High-Pressure Phase Behavior of Ionic Liquid/CO₂ Systems

TLDR

The study investigates the high‑pressure phase behavior of CO₂ with six ionic liquids, systematically varying anionic and cationic components to elucidate the underlying mechanisms of the unusual phase behavior. The authors examined how changing the anion and cation of each ionic liquid affects CO₂–ionic liquid phase behavior under high pressure. All six ionic liquids absorbed large amounts of CO₂ while negligible ionic liquid dissolved in CO₂, and the liquid‑phase volume expansion upon CO₂ introduction was negligible, unlike the large expansion seen in neutral organic liquids.

Abstract

This work presents the high-pressure phase behavior of CO2 with six ionic liquids: 1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]), 1-n-octyl-3-methylimidazolium hexafluorophosphate ([C8-mim][PF6]), 1-n-octyl-3-methylimidazolium tetrafluoroborate ([C8-mim][BF4]), 1-n-butyl-3-methylimidazolium nitrate ([bmim][NO3]), 1-ethyl-3-methylimidazolium ethyl sulfate ([emim][EtSO4]), and N-butylpyridinium tetrafluoroborate ([N-bupy][BF4]). We explored the effect of systematically changing the anionic and cationic components of the ionic liquid on the CO2−ionic liquid phase behavior. For all of the ionic liquids tested, large quantities of CO2 dissolved in the ionic liquid phase, but no appreciable amount of ionic liquid solubilized in the CO2 phase. In addition, the liquid phase volume expansion with the introduction of even large amounts of CO2 is negligible, in dramatic contrast to the large volume expansion observed for neutral organic liquids. Our results seek to elucidate the underlying physical mechanisms of this highly unusual phase behavior.