Mechanism insight into photocatalytic conversion of lignin for valuable chemicals and fuels production: A state-of-the-art review

The depletion of traditional fossil fuel reserves and ever-increasing environmental problems have prompted the demand for the exploration of renewable resources. Lignin is an earth-abundant but underestimated and underutilized renewable energy source, which has the potential to replace fossil fuels for the production of value-added aromatic derivatives. Despite great efforts have been devoted, fully unlocking the potential of lignin is still far from reaching due to its complex structure and recalcitrant nature. Recently, photocatalysis provides a marvelous platform for pursuing versatile lignin transformation and has attracted growing interests from the academic community. The unique photogenerated reactive species as well as the mild reaction conditions enable the selective cleavage of targeted bond in lignin, while other functional groups remain intact. In this critical review, we attempt to provide an overview of the state-of-the-art advances in the principal photocatalytic processes applied for the conversion of lignin using various photocatalysts. This review focuses on the latest investigation of lignin conversion mechanisms with emphasis on selective C–O and C–C bond cleavage. Introduction to lignin structure, technical lignin and lignin model compounds is highlighted. In addition, general mechanisms of photocatalysis are also classified and discussed in detail. We conclude the perspective on photocatalytic transformation of lignin with an eye toward the future challenges in this field.