Selective aerobic oxidation of 5‐(hydroxymethyl)furfural over heterogeneous silver‐gold nanoparticle catalysts

Abstract

Bimetallic silver‐gold alloy nanoparticles on zirconia with varying Ag/Au ratios were designed by a rational approach and tested as catalysts for the selective oxidation of the promising biomass platform molecule 5‐(hydroxymethyl)furfural (HMF). For this purpose, colloidal Ag$_{x}$Au$_{10-x}$ particles with molar compositions x=1/3/5/7/9 were prepared by laser ablation in liquids, a surfactant‐free method for the preparation of highly pure nanoparticles, before adsorption on zirconia. In‐depth characterization of the supported catalysts evidenced alloyed nanoparticles with distinct trends of the surface and bulk composition depending on the overall Ag/Au molar ratio as determined by X‐ray photoelectron spectroscopy (XPS) and X‐ray absorption spectroscopy (XAS), respectively. To uncover the synergistic effect of the Ag/Au ratio, the catalysts were further studied in terms of the catalytic activity and selectivity in HMF oxidation. Either the aldehyde moiety or both functional groups of HMF were selectively oxidized depending on the Ag/Au composition resulting in 5‐hydroxymethyl‐2‐furan‐carboxylic acid (HFCA) or 2,5‐furandicarboxylic acid (FDCA), respectively. Optimization of the reaction conditions allowed the quantitative production of HFCA over most catalysts, also after re‐use. Only gold rich catalysts Ag$_{1}$Au$_{9}$/ZrO$_{2}$ and particularly Ag$_{3}$Au$_{7}$/ZrO$_{2}$ were highly active in FDCA synthesis. While Ag$_{3}$Au$_{7}$/ZrO$_{2}$ deactivated upon re‐use due to sintering, no structural changes were observed for the other catalysts and all catalysts were stable against metal leaching. The present work thus provides fundamental insights into the synergistic effect of Ag and Au in alloyed nanoparticles as active and stable catalysts for the oxidation of HMF