CO2 capture in poly(ionic liquid) membranes: atomistic insight into the role of anions

We report the first atomistic simulation study to characterize poly(ionic liquid) (PIL) membranes and examine their capability for post-combustion CO2 capture. Four PILs based on 1-vinyl-3-butylimidazolium ([VBIM](+)) are examined with four different anions, namely bis(trifluoro-methylsulfonyl) imide ([TF2N](-)), thiocyanate ([SCN](-)), hexafluorophosphate ([PF6](-)) and chloride ([Cl](-)). Gas molecules (CO2 and N-2) in [VBIM](+)-based PILs interact with polycations more strongly than with anions. Therefore, the role of anions in gas solubility is insignificant, which is in remarkable contrast to monomeric ILs. The solubilities predicted in the four PILs are close and in good agreement with available experimental data. The sorption, diffusion and permeation selectivities of CO2/N-2 predicted from simulation are consistent with experiment. Particularly, the diffusion selectivities are approximately equal to one, implying that CO2/N-2 separation is governed by sorption. This study provides atomistic insight into the mechanisms of gas sorption, diffusion and permeation in [VBIM](+)-based PILs and reveals that polycations play a dominant role in determining gas-membrane interaction and separation.