Toward an Understanding of the Forces behind Extractive Desulfurization of Fuels with Ionic Liquids

In this study, the extractive desulfurization of model fuel oil with ionic liquids (ILs) has been studied in an attempt to gain insights into the dominant forces controlling the extraction efficiencies of aromatic sulfur compounds, thiophene and dibenzothiophene. This work investigates the intrinsic properties of a series of common ILs based on a constant (molar) amount of IL to directly draw insights into the intrinsic properties of each IL’s extraction capability. Experimentally both the cation and anion size influenced the efficiency of extraction, following the trend pyridinium > imidazolium > pyrrolidinium for the cation and [NTf2]⁻ > [OTf]⁻ > [PF6]⁻ > [BF4]⁻ for the anion. Similar trends are observed for both thiophene and dibenzothiophene. Density functional theory modeling, using the APFD method, was employed to quantify the complexation energies and corresponding dispersion contributions between thiophene and the cations as well as between thiophene and the anions used in this work, showing a similar trend to the experimental results. Through a combination of experimental and computational analyses it is suggested that the dominant force in extraction is dispersion-driven binding between the ions and S-compounds.