S-scheme Cs3Bi2Br9/Bi2WO6 heterojunctions for efficient photocatalytic cleavage of C–C bonds in β-1 lignin models

Abstract

The selective cleavage of C–C bonds is a crucial process for depolymerizing lignin to obtain high-value aromatic chemicals. However, achieving this under mild reaction conditions remains a formidable challenge due to the relatively high bond dissociation energy of C–C bonds. Herein, a lead-free Cs₃Bi₂Br₉/Bi₂WO₆ (CBB/BWO) photocatalyst was successfully constructed via electrostatic self-assembly. Compared to pure CBB and BWO, the resulting CBB/BWO exhibited a well-designed S-scheme heterojunction structure, which enhanced light absorption and the separation of electrons and holes due to the internal electric field between CBB and BWO. The CBB/BWO photocatalyst demonstrated excellent catalytic performance in the selective C_α–C_β bond cleavage of β-1 lignin models with reaction rates 6.8 times and 3.8 times higher than those of the single-component catalysts CBB and BWO, respectively. Various C_α–C_β model compounds were depolymerized into the corresponding aromatic aldehydes and aromatic acid products with yields ranging from 76% to 88%. Mechanistic studies revealed that the formation of a six-membered ring transition state by the C_β radical intermediate and superoxide anion radical played a crucial role in C_α–C_β bond cleavage.