Amorphous Carbon-Rich Carbon-Nitride-Based Photocatalyst for Selective C−C Bond Cleavage in Lignin β-O-4 Linkages

In this study, an amorphous carbon-rich carbon nitride photocatalyst was prepared by nitric acid treatment of pyrolyzed melamine followed by nitrogen annealing. The resulting semiconductor material exhibited an up-shifted valence band that enhances the oxidizing power of photogenerated holes, enabling selective C α −C β bond cleavage of 2-phenoxy-1-phenylethanol (2-POE). Using 1.5 g L −1 of the optimized photocatalyst in acetonitrile, 2-POE underwent first-order conversion with a reaction rate of 2.55 h −1 under a 9 W blue LED light source. Product analysis showed the formation of benzaldehyde as the major product, confirming C−C bond scission with 93.1% selectivity. In addition, ring-opening reactions were observed, producing fumaric and maleic acids as the by-products. Photocatalytic depolymerization of organosolv lignin further demonstrated ring opening of the β-O-4 linking motifs, forming dicarboxylic acids analogous to those generated from 2-POE. These demonstrate a metal-free photocatalytic platform capable of promoting β-O-4 bond cleavage in lignin model compounds and inducing photocatalytic structural modification of organosolv lignin, accompanied by the formation of low-molecular-weight aliphatic dicarboxylic acids.