Extraction–oxidation desulfurization by pyridinium-based task-specific ionic liquids

The Brönsted acidic ionic liquids N-carboxymethylpyridine hydrosulphate ([CH2COOHPy][HSO4]) and N-carboxyethylpyridine hydrosulphate ([(CH2)2COOHPy][HSO4]) were synthesized and used as the extractant and catalyst for the extraction–oxidation desulfurization of model oil. The structures of the ILs were confirmed by 1H NMR and 13C NMR. The density, viscosity and acid strength of the ILs were also investigated. The acid strength was in order of H2SO4 (98%) > [CH2COOHPy][HSO4] > [(CH2)2COOHPy][HSO4]. The [CH2COOHPy][HSO4] showed better activity during the removal of dibenzothiophene (DBT) in n-octane by a combination of extraction and oxidation, and the sulfur removal reached 99.9% under the conditions of Vmodel oil = 20 mL, VIL = 1.2 mL, T = 30 °C and H2O2/S molar ratio (O/S) = 6. In the same conditions, the removal of benzothiophene (BT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) by using [CH2COOHPy][HSO4] reached 82.5% and 89.1%, respectively. Ionic liquid [CH2COOHPy][HSO4] can be recycled 9 times without a significant decrease in the sulfur removal.