A Bouibes

A Zaoui
email: azaoui@polytech-lille.fr

Lgcge

High-pressure phase transitions of Forsterite from first-principles

Keywords: Forsterite, DFT, High pressure, Structural properties

From ab initio evolutionary algorithm, we predict a new high pressure phase transition of Mg2SiO4 (Forsterite) above 30 GPa. At this pressure, Forsterite changes into post-spinel structure with Pbam space group. This structure has attracted the interest of many previous works, but which have not been definitively conclusive. Forsterite is stabilized under further phase transition at 64.5 GPa. Above this pressure, the structure with Cmc21 space group becomes the most stable.

The modern first-principles calculation technique is a reliable tool for studying the electronic structure of materials under extreme conditions, such as high pressure and temperature. By performing ab initio simulations one can go far beyond the pressure and temperature range achieved experimentally. An important area, which can be dealt by using first-principles calculations, is the study of the structural stability and phase transformations at high pressure of Earth's interior constituents [START_REF] Arapan | Formation of sp 3 Hybridized Bonds and Stability of CaCO3 at Very High Pressure[END_REF]. The most important constituents of Earth are silicates which constitute 95 percent of the rocks of the Earth's crust. They are considered as dominant mineral phase of Earth's upper mantle, which occur widely in igneous and metamorphic rocks [2][START_REF] Ringwood | Phase transformations and their bearing on the constitution and dynamics of the mantle[END_REF]. In addition, these minerals could be found in other planets [START_REF] Mustard | Olivine and pyroxene diversity in the crust of Mars[END_REF]. The most astrophysically important silicate groups are olivines and pyroxenes [START_REF] Martínez-González | Multiscale Computational Simulation of Amorphous Silicates Structural, Dielectric, and Vibrational Spectroscopic Properties[END_REF]. Olivines are characterized to have the [SiO4] 4-tetrahedra linked by Mg 2+ and Fe 2+ and the general formula Mg2xFe(2-2x)SiO4 (x = 0-1), with the two end members named forsterite (Mg2SiO4) and fayalite (Fe2SiO4) [START_REF] Martínez-González | Multiscale Computational Simulation of Amorphous Silicates Structural, Dielectric, and Vibrational Spectroscopic Properties[END_REF]. Whereas, pyroxenes are formed by single chains of [SiO4] 4- intercalated by the divalent cations and have the general formula MgxFe(1-x)SiO3 (x = 0-1) [START_REF] Martínez-González | Multiscale Computational Simulation of Amorphous Silicates Structural, Dielectric, and Vibrational Spectroscopic Properties[END_REF]. This ferromagnesian pyroxene seems to have implications for enigmatic seismic feature beyond 2000 Km. Indeed, (Mg,Fe)SiO3 is unstable in the temperature and pressure higher than 2200 K and 95 GPa respectively, and forms Fe-free MgSiO3 perovskite phase and a Feriche phase with a hexagonal structure [START_REF] Zhang | Disproportionation of (Mg,Fe)SiO3 perovskite in Earth's deep lower mantle[END_REF]. On the other hand, the phase transitions in Mg-rich olivine and its polymorphs are believed to be responsible for the major seismic discontinuities at 410, 520, and 660 km depth [START_REF] Ringwood | Phase transformations and their bearing on the constitution and dynamics of the mantle[END_REF]. Olivine's creep behavior exerts a major control on mantle rheology and affects the interpretation of seismic anisotropy [START_REF] Karato | Rheology of the upper mantle-A synthesis[END_REF]. While olivine transforms to wadsleyite at a depth near 410 km (~14GPa) at normal mantle temperatures, it may persists metastably in cold subducting lithosphere below 410 km depth and plays a role in deep earthquake generation [START_REF] Stephen | C Metastable mantle phase transformations and deep earthquakes in subducting oceanic lithosphere[END_REF][START_REF] Kawakatsu | Metastable olivine wedge and deep dry cold slab beneath southwest Japan[END_REF]. There has also been interest in the behavior of amorphous and liquid Mg2SiO4 over a wide range of pressures as a model system for understanding partial melting of the mantle, production of basalts and komatiites, and the behavior of magma oceans [START_REF] De Koker | Thermodynamics, structure, dynamics, and freezing of Mg2SiO4 liquid at high pressure[END_REF][START_REF] Adjaoud | Transport properties of Mg2SiO4 liquid at high pressure: Physical state of a magma ocean[END_REF][START_REF] Finkelstein | Phase transitions and equation of state of forsterite to 90 GPa from single-crystal X-ray diffraction and molecular modeling[END_REF].

The question of stability of olivine at the Earth's mantles condition arose a great interest to researchers, where a series of experiments as well as theoretical investigations, have been carried out [2][START_REF] Ringwood | Phase transformations and their bearing on the constitution and dynamics of the mantle[END_REF][START_REF] Mustard | Olivine and pyroxene diversity in the crust of Mars[END_REF][START_REF] Martínez-González | Multiscale Computational Simulation of Amorphous Silicates Structural, Dielectric, and Vibrational Spectroscopic Properties[END_REF][START_REF] Zhang | Disproportionation of (Mg,Fe)SiO3 perovskite in Earth's deep lower mantle[END_REF][START_REF] Karato | Rheology of the upper mantle-A synthesis[END_REF][START_REF] Stephen | C Metastable mantle phase transformations and deep earthquakes in subducting oceanic lithosphere[END_REF][START_REF] Kawakatsu | Metastable olivine wedge and deep dry cold slab beneath southwest Japan[END_REF][START_REF] De Koker | Thermodynamics, structure, dynamics, and freezing of Mg2SiO4 liquid at high pressure[END_REF][START_REF] Adjaoud | Transport properties of Mg2SiO4 liquid at high pressure: Physical state of a magma ocean[END_REF][START_REF] Finkelstein | Phase transitions and equation of state of forsterite to 90 GPa from single-crystal X-ray diffraction and molecular modeling[END_REF]. The structure of Olivine has been found experimentally and theoretically at ambient conditions [START_REF] Finkelstein | Phase transitions and equation of state of forsterite to 90 GPa from single-crystal X-ray diffraction and molecular modeling[END_REF][START_REF] Smyth | Comparative crystal chemistry of orthosilicate minerals[END_REF][START_REF] Guyot | Pressure-induced structural modifications and amorphization in olivine compounds[END_REF][START_REF] Horiuchi | β-Mg2SiO4: Single-crystal X-ray diffraction study[END_REF][START_REF] Andrault | Compression and amorphization of (Mg, Fe)2SiO4 olivines: an X-ray diffraction study up to 70 GPa[END_REF][START_REF] Rouquette | Highpressure studies of (Mg0.9Fe0.1)2SiO4 olivine using Raman spectroscopy, X-ray diffraction, and Mössbauer spectroscopy[END_REF]. The Olivine structure has Pbnm space group and Z=4 formula with 28 atom in unit cell. It can be described as an expanded and distorted hexagonally closepacked (hcp) array of oxygen anions stacked along the a direction. Si cations are situated in tetrahedral sites, and Mg is in two distinct octahedral sites; Si-O tetrahedral are isolated and share corners with Mg-O octahedral, while the Mg-O octahedral sharer both corners and edges. There are three distinct oxygen: O1 and O2 are located on a mirror plane, while O3 is in a general position. Each oxygen anion is bonded to three octahedral cations and one tetrahedral cation [START_REF] Finkelstein | Phase transitions and equation of state of forsterite to 90 GPa from single-crystal X-ray diffraction and molecular modeling[END_REF][START_REF] Smyth | Comparative crystal chemistry of orthosilicate minerals[END_REF].

Under pressure, several theoretical and experimental studies report that the Olivine becomes metastable above 14GPa and take other forms. Wadsleyite and ringwoodite are reported to be the main forms of Mg2SiO4 at high pressure. These polymorphs are denoted as β-and γ-Mg2SiO4 phase, respectively. The phase transition, between β-Mg2SiO4 and γ-Mg2SiO4 phases, occurs around 17 GPa (520 Km depth). The transitions between these olivine phases induced by pressure and also temperature, the reported thermodynamic stable phase of Forsterite are presented in shematic phase pressure-temperature diagram in Fig. 1 [14 -20]. Recently, Finkelstein et al. investigated different phase transitions of Forsterite using single crystal X-ray diffraction and molecular modeling [START_REF] Finkelstein | Phase transitions and equation of state of forsterite to 90 GPa from single-crystal X-ray diffraction and molecular modeling[END_REF]. They revealed that Forsterite knows two phase transitions up to 90 GPa of pressure. The first one at 50 GPa, and the second one at 58 GPa. Before 50 GPa, Forsterite is stable under Pbnm space group structure, and between 50GPa and 58 GPa Forsterite stabilizes in P1 space group structure. Above 58 GPa, Forsterite is stable into Cmc21 space group structure [START_REF] Finkelstein | Phase transitions and equation of state of forsterite to 90 GPa from single-crystal X-ray diffraction and molecular modeling[END_REF]. We report here the new phase transitions to Forsterite at high pressure. To this end, we have used ab initio structures prediction.

In order to find the stable high-pressure structures of Forsterite, an ab initio evolutionary algorithm (EA), as implemented in the "Universal Structure Predictor: Evolutionary Xtallography" (USPEX) code, is employed [START_REF] Oganov | Evolutionary crystal structure prediction as a method for the discovery of minerals and materials[END_REF][START_REF] Oganov | Crystal prediction using ab initio evolutionary techniques: Principles and applications[END_REF][START_REF] Lyakhov | New developments in evolutionary structure prediction algorithm USPEX[END_REF][START_REF] Oganov | How evolutionary crystal structure prediction works -and why[END_REF]. The local optimization of the candidate structures were carried out using density functional theory (DFT) calculations with the generalized gradient approximation (GGA), and the Perdew-Burke-Ernzerhof (PBE) [START_REF] Perdew | Generalized gradient approximation made simple[END_REF] exchange-correlation functional, as implemented in the Vienna ab-initio Simulation Package (VASP) code [START_REF] Kresse | Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set[END_REF][31]. The structure prediction of Mg2SiO4 was performed at 1atm, 20GPa, 50 GPa, 55GPa, 70 GPa, 100GPa, 125GPa, 150 GPa all at zero Kelvin. In these variable-cell simulations, we consider the system with 14 and 28 atoms in the unit cell. The population size is fixed at 40 numbers of structures. Forsterite II found by Finkelstein et al. at 1000K and also Wadsleyite structure with 56 atoms in unit cell were added to the resulting structures [START_REF] Finkelstein | Phase transitions and equation of state of forsterite to 90 GPa from single-crystal X-ray diffraction and molecular modeling[END_REF][START_REF] Horiuchi | β-Mg2SiO4: Single-crystal X-ray diffraction study[END_REF].

The structural and mechanical properties of the obtained structures were calculated using PBE-D2 exchange-correlation functional with D2 dispersion correction (PBE-D2) [START_REF] Grimme | Semiempirical GGA-type density functional constructed with a long-range dispersion correction[END_REF], as implemented in VASP code [START_REF] Kresse | Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set[END_REF][31] . We used the all-electron projector-augmented wave (PAW) [START_REF] Kresse | From ultrasoft pseudopotentials to the projector augmentedwave method[END_REF]. The plane-wave kinetic-energy cutoff is 900 eV, and the k-point mesh resolution in reciprocal space is 2*0.03 A -1 . These settings enable excellent convergences of the energy differences, stress tensors, and structural parameters.

In Fig. 2, enthalpies of the obtained structures relative to the enthalpy of the forsterite phases versus pressure are shown. One can see immediately from Fig. 2.a that the P1 space group structure, reported by Finkelstein et al as Forsterite phase II, is metastable at 0K within the whole range of applied pressure, and one may disregard it from further consideration.

The most stable structure of Mg2SiO4 at ambient conditions is the Pbnm space group structure called α-Mg2SiO4, which fits perfectly the experiment [START_REF] Finkelstein | Phase transitions and equation of state of forsterite to 90 GPa from single-crystal X-ray diffraction and molecular modeling[END_REF]. The obtained lattice parameters as well as bulk modulus and its derivative are summarized in Table 1.

By comparing the enthalpies of the remaining structures, we estimated pressures at which phase transitions occurs. Fig. 2.b. shows the first and second phase transition pressures. The first transition from Forsterite Phase I to Forsterite phase II structure occurs at 12.6 GPa. The second one occurs at 16.3 GPa. The Forsterite phase II and phase III are β-wadsleyite structure, which has Imma space group, and ringwoodite (γ-spinel) structure, with Fd-3m space group, respectively. The obtained lattice parameters as well as bulk modulus and its pressure derivative of the optimized Forsterite phase III, at ambient conditions, are summarized in Table 2. These results fit perfectly previous several theoretical and experimental studies [START_REF] Smyth | Comparative crystal chemistry of orthosilicate minerals[END_REF][START_REF] Guyot | Pressure-induced structural modifications and amorphization in olivine compounds[END_REF][START_REF] Horiuchi | β-Mg2SiO4: Single-crystal X-ray diffraction study[END_REF][START_REF] Andrault | Compression and amorphization of (Mg, Fe)2SiO4 olivines: an X-ray diffraction study up to 70 GPa[END_REF][START_REF] Rouquette | Highpressure studies of (Mg0.9Fe0.1)2SiO4 olivine using Raman spectroscopy, X-ray diffraction, and Mössbauer spectroscopy[END_REF][START_REF] Suito | Phase transformations of pure Mg2SiO4 into a spinel structure under high pressure and temperatures[END_REF][START_REF] Katsura | The system Mg2SiO4-Fe2SiO4 at high pressures and temperatures: Precise determination of stabilities of olivine, modified spinel, and spinel[END_REF][START_REF] Tokar | Thermodynamic properties and phase stability of wadsleyite II[END_REF][START_REF] Inoue | Elastic properties of hydrous ringwoodite (γ-phase) in Mg2SiO4[END_REF][START_REF] Wentzcovitch | Thermodynamic properties and phase relations in mantle minerals investigated by first principles quasiharmonic theory[END_REF][START_REF] Dziewonski | Preliminary reference Earth model[END_REF].

By increasing pressure, Forsterite phase III is stable until 30 GPa where the new third phase transition occurs. Between 30 GPa and 64.5 GPa, the novel most stable structure has a space group Pbam including 28 atoms in the primitive cell. The Forsterite phase IV contains octahedral (SiO6) 4-ions in a parallel orientation as we can see in Fig. 3.a. In addition, Mg atom are located in octahedral sites. The obtained structural properties as well as bulk modulus and its pressure derivative of the optimized Forsterite phase IV, at ambient conditions, are summarized in Table 3.

Above 64.5 GPa, the structure of Cmc21 space group becomes more stable as shown in Fig. 2.c. Table 4 reported the obtained structural properties of the optimized Forsterite phaseV at ambient conditions as well as the obtained bulk modulus and its pressure derivative from the Birch-Murnaghan equation of state. The Forsterite phase V is presented in Fig. 3.b. We can see that Si atom is located in octahedral sites, and Mg1 atoms (yellow color) are located also in octahedral site. However Mg2 atoms (orange color) are located in face -sharing trigonal prism sites. Let us notice that the latter was also found by Finkelstein et al. [START_REF] Finkelstein | Phase transitions and equation of state of forsterite to 90 GPa from single-crystal X-ray diffraction and molecular modeling[END_REF].

In order to confirm our prediction, we studied the dynamical stability of new Forsterite phase IV and V at 50GPa and 70 GPa, respectively. To this end, we applied the Born stability criteria [START_REF] Born | Dynamical Theory of Crystal Lattices[END_REF]. The Born stability criteria for an orthorhombic system are: > 0

B1,ii= Cii > 0 (i=1-6) (1) 2,
Table 5 reports the calculated elastic constant of Pbam structure and Cmc21 structure at 50

GPa and 70 GPa. The three born criteria are respected. Thus, the predicted structures are dynamically stable.

In conclusion, we have predicted through this study the unknown post-spinel structures above 30 GPa. We have determined the Forsterite phase IV and Forsterite phase V, which are Pbam space group structure and Cmc21 space group structure respectively. Tables: a Experimental data reference [START_REF] Tokar | Thermodynamic properties and phase stability of wadsleyite II[END_REF] b Experimental data reference [START_REF] Inoue | Elastic properties of hydrous ringwoodite (γ-phase) in Mg2SiO4[END_REF] Table 3: structural parameters of new stable Mg2SiO4 structure (Phase IV) between 30 GPa and 64.5 GPa as well as obtained bulk modulus and its derivative according to the Birch-Murnaghan equation of state [START_REF] Vinet | Universal features of the equation of state of solids[END_REF].

Figure 1 :

 1 Figure 1: Shematic phase pressure-temperature diagram of Mg2SiO4. Figure2: Enthalpies of the best forsterite structures vs pressure (a), zoom of enthalpiespressure diagram: from 4 to 22 GPa (b) and from 20 to 80 GPa of pressure (c). Figure3: Forsterite phase IV (Pbam ) at 50 GPa: yellow spheres -Mg atoms, blue spheres-Si atoms, small red spheres -O atoms (a); Forsterite phase V (Cmc21): : yellow spheres -Mg1 atoms, Orang spheres: Mg2 atoms, blue spheres-Si atoms, small red spheres -O atoms (b).

Fig. 1

 1 Fig.1

  Fig.2

  Fig.3

Table 1 :

 1 Obtained forsterite Phase I at ambient conditions.

	a (Å)= 10.1750 (10.201) *
	b(Å)= 5.9860 (5.9819) *
	c(Å)= 4.7910 (4.7543) * V0(Å 3 )= 291.81 (290.1) * K0 * (GPa)= 129 GPa (130 a , 128 b , 128.8 c ) * K'0 * = 3.8 (4.12 a , 4.0 b , 4.2 c ) *
	Space group :	Pnma (Pbnm)
	Atoms	x	y	z
	Mg1	0.0000	0.0000	0.5000
	Mg2	-0.2230	0.2500	-0.0102
	Si	-0.4060	0.2500	0.4275
	O1	-0.4085	0.2500	-0.2331
	O2	-0.0545	0.2500	0.2219
	O3	-0.3364	0.0314	0.2769

* Experimental data reference

[START_REF] Finkelstein | Phase transitions and equation of state of forsterite to 90 GPa from single-crystal X-ray diffraction and molecular modeling[END_REF]

a Using single-crystal X-ray diffraction b Using polycrystalline X-ray diffraction c Using Brillouin spectroscopy

Table 2 :

 2 Stable Mg2SiO4 structure (Phase III) between 16.3 GPa and 30 GPa, as well as

	obtained bulk modulus and its derivative according to the Birch-Murnaghan equation of state
	[34].
	a(Å )= 8.0923 (8.129 a , 8.0649±1 b ) V0(Å 3 )= 529.94 (537.16 a , 524.56 b )
	K0 (GPa)= 183.6 GPa (184±2 b )
	K'0= 3.8
	Space group: Fd-3m
	Atoms	x	y	z
	Mg	0.0000	0.0000	0.0000
	Si	0.3750	0.3750	0.3750
	O	-0.2556	-0.2556	-0.2556

Table 4 :

 4 Stable Mg2SiO4 structure (Phase V) above 64.5 GPa.

	a(Å)=5.0150
	b(Å)=8.8720
	c(Å)=2.8250 V0(Å 3 )= 125.693
	K0 (GPa)= 182
	K'0= 3.8
	Space group : Pbam
	Atoms	x	y	z
	Mg	-0.4379	0.3197	0.0000
	Si	0.0000	0.5000	0.5000
	O1	-0.2721	0.0470	0.0000
	O2	-0.3647	-0.1843	0.5000
	a(Å)=2.7855
	b(Å)=9.4506
	c(Å)=9.4000 V0(Å 3 )= 247.4517
	K0 (GPa)= 197.12
	K'0= 3.4
	Space group : Cmc21
	Atoms	x	y	z
	Mg1	0.0000	0.3623	-0.1485
	Mg2	0.0000	-0.1084	0.1854
	Si	0.0000	-0.3728	-0.0020
	O1	0.0000	-0.4555	0.1612
	O2	0.0000	-0.2942	-0.1751
	O3	0.0000	-0.0058	0.4354
	O4	0.0000	-0.2477	-0.4555

Table 5 :

 5 Elastic constants of Forsterite phase IV and V at 50 GPa and 70 GPa respectively.

		Pbam	Cmc21
	C11	682.24	628.39
	C22	467.39	675.34
	C33	648.74	769.24
	C12	230.47	295.29
	C13	121.93	188.17
	C23	199.13	189.61


Achraf Sadier

Noémie Perret

D Perez

Michèle Besson

Catherine Pinel

Denilson Da

Silva Perez

Effect of carbon chain length on catalytic C-O bond cleavage of polyols over Rh-ReOx/ZrO2 in aqueous phase

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

Introduction

Hydrogenolysis reactions in aqueous medium of C4-C6 sugar alcohols including erythritol (ERY), xylitol (XYL), and sorbitol (SOR) are important pathways for the production of high valuable chemicals. XYL and SOR are considered among the twelve most relevant platform molecules according to the US Department of Energy (DOE) [START_REF] Werpy | Top Value Added Chemicals from Biomass. Volume I-Results of Screening for Potential Candidates from Sugars and Synthesis Gas[END_REF], since they are industrially obtained in large scale by catalytic hydrogenation of the carbonyl groups of xylose and glucose, respectively [START_REF] Corma | [END_REF]. They are also important intermediates during the conversion of cellulose or xylan [3][4][5]. Unlike the other polyols (XYL and SOR), the production of ERY through direct catalytic hydrogenation is without interest due to the high cost of erythrose substrate [6]. The standard industrial production routes of this polyol involve the microbial fermentation of sugar and sugar alcohols such as glucose and glycerol using various yeasts [7,8]. The selectivity of the catalytic hydrogenolysis reactions is a major issue because both C-C and C-O bonds cleavage can take place during the conversion of these highly functionalized molecules. The most challenging is the selective C-O bond cleavage without C-C bond cleavage in order to produce partially deoxygenated chemicals used for industrial applications. They are key components in materials such as polyesters, urethane foams and resins, and as intermediates for the preparation of surfactants for which they can be used as isolated compounds or as a mixture [9][10][11][12].

Under hydrogenolysis conditions, not only the desired C-O bond cleavages occur together with the undesired C-C bond cleavage but epimerization and cyclic dehydration reactions are also reported [13][14][15][16][17][18].

Starting from glycerol (GLY), mainly, propanediols (1,2-PrDO, 1,3-PrDO) and propanols (nPrOH, iPrOH) were obtained together with small amount of ethanol (EtOH) and methanol [19]. Taking ERY as a model molecule, butanetriols (1,2,3-and 1,2,4-BTO), butanediols (1,2-, 1,3-, 1,4-, and 2,3-BDO), and butanols (1-and 2-BuOH) were the main deoxy products obtained from C-O bond hydrogenolysis [20]. In addition, ethylene glycol (EG), GLY, 1,2-PrDO and 1-PrOH were the secondary products obtained from competing C-C bond cleavage.

Moreover, for polyols with four carbon atoms, intramolecular dehydration reactions took place easily and formed thermodynamically stable heterocycles [21]. Indeed, dehydration of ERY yielded 1,4-anhydroerythritol (1,4-AE), whose successive C-O hydrogenolysis generated 3-hydroxytetrahydrofuran (3OH-THF) and tetrahydrofuran (THF). Finally, ERY underwent epimerization reaction and formed threitol (THR) [20].

Increasing further the number of carbon will increase the complexity of the reaction resulting in the formation of more reaction products. (Scheme 1) Thus, C5 polyol (XYL) is converted to linear deoxygenated compounds (pentanetetraols,triols, -diols, and pentanols). Arabitol (ARB) and adonitol (ADO) are formed after epimerization reaction, via dehydrogenation/hydrogenation reaction [13]. In addition, intramolecular dehydration reactions of XYL yields 1,5-and 1,4-anhydroxylitol (1,5-and 1,4-AX); the successive C-O hydrogenolysis of 1,4-AX generates tetrahydrofurfuryl alcohol (THFA). Moreover, C-C bond hydrogenolysis reactions lead to many types of C1-C4 products such as methanol, EG, EtOH, GLY, 1,2-PrDO, PrOH, ERY, and BuOH.

In turn, in addition to the desired deoxy products (hexanepentaols, -tetraols, -triols, -diols, and hexanols), SOR can be converted to mannitol (MAN) and iditol (IDI) by epimerization. First dehydration yields 2,5-anhydrosorbitol (2,5-AS), 1,5-anhydrosorbitol (1,5-AS), and sorbitans (1,4-S and 3,6-S), the successive second dehydration of sorbitans generates isosorbide (IS). Also, the competing C-C bond hydrogenolysis of SOR leads to many types of C1-C5 products including methanol, EG, EtOH, PrOH, GLY, 1,2-PrDO, BDO, BuOH, pentanediols (PDO), pentanols (PeOH), etc [14].

While, there is a large number of reports dealing with selective hydrogenolysis of glycerol to propanediols [22][23][24], much less articles focused on the hydrogenolysis of C4, C5 and C6 polyols and the literature dealing with the production of linear deoxygenated C4-C6 products (up to alkane) is rather limited [15][16][17]. The understanding of the relative reactivity of each biobased polyols is mandatory for further applications. So, C-O bond hydrogenolysis of XYL or SOR is an interesting reaction to achieve and to compare with ERY hydrogenolysis.

Several groups reported that Cu-based catalysts are efficient for C-O bond hydrogenolysis [9,17,[25][26][27]. An hybridized Cu/CaO-Al2O3 catalyst (molar ratio of Cu/Ca 2+ = 5.4) gave a selectivity of 39% to 1,2-BDO at 85% conversion in the hydrogenolysis of 2 wt% aqueous solution of ERY at 230 °C under 28 bar of H2 [26]. Under optimum conditions (180 °C, 130 bar of H2), in the presence of a commercial CuO-ZnO (33:67) catalyst, a 21 wt% aqueous solution of SOR was hydrogenolyzed into a mixture of deoxyhexitols with a combined selectivity of 63% at 92% conversion [17]. A solution containing 25 wt% SOR, 25 wt% water, and 50 wt% 1,2-PrDO was hydrogenolyzed over a Raney ® Cu catalyst in a trickle-bed reactor (LHSV of 0.5 h -1 and hydrogen flow of 800 mL min -1 , 210 °C, 124 bar) [9]. This patent claimed 78% selectivity towards deoxygenated C6 products at 99% conversion. Very recently, the hydrogenolysis of 7.5 wt % aqueous solution of mannitol (MAN) over Raney ® Cu catalyst yielded a mixture of linear deoxygenated C6 products with a molar composition of 76% (carbon yield > 80%) at 180 °C under 200 bar of H2 [27].

Due to stability issues of Cu, alternative catalysts based on noble metals are good candidates in the hydrogenolysis reactions. Ru-based catalysts were efficiently reported for the hydrogenolysis of polyols either in the presence of base, acid or neutral media. Significant C-C cleavage were observed that could be reduced by addition of acid [18]. Furthermore, a number of previous studies showed that the cooperation between an oxophilic promoter (Re, W, or Mo) and a highly reducible noble metal (Rh, Ir, or Ru) enhanced the catalytic performance in the selective C-O bond hydrogenolysis of polyols or cyclic ethers (including tetrahydrofurfuryl alcohol and tetrahydropyran-2-methanol) [15,16,20,[28][29][30][31][32][33][34][35]]. An Ir-ReOx/SiO2 (nRe/nIr = 1) catalyst prepared by successive impregnation was investigated in the hydrogenolysis of XYL and SOR in biphasic conditions (1 g substrate + 4 mL H2O + 4 mL n-dodecane). After 24 h at 140 °C under 80 bar of H2, almost full conversion of XYL and SOR was achieved; the maximum selectivity towards linear deoxygenated C5 products from XYL reached 89%, and the selectivity to deoxy C6 products from SOR was 93% [16]. At 160 °C and under 80 bar of H2, a 2%ReOx-0.3%Pd/CeO2, was shown to perform hydrodeoxygenation of vicinal diols, however the solvent used was dioxane [15]. At nearly total conversion, the selectivity to BDO was 93% in hydrogenolysis of ERY and only 1-and 3-PeOH were detected with 99% selectivity from XYL, while SOR yielded 85% selectivity to HDO. In our previous work on ERY hydrogenolysis in the presence of a 3.7%Rh-3.5%ReOx/ZrO2 catalyst in aqueous phase, the selectivity to BTO and BDO reached 37% and 29%, respectively, at 80% conversion at 200°C under 120 bar [20].

The objective of this paper was to compare hydrogenolysis of three polyols (ERY, XYL, and SOR) in the presence of Re-promoted bimetallic catalysts, in particular Rh-ReOx/ZrO2, and to study the effect of some reaction parameters. The different number of carbon atoms in these polyols will allow us to compare the relative ability of these substrates to undergo parallel reactions such as epimerization, dehydration, C-C, and C-O bond hydrogenolysis reactions.

Results and Discussion

Identification of non-commercial deoxygenated products

Before being able to compare the catalytic hydrogenolysis results, it was necessary to develop the analysis set-up for the numerous products that can be obtained from the three reactants.

During ERY hydrogenolysis [20], four types of reactions were observed which yielded four families of commercially available products. HPLC analysis using Rezex ROA-Organic acid-H + and Rezex RCM-Monosaccharides-Ca 2+ column allowed to separate all possible products and detailed analysis of the products was performed (Figure S1) [20]. Increasing the number of carbon (XYL and SOR) will increase the possible formation of products especially dehydroxylated compounds (Scheme S1, Scheme 2). Starting from XYL, a large number of linear deoxygenated C5 products is generated (Scheme S1). Among all the possible intermediates (pentanetetraols, -triols, -diols, and pentanols), only a few of them were commercially available (1,2-, 1,4-, 1,5-, and 2,4-PDO, 1-, 2-, and 3-PeOH, Scheme S1). The further identification of the deoxygenated products issued from XYL was then based on the following observations. First, previous work on hydrogenolysis of ERY clearly showed that for a given number of carbon atoms, the more hydroxyl groups present, the lower the retention time on the H + column (Figure S1); in contrast, no specific elution order was observed using the Ca 2+ column. Second, the catalytic hydrogenation reaction of three commercially available deoxy-C5 sugars (2-deoxy-L-ribose, 3-deoxy-D-ribose, and 5deoxy-D-ribose) under mild conditions can yield the three pentanetetraols (1,2,3,4-PTeO, 1,2,3,5-PTeO, and 1,2,4,5-PTeO, respectively) shown in Scheme S1. Identification of the deoxygenated compounds was essentially based on the hydrogenation of 2-deoxy-L-ribose.

Hydrogenation of 2-deoxy-L-ribose under mild conditions (0.17 mol L -1 , 100°C, 50 bar of H2, 5%Ru/C) allowed the identification of the isomers of 1,2,3,5-pentanetetraols (Scheme 3). The direct hydrogenation of 2-deoxyribose leads to the formation of (2S, 3R)-1,2,3,5pentanetetraol [START_REF] Werpy | Top Value Added Chemicals from Biomass. Volume I-Results of Screening for Potential Candidates from Sugars and Synthesis Gas[END_REF]. The change in the conformation in position 2 and 3 yields the second pair of diastereoisomers (2R, 3R)-1,2,3,5-pentanetetraol (2) and (2S, 3S)-1,2,3,5-pentanetetraol (2'). On the other hand, C-O cleavage of a secondary alcohol in XYL leads to the direct formation of enantiomers (2) followed by isomer (1) after isomerization of one of the chiral centers. Furthermore, the C-O cleavage of one of the primary alcohol of xylitol produces both enantiomers (2R, 3R, 4S)-1,2,3,4-pentanetetraol (3) and (2S, 3S, 4R)-1,2,3,4-pentanetetraol (3'). The reaction medium of 2-deoxy-L-ribose hydrogenation after 24 h reaction was analyzed on the H + column. In the range 16.0-18.0 min, the major peak at 16.4 min was assigned to (2S, 3R)-1,2,3,5-pentanetetraol (1) and the minor peak at 17.0 min (shoulder) was attributed to the diastereoisomers of 1,2,3,5-pentanetetraol (2) and (2'). In addition, in the range 17.5-23.5 min, peaks with small areas started to appear from the beginning of the hydrogenation reaction that could be attributed to pentanetriols.

When harsh conditions were then used for hydrogenolysis of the above mixture of PTeO isomers (200 °C over Rh-ReOx/ZrO2 catalyst), the intensity of the main peak (1) at 16.4 min decreased to reach a minimum after 9 h (green line) while the intensity of the peak at 17.0 increased to a maximum after 7 h. In parallel, six products were formed that eluted between 17.5 and 23.5 min (Figure 1). They were associated with the family of pentanetriols, the monodeoxy compounds of PTeO. 1,2,3-, 1,2,5-, 1,3,5-, 1,3,4-PTrO are the possible triols formed from 1,2,3,5-PTeO and they correspond to the six different diastereoisomers (Scheme S1).

For the analysis of the pentanediols (Figure S2 The pentanediols (1,2-, 1,3-, 1,4-, 1,5-, and 2,3-PDO) can be obtained from 1,2,3,5-PTeO, while all these mentioned PDO as well as 2,4-PDO can be formed from XYL. The peaks observed in the zone (23.5-38.0 min) using H + column were thus attributed to the diastereoisomers of the pentanediols (Scheme S1). THFA and 1,2-PDO were eluted very close on the H + column, and additional analysis was done by GC.

On the other hand, the reaction of the most complicated model (SOR) yields an even larger number of deoxygenated C6 compounds. Among all the possible intermediates (hexanepentaols, -tetraols, -triols, -diols, and hexanols), again, only a few of them were commercially available (1,2,3-, and 1,2,6-HTrO, 1,2-, 1,5-, 1,6-, and 2,5-HDO, 1-, 2-, and 3-HeOH, Scheme 2). Similar strategy for the identification of the non-commercial linear deoxy C6 products was employed. The catalytic hydrogenation reaction of three commercially available monodeoxysugars (L-rhamnose (6-deoxy-D-glucose), 2-deoxy-D-glucose, and 3deoxy-D-glucose) was performed under mild conditions (0.02-0.55 mol L -1 sugar, 60 bar H2, 100 °C, 24 h, 5% Ru/C) to help in the identification of HPO, the monodeoxy compounds of SOR. In addition to the hydrogenation reaction of the carbonyl function, isomerization was observed at the different hydroxyl positions yielding eight diastereoisomers (with potentially 2 enantiomers for each) of 1,2,3,4,5-HPO starting from L-rhamnose, four diastereoisomers of 1,2,3,4,6-HPO starting from 2-deoxy-D-glucose, and four diastereoisomers of 1,2,3,5,6-HPO from 3-deoxy-D-glucose (Scheme 4).

As an example, the eight isomers of 1,2,3,4,5-HPO formed during hydrogenation of Lrhamnose were clearly separated using the Ca 2+ column (Figure 2a).

The HPLC evolution of the concentration of the different products as a function of time during hydrogenation of L-rhamnose allowed to clearly identify the different diastereoisomers (Figure 3.

The compound 4 was the main product with a maximum concentration after 1 h and was directly obtained by hydrogenation of L-rhamnose without epimerization. Then, 4 further isomerized to compound 5 followed by 6 and 7. These three products correspond to isomerization at C2, C3 and C4 carbons, respectively. Isomers 8 to 11 corresponding to isomerization of a second carbon were formed, albeit to a lesser extent. These results are in good agreement with previous work of Palkovits et al [18]. In addition, minor peaks (a to g) appeared after 24 h reaction time and can be attributed to further deoxycompounds (HTeO).

To confirm this attribution, the reaction medium resulting from the above hydrogenation reaction of L-rhamnose after 24 h (Figure 2a) and containing mainly 1,2,3,4,5-HPO was further hydrogenolysed under more severe conditions (150°C in the presence of Ru/C catalyst). It was then observed that (i) the isomerization reaction was more significant: the areas of signals 4 to 7 decreased while the area of signals 8 to 11 increased; (ii) the area of signals attributed to HTeO (a to g) increased and simultaneously the TOC value measured in aqueous phase decreased (22% loss after 7 h). Using the Ca 2+ column, clear identification of the 1,2,3,4,5-HPO isomers was established. Unfortunately, during hydrogenolysis of sorbitol, there was also major overlapping (i) between all the peaks related to C6 deoxygenated compounds, i.e. HPO, HTeO, HTrO, and HDO and also (ii) between these C6 deoxygenated compounds and the C-C hydrogenolysis products (C2-C5). This precludes any identification and quantification with this column which was not used furthermore in the study.

In parallel, analysis was carried out using the H + column. After mild hydrogenation of Lrhamnose, broad signals were observed between 14.5 and 17.5 min which were attributed to 1,2,3,4,5-HPO isomers (Figure 2b). Furthermore, under more severe hydrogenolysis conditions, the areas of these signals ( 14 Table S1 shows the deoxygenated products that can be obtained by successive C-O bond cleavage of 1,2,3,4,5-, 1,2,3,4,6-, and 1,2,3,5,6-HPO formed after the mild hydrogenation of L-rhamnose, 2-deoxy-D-glucose, and 3-deoxy-D-glucose, respectively. As mentioned previously, using H + column for analysis of ERY hydrogenolysis, the more polar a product is, the shorter the retention time is [20]. This was confirmed by analysis of commercially available products (1,2,3-HTrO: 24. andS2). Similarly, C6 products follow the same order: SOR and its isomers (13.5-14.0 min) < HPO (14.5-17.5 min) < HTeO (17.5-24.5 min) < HTrO (24.5-30.0 min) < HDO (32.0-56.0 min) (Figures 5b and S3 to S6). These retention time zones will be used to quantify the different families of products.

ZrO2-Supported catalyst: influence of the metal

As mentioned in the introduction, several works have shown that bimetallic catalysts based on a noble metal together with an oxophilic promoter such as rhenium were efficient for hydrogenolysis reaction. The effect of noble metal on sorbitol conversion and product selectivity is presented in Table 1. Over bare supports (ZrO2) the catalytic activity was negligible (5% SOR conversion after 24 h) as it was observed for ERY. [20] Only cyclic C6 compounds (1,5-AS, 2,5-AS, 1,4-S, 3,6-S, and isosorbide) were formed due to the intrinsic moderate acidity of the support. Neither C-O nor C-C bond cleavage products were formed due to the absence of metallic sites active in H2 dissociation.

ReOx/ZrO2 exhibited only 23% conversion after 24 h; the products analyzed were mainly cyclic C6 compounds with a 54% selectivity. Deoxygenated C6 products (26% selectivity) and isomers (10% selectivity) were also generated in addition to C-C bond cleavage products (8% selectivity). This result is in good agreement with the report by Shao et al [36] who investigated the hydrogenation of 7.65 wt% aqueous solution of 1,4-butanediol at 240 °C under 80 bar of H2 in the presence of a 4wt.%Re/C catalyst. THF was the main product formed through internal dehydration with selectivity of 83.1% at 65% conversion. Under the same reaction conditions, conversion was negligible (<1% ) for the hydrogenation of a 6.1wt.% solution of THF. In this cited work, Re-based catalysts were mainly efficient for cyclization, but they also catalyzed the dehydrogenation/hydrogenation steps (isomer products) and hydrogenolysis reaction (linear deoxygenated C6 products) to a much less extent. It was also reported that ReOx species exhibit surface acidity, as proved by IR spectra of pyridine adsorption and NH3-TPD [37,38]. This suggests that ReOx species can trigger the internal dehydration of SOR.

At 200 °C under 80 bar of H2, the catalytic activity for sorbitol conversion followed the order Rh ≈ Pt > Ir, and the reaction time needed for attaining ca 80% conversion was 3, 6, and 24 h, respectively. Whatever the noble metal, the selectivity to C1-C5 products (in the range 6-9%) remained roughly constant. Isomerization reaction was more important in the presence of Rh-ReOx/ZrO2 and Pt-ReOx/ZrO2 catalysts (selectivity of 11-13%). In addition, Ir-ReOx/ZrO2 triggered the internal dehydration of sorbitol to cyclic C6 products with a selectivity of 55% at 80% conversion. Finally, the selectivity to linear deoxygenated C6 products was favored in the presence of Rh-ReOx/ZrO2 with a selectivity of 54%. The catalytic selectivity to C6 products followed the order: Rh ≥ Pt > Ir. For that reason, Rh-based catalysts were chosen in the selective hydrogenolysis of polyols. The hydrogenolysis reactions were carried out at 200 °C and 80 bar of H2. Under these conditions, XYL and SOR were rapidly converted to attain full conversion after 6-6.5 h (Figures 6a and6b).Similar initial reaction rates were calculated for both substrates (ca 11 mol L -1 h -1 ) During XYL hydrogenolysis, linear deoxygenated C5 products appeared as the main products from the initial stage of the reaction (Figure 6a). Their concentrations increased to a maximum of 0.150 mol L -1 after 5 h (yield ~ 50%). In addition, the concentration of cyclic C5 compounds increased and attained a maximum of 0.066 mol L -1 after 4 h (yield ~ 22%). These cyclic compounds were relatively resistant to hydrogenolysis and they were still present after 6 h (0.050 mol L -1 ). Lower reactivity of cyclic compounds was previously reported by Palkovits et al via H/D exchange reaction [18]. Moreover, the concentration of XYL isomer (mainly ARB) increased to a maximum of 0.027 mol L -1 after 3 h, and then it was also converted. Finally, C2 (EG, EtOH), C3 (GLY, 1,2-PrDO, 1-PrOH), C4 (ERY and 1,2-BDO), and C1 (methanol, not shown) products were formed continuously in low concentrations (< 0.040 mol L -1 , ~ 5%).

Hydrogenolysis of XYL and SOR over

The same trend of evolution of the families of compounds was observed during SOR reaction (Figure 6b). Linear deoxygenated C6 products appeared from the initial stage of the reaction as main products and their concentration attained a maximum of 0.155 mol L -1 after 5 h (~ ca. 57% yield). Simultaneously, cyclic C6 compounds (mainly 1,4-S and IS) were detected and reached a plateau after 3 h (~ 0.040 mol L -1 or 15% of initial SOR). Isomerization of SOR to MAN and IDI occurred, after 3 h, the maximum concentration reached 10% of initial SOR concentration. In addition, products issued from C-C cleavage, including C5 (THFA, PDO, PeOH), C4 (1,2-BDO, BuOH), C3 (GLY, 1,2-PrDO, 1-PrOH), C2 (EG, EtOH), and C1 (methanol, not shown) were formed continuously in low concentrations as secondary products.

To summarize, similar products distribution was observed during hydrogenolysis of XYL and SOR, namely the products issued from C-O hydrogenolysis were formed predominantly up to 50-% yield. In addition, intramolecular dehydration occurred to give cyclic ether up to 15-22% yield. These cyclic ethers are much more resistant to C-O hydrogenolysis and once they are formed, they do not further react. At time zero, TOC measured was 18.5 and 19.5 gC L -1 , respectively, for the XYL and SOR solutions, respectively. The TOC values decreased during the reaction (15-18% loss after 6-6.5 h) for both substrates due to the formation of small amount of gaseous products via C-C hydrogenolysis as mentioned previously. However, it is worth noting that, as time proceeded, no significant difference was observed between the measured TOC values and the calculated TOC values, which indicates that the products present in aqueous phase were correctly analysed by HPLC and GC. The mass balance was always over ca. 90%.

Then, for each substrate (XYL or SOR), we focused on the distribution of the different deoxy C5 or C6 products (main compounds), respectively, as a function of reaction time, as reported in Figure 8.

Under 80 bar of H2, full conversion of XYL was attained after 6 h and PTeO (first C-O bond cleavage) is the major product among the categories of linear deoxygenated C5 compounds (Figure 8a). It appeared from the beginning of the reaction and the maximum concentration was 0.065 mol L -1 after 3 h (~ 23% yield). On the other hand, PTrO appeared after 0.5 h of the reaction to reach a plateau at 0.043 mol L -1 after 4 h (~ 15% yield), whereas PDO was observed after 0.5 h and its concentration attained 0.056 mol L -1 (~ 19% yield). Lastly, the concentration of PeOH was negligible.

Similarly, HPO (one C-O bond cleavage) was the major family of products formed from the beginning of reaction of SOR (Figure 8b). The concentration went through a maximum at 0.066 mol L -1 after 3 h, then it decreased to 0.029 mol L -1 at 6.5 h. On the other hand, HTeO started to appear after 0.5 h of the reaction, the concentration increased up to a plateau at 0.030 mol L -1 after 5 h. HTrO and HDO were detected only after 1 h and reached a concentration around 0.020 mol L -1 after 6.5 h. Lastly, HeOH was formed in traces. The formation of polydeoxygenated C5 compounds must be generated from the successive C-O bond hydrogenolysis of XYL through PTeO as intermediates. Similarly, polydeoxygenated C6 products from SOR should occur via successive hydrogenolysis via HPO.

As mentioned previously, the analytical method using H+ column did not allow separating the different isomers (PTeO's and HPO's from XYL and SOR, respectively). It is noteworthy during ERY hydrogenolysis [20] that primary and secondary C-O hydrogenolysis occurred simultaneously yielding to a mixture of 1,2,3-BTO and 1,2,4-BTO. Similar absence of primary/secondary C-O selectivity was expected during XYl and SOR hydrogenolysis.

Then, the influence of reaction conditions on activity and selectivity for the hydrogenolysis of C4 to C6 polyols was studied.

Effect of substrate concentration over Rh-ReOx/ZrO2

The effect of substrate concentration (in the range 5-13 wt%) on initial rate (Figure 9) and on product selectivity at 70-86 % conversion (Figure 10) was investigated for ERY, XYL, and SOR hydrogenolysis at 200°C, under 80 bar of H2 over 300-500 mg Rh-ReOx/ZrO2 (nRe/nRh = 0.5). In these experiments, the concentration of each substrate was adjusted by varying the amount of the polyol, keeping constant the volume of water (120 mL).

The reaction order with respect to all substrates (ERY, XYL, and SOR) concentration was almost zero (Figure 9). Indeed, this behavior was observed in the literature for the hydrogenolysis of 1,4-anhydroerythritol [39] and tetrahydrofurfurylalcohol [40], in which the reaction order with respect to 1,4-AE and THFA concentrations over Rh/SiO2 decreased from 0.6 and 0.4, respectively, to zero order over Rh-MoOx/SiO2 and Rh-ReOx/SiO2. The authors suggested that MoOx and ReOx species are responsible for the strong adsorption of 1,4-AE and THFA over Rh-MoOx/SiO2 and Rh-ReOx/SiO2 catalysts, respectively. Similar phenomenon could be suggested in the present work with adsorption of substrates on ReOx species of Rh-ReOx/ZrO2 catalyst.

At 200 °C and under 80 bar of H2, whatever the substrate concentration (5-13 wt%), the selectivity to C-C cleavage products (C1-C3, 11-12%; C1-C4, 6-8%; C1-C5, 5-9%) and to isomers (THR = 7-13%; ARB = 9%; MAN + IDI = 9-13%) remained roughly constant at similar conversion. Surprisingly, the selectivity to the corresponding C4, C5, and C6 cyclic compounds resulting from internal dehydration of substrates increased moderately from 12, 26, and 21% to 25, 41, and 38%, respectively, as substrate concentration increased from 5 to 13 wt%. Indeed, this is expected since the cyclic dehydration reaction is not catalyzed by metallic catalysts and it will be favored at high concentration at the detriment of the C-O bond hydrogenolysis. On the other hand, it is worth noting that the highest selectivity to linear deoxygenated C4 (BTO + BDO + BuOH), C5 (PTeO + PTrO + PDO + PeOH), and C6 (HPO + HTeO + HTrO + HDO + HeOH) products (58, 59, and 54%, respectively), was favored for the lowest substrate concentration (5 wt%). This trend is in line with the literature dealing with the selective hydrogenolysis of SOR to 1,2,5,6-hexanetetraols (200 °C, 136 bar of H2) over copper chromite catalyst [9,41]. In these studies, the authors reported an increase of the yield of 1,2,5,6-hexanetetraols from 28 to 38% as the concentration of SOR decreased from 9.4 to 2 wt%, in ethylene glycol monoethyl ether as solvent. Based on these results, substrate concentration of 5 wt% was chosen for hydrogenolysis experiments, due to highest selectively towards linear deoxygenated C4, C5, and C6 products.

Effect of reaction temperature over Rh-ReOx/ZrO2

The temperature was varied in the range of 150 °C to 240 °C, under 80 bar of H2, in the presence of 300-500 mg of Rh-ReOx/ZrO2. The effect on the selectivity to the reaction products at 70-86% conversion is presented in Figure 11 (Table S2). It is also worth mentioning that the carbon loss (obtained from TOC measurement) at the defined conversion of each substrate (ERY, XYL, and SOR) increased from 3, 8, and 4% to 20, 16, and 10%, respectively as temperature increased. The selectivity to C-C cleavage products was in the same range (5-12%), except for ERY at 240 °C. The selectivity reached 16%, but it must be noted that the mass balance in that case was lower due to significant formation of products in gas phase.

On the other hand, the highest selectivity to cyclic C4 from ERY, C5 from XYL and C6 from SOR was favored at 240 °C (selectivity = 25, 38 and 42%, respectively). These data are consistent with the observations during hydrogenolysis of 9.4 wt% of SOR in ethylene glycol monoethyl ether under 68 bar of hydrogen over a copper chromite catalyst [41]: the yield to 2,5-bis(hydroxymethyl)-tetrahydrofuran obtained from internal dehydration of 1,2,5,6hexanetetraol was 16.5% after 23 h at 200 °C; upon increasing the temperature from 200 to 260 °C, the yield to the cyclic compound increased to 31.4%, which confirms that the dehydration reactions are favored at high temperature; in addition, the heterocyclic products are thermodynamically stable [41]. Moreover, upon decreasing the reaction temperature from 240 to 160-150 °C, the selectivity to linear deoxygenated C4, C5, and C6 products increased from 33-34% to 80, 62, and 59%, respectively. The same trend was observed in the dehydroxylation of MAN over Cu-Raney® catalyst [27] in which the molar carbon selectivity to linear deoxygenated products increased from 35 to 75% as temperature decreased from 220 to 180 °C. These results confirm that upon decreasing the reaction temperature, the selectivity towards C-O bond hydrogenolysis will be enhanced. It is worth noting that the temperature has less influence on the selectivity towards C-O bond cleavage for longer carbon chain. In parallel, isomers formation was favored at low temperature; indeed the selectivity to THR (isomer of ERY), ARB (isomer of XYL), and MAN + IDI (isomers of SOR) increased from 6, 4, and 5% to 10, 13 and 16%, respectively, as the temperature decreased. This trend in selectivity to isomers is in accordance with the results obtained during hydrogenolysis of SOR to ethylene glycol and propylene glycol over Ru/C catalyst, in which the selectivity to C5 and C6 polyols (XYL + ARB + MAN + IDI) decreased from 10 to 7% with increasing temperature from 180 to 220 °C [42].

Effect of H2 pressure over Rh-ReOx/ZrO2

The effect of H2 pressure on initial rate and product selectivity was investigated for hydrogenolysis of the three substrates (ERY, XYL, and SOR) at 200 °C, over Rh-ReOx/ZrO2 catalyst. During ERY, XYL, and SOR reactions, the initial rate increased linearly from 300, 341, and 321 mmolSub gRh -1 h -1 to 800, 618, and 603 mmolSub gRh -1 h -1 , respectively, as pressure was varied from 30 to 80 bar.

The effect of hydrogen pressure (in the range 30-120 bar) on the selectivity to the different products is presented in Figure 12 (Table S2). First, concerning the carbon balance, there was no difference between TOC calculated and TOC measured in the hydrogenolysis of all substrates. Also, the mass balance was > 90% under the range of H2 pressure applied, which means that the formation of gaseous compounds such as CO2 or alkanes (methane, ethane) is limited, except for ERY at 30 bar. Under these reaction conditions, minor C-C cleavage occurred and the carbon selectivity to the products issued from C-C cleavage was < 12% regardless the substrate and the H2 pressure (30-120 bar). Secondly, the selectivity to cyclic C4, C5 and C6 products was favored at low pressure (30 bar) (21, 48, and 35%, respectively). These data are consistent with observations during the hydrogenolysis of 4.8 wt% aqueous solution of SOR to hexane over Pt/NbOPO4 catalyst at 250 °C. The yield to cyclic oxygenated products [1,4-sorbitan + isosorbide + 2-(tetrahydrofuran-2-yl)ethan-1-ol] decreased from 60 to 10% upon increasing the hydrogen pressure from 10 to 50 bar [43]. Indeed, these two reactions (intramolecular dehydration and hydrogenolysis) are parallel reactions and the latter being favoured by increasing hydrogen pressure. In parallel to the observed decrease of cyclic compounds, the selectivity to linear deoxygenated products of ERY, XYL, and SOR increased from 43, 32, and 43% to 71, 55, and 54%, respectively, when the pressure increased. More specifically, the selectivity to mono C-O hydrogenolysis product was higher when the pressure increased, regardless the nature of the polyol (Table 2). In parallel, the selectivity to the other consecutive C-O cleavage products remained roughly constant except for XYL, where it increased from 16 to 31% as the pressure increased from 30 to 80 bar. Even if isomerization reaction remained moderate, high pressure favors slightly the epimerization reaction of ERY to THR, XYL to ARB, and SOR to MAN + IDI; selectivity was of 9% at 120 bar starting from ERY, whereas under 80 bar of H2, from XYL and SOR, the selectivity to isomers reached 9 and 13%, respectively. This behavior is also in accordance with results obtained for the selective epimerization reaction of 33.3 wt% isosorbide solution to isoidide and isomannide at 220 °C over Ru/C; the selectivity to the isomers (isoidide + isomannide) increased from 5 to 57% as hydrogen pressure increased from 23 to 58 bar [44]. At low conversion < 40%, the total selectivity was in the range 105-115% due to the uncertainty of the analysis at low concentrations. Under 80 bar, the selectivity to BTO decreased regularly from 47 to 22% as conversion increased from 21 to 92%, while at the same time the selectivity to BDO remained constant around 26%. Differently, under 120 bar, the selectivity to BTO decreased slowly at first (for conversion < 80%), then sharply from 43 to 18%, as conversion increased up to 100%. At the same time, the selectivity to BDO increased from 25 to 35%. Moreover, the selectivity to BuOH increased from 0 to 4% under 80 bar, and from 4% to 8% under 120 bar. In addition, the selectivity to THR decreased from 22 to 5% and from 14% to 3% under 80 and 120 bar, respectively, with increasing conversion.

The selectivity to C3 did not vary with conversion (7-12% under 80 and 120 bar). Finally there was no effect of pressure on the selectivity to cycles which remained constant (9-12% under 80 bar, slightly lower at ca. 6-7% under 120 bar) as a function of conversion. Similar progression was observed during the hydrogenolysis of SOR. Figure 14 is an example of the evolution of carbon selectivity as a function of SOR conversion under 30 and 80 bar of H2, at 200 °C. Under 30 bar, as conversion increased from 23% to 89%, the selectivity to HPO decreased strongly from 40% to 18% whereas the selectivity to the other C6 linear derivatives (with less oxygenated functions) increased from 10% to 24%. Similarly, under 80 bar the selectivity to HPO decreased from 45% to 22% and the selectivity to deoxygenated linear C6 (without HPO) increased from 12% to 33% as conversion increased from 33% to 95%. IDI and MAN underwent the same types of reactions (dehydration, C-O and C-C bond cleavage) as SOR, but with lower reactivity compared with SOR (Figure 6b). Regardless of H2 pressure, the selectivity to cyclic C6 products decreased slightly with the increase of SOR conversion (from 22% to 17% under 80 bar and from 40% to 34% under 30 bar H2). Independent hydrogenolysis of cyclic isosorbide was conducted at 200 °C and exhibited low activity (26% conversion after 24 h). This is in good agreement with the relative stability of cyclic C6 compounds during the conversion of SOR. Finally, the selectivity to C1-C5 products increased from 8% and 4% to 12% and 15%, respectively. This is expected as there are secondary products obtained from C-C bond cleavage reactions. These results indicate that successive deoxygenation reactions occur with time. We have previously evaluated the hydrogenolysis of butanetriol: 1,2,4-BTO was almost as reactive as ERY while 1,2,3-BTO was much less reactive. [11] To evaluate the relative reaction rate of C5 and C6 polyols, we also compared the temporal evolution of monodeoxygenated C5 or C6 products with the corresponding XYL or SOR under identical reaction conditions (Rh-ReOx/ZrO2 at 200 °C, under 50 bar of H2). 1,2,3,5-PTeO and 1,2,3,4,6-HPO were obtained from the mild hydrogenation reaction over Ru/C at 100 °C under 50 or 60 bar of H2, of 2-deoxy-L-ribose or 2-deoxy-D-glucose, respectively (see experimental section).

Insight into the deoxygenation pathway

Let us first consider the case of XYL and PTeO. Under these conditions, 95 % XYL conversion was attained after 9 h with up to 40% carbon selectivity to linear deoxygenated products (Figure 15a) and PTeO (first C-O bond cleavage) is the major product among the linear deoxygenated C5 compound (Figure 15b). That compound appeared from the beginning of the reaction and the maximum concentration was 0.053 mol L -1 after 5 h (~ 18% yield). On the other hand, PTrO appeared after 0.5 h of the reaction to reach a plateau at 0.028 mol L -1 after 7 h (~ 10% yield), whereas PDO was observed after 1 h and its concentration attained 0.032 mol L -1 (~ 11% yield). Lastly, the concentration of PeOH was 0.003 mol L -1 (~ 1%) after 9 h of reaction. In comparison, reaction of PTeO (0.16 mol L -1 , nPTeO/nRh = 190) shows that it is a little less reactive than XYL and the conversion reached 85% after 9 h (Figure 15a). This lower conversion can be explained by the formation of only trace amounts of cyclic C5 compounds (mainly THFA, < 0.005 mol L -1 ) during PTeO reaction (Figure S7), whereas this reaction was observed from XYL (Figure 6a). It could be suggested that decreasing the number of hydroxyl groups in the molecule, lowered the probability to form cyclic compound. The evolution of each category of linear deoxygenated C5 products shows that in parallel, PTrO (first C-O bond cleavage) is the major product formed from PTeO (Figure 15c).

During the hydrogenolysis reaction, the concentration of PTrO attained a maximum of 0.061 mol L -1 after 7 h (~ 38% yield) and decreased to 0.033 mol L -1 at the end of the reaction. On the other hand, PDO appeared after 0.5 h of the reaction and reached a maximum of 0.059 mol L -1 (~ 37%), whereas traces of pentanols were only formed after 24 h of the reaction. The This result agrees with that found for the transformation of ERY to butanediols over Ir-ReOx/SiO2 [45] or Rh-ReOx/ZrO2 catalyst [20]. Butanediols were formed from ERY passing through butanetriols as intermediates.

The reaction pathway of formation of the linear deoxygenated C6 compounds was investigated, by similarly comparing the temporal evolution of each category of deoxygenated C6 products during the hydrogenolysis of SOR and 1,2,3,4,6-HPO. HPO is the major family of products formed from the beginning of reaction of SOR (Figure 15e). The concentration went through a maximum at 0.047 mol L -1 after 5 h, then it decreased to 0.032 mol L -1 at 9 h. On the other hand, HTeO started to appear after 0.5 h of the reaction, the concentration increased up to a plateau at 0.020 mol L -1 after 7 h. HTrO and HDO were detected only after 1 h and reached a concentration around 0.025 mol L -1 after 9 h. Lastly, HeOH was formed in traces.The final TOC measured was of 88% (i.e. 12% loss).

During the hydrogenolysis of a mixture of hexanepentaols (HPO) (Figure 15f), first of all, HPO was converted gradually up to 96 % conversion after 24 h. So, under similar reaction conditions, HPO is much less reactive than SOR (Figure 15d), the difference was more significant than in the case of XYL and PTeO (Figure 15a). Once again we attributed the difference to the absence of formation of cyclic compounds during HPO reaction (Figure S8). HTeO, the major monodeoxy family formed from HPO, appeared from the initial stage of the reaction and was further converted after a maximum of 0.042 mol L -1 after 9 h (yield of 26%).

HTrO and HDO appeared after 1 h of the reaction, while HTrO was obtained in larger amounts than HDO. Traces of hexanols were detected after 24 h of the reaction. The TOC measured after 24 h was 72%, so more gaseous compounds were formed compared to SOR reaction.

As expected, the evolution of the concentration of each category of C6 products during hydrogenolysis of SOR and HPO confirms the multiple consecutive C-O bond cleavages of SOR This multiple consecutive C-O bond cleavage was also described very recently for the hydrogenolysis of mannitol or xylitol at 180°C and 150 bar of H2 over Cu Raney® [27].

Moreover, this catalyst was shown to be selective for a specific C-O bond cleavage, i.e. it mainly occurred at the 3-position of mannitol to yield 1,2,4,5,6-HPO, and at a lesser extent at the 6-position (1,2,3,4,5-HPO). The authors proposed a preferential adsorption via vicinal OH-groups and cleavage of OH-groups in close proximity. These results are in good agreement with the evolution of the selectivity with polyol conversion (Figure 14).

Effect of support (TiO2 vs ZrO2)

The support may play a role on activity and selectivity of the reaction, so we evaluated TiO2 which is also stable under the reaction conditions. The effect of H2 pressure (in the range 30-120 bar) was also investigated for ERY, XYL, and SOR hydrogenolysis at 200 °C over a 3.4wt%Rh-2.7wt%ReOx/TiO2 catalyst. At low pressure (30 bar), during the hydrogenolysis of ERY, XYL, and SOR, initial reaction rates were of 211, 160, and 157 mmolSub gRh -1 h -1 , respectively. However, as pressure increased from 30 to 120 bar, the initial rate over Rh-ReOx/TiO2 increased slowly to 550, 319, and 296 mmolSub gRh -1 h -1 , respectively. These catalysts were used in a previous study and fully characterized by XRD, TEM, XPS and CO chemisorption. The difference in catalytic activity (initial rate) between the bimetallic catalysts over both supports can be explained by the difference in metallic dispersion of rhodium over both supports. The Rh-ReOx/ZrO2 displayed a higher CO/Rh ratio (0.80) than Rh-ReOx/TiO2 (0.25) [20].

Figure 16 presents the effect of H2 pressure on the selectivity to different products, using the TiO2-supported catalyst. With respect to the carbon balance, there is no difference between calculated TOC and measured TOC in the hydrogenolysis of all susbstrates. Also, the mass balance was > 85% under the range of H2 pressure applied (30 to 120 bar) in the presence of Rh-ReOx/TiO2 catalyst with < 10% carbon selectivity to C-C cleavage products. As over ZrO2 support, the selectivities to cyclic C4, C5 and C6 products were favored at low pressure over Rh-ReOx/TiO2 catalyst (13% at 30 bar, 24% at 30 bar, and 21% at 80 bar, respectively).

Significant high pressure (120 bar) was required to observe lower amounts of cyclization products. The selectivity to the monodeoxygenated products increased from 10-11% to 28-35%, when the pressure was increased. This enhancement went along with a small decrease in selectivity to polydeoxygenated products. Finally, the epimerization reactions occurred under high pressure (120 bar, selectivity of 12-14%). To conclude, there is no significant difference between both supports neither concerning the activity nor the selectivity to the different families of products.

Stability of bimetallic catalysts

The test of recyclability was investigated during SOR hydrogenolysis in the presence of 3.4wt.%Rh-3.2wt.%ReOx/ZrO2 (Figure 17). After the first reaction, the solid was collected, washed with distilled water, and dried at 110 °C overnight. The catalyst was re-activated under a flow of H2 at 450 °C before the second run. The initial catalytic activity was quite similar. Full conversion was attained after 6.5 h in the presence of fresh catalyst (run 1), vs. a conversion of 93% after 11 h during the second run. No appreciable leaching of rhodium and rhenium metals was detected in the aqueous phase (< 0.06%) after the first run. The deactivation of the catalyst may be due to the subsequent washing of the filtrated catalysts, several times with distilled water which caused the leaching of some rhenium as it was observed elsewhere with bimetallic catalysts used for succinic acid hydrogenation [46]. Analysis of the solid after washing revealed 2.5 wt.% Re vs. 3.2 wt.% in the fresh catalyst.

Aqueous solubilization of rhenium species was also reported In addition, no significant changes were observed concerning the selectivity (± 7%) to deoxy C6 products (C6 without HPO or HPO) in run 1 and 2.

Conclusion

The effects of some reaction parameters were investigated for polyol hydrogenolysis using three different substrates. Regardless of the substrate, the responses in terms of catalytic activity and product selectivity were similar. The polyols can undergo parallel reactions such as epimerization, dehydration, C-C, and C-O bond cleavage. At 80% conversion, the highest selectivity to linear deoxygenated C4, C5, and C6 products was favored at high pressure of H2 in the presence of 3.7wt%Rh-3.5wt%ReOx/ZrO2 catalyst (54-71% under 80 bar) at 200 °C.

The evolution of each family of linear deoxygenated C5 and C6 products during the hydrogenolysis reactions of (XYL or SOR) and (HPO or PTeO) confirms that the formation of polydeoxygenated C5 and C6 products was generated from the successive C-O bond hydrogenolysis of XYL and SOR through HPO and PTeO as an intermediates. As a general rule, the reactivity of partially deoxygenated compounds is lower compared to the alditols.

The optimization of reaction conditions are needed to improve furthermore the selectivity to linear deoxygenated products and further work are under progress.

Experimental Section

Preparation of catalysts

Rh-ReOx/ZrO2 and Rh-ReOx/TiO2 (nRe/nRh = 0.4-0.5) were prepared by consecutive impregnation, as reported previously [20]. In summary, the first impregnation step was conducted by suspending the support (ZrO2 MEL chemicals, SBET = 129 m 2 /g or DT-51D, Cristal, SBET = 91 m 2 /g) in an aqueous solution of RhCl3 (10 g L -1 ) in a flask; the solution was stirred at room temperature for 7 h. After evaporation of the solvent, the material was dried overnight in an oven at 110 °C and calcined under air (60 mL min -1 ) at 4 °C min -1 to 500 °C for 3 h. The second impregnation was conducted with an aqueous solution of NH4ReO4 (10 g L -1 ). After calcination, the solids were reduced under pure H2 (30 mL min -1 ) at 3.5° C min -1 to 450 °C for 3 h; after cooling and flushing with Ar, they were passivated under 1% v/v O2/N2 (30 mL min -1 ) at room temperature for 30 min. The loading amount of Rh in the bimetallics was between 3.4 and 3.7 wt.%. The additive (Re) was added to give a nominal molar ratio (Re/Rh) around 0.5. The monometallic 3.7 wt.% Rh/ZrO2 and 3.8 wt.% ReOx/ZrO2 were reduced directly after the first impregnation and calcination steps and then passivated. A commercial 5 wt.% Ru/C catalyst was purchased from Alfa Aesar.

3.4wt.%Pt-3.4wt.%ReOx/ZrO2 and 3.7wt.%Ir-3.9wt.%ReOx/ZrO2 catalysts were prepared by consecutive impregnation as Rh based catalysts. The basic characterization such as ICP, BET, and XRD were done. The percentage weights of noble metals (Pt and Ir) and Re were in the range 3.4-3.7wt% and 3.4-3.9wt%, respectively. The BET surface area of the catalysts were in the range 70-92 m 2 g -1 . Finally, the powder X-ray diffractions of Pt-ReOx and supported on ZrO2 (Figure S9) showed no diffraction peaks that can be assignable to the metallic phase suggesting small crystallite sizes (< 5 nm).

Catalytic testing

The hydrogenolysis reactions of ERY (Alfa Aesar, 99%), XYL (Sigma, 99%), and SOR (Sigma, 99%), were performed in a 300 mL batch Hastelloy Parr 4560 autoclave set-up. In a typical experiment, an aqueous solution (120 mL) of substrate (5 wt%) and 500 mg of catalyst were loaded into the reactor. After sealing, the autoclave was purged three times with Ar, pressurized with H2 to the required pressure (PH2 = 30-120 bar), and heated to the reaction temperature (T = 200 °C) under stirring (1000 rpm). Liquid samples (1 mL) were collected periodically during the course of the reaction. Small amounts of H2 were added periodically to maintain the H2 pressure constant. At the end of the reaction, the autoclave was cooled, the pressure was released, and the suspension was collected and filtered. 1,2,3,5-PTeO product was formed by the reduction reaction of 0.17 mol L -1 2-deoxy-L-ribose (2 g, 40 mL H2O) under 50 bar of H2 at 100 °C for 24 h over 5%Ru/C. The product was analyzed by H + column with a purity of ~97% after 24 h. Under similar reaction conditions (60 bar of H2, 100 °C, 24 h over 5%Ru/C). 1,2,3,4,5-, 1,2,3,4,6-, and 1,2,3,5,6-HPO were synthesized by hydrogenation of L-rhamnose, 2-deoxy-D-glucose, and 3-deoxy-D-glucose, respectively. The purity of the products analyzed by H + column after 24 h was in the range 95-97%.

Linear deoxygenated C4 (BTO and BDO), C5 (PTeO, PTrO, and PDO), C6 products (HPO, HTeO, HTrO, and HDO), in addition to C2-C5 products (ERY, THR, 1,4-AE, 3OH-THF, GLY, 1,2-PrDO, and EG) were analyzed using a Shimadzu LC 20A HPLC connected to a refractive index detector (RID-10A). The separation of the products was achieved using a column with a stationary phase (PS/DVB with SO 3-, Rezex ROA-Organic Acid H + , Ø 7.8 mm x 300 mm ) heated at 40 °C. A solution of H2SO4 (0.005 N) in ultra-pure ELGA water was used as a mobile phase at a flow rate of 0.5 mL min -1 . The separation of XYL and its isomer (ARB), SOR and its isomers (MAN and IDI), cyclic C5 and C6 products [(1,4-and 1,5-AX),

(1,4-and 3,6-S) and (2,5-and 1,5-AS)] were analyzed using a Shimadzu LC 20A HPLC connected to a refractive index detector (RID-10A). The separation of these products was achieved using a column with a stationary phase (PS/DVB with SO 3-, Rezex RCM-Monosaccharide Ca +2 (8%), Ø 7.8 mm x 300 mm) heated at 60 °C. Elution was performed by 0.4 mL min -1 with water as a mobile phase. The analysis of the C1-C6 mono alcohols in the liquid phase, in addition to other C4 and C5 products [THFA, 1,2-PDO, 1,5-PDO, 2-methyltetrahydrofuran (2-MTHF), and THF] was performed on a Shimadzu GC-2010 system equipped with a flame ionization detector and an autosampler. A Phenomenex FFAP 30 m x 0.25 µm column was used for separation of the products. The temperature was increased from 40 °C to 100 °C at 5 °C min -1 (hold for 2 min), and then up to 250 °C at 8 °C min -1 . The response factor was determined experimentally for the commercial compounds. The same value was then used for a family of compounds.

The concentration of Total Organic Carbon (TOC) was measured using a Shimadzu TOC-VCSH analyzer, to check the possible formation of gaseous products. This measurement was also compared to the calculated TOC which is the total concentration of carbon in the liquid phase quantified by HPLC and GC. If the values are close, the quantification by HPLC and GC is complete.

The substrate conversion was calculated from equation (1):

Conversion (%) = [ ] [ ] [ ] × 100 (1)
where [Sub]0 is the initial concentration of substrate, and [Sub]t is the concentration at time t.

The carbon selectivity S to a desired product was calculated according to equation (2):

S (%) = [ !"] # $ × % & '()*+,# $ ([-.] [-.] # ) × % & /+0 × 100 (2)
where [Product] " 6 is the concentration of product i formed at time t and n 7 8 !" 6 is the number of carbon atoms in the product i.

The initial reaction rate was calculated using equation (3) (slope of the curve at low conversion):

  Scheme S1 presents the successive C-O bond hydrogenolysis of XYL to different categories of linear deoxygenated C5 products. Without considering the configuration at each C, the first C-O cleavage of XYL yields three pentanetetraols (PTeO). In addition, there are six possible pentanetriols (PTrO) that can be formed through C-O bond cleavage of PTeO. Subsequently, the C-O bond hydrogenolysis of the mixture of pentanetriols generates six pentanediols (PDO). Finally, the hydrogenolysis of PDO yields 1-, 2-, and 3-pentanols (PeOH) and then pentane. Similarly, Scheme 2 presents the successive C-O bond hydrogenolysis of SOR with different categories of linear deoxygenated C6 products. At the beginning, the first C-O bond cleavage of SOR generates three hexanepentaols (HPO) without considering the stereogenic centers (HPO). Moreover, there are nine possible hexanetetraols (HTeO) that can be formed through C-O bond cleavage of these hexanepentaols. Successively, the C-O bond hydrogenolysis of the mixture of HTeO yields potentially ten hexanetriols (HTrO) and then, nine hexanediols (HDO). Hydrogenolysis of HDO yields 1-, 2-, and 3-hexanol (HeOH), and total hydrodeoxygation may finally produce hexane, that is to say, that 34 partially oxygenated hexane derivatives can be formed during the reaction.

  ), 2,4-PDO (two diastereoisomers, 25.5 + 26.1 min), 1,4-PDO (29.8 min), 1,2-PDO (34.2 min), and 1,5-PDO (36.1 min) are commercially available. So the signals in the region 23.5-38.0 min could be assigned to the PDO family.

  .5-17.5 min) decreased while new signals appeared with retention times between 18.0 and 25.0 min, which were attributed to HTeO, i.e. one C-O bond cleavage (Figure 4). Similar analysis was run starting from 2-deoxy-D-glucose and 3-deoxy-D-glucose. It was shown that 1,2,3,4,6-HPO isomers eluted from 15.0 to 17.5 min while 1,2,3,5,6-HPO isomers eluted from 14.5 to 17.5 min. Unfortunately with this column, there is no possibility to identify all the isomers formed during the transformation due to overlapping of the different signals.

  7 min, 1,2,6-HTrO: 27.7 min, 1,2-HDO: 49.4 min, 1,5-HDO: 41.3 min, 1,6-HDO: 54.3 min and 2,5-HDO: 32.5 min).Finally, as a result, Figure 5 shows the range of retention time of each family of linear deoxygenated C5 or C6 products using H + column during hydrogenolysis of XYL or SOR (Rh-ReOx/ZrO2, 200 °C, 50 bar H2). The different linear deoxy products exhibit retention times that are similar within a given family. The C5 products follow the order: XYL and its isomers (15.0-15.5 min) < PTeO (16.0-17.5 min) < PTrO (17.5-25.0 min) < PDO (25.0-38.0 min) (, Figures 1, 5a

  Figure 6shows the evolution of the concentrations of the different families of products defined in Scheme 1 as a function of time during hydrogenolysis of XYL (a) and SOR (b) over Rh-ReOx/ZrO2 under similar reaction conditions.

Figure

  Figure 7compares the evolution of TOC values (TOC measured and TOC calculated) during XYL and SOR hydrogenolysis as a function of time under these conditions.

Figure 13

 13 Figure 13 is an example of the evolution of carbon selectivity to the different products as a function of ERY conversion under 80 and 120 bar of H2, at 200 °C, in the presence of Rh-ReOx/ZrO2 catalyst.

  mass balance at the end of both reactions was around 90%. The evolution of the concentration of each category of C5 products during hydrogenolysis of XYL and PTeO confirms that the formation of linear deoxygenated C5 products occurred through consecutive C-O bond cleavages of XYL. The analysis of the sequence of appearance of the different categories of C5 deoxygenated products clearly showed the sequential deoxygenation pathway:

Scheme 1 :Scheme 2 :Scheme 3 :Scheme 4 :

 1234 Scheme 1: Reaction pathways from polyols under hydrogenolysis conditions. Scheme 2: Successive C-O bond hydrogenolysis of SOR under hydrogenolysis conditions. The framed molecules were commercially available. Scheme 3: Ru/C catalyzed hydrogenation of 2-deoxy-L-ribose and C-O bond hydrogenolysis of XYL to 1,2,3,5-PTeO. (2) and (2') are enantiomers, (3) and (3') are enantiomers. Scheme 4: Catalytic hydrogenation of sugars over Ru/C.

Figure 1 :

 1 Figure 1: Evolution of the peaks associated with the isomers of 1,2,3,5-pentanetetraol and pentanetriols during the hydrogenolysis of mixture of 1,2,3,5-PTeO over Rh-ReOx/ZrO2 at 200 °C, with H + column.

Figure 2 :

 2 Figure 2: Chromatogram observed during catalytic hydrogenation of L-rhamnose under mild condition (100 °C, 24 h), using (a) Rezex RCM-Monosaccharide Ca 2+ column, (b) Rezex ROA-organic acid H + column. Signals 4-11 are attributed to HPO, signals a-g are attributed to HTeO.

Figure 3 :

 3 Figure 3: Evolution of the concentration of L-rhamnose and the different isomers of 1,2,3,4,5-hexanepentaols as a function of time. (◆ ) L-rhamnose; (▲) 4; (★,✛,○) 5, 6, and 7; (□, ☆,▽ ,∆) 8, 9, 10, and 11 (Ca 2+ column analysis).

Figure 4 :

 4 Figure 4: Evolution of the peaks during hydrogenolysis of L-rhamnose over Ru/C at 100 °C and 150 °C, with H + column, in the range (a) 14.5-17.5 min and b) 17.5-25.0 min.

Figure 5 :

 5 Figure 5: Response observed during the hydrogenolysis of (a) XYL or (b) SOR over Rh-ReOx/ZrO2 at 200 °C under 50 bar of H2, with H + column.

Figure 6 :

 6 Figure 6: Evolution of the concentrations of the different families of products as a function of time during hydrogenolysis of a) XYL and b) SOR. (◆ ) XYL or SOR; (☓) Cyclic C5 or C6 products; (✛) Isomers; (▲) Deoxy C5 or C6 products; (•) C5, (▼) C4, (▬) C3, and (★) C2 products. Reaction conditions: reactant 0.27-0.30 mol L -1 , 120 mL H2O, 200 °C, 80 bar of H2, 500 mg Rh-ReOx/ZrO2, nreactant/nRh = 200-230.

Figure 7 :

 7 Figure 7:Comparison between (•,▲) TOC measured and (○, △) TOC calculated during XYL (△,▲) and SOR (○,•) hydrogenolysis over Rh-ReOx/ZrO2 (Reaction conditions: see Figure6).

Figure 8 .

 8 Figure 8. Evolution of the concentrations of each family of deoxygenated C5 and C6 products as a function of time during hydrogenolysis of a) XYL and b) SOR. (◆ ) XYL or SOR; (▽ ) PTeO or HPO; (□) PTrO or HTeO; (○) PDO or HTrO; (☆) PeOH or HDO; (⁕) HeOH. Reaction conditions: reactant 0.27-0.30 mol L -1 , 120 mL H2O, 200 °C, 80 bar of H2, 500 mg Rh-ReOx/ZrO2, nreactant/nRh = 200-230.

Figure 9 :

 9 Figure 9: Effect of substrate concentration on initial rate of the reaction. (▲) ERY, (◼) XYL, (•) SOR. Reaction conditions 120 mL H2O, 200 °C, 80 bar of H2, 300-500 mg Rh-ReOx/ZrO2.

Figure 10 :

 10 Figure 10: Effect of substrate concentration on carbon selectivities to reaction products during hydrogenolysis of ERY, XYL, and SOR at 70-86% conversion. (◼) C-C cleavage

Figure 11 :

 11 Figure 11: Effect of temperature on selectivities at 80-95% conversion. (▲) % TOC measured (◼) C-C cleavage products; (◼) Cyclic compounds; (◼) Isomers; (◼) Linear deoxygenated products. Reaction conditions: Reactant 0.27-0.40 mol L -1 , 120 mL H2O, 80 bar of H2, 300-500 mg Rh-ReOx/ZrO2 catalyst; nreactant/nRh = 200-450.

Figure 12 .

 12 Figure 12. Effect of H2 pressure on selectivity to the reaction products and %TOC measured at 80-95% conversion. (▲) % TOC measured; (◼) C-C cleavage products; (◼) Cyclic compounds; (◼) Isomers; (◼) Linear deoxygenated products. Reaction conditions: reactant 0.27-0.40 mol L -1 , 120 mL H2O, 200 °C, 300-500 mg Rh-ReOx/ZrO2 catalyst; nreactant/nRh = 200-450.

Figure 13 .

 13 Figure 13. Dependence of product selectivity on ERY conversion under a) 80 bar b) 120 bar. (╼) C3 products; (☓) Cyclic C4 products; (✛) Threitol; (○) BuOH; (□) BDO; (∆) BTO. Reaction conditions: ERY 0.40 mol L -1 , 120 mL H2O, 200 °C, 300 mg Rh-ReOx/ZrO2.

Figure 14 .

 14 Figure 14. Dependence of product selectivity on SOR conversion under a) 30 bar and b) 80 bar. (◇ ) C1-C5 products, (☓) Cyclic C6 products, (✛) Isomers, (∆) Deoxygenated C6 products without HPO, (▽ ) HPO. Reaction conditions: SOR 0.27 mol L -1 , 120 mL H2O, 200 °C, 500 mg Rh-ReOx/ZrO2 catalyst.

Figure 15 .

 15 Figure 15. Evolution of the selectivity as a function of time: a) carbon selectivity during XYL and PTeO hydrogenolysis; d) carbon selectivity during SOR and HPO hydrogenolysis. Evolution of the concentrations of each family of deoxygenated C5 products as a function of time during hydrogenolysis of b) XYL and c) 1,2,3,5-PTeO; (◆ ) XYL; (▽ ) PTeO; (▲) Deoxy C5 from XYL; (△) Deoxy C5 from PTeO; (□) PTrO; (○) PDO; (☆) PeOH. Evolution of the concentrations of each family of deoxy C6 products as a function of time during hydrogenolysis of e) SOR and f) 1,2,3,4,6-HPO. (◆ ) SOR; (▽ ) HPO; (▲) Deoxy C6 from SOR; (△) Deoxy C6 from HPO; (□) HTeO; (○) HTrO; (☆) HDO; (⁕) HeOH. Reaction conditions: 200 °C, 50 bar of H2; a) and b) XYL 0.30 mol L -1 , 120 mL H2O, 500 mg Rh-ReOx/ZrO2, nXYL/nRh = 230; a) and c) PTeO 0.16 mol L -1 , 33 mL H2O, 75 mg Rh-ReOx/ZrO2, nPTeO/nRh = 190; d) and e) SOR 0.27 mol L -1 , 120 mL H2O, 500 mg Rh-ReOx/ZrO2, nSOR/nRh = 200; d) and f) HPO 0.16 mol L -1 , 30 mL H2O, 50 mg Rh-ReOx/ZrO2, nHPO/nRh = 270.

Figure 16 .

 16 Figure 16. Effect of H2 pressure on selectivity to the reaction products and %TOC measured at 72-84% conversion. (▲) % TOC measured; (◼) C-C cleavage products; (◼) Cyclic compounds; (◼) Isomers; (◼) polydeoxygenated products C4 without BTO, C5 without PTeO, or C6 without HPO; (◼) monodeoxygenated products BTO, PTeO, or HPO. Reaction conditions: reactant 0.27-0.40 mol L -1 , 120 mL H2O, 200 °C, 300-500 mg 3.4wt%Rh-2.7wt%ReOx/TiO2 catalyst; nreactant/nRh = 200-450.

Figure 17 :

 17 Figure 17: Catalyst recycling study. (◼) Run 1; (□) Run 2. Reaction conditions: SOR 0.27 mol L -1 120 mL H2O, 200 °C, 80 bar of H2, 500 mg of Rh-ReOx/ZrO2. C6 products without HPO: HTeO + HTrO + HDO + HeOH

Table 1 .

 1 Conversion and carbon selectivity to the different families during hydrogenolysis of SOR. Reaction conditions: SOR 0.27 mol L -1 , 120 mL H2O, 200 °C, 80 bar of H2, 500 mg catalyst, nSOR/nRh = 200, nSOR/nPt = 342, nSOR/nIr = 377.

Table 2 .

 2 Effect of H2 on the carbon selectivity to the mono-and poly-deoxygenated products (reaction conditions seeFigure 12).

	Scheme 1

Table 1 .

 1

	Catalyst	Molar ratio Re/Rh	Time [h]	Conv. [%]	C1-C5	Carbon selectivity (%) Isomers Cyclic C6	C6
	ReOx/ZrO2		-	24	23	8	10	54	26
	Rh-ReOx/ZrO2	0.5	3	80	9	13	20	54
	Pt-ReOx/ZrO2	1	6	80	9	11	28	47
	Ir-ReOx/ZrO2	1	24	80	6	5	55	32
	Isomers = MAN + IDI, Cyclic C6 = 1,5-AS + 2,5-AS + 1,4-S + 3,6-S + IS, C6 = HPO + HTeO + HTrO + HDO
	+ HeOH

Table 2 .

 2

	PH2 (bar)	ERY (%)	XYL (%)	SOR (%)
		Mono	Poly	Mono	Poly	Mono	Poly
	30	13	30	16	16	20	23
	80	28	30	20	21	22	23
	120	37	34	24	31	31	23

V : ;mmol -. g ?@ A h A C = DD EF G -. DHII G ?@× "6DF

The calculated TOC in liquid phase is determined by the sum of the carbon concentrations of all compounds quantified by HPLC and GC at time t (4):

where MC is the molar mass of carbon (12 g mol -1 ), Ncarbon i is the number of carbon atom in compound i, and Ct i is the concentration of compound i at time t (in mol L -1 ). The comparison between TOC calculated and TOC measured gives an indication about carbon balance. At the end of each catalytic test, the final solution was analyzed by ICP to measure the % of leached metal.

Acknowledgements

The financial support of this work was provided by Région Rhône-Alpes, FCBA, and by ANR (French Agence Nationale de la Recherche, ANR-15-CE07-0017) within the French-Luxembourg Programme. Figure 17