(142e) Use of Room Temperature Ionic Liquids for Carbon Dioxide Separations

Room Temperature Ionic Liquids (RTILs) are solvents consisting entirely of ions resembling the ionic melts of metallic salts; however, RTILs are liquids at much lower temperatures (298 K). RTILs have unique properties including high thermal stability, high ionic conductivity, negligible vapor pressure, and non-flammability. These properties make RTILs potential benign solvent replacements for volatile organics traditionally used in organic synthesis and separation processes. However, future process development using RTILs will require an ability to predict gas solubilities and diffusion coefficients in RTILs since many reactions and separations involve permanent or condensable gases. We have developed an experimental apparatus that can measure both the solubility and diffusion coefficient of various permanent gases and organic vapors in RTILs. Regular Solution Theory has been successfully used to analyze and predict the solubility behavior using immidizolium-based RTILs. The diffusion coefficient is determined by the use of a semi-infinite permeation analysis. The Stokes-Einstein relationship is adequate to predict the diffusion coefficients. Results indicate that the separation selectivity is dominated by solubility selectivity. These results can be used to determine the best RTIL solvent for a given gas as well as an ?upper bound? line on a Robeson plot for ionic liquids to compare to polymer membranes for a given separation. We have successfully incorporated RTILs into polymer matrices and produced mechanically stable films. This approach holds the promise to obtain ?tunable? properties for composite structures for a given separation. One approach is to use hydrophilic polymers to prepare a homogeneous composite membrane. We have incorporated up to 70% ionic liquid into these structures and produced stable films. Another approach is to use ordered liquid crystal structures and incorporate the ionic liquid into the hydrophilic regions. These composite structures are being evaluated for a number of separations, including CO2 separation. CO2 separations are especially attractive since CO2 is very soluble compared to other gases and the separation is based on solubility and not diffusion differences. Commercial polymer membranes use diffusion differences as the primary separation mechanism.