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 mmolDFF gcat -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.

College of Chemical Engineering Nanjing Forestry University Longpan Road 159 Nanjing 210037 China

Department of Chemical and Pharmaceutical Sciences Center for Energy Environment and Transport Giacomo Ciamician ICCOM CNR Trieste Research Unit and INSTM Trieste Research Unit University of Trieste Trieste 34127 Italy

Institute of Sustainability for Chemicals Energy and Environment 1 Pesek Road Jurong Island Singapore 627833 Republic of Singapore

Leibniz Institut für Katalyse e 5 Albert Einstein Str 29a Rostock D 18059 Germany

Nanotechnology Centre Centre for Energy and Environmental Technologies VSB Technical University of Ostrava 17 listopadu 2172 15 Poruba Ostrava 708 00 Czech Republic

Regional Centre of Advanced Technologies and Materials Czech Advanced Technology and Research Institute Palacký University Olomouc Šlechtitelů 241 27 Olomouc 783 71 Czech Republic

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