Mixing poly(ionic liquid)s and ionic liquids with different cyano anions: Membrane forming ability and CO2/N2 separation properties

Abstract

In this work, poly(ionic liquid)–ionic liquid (PIL–IL) composite membranes were prepared using the solvent casting technique. The studied PILs have pyrrolidinium polycation backbone ([Pyr11]+), while the five ILs display either an imidazolium ([C2mim]+) or a pyrrolidinium ([Pyr14]+) based cation. Both the PIL and IL components comprised cyano-functionalized anions ([N(CN)2]–, [C(CN)3]– or [B(CN)4]–), being the anion for each component different from one another. The use of the [NTf2]– anion was also tested for comparison. Several experimental conditions for the solvent casting procedure were tested in order to prepare homogenous and free standing PIL–IL composite membranes. The CO2 and N2 permeation properties (permeability, diffusivity and solubility) were evaluated at a fixed temperature (293 K) and constant trans-membrane pressure differential (100 kPa) using a time-lag apparatus, so that trends regarding the different anions either on the PIL or IL could be obtained and evaluated. From all 42 PIL–IL combinations tested, 21 were suitable membranes (homogeneous and free standing) for gas permeation experiments and 4 of them were on top or surpassed the 2008 Robeson upper bound for CO2/N2 separation. The best performance membranes contain the [C(CN)3]– and [B(CN)4]– anions, enlightening therefore the promise these anions entail for future high performance membranes for post-combustion CO2 separation.