Inverse gas chromatography for crucial separations using ionic liquids



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Ionic liquids (ILs) are widely studied for gas separations because of their low vapor pressures and high gas solubilities. Volumetric and gravimetric apparatuses are used to accurately determine solubilities of gases in ILs; however, they can be incredibly time consuming. A faster method that can be used to screen ionic liquids for specific separation applications is gas chromatography (GC). It yields Henry’s Law Constants (k [subscript H], for gases) and infinite dilution activity coefficients (γ∞, for liquids), which are good measures of the IL-solute interactions. As a result, the GC method can be used to quickly and easily screen numerous ILs for the strength of their interactions with multiple gases and liquids. k [subscript H] and γ∞ values of various solutes are presented, including ethane, ethylene, propane, propylene, a family of C5-C8 linear alkanes and alkenes, and aromatics in imidazolium ionic liquids, including 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim][Tf₂N]), and 1-allyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([allylmim][Tf₂N]). Adding silver bis(trifluoromethylsulfonyl)imide [Ag][Tf₂N] at 6 concentrations up to 2.45 M dramatically improves the selectivity of olefin/paraffin pairs. Lastly, [allylmim][Tf₂N] with 1 M [Ag][Tf₂N] was contacted with cyclohexane/cyclohexene and cyclohexane/toluene mixtures to improve the selectivity over what can be achieved with pure ILs for liquid-liquid equilibrium applications. Together this work shows a comprehensive study of using the GC method to screen various ILs for separation applications.


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