Facet-dependent visible-light cleavage of lignin CC bonds over mixed-phase TiO₂

Lignin, one of the most prevalent organic polymers on Earth, is predominantly produced as a byproduct in the pulp and paper industry and is frequently disposed of as waste, posing substantial environmental challenges. The carbon‑carbon (CC) bonds in lignin are notably stable, complicating catalytic activation and cleavage. This study presents a significant advancement in the visible-light-driven cleavage of lignin's recalcitrant CC bonds (specifically the β-1 type) utilizing mixed-phase TiO₂ (P25) under ambient conditions. In contrast to conventional approaches that emphasize CO bond scission, the P25 catalyst achieves a 93 % conversion rate and 87 % selectivity towards benzaldehyde from 1,2-diphenylethanol without the use of noble metals or complex modifications. Density Functional Theory (DFT) calculations reveal that the rutile {110}/anatase {101} interface lowers the energy barrier for ·O₂^- generation, corroborating experimental selectivity trends. The synergy between these facets facilitates interfacial charge transfer, generating superoxide radicals (·O₂^-) as the dominant reactive species for CC activation. When applied to native lignin, β-1 and β-β bonds have been completely broken, with over 70 % of β-5 bonds also cleaved. In contrast, only 3 % of β-O-4 bonds were broken, validating scalability beyond model compounds. This work provides a solar-driven, metal-free route to valorize lignin waste, addressing both environmental and energy challenges.