Mechanistic Insights into the Selective Oxidation of 5-(Hydroxymethyl)furfural over Silver-based Catalysts

Silver-catalyzed oxidation of 5-(hydroxymethyl)furfural (HMF) to 5-hydroxymethyl-2-furancarboxylic acid (HFCA) was investigated using Ag/ZrO$_{2}$ and Ag/TiO$_{2}$ catalysts. The reaction proceeded very selectively without formation of the dicarboxylic acid in the presence of air and NaOH as a base. In situ X-ray absorption spectroscopy (XAS) performed systematically under varying reaction conditions in a specially designed cell evidenced that reduced silver particles are the catalytically active species in this reaction. Although an incomplete reduction of Ag/ZrO$_{2}$ and Ag/TiO$_{2}$ was observed after the catalyst preparation even after reduction in hydrogen, silver was reduced to the metallic state as soon as HMF was introduced to the reaction mixture at room temperature and stayed reduced throughout the reaction under conditions optimized for high HFCA yield. The degree of silver reduction and product formation differed for varying reaction conditions, indicating that reduced silver particles, a homogeneous base and oxygen are needed in order to achieve high HFCA yield. Based on the catalytic and spectroscopic experiments, a detailed reaction mechanism is proposed involving a dehydrogenation pathway of an intermediately formed geminal diol in basic aqueous solution.