3.2.1. Extraction of Hemicelluloses

The chemical composition of maritime pine wood used in this work (on a dry weight basis) was 41.4% glucan, 26.1% lignin, 13.1% mannan, 7.3% xylan, 4.6% extractibles, 2.5% galactan, and 1.3% arabinan (balance 96.3%).

Wood chips (1.2 kg) were ground and sieved to size particle below 1 mm. Wood powder was first delignified using sodium chlorite in an acetate buffer medium (liquor/wood weight ratio = 6) in a 5 L glass reactor, at 60 ◦C, under mechanical stirring and nitrogen gas flow [77]. Chlorite sodium was progressively added to the medium until complete delignification (around one week of extraction for each batch). The delignification was followed by the measurement of Kappa number (carried our using the ISO 302-2015 standard method) of the treated wooden materiel. At the end of the delignification, the wood particles were totally converted into fibres, which were thoroughly washed with demineralized water until neutral pH.

The hot water extraction of hemicelluloses was performed after elimination of lignin by a sodium chlorite treatment. Typically, the delignified wood chips (300 g, dry basis, particles below 1 mm) and deionized water (1800 mL) at a water/wood ratio of 6 were filled into a 5 L batch stainless-steel oil-heated autoclaves. Temperature was raised from 25 ◦C to 120 ◦C in 30 min, followed by a plateau at 160 ◦C for 30 min. Subsequently, the liquor was separated from the solid material by filtration over a 4-L Büchner porosity 2. The pH of the collected filtrate was acidic (due to the release of small amounts of acetic or uronic acids upon extraction) and its value was then raised to 6 with the addition of NaOH.

The extracted stream was then further fractionated. The supernatant solution was concentrated by evaporation, and subsequent addition of ethanol (1/3 water–2/3 ethanol) caused the precipitation of high molecular weight hemicelluloses polymers. The remaining aqueous extract, containing monosaccharides and low molecular weight oligosaccharides [78], was finally purified with active

Post-hydrolysis

 liquor  1229.9

carbon in order to remove all unsatured by-products. This liquor phase was used for the hydrogenolysis reactions after the removal of EtOH using a rotary evaporator at 40 ◦C under vacuum (175 mbar).

### 3.2.2. Analysis of Carbohydrates in Hemicellulose Extract

The low MW hemicelluloses extracted liquor was analysed by high performance liquid chomatography (HPLC, see below) for monosaccharide contents (glucose, xylose, mannose, galactose and arabinose). The carbohydrate composition was determined after a second acid post-hydrolysis of the liquor to depolymerize all remaining oligomers to monosaccharides using 37% HCl aqueous solution(pH1)for24hat90 ◦Cinanautoclave,accordingtotheprotocoldescribedin[79,80].

 Table 2 shows the chemical compositions of the unhydrolyzed and post-hydrolyzed hemicelluloses liquor.

 5575.5

 437.4  1088.8


 2195.2

**Table 2.** Amounts ofmonosaccharidesin the extracted purifiedliquor before and after acid post-hydrolysis.

The sugars in the non-hydrolyzed extraction liquor were essentially present as oligomers. Arabinose was extracted early as monosaccharide, consistent with results of Grénman et al. [81] in their study of aqueous extraction of hemicelluloses from spruce chips. These authors demonstrated that this sugar experienced further reaction and degradation during extraction.

The post-hydrolysis fraction consisted of a mixture of 7.24 g/<sup>L</sup> of hexoses and 3.28 g/<sup>L</sup> of pentoses, with a hexoses/pentoses ratio of 2.2. Mannose accounted for about 50% in the extracted purified liquor. This observation is in accordance with the fact that the main hemicelluloses of softwood are galactoglucomannans.

### *3.3. Catalyst Evaluation Experiments and Products Analysis*

The model substrates d-Xylitol (99%), d-sorbitol (99%), d-xylose (99%), and d-mannose (99%) were purchased from Sigma-Aldrich (St. Louis, MO, USA), d-glucose (99%), d-galactose (98%), and l-arabinose from Alfa Aesar. All reagents were used without further purification.

All the hydrogenolysis reactions of polyols, sugars, their mixtures, and the hemicelluloses liquor were performed in a batch Hastelloy Parr 4560 reactor (300 mL). For each run, the autoclave was loaded with 120 mL of aqueous solution and 0.5 g of catalyst. For the polyols (xylitol, sorbitol) and sugars (xylose, arabinose, glucose, mannose and galactose), 6 g of each product were previously dissolved in 120 mL of water. An amount of 120 mL of hemicelluloses liquor was loaded after elimination of ethanol. After sealing and flushing three times with 10 bar Ar to remove residual air, the reactor was pressurized with H2 and heated to 200 ◦C under stirring (1000 rpm). The pressure increased during heating; hence, the pressure given of 80 bar reflects the pressure under the reaction conditions. Liquid samples were regularly collected during the reaction.

A detailed description of the analytical procedure of the reaction medium during hydrogenolysis of polyols can be found in our previous study [71]. Hexanepentaols, hexanetetraols, hexanetriols, hexanediols, pentanetetraols, pentanetriols, isosorbide, butanediols (mainly 1,2-butanediol), glycerol (GLY), 1,2 propanediol (1,2-PrDO), ethylene glycol (EG) were analyzed using a Shimadzu LC 20 A HPLC equiped with a Rezex ROA-Organic Acid H<sup>+</sup> column (crosslinked sulfonated styrene divinylbenzene copolymer) with a 0.005 N H2SO4 mobile phase at a flow rate of 0.5 mL min−1) kept at 40 ◦C and connected to a RID detector.

The C1-C6 mono alcohols and tetrahydrofurfuryl alcohol (THFA), 1,2-pentanediol (1,2 PDO), 1,5-pentanediol (1,5 PDO), tetrahydrofurane (THF) present in the aqueous reaction medium were determined on a Shimadzu GC-2010 analyzer equiped with a Phenomenex FFAP 30 m × 0.25 μm

column (the temperature was increased from 40 ◦C to 100 ◦C at 5 ◦C min−1, and then to 250 ◦C at 8 ◦C min−1.

C6 and C5 sugars and sugar polyols (glucose, xylose, galactose, mannose, arabinose, sorbitol, xylitol, dulcitol, mannitol, arabitol, iditol) were analyzed on a Shimadzu LC 20 A HPLC using Rezex RCM-Monosaccharide Ca2+ column heated at 65 ◦C with water as eluent and with refractive index detector (RID-10A) [67]. In fact, this column allows analysis of sugars, polyols, and can additionally provide analysis of oligosaccharides with low DP < 6. Therefore, in addition to the monosugars, available soluble oligosaccharides were used as analytical reference standards to attempt to identify the structures or homologs released during extraction, such as cellobiose, the ra ffinose family of oligosaccharides comprising ra ffinose (trisaccharide), stachyose (-galactosido-1,6-ra ffinose) and verbascose ( α-galactosido-1,6-stachyose), but also sorbitan, 2-MeTHF, HMF, furfural, THFA, tetrahydropyran-2-methanol. Figure S2 presents the HPLC chromatogram of the hydrolysed extract, which shows that not only sugar monomers were formed, but also a high percentage of the hemicelluloses as dimers, trimers, and larger oligomers.

The calibration of products was carried out by injecting known concentrations of commercial standards. For a same family of product, the same response coe fficient as the one of the commercially available product was considered.

The substrate conversion was calculated from Equation (1):

$$\text{Conversion } (\%) = \frac{[Substrate]\_0 - [Substrate]\_t}{[Substrate]\_0} \times 100\tag{1}$$

where [*Substrate*]0 is the initial concentration of s, and [*substrate*]*t* the concentration at time t.

The yield of a given product was calculated according to Equation (2):

$$\text{Product yield} \left( \% \right) = \frac{\left[ Product \right]\_{t}}{\left[ Substrate \right]\_{0}} \times 100 \tag{2}$$

The carbon selectivity S*i t*to a desired product is based on Equation (3):

$$\mathbf{S}\_t^i(\%) = \frac{[\text{Product}]\_t^i \times \text{N}\_\text{C}^{\text{product i}}}{([\text{Substrate}]\_0 - [\text{Substrate}]\_t) \times \text{n}\_\text{C}^{\text{substrate}}} \times 100\tag{3}$$

where [Product] i t is the concentration of product i formed at time t, Nsubstrate C and Nproduct i C is the number of carbon atoms in the substrate and product I, respectively.

Total Organic Carbon (TOC) was measured using a TOC-VCSH Shimadzu analyzer. The values were compared with the TOC calculated from the analysis of the products in liquid phase by HPLC and GC by the following Equation (4):

$$\text{Calculated TOC} \left( \text{g L}^{-1} \right) = \text{M}\_{\text{C}} \left[ \left( \sum\_{\text{i}}^{\text{n}} \text{N}\_{\text{carbon}}^{\text{i}} \times \text{C}\_{\text{t}}^{\text{i}} \right) \text{HPLC} + \left( \sum\_{\text{i}}^{\text{n}} \text{N}\_{\text{carbon}}^{\text{i}} \times \text{C}\_{\text{t}}^{\text{i}} \right) \text{GC} \right] \tag{4}$$

where MC is the molar mass of carbon (12 g moL−1), N icarbon is the number of carbon atoms in compound i, and Ci t is the concentration of compound i at time t (in mol <sup>L</sup>−1). The comparison between TOC calculated and TOC measured provides an indication as to the carbon balance in liquid phase.
