Solvent properties of functionalized ionic liquids for CO₂ absorption

Ionic liquids can be used as solvents for gas absorption operations in order to improve the process economy and general efficiency of gas separations. This work investigates solvent properties of ionic liquids and compares them to amine solutions used for absorption of carbon dioxide (CO2). The CO2 solubility into six different room temperature ionic liquids (RTILs) was measured at temperatures between 298 K and 343 K and pressures up to about 1 MPa. The RTILs used were: [bmim]+[BF4]−, [bmim]+[DCA]−, and four imidazolium-based ionic liquids paired with [DCA] and [BF4], in which the cation was functionalized with either a primary, tertiary amine or a hydroxyl group. The density, viscosity and surface tension of the studied RTILs were measured at temperatures ranging from 293 K up to 363 K. The results showed that CO2 absorption behaviour was influenced by the functionalized chains appended to the RTILs cation. A chemical enhancement of the CO2 absorption was observed when functionalized RTILs were used as absorption solvents. It was possible to increase the ionic liquid volumetric gas load almost threefold by attaching functional groups to the ionic liquid, whereas for the traditional amine solutions the maximum gas load is stoichiometrically limited.