Prominently improved CO2/N2 separation efficiency by ultrathin-ionic-liquid-covered MXene membrane

MXene-based membranes exhibit great potential in the gas separation but still suffer from the poor selectivity for gases with similar dynamic diameters, such as CO2 and N2. Here, we propose a prototype of composite membrane in atomistic scale theoretically, which is constructed by covering MXene with an ultrathin layer of ionic liquid (IL) and potentially has very good performance on the separation of CO2/N2 mixtures. A series of molecular dynamics simulations were carried out to investigate the influences of the MXene-slit width and the IL-film thickness on CO2/N2 separation performance. The gas separation performance is significantly enhanced after covering the IL film, and the IL-film thickness acts as a major factor to determine the performance. With the amounts of the IL increased from one to two layers, the CO2 permeance decreases from 2.3 × 104 to 2.6 × 103 GPU, while the N2 permeance declines more sharply, resulting in an increased selectivity. Compared to the IL-film thickness, the MXene-slit width has less influence on the gas permeance. However, the CO2/N2 selectivity decreases as the slit width enlarges. Finally, we concluded that the composite membrane composed of the MXene with 8 Å slit and 40 pairs of IL (about one and a half layers) exhibits the best CO2/N2 separation performance, with the CO2 permeance up to 104 GPU, and no N2 passing through the membrane during the simulations. Furthermore, the interaction of gas-membrane and the penetration process of gas were also investigated. The excellent CO2/N2 separation performance is ascribed to the synergistic effect of IL on the surface of the MXene and anions in the slit.