Oxidative esterification of ethylene glycol in methanol to form methyl glycolate over supported Au catalysts

Abstract

Au/ZnO and Au/Al₂O₃ catalysts with various mean Au particle diameters (2.0–7.4 nm) were prepared by the deposition of pre-formed Au colloids. These catalysts were evaluated in the oxidative esterification of ethylene glycol to methyl glycolate. The results show that the catalytic activity per surface Au atom is independent of Au particle diameter in the range of 3–7.4 nm, whereas smaller Au particles (~2.0 nm) show an inferior activity. This behavior was observed on both Au/ZnO and Au/Al₂O₃ catalysts. This observed correlation between activity and Au particle diameter confirms the assertion that only exposed atoms are catalytically active. We prepared gold nanoparticles with a uniform mean diameter of ~3 nm loaded on various supports, i.e. ZnO, Al₂O₃, SiO₂, TiO₂ and CeO₂. Among these five catalysts, Au/ZnO gave the best catalytic activity in the reaction followed by Au/Al₂O₃. Au/SiO₂, Au/TiO₂ and Au/CeO₂ gave significantly lower activities. The variation in catalytic behavior of these gold catalysts on different supports originates from differences in the anchoring of the supported Au particles, the gold oxidation state, the gold–support interaction, and the acidity of the support.