Phase-Change Ionic Liquids for Postcombustion CO₂ Capture

Phase-change ionic liquids, or PCILs, are salts that are solids at normal flue gas processing temperatures (e.g., 40–80 °C) and that react stoichiometrically and reversibly with CO₂ (one mole of CO₂ for every mole of salt at typical postcombustion flue gas conditions) to form a liquid. Thus, the melting point of the PCIL–CO₂ complex is below that of the pure PCIL. A new concept for CO₂ separation technology that uses this key property of PCILs offers the potential to significantly reduce parasitic energy losses incurred from postcombustion CO₂ capture by utilizing the heat of fusion (Δ<i>H</i>_fus) to provide part of the heat needed to release CO₂ from the absorbent. In addition, the phase transition yields almost a step-change absorption isotherm, so only a small pressure or temperature swing is required between the absorber and the stripper. Utilizing aprotic heterocyclic anions (AHAs), the enthalpy of reaction with CO₂ can be readily tuned, and the physical properties, such as melting point, can be adjusted by modifying the alkyl chain length of the tetra-alkylphosphonium cation. Here, we present data for four tetrabutylphosphonium salts that exhibit PCIL behavior, as well as detailed measurements of the CO₂ solubility, physical properties, phase transition behavior, and water uptake for tetraethylphosphonium benzimidazolide ([P₂₂₂₂]­[BnIm]). The process based on [P₂₂₂₂]­[BnIm] has the potential to reduce the amount of energy required for the CO₂ capture process substantially compared to the current technology that employs aqueous monoethanolamine (MEA) solvents