Production of polyols from sugars in biorenewable alcohol: Selectivity of hydrogen transfer on metal catalysts

Abstract

Non-H2 hydrogen donors, such as alcohols, should be explored for their potential to enhance flexibility in hydrogenation reactions, particularly in contexts where alcohols can be fermentatively produced from biowaste. This study investigates the aqueous-phase catalytic transfer hydrogenation (CTH) of glucose to sorbitol, revealing the mechanism of the activation of isopropanol (IPA) as a liquid H donor over carbon-supported Ru (Ru/C) catalyst. The surface characterization of catalysts and mass spectra of sorbitol obtained in deuterated solvents suggest that glucose is hydrogenated through the transfer of two equivalent chemisorbed H atoms (H*) on metallic Ru0, instead of the Meerwein–Ponndorf–Verley mechanism. The co-generation of H2 gas hints that H* atoms are susceptible to combination among themselves instead of transferring to glucose. An expanded kinetic model including H-H combination as a parallel step to sorbitol formation can qualitatively predict CTH in varying reactant concentrations. The competition between H2 formation and substrate reduction could vary with the Ru metal nanostructure and substrate energetics: Ru/Al2O3 can largely suppress H2 in glucose CTH, whereas the H selectivity in maltose-to-maltitol CTH is double that in glucose-to-sorbitol. This study demonstrates a renewable H source that serves as a safer alternative to pressurized H2 used in conventional hydrogenation. More importantly, it informs the rational design of catalysts by pining down the significance of H-H combination, which should be moderated to improve H atom economy in chemical upgrading using non-H2 reducing agents.