(417c) Thermodynamics of Ionic Liquids and Gases

Ionic liquids (ILs) present intriguing possibilities for a variety of important gas separations, including removal of carbon dioxide from post-combustion flue gas, pre-combustion gases, air, and raw natural gas streams.  An important advantage of ILs is their low volatility, which prevents evaporation or loss of the IL to the gas stream.  Even by physical absorption, many ILs provide sufficient selectivity of CO₂ over N₂, O₂, CH₄ and other gases to be used for separations.  Here we will focus on the solubility of CO₂, H₂S, N_2, Ar, Xe and Kr in ILs with imidazolium, ammonium and phosphonium cations paired with a wide variety of different anions.  The gas solubilities are measured with both gravimetric and volumetric apparatuses.  We will explore the relative contributions to the solubility from both enthalpic and entropic effects, using densitometry and calorimetry to complement the information available from the temperature-dependent solubility isotherms.  In addition, we will demonstrate how CO₂ capacity (and, subsequently, selectivity over other gases) can be dramatically enhanced by incorporation of chemical functionality into the IL that can chemically react with CO₂.  Increasing CO₂ capacity is particularly important when CO₂ partial pressures are low, as is the case in post-combustion flue gas separation.  The thermodynamics of the chemical reaction between the IL and CO₂ are discerned both from the equilibrium solubilities and calorimetry.  The key outcome of this research is the tremendous tunability made available by the ionic liquid platform.