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

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 CO2/CH4/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 > COOH-CVMM > Ori-CVMM > C2O-CVMM > CO-CVMM > C-CVMM for CO2 and N-2 and OH-CVMM > CO-CVMM > C2O-CVMM > Ori-CVMM > C-CVMM > COOH-CVMM for CH4. The increasing microporous heterogeneity and binding sites and affinity induced from the OFGs interact more favorably with CO2 than CH4 due to the stronger quadrupole moment and polarizability of CO2, leaving a much more disadvantageous environment for CH4 adsorption. The inclusion of the OFGs can significantly enhance the selectivity of CO2 over CH4 (S-CO2/CH4), following the sequence of COOH-CVMM > OH-CVMM > Ori-CVMM > C2O-CVMM > CO-CVMM > C-CVMM. The electrostatic interactions are advantageous to the adsorption for CO2 and N-2 but disadvantageous for CH4 adsorption. Both the electrostatic contributions (ECs) for S-CO2/CH4 and S-CH4/N2 decrease with the increasing pressure when pressure < 6 MPa and then keep stable during 6-10 MPa. The electrostatic interactions induced from the carbon skeleton framework are significantly weaker than the Ori-CVMM and the functionalized CVMMs and is insensitive to the pressure. The inclusion of OFGs enhances the accessible surface area and enlarges the microporous spaces and diameters, especially for the carboxylation and carboxylation functionalized CVMMs, creating a more favorable microporous surrounding for adsorption. The adsorption energy (Delta Es) of pure CO2, CH4, and N-2 are -31.69 to -67.61 kcal/mol, -29.43 to -46.83 kcal/mol, and - 14.57 to -29.74 kcal/mol respectively, indicating that the CO2 interacts with the CVMM surfaces more strongly than CH4 and N-2 .