Fabrication of Z-scheme CdS/H5PMo10V2O40/g-C3N4 for the photocatalytic depolymerization of lignin into aromatic monomers

As the most abundant resource composed of aromatic groups in nature, lignin harbors significant potential for the production of bio-based fuels and chemicals. In recent years, solar-driven photocatalysis with its mild, low cost and environmental benign, has been employed for lignin depolymerization. Herein, we designed a Z-scheme CdS/H5PMo10V2O40/g-C3N4 (CdS/HPA-2/CN) photocatalyst via hydrothermal method for the photocatalytic depolymerization of lignin. The prepared CdS/HPA-2/CN presents a nanoflower-like morphology with the average diameter of ∼600 nm. With the presence of electron-mediated HPA-2, CdS/HPA-2/CN exhibits the enhanced separation and transfer capacity of photogenerated charge carriers through Z-scheme mechanism. Result shows that the total yield (Y) of vanillin and vanillic acid from lignin depolymerization over optimized CdS/HPA-2/CN can reach to 32.7 mg/g lignin in DMF/formic acid (v/v = 40/10) for 4 h. Reusability experiment of optimized CdS/HPA-2/CN displays that Y (26.5 mg/g lignin) can still reach to 81.0% of initial one after three cycles. Qualitative analysis of depolymerized products reveals that most components are aromatic compounds with the dominant products of vanillin and vanillic acid. Evidence also shows that photocatalysis promotes the reaction and small molecular products increased significantly after depolymerization. In addition, mechanism experiments and electron spin resonance tests exhibit that h+, ·O2− and ·OH took part in lignin depolymerization together and the effect sequence was ·O2− > ·OH > h+. In this work, as an electron mediator, HPA-2 provides the inspiration for construction of Z-scheme heterojunction, and CdS/HPA-2/CN exhibits enormous potential in lignin depolymerization by photocatalysis.