Oxidative esterification of ethylene glycol in methanol to methyl glycolate over Au/ZnO catalysts: effect of preparation methods†

The one-step oxidative esterification of ethylene glycol (EG) is an effective and promising route for methyl glycolate (MG) synthesis, during which the development of efficient catalysts is critically crucial. Herein, Au/ZnO catalysts were prepared using impregnation (IM), colloidal-deposition (CD), deposition–precipitation (DP) and co-precipitation (CP) methods. The results of catalytic activity revealed that the Au/ZnO-DP catalyst was far superior to the other three catalysts, achieving an EG conversion rate of 90.4% and a selectivity towards MG of 93.8%. The catalysts were systematically characterized by N₂ adsorption–desorption, XRD, TEM, XPS, H₂-TPR, EPR, O₂-TPD and CO₂-TPD. The results showed that a large number of Au–ZnO interfaces facilitated the adsorption and activation of O₂. Moreover, it was found that the basic sites promoted the cleavage of O–H bonds in the EG molecules, and the acidic sites were responsible for the selectivity of MG. This work offers a simple strategy for the design of gold catalysts in EG oxidative esterification.