Influence of functional groups on the microporous development and CO2-CH4 adsorption: A molecular simulation investigation

In this study, the influence of different functional groups (carbonyl, ether, carboxyl, and hydroxyl) on microporous development and competitive adsorption for CO2-CH4 was investigated using Connolly potential theory, molecular mechanics, molecular dynamics, and grand canonical Monte Carlo based on the macromolecular representation of coal vitrinite (CV). Results indicated that microporous diameter induced from the carboxyl (COOH) and hydroxyl (OH) was more sensitive to increasing temperature than carbonyl (CO) and ether (C2O). The COOH and OH could induce more micropores than the other functional groups. The adsorption capacities of OH-CV and COOH-CV were significantly higher than C2O-CV, CO-CV, and C-CV (coal vitrinite purified by carbon atoms) and they all decrease with the decreasing temperature. Both the calculation results of Locate- and Isotherm task suggested that OH and COOH have a significantly higher adsorption amount than other coal macromolecules and the others have a close adsorption amount. The adsorption selectivities of CO2 over CH4 ( SA/B) for all the functional groups were higher than 1 here, indicating the adsorption preference of CO2 over CH4. For the pressure dependence, the SA/B first decreases significantly for the pressure of < 4 MPa and then slightly for the pressures of >4 MPa with the increasing pressure, indicating that high pressure was not conductive to the replacement of CH4 by CO2. For almost all the pressures here, the SA/B follows the order of COOH-CV > OH-CV≈C2O-CV≈Ori-CV≈CO-CV≈C-CV, indicating the highest adsorption preference of CO2 over CH4 for COOH, and OH-CV.