Correlating fractional free volume to CO2 selectivity in [Rmim][Tf2N] ionic liquids

• Fractional free volumes of [Rmim][Tf2N] ILs were simulated in COSMOTherm.
• When ‘R’ is a polar group, FFV is reduced compared to when ‘R’ is alkyl.
• Reduced FFV correlates to observed improvements in CO2 solubility selectivity.
• This methodology using COSMOTherm may help to suggest new ILs with even smaller FFV.

Ionic liquids (ILs) with smaller molar volumes are typically correlated with improved CO2/N2 and CO2/CH4 solubility selectivity. However, it is the free volume (i.e. the “empty” space) present within this molar volume where gas molecules will dissolve. Thus, we seek to find further relationships between IL free volume and molar volume as a means to more intuitively understand CO2 selectivity. Previously, we considered the effect of 1-n-alkyl-3-methylimidazolium ILs with various anions (i.e. [Cnmim][X]) using COSMOTherm to rapidly calculate free volume. It was noted that ILs with the smallest molar volumes also had the smallest free volumes, indicating that a correlation between free volume and CO2 selectivity can exist. In our prior work, we considered 165 ILs, many of which had not yet been synthesized or are perhaps purely theoretical. Here, we extend our examination to functionalities other than n-alkyl groups such as ethers, nitriles, fluoroalkyls, etc. with the bis(triflamide) anion ([Tf2N]). The focus is devoted specifically to families of ILs that are readily synthesized and have CO2 solubility and selectivity data reported in the literature. Analysis of these [Rmim][Tf2N] ILs serves to further confirm that ILs with smaller free volumes also have improved CO2 selectivity, and that in most cases, polar functional groups provide better performance than the n-alkyl-functionalized analogs. Given that synthesis of functionalized imidazolium cations is typically more facile than novel anions, a deeper exploration of functional groups may be key to further improving CO2 selectivity in these types of ILs.