Transition From Lattice Oxygen to Radical-Mediated Oxidation in Ammonium-Intercalated Birnessite Catalysts for Selective Valorization of Biomass to Produce Formic Acid

The valorization of biomass is crucial for sustainable chemical production, with formic acid emerging as a valuable platform chemical for hydrogen storage and green synthesis. This study addresses the challenge of insoluble bimass oxidation by pioneering ammonium-intercalated birnessite (NH₄-Bir) as a paradigm-shifting catalyst for selective formic acid production via radical-mediated oxidation. The engineered Mn³⁺-oxygen vacancy dual-active sites enable control over reactive oxygen species (ROS), favoring superoxide radical generation over hydroxyl radical pathways. Unlike lattice oxygen-dependent catalytic systems, NH₄-Bir activates molecular oxygen via single-electron transfer at Mn³⁺-V_O interfaces, driving selective bond cleavage and oxidation in diverse biomass substrates, achieving up to 66% formic acid yield. The hierarchical mesostructure facilitates rapid mass transfer and hexagonal symmetry structure ensures exceptional durability. The surface-engineered abundance of Lewis acid sites and diminished Brønsted acid sites partially inhibit the formation of •OH and overoxidation. The high proportion and electron-rich Mn³⁺-V_O sites endow the NH₄-Bir-dominated radical catalytic system with exceptional oxidation capability towards insoluble biomass, significantly surpassing reactions dominated by lattice oxygen. This work suggests the potential of ROS in the selective oxidation of biomass to formic acid, providing new insights into the oxidation of insoluble biomass by heterogeneous catalysts.