Fuel Desulfurization via Ionic Liquid Extraction: Mechanistic Understanding from PMF Simulations

Regulatory mandates have imposed strict limits on allowable sulfur content in petroleum fuels to minimize atmospheric pollutants. While conventional desulfurization methods are highly effective at removing simple, reactive sulfur compounds, e.g., thiols and sulfides, they struggle in the removal of refractory sulfur compounds that are aromatic, sterically hindered, or electronically stabilized. Ionic liquids (ILs) are particularly well-suited to extract these problematic compounds, e.g., thiophenes and their derivatives, given their complementary physical properties. However, choosing and optimizing ILs that suit industrial desulfurization requirements has been challenging. In this work, potentials of mean force simulations on a biphasic oil-IL system have been performed to investigate IL-based extractive desulfurization (EDS) efficiency by examining the transfer of two aromatic sulfur compounds, thiophene (TH) and dibenzothiophene (DBT), and an oxidized derivative (DBTO2) from the model oil dodecane into 12 unique ILs. The ILs were constructed using combinations of 1-alkyl-3-methylimidazolium [CnMIM] cations, where n = 2, 4, and 8 (i.e., ethyl, butyl, and octyl), respectively, with the anions: thiocyanate [SCN], tetrafluoroborate [BF4], hexafluorophosphate [PF6], and bis(trifluoromethylsulfonyl)amide [NTf2]. The study investigates how interfacial charge density, cation ordering, electrostatic interactions, π-π stacking, free-volume formation, hydrophobicity, and oil dissolution into the IL phase collectively influence sulfur compound extraction efficiency and selectivity, while also evaluating the enhanced thermodynamics associated with combined oxidative and extractive desulfurization (ODS-EDS). Detailed molecular-level characterization of the IL-oil interfacial environment is provided to elucidate the key structural and energetic features governing sulfur compound partitioning and extraction behavior in imidazolium-based ILs.