Insights into freestanding MgO monolayer membranes for efficient purification of SF6, C2F6, and C3F8 from gas mixtures: Density functional theory calculations and molecular dynamics simulations

The efficient separation of SF6/N2, C2F6/N2, and C3F8/N2 gas mixtures using membrane technology is essential yet challenging, primarily due to the well-known trade-off between permeability and selectivity, which hinders the recovery of SF6, C2F6, and C3F8. To address this challenge, novel two-dimensional materials have been explored for the design of high-performance membranes. Here, we systematically investigated the highly selective separation of SF6/N2, C2F6/N2, and C3F8/N2 mixtures using freestanding graphenylene-like magnesium oxide (g-MgO) monolayer membranes through a combination of molecular dynamics (MD) simulations and density functional theory (DFT) calculations. MD results reveal N2 permeances of (1.80 ± 0.05) × 107, (1.78 ± 0.04) × 107, and (1.78 ± 0.03) × 107 GPU (i.e., gas permeation unit) for SF6/N2, C2F6/N2, and C3F8/N2 mixtures at 300 K, respectively, which significantly exceed those reported for conventional zeolite and ZIF-8 membranes. DFT calculations further demonstrate N2 molecules permeate the g-MgO membrane with a small energy barrier of 0.237 eV, which is attributed to the absence of electron overlap between N2 and membrane, resulting from the smaller kinetic diameter of N2 compared to the effective pore size (4.11–5.08 Å) of g-MgO. Moreover, the g-MgO membrane exhibits high selectivity for SF6/N2, C2F6/N2, and C3F8/N2, reaching orders of 1017, 103, and 1032, respectively, at 300 K, surpassing the performance of most existing membranes. Our results indicate that the g-MgO membrane simultaneously achieves high permeability and high selectivity for the separation of these mixtures, effectively overcoming the traditional permeability-selectivity trade-off. These findings position the freestanding g-MgO monolayers as highly promising candidates for the purification and recovery of SF6, C2F6, and C3F8.