Visible-Light-Driven Cleavage of C-O Linkage for Lignin Valorization to Functionalized Aromatics

Lignin is the most abundant source of renewable aromatics. Catalytic valorization of lignin into functionalized aromatics is attractive but challenging. Photocatalysis is a promising sustainable approach. The strategies for designing well-performing photocatalysts are desired but remain limited. Herein, a facile energy band engineering strategy for promoting the photocatalytic activity of zinc-indium-sulfide (ZnmIn2Sm+3) for cleavage of the lignol beta-O-4 bond under mild conditions was developed. The energy band structure of ZnmIn2Sm+3 could be tuned by controlling the atomic ratio of Zn/In. It was found that Zn4In2S7 performed best for cleavage of the beta-O-4 bond under visible-light irradiation, owing to its appropriate energy band structure for offering adequate visible-light absorption and suitable redox capability. Functionalized aromatic monomers with near 18.4 wt % yield could be obtained from organosolv birch lignin. Mechanistic studies revealed that the beta-O-4 bond was efficiently cleaved mainly through a one-step redox-neutral pathway via a C-alpha radical intermediate. The thiol groups on the surface of Zn4In2S7 played a key role in cleavage of the beta-O-4 bond.