Dr Shuo Chen

Dr Carmen Ciotonea

Karine De

Oliveira Vigier

François Jérôme

Dr Robert Wojcieszak

Franck Dumeignil

Dr Eric Marceau

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

Prof Sebastien Royer

Prof Vigier Karine

De Oliveira

Hydrogenation of 5-hydroxymethylfurfural to 2,5-dimethylfuran and 2,5-dihydroxymethyltetrahydrofuran over Ni/SBA-15

Keywords: HMF, hydroconversion, nickel, biofuels, incipient wetness impregnation

Selective hydroconversion of 5-hydroxymethylfurfural (HMF) is of great importance for future chemical and energy supply. Herein, we demonstrate that a monometallic Ni/SBA-15 catalyst, prepared by simple Incipient Wetness Impregnation (IWI) using nickel nitrate precursor, can selectively convert HMF to 2,5-dimethylfuran (DMF) or to 2,5-dimethyltetrahydrofuran (DMTHF) under optimized reaction conditions. The mesopore confined Ni particles facilitate the hydrogenolysis and furan ring hydrogenation reactions. Through the control over reaction time, high yields to DMF (71%, at conversion of 93%) and to DMTHF (97%, at conversion of 100%) can be achieved. The kinetic study affords the determination of a preferential reaction route from HMF to DMTHF through 5-methylfurfural (MFFR) formation as intermediate. Favored route then involves the hydrogenolysis of the hydroxyl group as first step (MFFR production), followed by aldehyde function hydrogenation step to methylfurfuryl alcohol (MFOL).

Introduction

The selective production of biofuels with high octane numbers and energy densities, such as 2,5-dimethylfuran (DMF) and 2,5dimethyltetrahydrofuran (DMTHF) is currently highly sought for. These two molecules derive from 5-hydroxymethylfurfural (HMF), a platform molecules produced by dehydration of hexoses [START_REF] Chen | How Catalysts and Experimental Conditions Determine the Selective Hydroconversion of Furfural and 5-Hydroxymethylfurfural[END_REF]. HMF contains a furan ring bearing both a pending aldehyde function at the C2-position, and a hydroxymethyl group at the C5position [START_REF] Van Putten | Hydroxymethylfurfural, a versatile platform chemical made from renewable resources[END_REF]. The transformation of HMF to DMF may go through two routes, both involving the hydrogenolysis of the side-groups, either starting with the hydrogenation reaction of the aldehyde function, giving 2,5-bis(hydroxylmethyl)furan (BHMF) as an intermediate product, or with the hydrogenolysis of the hydroxyl group, giving 5-methylfurfural (MFFR) as an intermediate product (Scheme 1) [START_REF] Gallezot | Conversion of biomass to selected chemical products[END_REF][START_REF] Lange | Furfural-A Promising Platform for Lignocellulosic Biofuels[END_REF][START_REF] Zeitsch | The Chemistry and Technology of Furfural and its Many By-Products[END_REF][START_REF] Dautzenberg | Bio based fuels and fuel additives from lignocellulose feedstock via the production of levulinic acid and furfural[END_REF]. The formation of DMTHF is usually supposed to take place via the furan ring hydrogenation of DMF, rather than from the hydrogenolysis of hydroxymethyl tetrahydrofurans [START_REF] Hansen | One-pot reduction of 5-hydroxymethylfurfural via hydrogen transfer from supercritical methanol[END_REF]. Competing polymerization and ring opening reactions may also occur.

HMF being a multifunctional compound, one challenge in its upgrading is to increase the selectivity to the valuable products, as undesired reactions can readily occur in this network of parallel and consecutive reactions. Catalysts based on noble metals such as Pt, Pd, and Ru have proved to be highly active and selective in the reductive transformation of HMF to DMF and DMTHF. For the production of DMF, a CuRu/C catalyst was first explored giving a yield of 76-79% [START_REF] Román-Leshkov | Production of dimethylfuran for liquid fuels from biomass-derived carbohydrates[END_REF]. The best performance was obtained with a Pd/C catalyst under supercritical conditions, achieving a total conversion of HMF to DMF in water at 80 °C, under 10 MPa CO2 and 1 MPa H2 after 2 h. For the synthesis of DMTHF, a sulfurmodified Pt/C catalyst was firstly proposed with a yield of 50% [START_REF] Jackson | Hydrodeoxygenation of Fructose to 2,5-Dimethyltetrahydrofuran Using a Sulfur Poisoned Pt/C Catalyst[END_REF]. However, noble metal-based catalysts suffer from the limited availability and high cost of the metals, which may hinder their commercial applications on a large scale [START_REF] Luo | Mechanisms for High Selectivity in the Hydrodeoxygenation of 5-Hydroxymethylfurfural over PtCo Nanocrystals[END_REF][START_REF] Hu | Selective Transformation of 5-Hydroxymethylfurfural into the Liquid Fuel 2,5-Dimethylfuran over Carbon-Supported Ruthenium[END_REF][START_REF] Li | Ruthenium supported on CoFe layered double oxide for selective hydrogenation of 5hydroxymethylfurfural[END_REF][START_REF] Nishimura | Selective hydrogenation of biomassderived 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 5hydroxymethylfurfural in supercritical carbon dioxide-water: a tunable approach to dimethylfuran selectivity[END_REF][START_REF] Luo | Base metal-Pt alloys: A general route to high selectivity and stability in the production of biofuels from HMF[END_REF]. Substitutes should be found in non-noble transition metals (e.g., Cu, Co or Ni), which are more abundant but generally less active. High yields of DMF and/or DMTHF generally require bimetallic combinations (Ni-Cu, Ni-Fe, Cu-Co, Cu-Zn, etc.), bifunctional catalysts containing both metallic and acidic active sites, and/or the use of supports that participate in the reaction via their specific surface properties (TiO2, ZnO) [START_REF] Chen | Carbon-coated Cu-Co bimetallic nanoparticles as selective and recyclable catalysts for production of biofuel 2,5-dimethylfuran[END_REF][START_REF] Bottari | Copper-Zinc Alloy Nanopowder: A Robust Precious-Metal-Free Catalyst for the Conversion of 5-Hydroxymethylfurfural[END_REF][START_REF] Guo | Efficient hydrogenolysis of 5-hydroxymethylfurfural to 2,5-dimethylfuran over a cobalt and copper bimetallic catalyst on N-graphene-modified Al2O3[END_REF][START_REF] Zhu | Efficient synthesis of 2,5-dihydroxymethylfuran and 2,5-dimethylfuran from 5hydroxymethylfurfural using mineral-derived Cu catalysts as versatile catalysts[END_REF][START_REF] Seemala | Support Induced Control of Surface Composition in Cu-Ni/TiO2 Catalysts Enables High Yield Co-Conversion of HMF and Furfural to Methylated Furans[END_REF][START_REF] Srivastava | Synergism studies on aluminasupported copper-nickel catalysts towards furfural and 5hydroxymethylfurfural hydrogenation[END_REF][START_REF] Kumalaputri | Tunable and Selective Conversion of 5-HMF to 2,5-Furandimethanol and 2,5-Dimethylfuran over Copper-Doped Porous Metal Oxides[END_REF][START_REF] Luo | Unraveling the surface state and composition of highly selective nanocrystalline Ni-Cu alloy catalysts for hydrodeoxygenation of HMF[END_REF][START_REF] Yang | Catalytic transfer hydrogenation/hydrogenolysis of 5-hydroxymethylfurfural to 2,5dimethylfuran over Ni-Co/C catalyst[END_REF][START_REF] Kong | Inclusion of Zn into Metallic Ni Enables Selective and Effective Synthesis of 2,5-Dimethylfuran from Bioderived 5-Hydroxymethylfurfural[END_REF][START_REF] Huang | Nickel-Tungsten Carbide Catalysts for the Production of 2,5-Dimethylfuran from Biomass-Derived Molecules[END_REF][START_REF] Chen | Perovskite type oxidesupported Ni catalysts for the production of 2,5-dimethylfuran from biomass-derived 5-hydroxymethylfurfural[END_REF][START_REF] Kong | Rational design of Ni-based catalysts derived from hydrotalcite for selective hydrogenation of 5-hydroxymethylfurfural[END_REF]. For example, Seemala et al. used TiO2 as support to selectively form strong Ni-TiO2 interactions in Cu-Ni/TiO2, which resulted in a deficit of Ni at the particle surface and a promotion of Cu hydrogenation activity without compromising selectivity [START_REF] Seemala | Support Induced Control of Surface Composition in Cu-Ni/TiO2 Catalysts Enables High Yield Co-Conversion of HMF and Furfural to Methylated Furans[END_REF]. As a result, Cu-Ni/TiO2 gave a DMF yield of 84% at complete conversion at 200 °C under 2.5 MPa in 1,4-dioxane after 8 h. Luo et al. showed that bimetallic carbon-supported NiCu3 nanocrystals prepared by a solvothermal method, and consisting of a Cu-rich core and a 1:1 molar Ni:Cu shell, led to an excellent DMF yield of 99% at complete conversion at 180 °C under 3.3 MPa in a continuous flow reactor [START_REF] Luo | Unraveling the surface state and composition of highly selective nanocrystalline Ni-Cu alloy catalysts for hydrodeoxygenation of HMF[END_REF]. Since an emphasis was put early on the need for an auxiliary metal or for surface acidic sites to obtain a selective catalyst, the catalytic properties of monometallic catalysts based on non-noble metals have been paradoxically less explored [START_REF] Luo | Comparison of HMF hydrodeoxygenation over different metal catalysts in a continuous flow reactor[END_REF][START_REF] Goyal | Studies of synergy between metal-support interfaces and selective hydrogenation of HMF to DMF in water[END_REF][START_REF] Kong | Switchable synthesis of 2,5-dimethylfuran and 2,5-dihydroxymethyltetrahydrofuran from 5-hydroxymethylfurfural over Raney Ni catalyst[END_REF]. Ni-based catalysts are known for their hydrogenation ability [START_REF] Kong | Ni Nanoparticles Inlaid Nickel Phyllosilicate as a Metal-Acid Bifunctional Catalyst for Low-Temperature Hydrogenolysis Reactions[END_REF], but successes in the selective conversion of HMF have mostly been achieved with multifunctional catalysts, e.g. Ni-W2C/C, Ni-Fe/CNTs (carbon nanotubes), Ni/LaFeO3 and others [START_REF] Yang | Catalytic transfer hydrogenation/hydrogenolysis of 5-hydroxymethylfurfural to 2,5dimethylfuran over Ni-Co/C catalyst[END_REF][START_REF] Huang | Nickel-Tungsten Carbide Catalysts for the Production of 2,5-Dimethylfuran from Biomass-Derived Molecules[END_REF][START_REF] Chen | Perovskite type oxidesupported Ni catalysts for the production of 2,5-dimethylfuran from biomass-derived 5-hydroxymethylfurfural[END_REF][START_REF] Yang | High-yield production of 2,5dimethylfuran from 5-hydroxymethylfurfural over carbon supported Ni-Co bimetallic catalyst[END_REF]. Kong et al. have published a series of papers [START_REF] Kong | Inclusion of Zn into Metallic Ni Enables Selective and Effective Synthesis of 2,5-Dimethylfuran from Bioderived 5-Hydroxymethylfurfural[END_REF][START_REF] Kong | Rational design of Ni-based catalysts derived from hydrotalcite for selective hydrogenation of 5-hydroxymethylfurfural[END_REF][START_REF] Kong | Switchable synthesis of 2,5-dimethylfuran and 2,5-dihydroxymethyltetrahydrofuran from 5-hydroxymethylfurfural over Raney Ni catalyst[END_REF][START_REF] Kong | Ni Nanoparticles Inlaid Nickel Phyllosilicate as a Metal-Acid Bifunctional Catalyst for Low-Temperature Hydrogenolysis Reactions[END_REF] in which the reactivity of monometallic Ni catalysts was investigated. Raney Ni was shown to be active in the hydrogenation of the aldehyde function and of the furan ring of HMF to BHMTHF at 100°C in dioxane, while the hydrogenation and hydrogenolysis of the side-groups to DMF was favored at a higher temperature, 180°C [START_REF] Kong | Switchable synthesis of 2,5-dimethylfuran and 2,5-dihydroxymethyltetrahydrofuran from 5-hydroxymethylfurfural over Raney Ni catalyst[END_REF]. The hydrogenation of the furan ring was not as predominant at 180°C as it was at 100°C, with DMTHF building only slowly from DMF at that temperature. It was concluded by the authors that the methyl groups of DMF inhibited its adsorption on the Ni surface. The presence of acidic groups on the catalyst surface was considered as critical to promote both hydrogenolysis reactions and the hydrogenation of the furan ring on Ni. In later works by the same team [START_REF] Kong | Rational design of Ni-based catalysts derived from hydrotalcite for selective hydrogenation of 5-hydroxymethylfurfural[END_REF]. Lewis acidic sites were formed through the incomplete reduction of Ni 2+ ions from a phyllosilicate precursor, or from an aluminic support derived from the decomposition of a hydrotalcite. The production of DMTHF was almost quantitative at 150°C in the first case, and at 180°C in the second case. In contrast, the hydrogenolysis properties of 20 and 36 Ni wt% catalysts supported on silica, a support that lacks of acidic groups, and prepared by impregnation, were quite poor, even at 180°C, and a wide slate of products was obtained. Finally, DMF and DMTHF were observed to be formed in a parallel manner on the hydrotalcite-derived catalyst at 180°C, and not consecutively as would be expected, and as was found on Raney Ni [START_REF] Kong | Rational design of Ni-based catalysts derived from hydrotalcite for selective hydrogenation of 5-hydroxymethylfurfural[END_REF][START_REF] Kong | Ni Nanoparticles Inlaid Nickel Phyllosilicate as a Metal-Acid Bifunctional Catalyst for Low-Temperature Hydrogenolysis Reactions[END_REF]. In the present paper, we propose to re-examine the catalytic properties of the monometallic Ni/SiO2 system, by using a Ni/SBA-15 catalyst prepared following the incipient wetness impregnationmild drying method (IWI-MD), the SBA-15 support being chosen for its high surface area. Reaction conditions were screened for the production of DMF and DMTHF from HMF. A kinetic profile for the formation of these two molecules has been obtained, evidencing the existence of reaction intermediates on this catalytic system. The results of this study not only prove that monometallic Ni catalysts can lead to the selective hydroconversion of HMF to biofuel products, but also provide an understanding of the processes leading to the production both of DMF and DMTHF. Figure 1 depicts the low angle X-ray diffraction domain for NiO/SBA-15. Both the oxidized catalyst and the support exhibit one intense diffraction peak and four less intense diffraction peaks, indexed to the (100), ( 110), ( 200), ( 210) and (300) planes. It indicates that the ordered hexagonal 2D structure of p6mm symmetry in SBA-15 is well preserved after the formation of nickel oxide after calcination, as already reported [START_REF] Ungureanu | Synthesis of highly thermostable copper-nickel nanoparticles confined in the channels of ordered mesoporous SBA-15 silica[END_REF]. The intensity decreases and the position shifts of the diffraction peaks of NiO/SBA-15 as compared to SBA-15 could be explained by the specific localization of the NiO NPs, mainly inside the support pores. Filling of pores reduces the electron density contrast between the pores and silica walls, affecting reflections positions and their intensities [START_REF] Ungureanu | Synthesis of highly thermostable copper-nickel nanoparticles confined in the channels of ordered mesoporous SBA-15 silica[END_REF][START_REF] Chen | Preparation of nickel (oxide) nanoparticles confined in the secondary pore network of mesoporous scaffolds using melt infiltration[END_REF]. N2 adsorption/desorption isotherms obtained for NiO/SBA-15 (calcined material) and Ni/SBA-15 (reduced material) are presented in Figure 2, and calculated textural properties are listed in Table 1. An isotherm shape of Type IV according to the IUPAC classification is obtained for NiO/SBA-15, as obtained for the bare SBA-15 support. This confirms the retaining of a well ordered mesopore structure after the impregnation step and calcination. The change of the hysteresis shape, observed over the impregnated sample, with a two-step desorption and a delayed closure at lower relative pressure (P/P0 = 0.46 vs. 0.61 for the support) is indicative of the formation of NiO nanoparticles confined in the main mesopores of the SBA-15 [START_REF] Thielemann | Pore structure and surface area of silica SBA-15: Influence of washing and scale-up[END_REF][START_REF] Voort | Plugged hexagonal templated silica: a unique micro-and mesoporous composite material with internal silica nanocapsules[END_REF]. Both open and NPs-filled cylindrical mesopores are expected to be present in the material. The B.J.H. pore size distribution (Figure 2 inset) shows consequently two maxima: a first maximum located at 6.6 nm, close to the diameter of the primary mesopores measured for the bare SBA-15 support; a second maximum located at 5.2 nm which is associated to the pores filled by the NiO particles. Both surface area and pore volume are observed to decrease after impregnation (>30% of decrease) which is coherent with the pore plugging phenomenon and the increase in material weight density after deposition. The further reduction step to obtain the Ni/SBA-15 catalyst does not induce significant change in either isotherm or hysteresis shapes. Only slight additional decreases in the surface area (-9%) and the pore volume (-6%) are observed while the pore size distribution remains almost unchanged, an evolution in part explained by the increase in material weight density with the formation of the metal phase. The reducibility of NiO species was investigated by H2-TPR (Figure 3(A)). The H2-TPR profile of NiO/SBA-15 presents two main hydrogen consumption peaks, located at 389°C and 495°C, respectively, with a small H2 consumption also visible at 300°C. According to the literature, the reduction of bulk NiO generally takes place at temperatures below 420°C [START_REF] Robertson | Determination of reducibility and identification of alloying in coppernickel-on-silica catalysts by temperature-programmed reduction[END_REF][START_REF] Brown | Temperature programmed reduction of alumina-supported iron, cobalt and nickel bimetallic catalysts[END_REF]. Main hydrogen consumption (300 -389°C) is then associated to NiO NPs behaving like bulk NiO: NiO NPs confined in the mesopores or large NiO particles located at the external surface of the silica grains. The consumption at 495°C, smaller, is associated to Ni species in stronger interaction with the silica support or to smaller size NiO NPs [START_REF] Louis | Characterization of nickel/silica catalysts during impregnation and further thermal activation treatment leading to metal particles[END_REF]. Quantification leads to a value of hydrogen consumed during the TPR experiment of 14.3 mmol/g. Such a value correspond to a >99% reduction degree of Ni(+II) into Ni(0). 16.5 [b] , 15.9 [c] Ni/SBA-15 468 0.66 5.2, 6.6 15.5 [b] , 14.7 [c] Ni/SBA-15used 242 0.50 4.9, 7.0 14.5 [b] , n.a. [d] [a] B.E.T. surface area, total pore volume, and B. The speciation of nickel after reduction at 550°C for 2 h was first determined using XRD analysis (Figure 3(B)). Diffraction peaks at 52.2° and 61.0°, as well as the corresponding interplanar spacing values (d111 = 0.20 nm, d200 = 0.18 nm), indicate the presence of the cubic Ni(0) phase (JCPDS 04-0850). The shape of the diffraction peaks suggests the formation of both large and small Ni(0) crystals, with an average crystal domain size (calculated according to Scherrer equation) of 15.5 nm (Table 1). Spectra of Ni 2p core level were registered for NiO/SBA-15 and Ni/SBA-15 and are presented in Figure 4. Only Ni 2+ species (binding energy of 854.7 eV) are identified on the NiO/SBA-15 catalyst (calcined at 550°C) [START_REF] Lemonidou | Carbon dioxide reforming of methane over 5wt.% nickel calcium aluminate catalystseffect of preparation method[END_REF]. A significant evolution of the spectra was observed after reduction at 550°C: the Ni 2p3/2 signal shifted to 852.5 eV, which indicates the formation of metallic Ni(0) [START_REF] Yu | Hydrogen Production by Ethanol Steam Reforming on NiCuMgAl Catalysts Derived from Hydrotalcite-Like Precursors[END_REF], and the absence of residual oxidized nickel after treatment. TPR, XRD and XPS results then indicate the formation of purely metallic catalyst without remaining ionic nickel species, after a reduction performed at 550°C. Dispersion of the nickel phase in the catalyst was further characterized by TEM (Figure 5). After activation of the catalyst under reducing conditions, the support always show a regular hexagonal arrangement of pores, in agreement with the conclusions issued form low angle XRD and N2 physisorption.

Results and Discussion

Catalyst characterization

Besides, both empty and full mesopores exist in the structure. Two types of nickel particles are observed: as minor phase, large nickel particles (20-50 nm) (re)crystallized on the external surface of the silica grain (Figure 5A); as major phase, small oblongshaped nickel NPs confined within the support channels, and having size of 8 nm (width) -15 nm (length) (Figure 5B,C). As shown in the particle size distribution of the Ni NPs in Figure 5D, the size of most of the Ni particles were in the range of 5-15 nm, these particles being located within the main mesopores of the support. Notably, there exists a large population (14%) of external Ni particles (with a size larger than 20 nm).

Catalytic properties

In order to find the proper reaction parameters for high DMF/DMTHF yields over Ni/SBA-15, a set of orthogonal experiments were designed. Dioxane was chosen as the solvent, considering its stability in reaction, the solubility of HMF in this solvent and the better yields to target products than when other solvents are used [START_REF] Seemala | Support Induced Control of Surface Composition in Cu-Ni/TiO2 Catalysts Enables High Yield Co-Conversion of HMF and Furfural to Methylated Furans[END_REF]. The process parameters (i.e., reaction temperature, H2 pressure and HMF/Ni molar ratio) were selected on the basis of the available literature [START_REF] Nakagawa | Catalytic Reduction of Biomass-Derived Furanic Compounds with Hydrogen[END_REF][START_REF] Hu | Chemoselective Hydrogenation of Biomass-Derived 5-Hydroxymethylfurfural into the Liquid Biofuel 2,5-Dimethylfuran[END_REF][START_REF] Hu | Catalytic conversion of biomass-derived carbohydrates into fuels and chemicals via furanic aldehydes[END_REF]. According to the literature, when the temperature is lower than 180°C, the hydrogenolysis of the C-O bond (the reaction affording the production of DMF/DMTHF) becomes slow [START_REF] Kong | Switchable synthesis of 2,5-dimethylfuran and 2,5-dihydroxymethyltetrahydrofuran from 5-hydroxymethylfurfural over Raney Ni catalyst[END_REF]. 180°C was thus selected as the minimum temperature. The selectivity to DMF/DMTHF generally decreases with an increase of the reaction temperature above 240°C, owing to the formation of byproducts such as humins [START_REF] Seemala | Support Induced Control of Surface Composition in Cu-Ni/TiO2 Catalysts Enables High Yield Co-Conversion of HMF and Furfural to Methylated Furans[END_REF][START_REF] Nagpure | Efficient Preparation of Liquid Fuel 2,5-Dimethylfuran from Biomass-Derived 5-Hydroxymethylfurfural over Ru-NaY Catalyst[END_REF]. Hence, the reaction temperature was set in the range of 180-240°C. The hydrogen pressure was chosen in the range 15-45 bar, considering that increasing hydrogen pressure increases the concentration of hydrogen in the solvent, which may benefit to the conversion rate. Maximum DMF/DMTHF yields were obtained below 50 bar [START_REF] Chen | How Catalysts and Experimental Conditions Determine the Selective Hydroconversion of Furfural and 5-Hydroxymethylfurfural[END_REF]. Hence, 45 bar was selected as the maximum pressure level.

Besides, the HMF/Ni molar ratio was varied in the range of 3-30, covering the range of the HMF/Ni molar ratios mentioned in the literature [START_REF] Chen | How Catalysts and Experimental Conditions Determine the Selective Hydroconversion of Furfural and 5-Hydroxymethylfurfural[END_REF]. Once reaction parameters determined, a L9 (3 (levels) ^ 3 (factors: T, P, HMF/Ni molar ratio)) orthogonal array was constructed according to the Taguchi method using software Minitab 17. Reaction parameters, HMF conversion, main products yields and carbon balances are summarized in Table 2. Catalysis optimization results show that: 2, Entry 4 and 9), due to possible polymerization reaction with BHMF and MFOL [START_REF] Nakagawa | Catalytic Reduction of Biomass-Derived Furanic Compounds with Hydrogen[END_REF].

Two results out of the nine tests are worthy of noting. In Table 2 -Entry 5, a DMF yield of 40.6% at 43.8% HMF conversion was obtained at 210°C under 30 bar H2 with a HMF/Ni molar ratio of 15, giving a maximum DMF selectivity (93%). However, the conversion of HMF is limited under these reaction conditions (low catalyst loading). In Table 2 -Entry 2, a DMTHF yield of 81.4% (100% HMF conversion) was obtained at 180 °C under 30 bar H2 with a HMF/Ni molar ratio of 3, suggesting that these conditions are suitable for the selective production of DMTHF over Ni/SBA-15. This result is quite interesting considering that only one report in the literature reported similarly high DMTHF yield obtained with Ni-based catalysts [START_REF] Kong | Rational design of Ni-based catalysts derived from hydrotalcite for selective hydrogenation of 5-hydroxymethylfurfural[END_REF]. Then, the high DMTHF yield obtained here suggests the possibility to optimize DMF yield under these reaction conditions according to the HMF  DMF  DMTHF pathway depicted in Scheme 1. Evolution of HMF conversion and products yields with reaction time over Ni/SBA-15 (180°C, 30 bar H2, HMF/Ni molar ratio of 3) is depicted in Figure 6. It shows that at short reaction time (30 min), HMF was primarily converted into DMF (70.5% yield) in a fast way and the yield of DMTHF was limited (12.2% yield). BHMF and MFOL were also detected at low concentrations (4.9% and 4.0% yields, respectively). Almost no MFFR was detected (only 0.8% MFFR yield). BHMF could be converted into MFOL and DMF through the selective cleavage of the C-O bond (Scheme 1), the conversion of BHMF to MFOL followed by the hydrogenolysis of MFOL to DMF is consequently a possible route. The second route for DMF formation via MFFR and MFOL involves -CH2OH hydrogenolysis followed by -CHO hydrogenation, even if MFFR is measured at lower yield than BHMF. The DMF yield obtained after 30 min is quite comparable to the results obtained over monometallic Ni-based catalysts reported in the literature, especially considering that most catalytic performances previously reported rely on the use of functional supports (Table 3, Line 4-5). Results reported here then show that a simple monometallic Ni catalyst, even when supported over a neutral support, exhibits adequate surface properties affording fast hydrogenolysis to selectively form DMF under optimized reaction conditions.

Prolonging the reaction time to 1 h leads to complete HMF conversion, with the conversion increase associated to a significant decrease of the DMF yield (57.0%) at the expense of the DMTHF yield (32.1%). The concentrations of BHMF and MFOL also decreased to negligible levels (yields <2%). During this period, the main reaction occurring is consequently the furan ring hydrogenation of DMF to produce DMTHF. When the reaction time is prolonged above 1 h, the yield of DMTHF continuously increased, reaching 96.6% at complete HMF conversion after 10 h. DMF yield is, after 10 h, at a negligible content (0.6%). Then, considering the evolution of intermediates and products yields with reaction time, HMF was rapidly converted into DMF via hydrogenolysis (BHMF and MFOL being main intermediates products identified). The furan ring hydrogenation of DMF to produce DMTHF is occurring at a slower rate, explaining the need of longer reaction time to maximize its yield. As compared to the reports available in the literature ( 3, Entry 11) after 20 h of reaction at 180°C under 12 bar and with a HMF/Ni molar ratio of 11 [START_REF] Kong | Rational design of Ni-based catalysts derived from hydrotalcite for selective hydrogenation of 5-hydroxymethylfurfural[END_REF].

Kinetic of the reaction

A simplified kinetic model was defined (Scheme 2), lying on the hypotheses as described in the Supporting Information file.

Scheme 2. Kinetic scheme proposed for the DMTHF production from HMF (kapp: apparent rate constant)

The transformation of HMF to DMF and DMTHF supposes the formation of MFOL as intermediate (Scheme 2). Two hydrogenolysis steps take place sequentially, however, the hydrogenolysis of the aldehyde group requires an initial step of hydrogenation to -CH2OH [START_REF] Zhu | Efficient synthesis of 2,5-dihydroxymethylfuran and 2,5-dimethylfuran from 5hydroxymethylfurfural using mineral-derived Cu catalysts as versatile catalysts[END_REF]. MFOL itself can then be produced following two routes:

(1) Hydrogenolysis of HMF to MFFR, followed by the hydrogenation of the aldehyde function of MFFR to MFOL.

(2) Hydrogenation of the aldehyde function of HMF to BHMF, followed by the hydrogenolysis of BHMF to MFOL. As DMTHF appears in the last stage, once DMF has been produced and accumulated, we suppose that the hydrogenation of the furan ring does not take place to a large extent on the minor intermediate products as BHMF and MFFR, and even MFOL [START_REF] Gawade | Biobased Green Process: Selective Hydrogenation of 5-Hydroxymethylfurfural to 2,5-Dimethyl Furan under Mild Conditions Using Pd-Cs2.5H0.5PW12O40/K-10 Clay[END_REF].

When reaction is performed at 180°C, both MFFR and BHMF are detected in small amounts at short reaction times. MFOL appears in parallel to BHMF, and not in a second stage as would be the case if it was produced only from BHMF. It is thus not possible to eliminate one of these two routes a priori. A fitting of the kinetic curve was performed by least-square minimization, according to the following steps [START_REF] Gawade | Biobased Green Process: Selective Hydrogenation of 5-Hydroxymethylfurfural to 2,5-Dimethyl Furan under Mild Conditions Using Pd-Cs2.5H0.5PW12O40/K-10 Clay[END_REF][START_REF] Liu | Effects of Water on the Copper-Catalyzed Conversion of Hydroxymethylfurfural in Tetrahydrofuran[END_REF]:

-HMF consumption was fitted using a first-rate law of rate constant (k1,app + k2,app):

[HMF] = [HMF]0 exp (-((k1,app + k2,app)t))

-As a primary intermediate product, MFFR neat production rate was defined as the difference between the production rate k1,app [HMF] (HMF being consumed by paths 1 and 2) and the consumption rate k3,app [MFFR], leading to Eq. ( 2):

[MFFR]= k 1,app [HMF] 0 k 3,app -(k 1,app +k 2,app ) [ exp (-(k 1,app +k 2,app )t) -exp(-k 3,app t) ] (2)
-As a primary intermediate product, BHMF neat production rate was defined as the difference between the production rate k2 app [HMF] and the consumption rate k4 app [BHMF], leading to Eq. ( 3):

[BHMF]= k 2,app [HMF] 0 k 4,app -(k 1,app +k 2,app ) [ exp (-(k 1,app +k 2,app )t) -exp(-k 4,app t) ] (3)
A first fitting of HMF consumption, and of MFFR and BHMF neat productions, led to the determination of the four first apparent rate constants, k1,app to k4,app, and to the calculation of r1 to r4. It appeared that r1 is approximatively 10 times higher than r2, and that r3 was larger than r4 by a factor of 20 to 40. The main production path to MFOL was thus identified as the one involving MFFR (Scheme 3).

-The neat production of the secondary intermediate, MFOL, was then based on the HMF → MFFR → MFOL pathway, of rate constants k1,app and k3,app, for the production rate, and k5,app for the consumption rate (Eq. 4):

[MFOL]=(k 1,app k 3,app )[HMF] 0 [ exp(-k 1,app t) (k 3,app -k 1,app )(k 5,app -k 1,app ) - exp(-k 3app t) (k 1,app -k 3,app )(k 5,app -k 3,app ) + exp(-k 5,app t) (k 1,app -k 5,app )(k 3,app -k 5app ) ]
(4) allowing the fitting of constant k5,app along with k1,app -k4,app. -As DMF is not the final product of reaction, it is not possible to model its production just by subtracting the concentrations of HMF, BHMF, MFFR and MFOL from [HMF]0. However, BHMF, MFFR and MFOL being formed in very low amounts with respect to DMF and DMTHF, it can be attempted to apply the steady state approximation to these intermediates, and consider that the production rate of DMF is close to the consumption rate of HMF. DMF thus becomes an intermediate between HMF and DMTHF, and its concentration follows the law:

[DMF]= (k 1,app +k 2,app )[HMF] 0 k 6,app -(k 1,app +k 2,app ) [ exp (-(k 1,app +k 2,app )t) -exp(-k 6,app t) ] (5)
making it possible to fit k6,app with the other constants. (6) Finally, DMTHF is calculated as the difference between [HMF]0, and the concentrations of all the other products, including the unknown ones (implicitly supposing that they derive from the last stages of the global reaction).

After least-square minimization, the values of the six apparent reaction constants are determined as presented in Scheme 3. A graphical comparison between the experimental and the modelled concentrations (Figure 7) raises the following comments:

(1) HMF consumption, MFFR production and BHMF production are correctly modelled, in particular the position of the maximum of production of MFFR and BHMF.

(2) In contrast, the production of MFOL is poorly reproduced, as its consumption starts too early. Decreasing k5,app shifts the maximum of production to longer times, but also increases considerably the amount of MFOL formed, which means that k3,app would also need to be better adjusted. There is thus a certain uncertainty on the values of k3,app and k5,app obtained by this model.

A new fitting based only on the concentrations of the minor species led to a decrease of the two rate constants, but at the expense of the quality of the fitting for BHMF. In any case, k3,app and k5,app were still large compared to k2,app and k4,app.

(3) The fits for DMF and DMTHF follow the same tendency as the experimental data, though at longer times the consumption of DMF and the production of DMTHF are overestimated. Some inhibition phenomena not taken into account in this simplified model can be postulated. In summary, this simplified model shows that both C=O hydrogenation and hydrogenolysis steps are fast compared to the hydrogenation of the furan ring. C-O hydrogenolysis and C=O hydrogenation reactions are in competition. The MFFR route seems to be predominant, confirming the capability of the Ni catalyst for the hydrogenolysis reaction. The apparent rate constants of the hydrogenation and hydrogenolysis steps involving intermediates containing a methyl group (MFFR, MFOL) are found to be much larger than the others steps, suggesting a fast adsorption/reaction of these molecules on the catalyst surface. However, uncertainties remain large concerning the intermediates detected in low amounts.

Stability of Ni/SBA-15

Recyclability of the SBA-15 supported Ni catalyst was evaluated (Figure 8). Between each reaction cycle, the catalyst was recovered by centrifugation after the reaction followed by washing with acetone. Then the catalyst was re-calcined at 500°C for 6 h and re-reduced at 550°C for 2 h before the next test. Each test was performed for 3 h at 180°C under 30 bar H2. HMF conversion was 100% with the DMF yield of 45% and DMTHF yield of 48% in the first run (Figure 8). After 3 cycles of catalysis, HMF conversion decreased to 75% while the DMF yield and DMTHF yield reduced to 37% and 10%, respectively. This evolution indicated that the catalyst progressively deactivated during the tests. After the reaction, the spent catalyst was analysed by different characterization techniques in order to identify the main reason of the catalyst deactivation. As a first result, no evident growth of Ni crystal size was measured, applying the Scherrer equation to the XRD pattern recorded to the Ni/SBA-15-used catalyst (Table 1). The absence of significant sintering during reaction was confirmed by microscopy analysis results (Figure 9A-F), with no visible degradation of the mesostructured quality. Indeed, the majority of the particles are confined in well-defined mesopores of SBA-15 support, with a few aggregated Ni particles outside the channels (Figure 9A,B). Consequently, the deactivation is not associated to modifications of the support characteristics or Ni phase dispersion. Another possible way of deactivation is the adsorption of high molecular weight products on the active sites during the reaction [START_REF] Chen | Immobilized Ru Clusters in Nanosized Mesoporous Zirconium Silica for the Aqueous Hydrogenation of Furan Derivatives at Room Temperature[END_REF]. The FT-IR spectra, shown in Figure 9E for the used catalyst, exhibits visible bands at:

-880 cm -1 (adsorption of furan compounds [START_REF] Liu | Crosslinked epoxy materials exhibiting thermal remendablility and removability from multifunctional maleimide and furan compounds[END_REF][START_REF] Gandini | Materials from renewable resources based on furan monomers and furan chemistry: work in progress[END_REF]) -890 and 1254 cm -1 (vibrations involving the C-H groups [START_REF] Moreno-Castilla | Effects of non-oxidant and oxidant acid treatments on the surface properties of an activated carbon with very low ash content[END_REF]) -1111, 1369 and 1430 cm -1 (C-O stretching vibration [START_REF] Kim | Raman and IR Spectroscopy of Chemically Processed Single-Walled Carbon Nanotubes[END_REF]) -1521, 1566, 1616, 1674 and 1712 cm -1 (assigned to C=O groups [START_REF] Kim | Raman and IR Spectroscopy of Chemically Processed Single-Walled Carbon Nanotubes[END_REF] These results, added to the results obtained by TG-DSC and demonstrating a 2.3 wt.% carbon loss during temperature programmed oxidation up to 900°C, indicate that the adsorption of organic intermediates on the catalyst surface [START_REF] Yu | Robust and Recyclable Nonprecious Bimetallic Nanoparticles on Carbon Nanotubes for the Hydrogenation and Hydrogenolysis of 5-Hydroxymethylfurfural[END_REF] can be a reason of the progressive deactivation during recycling tests.

Conclusions

Ni/SBA-15, prepared by the incipient wetness impregnation of nitrate precursor, is an active catalyst for the selective hydroconversion of 5-hydroxymethylfurfural (HMF) to 2,5dimethylfuran (DMF) and 2,5-dimethyltetrahydrofuran (DMTHF). The existence of a large population of mesopore-confined NiO particles leads to a high reactivity. High yields of DMF and DMTHF are achieved, 70.5% and 96.6% respectively, by simply modifying the reaction time under optimized reaction conditions (180 ˚C, 30 bar). The catalyst exhibits a fast hydrogenolysis ability and a high ability for the furan ring hydrogenation. The kinetic study demonstrated that HMF was probably converted via the route involving MFFR as an intermediate product. The competitive reactions, i.e. C-O hydrogenolysis and C=O hydrogenation, are fast as compared to the furan ring hydrogenation that is the reason of the possibility to reach high DMF yields. Further studies are however needed in order to improve the catalyst stability since 25% of the activity is loss after three reaction cycles, a deactivation preferably associated to carbon deposition on active element than to Ni nanoparticles sintering.

Experimental Section

Synthesis of Ni/SBA-15 catalyst SBA-15 support was synthesized according to classical procedure [START_REF] Zhao | Triblock Copolymer Syntheses of Mesoporous Silica with Periodic 50 to 300 Angstrom Pores[END_REF]. In a typical synthesis, 4.0 g of Pluronic P123 was dissolved in a 1.6 M solution of HCl at 40 °C. Thereafter, 8.5 g of TEOS was added dropwise to the solution, and the solution was submitted to magnetic stirring for 24 h. The resulting gel was submitted for 48 h to hydrothermal ageing at 100 °C. After recovery by filtration, the SBA-15 sample was washed with water, and dried at 100 °C for 24 h. The dried SBA-15 sample was calcined at 550 °C for 6 h in a muffle furnace, using a heating ramp of 1.5 °C min -1 .

The nickel catalyst was prepared using the incipient wetness impregnation mild drying method (IWI-MD) [START_REF] Ungureanu | Synthesis of highly thermostable copper-nickel nanoparticles confined in the channels of ordered mesoporous SBA-15 silica[END_REF]. To prepare on a Micromeritics Tristar II Plus instrument. The particle size and shape were analysed by transmission electron microscopy. For fresh catalysts, experiments were conducted on a JEOL 2100 UHR, operated at 200 kV with a LaB6 source and equipped with a Gatan 832 CCD camera. For the used catalyst, experiment was conducted on a TITAN Themis 300 S/TEM. The high angle annular dark field (HAADF) images were collected at angles between 50 and 200 mrad. H2-temperature programmed reduction (TPR) was performed on an Autochem analyzer (Micromeritics) equipped with a quartz U-shaped microreactor. Quantification of H2 consumed is performed using a TCD after trapping of the produced water. The FT-IR spectra was registered on an infrared spectrometer with total attenuation (IR-ATR) from Thermo Scientific (IS50). Thermogravimetry experiments were conducted on a system from Mettler Toledo (TGA/SDTA 851 model).

Experiment was conducted under 20%O2 in N2 flow at the rate of 50 mL/min, with temperature increase from 50°C to 900°c at an increase rate of 5 °C/min.

Hydroconversion of 5-Hydroxymethylfurfural

Preliminary catalysis tests were performed in an 8-parallel automated autoclave system equipped with 8 high-pressure batch reactors (100 mL).

Prior to the catalytic tests, the catalysts were reduced at 550 °C for 2 h (heating ramp of 10 °C/min) under 5 Vol.% H2 in Ar flow (50 mL/min). Prior to the reaction, the reactor was charged under inert atmosphere (glove box) with 0.15 mmol/mL HMF in 25 mL 1,4-dioxane as a solvent and 0.16 g of catalyst, sealed and purged with H2. Then, the reactor was pressurized with required H2 pressure (15-45 bar), heated at required temperature (180-240 °C) and stirred at a speed of 700 rpm for different reaction time.

The kinetic profiles were performed in a 45 mL Parr reactor: the prereduced catalyst (550°C, 2 h, pure H2 flow ) was inserted in the reactor together with 12.5 mL of 0.15 mmol/mL HMF solution in 1,4-dioxane. The reactor was closed, purged with hydrogen gas for several times, and then pressurized with H2 to obtain 30 bar of H2 under the reaction conditions (T = 180 °C). The pressure was monitored using a gauge connected to the reactor. The reaction was stirred by magnetic agitation. Samples of reaction medium were taken off at selected reaction times:

Title

((Insert TOC Graphic here; max. width: 11.5 cm; max. height: 2.5 cm))

Scheme 1 .

 1 Scheme 1. Hydrogenation of HMF to DMF and DMTHF and possible side reactions (dotted arrows represent side reactions).

Figure 1 .

 1 Figure 1. Low angle XRD patterns registered for SBA-15 and NiO/SBA-15.

Figure 2 .

 2 Figure 2. N2 physisorption isotherms and BJH pore size distribution (inset) obtained for SBA-15, calcined NiO/SBA-15 and reduced Ni/SBA-15 catalyst (vertical shift of 500 cm 3 /g between isotherms for better clarity).

  J.H. pore size obtained from N2 physisorption; [b] Average crystal size measured for NiO or Ni using the Scherrer equation; [c] Average size measured from statistical analysis on TEM images. [d] n.a.: not analysed.

Figure 3 .

 3 Figure 3. (A) H2-TPR profile for calcined NiO/SBA-15; (B) XRD pattern of reduced Ni/SBA-15 (vertical bars: JCPDS file n° 04-0850).

Figure 4 .

 4 Figure 4. XPS high resolution spectra of Ni 2p core level obtained for NiO/SBA-15 and Ni/SBA-15 catalyst.

Figure 5 .

 5 Figure 5. (A) Representative TEM images recorded for Ni/SBA-15; (B) Ni(0) particle size distribution obtained by statistical analysis of TEM images.

( 1 )

 1 DMF, DMTHF, MFFR and MFOL were detected after 8 h of reaction in addition to DMTHF and DMF as the major products. These results indicate that the hydrogenolysis of C-OH bond and/or the furan ring hydrogenation are favored over the Ni/SBA-15 catalyst; (2) low HMF/Ni molar ratios (≤ 15) or large catalyst quantity (≥ 0.052 g) are required for efficient conversion of HMF; (3) hydrogen pressure in the selected range (15-45 bar) has a weaker influence on the catalytic results than the HMF/Ni molar ratio and the reaction temperature; (4) lower carbon balances (decreasing up to 58%) are obtained when HMF/Ni molar ratio decreases, an effect more visible at high reaction temperatures (Table

Figure 6 .

 6 Figure 6. Evolution of HMF conversion and product yields with reaction time over Ni/SBA-15. Reaction conditions: 0.144 mmol/mL HMF in 25 mL 1,4dioxane, P(H2) = 30 bar, T = 180˚C, HMF/Ni molar ratio of 3.

Scheme 3 .Figure 7 .

 37 Scheme 3. Apparent rate constants for each reaction step of HMF conversion to DMTHF

Figure 8 .

 8 Figure 8. Recyclability of the Ni/SBA-15 catalyst for the HMF hydrogenation reaction. Reaction conditions: 0.15 mmol/mL HMF in 20 mL 1,4-dioxane, P(H2) = 30 bar, T = 180˚C, HMF/Ni molar ratio of 3, 3 h.

Figure 9 .

 9 Figure 9. HAADF image (A,C,E) and Ni (B,F) and Si (D) mapping of the Ni/SBA-15-used sample; (G) FT-IR spectra of the fresh and used Ni/SBA-15 catalyst.

  5 min, 10 min, 20 min, 30 min, 1 h, 2 h, 4 h, 6 h, 8 h and 10 h. At the end of the reaction, the autoclave was rapidly cooled, and the catalyst was recovered by filtration. The products were analyzed by GC (Shimadzu 2010 Plus) equipped with ZB-WAX Plus capillary column (30.0 m × 0.25 mm × 0.25 µm) and a flame ionization detector (FID), and by GC-MS (Shimadzu QP2010 Ultra EI) equipped with a ZB-1XT capillary column (15.0 m × 0.53 mm × 0.25 µm) and a FID detector.FULL PAPEREntry for the Table of Contents (Please choose one layout)

Table 1 .

 1 Physico

	Catalyst	SBET, [a]	Vt, [a]	Dp, [a]	dNi. nm
		m 2 /g	cm 3 /g	nm
	SBA-15	841	1.10	6.8	-
	NiO/SBA-15	519	0.70	5.2,
				6.6

-chemical properties of SBA-15, NiO/SBA-15, Ni/SBA-15 and Ni/SBA-15-used (after reaction).

Table 2 .

 2 Conversions and product distributions obtained under optimisation conditions[a]

				HMF/			yield/%
	No.	T /°C	P /bar	Ni mol. ratio	conv. /%	DMF	DMT HF	MFF R	MF OL	C.B. /% [b]
	1	180	15	30	7	1	0	1	2	97
	2	180	30	3	100	1	81	1	1	85
	3	180	45	15	34	12	0	2	10	90
	4	210	15	3	100	1	72	1	1	76
	5	210	30	15	44	41	1	2	0	100
	6	210	45	30	21	17	1	1	2	100
	7	240	15	15	32	21	3	8	0	100
	8	240	30	30	11	7	1	1	2	100
	9	240	45	3	100	0	55	1	1	58

[a] Catalytic tests were performed in batch-pressurized reactors, using a Autoplant-Chemspeed instrument, with 0.15 mmol/mL HMF in 25 mL 1,4dioxane solution for 8 h. [b] carbon balance.

Table 3

 3

, Entry 8-11), the DMTHF yield obtained in this work is among the highest reported. Only Kong et al. reported comparable result, with a bifunctional Ni-Al2O3 catalyst (derived from the calcination of hydrotalcite-like precursor at high temperature, 850°C), with a comparable DMTHF yield (Table

Table 3 .

 3 Catalytic performances of Ni-based catalysts for the conversion of HMF to DMF and DMTHF.

	No.	Catalyst	Feed conditions	P (bar)	T (˚C)	t (h)	X (%)	Y (%)	ref
	HMF to DMF
			12 mmol HMF;
	1	Raney Ni	35 mL 1,4-dioxane;	15	180	15	100	89	[31]
			0.5 g catalyst
			12 mmol HMF;
	2	Ni/Ni 2+ -silicate [a]	38 mL 1,4-dioxane;	15	130	3	100	73	[32]
			0.08 g catalyst
			2 mmol HMF; 20
	3	Ni/CN [b]	mL water;	30	200	6	100	99	[30]
			0.05 g catalyst
	4	Ni/LaFe O3	1 mmol HMF; 12 0.1 g catalyst mL ethanol;	50	230	6	99	98	[27]
			8 mmol HMF;
			100 mL 1-
	5	Ni/C	propanol;	33	180	-	92	53	[29]
			W/F = 2
			g.min/mL
			12 mmol HMF,
	6	Ni/Al2O3	35 mL 1,4-dioxane,0.1 g	12	180	4	100	92	[28]
			catalyst
			1.8 mmol HMF;
	7	Ni/SBA-15	12.5 mL 1,4-dioxane; 0.261 g catalyst	30	180	0. 5	93	71	this work
	HMF to DMTHF
			8 mmol HMF,
	8	Pd/C	100 mL 1-propanol	33	180	-	100	55	[29]
			4 mmol HMF, 20
	9	CuZn	mL CPME, 0.1 g catalyst	20	220	6	100	25	[17]
			12 mmol HMF,
	10	Ni/SiO2	38 mL 1,4-dioxane,0.08 g	15	150	3	100	26	[32]
			catalyst
			12 mmol HMF,
	11	Ni/Al2O3	35 mL 1,4-dioxane,0.1 g	12	180	20	100	97	[28]
			catalyst
			1.8 mmol HMF;
	12	Ni/SBA-15	12.5 mL 1,4-dioxane; 0.261 g catalyst	30	180	10	100	97	this work
	[a] Ni(0)/Ni(+II) catalyst from nickel phyllosilicate; [b]: Ni/CN, Ni supported on
	mesoporous nitrogen-rich carbon.

Acknowledgements

Chevreul institute (FR 2638), Ministère de l'Enseignement Supérieur et de la Recherche and Région Hauts-de-France are also acknowledged for supporting this work. The authors thank Martine Trentesaux and Pardis Simon for XPS analysis, Olivier Gardoll for physical and redox characterization, Laurence Burylo for XRD experiment, Maya Marinova for TEM analysis and Svetlana Heyte from the REALCAT platform for high throughput catalytic tests.


K Laclavetine

D Giovannacci
email: david.giovannacci@culture.gouv.fr

M Radepont

A Michelin

A Tournié

O Belhadj

Ch Andraud

W Nowik

Macro X-Ray Fluorescence (MA-XRF) scanning, multi-and hyperspectral imaging study of multiple layers of paintings on paneled vault in the church of Le Quillio (France)

    Macro X-Ray Fluorescence (MA-XRF) scanning, multi-and hyperspectral imaging study of multiple layers of paintings on paneled vault in the church of Le Quillio (France)

INTRODUCTION

The chapel of Rosary, located on the Southern side of the church of Notre-Dame-de-la-Déliverance in Le Quillio (Bretagne, France), was built between the end of the 15th century and the beginning of the 16th century. It was covered by a wooden vault of 5.20 m long, 6.50 m wide and 4.00 m high. The 32 m 2 of vault surface is composed of 529 panels. [START_REF]Anonymous, Le Quillio (22) -Église paroissiale -Étude des supports et des décors des lambris sous-jacents aux toiles[END_REF] In 1869, the original vault was covered with canvas painted by Raphaël Donguyen. These canvases hid former paintings done by Louis Le Corre in 1686. The 19th century canvas, removed during a recent restoration works, represents a series of Saints and Holy Virgin in the style characteristic of this period. These canvases reproduce in a simplified way the pictorial program from the 17th century. [START_REF] Bec | Eglise Notre Dame de la Délivrance. Etude préalable à la restauration des décors du coeur et de la chapelle du Rosaire[END_REF] Restoration works, during which it was possible to take a closer look at Le Corre painting, allowed identifying the presence of another, underlying, painting in areas where lacunas of more modern painting showed the difference of color or gloss. The uncovering done on the forehead of the Virgin, whose head was straight-positioned, displayed a large part of another face's right eye, with the head leaning to the left. This underlying painting, probably the original one, could have been executed on the vault just after its construction between the end of the 15th century and the beginning of the 16th century. Due to this fact, it was interesting to investigate if the conservation state of underlying painting would be satisfying and the extent of its remains were large enough to take the decision to uncover it.

The approach for the evaluation of the remaining original painting in Le Quillio had to fit several constraints: capability to display the extent of underlying painting and clearly differentiate it from overlaying layers, possibility to cover large examined surface in relatively short time, as well as transportability of the instrumentation on site and work on the scaffolding. In addition, being twisted or curved, the wooden panels required a very skilled approach. Several imaging and spectroscopic techniques could be applied to serve the purpose; most of them are mainly based on the detection of visible, infrared or X-ray radiations. [START_REF] Borg | [END_REF][4][5][6][7][8][9][10] Moreover, a recent study underlines the efficiency of photoacoustic detection for such investigation. [11] Visible (VIS) light and near infrared (NIR) photographies are used for macroscopic observation. In addition, NIR photography highlights the IR reflection properties of materials. [12] For example, it helps in the observation and study of carbon black underdrawings, which does not reflect IR radiation, showing dark pattern on the NIR image. [13][START_REF] Hernández | art'14 Conference[END_REF] VIS and NIR photographies are in situ non-invasive multispectral techniques.

Moreover, VIS and NIR images are used to produce false color PCA composite images. The false color PCA composite images are created by digitally editing the VIS and NIR images of the same subject. These images linked the visible color of the materials with their IR reflection properties. They are useful to distinguish the different compounds present on the painting.

Reflectance imaging spectroscopy or hyperspectral imaging spectroscopy is a powerful technique for the non-invasive study of cultural heritage objects. It combines imaging and reflectance spectrometer systems in the wavelength domain going from visible to short wave infrared (400-2500 nm). The image recorded is defined by two spatial dimensions (x,y) and one spectral dimension (z), meaning that each pixel of the image contains diffuse reflection spectrum of the visible and/or infrared spectral ranges. Spectral images contain spectral features, which are linked to compositional information and can hence be used to generate useful chemical maps. Paintings or illuminated manuscripts have been studied using hyperspectral imaging. [12,[START_REF] Kim | [END_REF][16] It reveals preparatory drawings or artist modifications that can no longer be seen and helps to distinguish the materials used on large surfaces.

X-ray fluorescence spectroscopy (XRF), being non-destructive and non-invasive, is a modern analytical technique to study materials used in cultural heritage objects. The imaging possibilities offered by MA-XRF scanners are particularly interesting for paintings, [17] and the technique proved to be especially useful to visualize hidden layers due to good penetration of X-rays in matter. [18][19] The aim of the study was to detect the extent of the original underlying painting. For that, it was necessary to find chemical elements corresponding to the pigments present on wooden panels and to differentiate between original and later pictorial layers by characterizing them according to their behavior in VIS, NIR and X-rays. All the analytical techniques used were expected also to contribute to the localization of restoration work done on the 17th century overlying painting before the covering of vault with painted canvas in 19th century In this pilot work, done on the series of unmounted panels, we strived to evaluate the methods in terms of their applicability to the research of underlying painting on the whole vault, considering their respective informative outcomes and surface coverage capacities.

MATERIALS AND METHODS

The painted panels' sample

A small part of the painted vault, consisting of three wooden panels (W ~ 91 cm, H ~ 8 to13 cm) (Figure 1), was transferred to the laboratory and analyzed with different imaging and spectroscopic techniques, detailed below.

FIGURE 1

The three wooden panels studied with covering areas of macro X-ray fluorescence (MA-XRF) and spectral imaging. The wooden panels are delimitated in continuous white lines, the rectangular uncovering window showing an eye of underlying portrait is delimited in continuous orange, and X-ray fluorescence (XRF) analyzed area in dotted blue lines.

FIGURE 2

Calibrated visible (VIS) image of the three painted wooden panels.

VIS and NIR photographies

For VIS and NIR photographies, a Nikon unfiltered CCD camera (model D800) was used with an f11 aperture, 1/60 s shutter speed at 200 ISO. Two flashlights were positioned at the 45º angle on each side of the painted panel sample at the same distance. The lighting homogeneity was controlled with the use of a Gossen ambient light meter (model Digipro F). For VIS photography, the BG38 filter was used to cut UV and IR radiations while, for NIR photography, the RG780 filter was placed in front of the camera in order to cut UV and visible spectrum. Finally, the images were examined with Capture One software and calibrated using QP card charts. As the painting covering the three wooden panels appeared quite dark on the VIS photography (Figure 2), we adjusted the brightness levels of the VIS image in order to enhance the contrast of the painting (Figure 1).

The false color PCA composite images

The false color PCA composite images are digitally generated with the VIS and NIR images using ImageJ software. Each image is composed of three channels (red, green and blue). First, the VIS and NIR channels are put together in a composite image, named "stack". Then, this stack of channels is processed with the plugin "Multivariate Statistical Analysis of image series" which allows reducing its dimensions. [START_REF]Multivariate Statistical Analysis of image series (MSA_514)[END_REF] Stack dimensionality reduction is performed according to Principal Components Analysis (PCA). The different channels of the new stack are the principal components (or the factorial images) of the original stack. Based on the new stack, a PCA image of three channels is generated. After that, this PCA image is processed with the plug-in DStretch. [START_REF] Richards | Remote sensing digital image analysis: an introduction[END_REF][START_REF] Domingo | [END_REF] DStretch implements on the PCA image a decorrelation stretch based on color space (YDS, LAB, etc.) and it produces a new set of false color PCA images.

HSI spectroscopy)

Hyperspectral images were acquired with two different imaging devices for the two spectral ranges under study but using the same pushbroom scanning-method systems purchased from Specim Corp. (Finland). The visible and near infrared range (VNIR) imager was a Specim HS camera operating from 400 to 1000 nm equipped with an ImSpector V10E2/3" spectrograph combined with an Imperx IPX-2M30 CDD detector (1600 spatial x 1200 spectral pixels' matrix). The second image device covered the short-wave infrared (SWIR) range from 970 to 2500 nm. This system combined an ImpSector N25E 2/3" spectrograph coupled with a cooled MCT detector (9.6 mm detector with 320 spatial x 256 spectral pixels' matrix).

The objective lenses used for the VNIR and SWIR cameras were OLE23 (focal length of 23 mm) and OLES56 (focal length of 56 mm), respectively. Spectral samplings were of 3 nm for the VNIR and 10 nm for the SWIR detectors. Spatial resolutions were 1 mm (VNIR) and 0.3 mm (SWIR) for the scanning working distances 600 mm and 550 mm respectively.

Two ramps of three 35 W-halogen lamps (one on each side of the detector) were used as illumination sources. They were fixed together with the camera and moved along a motorized bar. After the hyperspectral image scanning of the panel, a diffuse white reference (Spectralon®, 99% reflectance) covering the entire field of view of the detector was scanned to flat-field and calibrate the data. A dark reference was also acquired at each acquisition with a built-in shutter in the aim of quantifying the detector electronic noise.

Different ways to map materials in hyperspectral imaging were used. Mapping specific materials is generally more difficult than with other spectroscopic techniques because the characteristic features are often less obvious and a general appearance of the spectrum is difficult to map without interfering with other materials. It is sometimes necessary to process the data cube once it has been converted to pseudo-absorbance, calculating log(1/R), or once the first derivative has been calculated. This process allows highlighting a characteristic feature of a material. The material's maps were obtained by supervised methods, i.e. by providing one or more reference spectra.

On one hand, we compared reference spectra to those from the data cube using spectral difference. Several functions exist, such as the Spectral Angle Mapper (SAM) [START_REF] Kruse | Summaries of the Third Annual JPL Airborne Geoscience Workshop[END_REF] or the Euclidean distance. We used the SAM which calculates the angle between two spectra considered to be vectors. The SAM is mainly affected by shape difference between the spectral curves. [START_REF] Deborah | [END_REF] Studying the spectral shape variation generally gives enough information concerning chemical properties of materials to be able to classify spectra in regards to its components. The SAM map shows the spectra closest to the reference. The threshold was chosen looking at the histogram of distance and choosing the closest peak to the origin if more than one.

On the other hand, for mixture or for spectra in the SWIR range, a Mixture Tuned Matched Filtering (MTMF) classification was preferred. This linear unmixing algorithm is an effective tool for identifying the presence and abundance when the spectrum presents features from several materials. [START_REF] Boardman | Proceedings of the Summaries of the Seventh JPL Airborne Geoscience Workshop[END_REF][START_REF] Boardman | [END_REF] The reference spectra used here were directly extracted from the data cube in a specific material or they were end-members determined thanks to the hourglass paradigm developed by Boardman et al. [START_REF] Boardman | Proceedings of the Summaries of the Seventh JPL Airborne Geoscience Workshop[END_REF] and implemented in the ENVI image processing program. This way to process is very widespread in the analysis of cultural heritage objects. [27] Hyperspectral maps should be considered carefully as it is possible that different materials may be mapped at the same time (due to the proximity of the spectra to the reference, for example). So, the validity of the maps was checked by extracting reflectance spectra from the data cube.

MA-XRF scanning

Elemental cartographies were performed with the M6 Jetstream XRF scanner (Bruker) [19,28] containing an X-ray tube with a rhodium anode at 50 kV and 600 µA with a 100 µm thick beryllium window. Polycapillary optic was used to focus the X-ray beam. The X-ray detector was a 60 mm² SDD with a Peltier cooler, which 4096 channels were divided into 40 keV with a maximum count rate of 275 kcps. The target was placed horizontally at an average working distance of 2.4 cm. The beam diameter was 450 µm, the distance between each point was 450 µm and the time per point 100 ms. Data were processed with the Esprit software (Bruker) and Datamuncher developed by Alfeld. [29][30] The wooden panels presented several flight holes made by xylophages. In order to protect the other wooden cultural heritage objects conserved in the room where the XRF system was implemented, all XRF analyses were performed through hermetically sealed polyethylene bags in which panels were slipped. Preliminary tests showed that the polyethylene covering of wood panels did not affect acquisitions conditions and obtained results.

RESULTS AND DISCUSSION

In this study, MA-XRF maps were made on a limited area, panel by panel, including faces of the Virgin and the Child (Figure 1), because already this part (composed of 9 individual areas) needed about 14 hours to be scanned. Figure 3 shows the original element maps obtained by XRF. Hyperspectral and multispectral techniques were applied on the three panels together. These techniques, allowing faster imaging and covering a larger area in relatively short time: about 10 min for multispectral and 75 min for both hyperspectral wavelength domains. These techniques were used to confirm, complete and extrapolate the XRF results on larger area of examined panels.

17TH CENTURY PAINT AND 19TH CENTURY RESTORATIONS

As stated before, painting executed by Louis Le Corre in 1686 [START_REF] Hamoury | La peinture religieuse en Bretagne aux XVIIe et XVIIIe siècles[END_REF][START_REF] Pottier | Le Patrimoine religieux -Le Quillio, Terre sacrée[END_REF] and its later 19th century restorations were discovered hidden behind painted canvas nailed on the panels. Lead white was identified for the carnation of the Virgin and the Child by the presence of lead in XRF spectra (Figure 4) and confirmed by hyperspectral analysis showing the presence of hydrocerussite (Pb 3 (CO 3 ) 2 (OH) 2 ) which, along with cerussite (PbCO 3 ), is present in this artificial pigment. On reflectance spectra, hydrocerussite can be identified in particular by the presence of a narrow absorption feature centered near 1445 nm corresponding to the first overtone of ─OH stretching vibration (Figure 5). The best map of the hydrocerussite distribution was obtained by converting the SWIR hyperspectral cube of reflectance data to pseudo absorbance (log (1/R)) and calculating the distance (here SAM) from a hydrocerussite reference extracted from the data cube to the other cube spectra. The resulting map (Figure 6) shows the spectra closest to the reference. The SAM map allows visualizing only the lead of the most surface layer of the painting. This gives us the possibility to access the information of surface materials only, without that of the underlying layers. However, the detection of hydrocerussite by hyperspectral data was sometimes difficult in some areas (the Virgin hair or the lower part of her face, for example) because of the low signal intensity, likely caused by the presence of black materials absorbing in the region of hydrocerussite characteristic features.

FIGURE 5

Short-wave infrared (SWIR) reflectance spectrum (extracted from 400 pixels) in the Virgin face showing the presence of hydrocerussite compared to a reference of lead white oil painting.

FIGURE 6

Spectral Angle Mapper (SAM) map of the short-wave infrared (SWIR) image cube converted in pseudo-absorbance (log(1/R)) showing the presence of hydrocerussite on the surface. A spectrum extracted from the Virgin face is used as reference spectrum.

Vermilion or cinnabar was used for the lips, as deduced from the XRF results showing the presence of mercury (Hg) (Figures 7 and8) and from the reflectance spectra showing absorption bands with a sigmoid-shape characteristic of semiconductor materials such as minium, realgar, cinnabar and vermilion (Figures 9 and10). The reflectance spectra have an inflection point around 615 nm which is slightly shifted compared to pure vermilion reference (inflection point at 580-605 nm). [START_REF] Aceto | [END_REF] This shift could be explained by the presence of impurities, by specific morphological characteristics (particle size distribution and concentration of pigments) or by mixture with other red pigment. Such a high inflection point value has already been found for pure cinnabar by other teams. [34] XRF and VNIR results suggest that small quantities of vermilion are mixed with lead white for the carnation (Figures 8 and10).

FIGURE 8

X-ray fluorescence (XRF) false color mapping. Gold (Lα lines) is in red, mercury (Lα lines) in blue, mix of both in white. Orange line underlines the lips of the characters and the flower on the surface, pink lines are the lips of the underneath painting.

FIGURE 9

Visible and near infrared range (VNIR) reflectance spectrum (extracted from 100 pixels) from the Child lips showing the presence of vermilion compared to a reference of vermilion (Kremer).

MA-XRF maps highlight the shape of the flower in the right hand of the Child. The high concentration of lead and mercury indicates the presence of vermilion and lead white used for lights (also confirmed by VNIR and SWIR hyperspectral data). The stem is easily discernable in black on the SWIR hyperspectral image, while looking directly at this painting the stem is not easily discernable from the dark background (Figure 11). As the reflectance spectrum in the stem is almost zero, it suggests that an absorbing material such as carbon black is used in this area. Due to the conservation state of the painting layers, it is difficult to determine the flower specie, but the shape of the petals suggests that it could be a rose.

FIGURE 10

Visible and near infrared range (VNIR) image extracted at 615 nm from the reflectance first derivative data cube. The white areas underline the high concentration of vermilion.

FIGURE 11

Short-wave infrared (SWIR) false color image (R=2177 nm, G=2204 nm, B=2347 nm). Yellow arrows indicate the stem of the flower.

FIGURE 12

X-ray fluorescence (XRF) false color mapping. The iron (Kα lines) is in red, manganese (Kα lines) in blue and mix of both in white. Based on data extrapolated from false color principal components analysis (PCA) composite images, dotted pink lines highlight iron based pigments (hair and architectural columns), whereas dotted blue lines underline umber (the background).

The presence of manganese (Mn) and iron (Fe) is the footprint of umber used for the background (Figures12 and 13). No hyperspectral imaging signals were detected due to the presence of highly absorbent black material (probably a carbon black).

The red pigment of the architectural columns is probably hematite, due to the presence of iron (Figures 12 and14) and the specific characteristics observed on VNIR reflectance spectra (Figures 15 and17). In these areas, the spectra show a characteristic shape due to the presence of a-Fe 2 O 3 as chromophore. [START_REF] Aceto | [END_REF] However, different pigments (red ochre, burnt sienna, etc.) contain hematite. It is therefore not possible to identify more precisely the red iron oxide pigment used here. Gold (Au) and mercury (Hg) are present in the columns' decoration (Figures 8, 16 and 18). Most of the time, these two elements are correlated but it is possible to find small areas with only Au or only Hg (Figure19). Thus this golden material doesn't seem to be a gold-mercury amalgam. As no features specific to a mercury based pigment are visible in spectra extracted from VNIR hyperspectral data, mercury might not be on the surface but should come from an underlying layer. All these data rather attest to the use of gold applied on vermilion or cinnabar used as underpaint probably to give redder hue to the gold on the surface.

FIGURE 13

X-ray fluorescence (XRF) sum spectrum (52 pixels) extracted from the background showing the presence of iron and manganese composing umber.

FIGURE 14

X-ray fluorescence (XRF) sum spectrum (58 pixels) extracted from the column showing the presence of iron.

FIGURE 15

Visible and near infrared (VNIR) reflectance spectrum (extracted from 100 pixels) from the column showing hematite compared to a reference of hematite (Kremer).

FIGURE 16

X-ray fluorescence (XRF) sum spectrum (60 pixels) extracted from the gold decoration on the column showing the presence of gold and mercury.

FIGURE 17

Spectral Angle Mapper (SAM) map of the visible and near infrared range (VNIR) image cube. A spectrum of hematite extracted from the right column is used as reference spectrum.

FIGURE 18

Spectral Angle Mapper (SAM) map of the visible and near infrared range (VNIR) image cube. A spectrum of gold area extracted from the right column is used as reference spectrum.

FIGURE 19

Detail of figure 8, X-ray fluorescence (XRF) false color mapping. Gold (Lα lines) is in red, mercury (Lα lines) in blue, mix of both in white.

The presence of nickel and cobalt in the hair of the Virgin, as well as the background between her face and the hand of the Child were assigned to smalt (Figures 20 and21). The area where this pigment is detected is discontinued and covers some parts of the hair of the Virgin. As before, unfortunately, it is not possible to confirm this interpretation with hyperspectral imaging because the signal in this area is too weak probably due to the presence of a light-absorbing material like a carbon black.

In summary, the painter's pallet is classical for a paint of such period. Figure 22 resumes the results obtained for the 17th century painting.

FIGURE 20

X-ray fluorescence (XRF) sum spectrum (60 pixels) extracted from the hair of the Virgin showing the presence of cobalt and nickel assigned to the smalt.

FIGURE 21

X-ray fluorescence (XRF) map in false color, cobalt (Kα lines) appears in red, nickel (Kα lines) is in blue and the mix of both in white. Based on data extrapolated from false color principal components analysis (PCA) composite images, the dotted white line and the white arrows show the area with cobalt blue.

FIGURE 22

Schematic interpretation of the 17th century painting.

MA-XRF also reveals the presence of barium in different areas: on the carnation of the face and neck of the Virgin, on the shoulder of the Child, both on their hair and some parts of the flower (Figures 23 and24). The pigment used is rather the barium white (BaSO 4 ) than the lithopone (ZnS•BaSO 4 ), because zinc was not detected there. In the PCA image in false color (Figure 25), the green color of the dress is the same as on the hair of the Virgin where barium white is identified by XRF analysis. Reflectance spectra of the two areas confirmed this similarity (extracted from VNIR hyperspectral data, not shown). The mixture used to restore these two areas could be similar not only for the presence of barium white but also because of the use of a specific binding or unidentified pigments. Indeed, the barium white was mixed with other undetermined pigments in order to achieve the restoration (Figure 26).

The upper part of the hair of the Child is covered by an orange mixture composed of barium white and an iron based pigment containing hematite (confirmed by VNIR hyperspectral data, not shown). The reflectance spectrum of this mixture is similar to those of the dress and the hair of the Virgin. This mixture does not cover all the hair and let appeared a sublayer characterized by the presence of the iron only, and which seems to correspond to the original hair of the Child (Figure 24).

Barium white started to be sold as a pigment for painting in 1810-1820, so this narrow period is a terminus post quem for the restoration. [START_REF] Feller | Artist's pigments -A handbook of their history and characteristics[END_REF] The restoration should have then been done between this date and the covering of the panels in 1869 by oil-painted linen canvas. Also, the traces of iron nails are clearly visible on XRF iron map (Figures 3 and12) and on the SWIR hyperspectral image (not shown). The presence of chlorine is the footprint of the impact of corrosion of a nail on upper panel, which probably happened due to sea water salty aerosols, as the church is located in the region close to the ocean (Figure 27).

FIGURE 24

False color C-ray fluorescence (XRF) map. Barium (Lα lines) is in red, iron (Kα lines) is in blue and mix of both in white. The red and white areas represent restorations with barium white. Based on data extrapolated from false color principal components analysis (PCA) composite images, the dotted white lines underline the restored part of the hair of the Child and the dotted pink lines are the clothes of the Virgin.

FIGURE 25

Principal components analysis (PCA) image in false color (LDS color space). Dotted white lines underline the restored areas containing barium based material.

FIGURE 26

Visible (VIS) image. Semi-transparent white areas represent the restored areas containing barium based material on data obtained with X-ray fluorescence (XRF) (Figure 24) and principal components analysis (PCA) images (Figure 25).

FIGURE 27

Chlorine (Kα lines) X-ray fluorescence (XRF) map. The white area underlines the high concentration of chlorine around a nail hole. Blue points indicate the positions of such holes.

UNDERLYING PAINTING FROM THE END OF 15TH CENTURY

Under the surface of the 17th century paint, an older oil painting was discovered dating approximately from the end of 15th century. Indeed, a rectangular uncovering window, made by conservators, shows the presence of an eye (Figure 1). Furthermore, on the XRF cartography of lead, a three-quarter portrait appears enclosed between two phylacteries (Figure 28). The phylacteries are visible only on the upper panel while the face is visible also on the two lower panels. They are painted using lead white pigment (Pb) (Figure 28). The margins for the placement of the letters within the phylacteries were observed on Pb XRF map as darker lines. This is due to the mechanical removal of the lead white during the tracing of the margins. Letters on phylacteries are probably paint with a carbon black mixed with a copper-based siccative pigment, making them pretty readable on the XRF copper (Cu) map (Figure 29). The phylacteries like those we have seen on examined panels are partially visible in several parts of the entire vault, and especially above the image of the Virgin (Figure 30).

FIGURE 28

Lead (Lα lines) X-ray fluorescence (XRF) map. The yellow lines represent the limits of subjacent phylacteries, yellow arrows show the margins tracings and the white lines highlight the underlying face with long hair or veil.

FIGURE 29

X-ray fluorescence (XRF) mapping of copper (Kα lines). The red lines represent the letters of the phylacteries and the eye contour of the underlying face while blue lines indicate underlying elements which can correspond to hair or veil shadows.

FIGURE 30

Phylacteries of the original painting from the end 15th century under the 17th century overpaint.

The XRF mapping of lead also an underlying face with long hair or veil (Figure 28). Pupils, upper eyelids, the left eye arch and the bridge of the nose are visible. The eye contour is accentuated by a copper-based pigment (Figure 29). Several vertical oblong forms located on both sides of the Virgin face, detected with the XRF Cu mapping, appear as possible shadings of the hair or veil. The lips are painted with vermilion (Hg) (Figure 8). This figure could be a representation of a Virgin whose hair is covered with a veil (Figure 31). XRF mapping of the additional lower panels could help us identifying it more precisely by revealing complementary parts of this image. This would allow us to discuss the possible continuity of the iconographic program between the original painting from the end of the 15th century and the overlaid one from the 17th century.

The phylacteries are unequivocally earlier than the 17th century painting but it is questionable whether they are contemporary with the 15th century painting. In fact, the face is much enclosed between the phylacteries (Figure 31). Another example is visible on the vault with the phylacteries crossing behind the head of Saint Maurice. However, the visible Saint Maurice figure was painted during the 17th century campaign and it is impossible to say at the present moment if it covers another Saint's image, as we did not examine it. Since there is no written evidence of a third stage of painting, we will assume that all the hidden Virgin figure and phylacteries are contemporary with each other. A MA-XRF study or a full radiograph of the vault would be required to discuss this hypothesis. This would also allow us to check whether some of the panels have been replaced or moved over time, explaining the hypothetical discontinuities in the phylacteries.

FIGURE 31

Schematic interpretation of the elements of the underneath painting from the end of 15th century.

CONCLUSIONS

The aim of this study was to detect the extent of the original underlying painting on wooden panels. In addition, this work aimed to describe the second painting from century and its 19th century restorations. For this purpose, three wooden panels portraying the Virgin with a Child were chosen as the target of this work. The goals were achieved using non-destructive imaging techniques, which allowed to both find the original painting and to characterize the later painting and its restoration.

The more ancient 15th century painting is clearly detected by the MA-XRF mapping (Figure 31). The face of the Virgin and two phylacteries surrounding it were revealed by the XRF mapping while the copper map revealed the writing within the phylacteries. Largest campaign of analysis on the panels placed above the studied ones could allow to see larger text parts of the phylacteries conserved and to make their transcription.

Hyperspectral and multispectral techniques allowed faster imaging and covered larger area in relatively short time, circa 75 and 10 min respectively for whole 3 panels, in comparison with MA-XRF scanning, which needed more than 14h for about 1/3 of the surface. Hyper-and multispectral imaging confirmed, completed and allowed extrapolation of the XRF results of the examined panels. They particularly helped in understanding part of the painted iconography highly deteriorated like the case of the flower and its stem. Nonetheless, these techniques did not help revealing the original 15th century painting.

Indeed, the complementarity of MA-XRF mapping, multi-and hyperspectral imaging is clearly underlined with this study for the second painting from the 17th century (Figure 22) and its 19th century restorations (Figure 26). In particular, figure 22 illustrates the complementary results obtained by the combination of those three techniques. Based on these results, it is indeed possible to distinguish, with non-invasive techniques, the different painting and restoration campaigns in order to help the conservators in their decision to uncover or not the 15th century original painting.

The results get some information on the pigments used by the painters of the 15th and 17th century paintings. They show an interesting use of gold underlaid by a mercury based pigment used for the decoration of columns on both sides of the painted panels. They also allow dating the restoration work as done between 1810 and 1869 giving a terminus post quem due to the presence of barium white. Nevertheless, some questions are still pending, such as whether there is a lack of continuity of the phylacteries on the middle and lower parts of studied panels, or of the existence of the Child figure in the 15th century painting, which could attest the continuity of pictorial program in overlying painting. We expect that the answers to these questions could be given after the examination in situ of a larger area of the vault, using preferentially a smaller MA-XRF portable device.

FIGURE 3 X

 3 FIGURE 3X-ray fluorescence (XRF) maps of different chemical elements of interest (Kα or Lα lines).

FIGURE 4 X

 4 FIGURE 4X-ray fluorescence (XRF) sum spectrum (60 pixels) extracted from the forehead of the Virgin showing the presence of lead.

FIGURE 7 X

 7 FIGURE 7X-ray fluorescence (XRF) sum spectrum (60 pixels) extracted from the lips of the Child showing the presence of mercury.

FIGURE 23 X

 23 FIGURE 23X-ray fluorescence (XRF) sum spectrum (62 pixels) extracted from the face of the Virgin showing the presence of barium.

ACKNOWLEDGEMENTS

The authors would like to gratefully acknowledge the financial support of the Fondation des sciences du patrimoine (Labex Patrima, ANR-10-LABX-0094-01). This work was also supported by the Paris Ile-de-France Region through Domaine d'intéret majeur (DIM) programme:

"Matériaux anciens et patrimoniaux".