Solubility and diffusivity of CO2 in ionic polyimides with [C(CN)3]x[oAc]1−x anion composition

Abstract

Recently, mixtures of ionic liquids (ILs) containing acetate [oAc−] and tricyanomethanide [C(CN)3−] anions have demonstrated promising characteristics as solvents and as CO2 adsorbents. The anion composition can be optimized to obtain a significant improvement in the diffusivity while only slightly decreasing the solubility. Here, we explore a similar investigation applied to an ionic polyimide (i-PI) material in order to understand the molecular-level kinetic and thermodynamic characteristics that emerge when a polymeric material is used instead. The i-PI systems studied are part of an emerging class of “high-performance” ionene polymers that hold significant potential for applications in gas separation membranes. Here, neat i-PI systems with the two counter anions, [oAc−] and [C(CN)3−], at varying concentrations are modeled by a combination of molecular dynamics (MD) and grand canonical Monte Carlo (GCMC) simulations. Higher concentrations of [C(CN)3−] are predicted to improve CO2 diffusion, similar to the pure IL performance, while solubility remains relatively unchanged. The structural characteristics of the i-PI systems provide detailed insight into the effect that the anions have on the adsorption properties. The solubility is weakly related to the theoretical surface area, and the diffusivity is moderately correlated to the fractional free volume (FFV). Overall, the combination of different anions is predicted to be a viable strategy for improving the diffusivity throughout i-PI materials, but the behavior of pure ILs cannot be simply extrapolated to ion behavior in membranes.