**1. Introduction**

Until recently, chemicals have mostly been derived from fossil resources. The utilization of non-edible, abundant and renewable lignocellulosic biomass from agriculture and forestry has emerged as an interesting alternative for the chemical industry and polymer production in particular [1–6]. Lignocellulose consists of carbohydrate polymers (cellulose and hemicelluloses) embedded in a lignin matrix [4,7]. Cellulose (30–50%) is a water insoluble linear homopolysaccharide of β (1 → 4) linked glucose units with strong intra- and intermolecular hydrogen bonds and with a degree of polymerisation as high as 15,000 units. Different to cellulose, hemicelluloses (20–35%) are branched polymers made of structures of hexoses (d-galactose, d-mannose, d-glucose) and pentoses (d-xylose and l-arabinose) and some sugar acid subunits, with a low degree of polymerization of 80–200 units. The composition of hemicelluloses varies depending on the source of biomass: in general, softwood hemicelluloses are mainly composed of *O*-acetyl-galacto-glucomannans and some arabino-4-*O*-methyl-glucuronoxylan [8–10], whereas in hardwoods, *O*-acetyl-4- *O*-methylglucuronoxylans are the most abundant [11,12]. In addition, lignin is an aromatic polymer with a complex composition of phenylpropane units linked by ether bonds. In order to utilize the woody biomass e fficiently, all three main components should be used in a biorefinery to maximize economic returns [13].

Although cellulose has been much studied for the conversion to fuels and chemicals, there has been less e ffort in the conversion of hemicelluloses, which are the second most abundant polysaccharides. During the conventional Kraft process in the pulp and paper industry, the cellulose fraction is the core product to be used in papermaking, while most hemicelluloses are currently degraded and accumulated in the black liquor along with lignin, and are burnt to produce energy. However, if the hemicelluloses are pre-extracted or hydrolyzed to their structural C5 and C6 sugars—which are building blocks for numerous biomaterials or chemicals—with a minimum e ffect on subsequent pulping step, this makes a better valorization of this under-utilized fraction in paper mills feasible [14,15]. The potential of valorization is up to the millions of tons of wood treated by the Kraft process.

The relatively low degree of polymerization and the branched side groups allow a large fraction of hemicelluloses to dissolve in aqueous solution. Various e ffective methods have been proposed for hemicelluloses extraction from wood chips prior to cooking, including the use of acids, water, and alkaline solutions [16–27]. Many applications for the extracted hemicelluloses have been developed [28]. Moderate treatment provides polymeric hemicelluloses with su fficient molecular mass to be recovered by precipitation and valorized after chemical modification [29–31] as hydrocolloids [32,33], surfactants [34–36], coating materials [37], emulsion stabilizers [38] or as packaging materials [39], among others. In some more severe extraction processes where autohydrolysis takes place, the backbone of hemicelluloses is hydrolysed into oligomers with a low degree of polymerization, hexoses, pentoses, and uronic acids, which may be subsequently upgraded in further steps for various applications.

Though the pH may influence the composition of liquor after extraction (sugars, distribution of oligomers), extraction using only hot water (120–240 ◦C) has been shown to be an attractive environmentally friendly method [21,40]. Some auto-hydrolysis occurs due to release of acetic acid from the deacetylation of hemicelluloses and uronic acids. The glycosidic bonds between the sugar units of hemicelluloses are cleaved by a hydrolysis reaction to the partially hydrolyzed dimers, trimers and other oligomers or the sugar monomers, depending on the operating conditions and on hemicelluloses source [41]. Once separated, they can be further processed into chemicals. They may be a source for furans and their derivatives for ethanol [14,28,42–44], but also for the production of suitable monomers by catalytic hydrogenation [45–47] or oxidation [48] of the extracted sugars.

Like most of the biomass derivatives, the compounds in the extracted hemicellulosic stream are characterized by a much higher oxygen content compared with fossil-derived feedstocks. Usually, the target products, such as monomers for polymer synthesis, have a comparatively lower oxygen content. Triols and diols are useful as chemical intermediates for the production of polymers, agrochemicals and surfactants. For example, <sup>α</sup>,<sup>ω</sup>-diols, especially C5 and C6 <sup>α</sup>,<sup>ω</sup>-diols, can be used as plasticizers and as co-monomers in polyesters and polyurethanes manufacture [49–51]. Therefore, methodologies for upgrading of the hemicelluloses stream to monomers necessitate a controlled removal of oxygen—containing functional groups from sugars. One route is catalytic-selective hydrolysis/hydrogenolysis of soluble hemicelluloses/polysaccharides/monomers in the presence of supported metallic catalysts. Hydrogenolysis of C-O bonds has been applied to alcohols and polyols to produce more or less deoxygenated polyols. The transfer of hydrogenolysis has been applied to polyols over heterogeneous catalysts using H-donors (alcohols, formic acid) [52,53]. Under hydrogen pressure, although monometallic catalysts such as Cu, Ni, and Ru have been reported for dehydroxylation [54–57], a better control is obtained using catalysts that contain a noble metal (Pt, Pd, Rh, Ir) modified by an oxophilic metal (Re, Mo, W) [58]. These bimetallic catalysts have been found to be highly active and selective for —C-O- cleavage reactions of various biomass-derived substrates, including glycerol, 2-(hydroxymethyl)tetrahydropyran or tetrahydrofurfuryl alcohol (the full saturation product of furfural), and others [59,60]. Partial hydrodeoxygenation of glycerol to 1,2- or 1,3-propanediol was investigated intensively and reviewed [61–65]. It is much more difficult to selectively remove OH groups from substrates with four or more OH groups such as erythritol, xylitol, or sorbitol [66–72]. During this process, several products are very often obtained by unwanted reactions (hydrogenation, isomerization, dehydration, C-C bond cleavage by retro-aldol condensation, decarbonylation and decarboxylation).

The objective of this work was to evaluate the feasibility of catalytic hydrogenolysis of an hemicellulose extract from delignified maritime pine wood chips that contained simple sugars such as glucose or xylose or partially hydrolyzed dimers, trimers and other oligomers, to prepare more or less deoxygenated linear C6 and C5 polyols. We used ReOx-Rh/ZrO2 catalysts, which were particularly efficient in our previous work on the hydrogenolysis of erythritol, xylitol and sorbitol [68,69]. Before performing the catalytic hydrogenolysis of the hemicelluloses liquor, we studied the reaction of aqueous solutions of sugar polyols and sugars of different compositions.
