## 4 HPS-Based Catalysts for the Synthesis and Transformation of Platform Chemicals  
### Hydrolytic Hydrogenation of Cellulose to Hexitols  
Sugar alcohols, including sorbitol, are included in the list of substances with high added value obtained from biomass [3]. Sorbitol is a product of glucose hydrogenation and an industrial raw material [99; 100]. It is used in the food, textile, pharmaceutical, and cosmetology industries for the synthesis of vitamin C, surfactants, resins, glycols, lactic acid, etc. [101; 102]. The main industrial method for producing sorbitol is the hydrogenation of glucose, which can be obtained from components of plant biomass, like easily hydrolyzed starch or cellulose [103]. Cellulose has no nutritional value and comes in large quantities as a waste from the paper, woodworking, and agricultural industries that require disposal. Therefore, using it as a renewable raw material to produce sorbitol is in accordance with the principles of green chemistry, and can be economically beneficial. The presence of many hydroxyl groups in the structure of cellulose determines one of the most effective options for its processing, namely, conversion into hexitols through the hydrolytic hydrogenation [104; 105; 106], known as one-pot reactions of hydrolysis and hydrogenation (Figure 2). The products of hydrolytic hydrogenation of cellulose are mainly hexitols (sorbitol and mannitol) and some amounts of C\({}_{2}\)-C\({}_{5}\) polyols formed as a result of hydrogenolysis of monosaccharides and polyols. The study of simultaneous hydrolysis and hydrogenation of plant polysaccharides began in the 1950s by [PERSON] et al., who subjected cellulose to hydrolytic hydrogenation in the presence of mineral acids and Ru-, Pd-, and Pt-containing catalysts [108; 109]. For example, in the invention [110] high yield of sorbitol during the hydrolytic hydrogenation  
Figure 2: Scheme of hydrolytic hydrogenation of cellulose to hexitols [107]. of cellulose is claimed in the presence of suspended nickel catalyst at hydrogen pressure of 100-120 atm at 180-200 \({}^{\circ}\)C for 40-60 min. Several decades later, this topic was reconsidered due to the concepts of Green Chemistry [111]. The number of reports on the hydrolytic hydrogenation and hydrogenolysis of cellulose began to increase [112; 113]. For environmental reasons, in some cases (under subcritical conditions) the mineral acids as hydrolysis catalysts were replaced by cheap and environmentally friendly water [114; 115]. In the late 1990s, [PERSON] et al. [116; 117] studying the kinetics of dissolution and hydrolysis of cellulose in subcritical and supercritical water showed that in the supercritical region (at a temperature of 374 \({}^{\circ}\)C and above) the rate of cellulose hydrolysis is higher than the rate of thermal destruction of glucose, while in subcritical conditions the rate of glucose destruction significantly exceeds the rate of hydrolysis of cellulose macromolecules. Thus, the efficiency of the process of hydrolytic hydrogenation of cellulose carried out in a subcritical water environment will largely be determined by the activity of the catalyst, its ability to quickly and selectively hydrogenate hexoses, which is confirmed by the results of experiments without a catalyst. In the latter case, solutions are formed with an odor and color characteristic of products of the thermal destruction of glucose. Large numbers of heterogeneous catalytic systems for the direct conversion of cellulose to sorbitol have been proposed. An assessment of the activity of various catalysts in the conversion of biomass into fuel and chemicals showed that catalysts containing noble metals have the highest efficiency [118]. The ruthenium-based catalyst has the maximum activity and comes first in the series: Ru >> Pt \(\approx\) Pd \(\approx\) Au > Rh > Ir >> Os.