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, respectively. The catalytic activities results revealed that the Au/ZnO-DP catalyst was far superior than the other three catalysts, achieving an EG conversion rate of 90.4% and a selectivity towards MG of 93.8%. Catalysts were systematically characterized by N2 adsorption-desorption, XRD, TEM, XPS, H2-TPR, EPR, O2-TPD and CO2-TPD. The results showed that the large number of Au-ZnO interfaces facilitated the adsorption and activation of O2. Moreover, it is found that the basic sites promoted the cleavage of O-H bonds in the EG molecules, and the acidic sites are responsible for the selectivity of MG. This work offers a simple strategy for design of gold catalysts in EG oxidative esterification.