Absorption of n-butane in imidazolium and phosphonium ionic liquids and application to separation of hydrocarbon gases

In this report, the density, viscosity, and heat capacity of the tetraalkylphosphonium ionic liquids, containing newly synthesized trihexyltetradecylphosphonium p-dodecylbenzenesulfonate and bis(2-ethylhexyl)sulfobuta-nedioate, were measured at atmospheric pressure. Then, the n-butane absorption in the dialkylimidazolium and tetraalkylphosphonium ionic liquids were investigated at the n-butane pressure of 0.101 MPa and the temperatures of (298.15-353.15) K. In a series of bis(trifluoromethanesulfonyl)amide ionic liquids, the trihexyltetradecylphosphonium salt absorbed the largest amount of n-butane, followed by the triethyloctylphosphonium, 1-methyl-3-octylimidazolium, and 1-butyl-3-methylimidazolium salts. On the other hand, in the trihexyltetradecylphosphonium ionic liquids, the bis(2-ethylhexyl)sulfobutanedioate and p-dodecylbenzenesulfonate salts had the higher solubilities of n-butane than the bis(trifluoromethanesulfonyl)amide salt. In addition, the solubilities of water vapor in the trihexyltetradecylphosphonium salt were measured at 298.2 K and atmospheric pressure. Trihexyltetradecylphosphonium bis(trifluoromethanesulfonyl)amide was the most free from the moisture. We further performed the continuous gas absorption experiments of n-heptane and toluene using the present tetraalkylphosphonium ionic liquids. They removed successfully the vapors of hydrocarbons despite the very dilute concentrations of hydrocarbons, less than 1500 ppm. The initial absorption rates were calculated to discuss the absorption kinetics in the tetraalkylphosphonium ionic liquids.