A One-Step Synthesis of C6 Sugar Alcohols from Levoglucosan and Disaccharides Using a Ru/CMK-3 Catalyst

Sorbitol is an important commercially available chemical with a broad application range and is typically made by the catalytic hydrogenation of glucose. Here we report a high-yield synthesis of sorbitol from levoglucosan (1,6-anhydro-beta-D-glucopyranose) and cellobiose, two sugars present in pyrolysis liquids, using a mesoporous carbon-supported Ru catalyst (Ru/CMK-3). The hydrogenation reactions were performed in a batch autoclave setup under a hydrogen pressure of 50 bar and temperatures ranging from 120 to 180 degrees C in water. The hydrogenation of levoglucosan gave essentially quantitative yields of sugar alcohols, composed of 96.2 wt % of sorbitol and 3.8 wt % of mannitol (180 degrees C, 5 h). Ru/CMK-3 shows superior catalytic performance compared to a commercial Ru/C catalyst. A reaction pathway involving glucose as an intermediate and subsequent (hydrogenolysis) reactions of the desired sorbitol is proposed. Reactions with glucose and sorbitol were performed to define the reaction pathways and to highlight the differences between Ru/C and Ru/CMK-3. Disaccharides including cellobiose and sucrose were also tested, yielding up to 95 wt % of C6 sugar alcohols at 180 degrees C in 5 h for both substrates. Detailed catalyst characterization studies (N-2 physisorption, TEM, XRD, NH3-TPD, H-2-TPD) revealed that Ru/CMK-3 contains considerable amounts of strong acid sites (NH3-TPD). Catalyst stability was tested by catalyst recycling experiments using levoglucosan in batch. After three successive runs, the rate of the hydrolysis reaction of LG to glucose was about constant, though the subsequent hydrogenation reaction to sorbitol/mannitol was slightly retarded as evidenced from a slight increase in the remaining amounts of glucose at the end of reaction.