Thermodynamic Properties of Imidazolium and Phosphonium Based Ionic Liquid Mixtures with Water or Carbon Dioxide

Ionic Liquids (ILs) are polar compounds composed of poorly coordinating ions which are liquid below 100 oC and have potential as a clean replacement for common VOCs as solvents in chemical processes. ILs have many favorable characteristics such as a low vapor pressure, stability over a large liquid working temperature range, and ability to be designed to dissolve compounds of interest. This research addresses innovative solutions using ILs for two applications: absorption refrigeration and post combustion carbon capture. Ionic liquids (ILs) as absorbents in absorption refrigeration systems present the possibility of overcoming some of the safety and environmental concerns of current systems. In general, absorption refrigeration is attractive since electrical energy is replaced with low value heat energy. Many ILs are completely miscible with water, which leads the focus to investigating ILs and water for this application. The coefficient of performance (COP) is the cooling capacity divided by the energy input to the absorption refrigeration cycle and it can be used to gauge the potential success of IL and water systems. This research provides the necessary thermodynamic measurements and/or predictive modeling of the mixture properties and phase behavior (such as excess enthalpy, heat capacity, and vapor-liquid equilibria) in order to evaluate the IL and water systems for future use in absorption refrigeration. The calculations have shown that the IL and water systems have higher COPs than the conventional systems. The second application of this research is based upon the fact that ILs have shown great potential to replace the current carbon capture technology, which typically utilizes an aqueous amine solution. This research focuses on ILs which can chemically complex (or chemically react) to carbon dioxide for separation from flue gas. The heat of absorption of the gas in the IL is experimentally investigated by calorimetry, which is key to determine how much energy is required to regenerate the absorbents and the viability of the process. It is shown that the enthalpy can be varied significantly by careful choice of substituents on amine-functionalized ILs.