Metal-free oxidized carbon nitride for efficient sunlight-driven photocleavage of lignin β-O-4 bond

Abstract

Benefitting from mild reaction conditions and good reusability, photocatalysis has industrial potential for high-value utilization of lignin waste. Commonly used metal sulfide photocatalysts present the risk of contamination of aromatic products by heavy metal ions due to photocorrosion. Here, a metal-free oxidized g-C3N4 photocatalyst was developed to selectively cleave the β-O-4 bond of lignin. Simple high-temperature oxidation treatment reduced carbon nitride size and scavenged the terminal NH2 groups, resulting in higher oxygen adsorption capacity and valence band potential. In the photoelectric effect, oxidized g-C3N4 exhibited lower carrier migration resistance and a longer photogenerated electron lifetime. The high oxygen adsorption and specific surface area led to a 160 % increase in superoxide radicals. Two routes for the photocatalytic cleavage β-O-4 bond in g-C3N4 are proposed. The catalytic activity of oxidized g-C3N4 is 8.5 times higher than that of untreated g-C3N4, the selective photocleavage reactivity attributed mainly to more OOH radicals and higher valence band potential. This work will provide a green and efficient strategy for the high-value conversion of lignin.