Pyrolysis Mechanism of Metal-Ion-Exchanged Lignite: A Combined Reactive Force Field and Density Functional Theory Study

摘要

Three-dimensional structural models for lignite and metal-ion-exchanged lignite were constructed to investigate the impact of added metal species on their pyrolysis. The Wender model was used as the structural unit of the two models, and Ca(OH)_2 was used as the added metal species. Reactive force field molecular dynamics was employed to simulate the pyrolysis of the two models at 1000-2000 K over a period of 300 ps. Subsequently, cleavage pathways in the pyrolysis of the two models were determined. The characteristics observed in the simulation agree well with the known characteristics of the lignite structure. We found that the initial reactions in the pyrolysis of the two models are mainly decarboxylation and cleavage of the bridged C-O bond. These reactions were further confirmed by density functional theory calculations. Upon addition of Ca(OH)_2 to lignite, the carboxyl and phenolic hydroxyl could be deprotonated. Added metal species changed the pathways for both reactions from homolytic cleavage to heterolytic cleavage and decreased the bond dissociation energy and the energy of products, thereby accelerating lignite pyrolysis.