**4. Summary**

The pre-extraction of hemicelluloses from lignocellulosic materials before pulping processes generates sugars as feedstocks for hemicelluloses-based chemicals. In this work, the comparative hydrogenolysis of C5-C6 separate sugar polyols and sugars was studied over ReOx-Rh/ZrO2 catalysts before evaluating mixtures and a liquid stream of hemicelluloses-based sugars.

A complete conversion of sugars was achieved very rapidly: the predominant reaction was hydrogenation to the corresponding sugar alcohol to yield the corresponding polyols by hydrogenation, but direct hydrogenolysis of the monosaccharides was observed from the beginning of the reaction, yielding all the products formed by hydrogenolysis of the sugar-derived polyols. Due to higher C-C cleavage and mainly related to the retro-aldol and decarbonylation reactions, the maximum global selectivity to deoxy linear polyols was lower from sugars (28% and 32%, from glucose and xylose) than from polyols (35% and 49% from sorbitol and xylitol). However, sugars underwent deeper C-O bond hydrogenolysis reaction, which favored the rapid formation of diols.

Hydrogenolysis of mixtures of polyols and sugars allowed the further analysis of the complete product spectra. The comparative hydrogenolysis of epimers of glucose and xylose also suggested that orientation of the OH-groups may a ffect the selectivity to the deoxy-products.

In the end, hot water extraction of hemicelluloses from a delignified maritime pine wood was performed. Conversion of the low molecular weight mono- and oligosaccharides in the recovered extract was studied by hydrogenolysis and the results were compared to a synthetic aqueous solution of glucose/xylose mixture of same C6/C5 composition of 2/1. The global selectivity to linear deoxygenated polyols was found to be higher than in the reaction of the synthetic solution. The yield of the deoxy polyols from the extracted hemicelluloses liquor was as high as 65% whereas it was only 30% from the 2/1 glucose/xylose mixture. This selectivity di fference is mainly linked to the high concentration of mannose and galactose leading to improved selectivity compared to glucose. The presence of by-products from the extraction process, such as furfural or HMF formed from the dehydration of sugars, may play a role of selective poisoning adsorbate limiting the reduction of the undesired C-C bond breaking side reactions. These runs show the possibility of the production of value-added C6 and C5 tetraols, triols, and diols by catalytic hydrogenolysis of sugars and their oligomers present in the hemicellulose extracts. The next step should be to study the catalyst stability. Further research is also needed to develop kinetic models that describe adequately the experimental concentration of liquid products–time data and the complex networks of hydrogenolysis of either the polyols or the sugars. This kinetic analysis is essential for maximizing the yield of desired chemical products.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4344/9/9/740/s1, Figure S1: XRD pattern of support ZrO2 and of catalyst 9.4%Re–3.2%Rh/ZrO2, Figure S2: HPLC chromatogram of pine maritime hemicellulose after chlorite treatment, hot water extraction, and acid hydrolysis with HCl at 90 ◦C during 24 h using Rezex RCM-Monosaccharide Ca2+ HPLC column, Scheme S1: Reaction pathway during hydrogenolysis of sorbitol and glucose. Adapted from [71], Scheme S2: Reaction pathway during hydrogenolysis of xylitol and xylose, Scheme S3: C6 and C5 sugars in the hemicelluloses liquor Adated from [71].

**Author Contributions:** Conceptualization: D.D.S.P., M.B., and C.P.; Design of experiments: M.B. and N.P.; Carrying the experiments and analyzing the data: A.S., M.M.-D., and E.N.; Writing original draft: M.M.-D.; Writing, review and editing: M.B., N.P.; Supervision: C.P., N.P. and M.B.

**Funding:** This research was funded by the French Agence Nationale de la Recherche (ANR) within the French-Luxembourg program under Award Number ANR-15-CE07-0017-02.

**Acknowledgments:** The authors would like to express their grateful appreciation to the scientific platforms of IRCELYON and FCBA.

**Conflicts of Interest:** The authors declare no conflict of interest.
