ABSTRACT
Abstract In aqueous media, glycerol (GO), a low-cost renewables-based feedstock, can be catalytically converted under mild conditions to the commodity products propylene glycol (PG), lactic acid (LA) and ethylene glycol (EG). This report chronicles a comparative study of catalysts, solvents, and reaction conditions aimed at optimizing selectivity towards PG and at the same time augmenting our understanding of catalyst-substrate interactions. Two catalysts were evaluated: Ni/Re on carbon was more active and selective than Ru on carbon for GO hydrogenolysis to PG. Partial replacement of water with other hydroxylic solvents-the simple alcoholsincreased the selectivity and conversion in the order ethanol/water < water < isopropanol/water ~ tert-butanol/water, while unmixed solvents yield the following trend: water < ethanol < iso-propanol < tert-butanol, with a three-fold increase in PG yield in tert-butanol relative to water. We interpret these results as evidence of facilitated GO access to the catalyst surface in the presence of larger solvent molecules with a hydrophobic carbon backbone. The reaction is also pH limited because of formation of acids such as lactic and formic, which neutralize the base promoter. We discuss the relationship between solvent structure and PG yield in terms of interactions at the catalyst surface. Introduction Depletion of fossil oil resources and concern about environmental pollution, especially heavy metals and greenhouse gases, have brought biomass into focus as a renewable source of raw materials for large-scale chemicals and energy production (1). The “biomass refinery” of the future will require a powerful toolbox of processes for converting complex plant matter into useful commodity and specialty products.
