Efficient separation of CO2/CH4 by covering ultrathin ionic liquid film on COF membrane

Covalent organic framework (COF) membranes show great potential for gas separation, but their large pore size make it hard to effectively separate gases with similar kinetic diameters. A promising strategy is to cover the surface of COF membranes with an ultrathin film of ionic liquids (ILs) to improve the selectivity of gas permeation. Here, we investigated the CO2/CH4 separation performance of composite membranes composed of COF (CTF-1) and 20 kinds of ILs, respectively, using molecular simulations. The [C4mim][PF6]/COF membrane is found to exhibit the best separation performance, with CO2 permeability up to 7.93×104 GPU and CO2/CH4 selectivity of 12.33 when the IL thickness is 8 Å. By studying the microstructure of the composite membrane, it was found that there is a relatively dispersed distribution of cations and anions on the COF surface, which is beneficial to the diffusion of gas molecules. The gas permeation process results reveal that CO2 rapidly achieves equilibrium in both the adsorption and dissolution layers simultaneously, thereby enhancing the permeation rate of gas molecules. The analysis of interaction energy and PMF indicates that the improved selectivity is due to the stronger interaction between IL and CO2 than that for CH4.