Z-scheme heterojunction g-C3N4/CQD/CdZnS with high redox capability for enhancing visible light-driven photocatalytic depolymerization of lignin into aromatic monomers

Abstract

The photocatalytic depolymerization of lignin is a sustainable strategy to produce high-value aromatic chemicals. The selective and efficient cleavage of β-O-4 bonds under mild conditions remains one of the challenges in lignin valorization. Herein, an efficient and mild strategy was employed for one-step photocatalytic depolymerization of lignin and β-O-4 model compounds over novel Z-scheme heterojunction g-C₃N₄/carbon quantum dot/CdZnS (CN/CQD/CZS) under simulated solar light. The total yield of vanillin and vanillic acid from lignin depolymerization was 36.3 mg g⁻¹ lignin when the volume ratio of SDS-8 (sodium dodecyl sulfate solution with a mass fraction of 0.08%) to HCOOH was 4 : 1 within 2 h. The depolymerization mechanism of lignin was investigated using β-O-4 model compounds. The β-O-4 model compounds were converted into phenolic monomers over optimized CN/CQD/CZS, yielding 84% substrate conversion and ∼100% C–O bond cleavage selectivity. Three extractive natural lignin samples were applied to this system and successfully depolymerized. A combination of experimental results and theoretical calculations illustrated that CQDs played a crucial role in accelerating photogenerated electron transfer at the CN/CZS interface via the Z-scheme electron pathway, which effectively promoted the separation and transfer of photoexcited hole–electron pairs. Mechanistic studies demonstrated that the selective cleavage of the β-O-4 bond mainly involved a photogenerated electron–hole coupled redox mechanism. The construction of the Z-scheme CN/CQD/CZS heterojunction is essential to enhance the kinetics of electron transport and maintain strong redox ability. The approach in this work offers a useful guidance for the construction of Z-scheme heterojunction photocatalysts with a carbon-based electron mediator and the selective breakage of C–O bonds in lignin.