Theoretical study on desulfurization mechanisms of a coal-based model compound 2-methylthiophene during pyrolysis under inert and oxidative atmospheres

Density functional theory (DFT) was used to investigate the desulfurization mechanisms of 2-methylthiophene (2-MT) during pyrolysis under inert and oxidative atmospheres. 12 possible reaction pathways were calculated and the feasibility of each reaction was analyzed from the perspective of thermodynamics and kinetics. The calculated results show that a H and S can promote 2-MT pyrolysis under inert atmosphere. The reaction temperatures and the highest energy barriers decrease significantly when one H and S participating in 2-MT pyrolysis process. The reason is that H and S can eliminate aromaticity of 2-MT by anisotropy of the current-induced density (ACID) analysis and reduce structural stability of 2-MT molecule. Pyrolysis desulfurization of 2-MT is thermodynamically more favorable under oxidative atmosphere than inert atmosphere. Furthermore, for COS formation, the desulfurization mechanism is more kinetically favorable initiated by alkyl side chain than by thiophene ring. For SO2 formation, it is more kinetically and thermodynamically favorable for the desulfurization mechanism initiated by Diels-Alder cycloaddition between bimolecular 2-MT sulfones than by single 2-MT sulfone decomposition. Additionally, the rate constants of COS formation are much smaller than that SO2 formation at a certain temperature. This also explains why SO2, rather than COS, is the main sulfur-containing gas product during 2-MT pyrolysis under oxidative atmosphere.