Photocatalytic Lignin Depolymerization and Cross-Coupling With Alcohols to Produce Unsymmetric Aromatic Diols

Conversion of lignin into chemical products typically involves two separate steps, such as depolymerization and subsequent upgrading of monomers, which often require different catalytic systems or varying reaction conditions. Notably, the direct one-pot conversion of lignin into valuable compounds is crucial for advancing biomass utilization, aiming to streamline production by combining depolymerization and functionalization steps to maximize process-, atom-, and cost-efficiency. Here we report the photocatalytic approach for synchronized depolymerization and C-C coupling processes for the direct conversion of lignin and its derivatives into aromatic diols using CdZnS (Cd_0.3Zn_0.7S) catalyst. By coupling hole-driven radical generation with electron-driven regeneration of overoxidized benzylic intermediates, this strategy overcomes the intrinsic preference for homocoupling and enables selective cross-coupling between benzylic and aliphatic alcohol derivatives. This CdZnS-based photocatalytic process enables simultaneous hydrogen transfer, hydrogenolysis and cross-coupling of various lignin model compounds with alcohols to obtain unsymmetrical aromatic diols. In the case of real lignin, this depolymerization-cross-coupling protocol gives corresponding aromatic diols up to 10 wt% monomer yield. The resulting diol products are directly applicable to the synthesis of thermoplastic polyurethanes (TPUs) as well as fused polycyclic frameworks, functional materials, and bioactive molecules.