CO₂/CH₄ Separation Performance in Polymers of Intrinsic Microporosity: All-Atom Simulations on Functional Group Effects

The separation performance of the CO2/CH4 mixture in polymers of intrinsic microporosity (PIMs) is studied by using all-atom molecular dynamics simulations. Eight types of PIMs with different functional groups are considered, namely, cyano, amidoxime, hydroxyl, thioamide, amide, amine, carboxyl, and tetrazole groups. The separation performance of these membranes is mainly controlled by the preferential adsorption of CO2 over CH4. PIM with amidoxime (PIM-AO) and amine (PIM-AM) groups are the best two cases. The permeability selectivity of PIM-AO almost exceeds two times that of the original PIM membrane. The good separation performance is attributed to the strong adsorption of CO2 in membranes because of the interactions of CO2 molecules with −OH or −NH2 in amidoxime. In PIM-AM, the interaction strength of CO2 to the membrane is strong because of the coupling effect of covalent bonding and hydrogen bonding interactions. To give good separation performance, it is suggested to design PIMs with functional groups having strong interactions or multi-interaction sites to CO2.