# Production of Sorbitol via Catalytic Transfer Hydrogenation of Glucose
## 1 Introduction  
Lignocellulose biomass is considered a promising alternative source to fossil fuels for the sustainable production of useful chemicals and value-added products [1, 2]. The abundance of this feedstock makes biomass the only alternative to the traditional raw materials used in chemical industries, with most of them based on the exploitation of crude oil derivatives. Lignocellulose biomass is mainly composed of cellulose, hemicellulose, and lignin [3]--two polysaccharides (cellulose and hemicellulose), and a random polymer mainly composed of phenyl derivatives (lignin). Lignocellulosic biomass is the major component of agricultural waste and forest residue, which are inexpensive and abundant raw materials, as well as a primary source of monosaccharides, including glucose and fructose [4], which can be converted into a variety of different chemicals, including sugar polyols. Sugar polyols are interesting monosaccharide-derived products, which find applications in several fields, both as polyols and as starting materials for the preparation of different products. Sorbitol is a sugar polyol extensively used in nutrition, cosmetics, and medical or industrial applications. It is employed as a low-calorie sweetener, as a humectant in cosmetics and pharmaceutical products, and as an intermediate platform for the production of value-added molecules (e.g., 1,4-sorbitan, isosorbide, glycols, \(\upmu\)-ascorbic acid, etc.) [5, 6, 7]. This versatility led to its inclusion in the top derived value-added chemicals to be converted into high-value bio-based products from biomass [8]. Sorbitol production was reported to be carried out using cellulose in the presence of noble metals [9] and transition metal-based catalysts [10; 11]. However, a most extended alternative is its production starting from glucose. At an industrial scale, sorbitol is produced via glucose hydrogenation [2] as it is the most affordable precursor [12], whose production is liable to be obtained via cellulose hydrolysis [13; 14]. The reaction mechanism consists of the reduction of carbonyl groups of saccharides under hydrogen pressure in the presence of a solid metal catalyst based on Ni, Pd, Pt, or Ru [15; 16]. All these catalysts are easily recoverable and display good catalytic activity in terms of sorbitol yield, operating under aqueous-phase solution [6; 17; 18; 19; 20]. In recent years, Raney nickel and ruthenium catalysts made a clean sweep on sorbitol production. Ru presents higher activity and selectivity to sorbitol [16; 21] than Raney nickel catalysts, but this metal is much more expensive than nickel-based catalysts. In this way, Raney nickel was revealed as the most interesting active phase to be used in the industrial-scale hydrogenation of glucose [21; 22], mostly because of its low price [23]. Nevertheless, the lower catalytic activity of nickel as compared to other precious-metal-based conventional hydrogenation catalysts, such as ruthenium, results in the need for high hydrogen pressure and harsh operating conditions. Consequently, a significant energy demand must be satisfied compared to that required when using more active hydrogenation catalysts and, thus, higher environmental impacts are associated with the use of Raney nickel catalysts [24]. This work focuses on gaining insights into a developing alternative reaction pathway to the conventional hydrogenation procedure used to transform glucose into sorbitol by exploring the feasibility of carrying out the transformation through catalytic transfer hydrogenation (CTH) using a hydrogen donor. CTH is, a priori, a more sustainable alternative to conventional hydrogenation with molecular hydrogen because of the lower requirements for high-pressure conditions. CTH is usually performed in the presence of hydrogen donors, such as small alcohols and acids, which can be used under liquid phase conditions and under moderate operation conditions, and which are potentially easily prepared from renewable biomass feedstock [25; 26; 27].