A DFT study of the extractive desulfurization mechanism by [BMIM](+)[AlCl4](-) ionic liquid.

In this work, the interaction nature between BMIMAlCl4 ionic liquid (IL) and aromatic sulfur compounds (thiophene, benzothiophene, and dibenzothiophene) has been studied by means of density functional theory (M06-2X functional) combined with an implicit solvation model. Although BMIMAlCl4 is a metal-containing IL, its extractive desulfurization mechanism is different from other metal-containing ILs but similar to non-metal-containing ILs. Important reactions involved in extractive desulfurization (EDS) were systematically studied. Our results have demonstrated that both the cation and the anion play important roles in EDS. On the basis of the structure analysis, reduced density gradient analaysis (RDG), and energy decomposition analysis, BMIM cation affords a π-π interaction while AlCl4 anion provides a hydrogen bonding interaction. Electrostatic potential analysis implies the dominant π-π interaction and hydrogen bonding interaction are driven by electrostatic interaction between IL and aromatic sulfur compounds. Interaction energy between BMIMAlCl4 and thiophene (TH), benzothiophene (BT), and dibenzothiophene (DBT) follows the order TH < BT < DBT. Moreover, Al-containing IL with a high molar ratio of AlCl3 ([BMIMCl]/2[AlCl3]) has also been studied. Results show that Al2Cl7 species will be formed with excess AlCl3. However, the Al2Cl7-based IL cannot improve the EDS performance. Improvement of EDS performance with a high molar ratio of AlCl3 is credited to the Lewis acidity of AlCl3. Charge analysis reveals that there is no obvious charge transfer during the reaction, which is different from Fe-containing ILs as well as solid sorbents. In addition, CH-π interaction is not important for the current system.