Synergistic Effect of Gold and Hydrophobic Reagents Enhances Ester Formation from Aldehydes via One-Step Oxidative Esterification

Hydrophobic catalysts play a crucial role in promoting oxidative esterification. In such reactions, water, formed as a byproduct, can occupy active sites, thereby reducing catalytic activity. A series of hydrophobic catalysts (Au@M–Si-y, M = ZnO, Ce2O, or Al2O3) was designed for the oxidative esterification of methacrolein. These catalysts utilize metal oxides as supports and integrate trimethylchlorosilane as a hydrophobic group to produce hydrophobic metal oxide supports. The active component, Au, was immobilized on the hydrophobic supports using the deposition–precipitation technique, and then the Au-based hydrophobic catalyst was synthesized. Scanning electron microscopy and Fourier transform infrared spectroscopy analyses confirmed the grafting of Au and hydrophobic groups onto the metal oxide surface. The as-synthesized hydrophobic catalyst exhibited a water droplet contact angle of 71.6°, higher than that of the hydrophobic-group-free Au-based metal oxide catalyst. X-ray photoelectron spectroscopy revealed that the gold was mainly in a metallic state, facilitating oxygen activation and thus accelerating the reaction. For Au@ZnO–Si-y, the catalytic activity first increased and then decreased with increasing TMCS loading, indicating that the moderate degree of hydrophobicity of the catalyst promotes the reaction. Based on reaction kinetics, the activation energy was 29.42 kJ mol–1.