Vapor-Liquid Equilibria of Water and Amine-Functionalized Ionic Liquids

Water is present in many carbon dioxide (CO2)-rich streams, such as air and postcombustion flue gas, for which carbon capture treatment is desirable. For nonaqueous solvents, such as ionic liquids (ILs), water can drastically affect CO2 absorption. For a particularly promising class of ILs for carbon capture, those with aprotic N-heterocyclic anions (AHAs), the effect of water on CO2 capture has been characterized, but the water loading of the AHA ILs as a function of relative humidity is not known. We performed gravimetric vapor-liquid equilibrium (VLE) measurements at 308.2 K for triethyl(octyl)phosphonium 4-nitropyrazolide ([P2228][4-NO2Pyra]), 3-(trifluoromethyl)pyrazolide ([P2228][3-CF3Pyra]), 2-cyanopyrrolide ([P2228][2-CNPyr]), 4-bromopyrazolide ([P2228][4-BrPyra]), and bis(trifluoromethylsulfonyl)imide (a non-AHA) ([P2228][Tf2N]), as well as trihexyl(tetradecyl)phosphonium 2-cyanopyrrolide ([P66614][2-CNPyr]) for water activities from 0.05 to 0.90. These VLE data were fit with the nonrandom two liquid (NRTL) activity coefficient model. The highly basic [P2228][4-BrPyra] was the most hydrophilic. [P66614][2-CNPyr], with its bulkier hydrocarbon chains, was much less hydrophilic than [P2228][2-CNPyr]. Three of the IL-water mixtures exhibited liquid-liquid equilibrium (LLE) at 308 K, with water solubility in the IL-rich phase ranging from 0.15 mole fraction for [P2228][Tf2N] to 0.95 mole fraction for [P2228][4-NO2Pyra].