Molecular dynamics studies on separation of CO2/CH4 by the ionic liquids encapsulated ZIF-8

The utilisation of ZIF-8 membrane with encapsulated ionic liquids (IL@ZIF-8 composite membrane) has been demonstrated as one of the most efficient approaches for CO2/CH4 separation. However, compared to pure ZIF-8 membrane, the mechanisms behind the CO2/CH4 selectivity enhancement caused by the encapsulation of ILs still remain indistinct. Here, molecular dynamics (MD) simulations have been performed to investigate the separation and diffusion of gas molecules in the IL@ZIF-8. The simulated results show that the selectivity enhancement is because the adsorbed CO2/CH4 ratio has been increased when ILs are encapsulated into ZIF-8. Detailed analysis indicates that the diffusion of CO2/CH4 molecules follows three different pathways of the hexagonal microstructure of IL@ZIF-8 composite, of which the route to the nearest composite cells (route I) has the highest possibility, and gives the smallest mean diffusion displacement. Lastly, the effect of CO2/CH4 ratio in the initial gas mixture has been explored, and we found that CO2/CH4 selectivity is related to the concentration of CO2, where three regions with different slopes have been classified.The diffusion mechanisms of CO2/CH4 in IL@ZIF-8 composite were investigated by MD simulations, including diffusion routes and effects of CO2 concentration. [Display omitted]•The diffusion mechanisms of CO2/CH4 molecules in IL@ZIF-8 composite were fully investigated by MD simulations.•A route with minimum length for gas molecules diffusion has been identified based the mean diffusion displacement analysis.•MD simulation reveals a general relationship between the CO2/CH4 selectivity and CO2 concentration