Fabrication of S-Scheme g-C3N4/Zn4In2S7 Heterojunction Photocatalyst for Enhancing Selective Cleavage of β-O-4 Bond in Lignin Model Compounds and Lignin

The photocatalytic cleavage of the C–O bonds between the β-O-4 structure in natural lignin is still very challenging. Regrettably, the efficiency and selectivity of the photocatalysts for lignin valorization need to be further improved due to the fast recombination of the photogenerated carriers and unsuitable redox potential. To address these challenges, an efficient approach for promoting the one-step Cβ–O bond cleavage in β-O-4 lignin model compounds and lignin by using a g-C3N4/Zn4In2S7 (CN/ZIS) photocatalyst with a step-scheme (S-scheme) heterojunction was employed under visible-light illumination. The construction of the S-scheme CN/ZIS heterojunction was essential to efficiently separate the photoexcited carriers and then improve their redox capability. Due to its ingenious design, the lignin β-O-4 model compound was completely converted under mild conditions with 99% C–O bond cleavage selectivity, which affords high selectivity of phenol (93.4%) and acetophenone (75.2%). The mechanistic studies and density functional theory calculation demonstrated that the efficient cleavage of Cβ–O bonds mainly involved a photoexcited electron–hole-coupled redox mechanism. After optimizing the reaction conditions, real lignin samples were applied to this system and successfully depolymerized. This study elucidated the high-performance charge transfer mechanism of heterogeneous interfaces and provided an effective strategy to produce aromatic resources by utilizing both lignin and photoenergy.