Why Is CO_2 So Soluble in Imidazolium-Based Ionic Liquids?

摘要

Experimental and molecular modeling studies are conducted to investigate the underlying mechanisms for the high solubility of CO_2 in imidazolium-based ionic liquids. CO_2 absorption isotherms at 10, 25, and 50 ℃ are reported for six different ionic liquids formed by pairing three different anions with two cations that differ only in the nature of the "acidic" site at the 2-position on the imidazolium ring. Molecular dynamics simulations of these two cations paired with hexafluorophosphate in the pure state and mixed with CO_2 are also described. Both the experimental and the simulation results indicate that the anion has the greatest impact on the solubility of CO_2. Experimentally, it is found that the bis(trifluoromethylsulfonyl)imide anion has the greatest affinity for CO_2, while there is little difference in CO_2 solubility between ionic liquids having the tetrafluoroborate or hexafluorophosphate anion. The simulations show strong organization of CO_2 about hexafluorophosphate anions, but only small differences in CO_2 structure about the different cations. This is consistent with the experimental finding that, for a given anion, there are only small differences in CO_2 solubility for the two cations. Computed and measured densities, partial molar volumes, and thermal expansion coefficients are also reported.