# Production of Sorbitol via Catalytic Transfer Hydrogenation of Glucose
## 3 Results and Discussion
### Influence of Sacrificing Alcohol
This is probably due to the better performance of ethyl and 2-propyl moieties as leaving groups, and the consequent lower stability of the resultant alkyl glucopyranosides under the tested reaction conditions. In this way, the achieved sorbitol yield in tests performed in the presence of different alcohols varies according to the following trend: ethanol \(>\) 2-propanol \(>\) methanol. Unlike methanol, ethanol and 2-propanol provide sorbitol yield values higher than those obtained in aqueous phase conditions, suggesting the existence of a net contribution to the formation of sorbitol coming from the hydrogen transfer from these alcohol solvents to the sugar substrate. To assess this possibility, several additional experiments were performed. Firstly, the analysis of the reaction media using gas chromatography analysis confirmed the presence of acetaldehyde and acetone evolved during the dehydrogenation of ethanol and 2-propanol, respectively. On the contrary, formaldehyde was not detected, evidencing that, unlike for ethanol and 2-propanol, the catalytic transfer hydrogenation from methanol does not occur under the tested conditions. Secondly, several catalytic tests were also performed, this time under the very same reaction conditions as those already described for glucose, instead using fructose as the reaction substrate. Fructose, unlike glucose, cannot undergo disproportionation because the quaternary carbon at the furanose sugar ring cannot donate an H\({}_{2}\) molecule. Figure 1: Cumulative product yields (**left** axes) and substrate conversion (**right** axes) in the catalytic transfer hydrogenation of (**a**) glucose and (**b**) fructose, with different solvents in the presence of Raney nickel catalysts. Reaction conditions: temperature = 150 \({}^{\circ}\)C; substrate concentration = 90 mM; substrate to catalyst mass ratio = 1:1; reaction time = 6 h.  
Thus, the reduction of fructose under the tested conditions should come from the catalytic hydrogen transfer from the sacrificing alcohol. Under these conditions, methanol only provided traces of hexitols, confirming that methanol is not a good option as a hydrogen donor for glucose CTH. On the contrary, both ethanol and 2-propanol led to quantifiable amounts of sorbitol and mannitol, present in almost equimolar amounts as that corresponding to the non-stereo controlled hydrogenation of fructose. Nevertheless, ethanol provided lower amounts of mannitol, probably as a consequence of the lower extent of isomerization reactions in the presence of this solvent. Furthermore, ethanol seems to be a better hydrogen donor than 2-propanol under the tested conditions, bearing in mind the higher hexitol yields obtained with the primary alcohol. This behavior is probably due to the neutral conditions used during the catalytic tests, which favor the dehydrogenation of primary alcohols over secondary ones [33]. The better performance of ethanol as a sacrificing alcohol as compared to methanol or 2-propanol introduces an additional advantage for the CTH reduction of glucose to sorbitol, which is the possibility to obtain ethanol from renewable biomass via fermentation through well-known industrially scaled processes.