(367f) Free Volume Basis of Gas Solubility and Selectivity in Functionalized Imidazolium-Based Ionic Liquids | AIChE

(367f) Free Volume Basis of Gas Solubility and Selectivity in Functionalized Imidazolium-Based Ionic Liquids


Shannon, M. S. - Presenter, University of Alabama
Bara, J., University of Alabama
Horne, W. J., University of Alabama

Due to the tunable structures and functionalities of ionic liquids (ILs), current research efforts have introduced several unique materials for industrial applications, including solvents and media for CO2 capture.  Fundamental conclusions and aspects of these IL materials and their physical properties are imperative to understanding governing driving-forces for gas separations and underlying molecular structures.  Recently, we investigated fractional free volume (FFV)-dependence of gas solubility in 165 existing and theoretical 1-n-alkyl-3-methylimidazolium-based ionic liquids (ILs) via COSMOtherm, a very powerful and rapid software utilized commonly now for predicting IL thermophyscial properties. 

Our present study investigates the effects on gas solubility of novel functional groups by replacing n-alkyl chains with nitriles (CnH2nC≡N) and ethers or oligo(ethylene glycols) (-C2H4O-)n.  Initial computational results with these new functional groups via COSMOtherm reflect deviations in FFV from previously studied [Cnmim] ILs.  Polar substituents provide a higher CO2 binding affinity and repulsion of non-polar gases (i.e. CH4, N2, etc.) leading toward improved CO2 separation performance.  Initial results suggest decreased FFV values for appended nitrile and ether groups when compared to previous [Cnmim] IL data, which would also imply decreased gas solubility.  However, with this observed decrease in FFV due to chain aggregation of polar entities, ideal gas selectivity is shown to increase with these materials being more size selective within these microdomain/voids and unable to accommodate the larger CH4 and N2 molecules.

Previous models presented for gas solubility in ILs consist of a molar volume-basis for families of 1-n-alkyl-3-methylimidazolium ILs.  Free volume within these ILs as well as the currently studied nitrile- and ether-based ILs was shown to be the underlying property driving gas solubility and selectivity.  For light gases (i.e. CH4 and N2) considered in CO2 separations, solubility increases with increasing FFV implying direct proportionality with free volume.  However, solubility of CO2 was observed to decrease with increasing FFV, with CO2 selectivity being a function of free volume to the (-3/2) power.  Controlling IL FFV via cation/anion combinations, as well as, cation functional groups can lead to improvements in CO2 solubility and selectivity and permeability in IL-based membranes.