Fluorine-Free Imidazolium-Based Ionic Liquids with a Phosphorous-Containing Anion as Potential CO2 Absorbents

Abstract

Solubilities of carbon dioxide (CO2) in a series of fluorine-free room temperature ionic liquids (RTILs), dialkylimidazolium dialkylphosphates and dialkylimidazolium alkylphosphites, were measured at 313 similar to 333 K and pressures up to 5 MPa. Henry's law coefficients as the solubility parameter of CO2 in RTILs were derived from the isotherm of fugacity versus CO2 mole fraction. The CO2 Solubility in a phosphorus-containing RTIL was found to increase with the increasing molar volume of the RTIL. In general, dialkylimidazolium dialkylphosphate exhibited higher absorption capacity than dialkylimidazolium alkylphosphite as long as the RTILs possess an identical cation. Among RTILs tested, 1-butyl-3-methylimidazolium dibutylphosphate [BMIM][Bu2PO4] and 1-butyl-3-methylimidazolium butylphosphite [BMIM][BuHPO3] exhibited similar Henry's law coefficients to 1-butyl-3-methylimidazolium his (trifluoromethylsulfonyl)imide ([BMIM][Tf2N]) and 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]), respectively. The Krichevsky-Kasamovsky equation was employed to derive the CO2 solubility parameter (Henry's law coefficient) from the solubility data measured at elevated pressures.