Oxygen vacancies-promoted oxidative esterification of ethylene glycol to methyl glycolate over Au/ZnO catalyst

Abstract

Au/ZnO-x with different morphologies were prepared by controlling the amount of (NH₄)₂CO₃ (x) using deposition precipitation method and applied for the one-step oxidative esterification of methanol and ethylene glycol (EG) to methyl glycolate (MG). The catalytic performance of Au/ZnO-x firstly increases and then decreases with the increase of the amount of (NH₄)₂CO₃, and the maximum is achieved in Au/ZnO-4, with EG conversion of 89.6 % and 96.4 % selectivity to MG, significantly higher than the reported Au-based catalysts. The superior catalytic behavior is mainly derived from the most oxygen vacancy concentration (V_o⁺) accompanied with the highest I₁₀₀/I₁₀₁ ratio at moderate (NH₄)₂CO₃ dosage, promoting the adsorption and dissociation of O₂ molecular. Specifically, different preferential growth ratio between the (100) and (101) crystal plane accounts for Au/ZnO-2, Au/ZnO-4, and Au/ZnO-6 with lamellar stacking morphology, orderly layer structure, and irregular clusters, respectively. Significantly, it was found that there was a linear positive correlation between the V_o⁺ and the EG conversion. Innovatively, the mechanism of the oxygen vacancy-catalyzed oxidative esterification of EG was proposed, which provided a new idea for the design of efficient catalysts for the reaction.