Ultra-Dense Supported Ruthenium Oxide Clusters via Directed Ion Exchange for Efficient Valorization of 5-Hydroxymethylfurfural

Abstract

Maximizing the loadings of active centers without aggregation for a supported catalyst is a grand challenge but essential for achieving high gravimetric catalytic activity, especially toward multi-step reactions. The oxidation of 5-hydroxymethylfurfural (HMF), a key biomass-derived platform molecule, into 2,5-furandicarboxylic acid (FDCA), a promising alternative to polyester monomer, is such a multi-step reaction that involves 6 proton and electron transfers. This process often demands strong alkaline environment but also suffers from the alkali-driven polymerization side-reaction. Meanwhile, neutral media ameliorates the polymerization, but lacks efficient catalyst toward deep oxidation. Herein, we devised a strategy of creating ultra-dense supported Ru oxide clusters via directed ion exchange in a Co hydroxyanion (CoHA) support material. Pyrimidine ligands were first incorporated into the CoHA interlayers, and the subsequent evacuation of pyrimidines created porous channels for the directed ion exchange with the built-in anions in CoHA, which allowed the dense and mono-disperse functionalization of RuCl₆²⁻ anions and their resulting Ru oxide clusters. These ultra-dense Ru oxide clusters not only enable high HMF electrooxidation currents under neutral conditions but also create microscopic channels in-between the clusters for the expedited re-adsorption and oxidation of intermediates toward highly oxidized product, such as 5-formyl-2-furoic acid (FFCA) and FDCA. A two-stage HMF oxidation process, consisting of ambient conversion of HMF into FFCA and FFCA oxidation into FDCA under 60 °C, was eventually developed to first achieve a high FDCA yield of 92.1 % under neutral media with significantly reduced polymerization.