Catalytic valorization of glycerol in the absence of external hydrogen: Effect of the Cu/ZrO2 catalyst mass and solvent

Abstract

The glycerol valorization by heterogeneous catalysis to produce high-value-added chemicals is mainly carried out in the presence of high pressures of molecular hydrogen, a highly flammable gas whose main production in our days depends on fossil fuels. Therefore, an attractive alternative to explore is hydrogen in-situ generation in the catalytic process. Herein, we investigated the liquid-phase catalytic valorization of 80% wt. glycerol in different solvents such as water, 2-propanol, and acetone using a Cu/ZrO2 catalyst in the absence of external hydrogen. Firstly, the mass effect of the Cu/ZrO2 in the glycerol aqueous medium was evaluated finding an optimum mass of 250 mg for glycerol conversion. At these conditions, an initial reaction rate value of 2.63 × 10−3 molgly·gcat−1·min−1 was observed while the main products were hydroxyacetone (HA) and 1,2-propanediol (1,2-PDO). The use of 2-propanol as solvent and hydrogen donor molecule in the glycerol conversion induced the hydrogenation of hydroxyacetone towards 1,2-PDO. The ratio between the selectivity of 1,2-PDO/HA was found to be higher than that observed for the catalytic experiment with water. For the conversion of glycerol in the presence of acetone as a solvent it was noted the occurrence of acetalization reaction of glycerol with acetone and the main product observed was 4-hydroxymethyl-2,2-dimethyl-1,3-dioxalane or solketal. Density functional theory (DFT) calculations were included to gain insight into the reaction mechanisms of the hydroxyacetone hydrogenation to the formation of 1,2-PDO on Cu/ZrO2 catalyst. Four possible pathways were considered. The elementary steps include several intramolecular hydrogen atom transfer steps and two hydrogen addition steps. The hydrogen sources would come from adsorbed hydroxyl species of the aqueous reaction medium on ZrO2 support.