Enhanced photocatalysis efficiency of flower-like Ni/ZnIn2S4 coupling with supplying-hydrogen alcohols to catalyze lignin β-O-4 models into aromatic monomers

Abstract

Owing to the fussy depolymerization conditions and the limited photocatalytic efficiency, enhancing lignin valorization into aromatic monomers still remains challenging. Herein, the combination of photocatalyst optimization and solvent effect was reported to efficiently scissor the C_β-O bond in lignin β-O-4 model (2-phenoxy-1-phenylethanol, PP-ol) into ketones and phenols. The doped Ni species in ZnIn₂S₄ acting as electron-trapping sites inhibited the recombination of photogenerated carriers, leading to an enhanced photo-quantum efficiency. During the generation of 2-phenoxy-1-acetophenone (PP-one), the key intermediate of ·C_α radical was formed, which significantly decreased the dissociation energy of C_β-O bond to 66.9 kJ/mol. The competition between hydrogenolysis and dehydrogenation for these ·C_α radicals determined the products distribution of PP-one, phenol and acetophenone. Solvent effect played an important role in the above process. The concentration of atomic hydrogen species was highly increased assisted with EtOH as supplying-hydrogen agent, promoting the hydrogenolysis of C_β-O bonds in ·C_α radicals to aromatic monomers rather than their further dehydrogenation to PP-one. Based on the structure characterization, trapping and control experiments, the photocatalytic mechanism for lignin models depolymerization was proposed. Current work provides a simple and universal method in combination with photocatalyst optimization and solvent effect to accomplish the lignin valorization under solar drive.