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-C₃N₄ (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 H₂O₂ 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 ·O₂⁻ to form ¹O₂. The presence of ¹O₂ 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.