Photocatalytic hydrogenolysis of C–O bonds in lignin model compounds and lignin bio-oil over Ru-supported holey g-C3N4 exfoliated nanosheets

Abstract

Lignin valorization into fuel-range hydrocarbons and value-added chemicals remains hindered by the intrinsic complexity of the ether linkage and strong C–O bond in lignin. Among various ether linkages, the 4–O–5 bond presents a major challenge due to its high bond dissociation energy. Herein, a Ru-decorated holey carbon nitride (Ru/ECNHS) photocatalyst is synthesized by the double exfoliation process, resulting in excellent efficiency for the selective hydrogenolysis of the C-O bond in aryl ethers (lignin model compounds), model compounds bearing typical lignin functionalities, including –OH and –CH3 substituents, and native lignin under visible light photocatalytic reaction. In situ exfoliation of bulk carbon nitride (BCN) results in enhanced charge separation and prolonged lifetime of photogenerated charge carriers, as revealed by optoelectronic characterizations, thereby improving photocatalytic performance. The Ru NPs facilitate the activation of H2, which subsequently participates in the hydrogenolysis of 4-O-5 (and α-O-4 and β-O-4) to produce aromatics and cyclic hydrocarbons with 90-96 % yield. Mechanistic insights elucidated through various control experiments and radical-scavenging studies confirmed the role of photogenerated charge carriers in the hydrogenolysis pathway. Moreover, the photocatalyst also transforms simulated bio-oil and native lignin into arenes and hydrocarbons. Green chemistry metrics further confirm the high atom efficiency and low environmental impact of the present catalytic process. The study highlights the potential of Ru/ECNHS as a sustainable, reusable, and efficient photocatalyst for lignin valorization and C–O bond cleavage under visible-light conditions.