Ionic Liquids for Hydrocarbon/Thiophene Separation: Extensive Analysis of Structure–Property Correlations

Abstract The development of energy-efficient technologies for the deep desulfurization of transport fuels remains a paramount challenge in modern refining. Ionic liquids (ILs) are recognized as promising solvents for extractive desulfurization. The choice of particular ILs for separation processes should be guided by thermodynamic data and models. In this study, we present an analysis of the performance of 192 distinct ILs in the separation of thiophene from hydrocarbons. A dataset of 1597 experimental infinite dilution activity coefficients of thiophene and a number of hydrocarbons in ILs was assembled. These data allowed to uncover the fundamental structure-property relationships that govern the limiting activity coefficient values and hydrocarbon/thiophene selectivity of ILs. The volume density of ion charges quantified via the reciprocal molar volume was found to be the most important factor, with the polarity of the substituents in the cation being the second most important one. Predictive correlations with the two descriptors reflecting these factors reach R2 = 0.8 and above in most cases. The best selectivity is achieved with ILs that contain cations with short-chain polar substituents and small anions, significantly exceeding the performance of conventional molecular solvents. To facilitate the virtual screening of ILs, we introduce a computationally inexpensive, atom-contribution-based approach to estimating IL molar volumes, which achieves high accuracy without the need for experiments or quantum chemical calculations.