Abstract:

Ionic liquids are non-volatile organic salts that have low melting points, frequently below room temperature. Typical compounds are comprised of a quaternary ammonium, quaternary phosphonium, imidazolium or pyridinium cation with a wide variety of common anions. The large difference in the solubility of different gases in many ionic liquids, their low volatility, and their large liquidus range make them ideal candidates for gas separations. Potential applications include separation of carbon dioxide from natural gas and combustion flue gases, olefin/paraffin separations, the removal of nitrogen or oxygen impurities from carbon dioxide or methane, and many others. We have particularly focused our work on the design of ionic liquids for removal of carbon dioxide from flue gas. This is a particular difficult separation because of the low partial pressure of CO₂ available. It is estimated that the capture and sequestration of CO₂ from coal fired power plants using conventional capture technology (aqueous amines) would add 80% or more to the cost of electricity. This increase in cost is primarily from the separation process. We present measurements of the solubility of a variety of gases in ionic liquids using both low pressure and high pressure gravimetric microbalances. We show how various functionality in the anion and the cation influences the solubility of these gases and discuss the range of properties and behaviors that need to be optimized for an industrial separation process. In addition, we show how the compounds developed as part of this project can be used very successfully for other challenging CO₂ separations.