Reduction of the photogenerated electron transport path by modulating the conduction band region to achieve the highest lignin β–O–4 bond photocleavage efficiency under xenon excitation

Photocracking of lignin β–O–4 bonds is a green approach for the high-value modification of lignin. Graphitic carbon nitride (g-C 3 N 4 ) that can photocleave the β–O–4 bond is presently the only stable, metal-free photocatalyst owing to the residual metal ions generated due to photocorrosion of metal sulfides. Herein, g-C 3 N 4 with a N–C 3 vacancy structure is prepared by etching site–engineering strategy based on the protection of C–N=C structure by chlorine atom. The N–C 3 vacancy structure can reduce the transport path of the photogenerated electrons by tuning the conduction band (CB) region to the edge of the photocatalyst, which impedes the critical complexation of the photogenerated electron–hole pairs and enhances the photoelectric conversion efficiency five-fold. Combined with the unique chemisorption capacity (137.82 kJ/mol) of N–C 3 vacancies for oxygen, the g-C 3 N 4 photocatalysts with N–C 3 vacancy structure can generate 9.6-fold higher superoxide radicals and 13-fold higher lignin β–O–4 bond photocleavage efficiency lignin compared with untreated g-C 3 N 4 . Based on the reaction enthalpy changes and free energies investigated through simulations, a route of lignin C α –C β bonds being oxidized and dehydrogenated twice by photogenerated cavities is proposed for the first time. This work reveals the relation between the properties of g-C 3 N 4 and the photocleavage efficiency of lignin β–O–4 bonds.