Experimental Measurements of Co2 Absorption in Ionic Liquids and Its Effect On Viscosity

Ionic liquids (ILs) were synthesized and investigated as potential absorbents for CO2 capture from postcombustion flue gas. All ILs used a phosphonium based cation and either an amino acid based anion (AAs) or an aprotic heterocyclic anion (AHAs). The CO2 absorption was measured using a volumetric method and the results were modeled with two different Langmuir-type absorption models. All of the AAs had greater than 0.5 moles of CO2 per mole of IL, while the AHAs showed a range from only physical absorption to near 1 mole of CO2 per mole of IL. The chemical absorption of CO2 into AAs, except for [P66614][Pro], caused a dramatic increase in the viscosity. The AHAs and [P66614][Pro] had a smaller increase ([P66614][CNpyr] and [P66614][CApyr] had no detectable increase) in viscosity when CO2 was absorbed. Water was investigated as a solvent to further reduce the viscosity. Small amounts of water significantly reduce the viscosity of both the neat and CO2 saturated IL. Finally, preliminary experiments with changing the cation size had little impact in the CO2 capacity, but more interesting, ILs with the [P4444]+ cation were observed to phase change upon the reaction with CO2.