Molecular simulations insight into oxygen-containing groups' impact on CO2, CH4 and N2 adsorption in Ordos Basin deep coal, China.

CO2/N2-enhanced coalbed methane recovery serves as a key approach to boosting coalbed methane production. Gaining insight into the adsorption behavior and microscopic interaction mechanisms of CO2, CH4, and N2 in coal, along with their impact on coal properties, is therefore crucial. In this study, a series of functionalised coal macromolecular models (CMMs) were developed based on a previously validated anthracite prototype from the Daning-Jixian region. By employing a grand canonical Monte Carlo and density functional theory approach, combined with electrostatic potential (ESP) analysis, we systematically explored how a coal macromolecular model (CMM) modified with oxygen-containing functional groups (OFGs; i.e., -C = O, -COC-, -C, -OH, and -COOH) affects the adsorption of CO2, CH4, and N2. Adsorption energy rankings revealed that OFGs promote CO2 and N2 adsorption (COOH-functionalized CMM (COOH-CMM) shows high energy for both) but hinder CH4 adsorption. Moreover, CO2 exhibits the strongest adsorption affinity among the three gases, with a maximum adsorption energy slightly higher than that of CH4 and approximately 2.4 times that of N2. OFGs, particularly COOH, increase the accessible surface area, the number of binding sites, and affinity, thus facilitating CO2 adsorption in micropores. This study enriches the understanding of the adsorption potential and microscopic interaction mechanisms of CO2, CH4, and N2 in coal and reveals the crucial role of oxygen-containing functional groups, providing new molecular-level insights into selective CO2 capture and CO2/N2-enhanced coalbed methane recovery.