Extractive desulfurization and denitrogenation of fuels using ionic liquids

Two types of ionic liquids, 1-alkyl-3-methylimidazolium [AMIM] tetrafluoroborate and hexafluorophosphate and trimethylamine hydrochloride (AlCl3-TMAC), were demonstrated to be potentially applicable for sulfur removal from transportation fuels. EMIMBF4 (E = ethyl), BMIMPF6 (B = butyl), BMIMBF4, and heavier AMIMPF(6) showed high selectivity, particularly toward aromatic sulfur and nitrogen compounds, for extractive desulfurization and denitrogenation. The used ionic liquids were readily regenerated either by distillation or by water displacement of absorbed molecules. The absorbed aromatic S-containing compounds were quantitatively recovered. Organic compounds with higher aromatic pi-electron density were favorably absorbed. Alkyl substitution on the aromatic rings was found to significantly reduce the absorption capacity, as a result of a steric effect. The cation and anion structure and size in the ionic liquids are important parameters affecting the absorption capacity for aromatic compounds. At low concentrations, the N- and S-containing compounds were extracted from fuels without mutual hindrance. AlCl3-TMAC ionic liquids were found to have remarkably high absorption capacities for aromatics.