Redox-Switchable Single-Atom Catalyst Enables Efficient Aqueous Hydroxymethylfurfural Oxidation

The selective aerobic oxidation of biomass-derived 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF) is a pivotal step toward biobased polymers, pharmaceuticals, and fuels. Yet, most high-performance catalysts require noble metals and organic solvents and lose activity in water. Here, we report a robust and recyclable heterogeneous catalyst comprising mixed-valence single-atom iron dimers anchored on nitrogen-doped graphene acid (Fe-NGA), which mimics the powerful oxidation center in nonheme diiron oxidases. Spectroscopic and theoretical studies reveal a redox-flexible Fe²⁺/Fe³⁺ manifold that, under basic aqueous conditions, evolves into a Fe³⁺-Fe⁴⁺ ferryl species capable of highly selective proton-coupled two-electron oxidations. Fe-NGA achieves 97% HMF conversion with 95% DFF selectivity, a turnover frequency of 17.3 h^-1, and a specific productivity of 12.5 mmol_DFF g_cat ^-1 h^-1 in pure water, surpassing state-of-the-art homogeneous and heterogeneous catalysts. The catalyst is stable with very low performance loss for at least six reactions. By merging such functionalities within a stable and reusable heterogeneous framework, Fe-NGA provides a benchmark earth-abundant catalyst for the effective oxidation of renewable feedstocks.