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.