(314c) Lessons Learned from “Not Entirely Successful” Attempts to Use Room Temperature Ionic Liquids (R T I Ls) for Electrochemical Separations

Room Temperature Ionic Liquids (RTILs) are organic salts that are liquids at ambient conditions consisting entirely of ions. RTILs possess a number of unique properties for electrochemical processes including: negligible vapor pressure, high thermal stability, non-flammability, and high ionic conductivity. It was this last property, we hoped to exploit in our work to develop an Electrochemically Modulated Complexation (EMC) separation system for CO2. EMC is a process that combines a reversible set of redox reactions with absorption/desorption steps to achieve selective separation of gas mixtures. In this paper, we will discuss our efforts and failures in selecting, designing, and using RTILs for EMC. For example, all attempts to increasing the redox-active carrier solubility in the RTIL resulted in increased solution viscosity. Therefore, we were faced with a trade-off selection process. Furthermore, in the reduction-to-practice, the carrier concentration needs to be greater than 0.5 mol/liter. In complex solvents, like RTILs, this is a mole-fraction of at least 0.15. This leads to a number of complications due to dynamic changes in voltage control, carrier solubility, viscosity etc. Consider the difficulty of voltage control (i.e. reference electrode meaning) in a system were 15 mole-% of the solution changes oxidation state during the process. We also encountered changes in carrier solubility and increased viscosity as the oxidation state of 15 mole-% of the solution changed. The widely-studied imidiazolium-based RTILs initially were our choice for the EMC system. We will discuss how other non-imidiazolium-RTILs can alleviate some of the above complications.