Multiphase Hydrogenation of d-Glucosamine Hydrochloride, N-Acetyl-d-Glucosamine, d-Glucose, and d-Maltose over Ru/C with Integrated Catalyst Recovery

A multiphase (MP) system composed of two immiscible liquids, water and isooctane, and commercial 5% Ru/C as a catalyst, both with and without an additional organic liquid (OL: tetrahydrofuran (THF), 2-methyl-THF, methyl isobutyl carbinol, and cyclopentyl methyl ether) was designed and investigated for the chemoselective catalytic hydrogenation of four model examples of biobased sugars and amino/amido-sugars. At 110 °C and 40 bar of H2, d-glucosamine hydrochloride and N-acetyl-d-glucosamine were converted selectively into their corresponding hydrogenated derivatives, 2-amino-d-sorbitol and 2-acetamide-d-sorbitol, respectively, isolated in >99% yields. Both the reagents and the products were converted and formed in the aqueous phase, respectively, while, by tuning the relative proportions of water, isooctane, and the third added liquid (particularly THF), the catalyst (Ru/C) was perfectly segregated in the organic layer, where it could be recycled and reused up to nine times without any loss of activity and selectivity, in a semicontinuous mode. Under such conditions, the reaction was implemented on a gram scale with a productivity up to 0.89 mmol 2-amino-d-sorbitol/(gcat h). The same hydrogenation efficiency and reagent/product/catalyst separation were achieved during the MP reactions of d-glucose and d-maltose. In this case, however, results were independent of the MP composition: at 120 °C and 20-40 bar of H2, using either H2O/isooctane or H2O/isooctane/OL systems, a quantitative conversion of d-glucose and d-maltose was reached with a selectivity up to 78 and >99% toward sorbitol and maltitol, respectively. Ru/C was perfectly separated out of the aqueous phase in both MP mixtures, with a negligible metal leaching, below 0.01 wt %. The multiphase approach for all the tested substrates proved not only to be an original and robust protocol to improve the product isolation and catalyst recycling, but also effective in preventing metal contamination in the synthesis of final derivatives.