Four low-viscosity ionic liquids (ILs) based on the dicyanamide anion ([N(CN) 2 ] -), i.e., 1-butyl-3-methylimdazolium ([BMI][N(CN) ), have been investigated to determine their extraction capability for thiophene (TS) and dibenzothiophene (DBT) from model fuel oils. Aromatic imidazolium is more efficient than cyclic thiophenium and tetrahedral trialkylsulfonium; specifically, the S-extraction ability follows the orderwith DBT being more efficiently extracted than TS. The S-extraction of [BMI][N(CN) 2 ] has been investigated as a representative with respect to the influence of extraction temperature, IL : oil mass ratio, initial S-content, multiple extractions and reusability, along with its mutual solubility in oil. The percentage of S-removal from gasoline and diesel fuel were 48.5 and 68.7%, respectively, in a single extraction at 25 • C, 1 : 1(w/w) IL : oil, 5 min; the S-content in gasoline decreased from 599 ppm to 4 ppm after 5 extraction cycles and in diesel fuel decreased from 606 ppm to an undetectable value after 4 cycles. The mutual solubility is not pronounced and the extraction efficiency is not conspicuously changed after 6 regeneration cycles. It is worth noting that a short extraction time of < 5 min is observed for all the ILs at room temperature, which is understood by their low viscosities and effective mass transfer. This work may offer a new option for the deep desulfurization of fuel oils.

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In this work, the extractive and oxidative deep desulfurizations of model fuel oils using a low-viscosity ionic liquid, i.e., 1-ethyl-3-methylimidazolium dicyanamide ([C2mim][N(CN)2]), are investigated. [C2mim][N(CN)2] is capable of effectively extracting thiophene (TS) and dibenzothiophene (DBT) from oils. The sulfur content in the raffinate phases is only ∼10 ppm after a few extraction steps. A short extraction equilibrium time of <5 min is observed. The extraction operation is insensitive to temperature, and it can be effectively performed at or around room temperature. Unexpectedly, the oxidative removal of DBT by such a dicyanamide-based ionic liquid is not effective and is not as good as the corresponding extraction operation. Such an undesirable oxidative desulfurization is understood at a molecular level from ab initio calculations, and it may be ascribed to the strong intermolecular interaction between CH3COOH or CH3COOOH and [C2mim][N(CN)2] phase. Therefore, such a dicyanamide-based ionic liquid is efficient for direct extractive desulfurization, while it is less efficient for oxidative desulfurization.

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Ionic liquids (ILs), a new class of green solvents, have recently been undergoing intensive research on the removal of thiophenic sulfur species (e.g., dibenzothiophene) from fuels because of the limitation of the traditional hydrodesulfurization method in removing these species. In this work, deep oxidative desulfurization of diesel fuels by six functional acidic ILs are studied, in which ILs are used as both extractant and catalyst, and 30 wt % H2O2 solution as oxidant. These ILs include both Lewis acidic species such as 1-butyl-3-methylimidazolium chloride/2ZnCl2 ([C4mim]Cl/2ZnCl2 and [C4mim]Cl/ZnCl2) and Brønsted acidic species such as 1-methyl-3-ethylcarboxylic acid imidazolium hydrogen sulfate ([CH2COOHmim]HSO4), 1-methyl-3-(butyl-4-sulfinate) imidazolium hydrogen sulfate ([SO3H–C4mim]HSO4), [Hmim]HSO4, and [C4mim]HSO4 where different acidic groups such as −H, −COOH, and −SO3H are appended to the cations. Except for [CH2COOHmim]HSO4, both Brønsted and Lewis acidic ILs are capable of effectively removing dibenzothiophene from model diesel fuels, where 100% sulfur removal is obtained for [C4mim]Cl/2ZnCl2 and [SO3H–C4mim]HSO4. The effects of temperature, molar ratio of O/S, mass ratio of IL/oil, and IL regeneration on desulfurization are investigated systematically for [C4mim]Cl/2ZnCl2 and [SO3H–C4mim]HSO4. The desulfurization ability is not sensitive to the mass ratio of IL/oil, which is desired for reducing IL dosage in industrial application; the ILs can be recycled six times with merely a negligible loss in activity. [C4mim]Cl/2ZnCl2 can reduce the sulfur content in real commercial diesel fuel from 64 to 7.9 ppm with a sulfur removal of 87.7%; however, it is not too effective for coke diesel fuel with high initial sulfur content of 5380 ppm. This work tends to show that diesel fuels can be purified to sulfur-free or ultralow sulfur fuels by further deep oxidative desulfurization by using ILs after hydrodesulfurization.