CO2 Separation Using Room-Temperature Ionic Liquids with the Tetracyanoborate Anion

Room-temperature ionic liquids offer a unique method to separate CO₂ from gas streams for both carbon capture and membrane separations because of the ability to tailor the physical and chemical properties through selection of the cation and anion.  We report on the use of ionic liquids containing the tetracyanoborate (TCB) anion for membrane gas separations, more specifically CO₂/N₂ separation.  The TCB anion was coupled to a variety of cations including imidazolium, pyridinium, pyrrolidinium and a unique ring-opened cation that resembles an opened imidazolium structure creating a set of low viscosity ionic liquids for carbon separation.  CO₂ solubility and permeability values as well as CO₂/N₂ selectivity values were measured under various conditions.  In general, all of the ionic liquids containing the TCB anion, regardless of the cation, were near to or exceeded the Robeson limit.  In addition, the CO₂ solubility values were exceptionally high ranging from 0.131 to 0.151 mol·L^-1·atm^-1.  The results of the experimental measurements were compared to computational results using modified regular solution theory, the quantum chemical Conductor-like Screening Model for Real Solvents (COSMO-RS) method, and classical molecular dynamics simulations to achieve a greater understanding of the properties that control CO₂ separations.  The role of cation/anion interaction and anion/CO₂ interaction in determining the CO₂ solubility will be discussed.  This set of ionic liquids shows excellent potential for CO₂ separation and provides a potential path for tailored ionic liquids with improved separations capabilities.  This work was sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy.