(502e) 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 we report the progress of understanding the mechanisms from examining the ZIF-8 aperture configuration variation using density functional theory (DFT) and molecular dynamics (MD) simulations. DFT results indicate that the pristine aperture configuration exhibits the best separation performance, and the addition of ILs prevents the apertures from large swing. Subsequently, the effect of IL viscosity on the ZIF-8 configuration variation was investigated, and results show that the pristine aperture configuration is more stabilized by ILs with large viscosity (0-87Cp). Molecular dynamics (MD) simulations 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.