Hydrothermal Redox Driven Phase and Defect Engineering of MnO₂ for Enhanced HMF Oxidation at Ambient Oxygen Pressure

Abstract Efficient oxidation of biomass‐derived 5‐hydroxymethylfurfural (HMF) to 2,5‐furandicarboxylic acid (FDCA) under low temperature and atmospheric pressure using non‐noble metal catalysts is highly desirable yet challenging. This study employs a temperature‐controlled hydrothermal redox strategy to synthesize a series of MnO₂ nanorods (MnO₂‐R) and investigates their catalytic performance in HMF oxidation under ambient oxygen pressure. Results demonstrate that increasing the hydrothermal temperature not only sequentially forms β, α/β, and α phases of MnO₂, but also continuously enriches oxygen vacancy and Lewis acid. The higher‐defect α‐MnO₂‐R, exhibiting a lower Mn─O coordination number, displays a 96.7% FDCA yield and an initial productivity up to 365.5 µmol_FDCA g_cat⁻¹ h⁻¹, which are 1.5 times and 2.7 times higher than that of β‐MnO₂‐R, respectively. DFT calculations and spectroscopic experiments reveal that high‐temperature drives phase transition also intensifies redox reactions, leading to lower Mn coordination numbers, which result in stronger adsorption energy toward C─O bond, faster superoxide radical generation, and C─O bond oxidation. α‐MnO₂‐R exhibits good recyclability and excellent substrate universality toward various aromatic aldehydes/alcohols. This work provides a new perspective on designing efficient oxide catalysts, focusing on coordination/defect reconstruction of metal cations, thus deepening understanding of how to modulate these centers to enhance the catalytic oxidation efficiency.