Confined Ionic Liquid in an Ionic Porous Aromatic Framework for Gas Separation

Ionic liquids (ILs) and porous frameworks are commonly employed in gas separation, but rarely together. To leverage their combined advantages, we designed a novel system comprising an ionic liquid inside a charged porous-polymer framework. A positively charged ionic porous aromatic framework was impregnated with an ionic liquid to confine the cations and anions inside the polymeric framework via electrostatic interactions. Molecular dynamics and free energy simulations were performed to determine the solubility, diffusivity, and hence permeability of CO2 and CH4 through the composite material. We found that the polymeric framework introduces nanoscale space in the confined IL, improving gas solubility, while the IL promotes gas transport in the porous structure. The confinement impacts CO2 and CH4 very differently regarding their solubility and diffusivity. Through tuning of the ionic-liquid loading, both high CO2 permeability and high CO2/CH4 selectivity could be achieved, overcoming the Robeson upper bound. This work highlights the advantages of confined ionic liquids in an ionic porous-polymer framework in enhancing selective gas separation.