Tunable Ionic Liquids for Gas Separation: Solubility Studies for Thermodynamic Analysis

Ionic liquids (ILs) are salts with a melting temperature below 100°C and a large liquidus range over 300°C. ILs have many beneficial properties; most notably they are negligibly volatile under normal operating conditions. ILs are being investigated for a variety of applications including reaction media, separation solvents, non-volatile electrolytes, heat transfer fluids, and gas capture. Our research interest focuses specifically on how ILs can be designed for specific and selective gas separations. One potential application for this research is the separation of industrial flue gases for removal of environmentally hazardous gases. The ILs' negligible vapor pressure and tunable properties make them ideal to replace volatile and/or corrosive solvents currently being used for these processes. To this end, gases of interest to our study include carbon dioxide, sulfur dioxide, oxygen, nitrogen, and light hydrocarbons, as well as mixtures of these gases.

Selection of different combinations of anions and cations influences the physical properties and functionality of an IL. Thus, one objective of this work was to determine the thermophysical property relationships between ILs and the gases of interest by making systematic changes to both the anion and cation. Overall, the anion was found to influence the thermophysical properties to a larger degree than changes to the cation. Gases dissolve in ILs via a physical absorption mechanism, a chemical complexation mechanism, or a combination of the two. The physical absorption of gas increased with increased fluorination; the effect was most pronounced with fluorination on the anion. Additionally, even common ILs like 1-hexyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide ([hmim][Tf2N]) were found to have excellent selectivity for CO2 relative to N2 and other components in flue gas based on pure physical absorption. Importantly, ILs also exhibit high absorption capacity for SO2 without degradation of the IL.

However, ILs designed for chemical absorption, those which contain acetate or amine functional groups, were found to be superior to physical absorbing ILs. These efforts have resulted in a ten fold increase in the carrying capacity of ILs for CO2 while maintaining good selectivity relative to other flue gas components. However, preliminary calculations suggest that successful competition with amine-based scrubbing techniques requires a higher carrying capacity for CO2.