The selectivity for sulfur removal from oils is an important topic. In this work, the selectivity for different sulfur removal methods has been studied by conceptual density functional theory (CDFT) at the B3LYP/6-31111G(3df,2p) level of theory. In principle, the selectivity is directly related to the mechanisms of sulfur removal. It cannot be precisely elucidated until the mechanisms are totally known. However, current work shows that relationships can be constructed between CDFT and the selectivity. That is, for hydrodesulfurization, good descriptors will be ionization energy, hardness, and bond lengths of SAC; for adsorptive desulfurization, the hardness is a good descriptor; for oxidative desulfurization, good descriptors are electron density and Fukui function. And for extractive desulfurization (nonmetal-based ionic liquids), electron affinity and electrophilicity may be good descriptors. In addition, structures and frontier orbitals of various sulfides have also been discussed. It is hoped that these relationships between CDFT and selectivity can give useful information to develop highly efficient sulfur removal methods for specific sulfides, like 4,6-dimethyldibenzothiophene, and 4-methyldibenzothiophene.

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In this work, the interaction nature between [BMIM](+)[AlCl4](-) 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 [BMIM](+)[AlCl4](-) 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 [BMIM](+)[AlCl4](-) 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.