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/Zn2+ 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 (OV) promote the generation of activated oxygen species (O delta-), which facilitate the cleavage of C-H bonds in alkoxide intermediate methylene (-CH2) 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.