Microscopic mechanism of Ca-catalyzed coal pyrolysis affecting CH₄ generation: A combined ReaxFF MD and DFT study

Exploring methane formation is critical for elucidating the catalytic pyrolysis mechanisms and regulating high value-added chemical production. This paper aims to propose a strategy that combines ReaxFF molecular dynamics (ReaxFF MD) with density functional theory (DFT) to systematically tackle the micro-mechanisms of methane generation influenced by Ca-catalyzed coal pyrolysis. This research determines the contribution of three-phase products (char, tar, and gas) and precursors to methanogenesis for the first time, as well as explores the changes in reactivity and micro-path energy barrier of calcium on critical precursors. The results indicate that char serves as the primary product source of methanogenesis, while methoxyl groups are essential structural contributors to its generation. The pivotal factor in calcium impacting methane release is the inhibition of methoxy cleavage. It was found that calcium intensified the difficulty of CH₃· desorption from the carbonaceous surface by reducing the electron density in the methoxyl region and enhancing the Laplace bond order (LBO) of the methoxyl O-C bond, elevating the energy barrier value of the rate-determining step in the methane generation. This reduced the contribution of methoxy to methane generation from 40% to only 4.34%. The work bridges the inadequacy of the current microscopic insights into metal-influenced product formation and provides a comprehensive and in-depth theoretical basis for the development of coal-catalyzed pyrolysis technology.