(112a) The Solubility of Carbon Dioxide, Hydrogen and Their Mixture in the Ionic Liquids 1-Alkyl-3-Methylimidazolium Bis(trifluoromethanesulfonyl)Amide ([CnMIM+][TF2N-])

Ionic liquids are being investigated as potential solvents for pre- and post-combustion CO₂ capture. In pre-combustion applications, knowledge of the selectivity of CO₂ over H₂ in high pressure and high temperature gas mixtures is crucial. In the present work, the solubility of carbon dioxide, hydrogen and their mixture in the ionic liquids 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C_nMIM⁺][TF2N^-]) is computed using the isothermal-isobaric Gibbs ensemble Monte Carlo (NPT-GEMC) method. In the case of 1-3-dimethylimidazolium bis(trifluoromethanesulfonyl)amide ([DMIM⁺][TF₂N^-]) and 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([EMIM⁺][TF₂N^-]), the predicted absorption isotherms for CO₂ are compared with the experimental results of Ren et al. [J. Chem. Thermodyn., 42, 2010, 305]. The predicted solubility for CO₂ in 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([BMIM⁺][TF₂N^-]) and 1-hexyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([HMIM⁺][TF₂N^-]) are compared with the experimental results of Maurer et al. [J. Chem. Thermodyn., 2006,38,1396] and Brennecke et al. [J. Phys. Chem. B, 2004, 108, 20355] at 333 K. It is found that the predicted absorption isotherms for pure CO₂ in the studied ionic liquids are in good agreement with the experimental results.

 The absorption isotherms for H₂ in the ionic liquid [HMIM⁺][TF₂N^-] are computed at various temperatures ranging from 313 to 573 K. The computed absorption isotherms and Henry’s law constantagree well with the experimental results of Maurer et al. [J. Chem. Eng. Data, 51, 2006, 1364], Peters et al. [J. Chem. Eng. Data, 56, 2011, 1105], and Costa Gomes [J. Chem. Eng. Data, 52, 2007, 472 but there is a large discrepancy with the experimental results of Noble et al. [Ind. Eng. Chem. Res., 47,2007,3453]. The partial molar enthalpy was calculated for H₂ using the van’t Hoff relationship and a value of 1.49 kJ/mole was determined. The value is in reasonable agreement with the experiment value of 4.09 kJ/mole reported by Maurer et al. [J. Chem. Eng. Data, 51, 2006, 1364]. Unlike CO₂ , which has a negative enthalpy of absorption (and thus the solubility increases with decreasing temperature), the simulations correctly predict that the solubility of H₂ increases with increasing temperature. The absorption isotherms for H₂ are also computed in [DMIM⁺][TF₂N^-], [EMIM⁺][TF₂N^-], and [BMIM⁺][TF₂N^-] but unfortunately no experimental data are available for comparison. The studies have shown that hydrogen is sparingly soluble in ionic liquids, making experimental measurement difficult.

Mixed gas (CO₂/H₂) experiments are much harder to conduct that pure gas experiments and so it is usually assumed that absorption selectivity is ideal. To test this, mixed gas simulations were also carried out using NPT-GEMC method. The simulations indicate that the solubility selectivity of CO₂ over H₂ in the [HMIM⁺][TF₂N^-] is greater than that predicted by assuming an ideal mixture. The selectivity decreases from about 45 at 333 K to about 3 at 573 K, consistent with the simulation study of Shi et al. [J. Phys. Chem. B 2010,114,6531]. This is due to the fact that CO₂ solubility decreases with increasing temperature whereas H₂ solubility increases. The solubility selectivity of CO₂ over H₂ is computed for the remaining ionic liquids and the effect of the cation alkyl-chain is determined. These systems show more ideal behavior than liquid [HMIM⁺][TF₂N^-].