Extraction and separation of benzene/n-hexane using a deep eutectic solvent consisting of a dicationic ionic liquid and a polyol

The boiling points of benzene and n-hexane are similar, and they readily form an azeotropic mixture. Current separation processes have high energy consumption and low separation efficiency. In order to efficiently separate the benzene/n-hexane system, an asymmetric dicationic ionic liquid 1-(3-(trimethylammonium)propan-1-yl)-3-methylpiperidine dibromide ([N₁₁₁C₃MPi]Br₂) (a hydrogen bond acceptor) was synthesized. [N₁₁₁C₃MPi]Br₂ was subsequently mixed with ethylene glycol (a hydrogen bond donor) in different proportions to prepare a deep eutectic solvent [N₁₁₁C₃MPi]Br₂/ethylene glycol. The density and viscosity of the deep eutectic solvent were measured in the temperature range 288.15-323.15 K. The results show that the density of the deep eutectic solvent decreased linearly with increasing temperature, and the viscosity decreased gradually. At 313.15 K, the viscosity of [N₁₁₁C₃MPi]Br₂/ethylene glycol (molar ratio of 1∶6) was only 25.57 mPa·s. Using the deep eutectic solvent as the extractant, the conditions for separation of benzene/n-hexane were optimized by orthogonal tests, namely a separation time of 1.5 h, separation temperature of 313.15 K, feed ratio (volume ratio of deep eutectic solvent to benzene/n-hexane solution) of 1∶1. Under the optimized conditions, the distribution coefficient and selectivity of benzene were 9.29 and 120.83, respectively, which were significantly higher than those reported using sulfolane and other ionic liquid systems.