Sunlight-driven superoxide radicals generation from double-modified carbon nitride for efficient lignin β-O-4 bonds photocleavage

Lignin has become the largest renewable aromatic resource, and photocatalytic breaking of lignin linkage bonds has emerged as a promising green oxidation technology initiated by solar energy at room temperature. Wherein, carbon nitride (g-C3N4) photo-cracking of lignin has the advantage of stability and reusability (no photo-corrosion), but is inefficient compared to conventional metal sulfides. In this work, g-C3N4 was dual-modified with Cl doping and high-temperature oxidation, to maximum retain the π-π conjugation that can absorb the benzene ring structure in lignin. The inhibitor experiments and product composition analysis indicated that superoxide radicals acted as the most crucial active species, which was 4 times higher than that of g-C3N4. The attachment sites of doped Cl on carbon nitride were calculated by comparing the density of states with the actual band gap. The as-fabricated g-C3N4-Cl-A5 photocatalyst showed a longer carrier lifetime and higher oxygen adsorption capacity and demonstrated excellent benzaldehyde production (1.74 mg h−1) from the lignin model under sunlight. The work provides insights into the enhanced superoxide radicals generation via photocatalysis and guidance for the selective breaking of lignin β-O-4 bonds.