Design and Performance of Di- and Tricationic Poly(ionic liquid) plus Ionic Liquid Composite Membranes for CO2 Separation

Thesynergy of ionic liquids (ILs) and polymers has given riseto the design and study of poly(IL) materials, which has opened vastnew design spaces in the area of polyelectrolytes. When used as membranes(i.e., thin films), poly(ILs) can efficiently separate CO2 from other species. Some poly(IL) and poly(IL)-IL compositemembranes have been shown to exceed the "upper bounds"of Robeson plots, particularly for CO2/CH4.While some guidelines for structure-property relationshipsin poly(IL) membranes have been developed, the vast design space leavesmany opportunities to more deeply probe the factors underlying gaspermeability and selectivity. In this work, we have designed morecomplex poly(ILs) comprising di- and tricationic vinylimidazoliumIL monomers with two different aliphatic linkers (i.e., tetramethyleneand hexamethylene) along with different pendant substituents, includingtwo types of aromatic imides that have not been previously consideredwithin the context of ILs or poly(ILs). Dense poly(IL)-IL compositemembranes were prepared via photopolymerization of a mixture containingIL monomer, a corresponding amount of free IL ([C(4)mIm][Tf2N], 1-butyl-3-methylimidazolium bistriflimide), and a bisimidazoliumcross-linker (CL). The physical and thermal properties of the poly(IL)-ILcomposite membranes were extensively characterized. Membranes of thesepoly(ILs) were studied for their gas separation performances (CO2/N-2, CO2/H-2, and CO2/CH4). The gas permeabilities and diffusivities are greatlydependent on the contents of the poly(IL)-IL composite. TheCO(2) permeability of these poly(IL)s with di-(alkyl or oligomer)and tricationic pendants was found to be higher than those with aromaticpendants, which seemingly had slow diffusion. Further, the increasein the CL amount promotes a decrease in the permeability and diffusivityof the materials. Overall, the poly(IL)-IL composite membranesexhibit a high CO2 permeability up to 110 barrer with goodCO(2)/N-2 (16.3-30.8), CO2/CH4 (10.3-20.7), and CO2/H-2 (2.7-5.5)permselectivities.