Metal‐Free Photocatalysis for Oxidation of 5‐Hydroxymethylfurfural to 2,5‐Furandicarboxylic Acid Over O‐Doped C ₃ N ₅ With N Vacancies

Selective oxidation of 5‐hydroxymethylfurfural (HMF) to 2,5‐furandicarboxylic acid (FDCA) under mild conditions represents a sustainable approach for converting biomass into high‐value derivatives. In this work, we report a metal‐free photocatalyst, O‐doped C 3 N 5 nanosheets with N vacancies (C 3 N 5‐ x ‐O y ), for visible‐light‐driven oxidation of HMF to FDCA. Under 415 nm LED irradiation, 1 bar O 2 , and 45 °C in aqueous medium, this catalyst achieves 96% FDCA yield and retains stable performance over eight cycles. Mechanistic studies reveal that photogenerated holes are the primary species driving the conversion of the intermediate HMFCA to FDCA, while hydroxyl radicals and singlet oxygen contribute to the initial oxidation step from HMF to HMFCA. The dual defects enhance light absorption, carrier separation, and O 2 activation. The catalyst also exhibits broad substrate compatibility, accommodating 20 aromatic aldehydes. This work offers an efficient and sustainable route for biomass valorization under metal‐free and mild photocatalytic conditions.