Deep Desulfurization of Fuels by Extraction with 4-Dimethylaminopyridinium-Based Ionic Liquids

In this work, three 4-dimethylaminopyridinium-based ionic liquids (ILs), N-ethyl-4-dimethylaminopyridinium dicyanamide ([C 2 4 DMAPy][N(CN) 2 ]), N-butyl-4-dimethylaminopyridinium dicyanamide ([C 4 4 DMAPy][N(CN) 2 ]) and N-hexyl-4-dimethylaminopyridinium dicyanamide ([C 6 4 DMAPy][N(CN) 2 ]), were synthesized and then demonstrated to be efficient for aromatic sulfur compounds extraction from fuels. The mutual solubility evaluating results indicated that 4-dimethylaminopyridinium-based ILs hardly dissolved in the fuels, while the solubility of 97 # gasoline in ILs varies from 7.0 wt % for [C 2 4 DMAPy][N(CN) 2 ] to 12.4 wt % for [C 6 4 DMAPy][N(CN) 2 ] and the solubility of 0 # diesel in ILs varies from 6.7 wt % for [C 2 4 DMAPy][N(CN) 2 ] to 9.7 wt % for [C 6 4 DMAPy][N(CN) 2 ]. Also, the sulfur partition coefficient was evaluated. In the case of model gasoline, the thiophene (TS) partition coefficient in model gasoline varies from 0.885 for [C 2 4 DMAPy][N(CN) 2 ] to 1.218 for [C 6 4 DMAPy][N(CN) 2 ]. In the case of model diesel, 4-dimethylaminopyridinium-based ILs exhibit a relatively higher sulfur partition coefficient, compared to alkyl modified pyridinium-based ILs. 1 H NMR results confirmed that the high aromatic π-electron density of the dimethylaminopyridinium cation was the main reason for the good extraction performance. 1 H NMR results also confirmed the K N sequence for each IL ([C 2 4 DMAPy][N(CN) 2 ] < [C 4 4 DMAPy][N(CN) 2 ] < [C 6 4 DMAPy][N(CN) 2 ]) is mainly due to the sulfur compounds and ILs structures. Furthermore, the extractive selectivity results indicated a more preferable extraction of TS than toluene with 4-dimethylaminopyridinium-based ILs. To have a better evaluation of the overall sulfur extraction performance, the three 4-dimethylaminopyridinium-based ILs were compared to other typical ILs (e.g., pyridinium-based ILs and imidazolium-based ILs), which suggested our synthesized 4-dimethylaminopyridinium-based ILs exhibited good balance between mutual solubility and the sulfur partition coefficient. ILs after use were regenerated by a water dilution process. Based on these results, 4-dimethylaminopyridinium-based ILs can be used as potential extractants for EDS processes.