The separation of CO₂ from a mixture of CO₂ and N₂ using a porous graphene membrane was investigated using molecular dynamics (MD) simulations. The effects of chemical functionalization of the graphene sheet and pore rim on the gas separation performance of porous graphene membranes were examined. It was found that chemical functionalization of the graphene sheet can increase the absorption ability of CO₂, while chemical functionalization of the pore rim can significantly improve the selectivity of CO₂ over N₂. The results show that the porous graphene membrane with all-N modified pore-16 exhibits a higher CO₂ selectivity over N₂ (∼11) due to the enhanced electrostatic interactions compared to the unmodified graphene membrane. This demonstrates the potential use of functionalized porous graphene as single-atom-thick membrane for CO₂ and N₂ separation. We provide an effective way to improve the gas separation performance of porous graphene membranes, which may be useful for designing new concept membranes for other gases.

First-principle density functional theory (DFT) calculation and molecular dynamic (MD) simulation are employed to investigate the hydrogen purification performance of two-dimensional porous graphene material (PG-ESX). First, the pore size of PG-ES1 (3.2775 Å) is expected to show high selectivity of H2 by DFT calculation. Then MD simulations demonstrate the hydrogen purification process of the PG-ESX membrane. The results indicate that the selectivity of H2 over several other gas molecules that often accompany H2 in industrial steam methane reforming or dehydrogenation of alkanes (such as N2, CO, and CH4) is sensitive to the pore size of the membrane. PG-ES and PG-ES1 membranes both exhibit high selectivity for H2 over other gases, but the permeability of the PG-ES membrane is much lower than the PG-ES1 membrane because of the smaller pore size. The PG-ES2 membrane with bigger pores demonstrates low selectivity for H2 over other gases. Energy barrier and electron density have been used to explain the difference of selectivity and permeability of PG-ESX membranes by DFT calculations. The energy barrier for gas molecules passing through the membrane generally increase with the decreasing of pore sizes or increasing of molecule kinetic diameter, due to the different electron overlap between gas and a membrane. The PG-ES1 membrane is far superior to other carbon membranes and has great potential applications in hydrogen purification, energy clean combustion, and making new concept membrane for gas separation.

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It is demonstrated that the fluorine-modified porous graphene membrane has excellent selectivity for CO2/N2 separation by using molecular dynamic (MD) simulations. We also investigated in detail the mechanism of the fluorine-modified porous graphene membrane for CO2/N2 separation by using first-principles simulations. We find that the diffusion barriers for CO2 and N2 to pass through the pore-22 (with 22 carbon atoms drilled out) graphene membrane are relatively small, which indicates that the pore-22 has a low selectivity for CO2/N2 separation. After fluorine modification, the diffusion barrier for CO2 to pass through decreases to 0.029 eV, while the diffusion barrier for N2 greatly increases to 0.116 eV. Therefore, N2 gets more difficult, while CO2 gets easier to penetrate through the fluorine-modified pore-22. The fluorine-modified pore-22 porous graphene shows a great enhancement of selectivity for CO2/N2 separation, which is consistent with the MD results. Our studies show that first-principles simulations can be well used to understand the MD results and propose an economical and efficient means of separating CO2 from N2, which may be useful for designing new concept membranes for gas separation, like CO/N2 and SO2/N2 separations.