Catalytic depolymerization of lignin by N/S modified bagasse-based hierarchical porous carbon

Depolymerization of lignin, a complex aromatic biopolymer, presents a significant challenge in biomass conversion. While metal-based catalysts have been extensively investigated for lignin oxidative degradation, hierarchical porous carbon catalysts remain relatively underexplored. In this study, we introduce a novel approach employing nitrogen and sulfur co-doped hierarchical porous carbon (NSHPC), synthesized from sugarcane bagasse, calcium chloride (CaCl₂), and thiourea at a mass ratio of 1:2:2. The mixture underwent thermal treatment at 85 °C for 12 h, followed by drying and calcination under nitrogen atmosphere at 800 °C for 2 h. Subsequently, impurities were removed via dilute hydrochloric acid treatment. The resultant NSHPC catalyst effectively depolymerized lignin, specifically cleaving C_α-C_β and C_β-O bonds, at 140 °C for 4 h in methanol under 1 MPa oxygen pressure, achieving a high conversion rate of 98.7 % for lignin model compounds. The yields of key products were as follows: 67.8 % for phenol, 25.9 % for benzoic acid, and 33.2 % for methyl benzoate. Mechanistic investigations revealed that NSHPC facilitates the generation of superoxide anions from oxygen and active hydrogen species from methanol, which are crucial for bond depolymerization. Furthermore, experimental validation using authentic lignin samples corroborated the potential of NSHPC as an efficient catalyst for lignin depolymerization.