst[END_REF][START_REF] Chatterjee | Hydrogenation of 5-Hydroxymethylfurfural in Supercritical Carbon Dioxide-Water: A Tunable Approach to Dimethylfuran Selectivity[END_REF][START_REF] Thananatthanachon | Efficient Production of The Liquid Fuel 2,5-Dimethylfuran from Fructose using Formic Acid as A Reagent[END_REF][START_REF] Zu | Efficient Production of The Liquid Fuel 2,5-Dimethylfuran from 5-Hydroxymethylfurfural over Ru/Co 3 O 4 Catalyst[END_REF][START_REF] Gawade | Biobased Green Process: Selective Hydrogenation of 5-Hydroxymethylfurfural to 2,5-Dimethyl Furan under Mild Conditions Using Pd-Cs 2.5 H 0.5 PW 12 O 40 /K-10 Clay[END_REF][START_REF] Kong | Ni Nanoparticles Inlaid Nickel Phyllosilicate as a Metal-Acid Bifunctional Catalyst for Low-Temperature Hydrogenolysis Reactions[END_REF][START_REF] Guo | Selective Hydrogenolysis of 5-Hydroxymethylfurfural to Produce Biofuel 2, 5-Dimethylfuran over Ni/ZSM-5 Catalysts[END_REF][START_REF] Han | Selective Hydrogenolysis of 5-Hydroxymethylfurfural to 2,5-Dimethylfuran Catalyzed by Ordered Mesoporous Alumina Supported Nickel-Molybdenum Sulfide Catalysts[END_REF][START_REF] Saha | Zinc-Assisted Hydrodeoxygenation of Biomass-Derived 5-Hydroxymethylfurfural to 2,5-Dimethylfuran[END_REF] . The conversion of HMF to DMF over metal-acid catalysts is performed by metallic sites responsible for hydrogenation of carbonyl groups, while the acid sites of support are required for C-OH bond activation and simultaneous hydrogenolysis on metallic sites. The intimacy of metal and acid sites in the bifunctional catalysts seems to be primordial for deoxygenation. Insufficient contact between metal and acid sites limits the deoxygenation efficiency of HMF to DMF.

Up to date, the most active bifunctional metal-acid catalytic system for the HMF deoxygenation has been the combination of heterogeneous metal catalyst with homogenous acid (Table 3, Entry 12-14) [START_REF] Mitra | Pd/C-Catalyzed Reactions of HMF: Decarbonylation, Hydrogenation, and Hydrogenolysis[END_REF][START_REF] Nishimura | Selective Hydrogenation of Biomass-Derived 5-Hydroxymethylfurfural (HMF) to 2,5-Dimethylfuran (DMF) under Atmospheric Hydrogen Pressure over Carbon Supported PdAu Bimetallic Catalyst[END_REF][START_REF] Chatterjee | Hydrogenation of 5-Hydroxymethylfurfural in Supercritical Carbon Dioxide-Water: A Tunable Approach to Dimethylfuran Selectivity[END_REF][START_REF] Thananatthanachon | Efficient Production of The Liquid Fuel 2,5-Dimethylfuran from Fructose using Formic Acid as A Reagent[END_REF] . In the presence of acetic acid, the yield of DMF increased from 0 to 85% over Pd/C at 120 o C [START_REF] Mitra | Pd/C-Catalyzed Reactions of HMF: Decarbonylation, Hydrogenation, and Hydrogenolysis[END_REF] . Bimetallic PdAu/C exhibited a significant yield of over 99% for selective hydrogenation of HMF toward DMF in the presence of homogeneous HCl at 60 o C [START_REF] Nishimura | Selective Hydrogenation of Biomass-Derived 5-Hydroxymethylfurfural (HMF) to 2,5-Dimethylfuran (DMF) under Atmospheric Hydrogen Pressure over Carbon Supported PdAu Bimetallic Catalyst[END_REF] . The homogenous acids provide intimacy of acid sites around the metal surface, which could protonate the substrate hydroxyl group and polarize the C-O bond and favor the cleavage of the C-O bond over Pd [START_REF] Volkov | Mild Deoxygenation of Aromatic Ketones and Aldehydes over Pd/C Using Polymethylhydrosiloxane as the Reducing Agent[END_REF] . However, the addition of homogeneous organic or inorganic acid at the same time could result in the side self-condensation of HMF, which lowers the DMF productivity [START_REF] Shen | Evolution Process and Controlled Synthesis of Humins with 5-Hydroxymethylfurfural (HMF) as Model Molecule[END_REF] . Besides, the presence of liquid acids in the reaction mixture leads to the corrosion of equipment.

The TOF numbers towards DMF formation were calculated based on the amount of active sites reported in the literature (Table 3). Pd-Br sites exhibited higher activity to DMF (TOF = 181.7 h -1 ) than most of the catalysts from the literature.

Thus, the elaboration of efficient catalysts for deoxygenation of biomass-derived compounds faces the following dilemma. Heterogeneous metal catalysts with acidic support do not have require metal-acid intimacy, while the combination of the metal catalysts with homogenous acid suffers from the side-reactions and corrosion issues.

Our results demonstrate that the supported bifunctional metal catalysts promoted with bromine (Table 3) exhibit several advantages:



Conclusion

In this paper, an efficient Pd-Br bifunctional catalyst with intimate metal and acid sites, which are both localized over the same metal nanoparticles, was developed and exhibited extremely high yield (96 %) to DMF from HMF at ambient temperature.

The promotion with bromine results in double effect on the catalytic performance. On the one hand, it tunes the hydrogenation activity of Pd active sites. On the other hand, the promotion bromine creates Brønsted acid sites, which are situated in extremely close proximity to the metal sites. The adsorbed Br atoms on the Pd surface can prevent the hydrogenation of furan-ring, while the hydrogenation of carbonyl groups is not affected. 2. The reaction was performed at 30 o C with 5 bars of external H2 pressure for 6 hours.

3. 15 mg NH4Br, 15 mg C16H33Br, 5 mg Br2, or 5 mg HBr were added to the reaction mixture, respectively.

4. ClBen and IBen were added for 10 µl in the reaction mixture, respectively.

  Pd/Al 2 O 3 using BrBen, the reaction mixture after catalysis has been analyzed by GC-MS. The presence of benzene in the reaction mixture indicates the dehalogenation of BrBen over the Pd surface (Figure S1, SI). Therefore, it is rationally to suspect that the hydrogenolysis of BrBen over Pd surface leads to the modification of Pd surface with Br atoms and the newly formed Pd-Br sites could be responsible for the changes of selectivity during the conversion of HMF. The heterogeneous nature of the Pd-Br catalyst was further confirmed by a hot filtration test (Figure S2, SI). The catalyst has been filtered out and the test has been continued without additional activity. It indicates that the reaction was catalyzed by heterogeneous Pd-Br sites. This conclusion has been further supported by HMF conversion over homogeneous Pd 0 (PPh 3 ) 4 catalyst in the presence and without BrBen.

  The high reactivity of elemental Br 2 and HBr can result in the reaction of bromine source with support with the generation of non-selective sites and partial dissolution of support. Pre-treatment by BrBen provides a selective tool for modification of only Pd surface in the presence of H 2 . As illustrated in Figure1, the oxidative addition of BrBen to the surface Pd atoms leads to the formation of (Br)-Pd-Ben, which further reacts with H 2 to form Pd(H)(Br) and release of benzene (FigureS4, SI). At the same time, pure Al 2 O 3 support cannot perform the hydrogenolysis of BrBen at the same reaction conditions.

  I<Br<Cl with zero activity of the iodine-promoted catalysts, because of full coverage of Pd by iodine and low effect of chlorine on the Pd catalyst. The increase in the electronegativity of the elements in this row should also lead to an increase in the polarity of the Pd-Hal bond. Bromine demonstrates intermediate activity with significant enhancement of selectivity toward deoxygenation instead of the aromatic ring hydrogenation. Moreover, the stability of Cl on the Pd surface was verified by reusing the Cl-Pd/Al 2 O 3 catalyst consequently for three cycles. As indicated in Figure S5, SI, the deoxygenation ability of Cl-Pd catalyst significantly decreased in the second cycle and the selectivity in the third cycle was similar to the initial Pd/Al 2 O 3 .

has been performed at 30

 30 o C and 5 bar of H 2 over the Br-Pd/Al 2 O 3 catalyst in three consecutive cycles with intermediate separation of the catalyst (Figure 2). The catalyst demonstrates comparable activity in HMF transformation with a slight decrease in the 2 and 3 cycles, which could be explained by the loss of catalysts during separation. The selectivity curves are very similar for all three cycles with the same distribution of the products and high selectivity to DMF (>90%) at high conversions. No catalyst deactivation was observed. Moreover, additional stability tests have been performed for 5 cycles at high conversion at ambient temperature (Figure S7, SI). Despite a slight decrease of conversion due to the loss of catalyst during separation, the Pd-Br catalyst showed similar product selectivity, confirming the high stability of the Pd-Br catalyst.

  to the superposition of the main Br and Al edges in an EDS spectrum, Br-Pd/SiO 2 with a similar promotion effect was selected as a model catalyst for EDS mapping analysis. EDS mapping clearly shows the preferential localization of Br on the surface of palladium nanoparticles (Figure3). The signal of Br atoms (in blue) is localized at the same places as Pd (in green) indicating spatial coexistence of both elements and a possible modification of Pd surface by the presence of Br atoms. Selective area EDS analysis of Br-Pd/SiO 2 also confirmed the localization of Br on Pd, as the Br signal mostly appears on the Pd nanoparticles and not on the support (FigureS8, SI).The amount of Br on the Br-Pd/Al 2 O 3 catalysts was measured by XRF analysis and the results are summarized in

  5 eV) indicates an increase in the electronic density in comparison with PdBr 2 due to the transfer of electronic density from Pd metallic nanoparticles to Br. The red shift of the Br 3d5/2 XPS peak is consistent with the increase in the electron BE of Pd metal atoms due to the withdrawal of the electron density from palladium towards the bromine atoms. It has to be noted that the XPS spectra of Pd and Br of the Br-Pd/Al 2 O 3 catalyst are not affected by conducting the reaction. Pyridine adsorption has been carried out on non-promoted Pd/Al 2 O 3 and Br-Pd/Al 2 O 3 to study the catalyst acidity in the absence and in the presence of H 2

  electronegativity of bromine in comparison with iodine, which should result in the stronger stabilization of in-situ generated H + over the Br atoms. The same experiment over non-promoted Pd/Al 2 O 3 does not show any generation of the Brønsted acidity (Figure 6). Moreover, the effect of H 2 O on the generation of Brønsted acidity has been studied by Py-FTIR. As shown in Figure S10, SI, there is no Brønsted acidity generation in the simultaneous presence of Pyridine and H 2 O over Pd-Br catalyst.

  additional model experiments over Pd/Al 2 O 3 and Br-Pd/Al 2 O 3 have been performed. These model experiments include hydrogenation of DMF, 3-phenylpropanal and benzyl alcohol. The goal was to demonstrate selectivity of different sites over non-promoted and Br-promoted Pd nanoparticles for hydrogenation of furan ring, carbonyl group and hydrodeoxygenation of hydroxyl group, respectively.

Figure 7

 7 Figure 7 demonstrates higher activity of the non-promoted Pd/Al 2 O 3 catalyst for



  Hydrogenation at ambient temperature;  High-selectivity due to only hydrogenation of the C=O bonds;  High-efficiency due to simultaneous activation and hydrogenolysis of C-OH bonds over metal and acid sites located in close proximity of metal palladium nanoparticles Heterogenization of the catalyst and easy separation of the catalyst and reaction products. Indeed, heterolytic dissociation of hydrogen over the Pd-Br sites should favor hydrogenation of the polar C=O and C-OH groups and should hinder hydrogenation of non-polar C=C bonds as demonstrated in the model reactions (Figure 7). At the same time, the generation of Brønsted acid sites over Pd-Br sites simplifies protonation and hydrogenolysis of the BHMF alcohol with the direct generation of DMF. Thus, Pd-Br catalyst has dual metal-acidic properties and can be used for direct deoxygenation of biomass-derived alcohols and aldehydes. The conventional bifunctional catalysts such as metal-zeolite or metal oxide composites contain metal sites over metal nanoparticles and acid sites over the zeolite or oxide support. The metal and acid sites in these catalysts are often sterically separated and suffer from insufficient intimacy. Different to previously developed metal-acid bifunctional catalysts, the Pd-Br systems have both metal and acid sites located on the surface of the same metal nanoparticles. This results in the unique intimacy of metal and acid sites and in the spectacular enhancement of the catalytic performance and more particularly the reaction selectivity.

H 2 Figure 1 .Figure 2 .

 212 Figure 1. Kinetics of hydrogenation of HMF over Br-Pd/Al 2 O 3 . Reaction conditions:

Figure 3 .Figure 4 . 3 Figure 5 .

 3435 Figure 3. TEM-EDS analysis (a) STEM image and (b-e) corresponding EDS

Figure 6 .

 6 Figure 6. Pyridine-FTIR for Pd/Al 2 O 3 and Br-Pd/Al 2 O 3 with and without hydrogen pre-treatment.

Figure 7 .

 7 Figure 7. Model reactions of hydrogenation of DMF, 3-phenylpropanal, and benzyl

  prepare the BrBen pre-treated Pd catalysts, typically, 200 mg of Pd/Al 2 O 3 ,

	Pd/C, Pd/SiO 2 , or Pd black catalyst, together with 50 µl BrBen and 5 g THF were
	added in the autoclave reactor. Afterwards, the reactor was sealed and pressurized by
	20 bar of hydrogen, followed by heating up to 60 o C with continuous magnetic
	stirring for 1 h. Finally, the catalyst and liquid phase were separated by centrifugation
	and washed with THF for 3 times and dried at 60 o C overnight.
	Br 2 -Pd/Al 2 O 3 and HBr-Pd/Al 2 O 3 catalysts were prepared by modification of
	Pd/Al 2 O 3 under ambient conditions. Typically, 200 mg Pd/Al 2 O 3 catalyst, an
	equivalent amount of Br 2 or HBr (0.3 mmol of Br), and 5 g THF were stirred together
	under ambitious conditions for 15 min. Afterwards, the catalysts were separated by
	centrifugation and washed with THF for 3 times. Finally, the catalysts were dried at
	60 o C overnight.

Table 1

 1

and the detailed other product description is shown in Table

S1

, SI. Surprisingly, when 10 µl of BrBen was added, the product distribution has changed to mainly DMF with 96.7% yield (Table

1, Entry 2

). The promotion effect of BrBen could be observed as well for other Pd catalysts supported over SiO 2 (Pd/SiO 2 ) and C (Pd/C) (Table

1

, Entry 5-10). The non-supported Pd without promoter also provides BHMTHF as the main product (selectivity of 81.3%), while DMF was the dominant product in the presence of BrBen (selectivity of 82.1%). It is worth noting that without Pd, BrBen cannot catalyze the transformation of HMF (Table

1

, Entry 11). These results confirm that

  o C and 20 bar of H 2 . Even at such mild conditions, HMF could be converted to DMF with yield of 95.3 % over the Br-Pd/Al 2 O 3 catalyst prepared by pre-treatment of the parent Pd/Al 2 O 3 with BrBen. A similar performance has been observed in the presence of BrBen added to the reactor. These results demonstrate that the hydrodeoxygenation of HMF could be carried out over Pd-Br catalyst with high selectivity and conversion at ambient temperature. To the best of our knowledge, this is the first ever reported heterogeneous catalyst for hydrodeoxygenation of HMF with H 2 at ambient temperature.

Hydrogenation of HMF at mild reaction conditions over Br-Pd/Al 2 O 3 catalyst has been performed at 30 o C and 5 bar of H 2 for 6 h (Table 1, Entry 16-18). These are rather mild conditions in comparison with the 1 h tests performed at 60

  HBr over Pd surface. Moreover, elemental Br 2 and HBr were used as bromine source to pre-treat the initial Pd/Al 2 O 3 catalyst. It is interesting to note that Br 2 -Pd/Al 2 O 3 produces DMF as the main product (Table S2, SI), indicating the modification of Pd surface with Br at ambient conditions in comparison with no effect of Br 2 added during reaction.

However, HBr-Pd/Al 2 O 3 catalyst produces DMTHF as the main product (selectivity is 61.9%), which can be explained by the modification of Al 2 O 3 with HBr pre-treatment.

  2 O 3 due to the fast hydrogenation of the carbonyl group in the HMF molecule. Afterward, this product has been transformed into MFA and DMF which seem to be secondary products MFA is produced by deoxygenation of one of the hydroxyl groups of BHMF, while DMF is produced by simultaneous deoxygenation of both BHMF hydroxyl groups. Finally, MFA decreases at higher reaction time due to transformation to DMF.It is worth to note that when the reaction time progresses, the rapid decrease of BHMF and the slow increase of MFA and DMF confirmed the high deoxygenation ability of Pd-Br catalyst, which can effectively deoxygenate hydroxyl groups. Thus, as indicated in Table1, the conversion of HMF molecules proceeds with intermediate hydrogenation of the carbonyl group and subsequent hydrodeoxygenation of hydroxyl groups to methyl groups. For comparison, the reaction kinetics at the same reaction conditions has been measured for Pd/Al 2 O

3 (Figure S6, SI). The Pd-Br catalyst yields a high amount of BHMF at initial reaction time, however, Pd without modification yields mainly BHMTHF for the whole reaction time (Figure

Table 2 .

 2 The -pulse chemisorption has been conducted to evaluate the amount of available surface Pd sites on the catalyst. Table2shows that the amount of adsorbed CO over

	the initial Pd/Al 2 O 3 catalyst is 0.16 mmol/g, however, after pretreatment with BrBen,
	the CO adsorption amount decreases to 0.059 mmol/g. The dispersion of Pd (Pd CO /Pd)
	in the non-promoted catalyst corresponds to 33 % according to CO adsorption. This
	decline of the CO adsorption demonstrates the modification of Pd surface with the Br
	atoms, which prevents the adsorption of CO on Pd. The coverage of Pd by Br has
	been calculated by dividing the amount of Br (0.014 mmol/g) by the amount of

amount of Br in the catalyst (0.11 wt. %) is significantly lower in comparison with the amount of Pd. COsurface Pd (0.16 mmol/g), which is equal to 8.7 %. Notably, the Br loading could be tuned by the Pd metal dispersion. As shown in Table

S4

, SI, Pd/C with higher metal dispersion (40%) than Pd/Al 2 O 3 (33%) accumulates a higher amount of Br on the catalyst confirming the localization of Br on the Pd surface. Moreover, the coverage of Br on Br-Pd/C (18.4%) catalyst is significantly higher in comparison with

  electronic state of Pd and Br in the Br-Pd/Al 2 O 3 catalyst has been studied by XPS. Figure 5 demonstrates the Pd 3d and Br 3d core-level spectra of Br-Pd/Al 2 O 3 before and after reaction in comparison with the parent Pd/Al 2 O 3 . The Pd 3d spectrum of Pd/Al 2 O 3 catalyst, as shown in Figure

Br Support Bromobenzene H H 96% yield 30 o C H 2 H 2 Pd Pd

  The formed Pd-Br sites on the Pd surface provide heterolytic dissociation of H 2 for deoxygenation of the hydroxyl groups to DMF. To summarize, this paper describes design of a new type of bifunctional catalysts with enhanced intimacy of metal and acid sites. It deepens the understanding of how halogens can be used for the modification of metal surface for the generation of new types of active sites for activation of hydrogen in a heterolytic way with generation of the Brønsted acidity.

	H -	H +
	Support
	Scheme 1. HMF deoxygenation over Pd-Br catalyst via heterolytic dissociation of
	hydrogen.

Table 1 .

 1 Conversion of HMF over palladium catalysts under different conditions.Unless stated otherwise, the reaction conditions were as follows: 50 mg catalyst, 10 µl BrBen, 5 g THF, 100 mg HMF, 20 bar H 2 , 60 o C, 1h.

	Entry	Catalysts	Ad.	Conv.			Selectivity
				HMF	BHMF	MFA	DMF	BHMTHF	HT	Oth.
				Pd catalysts with and without BrBen
	1	Pd/Al2O3	-	100	0	0	0	60.8	29.5	9.7
	2	Pd/Al2O3	BrBen	100	0	1.2	96.7	0	0	2.1
	3	Pd/Al2O3 1		85.1	0	0	0	72.1	26.9	1.0
	4	Pd/Al2O3 1	BrBen	76.5	16.0	30.8	52.8	0	0	0.4
	5	Pd/C	-	100	0	0	0	42.8	41.3	15.9
	6	Pd/C	BrBen	100	0	1.4	94.9	0	0	3.7
	7	Pd/SiO2	-	100	0	0	0	49.8	45.6	4.6
	8	Pd/SiO2	BrBen	100	0	2.5	97.1	0	0	0.4
	9	Pd black	-	94.2	0	0	0	81.3	18.6	0.1
	10	Pd black	BrBen	85.6	4.4	12.5	82.1	0	0	1.0
	11	-	BrBen	0.7	-	-	-	-	-	-
				Heterogeneous Pd-Br catalysts
	12	Br-Pd/Al2O3	-	77.6	47.9	11.2	40.8	0	0	0.1
	13	Br-Pd/C	-	66.9	63.6	8.5	26.8	0	0	1.1
	14	Br-Pd/SiO2	-	86.5	19.0	34.3	46.5	0	0	0.2
	15	Br-Pd black	-	34.7	57.4	18.6	23.1	0	0	0.9
				Ambient temperature 2
	16	Pd/Al2O3		100	0	0	0	90.2	5.6	4.2
	17	Pd/Al2O3	BrBen	100	0.6	1.5	95.3	0	0	2.6
	18	Br-Pd/Al2O3		100	0	2.6	96.1	0	0	1.3
					Other Br sources 3
	19	Pd/Al2O3	NH4Br	79.1	40.3	0.8	0	37.1	17.3	4.5
	20	Pd/Al2O3	C16H33Br	61.9	12.7	0	0	62.2	21.2	3.9
	21	Pd/Al2O3	Br2	76.7	0.4	5.4	2.1	67.1	18.0	7.0
	22	Pd/Al2O3	HBr	81.0	0.3	1.7	75.9	1.1	10.3	10.7
					Other halogens 4

Table 2 .

 2 Summary of the results obtained from the characterizations of Pd/Al 2 O 3 and Br-Pd/Al 2 O 3

				Br amount	CO amount
		Element analysis (wt. %)
	Catalyst			(mmol/g)	(mmol/g)
		Pd	Br
	Pd/Al2O3	4.7	0	0	0.16
	Br-Pd/Al2O3	5.1	0.11	0.014	0.059

Table 3 .

 3 Hydrodeoxygenation of HMF to DMF over various catalytic systems. TOF number towards DMF formation was calculated based on the number of active sites reported in the literature.

			Conditions	Results
	Entry
		Catalyst			HMF	DMF	TOF 1	Ref.
			Solvent	T, o C
					Conv., %	Sel., %	h -1
								This
	1	Br-Pd/Al2O3	THF	30	100	96.1	181.7
								work
				Metal catalysts
	2	CuRu/C	1-butanol	220	100	79	-	14
	3	Ru/C	isopropanol	190	100	80	11.5	19
	4	Ni/SBA-15	Dioxane	180	95	74.2	-	22
	5	CoCu@C	Ethanol	180	100	99.4	-	23
	6	PtCo/ MWCNTs	1-butanol	160	100	92.3	24.7	18
	7	Ru/CNT	Dioxane	150	97	86.1	472.4	20
			Bifunctional metal-acid catalysts
	8	Pd/C/Zn	THF	150	100	85	70.2	33
	9	Ru/Co3O4	THF	130	100	93.4	-	28
	10	NiSi-PS	Dioxane	130	100	72.9	101.5	30
		Pd-Cs2.5H0.5PW12O40
	11		THF	90	98	80.6	10.8	29
		/K-10 clay
			H2O-Dixoan
	12	Pd/C + Acetic acid		120	95	89.5	8.5	24
			e
	13	Pd/C + HCl	THF	60	99	100	-	25
	14	PdAu/C + HCl	THF	60	99	100	-	25
	1

AUTHOR INFORMATION

Corresponding Author

Andrei Khodakov: E-mail:andrei.khodakov@univ-lille.fr;

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ASSOCIATED CONTENT

Supporting Information. Description of side-products; GC and GC-MS images; IR spectrum; TEM images; Supplementary catalytic test; Kinetic studies.

ACKNOWLEDGMENT

The authors thank Solvay and Lille University for stipend for the PhD research of D.W. and financial support of this work.