Application of [HMim][NTf2], [HMim][TfO] and [BMim][TfO] ionic liquids on the extraction of toluene from alkanes: Effect of the anion and the alkyl chain length of the cation on the LLE

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Abstract

In this paper, the separation of toluene from the aliphatic hydrocarbons heptane and cyclohexane employing the ionic liquids 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [HMim][NTf2], 1-hexyl-3-methylimidazolium trifluoromethanesulfonate, [HMim][TfO] and 1-butyl-3-methylimidazolium trifluoromethanesulfonate, [BMim][TfO], as solvents was studied and discussed. Liquid–liquid equilibrium data for the ternary systems {heptane, or cyclohexane + toluene + [HMim][NTf2], or [HMim][TfO], or [BMim][TfO]} and {heptane + cyclohexane + [HMim][NTf2], or [HMim][TfO], or [BMim][TfO]} were measured at T = 298.15 K and atmospheric pressure. The degree of consistency of the tie-lines was tested using the Othmer–Tobias equation. The solute distribution ratio and selectivity, derived from the experimental tie-lines, were used to determine if these ionic liquids can be used as potential solvents on the extraction of toluene from aliphatic hydrocarbons; a comparison with literature data where sulfolane is used as solvent was also included. Finally, the experimental data were correlated with the NRTL and UNIQUAC thermodynamic models.

Highlights

► Several ionic liquids were studied as solvent to extract toluene from heptane and cyclohexane. ► (Liquid + liquid) equilibrium data were measured at 298.15 K and atmospheric pressure. ► Selectivity and solute distribution ratio were calculated and compared with those found in literature for sulfolane. ► Experimental data were correlated using NRTL and UNIQUAC thermodynamic models.

Introduction

The unique properties of ionic liquids (ILs) such as their negligible vapor pressure, large liquid range, high thermal and chemical stability and strong solvent properties [1], have made these compounds promising solvents to replace traditional organic solvents in liquid extraction processes. In petrochemical industry, these separation processes use environmentally unfriendly solvents such as sulfolane, diethylene glycol or dimethyl sulfoxide [2], [3], [4], [5]; moreover, additional process steps are necessary in order to recover the solvent used. Consequently, additional investments and large energy consumption are also required. On the contrary, the employment of ILs allows the simplification of the processes resulting in an easier recovery of the IL, due to their very low vapor pressure [6], [7].

In recent years ILs have attracted the attention of numerous researchers, and different works on liquid extraction of aromatics from aliphatic hydrocarbons using ILs have been published [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]. However, there are still few works published of liquid–liquid equilibrium (LLE) where a comparative study between ILs with different anion and cation was made.

This work is a continuation of our previous studies on the extraction of aromatics from aliphatic hydrocarbons using ILs [21], [22], [23], [24], [25], [26], [27]. In this case, the selected ionic liquids were 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [HMim][NTf2], 1-hexyl-3-methylimidazolium trifluoromethanesulfonate, [HMim][TfO] and 1-butyl-3-methylimidazolium trifluoromethanesulfonate, [BMim][TfO]. LLE data were experimentally determined for the ternary systems {heptane, or cyclohexane (1) + toluene (2) + [HMim][NTf2], or [HMim][TfO], or [BMim][TfO] (3)} and {heptane (1) + cyclohexane (2) + [HMim][NTf2], or [HMim][TfO], or [BMim][TfO] (3)} at T = 298.15 K and atmospheric pressure. The solute distribution ratio and the selectivity for the ternary systems were obtained from LLE data and were compared with those obtained using sulfolane as solvent [3].

The reliability of the experimental LLE data was tested by Othmer–Tobias equation [28] and correlated with the Non Random Two Liquid (NRTL) model [29]. Besides, the ternary systems containing [BMim][TfO] were also correlated with the UNIversal QUAsiChemical (UNIQUAC) model [30].

Section snippets

Chemicals

Cyclohexane, heptane and toluene were supplied by Sigma–Aldrich with mass fraction purity greater than 0.999, 0.995 and 0.999, respectively. The ionic liquids [HMim][NTf2], [HMim][TfO] and [BMim][TfO] were supplied by IoLiTec GmbH (Germany) with mass fraction purity greater than 0.990. Prior to their use, the ILs were subjected to vacuum (p = 0.2 Pa) at moderate temperature (T = 343.15 K) until the density value was constant, to reduce the water content and volatile compounds to negligible values.

Experimental LLE data

The experimental LLE data for the ternary systems {heptane, or cyclohexane (1) + toluene (2) + [HMim][NTf2], or [HMim][TfO], or [BMim][TfO] (3)} and {heptane (1) + cyclohexane (2) + [HMim][NTf2], or [HMim][TfO], or [BMim][TfO] (3)} at T = 298.15 K and atmospheric pressure are given in table 2, and the corresponding ternary diagrams, in mole fraction, are plotted in figure 1. As it can be seen in this figure, toluene is more soluble in the tested ILs than the studied alkanes (heptane, or cyclohexane).

Conclusions

LLE data for the ternary systems {heptane, or cyclohexane + toluene + [HMim][NTf2], or [HMim][TfO], or [BMim][TfO]} and {heptane + cyclohexane + [HMim][NTf2], or [HMim][TfO], or [BMim][TfO]} were experimentally measured at T = 298.15 K and atmospheric pressure. The solute distribution ratio and selectivities were ascertained and compared with those reported in literature for sulfolane.

The S values for the studied ILs were higher than the unity, so these ionic liquids could be used as solvents for the

Acknowledgments

The authors are grateful to Xunta de Galicia (Spain) for the financial support via the project INCITE09314258PR. S.C. is also thankful to Xunta de Galicia for her predoctoral grant (Plan I2C). N.C. and E.J.G. acknowledge the financial support from Fundação para a Ciência e a Tecnologia (Portugal) through their postdoctoral grants (SFRH /BPD/37775/2007 and SFRH /BPD/70776/2010, respectively).

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