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 Fe-III 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 desulfurizadon (EDS), where ILs containing anionic Fe-III 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 t(2g) 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. (C) 2012 Elsevier B.V. All rights reserved.