The mechanism for residual solid (char) formation in lignin depolymerization is proposed. The results demonstrate that the lignin-char is composed of a multiplicate layer of alternate lignin and coke. During the lignin conversion, residual solid is generated from the G and S units in phenolic oligomer via the pathways of alkyl aryl ether rearrangement, α-hydroxyl coupling reaction, and aldol condensation.

Abstract

Current techniques of lignin conversion are challenged by the low carbon utilization efficiency resulting from the severe generation of residual solid (char). Therefore, a better understanding of pathway for char formation is significant and highly desired for lignin valorization. In this work, we propose a fundamental mechanistic insight into char formation in lignin depolymerization, using hydrothermal decomposition as model reaction. The results demonstrate that the char featuring a multi-layer construction of coke and oligomer contains mainly G units, primarily generated from native G-lignin and demethoxylation of S-lignin. Instead, H-lignin contributes to the formation of volatile monophenols. Furthermore, new methylene bridges form between the benzene rings in lignin, which consequently results in the formation of recalcitrant char. Based on these observations, a plausible mechanism for char formation is proposed and verified by the density functional theory calculation.