Photocatalytic valorization of lignin: radical-mediated scission of recalcitrant bonds to aromatics

Abstract

Lignin, a three-dimensional aromatic polymer formed by the cross-linking of arylpropanol units via C–C/C–O bonds, faces the critical scientific challenge of achieving selective cleavage of these bonds to enable its valorization and the directional synthesis of renewable aromatic compounds. Photocatalysis offers a novel strategy for targeted bond dissociation through precise regulation of electron transfer pathways and radical generation modes. This review systematically elucidates the mechanisms underlying photocatalytic C–C and C–O bond cleavage in lignin systems. For C–C bond cleavage, three primary pathways include: generation of oxygen-centered radicals via ligand-to-metal charge transfer (LMCT) or proton-coupled electron transfer (PCET) processes, inducing β–C–C bond cleavage; generation of carbon-centered radicals via hydrogen atom transfer (HAT) or single-electron transfer (SET) processes, followed by C–C bond cleavage via oxygen participation. For C–O bond cleavage, the main pathways are: a stepwise oxidation-reduction mechanism driven by HAT or SET; generation of carbon-centered radicals via HAT or SET, inducing β–C–O bond cleavage; or activation of lignin models or auxiliary reagents via SET to form reactive radicals inducing C–O bond cleavage. These pathways universally rely on photocatalytically generated radicals (e.g., oxygen/carbon-centered radicals), which redistribute electrons and significantly weaken β–C–C and β–C–O bonds. Based on these insights, we propose feasible strategies for efficient native lignin depolymerization through catalyst electronic structure optimization and reaction microenvironment modulation, providing a theoretical framework for the photoelectrocatalytic valorization of lignin resources.