Base-Free Oxidative Esterification of HMF Using Mg–Al Hydrotalcite-Supported Gold Catalyst

Gold-catalyzed oxidative esterification of 5-hydroxymethylfurfural (HMF) under basic conditions is a promising process to produce dimethyl-2,5-furan dicarboxylate (DMFD). However, it has been overlooked that the DMFD readily hydrolyzes in the presence of soluble bases. Herein, we report solid base-supported gold catalysts for the base-free conversion of HMF. The catalysts' basic sites (BSs) were found to facilitate the esterification of aldehydes and oxidation of alcohols to aldehydes. Moreover, the properties of the BSs strongly influenced the catalyst performance, as excessive and insufficient BS strengths resulted in extensive hydrolysis of the DMFD and low catalytic activity, respectively. A larger quantity of BSs is beneficial for complete conversion of the reaction intermediate. The hydrotalcite (HT)-supported catalyst of Au/HT-0.2 possesses a large quantity and moderate strength of BSs, therefore providing an excellent DMFD yield of 94.5%. Our results reveal for the first time the issue of DMFD hydrolysis in gold-catalyzed oxidative esterification of HMF. Replacement of a soluble base by a solid base-supported catalyst to enhance product stability provides insights for similar reactions.