Oxidative esterification of 1,2-propanediol to methyl lactate over Cu modified Au/γ-Al2O3 catalysts in an alkali-free system

Abstract

Lactic acid is an important organic acid with wide applications, and preparation of lactic acid from biomass-derived 1,2-propanediol was considered a green reaction route. In this paper, a Cu modified Au/γ-Al₂O₃ catalyst was designed for the oxidative esterification of 1,2-propanediol to methyl lactate in an alkali-free methanol system. The structural properties of the Au–Cu/γ-Al₂O₃ catalyst were characterized by various characterization methods, and interaction between Au nanoparticles and Cu species was discovered. Moreover, the catalytic tests showed that the Cu modified Au/γ-Al₂O₃ catalysts exhibited higher methyl lactate selectivity than the monometallic Au/γ-Al₂O₃ catalyst. It was revealed that the synergistic effect between the Au nanoparticles and Cu species could promote the oxidation of primary hydroxyl groups of 1,2-propanediol to produce methyl lactate and reduce the generation of by-products. In addition, a reasonable reaction pathway for the oxidative esterification of 1,2-propanediol was proposed based on the comparative experiments. These findings may provide favorable guidance for design of Au based nanocatalysts for the oxidative esterification of alcohols to esters and carboxylic acids.