Elsevier

Fluid Phase Equilibria

Volume 345, 15 May 2013, Pages 18-22
Fluid Phase Equilibria

Phase behaviour of ionic liquid 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate with alcohols, water and aromatic hydrocarbons

https://doi.org/10.1016/j.fluid.2013.01.027Get rights and content

Abstract

The phase diagrams for the binary systems of {1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate ([BMPYR][FAP]) + an alcohol (1-butanol, 1-hexanol, 1-octanol, 1-decanol, 1-dodecanol), or + water, or + aromatic hydrocarbons (benzene, toluene, ethylbenzene, propylbenzene, or thiophene)} have been determined at atmospheric pressure using a dynamic method. The influence of an alcohol chain length on the liquid–liquid equilibrium diagrams was discussed. A systematic decrease in the solubility was observed with an increase of the alkyl chain length of an alcohol and of the substituent on aromatic ring. The correlation of the experimental data has been carried out using the NRTL equation. The influence of the cation and anion on the phase behaviour has been discussed. The phase diagrams reported here have been compared to the systems published earlier with the 1-butyl-1-methylpyrrolidinium, or tris(pentafluoroethyl)trifluorophosphate-based ionic liquids.

Highlights

► The solubility of alcohols, water and aromatic hydrocarbons in [BMPYR][FAP] was measured. ► High solubility of benzene and thiophene in [BMPYR][FAP] was found. ► [BMPYR][FAP] may be proposed as entrainer for the separation processes. ► Extraction of sulphur-compounds from alkanes was proposed. ► Separation of aromatic/aliphatic hydrocarbons was proposed.

Introduction

Ionic liquids (ILs) may be used as new solvents in catalysis [1] and in the separation processes for the close boiling mixtures by extractive distillation, or in binary and ternary liquid-liquid extraction, or in extraction of solid-soluble in the IL compounds [2], [3], [4]. It is one of the most rapidly growing field in the past years linked to possible large scale industrial applications. ILs can offer the set of favourable properties for extraction and separation due to their unique and tailorable properties. In particular, ILs can offer technologically and environmentally favourable alternative to common organic solvents in pharmaceutical separation processes [5], [6], [7], or in biochemistry processes [8], [9].

The ILs with highly fluorinated anion as trifluorotris(perfluoroethyl)phosphate, [FAP] have a broad temperature range liquid phase, extremely low vapour pressure, high heat capacity, low viscosity, high electrochemical and thermal stability and structure similarity to popular bis{(trifluoromethyl)sulfonyl}imide, [NTf2]-based ILs. The [FAP]-based ILs are potentially useful in many technological processes.

ILs can interact via cation and/or anion with inorganic, organic and biomaterials. In this regard, perfluorinated anion [FAP] has become the subject of increasing investigations [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]. Recently, the activity coefficients at infinite dilution, γ13 of many solutes were measured for few [FAP]-based ILs [10], [11], [12], [13], [14]. The results obtained with the 4-(2-methoxyethyl)-4-methylmorpholinium trifluorotris(perfluoroethyl)phosphate, [COC2MMOR][FAP] were very promising for the selectivity and capacity of aliphatic hydrocarbons/aromatic hydrocarbons separation problem [13] and those of 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate [BMPYR][FAP] for aliphatic hydrocarbons/thiophene separation problem, which can be used in desulphurization of fuels [14]. The possible extraction of polycyclic aromatic hydrocarbons was examined with [FAP]-based ILs including the [BMPYR][FAP] IL [15], [16]. The thermal conductivity and heat capacity data were presented in the open literature for 1-butyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate, [BMIM][FAP] [17]. The density, viscosity, conductivity and heat capacity for 1-hexyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate, [HMIM][FAP] were also measured [18]. The double values in heat capacities were found for trihexyl-tetradecylphosphonium tris(pentafluoroethyl)trifluorophosphate in comparison with chloride based phosphonium IL [19].

The measurements of phase equilibria of [FAP]-based ILs are rare. The only data known are from our laboratory [20], [21]. It was shown that solubility of hydrocarbons and especially aromatic hydrocarbons is much higher in [FAP]-based ILs than in [NTf2]-based ILs [20]. The measurements of liquid–liquid equilibria (LLE) of 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate, [EMIM][FAP] has shown higher solubility in octan-1-ol than in water [21].

From the technological point of view, entrainer should reveal a high selectivity, high solute distribution ratio in the extraction process, a low viscosity, a high flash point and a boiling point higher that those of the separated components. Furthermore, it should be non-toxic, non-flammable, non-corrosive and recyclable. The [FAP]-based ILs were found to show the suitability as entrainers for the separation of thiophene from aliphatic hydrocarbons [22]. In ternary LLE measurements much better selectivity of [FAP]-based ILs than that of [NTf2]-based ILs was observed [22].

Knowledge of the phase equilibrium is important if ILs are to be considered as solvents in technological processes. Essentially, all of the solvents available for industrial processes, performing separations and processing materials are usually common liquids such as water, ethanol, and aromatic hydrocarbons. After years of measuring the phase equilibria of mixtures including ILs it is easy to conclude that aliphatic hydrocarbons are weakly soluble in ILs, whereas the aromatic compounds are much better soluble in many ILs [5], [8], [20], [23], [24]. Phase equilibria results are also important in expanding our knowledge about the nature of ILs and in assisting in the systematic study of their thermodynamic properties.

The aim of this work was to measure solubilities of [BMPYR][FAP] in alcohols, or water, or aromatic hydrocarbons at atmospheric pressure using a dynamic method. The investigation includes the effect of the alkyl chain length of the alcohol and the alkyl chain length of substituent on the benzene ring.

Section snippets

Materials

The ionic liquid [BMPYR][FAP] (CAS: 851856-47-8) was supplied by Merck, KGaA. The sample was dried for several days at 350 K under reduced pressure to remove volatile impurities and trace water. The water content was of <0.0004 mass fraction, as determined by Karl Fisher titration. The structure of the IL is shown in Fig. 1. Most of high purity solvents were fractionally distilled over different drying reagents to mass fraction purity better than 99.8 wt% and were stored over freshly activated

Results and discussion

The phase equilibria LLE diagrams result from different interactions between the IL and the solvent and are very similar for different ionic liquids with alcohols. It is typical for the polar cation, or anion capable of forming hydrogen bonding with solvent to reveal the complete miscibility in the liquid phase. The complete miscibility for the liquid at room temperature ILs is observed in the short chain alcohols. The simple eutectic systems with complete miscibility in the liquid phase for

(Liquid + liquid) phase equilibria correlation

The (liquid + liquid) phase equilibria was described at ambient pressure in terms of the general thermodynamic relations adopting model for the excess Gibbs free energy. The widely known NRTL model proposed by Renon and Prausnitz [38] was used. The equilibrium condition for (liquid + liquid) phase equilibrium in binary system is as follows [39]:GERT=x1x2τ21G21x1+x2G21+τ12G12G12x1+x2whereτ12=(g12g22)/RTτ21=(g21g11)/RTG12=exp(τ12τ12)G12=exp(τ12τ21)and the activity coefficient is express by the

Conclusions

Solubilities of the ionic liquid [BMPYR][FAP] in alcohols, water and aromatic hydrocarbons have been determined over a range of temperatures and the whole mole fraction. The pyrrolidinium-based IL with highly fluorinated anion has the influence on the higher solubility of aromatic hydrocarbons and lower solubility of alcohols and water in comparison with [NTf2]-based ILs. The results of the correlation of LLE with two parameters NRTL equation were with acceptable standard deviation.

The

Acknowledgements

This work was supported by the National Science Center project 2011/01/B/ST5/00800. Author E.V. Lukoshko wish to thank for the project MPD 2010/4 “Towards Advanced Functional Materials and Novel Devices-Joint UW and WUT International PhD Programme”. Authors wish to thank Merck for the free sample of [BMPYR][FAP].

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