The oxidative esterification of glycerol to methyl glycerate in methanol using gold on oxidic supports: an insight in product selectivity

Abstract

Gold nanoparticles on different oxidic supports (TiO₂, Al₂O₃ and ZnO) have been studied for the oxidation of glycerol in methanol, using molecular oxygen as the oxidizing agent in a batch set-up. The main oxidation products are methyl glycerate and dimethyl mesoxalate in over 95% selectivity at high glycerol conversion, indicating that C–C bond scission occurs at a significantly lower extent compared to glycerol oxidations in water. The product selectivity is a function of the support. Highest selectivity (82% at 72% conversion) to methyl glycerate is observed in the case of Au/TiO₂ as the catalyst. The use of a base is not essential for the glycerol oxidation reaction to occur, although for TiO₂ and Al₂O₃ higher initial activities are found in the presence of sodium methoxide. Au/ZnO gives comparable activity and selectivity both in the presence and absence of a base. Oxidation experiments with reaction intermediates indicate that oxidation of methyl glycerate to higher oxygenates does not occur to a significant extent in methanol. An alternative pathway for the formation of dimethyl mesoxalate involving dihydroxyacetone is proposed.