# Production of Sorbitol via Catalytic Transfer Hydrogenation of Glucose
## 3 Results and Discussion
### Influence of the Reaction Temperature and Catalyst Loading  
Blank reactions were performed to assess the different side reactions taking place in the absence of Raney nickel catalysts. These tests were performed in the temperature range 130-190 \({}^{\circ}\)C. Results are depicted in Figure 2. Regardless of the temperature, fructose and mannose were obtained even in the milder conditions, most probably coming from the isomerization of the starting glucose. The extension of these transformations increases as the temperature rises, although, at 190 \({}^{\circ}\)C, lower yields toward both mannose and fructose were detected. This is probably related to the existence of sugar monosaccharide-consuming side reactions, such as the dehydration of glucose/fructose toward 5-hydroxymethyl furfural [34]. Finally, blank tests also served to confirm the true catalytic activity of nickel in the above-observed hydrogen transfer reduction from glucose to sorbitol, as, in the absence of the metal catalyst, no sorbitol was detected. The influence of several operational parameters (reaction temperature and catalyst loading) was assessed in the catalytic transfer hydrogenation of glucose to sorbitol, using ethanol as a sacrificing alcohol. Figure 3 depicts the product yields and substrate conversion achieved in the catalytic tests conducted at different temperatures, within the range 130-190 \({}^{\circ}\)C, and in the presence of two catalyst loadings (glucose to catalyst mass ratios = 8:1 and 1:1). The influence of both operation parameters on the glucose conversion rate was as expected, i.e., larger catalyst loading and temperature led to faster substrate conversion. As for product distribution, sorbitol was present as the main product in all tested conditions. However, the influence of the temperature and catalyst loading on the selectivity to the different products provided quite distinct results, as a consequence of a different effect of both operation parameters on the extension of not only the main reaction but also of the different side reactions taking place under the tested conditions. The catalyst loading exerted a dramatic influence on the extension of glucose isomerization to fructose and mannose. Assays performed with a lower catalyst loading resulted in a poorer selectivity toward the desired sorbitol, and a large amount of side products, such as mannose and fructose, were observed (Figure 3a-d). This influence was even higher when increasing the applied temperature. Increasing the catalyst loading led to substantial suppression of the extension of sugar isomerization reactions, and only a limited production of fructose and mannose, below 10% total yield, was observed, even in the highest-temperature conditions. These results, together with those achieved from the blank tests, confirmed that the conversion of glucose into its different isomers is, under the tested reactions conditions, a side transformation not related to the Raney nickel catalyst but to a thermal effect. As for the production of sorbitol, the extension of the catalytic transfer hydrogenation of glucose to this hexitol seems to be highly dependent on the catalyst loading. Thus, a substrate to catalyst mass ratio of 8:1 provided a limited production of sorbitol, even in the highest tested temperature conditions, leading to sorbitol yields always below 25% of the starting glucose. Increasing the catalyst loading to a substrate to catalyst mass ratio of 1:1 led to much more efficient conversion of the sugar monosaccharide into sorbitol.