A Monte Carlo simulation study to predict the solubility of H₂S in ionic liquids with 1-butyl-3-methylimidazolium ([C₄mim⁺]) cation and tetrafluoroborate ([BF₄⁻]), hexaflorophosphate ([PF₆⁻]) and bis(trifluoromethanesulfonyl)amide ([Tf₂N⁻]) anions

Abstract

The solubilities of H₂S in ionic liquids, 1-butyl-3-methylimidazolium tetrafluoroborate ([C₄mim⁺][BF₄⁻]), 1-butyl-3-methylimidazolium hexaflorophosphate ([C₄mim⁺][PF₆⁻]) and -butyl-3-methylimidazolium tetrafluoroborate bis(trifluoromethanesulphonyl)amide ([C₄mim⁺][Tf₂N⁻]) are predicted using isothermal–isobaric Gibbs ensemble Monte Carlo method (NPT-GEMC) at temperatures ranging from 333 to 453 K and pressure up to 20 bar. The low pressure points (up to 3 bar) of the absorption isotherms are fitted to a straight line to get a rough estimation of the Henry’s law constants. The van’t Hoff relationship is used to estimate the partial molar enthalpy of the absorption. The obtained results are in a good agreement with Jou and Mather [18], and Jalili et al. [13]. When comparing the solubility in ILs, it is found that H₂S solubility is highest for [C₄mim⁺][Tf₂N⁻], followed by [C₄mim⁺][PF₆⁻]. The lowest solubility is observed in [C₄mim⁺][BF₄⁻]. The highest solubility in [C₄mim⁺][Tf₂N⁻] is consistent with Jalili et al. [13]. However, their results indicate slightly higher solubility in [C₄mim⁺][BF₄⁻] than in [C₄mim⁺][PF₆⁻], and do not agree with the predictions. Upon absorption, the molar volumes of the mixtures decrease linearly, showing only small changes in volume. The effect of H₂S absorption on ILs is further studied by calculating the radial distribution functions between the ions. The results indicate that the solute molecules accommodate themselves in the cavities without significantly disturbing the ionic arrangement of the ions, similar to CO₂ absorption in ILs. The spatial distribution functions show similar spatial distribution for H₂S around cation in all of the studied ILs, whereas the distribution around anion depends on the shape and flexibility of the anion. The mechanism of H₂S absorption is studied by computing the van der Waals (VDW) and electrostatic (ELEC) energies. It is observed that the solubility of H₂S in the studied ILs is primarily controlled by VDW interaction. When comparing the interaction of H₂S with the ions, it is found that solute molecules interact with cations mainly due to VDW interaction. Both VDW and ELEC energies contribute in the interaction between H₂S and anions.

Keywords:

• Gibbs ensemble Monte Carlo simulation
• ionic liquid
• hydrogen sulphide
• physical absorption

Acknowledgements

I thank Professor Edward J. Maginn at the University of Notre Dame for providing Cassandra Monte Carlo code. I also thank Eliseo T. Marin Rimoldi at the University of Notre Dame for his assistance with interaction energy calculations.