Selective transformation of glucose into propylene glycol on Ru/C catalysts combined with ZnO under low hydrogen pressures

Abstract

Recently, biomass resources have garnered significant attention as sustainable and renewable raw materials for the production of chemicals. Propylene glycol (PG) is a valuable chemical product that can be synthesized from biomass. Herein, the selective transformation of glucose into PG was investigated on a carbon-supported 5wt% Ru catalyst (5%-Ru/C) combined with solid acid–base catalysts under low hydrogen pressures. The reaction conditions, namely the amount of ZnO, temperature, and hydrogen pressure, were also evaluated. At a hydrogen pressure of 0.4MPa, a full conversion of glucose and 38% yield of PG were obtained using the ZnO+Ru/C system at 453K for 20h, while the yield of PG on Ru/C alone was only 9.3% under the same conditions. Studies on the reaction mechanism indicated that the transformation of glucose into PG consisted of the isomerization of glucose to fructose, retro-aldol reaction of fructose to triose (dihydroxyacetone and glyceraldehyde), dehydration of glyceraldehyde to pyruvaldehyde, and successive hydrogenation of pyruvaldehyde to PG via hydroxyacetone. The ZnO catalyst promoted both the isomerization and retro-aldol reaction steps, and the Ru catalyst promoted the hydrogenation steps. The retro-aldol reaction, which included the cleavage of C3C4 bond in glucose, enabled the selective transformation of glucose into PG under low hydrogen pressures. These findings provide novel insights into the efficient synthesis of PG from glucose, which could be achieved by combining and optimizing the retro-aldol and hydrogenation steps.