# Production of Sorbitol via Catalytic Transfer Hydrogenation of Glucose
## 3 Results and Discussion
### Influence of Catalyst Doping Agents  
Unlike for glucose hydrogenation, where extensive literature can be found, studies devoted to ascertaining the influence in CTH of different dopants in the catalytic activity of Raney-type nickel catalysts are not yet reported. Thus, this section is focused on evaluating the influence of typical metal promoters for Raney nickel catalysts, like Fe-Cr and Mo (Table 1), on their ability to promote glucose conversion to sorbitol via CTH from ethanol. Nickel, like other metals from the VIII group, can interact with glucose through the carbonyl group, which is the basis of the catalytic activity of nickel in the hydrogenation of glucose. Fe-Cr and Mo species in Raney nickel catalysts act as Lewis acid sites, providing oxophilicity to the catalyst [15; 22; 35; 36; 37], and they modify the affinity of nickel for the C=O group, necessarily conditioning its activity in hydrogenation or hydrogen transfer reactions. Figure 4 depicts the kinetic profiles obtained in the conversion of glucose to sorbitol, as well as the evolution of side products, performed at 150 \({}^{\circ}\)C, and in the presence of different Raney nickel catalysts (Table 1). The conversion of glucose in the presence of unpromoted Raney nickel catalyst (Figure 4a) reached 64.9% after reacting for 6 h. Sorbitol evolved as the main product, together with small quantities of fructose. Fe-Cr-doped Raney nickel led to an almost identical profile for glucose conversion, maybe slightly faster during the early stages of the reaction (Figure 4b). However, this catalyst provided lower substrate conversion (~60%) than the unpromoted catalyst, probably as a consequence of a deactivation phenomenon, conventionally occurring on Fe-Cr-doped Raney nickel through the leaching of the dopant agents [22]. The achieved sorbitol yield was much lower in the presence of Fe-Cr-doped catalyst than in the unpromoted one and, thus, only 11.5% of the starting glucose was obtained as sorbitol. Interestingly, a major fraction of the evolving sorbitol was obtained during the early stages of the reaction (up to 1 h), resulting in the concentration of this product becoming almost constant during the rest of the reaction. This low amount of sorbitol was accompanied by the evolution of fructose, which was produced after the first hour, reaching yield values around 10%-15%. This poor performance of the Ni-Cr-doped catalyst seems to be related to the leaching of the iron and chromium species and their dissolution in the solvent of the reaction [22]. Iron and chromium act as hydrogenation promoters for nickel, but their lixiviation eliminates their capacity to bind glucose molecules to the surface of the catalyst, although their Lewis acid features remain intact. In this way, the leached, Lewis acid Cr and Fe species do not promote catalytic transfer hydrogenation, but the isomerization of glucose to fructose [38; 39], which seems to become the dominant reaction after the first hour. Fructose is, unlike glucose, a substrate more resistant to undergoing CTH and, thus, only scarce evolution of sorbitol was detected after its production. The different kinetic profiles achieved for the products obtained in the presence of the Fe-Cr-promoted catalyst suggest that the deactivation of this material occurred during the first hour of the catalytic test. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Catalogue \#** & **Ni (wt.\%)** & **Al (wt.\%)** & **Fe (wt.\%)** & **Mo (wt.\%)** & **Cr (wt.\%)** \\ \hline A-5000 & 93.9 & 5.5 & — & — & — \\ A-7063 & 93.3 & 5.8 & — & 1.1 & — \\ A-4000 & 85.6 & 8.6 & 2.3 & — & 2.5 \\ \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of Raney nickel catalyst used in this work. The molybdenum-promoted Raney nickel displayed a completely different catalytic performance to that shown by the Fe-Cr-doped material. In this case, a faster and more selective transformation of glucose into sorbitol was observed (glucose conversion of \(\sim\)78.5% and sorbitol yield of 38.4%).