Benzyl- and Vinyl-Functionalized Imidazoium Ionic Liquids for Selective Separating Aromatic Hydrocarbons from Alkanes

The separation of aromatics and aliphatic hydrocarbons is one of the most challenging and energy consuming operations in the petrochemical industry. In this study, two ionic liquids (ILs), N-benzyl-N-methylimidazoium bis(trifluoromethylsulfonyl)imide (IL-a) and N-benzyl-N-vinylimidazolium bis(trifluoromethylsulfonyl)imide (IL-b), were synthesized. Six ternary systems, i.e., toluene–heptane–IL (IL-a or IL-b), benzene–hexane–IL (IL-a or IL-b), and benzene–cyclohexane–IL (IL-a or IL-b), were studied in terms of both quantum chemical calculation and liquid–liquid extraction (LLE). The quantum calculation results showed that both ILs had stronger interaction with aromatics than that with alkanes. For both ILs, stronger binding energy was obtained with toluene than that with benzene. The liquid–liquid extraction experiments were conducted at 298.2 K and atmospheric pressure. The distribution coefficients of aromatic compounds (benzene and toluene) were over 0.72 when IL-a was used as extractant and above 0.75 when IL-b was used. When IL-a was used in the ternary system with benzene and hexane, the selectivity was more than 30, and the distribution coefficient of benzene was over 2.2. Both ILs could be reused more than 10 times without significant loss of selectivity and decrease of distribution coefficients. Better separation performance was obtained at 298.2 K than that at 318.2 K. Another three ternary systems with long-chain alkanes were also tested. Experimental LLE data of these studied ternary systems could be correlated adequately using the NRTL thermodynamic model.