Alina Tirsoaga

Magdi El Fergani

Nicolas Nuns

Pardis Simon

Pascal Granger
email: pascal.granger@univ-lille.fr

Vasile I Parvulescu
email: vasile.parvulescu@chimie.unibuc.ro

Simona M Coman
email: simona.coman@chimie.unibuc.ro

Multifunctional nanocomposites with non-precious metals and magnetic core for 5-HMF oxidation to FDCA

Keywords: 5-hydroxymethylfurfural (HMF), 2, 5-furandicarboxylic acid (FDCA), core-shell magnetic nanocomposite, niobia-mesoporous silica shell, non-precious metal oxides

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

Introduction

5-hydroxymethyl-furfural (HMF) has been identified among the top platform molecules derived from cellulose and as one of the promising starting substrates for a wide spectrum of chemicals. These transformations involve various reactions such as oxidation, hydrogenation, hydration, decarbonylation, or etherification [START_REF] Pichler | Ruthenium Supported on High-Surface-Area Zirconia as an Efficient Catalyst for the Base-Free Oxidation of 5-Hydroxymethylfurfural to 2,5-Furandicarboxylic Acid[END_REF]. In this context, 2,5-furandicarboxylic acid (FDCA) affording polyethylene 2,5 furan-dicarboxylate (PEF) has the potential to substitute petroleumderived terephthalic acid (TPA) in the production of polyethylene terephthalate (PET)-based plastics and therefore became a very attractive feedstock for the "green" polymers [START_REF] Jong | Furandicarboxylic Acid (FDCA), A Versatile Building Block for a Very Interesting Class of Polyesters[END_REF]. However, the classical synthesis of FDCA requires harsh reaction conditions such high pressures and temperatures, metal salts as catalysts and organic solvents, rendering the process into a quite expensive and polluting one [START_REF] Partenheimer | Synthesis of 2,5-diformylfuran and furan-2,5-dicarboxylic acid by catalytic air-oxidation of 5-hydroxymethylfurfural. Unexpectedly selective aerobic oxidation of benzyl alcohol to benzaldehyde with metal/bromide catalysts[END_REF][START_REF] Ribeiro | Cooperative effect of cobalt acetylacetonate and silica in the catalytic cyclization and oxidation of fructose to 2,5-furandicarboxylic acid[END_REF]. Therefore, in order to be competitive with TPA, highly efficient routes for the FDCA production are mandatory. With this aim great efforts have already been spent merely using noble metals (ie, Pt, Au, Pd, or Ru) catalysts supported on different carriers and in the presence of aqueous alkali solutions [START_REF] Rass | Selective aqueous phase oxidation of 5hydroxymethylfurfural to 2,5-furandicarboxylic acid over Pt/C catalysts: influence of the base and effect of bismuth promotion[END_REF][START_REF] Pasini | Selective oxidation of 5-hydroxymethyl-2-furfural using supported gold-copper nanoparticles[END_REF]. However, the presence of an inorganic base in the homogeneous phase led to serious environmental concerns. Therefore, a first progress was made by developing catalytic systems able to work in their absence. For instance, CeO2 and TiO2-supported Ru(OH)x catalysts are able to drive this reaction under base-free conditions, with a 60% yield to FDCA [START_REF] Gorbanev | Effect of Support in Heterogeneous Ruthenium Catalysts Used for the Selective Aerobic Oxidation of HMF in Water[END_REF], while the cationic Ru(4wt%)-Fe3O4@SiO2 catalyst, recently developed in our group [START_REF] Tirsoaga | Upgrade of 5-Hydroxymethylfurfural to Dicarboxylic Acids onto Multifunctional-Based Fe3O4@SiO2 Magnetic Catalysts[END_REF] led to as high selectivities as 80.6% for a 92% conversion of HMF. However, the high price of the noble metals still demands innovating cheap alternatives using non-noble transition metal-based heterogeneous catalysts [START_REF] Delidovich | Alternative Monomers Based on Lignocellulose and Their Use for Polymer Production[END_REF].

In accordance to the structural similarity, FDCA and the intermediate of its oxidation, 5formyl furan-carboxylic acid (FFCA), may combine limiting a further oxidation to FDCA. Also, FFCA may affect the further polymerization to PEF acting as a chain termination agent. Therefore, the absence of FFCA in the final oxidation product is mandatory in a FDCA competitive market. To achieve this goal a great deal of attention has been paid for the production of FDCA from HMF by (i) improving the catalytic strategies towards high FDCA yields [START_REF] Donoeva | Carbon Support Surface Effects in the Gold-Catalyzed Oxidation of 5-Hydroxymethylfurfural[END_REF] and (ii) replacing precious (e.g. Au, Pd) with less expensive and more available non-noble metals (e.g. Mn, Co, Fe) [START_REF] Pal | Recent Advances in the Development of 5-Hydroxymethylfurfural Oxidation with Base (Nonprecious)-Metal-Containing Catalysts[END_REF]. Using molecular oxygen and MnO2 in a weak basic medium (NaHCO3) Hara et al. reported conversions of 17-58% [START_REF] Hayashi | Heterogeneously-Catalyzed Aerobic Oxidation of 5-Hydroxymethylfurfural to 2,5-Furandicarboxylic Acid with MnO2[END_REF]. Under similar conditions less acidic catalysts as Mn 2+ and Mn 3+ (ie, Mn2O3, Mn3O4, and MnO) produced only extremely low yields of FDCA (0-5%). However, Mn3O4 is very efficient for the further oxidation of HMF to 2,5-diformylfuran (DFF, 82.1% yield for 100% conversion of HMF) [START_REF] Liu | Efficient aerobic oxidation of biomassderived5-hydroxymethylfurfural to 2,5-diformylfuran catalyzed by magneticnanoparticle supported manganese oxide[END_REF]. Other less basic oxide catalysts such as Fe2O3, Fe3O4, FeO, Co3O4, CoO, NiO, CuO, and Cu2O are also not effective affording only moderate HMF conversions (16-40%) [START_REF] Hayashi | Heterogeneously-Catalyzed Aerobic Oxidation of 5-Hydroxymethylfurfural to 2,5-Furandicarboxylic Acid with MnO2[END_REF]. On the other side high loadings of CoOx and MnOx (ie 10wt%) deposited onto silica in the presence of molecular oxygen and a homogeneous base orientate this oxidation rather to succinic or maleic acid than to FDCA [START_REF] Tirsoaga | Upgrade of 5-Hydroxymethylfurfural to Dicarboxylic Acids onto Multifunctional-Based Fe3O4@SiO2 Magnetic Catalysts[END_REF].

Based on this state of the art we orientate our research in the direction of the utilization of more acidic oxide catalysts. In this respect, Tanabe identified niobia as one of the most recommended exhibiting, in addition of a high acidity, noticeable water tolerant properties [START_REF] Chen | Niobic acid as an efficient catalyst for vapor phase esterification of ethyl alcohol with acetic acid[END_REF].

However, its catalytic performances are closely related to the local structure which is highly influenced by the preparation method [START_REF] Yoshida | Control of the structure of niobium oxide species on silica by the equilibrium adsorption method[END_REF][START_REF] Shirai | Preparations and catalytic properties of single, pair, and monolayer niobium catalysts[END_REF][START_REF] Shirai | Monolayer structures of niobic acids supported on silica and their catalytic activities for esterification of acetic acid with ethanol[END_REF][START_REF] Asakura | Synthesis, characterization, and catalytic properties of silica-attached one-atomic-layer niobium oxide catalysts[END_REF][START_REF] Brayner | Niobium pentoxide prepared by soft chemical routes: morphology, structure, defects and quantum size effect[END_REF], the precursors [START_REF] Nishimura | New SiO2-supported niobium monomer catalysts for dehydrogenation of ethanol[END_REF][START_REF] Nishimura | Carbon Monoxide Hydrogenation on SiO2 -, Al2O3 -, or TiO2-Attached Nb-Monomer Catalysts[END_REF][START_REF] Ichikuni | Structures and catalysis of new Nb dimers on SiO2[END_REF][START_REF] Bayot | Water-soluble niobium peroxo complexes as precursors for the preparation of Nb-based oxide catalysts[END_REF] and the nature of the support is deposited [START_REF] Yoshida | Control of the structure of niobium oxide species on silica by the equilibrium adsorption method[END_REF][START_REF] Jehng | Molecular structures of supported niobium oxide catalysts under in situ conditions[END_REF][START_REF] Jehng | The molecular structures and reactivity of supported niobium oxide catalysts[END_REF][START_REF] Jehng | Molecular structures of supported niobium oxide catalysts under ambient conditions[END_REF][START_REF] Somma | Niobia-silica aerogel mixed oxide catalysts: Effects of the niobium content, the calcination temperature and the surface hydrophilicity on the epoxidation of olefins with hydrogen peroxide[END_REF]. Indeed, different niobia species were evidenced such as highly dispersed monomeric and oligomeric fourfold (NbO4), fivefold (NbO5) and sixfold (NbO6) coordinated species [START_REF] Jehng | Molecular structures of supported niobium oxide catalysts under in situ conditions[END_REF][START_REF] Yoshida | The local structures and photocatalytic activity of supported niobium oxide catalysts[END_REF][START_REF] Yoshida | Analysis of XANES for identification of highly dispersed transition metal oxides on supports[END_REF][START_REF] Tanaka | Preparation of highly dispersed niobium oxide on silica by equilibrium adsorption method[END_REF] with a relative distribution depending on the Nb loading and the nature of the oxide support. However, the relation between the local structure of the supported niobium species on different carriers and its particular catalytic behavior [START_REF] Coman | NbF5-AlF3 Catalysts: Design, Synthesis, and Application in Lactic Acid[END_REF][START_REF] Podolean | Efficient magnetic recoverable acid-functionalized-carbon catalysts for starch valorization to multiple bio-chemicals[END_REF][START_REF] Candu | Direct conversion of cellulose to α-hydroxy acids (AHAs) over Nb2O5-SiO2-coated magnetic nanoparticles[END_REF][START_REF] Fergani | Nb-Based Zeolites: Efficient bi-Functional Catalysts for the One-Pot Synthesis of Succinic Acid from Glucose[END_REF][START_REF] Candu | Efficient glucose dehydration to HMF onto Nb-BEA catalysts[END_REF] is also dictated by the niobia content and the preparation route.

For Nb-Beta zeolites catalysts prepared by a post-synthesis methodology the wet oxidation of glucose occurred with a selectivity to succinic acid (SA) as high as 84% for a total conversion [START_REF] Fergani | Nb-Based Zeolites: Efficient bi-Functional Catalysts for the One-Pot Synthesis of Succinic Acid from Glucose[END_REF]. This excellent catalytic performance was attributed to extra-framework isolated Nb(V)O-H and Nb2O5 pore-encapsulated clusters along to residual framework Al-acid sites. However, a direct synthesis procedure leads to tetrahedral species corresponding to niobyl bonds (-Nb=O) with a different behavior [START_REF] Corma | Water Resistant, Catalytically Active Nb and Ta Isolated Lewis Acid Sites, Homogeneously Distributed by Direct Synthesis in a Beta Zeolite[END_REF]. Niobium incorporated in mesoporous siliceous matrixes, such as MCM-41 or MCM-48 [START_REF] Gao | Structural and Reactivity Properties of Nb-MCM-41: Comparison with That of Highly Dispersed Nb2O5/SiO2 Catalysts[END_REF][START_REF] Ziolek | Oxidative properties of niobium-containing mesoporous silica catalysts[END_REF][START_REF] Ziolek | Acidity study of Nb-containing MCM-41 mesoporous materials. Comparison with that of Al-MCM-41[END_REF][START_REF] Schumacher | Novel synthesis of spherical MCM-48[END_REF][START_REF] Parvulescu | Mono (V, Nb) or bimetallic (V-Ti, Nb-Ti) ions modified MCM-41 catalysts: synthesis, characterization and catalysis in oxidation of hydrocarbons (aromatics and alcohols)[END_REF][START_REF] Nowak | Epoxidation of cyclohexene on Nbcontaining meso-and macroporous materials[END_REF][START_REF] Hartmann | Characterization and catalytic evaluation of mesoporous and microporous molecular sieves containing niobium[END_REF][START_REF] Nowak | Catalytic behavior of niobium-containing SBA-15 molecular sieves in the oxidation of cyclohexene[END_REF][START_REF] Nowak | Effect of texture and structure on the catalytic activity of mesoporous niobosilicates for the oxidation of cyclohexene[END_REF] corresponds only to incorporated Nb 5+ cations as isolated NbO4 species; consequently, they exhibit rather redox characteristics than acidic properties displaying therefore unique oxidation properties. However, these isolated species are sensitive to the environmental conditions, being demonstrated that, through hydration, the coordination number of the surface niobia species may increase from four-fold to six-fold due to the adsorption and coordination of water molecules [15; 29]. The ability of niobium to combine with other elements was recently reviewed by Nowak [START_REF] Nowak | Frontiers in mesoporous molecular sieves containing niobium: From model materials to catalysts[END_REF] and Ziolek and Sobczak [START_REF] Ziolek | The role of niobium component in heterogeneous catalysts[END_REF]. This ability opens new opportunities to design and develop multifunctional materials with catalytic features able to carry out specific reactions.

Based on this state of the art, the main purpose of the present study was to design and develop an efficient multi-functional magnetic nanocomposites catalytic system for the selective HMF oxidation to FDCA by combining Mn, Co and Fe oxides with NbMCM-41 as a shell of the MNP core. These materials combine unique oxidation properties with practical separation advantages.

Experimental section

Catalysts preparation

The multi-functional magnetic nanocomposites were prepared by covering nano-magnetic cores (MNP) with Nb2O5-SiO2 mesoporous shells (denoted xNb@MNP, where x represents the Si/Nb ratio) followed by CoOx, MnOx or FeOx (10 wt%) deposition by impregnation-precipitation.

MOx-SiO2@MNP

MOx-SiO2@MNP (MOx =CoOx, MnOx or FeOx (10 wt%)) nanocomposites were prepared using a previously reported methodology [START_REF] Tirsoaga | Upgrade of 5-Hydroxymethylfurfural to Dicarboxylic Acids onto Multifunctional-Based Fe3O4@SiO2 Magnetic Catalysts[END_REF]. Briefly, SiO2@MNP was first synthesized as follow: 10 mL NH3OH (25%) and 3.5 mL of TEOS were added to magnetic particles dispersed into ethanol and the mixture was stirred overnight at 40°C. Then, the coated particles were washed with water and ethanol and dried for 24h, at 80 °C. Obtained Fe3O4@SiO2 particles were added to a solution containing the corresponding amount of M(II) acetate dissolved in 10 mL of distilled water, under stirring. After 5 min an aqueous solution of NaOH (1M) was dropwise added until a pH of 10.5 and the mixture was heated at 60 °C for 4h. After filtration, the solid was washed with distilled water until neutral pH, then dried in air at 80 °C for 24h, then under vacuum at 110 °C for 6h, and finally calcined in static air at 500 °C (heating rate 1 °C/min) for 3h.

Nb2O5-SiO2@MNP (xNb@MNP)

The Nb incorporation inside the pore walls was carried out through the Atrane method, versatile in developing mesoporous materials with uniform and high dispersed heteroelements in the silica hydrophilic matrix [START_REF] Coman | New heterogeneous catalysts for greener routes in the synthesis of fine chemicals[END_REF]. Water needed for TEOS hydrolysis was added simultaneously with the corresponding amount of MNPs in order to ensure an uniform packaging with the mesoporous Nb2O5-SiO2 shell. In this way stable links between the -OH groups existing on the MNPs surface and those generated by TEOS hydrolysis were obtained. For the synthesis of the 22Nb@MNP (Si/Nb = 22) sample the molar ratio of reagents in the native solution was adjusted to 2-xSi:xNb:7TEAH3:0.5NaOH:0.52CTAB:180H2O, where x = 0.087, at a pH of 11. The following methodology was used: 0.5 g (0.0125 mol) of NaOH were dissolved in 23 mL (0.172 mol) triethanolamine (TEAH3) at 60°C. Immediately, 10.54 mL (0.046 moles) of tetraethyl ortho-silicate (TEOS) and 0.544 mL (0.002 moles) of Nb(OEt)5 were added and the obtained mixture was stirred at 130 °C for 5 min. Then the temperature was reduced at 110 °C and 4.68 g (0.013 moles) cetyltrimethyl-ammonium bromide (CTAB) were added under stirring. After a further adjusting of the temperature at 60 °C, 80 mL (4.44 moles) of water and 0.5 g of MNP were added and, in a very short time, a white suspension was formed. This was left to ripen at room temperature, for 24 hours.

The resulted solid was filtered, washed with water and ethanol and dried in air. Finally, to release the mesoporous system, the surfactant (CTAB) was extracted from the solid with a solution of acetic acid/ethyl alcohol. For this, approximately 1 g of solid was suspended in a solution of 8 mL CH3COOH (80%) and 120 mL ethyl alcohol (99%), and the obtained mixture was refluxed at 60 °C for 2 h. To complete the extraction process, the suspension was refluxed at 60 °C for another 16 h, with a new mixture of CH3COOH/ethyl alcohol. The obtained material was collected by filtration, washed with ethanol and dried at 80 °C for 24h and then dried under vacuum at 110 °C for 6h. Part of the samples was calcined in a static atmosphere, at 500 °C for 3h. In a similar way a sample with a Si/Nb ratio of 50 was synthesized by adjusting the amounts of TEOS and Nb(OEt)5 (x = 0.039). These samples were denoted xNb@MNP, where x represents the Si/Nb ratio of 22 and 50, respectively.

10M@xNb@MNP mutifunctional magnetic nano-composites

To an aqueous solution of M(II) acetate (where: M(II) = Co, Mn or Fe), obtained by dissolving the salt in 10 mL of water, at room temperature, the xNb@MNP was added under strong stirring. After 5 min, an aqueous solution of NaOH (1M) was added drop-wise till pH = 10.5 and the obtained mixture stirred for 4h at 60 °C. After reaction the solid was separated by filtration, washed till neutral pH, dried in air, at 80 °C, for 24h, then under vacuum at 110 °C, for 6h and calcined in static atmosphere, at 500 °C for 3h. The final samples were denoted as: 10M@xNb@MNP, where: M = Co, Mn or Fe and x = Si/Nb ratio of 22 or 50. Theoretical loadings of Nb were 0.825 wt% (0.010 mol%) for the catalysts with a Si/Nb ratio of 50, and 1.82 wt% (0.02 mol%) for the catalysts with a Si/Nb ratio of 22. The loading of MOx (ie, CoOx, MnOx or FeOx) was 10 wt%, irrespective of the Si/Nb ratio.

Catalysts characterization

The prepared catalysts were exhaustively characterized using different techniques, such as BET, CO2-and NH3-TPD measurements, XRD, XPS, SEM-EDX, ToF-SIMS, ICP-OES and DRIFT spectroscopy.

The textural properties were determined from nitrogen adsorption-desorption experiments at -196°C using a Micrometrics Tristar 3020 apparatus. Prior to adsorption, all samples were systematically degassed at 200 °C under primary vacuum for 4 hours. The specific surface area was calculated from the BET equation and pore size distribution was investigated based upon Barret-Joyner-Halenda (BJH) theory.

CO2-and NH3-TPD measurements were carried out using the AutoChem II 2920 station.

The samples (100-200 mg), placed in a U-shaped quartz reactor with an inner diameter of 0.5 cm, were pre-treated under He (Purity 5.0) at 80 ºC for 1 h, and then exposed to a flow of CO2 or a flow of 1vol% NH3 in helium, for 1 h. After that, the samples were purged with a flow of He (50 mL min -1 ) for 20 min at 25 ºC in order to remove the weakly adsorbed species. TPD was then started, with a heating rate of 10 ºC min -1 till 500 ºC where was maintained for 30 min. The desorbed products were analyzed by GC-TCD chromatography. The desorbed CO2/NH3, expressed as mmoles of CO2/NH3 per gram of catalyst, was determined using a calibration curve.

DRIFT spectra were collected with a Thermo spectrometer 4700 in the range of 500-4000 cm -1 . The final spectra were averaged from 400 scans at a resolution of 4 cm -1 .

Powder X-ray Diffraction patterns were collected at room temperature using a Shimadzu XRD-7000 apparatus with the Cu Kα monochromatic radiation of 1.5406 Å, 40 kV, 40 mA at a scanning rate of 0.1 2θ min -1 , in the 2θ range of 5 o -80 o .

Scanning Electron Microscopy (SEM) measurements were carried out using a Hitachi S-4700 Cold Field Emission Gun Scanning Electron Microscope operating at an acceleration voltage of 15kV with a resolution of 1.5 nm. Two secondary electron detectors were utilized above and below the objective lens. Elemental analyzing and mapping were achieved by Energy-dispersive Xray Spectroscopy (EDS). An Oxford EDS system was employed to analyze elemental composition of the prepared catalysts.

X-ray photoelectron spectroscopy analysis was carried out on a Kratos Axis Ultra spectrometer equipped with a mono-chromatized Al-Kα source (15 mA, 14 kV). C 1s core level (285.0 eV) was used as a reference to calibrate Binding Energy (BE) values. Spectra analysis was carried out using Casa XPS software and the Gaussian(70)-Lorentzian(30) line profile was adopted for the peak deconvolution. Nb 3d, Co 2p, Mn 2p, Fe 2p, Si 2p and O 1s core levels were examined for 10M(II)@22Nb@MNP samples with M(II) = Co, Mn or Fe.

ToF-SIMS measurements were carried out on IONTOF GmbH Germany Time of Flight-Secondary Ion Mass Spectrometer equipped with a bismuth liquid metal ion gun (LMIG) for primary ions. Samples in powder form were compacted prior to analysis to reduce surface topography and thus improve mass resolution. High current bunch mode was used for analysis in both polarities using Bi3 + primary ion beam (25keV, 0.25 PA) over a 500×500 µm 2 exposed surface area. With 50 scans and 128×128 pixels, the total fluence did not amount up 10 12 ions/cm 2 ensuring static conditions. Charge effects induced by primary ion beam were compensated by means of a 20-eV pulsed electron flood gun. All these experimental parameters allowed us to reach a mass resolution (m/∆m) of about 4000 at m/z = 60 for SiO2 -.

To investigate the chemical stability of the catalysts the leached metal into the reaction liquid was analyzed by ICP-OES (Agilent Technologies, 700 Series).

Catalytic tests

Catalytic experiments were performed under a vigorous stirring in a stainless steel autoclave (15 mL, HEL Instruments) under the following conditions: 25 mg HMF, 4.4 mL solvent (ie, acetonitrile (ACN), water or water:methanol (50:50 v/v) mixture), oxidation agent (t-BuOOH (0.18 mL, 70 wt% in water), H2O2 (0.16 mL, 30 wt%) or molecular oxygen) and 25 mg catalyst (ie, MOx-SiO2@MNP, xNb@MNP and 10M@xNb@MNP). The catalytic experiments were carried out under autogenic pressure, at 80-100 °C for 2-24h. The tests in the presence of molecular oxygen were carried out at 10-18 atm and temperatures in the range 110-180°C, for 2-24h. Before running the experiments, the reactor was sealed and purged three times with oxygen to remove the residual air. After reaction the reactor was cooled at room temperature, the catalyst was magnetically recovered by placing a permanent magnet on the reactor wall and the products were separated by distillation under vacuum. The recovered products were silylated, diluted with 1 mL of toluene and analyzed by GC-FID chromatography (GC-Shimadzu apparatus). The identification of the products was made using a GC-MS (Carlo Erba Instruments QMD 1000) equipped with a Factor Four VF-5HT column.

Results and discussion

Catalysts characterization

which the base metal oxides (ie, MnOx, CoOx and FeOx, respectively) were incorporated into a previously synthesized mesoporous niobium-containing silica. The incorporation of Nb inside the silica pore walls was realized following the already reported Atrane method [START_REF] Cabrera | Generalised syntheses of ordered mesoporous oxides: the atrane route[END_REF].

The adsorption-desorption isotherms of nitrogen at 77 K for these samples correspond to multi-modal porous materials (Figures 1A-C, Table 1). According to the IUPAC classification the isotherms are of Type IV corresponding to mesoporous materials. However, the pore packing motif is disordered hexagonal in all cases with the simultaneous presence of internal pores and irregular secondary inter-crystalline cavities [START_REF] Cabrera | Generalised syntheses of ordered mesoporous oxides: the atrane route[END_REF]. The surface area was pretty high for such kind of materials and decreased in the order Co>Fe>Mn and with the Si/Nb ratio (Table 1). The pore volume varied in the same Co>Fe>Mn order. However, the pore volume was higher for a lower loading of Nb that also corresponded to an increased size of the mesopores. c.50Nb@MNP According to Schuchardt et al [START_REF] Gallo | Cyclooctene epoxidation using Nb-MCM-41 and Ti-MCM-41 synthesized at room temperature[END_REF] the insertion of Nb into the mesoporous MCM-41 increases the d100 spacing thus changing the unit cell parameter to a0 = 2d100/√3. Such an effect has been attributed to the larger ionic radius of Nb 5+ (0.064 nm) and the longer Nb-O bond length (0.169 nm). However, a decrease in the unit cell parameter and d100 spacing was also observed for higher Nb contents, e.g. Si/Nb = 16. This has been interpreted as a result of a strong interaction of Nb with the inner silica walls and the dispersion of Nb onto the high surface of MCM-41. No specific diffraction lines characteristic to the niobia phases were detected for the xNb@MNP samples that might be the effect of the conservation of the amorphous phase during the calcination.

XRD patterns of the 10M(Mn, Co)@xNb@MNP samples (Figures 4 and5) showed an alteration of the small-angle XRD patterns (Figure 6). 10Co@22Nb@MNP; c) 10Co@50Nb@MNP Figure 6. X-ray diffraction small-angle patterns of 10M@xNb@MNP materials, a)

10Co@22Nb@MNP non-calcined, b) 10Co@22Nb@MNP calcined, c) 10Co@50Nb@MNP noncalcined, d) 10Co@50Nb@MNP calcined, e) 10Mn@22Nb@MNP non-calcined, and f) 10Mn@22Nb@MNP calcined.

Such a behavior is the effect of the presence of NaOH in the precipitation/deposition stage.

According to previous reports [START_REF] Wark | Growth and reactivity of zinc and cadmium oxide nano-particles in zeolites[END_REF] this can be attributed to a partial removal of the silicon atoms from the mesoporous structure. However, this process has been limited by the association of niobium with silica which inhibited the extraction of silica [START_REF] Tao | Mesopore-Modified Zeolites: Preparation, Characterization, and Applications[END_REF]. On the other hand, the addition of Co 2+ , Mn 2+ or Fe 2+ ions may also facilitate the rupture of the Si-O bonds in the mesoporous silica shell structure, most likely due to the energies released during the exothermic precipitation of the producing hydroxides.

The absence of the diffraction lines characteristic to CoOx or MnOx in the XRD patterns of 10M@xNb@MNP provides a good indication of a high dispersion, as nano-sized oxides.

Contrarily, the deposition of these elements onto the SiO2@MNP was detected by XRD. For manganese were detected three additional diffraction lines at 2θ = 37.4, 42.6 and 56.4°, characteristic for akhtenskite (ε-MnO2), while for cobalt seven additional diffraction lines at 2θ = 18.9, 31.8, 36.9, 38.6, 44.9, 59.3 and 65.3° assigned to Co3O4 species (JCPDS card no. 42-1467) [START_REF] Tirsoaga | Upgrade of 5-Hydroxymethylfurfural to Dicarboxylic Acids onto Multifunctional-Based Fe3O4@SiO2 Magnetic Catalysts[END_REF]. One therefore may stress the possibility that the presence of niobium acts also as a structural promoter which limits the agglomeration of base metals in a similar way as previously evidenced for copper [START_REF] Ziolek | The role of niobium component in heterogeneous catalysts[END_REF].

SEM-EDS analysis complemented the XRD data. As seen in Figures S1-S3, irrespective of the composition, quite similar observations in terms of size and morphology of the particles were observed. Clearly strong heterogeneities appeared. Irregular shaped particles were also reported for NbMCM-41 prepared by hydrothermal synthesis [START_REF] Nowak | Epoxidation of cyclohexene on Nbcontaining meso-and macroporous materials[END_REF]. In addition, as seen in the distribution of the different elements, for 10Co@50Nb@MNP (Figure 7) the comparison between Si and Fe indicated sharp contrasts, suggesting that, indeed, a thin layer of silica was formed onto magnetic iron oxide particles but the depth of this layer is not homogeneous with the distinction of Fe-and Si-rich zones. It is worthwhile to note that niobium is randomly distributed that is in accordance to the protocol synthesis which stabilized niobium inside the silica framework. It is also obvious that a subsequent Co, Fe and Mn incorporation into the xNb@MNP through a coprecipitation procedure does not lead to sharp spots indicative of a significant aggregation process. On the contrary, a rather good dispersion seems to prevail. Such observations perfectly reproduce for the different samples irrespective of the nature of the active cation and the Nb/Si ratio.

The elemental analysis was performed on two different zones for each sample (Table S1).

The results confirmed previous comments related to heterogeneity in composition likely due to significant change in the thickness of the silica shell structure. Average compositions are reported in Table 2 which does not reveal significant discrepancy on the calculated atomic ratio. The only divergences appeared for the Fe/Nb and Fe/Si ratios and are due to the overestimation of iron coming from the magnetic core structure. XPS measurements were performed to determine the oxidation state of elements and their relative surface atomic concentration in the analyzed depth, ie 5-10 nm. The corresponding photopeaks are reported in Figures S4 andS5. Data from the semiquantitative analysis are collected in Table 3 and compared to elemental analysis from EDS analysis. Firstly, the calculation of the atomic Nb/Si showed a good consistency with this revealing comparable values varying within the margin of errors. This confirms no concentration gradiant for nobium and is in concordance to protocol synthesis. The surface concentration of the metal M (Co, Mn and Fe) does not vary significantly and seems weakly dependant on the nobium content thus suggesting that the acidic properties of the support does not exhibit a drastically influence on the dispersion.

The Si 2p level located at 103.2-103.8 eV corresponds to binding energies of Si(IV) in the silica layer structure (Table 2). The XP spectrum of the O 1s level showed for all the catalysts two components, one associated to oxygen in mesoporous silica at 532.3-532.6 eV [START_REF] Koscielska | Structural investigations of nitrided Nb2O5 and Nb2O5-SiO2 solgel derived films[END_REF] and another to oxygen connected to the deposited transition metal [START_REF] Ingo | Thermal and Microchemical Characterisation of Sol-Gel SiO2, TiO2 and xSiO2-(1-x)TiO2 Ceramic Materials[END_REF][START_REF] Paparazzo | XPS analysis of oxides[END_REF] or niobium at cca 530.1-531.0 eV [START_REF] Nowak | Frontiers in mesoporous molecular sieves containing niobium: From model materials to catalysts[END_REF].

The binding energy of the Nb 3d5/2 level (207.5-207.9 eV) corresponded to Nb(V) species (Table 2). According to literature, these values indicate an isomorphous substitution of niobium into the amorphous silica [START_REF] Trejda | New Nb and Ta-FAU zeolites-Direct synthesis, characterisation and surface properties[END_REF].

For the transition metals, the Co2p3/2 satellite located at ca 781.5 eV indicates the presence of Co(II) species (Table 2) [START_REF] Tan | XPS studies of solvated metal atom dispersed (SMAD) catalysts. Evidence for layered cobalt-manganese particles on alumina and silica[END_REF]. For manganese the binding energy of 642.2 eV may correspond to Mn(II) [START_REF] Franzen | XPS spectra of some transition metal and alkaline earth monochalcogenides[END_REF] but in most of the reported examples such an oxidation state is merely reflected by binding energies around 641.7 eV [START_REF] Tan | XPS studies of solvated metal atom dispersed (SMAD) catalysts. Evidence for layered cobalt-manganese particles on alumina and silica[END_REF]. A binding energy of 642.2 eV better fits an oxidation state of Mn(III) [START_REF] Dicastro | XPS study of MnO oxidation[END_REF]. The symmetrical band of the Mn 2p3/2 level also comes to confirm the presence of a single valence [START_REF] Kang | Manganese oxide catalysts for NOx reduction with NH3 at low temperatures[END_REF][START_REF] Raj | XRD and XPS characterization of mixed valence Mn3O4 hausmannite thin films prepared by chemical spray pyrolysis technique[END_REF]. Finally, for iron, the binding energies of 711.2 -711.5 eV (Table 2) corresponded to the Fe 2p level of Fe 3+ species [START_REF] Oku | X-ray photoelectron spectroscopy of Co3O4, Fe3O4, Mn3O4, and related compounds[END_REF].

Besides the results presented in Table 3, time of flight-secondary ion mass spectrometry (ToF-SIMS) analysis is a complementary technique affording the analysis of the composition of the solid surfaces. The main interest of ToF-SIMS lies in obtaining information on the chemical environment of atoms at the surface of samples through the detection and identification of molecular ions. Advanced analysis of the nature of the ionic fragments as well as a semiquantitative approach can provide valuable information on the core-shell structure of the samples as well as on the dispersion of active elements at the outermost layer. Hence, molecular information can complement those obtained from XPS analysis. ToF-SIMS spectra were recorded in positive and negative polarity as illustrated in Figures S6 and S7 in Supplementary Materials. All the molecular ions containing Mn, Fe, Co and Nb as well as most of the ions related to silicon have been identified. We relied on the mass resolution of our spectra as well as on the isotopic patterns of the ion fragments to get data collected in Tables S2 andS3 in Supplementary Materials. Measurements were repeated on different zones, labeled Z1 and Z2, showing a good consistency emphasizing quite homogeneous distribution of Mn, Fe, Co and Nb on the surface analyzed in agreement with SEM-EDS analysis. It is worthwhile to note that similar trends were observed through the evolution of the intensity of ionic fragments containing only the metal M + or MOx -respectively in positive and negative polarity. By way of illustration, Co + is significantly detected only on 10Co@xNb@MNP corresponding to a sharp increase in intensity of ∼ 3 order of magnitude with respect to the background signal. Changes in intensity in the same extent are also noticeable for Mn + on 10Mn@xNb@MNP. On the other hand, much lower deviations compared to the background signal were observed for Fe + or FeOx -on 10Fe@xNb@MNP which clearly indicates a significant contribution of iron coming from the magnetic iron MNP as previously emphasized. As similar trends arise irrespective of the polarity then we have illustrated, in Table 4, the semi-quantitative exploitation from spectral features obtained in negative polarity. It consisted in the calculation of intensity ratio of ionic fragments containing only the metal in MO2 -with M = Co, Fe or Mn and when M is associated to Si in the ionic fragment M2O4SiHx -. At a first glance, it is expected that MO2 -would not be able to discern the chemical environment of the metal, i.e. in the symmetry of the segregated single oxide MOx or isolated M n+ in specific coordination with the silica substrate.

On the other hand, M2O4SiHx -would reflect a closer interaction between M and the silica substrate.

The results in Table 4 reveal similar values for M/M-Si ratio on 10Co@xNb@MNP irrespective of niobium content. On the other hand, discernible changes appear on 10Mn@xNb@MNP according to niobium content with slightly higher values obtained on 10Mn@22Nb@MNP irrespective of the zone analyzed which could suggest a slightly greater sensitivity of manganese towards agglomeration of MnOx in weak interaction with the silica support. Comparison with XPS data in Table 3 did not reveal drastic changes on the atomic Mn composition according Nb content which 21 could be consistent with a greater sensibility of manganese to aggregate. For iron species, no clear observation appears likely due to the contribution of the magnetic iron NMP. Nevertheless, while reproducible values are obtained on 10Fe@22Nb@MNP strong divergences are remarkable on 10Fe@50Nb@MNP which could be inherent to the depth analyzed by this technique and to none uniformity of the film thickness of silica in 10Fe@50Nb@MNP.

Regarding the ionic fragment containing niobium, first the ratio Nb/Nb-Si where Nb stands for NbO2 -and Nb-Si for NbO7Si2 -were analyzed. As seen in Table 4, the values are of the same order of magnitude which corroborates previous SEM observation showing that niobium is randomly distributed in the thickness of the silica film. In a second stage, the ratio M-Si/Nb-Si has been calculated and compared to the surface M/Nb ratios calculated from XPS. It is worthwhile to note that their evolution coincides emphasizing that the atomic ratio calculated from XPS mainly reflect preferential interaction of M and Nb with silica support instead of weak interaction related to aggregation of both M and Nb.

The presence of Nb in the mesoporous silica hydrophilic matrix has also been confirmed by DRIFT spectroscopy (Figure 8). In accordance to the DRIFT spectroscopy results, the most probable state of niobium is that corresponding to Nb(V) where niobium is linked to the mesoporous walls via Nb-O-Si bonds [START_REF] Fergani | Nb-Based Zeolites: Efficient bi-Functional Catalysts for the One-Pot Synthesis of Succinic Acid from Glucose[END_REF]. The band centered at 986 cm -1 gives information on this. In the same time positive and negative Nb-charged species were identified by ToF-SIMS technique (Tables S2 andS3). However, no additional band at 3743 cm -1 , characteristic to NbO-H (Nb (V)) groups, or at 600-500 cm -1 , assigned to the Nb-O-Nb stretching mode were detected.

Nb(V) generally belongs to hardly reducible species. Therefore, as expected, no absorption bands at 3555-3602 cm -1 were evidenced, also confirming the absence of the Nb (IV) species in these materials. Such species were also identified in the IR spectra through the presence of two weak bands at around 955 and 980 cm -1 , while the non-activated sample exhibited only an IR band at 966 cm -1 [START_REF] Ziolek | Oxidative properties of niobium-containing mesoporous silica catalysts[END_REF].

CO2-and NH3-TPD analyses (Figures S8 andS9) revealed the acid-base properties of the investigated catalysts. Table 5 provides more detailed information for M(Co, Mn, Fe)@22Nb@MNP samples. These catalysts contain only a small concentration of acid sites, mostly weak, and a relatively high density of strong basic sites. As Table 5 shows, both the total basicity and the concentration of the middle-strong base sites (temperature range 220-400 0 C) decreases in the same order: Fe > Co >> Mn. However, the base/acid sites ratio decreases in order Co (8.8) > Fe (8.5) > Mn (5.9). According to our characterizations the base properties correspond to Nb-OH hydroxyls that are also in concordance to previous literature reports [START_REF] Candu | Direct conversion of cellulose to α-hydroxy acids (AHAs) over Nb2O5-SiO2-coated magnetic nanoparticles[END_REF][START_REF] Ziolek | Oxidative properties of niobium-containing mesoporous silica catalysts[END_REF][START_REF] Tielens | What Do the Niobium Framework Sites Look Like in Redox Zeolites? A Combined Theoretical and Experimental Investigation[END_REF].

Catalytic behavior

Previously work developed in our group allowed us to develop a series of shell-core magnetic catalysts with CoOx-SiO2 and MnOx-SiO2 shells for the oxidation of HMF in water, at 110-180 °C and 10-18 bar O2, using butylamine or NaHCO3 as a base, [START_REF] Tirsoaga | Upgrade of 5-Hydroxymethylfurfural to Dicarboxylic Acids onto Multifunctional-Based Fe3O4@SiO2 Magnetic Catalysts[END_REF]. However, under these conditions the HMF oxidation led to succinic and/or maleic acids. For instance, CoOx-SiO2@MNP (10wt% CoOx) catalyst provided a selectivity to succinic acid (SA) of 92.7% for a HMF conversion of 78.6%, while the MnOx-SiO2@MNP (10wt% MnOx) led to a 72% selectivity to maleic acid (MAc) for a HMF conversion of 5%.

Interesting enough, in the presence of 10M@xNb@MNP catalysts, experimental tests conducted under similar reaction conditions (ie, water solvent, 110-180 °C, 10-18 bar O2, using butylamine or NaHCO3 as a base) failed, the only products identified being a large spectrum of degradation compounds as lactic and glycolic acids, C5-COOH, C5, C6 fucose and humins.

Changing the nature of base from butylamine to NaHCO3, the reaction pathway goes through HMFA as intermediate (Scheme 1) leading to high amounts of glycolic acid (90-55%) and lactic acid (around 10%) for HMF conversions of 58-69%. However, no di-carboxylic acids were identified in the reaction products.

Scheme 1. Oxidation of HMF to FDCA

The failure of the above attempts made us to move from the wet oxidation conditions to a catalytic oxygen transfer methodology. This pathway is heterolytic in nature and requires oxygen donors such as H2O2 or RO2H. In the presence of H2O2, as oxygen donor, and acetonitrile, as solvent, no oxidation products were detected in the reaction products. This behavior could be assigned to the very high activity of these catalysts for the H2O2 decomposition at the investigated temperatures. A similar effect was also reported by Choudhary et al. [START_REF] Choudhary | Solvent-free oxidation of benzyl alcohol to benzaldehyde by tert-butyl hydroperoxide using transition metal containing layered double hydroxides and/or mixed hydroxides[END_REF] for the benzyl alcohol oxidation on Mn-Al, Mn-Fe, and Mn-Cr-hydroxides catalysts.

Table 6 presents the main catalytic results using mono-functional catalysts and t-butyl hydroperoxyde (t-BOOH) as an oxygen transfer agent.

mg catalyst; a -24h

As Table 6 shows monofunctional catalysts (ie, xNb@MNP and MOx-SiOx@MNP) display from moderate to high HMF conversions (Table 6, entries 1-6). However, after 12 h of reaction no FDCA product was identified into the reaction products, irrespective of the catalyst, but only oxidation intermediates as HMFA and FFCA. For xNb@MNP samples the distribution of the reaction products was highly influenced by the loading of niobium: the higher the loading led to the higher selectivity to FFCA (Table 6, entries 1 and 2). The incorporation of Nb(V) in the MCM-41

structure generates an excess of positive charge in the framework (Nb + which plays the role of Lewis acid sites), balanced by Nb-O -sites (base sites) during dehydroxylation, as already Ziolek et al [START_REF] Ziolek | Nb-containing mesoporous molecular sieves -a possible application in the catalytic processes[END_REF][START_REF] Ziolek | Physico-chemical and catalytic properties of MCM-41 mesoporous molecular sieves containing transition metals (Cu, Ni, and Nb)[END_REF] demonstrated (Scheme 2).

Scheme 2. NbMCM-41 dehydroxylation with the generation of Nb + and Nb-O -species [START_REF] Ziolek | The role of niobium component in heterogeneous catalysts[END_REF] The oxidative properties of NbMCM-41 [START_REF] Ziolek | Oxidative properties of niobium-containing mesoporous silica catalysts[END_REF] were attributed to the presence of active lattice oxygens concerted by the formation of Lewis acid sites generated during the dehydroxylation of the niobium-containing molecular sieve. It is thus probable that niobium-containing catalysts act like vanadium, i.e. react via oxometal or peroxometal intermediates, depending on the substrate [START_REF] Ziolek | Oxidative properties of niobium-containing mesoporous silica catalysts[END_REF].

However, even if monofunctional samples (22Nb@MNP and 50Nb@MNP) were active in the oxidation of HMF the presence of these species was not enough to run the oxidation of HMF to FDCA, and the product mixture contained HMFA and FFCA intermediates (Table 6).

For MOx-SiOx@MNP samples, higher amounts of FFCA were produced by FeOx-SiO2@MNP (Table 6, entry 7). However, increasing the reaction time from 12 to 24h led to the diminution of FDCA, this being identified only with CoOx-SiO2@MNP (Table 6, entry 4). Also important, no DFF intermediate was evidenced irrespective of the catalytic active phase nature.

However, the presence of Co 2+ /Co 3+ and Mn 4+ , as determined by XRD, seems to not favor the FDCA formation neither in catalytic wet oxidation conditions [START_REF] Tirsoaga | Upgrade of 5-Hydroxymethylfurfural to Dicarboxylic Acids onto Multifunctional-Based Fe3O4@SiO2 Magnetic Catalysts[END_REF] nor in the presence of an oxygen donor like t-BOOH, in this work.

Finally, mixing the two catalytic phases (ie, Nb and a metal oxide) led to unexpected active and selective multifunctional materials. Thus, the 10M@xNb@MNP (where: M = Co, Mn or Fe) catalysts showed a higher catalytic efficiency for the selective oxidation of HMF to FDCA (Scheme 1, Table 7) than any other catalytic system based on non-precious metals reported until now [START_REF] Pal | Recent Advances in the Development of 5-Hydroxymethylfurfural Oxidation with Base (Nonprecious)-Metal-Containing Catalysts[END_REF]. As Table 7 shows the most appropriate solvent for HMF oxidation to FDCA by t-BOOH is acetonitrile. In water a high amount of HMF is decomposed with the formation of glycolic acid (Table 7, entries 3 and 8), while in the water:methanol mixture (Table 7, entry 4) the oxidation stops to FDCA intermediates (ie, HMFA and FFCA). No DFF intermediate was identified.

Irrespective of the MOx nature, an increased amount of Nb (from Si/Nb=50 to Si/Nb=22) led to increased selectivities to FDCA, although in different proportions (Table 7, entries 1, 5, 6, 9, 10 and 12). For samples with Si/Nb=22, the selectivity to FDCA decreased in the order: Mn (76.6%) > Co (20.1%) > Fe (9.3%).

After 24h of reaction, the selectivity in FDCA was highly improved for the conversions of HMF higher than 96% (Table 7, entries 2, 7 and 11). Accordingly, for the samples with Si/Nb=22, the selectivity to FDCA increased to values of 87.0 -96.5% decreasing in the following order: Co (96.5%) > Fe (94.1%) > Mn (87.0%). Lower loadings of Nb (ie, Si/Nb = 50) exhibited a detrimental effect upon the selectivity to FDCA. Accordingly, it decreased from 94.1% (10Fe@22Nb@MNP) till 64.9% (10Fe@50Nb@MNP), while the conversion of HMF recorded an insignificant variation from 99.7% (10Fe@22Nb@MNP) to 98.7% (10Fe@50Nb@MNP) (Table 7, entries 11 and 13).

The correlation between textural properties and selectivity to FDCA has been proven by correlation with the XRD and BET results. The high active samples, irrespective of the niobium loading or the nature of the MOx phase, corresponded to a lack of the diffusion effects. As the adsorption-desorption isotherms of nitrogen at 77 K measurements showed, all samples display a disordered hexagonal pore packing with the simultaneous presence of internal pores and irregular secondary intercrystalline cavities, with sizes large enough to not impose restrictions to HMF diffusion. However, the textural properties correlate to the FDCA selectivity. The selectivity to FDCA follows the same tendency as the evolution of the surface area and the pore volume of the catalysts. Both decrease in the same order: Co > Fe > Mn (Tables 2 and7). In conclusion, the higher the surface area and the pore volume, the higher was the selectivity to FDCA.

In addition, the comparison of the XPS, SEM-EDX and ToF-SIMS results with the catalytic results provides a lift for the correlation between the structural properties and the selectivity to FDCA product. In the presence of the bifunctional catalysts the DFF intermediate was not identified, irrespective of the solvent nature. Therefore, the reasonable reaction pathway of the HMF oxidation over 10M@xNb@MNP should follow the HMF → HMFCA→FFCA→ FDCA route, as depicted in Scheme 1. Interestingly enough, this reaction pathway for the FDCA production coincides with that proposed for the supported Au and Pd catalysts [START_REF] Donoeva | Carbon Support Surface Effects in the Gold-Catalyzed Oxidation of 5-Hydroxymethylfurfural[END_REF][START_REF] Lei | Facet Effect of Single-Crystalline Pd Nanocrystals for Aerobic Oxidation of 5-Hydroxymethyl-2-furfural[END_REF][START_REF] Wan | Base-Free Aerobic Oxidation of 5-Hydroxymethyl-furfural to 2,5-Furandicarboxylic Acid in Water Catalyzed by Functionalized Carbon Nanotube-Supported Au-Pd Alloy Nanoparticles[END_REF] but is totally different from that indicated for non-noble metal oxides (i.e. MnOx-CeO2, Fe3O4-CoOx) [START_REF] Wang | Catalytic Conversion of Fructose and 5-Hydroxymethylfurfural into 2,5-Furandicarboxylic Acid over a Recyclable Fe3O4-CoOx Magnetite Nanocatalyst[END_REF][START_REF] Han | Selective oxidation of 5-hydroxymethylfurfural to 2,5furandicarboxylic acid over MnOx-CeO2 composite catalysts †[END_REF] and Ru-based catalysts [START_REF] Tirsoaga | Upgrade of 5-Hydroxymethylfurfural to Dicarboxylic Acids onto Multifunctional-Based Fe3O4@SiO2 Magnetic Catalysts[END_REF][START_REF] Yi | Base-free conversion of 5-hydroxymethylfurfural to 2,5furandicarboxylic acid over a Ru/C catalyst †[END_REF]. This one follows the route 2 (HMF→DFF→FFCA→ FDCA) in Scheme 1. The distinction in this reaction pathway is the consideration of a preferential adsorption of -CHO side chain of HMF, which requires the presence of a basic and positive surface of the catalyst [START_REF] Donoeva | Carbon Support Surface Effects in the Gold-Catalyzed Oxidation of 5-Hydroxymethylfurfural[END_REF]. In this respect, the CO2-TPD analysis indicated the presence of both basic sites and acid sites at temperature the reaction is carried out (Table 6). These basic sites are due to the presence of Nb-O -species formed together with the positive Nb + species during the catalyst calcination. Such species are also detected by ToF-SIMS in bifunctional samples (Table S2 andS3).

The XPS analysis indicated the existence of Nb 5+ , Co 2+ , Mn 3+ and Fe 3+ . As in the case of mono-functional MOx-SiOx@MNP samples, species as Co 2+ /Co 3+ and Mn 4+ were also evidenced [START_REF] Tirsoaga | Upgrade of 5-Hydroxymethylfurfural to Dicarboxylic Acids onto Multifunctional-Based Fe3O4@SiO2 Magnetic Catalysts[END_REF]. The presence of niobium in these catalysts can influence the electronic properties of cobalt and manganese. The source of nucleophilicity in Nb-MCM@MNP is the defect holes (ie, oxygen vacancies) generated by the introduction of niobium to the framework and the NbO -species, both able to donate electrons [START_REF] Trejda | Niobium rich SBA-15 materialspreparation, characterisation and catalytic activity[END_REF]. It implied, therefore, that the presence of Nb in the mesoporous framework is responsible for its high ability to transfer electrons towards cobalt and manganese oxides. Scheme 3 suggests such an electron transfer process for the Mn@xNb@MNP catalyst.

Scheme 3. Electronic transfer between Mn and Nb species in the Mn@xNb@MNP catalyst A similar electron transfer mechanism seems to take place onto the Nb@MNP stabilizing the cobalt species as Co 2+ (Co@xNb@MNP). It explains not only the reduced oxidation state of Mn (or Co) when deposited on the Nb@MNP support, but also the formation of basic sites, as evidenced by CO2-TPD.

The decomposition of t-BOOH plays a crucial role in the oxidation of HMF. The oxidizing activity of metal oxides (ie, Co, Mn and Fe oxides) is determined by their reverse oxidationreduction properties. Niobium is hardly reducible and therefore it is a good candidate to initiate oxidation via the formation of a peroxo metal complex (Scheme 4).

As Scheme 4 shows the Mn 3+ species decompose t-BuOOH with the formation of oxidized Mn 4+ , radicalic t-BuO˙ species and OH -ions which may remain in the solution or, subsequently, can be adsorbed by the Nb + sites. Thus, oxidized Mn 4+ is reversely reduced to Mn 3+ by interaction with t-BuOOH, forming in addition peroxo species t-BuOO˙ and generating protons. Further, peroxo t-BuOO˙ species may generate t-BuO˙ species and liberate oxygen. These species will interact with Nb-O -sites with the formation of Nb-O-O-tBu peroxo species, which by interaction with protons restore the initial Nb-O -sites and liberate t-BuOH alcohol. Scheme 5. Overall HMF oxidation mechanism to FDCA adapted from Donoeva et al [START_REF] Donoeva | Carbon Support Surface Effects in the Gold-Catalyzed Oxidation of 5-Hydroxymethylfurfural[END_REF] In this work HMF oxidation was performed under weakly basic conditions ensured by the acetonitrile solvent but no homogeneous bases were added to the reaction mixture. Moreover, the catalysts are built on a magnetite core ensuring an easy separation of the catalyst at the end of reaction.

On the base of the proposed mechanism (Schemes 4 and 5) and catalysts characterization results we can explain the observed differences between the mono-and bifunctional catalysts through the synergetic effect of metal oxides particles and niobium species in the decomposition of the oxidation agent and supplying a high concentration of OH -species. These adsorb onto the catalytic sites generating the active species required for an efficient oxidation of HMF to FDCA.

Accordingly, the selectivity to FDCA must be mainly related to the charge of xNb@MNP supports by basic groups furnished by MOx species. The positively charged surface of xNb@MNP support facilitates the adsorption of the OH-ions onto the Nb + species, in a closed vicinity of MOx particles.

Since niobium catalyzes the dehydrogenation (Scheme 5) only in the presence of adsorbed OH -, a higher concentration of Nb in catalytic materials ensures a higher concentration of adsorbed OH - and, automatically, a higher rate of dehydrogenation of oxidation intermediates (HMFCA and FFCA) to FDCA. This also explains the superiority of the catalyst with a Si/Nb ratio of 22 compared to the Si/Nb = 50 one. Moreover, the absence of FDCA in the reaction products using mono-functional catalysts can be due to the presence of the preponderantly acidic sites which prevent a further oxidation of the intermediates to FDCA.

The catalysts recycling followed the procedure reported previously by Wang and co-workers [START_REF] Wang | Catalytic Conversion of Fructose and 5-Hydroxymethylfurfural into 2,5-Furandicarboxylic Acid over a Recyclable Fe3O4-CoOx Magnetite Nanocatalyst[END_REF] and also applied by us in a previous study on HMF oxidation over monofunctional Co-and Mn-based catalysts [START_REF] Tirsoaga | Upgrade of 5-Hydroxymethylfurfural to Dicarboxylic Acids onto Multifunctional-Based Fe3O4@SiO2 Magnetic Catalysts[END_REF]. After each cycle, the catalyst was separated from the reaction mixture with the aid of an external commercial Nd-magnet, washed three times with distilled water, dried under vacuum at 50°C, for 12 h, and then re-used in the next cycle. These catalysts showed no significant loss in the catalytic activity during five recycling experiments, thus confirming their stability under these reaction conditions (Figure S10). In addition, the ICP-OES analysis showed no metallic species in the reaction solution, which represents a second confirmation of their stability.

Conclusions

Figure 1 .

 1 Figure 1. Adsorbtion-desorption isotherms of nitrogen at 77K for the 10M@xNb@MNP catalysts

Figure 2

 2 Figure2shows the XRD small-angle patterns of the xNb@MNP catalysts. Both materials display the characteristic (100) line centered at 2θ = 2° and other three lines in the range of 2θ = 3-

Figure 2 .

 2 Figure 2. X-ray diffraction small-angle patterns of xNb@MNP materials, a) 22Nb@MNP, b) 50Nb@MNP.

Figure 3 .

 3 Figure 3. X-ray diffraction patterns of xNb@MNP materials a. MNP, b. 22Nb@MNP;

Figure 4 .Figure 5 .

 45 Figure 4. X-ray diffraction patterns of 10Mn@xNb@MNP materials: a) MNP, b)10Mn@22Nb@MNP; c) 10Mn@50Nb@MNP

Figure 7 .

 7 Figure 7. Elemental mapping of 10Co@50Nb@MNP. Image Resolution: 512 by 384 -Image Pixel Size: 0.13 µm -Map Resolution: 256 by 192 -Map Pixel Size: 0.25 µm -Acc. Voltage: 15.0 kV -Magnification: 2000.

2 - 2 -Figure 8 .

 228 Figure 8. DRIFT spectra of MNP (black), MCM@MNP (red) and 50Nb@MNP (blue) samples

Scheme 4 .

 4 Scheme 4. Decomposition of t-BuOOH in the presence of the Mn@xNb@MNP catalyst

Table 1 .

 1 Texturale characteristics of the 10M@xNb@MNP multifunctional catalysts

		BET surface	BJH pore volume	Pore size distribution
	Catalyst	(m 2 /g)	(cm 3 /g)	(nm)
				2.5; 14.5; 25.3; 50.3;
	10Co@22Nb@MNP	285	0.38
				94.0; 194.4
				2.4; 15.6; 25.2; 50.3;
	10Co@50Nb@MNP	179	0.42
				199.0

Table 2 .

 2 Average atomic compostion from SEM-EDS analysis

			Atomic composition (at.%)
	Catalyst						M/Nb	Nb/Si	M/Si
		Si	Nb	Co	Mn	Fe
	10Co@22Nb@NMP	62.8	3.0	11.6		22.6	3.8	0.048	0.18
	10Co@50Nb@NMP	47.9	1.0	9.8		41.3	10.3	0.020	0.21

Table 3 .

 3 Binding energies (BE) and semiquantitative analysis from XPS analysis on 10M@xNb@MNP where M stands for Co, Mn or Fe and x being the niobium composition.

	Binding Energy (eV)
	Catalyst

a -values determined from EDX analysis 20

Table 4 .

 4 Poisson corrected intensity of characteristic ionic fragments recorded in negative polarity from Tof-SIMS analysis recorded

	Catalyst	SiO 2	-	MnO 2	-	CoO 2	-	FeO 2	-	NbO 2	-

Mn 2 O 4 SiH 5 -Co 2 O 4 SiH -Fe 2 SiO 4 H 4 - NbO 7 Si 2 -Co/Co-Si a Fe/Fe-Si b Mn/Mn-Si c Nb/Nb-Si d M-Si/Nb-Si

												e
	10Co@22Nb@NMP-Z1 42395	43	1217	1595	117	34	1629	1019	1400	0.75	0.084	1.16
	10Co@22Nb@NMP-Z2 33187	22	815	1123	69	24	1054	726	889	0.77	0.078	1.19
	10Co@50Nb@NMP-Z1 41098	20	1166	1749	56	40	1535	1412	708	0.76	0.079	2.17
	10Co@50Nb@NMP-Z2 36984	24	1058	1638	53	26	1376	1339	650	0.77	0.082	2.12
	10Fe@22Nb@NMP-Z1 107450	199	208	10470	389	302	475	11475	4792	0.91	0.081	2.39
	10Fe@22Nb@NMP-Z2 75381	116	173	6660	279	171	249	7114	2790	0.94	0.100	2.55
	10Fe@50Nb@NMP-Z1 149675	279	418	23386	297	438	791	18976	3267	1.23	0.091	5.81
	10Fe@50Nb@NMP-Z2 151288	243	276	16652	293	400	720	19196	4268	0.87	0.069	4.50
	10Mn@22Nb@NMP-Z1 122938 4935	86	4900	266	8451	146	5709	4383	0.58	0.061	1.93
	10Mn@22Nb@NMP-Z2 132875 5880	82	5187	345	10214	172	6718	5486	0.58	0.063	1.86
	10Mn@50Nb@NMP-Z1 130176 4373	112	6793	246	9485	210	8021	3803	0.46	0.065	2.49
	10Mn@50Nb@NMP-Z2 130867 4012	90	6777	256	8737	185	7812	3570	0.46	0.072	2.45

a Co/Co-Si = CoO

Table 5 .

 5 Base/acid sites concentration determined through CO2-/NH3-/TPD for the M(Co, Mn,

	Fe)@22Nb@MNP catalysts
	Catalyst@MNP		Range of temperature, 0 C
	Acid sites concentration (mmols NH3/g catalyst)
		100-150 0 C 150-210 0 C	-	400-500 0 C	Total
	10Co@22Nb@MNP	0.036	0.029		0.003	0.068
	10Mn@22Nb@MNP	0.037	0.012		0.004	0.053
	10Fe@22Nb@MNP	0.058	0.018		0.002	0.078
	Base sites concentration (mmols CO2/g catalyst)
		50-150 0 C	150-210 0 C	220-400 0 C	-	Total
	10Co@22Nb@MNP	0.068	0.209	0.325		0.602
	10Mn@22Nb@MNP	0.059	0.137	0.120		0.316
	10Fe@22Nb@MNP	0.073	0.214	0.380		0.667

Table 6 .

 6 The oxidation of HMF in the presence of monofunctional catalysts

	Entry Catalyst	C, %	SHMFA, %	SFFCA,%	SFDCA,%
	1	22Nb@MNP (Si/Nb = 22)	80.8	0	100	0
	2	50Nb@MNP (Si/Nb = 50)	85.2	90.7	9.3	0
	3	CoOx-SiO2@MNP	91.1	87.8	12.1	0
	4 a	CoOx-SiO2@MNP	100	12.1	72.0	15.8
	5	MnOx-SiO2@MNP	88.1	75.7	24.3	0
	6 a	MnOx-SiO2@MNP	100	0	100	0
	7	FeOx-SiO2@MNP	79.1	0	100	0
	Reaction conditions: HMF (0.2 mmoli; 25 mg); ACN 4.4 mL; 12h; 100 0 C; 0.18 mL 70% t-BuOOH,

Table 7 .

 7 The main results obtained in the oxidation of HMF in the presence of bifunctional

	catalysts
	Entry Catalyst	C, %	SHMFA, %	SFFCA,%	SFDCA,%
	1	10Co@22Nb@MNP	94.5	2.1	77.8	20.1
	2 a	10Co@22Nb@MNP	96.6	0.8	2.7	96.5
	3 b	10Co@22Nb@MNP	11.9	55.2	0	0
	4 c	10Co@22Nb@MNP	12.0	60.8	39.2	0
	5	10Co@50Nb@MNP	96.3	13.1	86.2	0.7
	6	10Mn@22Nb@MNP	97.7	2.5	20.9	76.6
	7 a	10Mn@22Nb@MNP	98.7	5.8	7.25	87.0
	8 b	10Mn@22Nb@MNP	36.6	63.8	0	0
	9	10Mn@50Nb@MNP	97.6	14.4	83.9	1.7
	10	10Fe@22Nb@MNP	96.7	69.8	20.8	9.3
	11 a	10Fe@22Nb@MNP	99.7	3.03	2.9	94.1
	12	10Fe@50Nb@MNP	98.8	77.1	16.6	6.3
	13 a	10Fe@50Nb@MNP	98.7	5.5	29.6	64.9
	Reaction conditions: HMF (0.2 mmoles; 25 mg); ACN 4.4 mL; 12h; 100 0 C; 0.18 mL 70% t-
	BuOOH, 25 mg catalyst; a -24h; b -water, 4.4 mL ; c -water:methanol (50:50 vol/vol), 4.4 mL. In
	water, the difference in selectivity till 100% corresponds to glycolic acid.

Acknowledgements

Authors are grateful to Dr. Bogdan Cojocaru for the TPD measurements. Simona M. Coman kindly acknowledges UEFISCDI for the financial support (project PN-III-P4-ID-PCE-2016-0533, Nr. 116/2017). Magdi El Fergani kindly acknowledges ERASMUS PICS "Nanocat" N°230460. Chevreul Institute (FR 2638), Ministère de l'Enseignement Supérieur et de la Recherche, Région Nord -Pas de Calais and FEDER are acknowledged for supporting and funding partially this work.

catalytic systems for the oxidation of HMF to FDCA. In our best knowledge, this is the first time when a catalytic system based on transitional metals follows a characteristic mechanism for noble metals, taking place through the adsorption of an aldehyde C-HO group through a HMF → HMFCA→FFCA→ FDCA sequence. The advantage of this new bifunctional system over the use of noble metals is that such a mechanism does not claim the presence of a homogeneous base.

The obtained results support a concerted effect of both textural and structural characteristics of the catalysts upon the produced FDCA selectivity. The magnitude of the FDCA selectivity follows the surface areas and the pore volumes decreasing in the same order: Co > Fe > Mn. In other words, a higher the surface area and the pore volume correspond to a higher selectivity to FDCA. Moreover, combining MOx (CoOx, MnOx or FeOx) with niobium species generate a synergetic effect in which MOx acts as a co-catalyst for niobium sites by furnishing the hydroxyl species through the t-BuOOH decomposition. Once formed these hydroxyl groups are catch by the Nb + species generating the active sites for the dehydrogenation of oxidated intermediates to FDCA. Following this pathway the 10Co@22Nb@MNP catalyst is able to produce FDCA with a selectivity of 96.5%, for a HMF conversion of 96.6%, while 10Fe@22Nb@MNP led to a selectivity of 94.1% to FDCA for a HMF conversion of 99.7%. The catalysts were magnetically separable, and showed no significant loss in the catalytic activity during several recycling experiments.


2

• Harmonize the methodologies and propose guidelines for performing experiments

• Validate in vitro models towards in vivo data (animal and/or human)

• Identify the beneficial/deleterious components that are released in the gut during food digestion

• Determine the effect of the food matrix on the bioavailability of food nutrients and bioactive molecules

• Develop new in vitro, in vivo and in silico digestion models including some for specific populations (infant, elderly)

.04

Industry involvement



✓

  encapsulation with food structures 2. Application of INFOGEST in vitro models in pharmaceutical sciences 3. Building of advanced in silico models 4. In vitro gut barrier models to study permeation in different populations 5. Drug effects on GI physiology 6. Imaging (MRI, scintigraphy, ultrasonography) Rate of nutrient digestion ✓ Structural changes in stomach Layering Coagulation

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Acknowledgments

Linda Giblin

Andre Brodkorb And all the TEAGASC members!

.017

In silico models of digestion