In the last decade, much attention has been paid to the deep desulfurization of fuels due to more stringent environmental regulations.^[1]Although hydrodesulfurization (HDS) is highly efficient in removing thiols, sulfides, and disulfides. It is difficult to reduce refractory sulfur-containing compounds such as dibenzothiophene (DBT) and its derivatives to an ultra-low level. Therefore, the development of alternative ultra-deep desulfurization processes, such as adsorption, biodesulfurization, oxidation, is desired. Ionic liquids (ILs) have attracted intensive interest because of their unique chemical and physical properties^[2]such as negligible vapor pressure, low toxicity, high chemical and thermal stabilities, and ability to dissolve a wide range of organic and inorganic compounds. Ionic liquids have been used as alternatives to classical molecular solvents in a range of fundamental researches and applications, including synthesis, catalysis, separation and so on. Extraction of fuels using ionic liquids to remove sulfur compounds has been reported. ^[3,4] Most of them are neutral ionic liquids, and their efficiencies of sulfur removal are rather low. Although AlCl₃-based Lewis-acidic ionic liquids have excellent desulfurization results, they are sensitive to moisture and air.

In this work, a kind of Lewis-acidic ionic liquid 1-butyl-3-methylimidazolium tetrahalogenoferrate(III) ([BMIM][FeCl₄]) and other two ionic liquids 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF₆]) and 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF₄]), are studied for the extraction sulfur compounds from model diesel and diesel fuel, and the feasibility of their regeneration is also investigated. Unlike AlCl3-based ionic liquids, [BMIM][FeCl₄] is air and moisture stable, and is immiscible with water. The electronic configuration of Fe³⁺is1s²2s²2p⁶3s²3p⁶3d⁵4s⁰. According to π-complexation mechanism the cations with empty s-orbitals and the electron density can form the usual σ bonds with their s-orbitals and, in addition, their d-orbitals can back-donate electron density to the anti-bonding π-orbitals of the sulfur rings. Fe³⁺ can form π-complexation bonding with aromatic sulfur compounds. [BMIM][FeCl₄] demonstrated effective for removal of aromatic sulfur compounds from diesel. It can be regenerated through reextraction by hexane, and might be used as a promising solvent for the desulfurization of diesel by an extractive desulfurization process.

Keywords: desulfurization, extraction, ionic liquid, Lewis-acidic, diesel

References

[1] Song C S. An overview of new approaches to deep desulfurization for ultra-clean gasoline, diesel fuel and jet fuel. Catal. Today, 2003, 86: 211-263.

[2] Welton T. Room-temperature ionic liquids: Solvents for synthesis and catalysis. Chem. Rev., 1999, 99: 2071-2084.

[3] Bösmann A, Datsevich L, Jess A, Lauter A, Schmitz C, Wasserscheid P. Deep desulfurization of diesel fuel by extraction with ionic liquids. Chem. Commun., 2001, 23: 2494-2495.

[4] Alonso L, Arce A, Franciso M, Rodrĺguez O, Soto A. Gasoline desulfurization using extraction with [C8mim][BF₄] ionic liquid. AIChE Journal, 2007, 53: 3108-3115.