Transformation of Glucose into Sorbitol on Raney Nickel Catalysts in the Absence of Molecular Hydrogen: Sugar Disproportionation vs Catalytic Hydrogen Transfer
Raney nickel catalysts have been tested in the transformation of glucose into sorbitol through a hydrogen transfer pathway in the presence of short chain alcohols. Comparison between different sacrificing alcohols evidenced that catalytic hydrogen transfer (CHT) was only possible from ethanol under the tested neutral conditions. Catalytic tests showed that together with CHT route, sorbitol was also produced by means of sugar disproportionation, with the simultaneous production of gluconolactone, which takes place easily in the presence of the Raney Ni catalysts. Studies on the influence of the catalyst loading on the production of sorbitol revealed the existence of a catalyst activation step, attributed to the generation of metal-hydride species, the truly catalytic sites for hydrogenation. However, a catalyst deactivation phenomenon was detected as well. In this case, TGA and FTIR analysis allowed ascribing the adsorption of organic species, coming from the oxidation of glucose (such as gluconic acid), onto the catalyst surface, to the most plausible cause for the deactivation of the catalyst. Catalyst recycling tests evidenced the deactivation occurred mainly during the first use of the Raney Ni catalyst.
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