Structural effect and reaction mechanism of Au/ZnO catalyst in the oxidation esterification of ethylene glycol in methanol

A series of ZnO nanostructures with different morphologies were prepared with hydrothermal synthesis method by adjusting the NaOH/Zn 2+ molar ratio. The Au/ZnO catalysts were further synthesized and investigated in the oxidative esterification of ethylene glycol (EG) in methanol to methyl glycolate (MG). As compared with prismatic Au/ZnO(2.0) and fusiform-shaped Au/ZnO(8.0), the flaky-shaped Au/ZnO(3.5) catalyst shows a significantly decreased Au particle size of 3.4 nm as well as superior catalytic performance with the EG conversion of 94.5 % and the selectivity of 91.8 % towards MG. The characterization results imply that the reaction undergoes three successive steps including glycol oxidation (the rate-determining step), nucleophilic addition of glycolaldehyde with methanol and oxidative dehydrogenation of hemiacetal. The small Au nanoparticles and abundant defective surface oxygen species (O V ) promote the generation of activated oxygen species (O δ- ), which facilitate the cleavage of C-H bonds in alkoxide intermediate methylene (–CH 2 ) as well as hemiacetal aldehyde groups (–CHO) and thereby boost the formation of MG. Moreover, base sites adsorbed the alcohol to facilitate the O-H bond cleavage and acid sites adsorbed hemiacetal or intermediate alkoxide to drive the reaction. The strong interaction at the Au-ZnO interface affords the superior catalytic performance of Au/ZnO(3.5) in the oxidative esterification of EG in methanol to MG.