Controllable depolymerization of lignin using carbocatalyst graphene oxide under mild conditions

• Published in: Fuel (Guildford) Vol. 267; no. C; p. 117100
• Main Authors: Zeng, Jijiao, Tong, Zhaohui, Bao, Hanxi, Chen, Nusheng, Wang, Fei, Wang, Yigui, Xiao, Dequan
• Format: Journal Article
• Language: English
• Published: Kidlington Elsevier Ltd 01.05.2020; Elsevier BV; Elsevier
• Subjects: Aromatic compounds; Carbocatalyst; Catalysis; Cations; Density functional theory; Depolymerization; Dimers; Energy diagram; Enol ether; Graphene; Graphene oxide; Guaiacol; Hydroxyl groups; Lignin; Lignin depolymerization; Monomers; Oligomers; Oxidizing agents; Phenolic compounds; Phenols; Quantum chemistry; Ring opening; Selectivity; Carbocatalyst; Graphene oxide; Enol ether; Energy diagram; Lignin depolymerization
• ISSN: 0016-2361; 1873-7153
• DOI: 10.1016/j.fuel.2020.117100

[Display omitted] •Carbocatalyst graphene oxide depolymerizes lignin without an additional oxidizer under mild conditions.•Depolymerization is catalyzed via an oxidative mechanism through an enol-ether intermediate.•Phenolic OH prohibits ring open reaction from the enol ether intermediate.•Carbocatalyst graphene oxide is recyclable. Many efforts have been devoted to depolymerizing lignin to high-value aromatic monomers. However, a controllable and complete lignin inter-unit bond cleavage remains difficult because of the randomness in lignin structure and low selectivity in catalysis. Herein, we demonstrate that a sole carbocatalyst graphene oxide (GO) can selectively oxidize aryl- and alkyl-hydroxyl groups of lignin without the presence of any additional oxidizers, leading to lignin degradation under mild conditions. The results from lignin dimers, lignin oligomers, and organosolv lignin indicate that the depolymerization is catalyzed via an oxidative mechanism through an enol ether intermediate, a stable intermediate that has been chemically isolated here. Quantum chemistry calculations at the density functional theory level indicate that the phenolic –OH group in lignin facilitates the formation of radical cations and thus prohibits ring opening reactions, leading to the complete conversion of lignin dimers to guaiacol through the enol ether intermediate.