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

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 ([Pyr(11)](+)), while the five ILs display either an imidazolium ([C(2)mim](+)) or a pyrrolidinium ([Pyr(14)](+)) 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 N-2 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/N-2 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.