(Liquid + liquid) equilibria in the binary systems (aliphatic, or aromatic hydrocarbons + 1-ethyl-3-methylimidazolium ethylsulfate, or 1-butyl-3-methylimidazolium methylsulfate ionic liquids)

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Abstract

(Liquid + liquid) equilibria of 14 binary systems composed of n-hexane, n-heptane, benzene, toluene, o-xylene, m-xylene, or p-xylene and 1-ethyl-3-methylimidazolium ethylsulfate, [emim]EtSO4, or 1-butyl-3-methylimidazolium methylsulfate, [bmim]MeSO4, ionic liquids have been done in the temperature range from (293.2 to 333.2) K. The solubility of aliphatic is less than those of the aromatic hydrocarbons. In particular, the solubility of hydrocarbons in both ionic liquids increases with the temperature in the order n-heptane < n-hexane < m-xylene < p-xylene < o-xylene < toluene < benzene. Considering the high solubility of aromatics and the low solubility of aliphatic hydrocarbons as well as totally immiscibility of the ionic liquids in all hydrocarbons, these new green solvents may be used as potentials extracting solvents for the separation of aromatic and aliphatic hydrocarbons.

Introduction

Aromatics are key chemicals in the petrochemical and chemical industries. They are used either as solvents or as raw materials for many intermediates of commodity petrochemicals and valuable fine chemicals. Among all aromatics, benzene, toluene, and xylenes (BTX) are the most important ones [1].

Liquid–liquid extraction in itself or in conjunction with other separation processes is the most widely used method for the isolation of the BTX components from crude BTX products. All processes involve the extraction of aromatics from non-aromatic materials by the use of a polar solvent having a high-selective affinity for the former compounds. Numerous extraction processes have been proposed and developed for the separation of pure aromatics from BTX feedstocks [2]. Nevertheless, no feasible processes are available from an economical point of view for the extraction of aromatic hydrocarbons in the range below aromatic mass fractions 0.20 in the feed mixture [3]. In early works, this group studied the extraction of several aromatic hydrocarbons but using supercritical carbon dioxide as extracting medium [4], [5], [6]. In this paper, ionic liquids (ILs) are investigated as new green solvents for BTX liquid–liquid extraction processes.

ILs have been identified as one of the new classes of solvents that offer opportunities to substitute conventional solvents in liquid–liquid extraction [7]. Compared to conventional organic solvents, the use of ILs for extraction has a number of advantages determined by the unique combinations of their properties. ILs are non-flammable, have a high thermal stability and negligible vapour pressure. These properties are significant when addressing the health and safety concerns associated with many solvent applications. They have liquid range of more than 300 K, which is helpful in temperature dependent separation processes as extraction. ILs are little denser than water and miscible with substances having very wide range of polarities and can simultaneously dissolve organic and inorganic substances. These features of ILs offer numerous opportunities for modifications of existing and for the development of new extraction processes.

The potentials of ILs as extractants for the separation of aromatic and non-aromatic hydrocarbons have already been explored in a few papers by measurements of (liquid + liquid) equilibrium (LLE) for a variety of (IL + hydrocarbon) systems [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. In these papers, ILs based mainly on substituted imidazolium cations have been used [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], even though other ones based on pyridinium [20], [21] and ammonium [22], [23] cations have been also investigated. These cations were combined with halide [8], [9], hexafluorophosphate [10], [11], tetrafluoroborate [11], [12], [13], [20], [22], alkylsulfates [14], [15], [16], [17], [21], and bis(trifluoromethylsulfonyl)imide [12], [18], [19], [23] anions to form the ILs.

In order to promote the use of ILs for industrial applications, apart from the thermodynamically favourable behaviour for the extraction, chemical stability, low toxicity, non-corrosivity, availability, and cost issues need to be addressed. In these regards, halide-based ILs are corrosive and, therefore, not very suitable for industrial applications [24], [25]. ILs with hexafluorophosphate anions can decompose in the presence of water and at elevated temperatures to form HF [26]. Bis(trifluoromethylsulfonyl)imide-based ILs have low viscosity and does not present problems of decomposition or corrosion, and some of them have better aromatic distribution ratios and selectivities than those for sulfolane, which is the most popular solvent in aromatic extraction [19]. However, they are the most expensive at all [27]. Thus, their high price will probably prevent that they can be used in large-scale applications. On the other hand, alkylsulfate-based ILs are less expensive, less viscous, more hydrolytically stable, and more environmental friendly than other ILs [28]. Unfortunately, some alkylsulfate-based ILs are known to be thermally less stable [29].

Bearing these aspects in mind and the scarce availability of experimental LLE data for alkylsulfate-based ILs, here we present the LLE data of 14 binary systems composed of n-hexane, n-heptane, benzene, toluene, o-xylene, m-xylene, or p-xylene and 1-ethyl-3-methylimidazolium ethylsulfate, [emim]EtSO4, or 1-butyl-3-methylimidazolium methylsulfate, [bmim]MeSO4, ILs at the temperature range from (293.2 to 333.2) K, in which extraction at atmospheric pressure has a practical application. The ILs have been selected according to their low melting point, relatively low viscosity, availability, and low cost.

Section snippets

Chemicals

Hydrocarbons, n-hexane (mass fraction  0.990), n-heptane (mass fraction  0.995), benzene (mass fraction  0.995), toluene (mass fraction  0.997), o-xylene (mass fraction  0.990), m-xylene (mass fraction  0.990), and p-xylene (mass fraction  0.990), were purchased from Fluka and used as received, without further purification. The [emim]EtSO4 (mass fraction  0.95) and [bmim]MeSO4 (mass fraction  0.95), both produced by BASF, were obtained from Sigma–Aldrich. They were purified by heating at T = 353 K and p = 10 

Experimental LLE data

The experimental LLE data in molar fraction for the binary systems (hydrocarbons + [emim]EtSO4) and (hydrocarbons + [bmim]MeSO4) in the IL-rich phase are shown in TABLE 1, TABLE 2, respectively.

The solubilities of both ILs in the hydrocarbon-rich phases were estimated in all cases less than the uncertainty of the GC analysis. So in an attempt to measure the presence of IL traces in the hydrocarbon-rich phase, 50 g of hydrocarbon was put inside the 100 mL jacketed vessel, magnetically stirred and

Conclusions

(Liquid + liquid) equilibria of 14 binary systems composed of n-hexane, n-heptane, benzene, toluene, o-xylene, m-xylene or p-xylene and 1-ethyl-3-methylimidazolium ethylsulfate, [emim]EtSO4, or 1-butyl-3-methylimidazolium methylsulfate, [bmim]MeSO4, ILs have been reported over the temperature range between (293.2 and 333.2) K.

The [emim]EtSO4 and [bmim]MeSO4 ILs are totally immiscible in the tested aromatic and aliphatic hydrocarbons. On the other hand, in the IL-rich phase, the solubility of

Acknowledgements

The authors are grateful to the Ministerio de Ciencia e Innovación (Spain) for the financial support through project CTQ2008-01591. J.S. Torrecilla is grateful to the Ministerio de Educación y Ciencia (Spain) for financial support via Ramón y Cajal Programme. We would also like to thank Dr. José Palomar for the calculation of the ionic liquid structural parameters for UNIQUAC.

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