(104a) Desulfurization of Diesel Fuel by Extraction with Lewis-Acidic Ionic Liquid
In the last decade, much attention has been paid to the deep desulfurization of fuels due to more stringent environmental regulations.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 propertiessuch 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
 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.
 Welton T. Room-temperature ionic liquids: Solvents for synthesis and catalysis. Chem. Rev., 1999, 99: 2071-2084.
 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.
 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.