Enhanced Photocatalytic Valorization of Lignin Model Compounds via Nanoarchitectonic Engineering of NH2-MIL-125(Ti)/Cd0.6Zn0.4S-SH Heterojunction

Photocatalytic lignin depolymerization represents a promising route for the sustainable production of value-added chemical feedstocks and high-quality fuel alternatives. However, challenges still exist in charge separation and promoting the interaction between the substrate and the catalyst. In this study, we designed an NH2-MIL-125(Ti)/Cd0.6Zn0.4S-SH (NM/Cd0.6Zn0.4S-SH) heterojunction photocatalyst for efficient cleavage of the Cβ-O bond in lignin under mild conditions. Surface modification of Cd0.6Zn0.4S-SH with sulfhydryl (-SH) groups significantly improved photocatalyst dispersibility and facilitated interaction between the photocatalyst and lignin substrate. Meanwhile, the construction of type-II heterojunction at the NM/Cd0.6Zn0.4S-SH interface enhanced the separation and transfer of photogenerated charge. Under optimized reaction conditions, the NM/Cd0.6Zn0.4S-SH photocatalysts achieved superior depolymerization performance, with approximately 100% conversion of PP-ol within 40 min while achieving 96% phenol (Phol) and 92% acetophenone (AP) as primary products. Control experiments combined with density functional theory (DFT) calculations revealed that the synergistic interaction between surface modification and optimized interfacial charge transfer pathways enables efficient cleavage of the Cβ-O bonds. Mechanistic investigation established that the reaction is initiated by Cα-H oxidation to the Cα• radical intermediate, culminating in the reductive cleavage of the Cβ-O bond to generate aromatic monomers. Collectively, this study achieved efficient and selective cleavage of lignin Cβ-O bonds, providing new insights into photocatalyst design for lignin photocatalytic depolymerization processes.