Synthesis, characterization and application of some non-halogen ionic liquids as green solvents for deep desulfurization of diesel oil

Abstract

Some non-halogen ionic liquids (1-n-butyl-3-methylimidazolium hydrosulphates ([BMIM][[HSO4]), 1-n-butyl-3-methylimidazolium acetate ([BMIM][CH3COO]), 1-n-butyl-3-methylimidazolium dibutyl phosphate ([BMIM][DBP]), 1-n-octyl-3-methylimidazolium hydrosulphates ([OMIM][HSO4]), and 1-n-octyl-3-methylimidazolium acetate ([OMIM][CH3COO])) were synthesized, characterized, and used as solvents for extraction of sulfur compounds in model oils. All investigated ionic liquids show high desulfurization performance from the model oils. The influence of factors such as the structures of ionic liquid anions and cations, and the structure of sulfur compounds on the deep desulfurization was investigated. Some ionic liquids with high, mild and low desulfurization ability ([OMIM][CH3COO], [BMIM][DBP], and [BMIM][[HSO4], respectively) were used as solvents to extract sulfur compounds in diesel oil. The residual sulfur contents in diesel oil meet Euro III and Euro IV standards after five or six extraction cycles corresponding to a desulfurization efficiency of approximately 97%. The combination of oxidation reagents and [BMIM][HSO4] or [BPy][HSO4] increases the desulfurization efficiency. The used ionic liquids were recovered by re-extraction of sulfur compounds and oxidation products of DBT from ionic liquid phase to ethyl acetate or cyclohexane. The formation of sulfones and sulfoxides during oxidative desulfurization was confirmed by FT-IR spectra of ethyl acetate that was used to recover ionic liquids. The slight decrease in desulfurization efficiency of recovered ionic liquids shows their high potential of reusability. Used ethyl acetate and cyclohexane were recovered by distillation and sulfur compounds can be used for other purposes. The extraction process was performed at room temperatures and under normal pressures; no high hydrogen pressure was needed.