Enhancing oxidation ability of graphitic carbon nitride photocatalysts to promote lignin Cα–Cβ bond cleavage in micellar aqueous media

As the most abundant aromatic resource, lignin is an appreciated biomass material to obtain aromatic high-value chemicals. However, the selective cleavage of lignin Cα-Cβ bonds under mild conditions constitutes a challenge. Herein, a photocatalyst having high oxidation capacity was successfully synthesized by codoping S and Cl atoms into graphite carbon nitride (g-C3N4). The resulting S,Cl/CN-1.5 photocatalyst exhibits enhanced photogenerated electron-hole separation ability and higher valence band potential than g-C3N4. S,Cl/CN-1.5 can efficiently break lignin Cα-Cβ bonds in micellar aqueous medium to produce benzaldehyde and benzyl alcohol as the main products. Mechanism studies show that the photocatalytic cleavage of lignin Cα-Cβ bonds proceeds via single-electron transfer and Cβ radical mechanisms in which hydroxyl radicals and photogenerated holes play an important role. Isotopic experiments show that the O atoms required for the photocatalytic cleavage of lignin Cα-Cβ bonds come from water in the micellar aqueous medium based on the full contact between water and substrate. Although O2 atmosphere is beneficial for the photocatalytic efficiency, O2 is not necessary for the photocatalytic cleavage of lignin Cα-Cβ bonds. This research provides a useful guide for designing efficient photocatalysts to depolymerize lignin into high-value chemicals.