Molecular analysis of selective gas adsorption within composites of ionic polyimides and ionic liquids as gas separation membranes

The CO2 separation characteristics of ionic polyimides (i-PIs) are modeled using molecular dynamics simulations in combination with grand canonical Monte Carlo calculations. The performance of neat i-PI systems is evaluated, as well as composite structures containing both i-PIs and various ionic liquids (ILs). The i-PI + IL composites are based on combinations of 1-n-butyl-3-methylimidazolium ([C4mim+]) cations with three common molecular anions: (bis(trifluoromethylsulfonyl)imide ([Tf2N−]), tetrafluoroborate ([BF4−]), and hexafluorophosphate ([PF6−]). It is found that 50 mol% IL inclusion can increase CO2/CH4 selectivity by 16% in [BF4−]-based materials and by 36% in [PF6−]-based materials from mixtures of 5% CO2/95% CH4. While the [BF4−]-based system shows higher CO2/CH4 selectivity, the [Tf2N−]-based system shows higher CO2/N2 gas selectivity. A comprehensive structural analysis (fractional free volume (FFV), pore size distribution, surface area, etc.) is used to highlight the underlying differences among the different i-PI + IL systems that lead to the different adsorption properties.