Visible light driven g-C₃N₄/TiO₂ photocatalysis for selective delignification of hemp biomass under mild conditions

The inherent recalcitrance of lignocellulosic biomass, primarily attributed to the dense lignin-carbohydrate complex (LCC), restricts the efficient valorization of plant fibers. Conventional alkaline degumming strategies suffer from high energy consumption, severe pollution, and non-selective cellulose degradation. Herein, a mild, alkali-free pretreatment strategy was developed using a visible-light-driven g-C₃N₄/TiO₂ heterojunction to achieve efficient and selective delignification of hemp fibers. Under ambient conditions (30 °C, neutral pH), the system achieved a delignification efficiency of 69.08 % while preserving the cellulose crystallinity (CrI increased to 78.64 %). Crucially, 2D-HSQC NMR and EPR analyses revealed a selective oxidative mechanism: photogenerated superoxide radicals (•O₂^-) acted as specific nucleophiles, preferentially cleaving the labile lignin β-O-4 ether bonds (from 40.9 % to 13.5 %), while leaving the chemically robust C-C linkages and the cellulose backbone largely intact. Consequently, the pretreated fibers exhibited superior mechanical strength (4.13 cN/dtex) compared to traditional alkaline-treated samples (3.56 cN/dtex). This work provides a sustainable, low-severity paradigm for biomass fractionation, offering fundamental insights into the photocatalytic cleavage of lignin inter-unit linkages.