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.