Evaluation of the ionic liquids 1-alkyl-3-methylimidazolium hexafluorophosphate as a solvent for the extraction of benzene from cyclohexane: (Liquid + liquid) equilibria

doi:10.1016/j.jct.2011.12.006

The Journal of Chemical Thermodynamics

Volume 48, May 2012, Pages 145-149

https://doi.org/10.1016/j.jct.2011.12.006 Get rights and content

Abstract

In the catalytic hydrogenation of benzene to cyclohexane, the separation of unreacted benzene from the product stream is inevitable and essential for an economically viable process. In order to evaluate the separation efficiency of ionic liquids (ILs) as a solvent in this extraction processes, the ternary (liquid + liquid) equilibrium of 1-alkyl-3-methylimidazolium hexafluorophosphate, [C_nmim][PF₆] (n = 4, 5, 6), with benzene and cyclohexane was studied at T = 298.15 K and atmospheric pressure. The reliability of the experimentally determined tie-line data was confirmed by applying the Othmer–Tobias equation. The solute distribution coefficient and solvent selectivity for the systems studied were calculated and compared with literature data for other ILs and sulfolane. It turns out that the benzene distribution coefficient increases and solvent selectivity decreases as the length of the cation alkyl chain grows, and the ionic liquids [C_nmim][PF₆] proved to be promising solvents for benzene–cyclohexane extractive separation. Finally, an NRTL model was applied to correlate and fit the experimental LLE data for the ternary systems studied.

Highlights

► Measured ternary LLE for {cyclohexane (1) + benzene (2) + [C_nmim][PF₆] (3)}. ► Well correlated the LLE data with the NRTL model. ► Determined the benzene distribution coefficient and solvent selectivity. ► Very promising for the extraction of benzene from cyclohexane. ► Very helpful for overcoming drawbacks of the conventional separation process.

Introduction

As an important chemical raw material for the production of cyclohexanol, hexane diacid, and caprolactam, cyclohexane is generally produced by catalytic hydrogenation of benzene. The separation of the unreacted benzene from the product stream is inevitable and essential for an economically viable process. However, due to the very close boiling temperatures of benzene and cyclohexane (ΔT_b = 0.6 °C at atmospheric pressure), it is not practical to separate such a binary mixture by conventional distillation processes. In the chemical industry, this separation is commonly performed by extraction using organic compounds as solvents [1], such as sulfolane, dimethylsulfoxide, N-methylpyrrolidone, or N-formylmorpholine. Although these separation processes are highly optimized, they suffer from process complexity and high energy consumption due to the low extraction selectivity, considerable volatility, and high solvent solubility in the raffinate. These current drawbacks are the motivation for our work where we explore and evaluate the potential of ionic liquids as innovative solvents in the benzene–cyclohexane extraction process.

Ionic liquids (ILs) are molten salts that exist as a liquid at or near room temperature. Among their various applications, there is a substantial interest of replacing conventional volatile organic solvents in liquid–liquid extraction processes by ionic liquids due to their attractive properties, such as non-volatilization, non-flammability, and high separation selectivity. Until now, a considerable amount of research has focused on the features of ionic liquids as solvent for the extraction of aromatic compounds from their mixtures with the aliphatic hydrocarbons in terms of the experimental measured (liquid + liquid) equilibrium (LLE) [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38]. However, to the best of our knowledge, the ternary LLE data with respect to the benzene and cyclohexane separation system is still limited [25], [26], [27], [37], [38].

Among the most widely used ionic liquids, 1-alkyl-3-methylimidazolium hexafluorophosphate [C_nmim][PF₆] is easy-to-prepare and commercially available, and shows potential for separating benzene from its mixtures with cyclohexane. In this work, the LLE data for the ternary systems {cyclohexane (1) + benzene (2) + [C_nmim][PF₆] (3)}, where the alkyl C_n refers to a butyl (b), a pentyl (p), and a hexyl (h) group, were determined at T = 298.15 K and atmospheric pressure. From the experimental data, the benzene distribution coefficient (β) and the solvent selectivity (S) were calculated. The values obtained were compared with literature data available for other solvents. Moreover, the effect of the different alkyl group in the imidazolium ring on the benzene extraction was analyzed. Finally, the experimental LLE data were correlated with the NRTL model [39].

Section snippets

Chemicals

The sample provenance and mass fraction purity are shown in table 1. The ionic liquids 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF₆]), 1-pentyl-3-methylimidazolium hexafluorophosphate ([pmim][PF₆]), and 1-hexyl-3-methylimidazolium hexafluorophosphate ([hmim][PF₆]) were purchased with water mass fraction less than 1 · 10⁻³, and halide mass fraction less than 2 · 10⁻⁴. The reported densities of the ionic liquids at T = 298.15 K are 1.368 g · cm⁻³, 1.326 g · cm⁻³, and 1.292 g · cm⁻³, respectively

Experimental LLE data

The experimental (liquid + liquid) equilibrium data for the systems in this study at T = 298.15 K and atmospheric pressure are presented in table 2 and displayed in the triangular diagram shown in figure 1. For all the systems studied, benzene shows a much higher solution affinity in the extract phase than cyclohexane, which indicates that the ionic liquids [C_nmim][PF₆] are potential solvents for the extraction of benzene from its mixture with cyclohexane. Moreover, the fact that no ionic liquids

Conclusions

In order to explore and evaluate the potential of the ionic liquids [C_nmim][PF₆] (n = 4, 5, 6) as promising solvent candidates for the extraction of benzene from cyclohexane, the ternary LLE data for systems {cyclohexane (1) + benzene (2) + IL (3)} were determined at T = 298.15 K and atmospheric pressure. The high reliability of the experimental tie-line data was confirmed by applying the Othmer–Tobias correlation. Based on the obtained LLE data, the benzene distribution coefficients and solvent

Acknowledgements

This research is supported by Major State Basic Research Development Program of China (973 Program 2012CB720500), National Natural Science Foundation of China (NSFC 21076074, 21006029), Shanghai Pujiang Talents Program (10PJ1402400), Shanghai Natural Science Foundation (10ZR1407200), the Program of Introducing Talents of Discipline to Universities (111 Project: B08021), and the Fundamental Research Funds for the Central Universities of China. The financial support by the Max Planck Partner

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