Extractive–oxidative desulfurization of model fuels using imidazole-based dicationic ionic liquids as extractants

Abstract

A series of bis-imidazole ionic liquid extractants [C_n(MIM)₂]X₂ (n = 2, 4, 6; X = HSO₄⁻, p-CH₃C₆H₄SO₃⁻, CF₃COO⁻) with different acidities were designed, synthesized and applied to the extractive–oxidative desulfurization system with H₂O₂ as an oxidant. The extractants were characterized by UV-vis spectroscopy and other methods. The effects of different factors on the desulfurization efficiency were investigated. The results showed that the desulfurization efficiency was highest when bis-(1-butyl-3-methylimidazole p-toluenesulfonate) [C₄(MIM)₂](p-TSA)₂ was used as an extractant. Under the optimal reaction conditions, the removal rate of dibenzothiophene (DBT) reached 98.35%, and its good extraction performance depended on the length of the carbon chain in the cation and the acidity of the anion. Increasing the length of the carbon chain in the cation and choosing the anion with strong acidity could improve the extraction efficiency of sulfide. The removal order of different sulfides by [C₄(MIM)₂](p-TSA)₂ was DBT > 4,6-dimethyldibenzothiophene (4,6-DMDBT) > benzothiophene (BT), and the extraction performance of [C₄(MIM)₂](p-TSA)₂ which had been used 6 times was recovered after back-extraction. Under the same reaction conditions, the removal rate of DBT could still reach 92.51%. After the discussion on the reactive oxygen species and desulfurization mechanism, it was found that ˙OH and ˙O₂⁻ played a major role in the oxidation of sulfide. At the same time, the interaction between the extractant and sulfide was the key factor for the extractant to show great extraction performance.