Depolymerization of lignin model compounds by reactive oxygen species generated from peroxymonosulfate under mild conditions

Directional cleavage the linkages bonds between the lignin units is of great scientific interest although technically challenging. In this work, free radical depolymerization of lignin was investigated by using three model compounds. Peroxymonosulfate (PMS) was activated by N-doped carbon and thermal method, and could generate reactive oxygen species (ROS), including hydroxyl radical (OH∙), sulfate radical anion (SO4∙−), superoxide anion (O2∙−) and singlet oxygen(O12). pH of solution was used to regulate the types of ROS. The results showed that 1) pH value can tune the reaction pathway by regulating the type of ROS; 2) There should be an optimal temperature for maximum selectivity of monomer products in the depolymerization proces of lignin model compounds by ROS; 3) Hydroxyl group and Methoxy group can activate the reactivity of lignin model compounds, which is conductive to the directional generation of monomer products. In addition, the depolymerization mechanism of lignin model compound by PMS was proposed. This work illustrated the reaction characters and the influence factors of the lignin depolymerization by PMS, the results would facilitate the optimization of the lignin valorization process.