Synergistic selective photocleavage of lignin C–C bonds by triazine-rich tubular carbon nitride and high-temperature oxidation treatment

Abstract

The key issue in lignin conversion depends on cutting the β-O-4 linkage bonds between units while keeping the aromatic ring intact. Triazine-heptazine homopolymers of carbon nitride are more prone to break the Cβ-H bond in the β-O-4 bond and generate the critical Cβ radical compared to the triazine or heptazine structure. Triazine-rich g-C3N4 was successfully prepared by ball milling-thermal polycondensation of melamine and cyanuric acid, which has the advantage of low cost and no desalting compared to the conventional eutectic salt method. The triazine-rich g-C3N4 has higher photocurrent density, lower photogenerated electron-transfer resistance, and less electron-hole recombination. Combined with high-temperature oxidation treatment, g-C3N4-O-Q-A5 has an excellent selective photocleavage efficiency of lignin β-O-4 bonds, which is 340 % higher than that of untreated g-C3N4. This paper reveals that the localization of holes in the photocleavage β-O-4 bond reaction, and the amount of superoxide radical generation are the main factors limiting the photocleavage efficiency.