Effects of Branched and Cycloalkyl Functionality on CO2 Separation Performance of Poly(IL) Membranes

The separation abilities of polymerizable ionic liquids (poly(ILs)) have been studied for the past several years, with a specific focus on their capabilities regarding CO₂ removal. The structure of ILs allows for a wide degree of flexibility regarding the addition of functional groups, important because the molecular composition of IL monomers directly correlates to the monomer’s gas transport properties. Here, a variety of IL monomers were synthesized, with varying branched- and cycloakyl groups appended to the imidazolium ring. These monomers were used to form poly(IL) membranes. The permeabilities of each of these membranes to CO₂, N₂, and CH₄ were then analyzed, and their selectivites for CO₂ relative to other gases determined. Poly(ILs) with branched and cyclic functionalities exhibited ~20% larger CO₂/N₂ and CO₂/CH₄ selectivities than n-alkyl poly(ILs), while exhibiting a reduction in gas permeability of almost an order of magnitude. It is proposed that the greater selectivity of branched and cyclic functional groups is due to smaller fractional free volumes (FFV) than analogous n-alkyl chains. Computational analysis using COSMOTherm supports this.