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Structure-Property Relationships for the Optimization of Reversible Ionic Liquid Solvents

Ryan Hart, Charles L. Liotta, and Charles A. Eckert. School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive NW, Atlanta, GA 30332-0100

We show how the molecular structure of our reversible ionic liquid solvent systems affects their thermodynamic properties, namely the equilibrium constant (K), enthalpy of reaction (ΔHrxn), and Gibb's free energy (ΔGrxn). From these data we can design economically and environmentally optimized solvent systems. Addition of carbon dioxide (CO2) to an amidine-alcohol or guanidine-alcohol mixture causes the conversion of the molecular liquid to a room-temperature ionic liquid, which is easily reversed by stripping with an inert gas or the addition of heat. This molecular-to-ionic liquid conversion causes a drastic change in the chemical and physical properties of the solvent, advantageous for the development of sustainable processes. This property switch allows us to design ionic liquid solvent systems with a built-in separation capability, solving the intense separation problem observed with conventional room temperature ionic liquids that have negligible vapor pressure. We have systematically examined the effect of altering the cationic and anionic structure on K by using ATR-FTIR at CO2) pressures up to 70 bar and temperatures up to 60C. Knowledge of K as a function of temperature and pressure allows for determination of ΔHrxn and ΔGrxn at different processing conditions, critical values for the optimization of solvent systems that can be economically viable for commercial applications. Such reversible ionic liquids have been shown to combine a homogeneous reaction with heterogeneous separation, for facile product purification and catalyst recycle. Another application that we are investigating is the recovery of CO2) from large point-source emitters. Our reversible ionic liquids combine the notable CO2) carrying capacities of conventional ionic liquids with chemical sorption through reaction, and can be optimized for a given feed stream by use of structure-property relationships.