Effect of Anions on CO2 Solubility of Ionic Liquids Using a Magnetic Suspension Balance

Room-temperature ionic liquids (RT-ILs) generally have negligibly small vapor pressure, high thermal and chemical stability, and can have higher solubilities of acidic gases such as CO₂, H₂S, NO_x and SO_x than those of normal gases, N₂, H₂, and O₂.  These attractive properties could lead the way to clean gas separation technologies that eliminate emissions of the liquid absorbent into the atmosphere.  We have proposed an effective CO₂ separation process from gas mixtures by physical absorption using ionic liquids.  Since the efficiency in the physical absorption process is predominantly determined by the gas solubility in a unit volume/weight of absorbent, it is of primary importance to develop highly CO₂ absorbing ILs.

In the present study, we have investigated the effect of anions (i.e., TFSA^-; = bis(trifluoromethanesulfonyl)amide, BETA^-; = bis(tetrafluoroethanesulfonyl)amide, and NFBS^-; = nonafluorobutanesulfonate) for 1-ethyl-3-methylimidazolium cation [Emim] on CO₂ absorption using a Magnetic Suspension Balance at 313.2 K and up to 6 MPa.

The CO₂ solubility in ionic liquids increased with increasing pressure.  [Emim][BETA] showed highest CO₂ solubility compared with the other ones, especially, the solubility is increased about 4% in comparison with [TFSA] which have a similar structure.  On the other hand, [Emim][NFBS] showed 30% higher CO₂ solubility at 3 MPa, compared with [Emim][OTf] (1-ethyl-3-methylimidazolium trifluoromethanesulfonate).  The symmetrical structure such as TFSA^- and BETA^- shows a higher CO₂ solubility rather than the asymmetric structure such as NFBS^- and OTf^-.  CO₂ solubility was increased with the extension of the perfluoroalkyl chain length of the anion.