Design and Performance of Di- and Tricationic Poly(ionic liquid) + Ionic Liquid Composite Membranes for CO<sub>2</sub> Separation
Sudhir Ravula, Kevin W. Wise, Pravin S. Shinde, Jason E. Bara
Abstract
The synergy of ionic liquids (ILs) and polymers has given rise to the design and study of poly(IL) materials, which has opened vast new design spaces in the area of polyelectrolytes. When used as membranes (i.e., thin films), poly(ILs) can efficiently separate CO 2 from other species. Some poly(IL) and poly(IL)–IL composite membranes have been shown to exceed the “upper bounds” of Robeson plots, particularly for CO 2 /CH 4 . While some guidelines for structure–property relationships in poly(IL) membranes have been developed, the vast design space leaves many opportunities to more deeply probe the factors underlying gas permeability and selectivity. In this work, we have designed more complex poly(ILs) comprising di- and tricationic vinylimidazolium IL monomers with two different aliphatic linkers (i.e., tetramethylene and hexamethylene) along with different pendant substituents, including two types of aromatic imides that have not been previously considered within the context of ILs or poly(ILs). Dense poly(IL)–IL composite membranes were prepared via photopolymerization of a mixture containing IL monomer, a corresponding amount of free IL ([C 4 mIm][Tf 2 N], 1-butyl-3-methylimidazolium bistriflimide), and a bisimidazolium cross-linker (CL). The physical and thermal properties of the poly(IL)–IL composite membranes were extensively characterized. Membranes of these poly(ILs) were studied for their gas separation performances (CO 2 /N 2, CO 2 /H 2, and CO 2 /CH 4 ). The gas permeabilities and diffusivities are greatly dependent on the contents of the poly(IL)–IL composite. The CO 2 permeability of these poly(IL)s with di-(alkyl or oligomer) and tricationic pendants was found to be higher than those with aromatic pendants, which seemingly had slow diffusion. Further, the increase in the CL amount promotes a decrease in the permeability and diffusivity of the materials. Overall, the poly(IL)–IL composite membranes exhibit a high CO 2 permeability up to 110 barrer with good CO 2 /N 2 (16.3–30.8), CO 2 /CH 4 (10.3–20.7), and CO 2 /H 2 (2.7–5.5) permselectivities.