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 β-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 β-O-4 bond in lignin is cleaved by an electron–hole coupled photoredox mechanism based on a Cα radical intermediate, in which both photogenerated electrons and holes participate in the reaction. Lignin-first approaches, which prioritize lignin upgrade over cellulose, can open the way to full biomass valorization, but are still hampered by the need of harsh reaction conditions and difficulties in catalyst recovery. Now, a photocatalytic strategy based on the use of cadmium sulfide quantum dots is reported that overcomes these limitations.