DFT study of the interaction between ethanethiol and Fe-containing ionic liquids for desulfuration of natural gasoline

The interaction between ethanethiol molecule and either anhydrous FeIII chloride anions or 1-butyl-3-methylimidazolium ([BMIM]+) cations of ionic liquids (ILs) was investigated using Density Functional Theory approach and results were correlated with our previous experimental results on extractive desulfurization (EDS), where ILs containing anionic FeIII species show excellent performance to remove sulfur compounds from natural gasoline, especially when there exists an excess of FeCl3 in the [BMIM]+[FeCl4]− IL since this mixture contains binuclear anions [Fe2Cl7]−, whose Fe―Cl―Fe bonds are larger and less strong than Fe―Cl bonds of mononuclear anions [FeCl4]−, being then the former bonds activated for ethanethiol chemisorption. Molecular frontier orbitals and Mulliken atomic charges reveal that the high desulfurization performance could be due to a Dewar–Chatt–Duncanson-like mechanism of electron donation–backdonation among ethanethiol sulfur and transition metal centers of [Fe2Cl7]− anions, and this mechanism is promoted because of the symmetry affinity among the ethanethiol HOMO and the atomic orbital t2g on Fe sites in [Fe2Cl7]− LUMO. On the other hand, when there is no excess of FeCl3, EDS increases with the size of N-alkyl substitutes in [BMIM]+ due to ethanethiol physisorption by these cations.