Construction of Lignin‐Assisted Nitrogen Defective Engineering Graphitic Carbon Nitride for HMF Selective Conversion to Maleic Acid Through Photo‐Thermal Processes

Abstract Design engineering for the preparation of green and inexpensive catalysts assisted by lignocellulosic biomass or biomass‐based derivatives has garnered significant attention. Herein, alkali lignin (AL) is utilized as a carbon precursor to prepare porous coral‐like g‐C ₃ N ₄ (x% PLCN) with nitrogen defects and biochar persistent free radicals (biochar‐PFRs). This catalyst is applied for photo‐thermal catalysis of HMF to produce maleic acid (MA). The optimal yield of MA (50.43%) is achieved using 1.5% PLCN in a GVL/FAc (1:1, V/V) solution at 90 °C for 4 h under visible light irradiation. Results show alkali lignin plays a key role in forming biochar components and introducing nitrogen defects into the catalysts, which significantly boosts H ₂ O ₂ generation (156.68 µmol L ⁻¹ h ⁻¹ ). Under photocatalytic conditions, biochar‐PFRs induce the generation of ·OH through a metal‐free Fenton‐like reaction and provide active sites for the recombination of ·OH and ·O ₂ ⁻ to form ¹ O ₂ . The presence of ¹ O ₂ greatly enhances the selective generation of MA. Additionally, the increased Lewis acidity of x% PLCN activates the C─C bond in HMF under thermal catalytic conditions, further facilitating HMF conversion. This work presents a new approach for biomass‐based material refinement through thermal‐photo catalysis.