On the Extraction of Aromatic Compounds from Hydrocarbons by Imidazolium Ionic Liquids

Abstract

The liquid-liquid equilibrium for the ternary system formed by n-octane andaromatic (alkylbenzenes) and heteroaromatic compounds (nitrogen and sulfur containingheterocyles) and 1-alkyl-3-methylimidazolium ionic liquids (ILs) associated with variousanions has been investigated. The selectivity on the extraction of a specific aromaticcompound is influenced by anion volume, hydrogen bond strength between the anion andthe imidazolium cation and the length of the 1-methyl-3-alkylimidazolium alkyl side chain.The interaction of alkylbenzenes and sulfur heterocyles with the IL is preferentially throughCH-π hydrogen bonds and the quantity of these aromatics in the IL phase decreases withthe increase of the steric hindrance imposed by the substituents on the aromatic nucleus. Inthe case of nitrogen heterocycles the interaction occurs preferentially throughN(heteroaromatic)-H(imidazolium) hydrogen bonds and the extraction process is largelycontrolled by the nitrogen heterocycle pKa. Competitive extraction experiments suggestthat benzene, pyridine and dibenzothiophene do not compete for the same hydrogen bondsites of the IL.