Solar energy-driven lignin-first approach to full utilization of lignocellulosic biomass under mild conditions

The lignin-first concept offers an opportunity to utilize the entire lignocellulosic biomass efficiently. However, current conversion strategies rely on high-temperature hydrogenolysis by supported metal catalysts, leading to low-functionalized products or difficulty in separation of solid catalyst from cellulose/hemicellulose. Here, we report the fractionation and valorization of lignocellulose via solar energy-driven conversion of native lignin at room temperature. We found that cadmium sulfide quantum dots not only catalyse the cleavage of beta-O-4 bonds in lignin models quantitatively but also are efficient for the conversion of native lignin within biomass into functionalized aromatics under visible light, while cellulose/hemicellulose remain almost intact. Further, the colloidal character of quantum dots enables their facile separation and recycling by a reversible aggregation-colloidization strategy. The beta-O-4 bond in lignin is cleaved by an electron-hole coupled photoredox mechanism based on a C-alpha radical intermediate, in which both photogenerated electrons and holes participate in the reaction.