Competitive adsorption of CO2/N2/CH4 onto coal vitrinite macromolecular: Effects of electrostatic interactions and oxygen functionalities

Abstract

Abstract Here, the influences of oxygen functional groups (OFGs, C O, C O C , C O, and COOH) and the effects of electrostatic on the binarily competitive adsorption of CO 2 /CH 4 /N 2 onto the coal vitrinite molecular model (CVMM) were systematically investigated through the giant canonical Monte Carlo (GCMC) and density functional theory including dispersion correction (DFT-D3) method. The absolute adsorption amounts of the origin CVMM (Ori-CVMM), carbonized CVMM (C-CVMM), and functionalized CVMMs follow the sequence of OH-CVMM\u202f>\u202fCOOH-CVMM\u202f>\u202fOri-CVMM\u202f>\u202fC2O-CVMM\u202f>\u202fCO-CVMM\u202f>\u202fC-CVMM for CO 2 and N 2 and OH-CVMM\u202f>\u202fCO-CVMM\u202f>\u202fC2O-CVMM\u202f>\u202fOri-CVMM\u202f>\u202fC-CVMM\u202f>\u202fCOOH-CVMM for CH 4 . The increasing microporous heterogeneity and binding sites and affinity induced from the OFGs interact more favorably with CO 2 than CH 4 due to the stronger quadrupole moment and polarizability of CO 2 , leaving a much more disadvantageous environment for CH 4 adsorption. The inclusion of the OFGs can significantly enhance the selectivity of CO 2 over CH 4 ( S CO2/CH4 ), following the sequence of COOH-CVMM\u202f>\u202fOH-CVMM\u202f>\u202fOri-CVMM\u202f>\u202fC2O-CVMM\u202f>\u202fCO-CVMM\u202f>\u202fC-CVMM. The electrostatic interactions are advantageous to the adsorption for CO 2 and N 2 but disadvantageous for CH 4 adsorption. Both the electrostatic contributions (ECs) for S CO2/CH4 and S CH4/N2 decrease with the increasing pressure when pressure E s) of pure CO 2 , CH 4 , and N 2 are −31.69 to −67.61\u202fkcal/mol, −29.43 to −46.83\u202fkcal/mol, and −14.57 to −29.74\u202fkcal/mol respectively, indicating that the CO 2 interacts with the CVMM surfaces more strongly than CH 4 and N 2 .