Mechanistic insights into the photocatalytic valorization of lignin models via C−O/C−C cleavage or C−C/C−N coupling

Serving as an earth-abundant, aromatic structure-rich but underutilized lignocellulosic component, the effective conversion of lignin into high-value-added chemicals is considered a major countermeasure for an energy-efficient and low-carbon future. Encouraged by the unique reaction pattern induced by photo-generated charge carriers or reactive species, the environmentally benign and low-energy dissipative photocatalytic technology emerges as a marvelous strategy for lignin valorization and has received increasing attention and extensive research. By focusing on the mechanistic perspectives, this review provides a comprehensive summary of the state-of-the-art developments in (1) the photocatalytic depolymerization of lignin models by precisely breaking targeted C−O and/or C−C bonds to obtain aromatic monomers and (2) the upgradation of lignin-derived aromatics via C−C or C−N bonds coupling into high-value-added coupling compounds. We then present the opportunities for photocatalytic conversion of native lignin, as well as the future challenges for achieving a photocatalytic system with profitability and sustainability.