A simple and feasible separation process of toluene from n-heptane with imidazolium-based switchable solvents

Abstract

The separation of aromatics and aliphatic hydrocarbons is one of the most challenging and energy consuming operations in the petrochemical industry. In this work, two switchable solvents, 1,8-diazabicyclo[5.4.0]undec-7-ene imidazole ([DBU][Im]) and 1,1,3,3-tetramethylguanidine imidazole ([TMG][Im]), were synthesized and used for phase-change separation of toluene from n-heptane. The structures of these two switchable solvents were analyzed by 1H Nuclear Magnetic Resonance (NMR) and Fourier Transform Infrared (FT-IR) spectroscopy. The miscibility of various components in these two ILs before and after treatment with CO2 were studied, the results of which indicated that both these two switchable solvents could be utilized to achieve the effective separation of toluene from n-heptane. Among them, [TMG][Im] was preferable to be used at a low toluene content in the initial toluene/n-heptane system through feasibility study. The effects of extraction time, switchable solvent dosage and molar fraction of toluene on separation performance and the reusability were also investigated, demonstrating that [TMG][Im] could be used simply and feasibly for phase-change separating toluene from n-heptane and showed very good reusability and stability. Finally, the mechanism of reversible phase-change extraction process with [TMG][Im] was explored, indicating that the formation of [TMG][ImCO2] could greatly increase the polarity of the switchable solvent, which is conducive to separating toluene and recovering [TMG][Im] by removing CO2.