Mechanism exploration of surface functional groups and pore sizes on CO2 adsorptive separation by GCMC and DFT simulations

The purpose of this study is to analyze the mechanistic effect of surface functional groups and pore sizes of graphene on the CO2 adsorption and separation behaviors by grand canonical Monte Carlo (GCMC) simulation and density functional theory (DFT) calculation. For the surface functional groups, the M−doped (M = N, P, S, O) functionalized graphene materials (G-Rs) exhibit a higher CO2/N2, CO2/H2 and CO2/CH4 adsorption selectivity than pristine graphene (G-None) in the entire pressure range. Especially, due to a stronger electrostatic interaction, the P-doped functionalized graphene materials (P-G-Rs) present the best CO2 separation performance. For the pore size, the influence of pore size on the CO2 adsorptive separation process depends on pressure ranges. At low pressures, 0.8 nm pore shows the highest CO2 adsorption selectivity. However, as for high pressure, the actual pore volume plays the dominant role. Furthermore, by analyzing the isosurface maps of electrostatic potential and van der Waals (vdW) potential, we conclude that the electrostatic effect determines the optimal adsorption sites for polar gas molecules (CO2) rather than non-polar molecules (N2, H2 and CH4), while the vdW interaction plays a secondary role in the intermolecular interaction. This study provides a novel theoretical guidance in designing and preparing modified graphene for CO2/other gas molecules mixture separation.