(341c) Ionic Liquids for Chemical Separations

Ionic liquids (ILs) present intriguing possibilities for numerous chemical separations because their functionality can be tuned to enhance specific interactions. We will describe four examples, two involving gas separations and two on liquid separations. ILs are particularly attractive for gas separations because their exceedingly low vapor pressures mean that they will not contaminate the purified gas stream. First, we will focus on designing IL systems for post-combustion CO₂ capture. Even by physical absorption, many ILs provide sufficient CO₂ selectivity over N₂, O₂, CH₄ and other gases. However, when CO₂ partial pressures are low, the incorporation of functional groups to chemically react with the CO₂ can dramatically increase capacity, while maintaining or even enhancing selectivity. Guided by quantum calculations, molecular modeling and process modeling, we have been able to design, synthesize and test ILs that reversibly bind up to one mole of CO₂ per mole of IL, with the desired enthalpy of reaction. Currently we are developing supported IL membranes and composite IL/polymer membranes for olefin/paraffin separations, which are necessary in the dehydrogenation of light hydrocarbons to make polymers, fuels and chemicals. While most ILs do not have sufficient inherent olefin/paraffin selectivity, the addition of carriers to achieve facilitated transport can enhance the selectivity to very attractive levels. We will show that the advantages of using ILs for olefin/paraffin separations goes well beyond the stability and non-volatility of the IL membrane. For liquid separations, we will show how ILs can be designed to extract aromatics from aliphatics with very little loss of the solvent to the raffinate phase, along with a novel method using CO₂ as an antisolvent to separate the aromatic from the IL. The final example will demonstrate the use of ILs as an entrainer to break azeotropes. Overall, we hope to show the versatility of this unique materials platform for a wide variety of industrially relevant chemical separations.