Competitive sorption of CO₂/CH₄ and CO₂ capture on modified silica surfaces: A molecular simulation

The effect of functionalization of modified silica (FMS) surfaces on the adsorption behaviors for CH4, CO2 and the mixture have been studied by combining a grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) methods. The isotherms, gas distribution, sorption sites, isosteric heat, interaction energy and adsorption selectivity for CO2/CH4 mixture are computed and discussed. And the effects of pressure and temperature on adsorption behavior are also studied. The adsorption capacity (C) of surface with various functional groups for CH4 is as follows: C(SiO2-COOH) > C(SiO2-SH) > C(SiO2-OH) > C(SiO2-CH3) > C(SiO2-NH2) > C(SiO2-H) and for CO2 is as follows: C(SiO2-COOH) > C(SiO2-SH) > C(SiO2-NH2) > C(SiO2-OH) > C(SiO2-CH3) > C(SiO2-H). Increasing temperature is not advantageous for gas adsorption on all FMS surfaces. The CO2/CH4 selectivity (S) for FMS surfaces decrease in the following order of S(SiO2-COOH) > S(SiO2-NH2) > S(SiO2-OH) > S(SiO2-SH) > S(SiO2-CH3) > S(SiO2-H). As the pressure increase, the CO2/CH4 selectivity decrease early and then flatten out to a constant value. The isosteric heat of adsorption (Q(st)) for gas adsorbed on each FMS surface are also calculated, the value of isosteric heat for CO2 is much higher than that for CH4 due to the stronger interaction between CO2 molecules and functional groups of FMS surface. The result of the smallest self-diffusion coefficients of gas in SiO2-COOH slit pores is consistent with the adsorption capacity in the slit pore with corresponding functional group. The electronegative atoms in the functional group act as basic adsorption sites for gas adsorbate molecules, which generates a larger electrostatic contribution on the selectivity of CO2 over CH4.