A cation-gating mechanism for enhanced CO2/N2 separation by porous nanostructure supported ionic liquid membrane

2D porous material supported ionic liquid membranes (SILMs) have demonstrated great potential for CO2 separation and purification, outperforming the original porous material. However, the working mechanism behind their enhanced CO2 selectivity remains unclear. In this study, we have conducted molecular dynamics simulation to investigate the CO2/N2 separation performance and the underlying mechanism of SILMs taking 2D rhombic N-graphdiyne (r-N-GDY) with intrinsic high thermal stability and porous structure covered with 1-butyl-3-methylimidazolium tetrafluoroborate as the representative SILM model. We found that the increase in the SILM thickness can decrease the permeance of CO2 and N2 but can effectively increase the CO2/N2 selectivity. The optimal SILM thickness is found to be 0.6 nm with the permeance reaching 5.7 × 105 GPU for CO2 and the selectivity being up to 25.8, which is 15 times higher than the 1.7 of bare r-N-GDY. This is because CO2 encounters a much lower transmembrane energy barrier than N2. At the molecular level, it is fascinating to observe a cation-gating mechanism, where IL cations play a determinative role in CO2 selectivity. More specifically, the IL cations normally bind at the pore site, like a closed gate for gas. When a CO2 molecule approaches the pore, the IL cation moves away; thus, the gate is opened for CO2 translocation. In contrast, N2 molecules are incapable of opening the cation gate. Such a cation-gating process guarantees the high selectivity of SILMs. This study offers insight into enhanced CO2 selectivity and provides theoretical guidance for designing nanocomposite membranes for gas or water treatment.