Enhancing oxidation ability of graphitic carbon nitride photocatalysts to promote lignin C-C 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 C-C 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-CN). The resulting S,Cl/CN-1.5 photocatalyst exhibits enhanced photogenerated electron-hole separation ability and higher valence band potential than g-CN. S,Cl/CN-1.5 can efficiently break lignin C-C bonds in micellar aqueous medium to produce benzaldehyde and benzyl alcohol as the main products. Mechanism studies show that the photocatalytic cleavage of lignin C-C bonds proceeds via single-electron transfer and C 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 C-C bonds come from water in the micellar aqueous medium based on the full contact between water and substrate. Although O atmosphere is beneficial for the photocatalytic efficiency, O is not necessary for the photocatalytic cleavage of lignin C-C bonds. This research provides a useful guide for designing efficient photocatalysts to depolymerize lignin into high-value chemicals.