Liquid-liquid separation of n-hexane/1-hexene and cyclohexane/cyclohexene using deep eutectic solvents

Abstract

In this study, the feasibility of twelve different deep eutectic solvents (DESs) in separating olefins from paraffins was investigated. The DESs were prepared by combining choline chloride (ChCl) or tetrabutyl ammonium bromide (TBAB) with six different hydrogen bond donors. The preliminary COSMO-RS screening revealed that TBAB/TEG (1:4) DES has the high performance index (PI) for both systems. This performance was confirmed by experimental tests at 20% molar ratio of olefins in the feed mixture with the twelve DESs. When TBAB is used, the best distribution ratios were obtained for cyclohexane/cyclohexene system, however for hexane/1-hexene system high selectivities were gotten. In comparison with the performance of ionic liquids, TBAB-based DESs showed better distribution ratios and comparable selectivity. For the cyclohexane/cyclohexene system, TBAB-based DESs exhibited high distribution ratios because of their hydrophobic character represented by butyl chain. In the case of hexane/1-hexene system, TBAB-based DESs are more selective to 1-hexene due to the affinity of TBAB chain and positive charge bringing by ammonium group and 1-hexene. The liquid–liquid equilibrium (LLE) was explored for two ternary systems of {hexane (1) + 1-hexene (2) + DES (3)} and {cyclohexane (1) + cyclohexene (2) + DES (3)} at 298.15 K and 101 kPa using TBAB/TEG (1:4) DES. Furthermore, the NRTL model was successfully used to correlate the experimental LLE data and good predictions were obtained with COSMO-RS for both selected systems. Finally, the extraction mechanism was elucidated by applying 1H NMR analysis and no DESs were found in the raffinate phase.