COSMO-RS prediction, liquid-liquid equilibrium experiment and quantum chemistry calculation for the separation of n-butanol and n-heptane system using ionic liquids

Abstract

Ionic liquids (ILs) have been extensively used in the separation of alkane + alcohol azeotropes as entrainers by liquid–liquid extraction. n-Butanol is widely added to the fuel because of its high calorific value and low volatility. It is of significance to recovery n-butanol from the azeotropic mixture containing n-butanol. In this work, ionic liquids, formed by combining 17 cations and 12 anions, were screened based on the COSMO-RS model. 1-ethyl-3-methylimidazolium dicyanamide [EMIM][DCA], 1-butyl-3-methylimidazolium dicyanamide [BMIM][DCA] and 1-hexyl-3-methylimidazolium dicyanamide [HMIM][DCA] were selected as entrainers. The liquid–liquid equilibrium data of the ternary system {n-heptane + n-butanol + [EMIM][DCA], n-heptane + n-butanol + [BMIM][DCA], n-heptane + n-butanol + [HMIM][DCA]} were measured at 298.15 K and 101.3 kPa. The quality of experimental data was tested by Othmer-Tobias, Bachman and Hand equations. The distribution coefficient (β) and selectivity (S) were calculated to evaluate the separation effect of the three ILs. The NRTL model was used to correlate liquid–liquid equilibrium experimental data. Finally, quantum mechanics calculations were performed to study the interactions between ILs and n-butanol through the reduced density gradient (RDG) function and quantum theory of atom in the molecule (QTAIM). The results of quantum chemistry calculation showed that the separation of n-butanol and n-heptane mixture by ionic liquids with [DCA] anions were dominated by hydrogen bonding. The research process is devoted to the development of green entrainers.