# Production of Sorbitol via Catalytic Transfer Hydrogenation of Glucose
## 3 Results and Discussion  
### Influence of Sacrificing Alcohol  
The capability of Raney nickel to drive the catalytic transfer hydrogenation of glucose to sorbitol was assessed using different short-chain alcohols as solvents and sacrificing alcohols. For this purpose, methanol, ethanol, and 2-propanol were used as potential hydrogen donors in the presence of a conventional nickel sponge catalyst. These alcohols were previously reported to be suitable hydrogen donors in different CTH reactions in the presence of nickel-based catalysts. Figure 1 depicts the results achieved (cumulative product yields and substrate conversion) in the transformation of glucose and fructose solutions prepared from the tested reaction solvents, treated in the presence of Raney nickel catalyst. The cumulative product yields and substrate conversion are depicted for all the tested short-chain alcohols. Substrate conversion was almost complete after reacting for 6 h, regardless of the used solvent. The results from the blank test, which was accomplished by treating an aqueous solution of glucose in the absence of any hydrogen donor, but in water, revealed that this substrate can be transformed in the presence of Raney nickel catalysts into hexitols (sorbitol and mannitol) in quite a large extension. This behavior was previously described to occur because of the reducing capability of glucose, coming from its ability to donate a hydrogen molecule through the oxidation of the hydroxyl group located in the \(\alpha\) position, thus leading to the formation of gluconolactone. The H\({}_{2}\) molecule is then transferred, mediated by the nickel catalyst, to a different molecule of glucose, thus leading to the formation of sorbitol via ring-opening hydrogenation, in a sort of disproportionation transformation of glucose (one molecule is oxidized whereas another one is reduced). This phenomenon was previously described to occur in the presence of metal catalysts such as Pd [29] and, more recently, Ru [32]. Nevertheless, the auto reduction of glucose is not the only transformation taking place in the reaction medium as, together with sorbitol, mannitol, and fructose are also produced through side reaction pathways. Fructose is most probably produced by the isomerization of glucose, a transformation which can be mediated by the aluminum traces remaining in the catalysts after the preparation of the sponge-like catalysts (which account for more than 5%) or simply because of a thermal effect. Mannitol, on the contrary, is not produced via the isomerization of sorbitol, but through (catalytic transfer) hydrogenation of fructose. All the detected chemical transformations are depicted in Scheme 1. The ring-opening hydrogenation of this monosaccharide produces, unlike in the case of glucose, two different hexitol isomers, sorbitol and mannitol, in equimolecular amounts, depending on which side of the sugar ring is approached by the nickel catalyst to be reduced. As for the influence of the type of short-chain alcohol used as a hydrogen donor, results evidenced the formation of the same reaction products already described for the test performed in water. Large differences are evident between the product distributions achieved in the presence of each short-chain alcohol, showing that the alcohol media do exert an influence on the product distribution. Thus, methanol provided a substantially lower amount of hexitols as compared to ethanol or 2-propanol. In this case, the presence of highly stable methyl glucopyranosides was detected in the reaction media, coming from the etherification of the sugar monosaccharide in the \(\alpha\) position with the alcohol solvent. This transformation avoids glucose undergoing dehydrogenase oxidation toward gluconolactone, depressing the extension of the sugar disproportionation and the consequent production of sorbitol. On the other hand, alkyl glucopyranosides were detected in trace levels when using ethanol and 2-propanol as hydrogen donors and reaction solvents.