Activity coefficients at infinite dilution and physicochemical properties for organic solutes and water in the ionic liquid 4-(2-methoxyethyl)-4-methylmorpholinium trifluorotris(perfluoroethyl)phosphate

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Abstract

This paper presents new data on activity coefficients at infinite dilution, γ for 62 different solutes: alkanes, alkenes, alkynes, cycloalkanes, aromatic hydrocarbons, alcohols, thiophene, ethers, ketones, and water in the ionic liquid 4-(2-methoxyethyl)-4-methylmorpholinium trifluorotris(perfluoroethyl)phosphate, [COC2mMOR][FAP]. The data were determined by inverse gas chromatography at six temperatures from T = 318.15 K to T = 368.15 K. These values were compared with ionic liquid based on the same cation and bis(trifluoromethylsulfonyl)-amide anion, [COC2mMOR][NTf2]. Additionally the partial molar excess Gibbs free energies ΔG1E,, enthalpies ΔH1E,, and entropies ΔS1E, at infinite dilution were calculated from the experimental γ values obtained over the temperatures range. The selectivities and capacities at infinite dilution for hexane/benzene, cyclohexane/benzene, and heptane/thiophene extraction problems were calculated from the γ to verify the possibility of investigated ionic liquid as an entrainer in liquid–liquid extraction and desulphurization of fuels. Obtained values were compared with literature data of other ionic liquids based on trifluorotris(perfluoroethyl)phosphate and bis(trifluoromethylsulfonyl)-amide anions and industrial solvents: NMP and sulfolane. Also the Abraham solvation parameter model was used for evaluation of the solvation characteristic of the ionic liquid. Linear free energy relationship (LFER) system constants as a function of temperature were calculated.

Highlights

► The γ13 and KL for 62 solutes in the IL [COC2mMOR][FAP] were determined by IGC. ► Partial molar excess thermodynamic functions at infinite dilution were calculated. ► The selectivities and capacities for selected separation problems were calculated. ► LFER system constants as a function of T for [COC2mMOR][FAP] were calculated.

Introduction

Currently often toxic and volatile solvents are used in separation, extraction, and extractive distillation, what causes unfavorable environmental impact. Therefore more ecological replacements are needed. At the beginning of 20th century there were recognized new classes of compounds – ionic liquids (ILs). However only since the 1990s they have received closer attention and become more studied. It was found that this unsymmetrical salts with the melting temperature at relatively low temperatures exhibit remarkable properties such as negligible vapor pressure, liquid phase existence over a wide temperature range, high thermal and chemical stability. It allows considering them as ‘green’ compounds which can be used as entrainers in mentioned industrial processes. Due to their non volatility this solution can lead not only to reduce the risk of loss to the atmosphere but also provide to no traces of entrainers in the distillate in extractive distillation. Consequently ILs can be easy recycle and it minimizes costs. There are also called ‘designer solvents’ due to their large number. It allows adjusting IL properties for specific application by appropriate selection of cation and anion structure. It is estimated that more than 1014 combination is possible [1].

In order to use ILs as entrainers in industry or design new ones the knowledge of interaction with different compounds is needed. It can be made by determination of basic physicochemical properties such as activity coefficients at infinite dilution γ and subsequently calculation from these values both properties of extractant, namely selectivity (Sij=γi/γj) as well as capacity (kj=1/γj).

This paper is a continuation of our systematic studies on ILs. There are searched the solvents with high values both selectivity and capacity. Unfortunately, generally when selectivity increases, capacity decreases, and vice versa. Previous investigation in our laboratory shows good selectivities for 4-(2-methoxyethyl)-4-methylmorpholinium bis(trifluoromethylsulfonyl)-amide, [COC2mMOR][NTf2] [2]. In turn, the research of trifluorotris(perfluoroethyl)phosphate anion [FAP] based ILs indicate high capacities [3], [4], [5], [6]. A combination of cation containing oxygen in structure and [FAP] anion should reveal both properties of extractant on good level. It was confirmed in the case of 1-(3-hydroxypropyl)pyridinium trifluorotris(perfluoroethyl)phosphate, [N-C3OHPY][FAP] [7]. Therefore 4-(2-methoxyethyl)-4-methylmorpholinium trifluorotris(perfluoroethyl)phosphate [COC2mMOR][FAP] was selected. The activity coefficients at infinite dilution γ, gas–liquid partition coefficients, KL, partial molar excess Gibbs free energies ΔG1E,, enthalpies ΔH1E,, and entropies ΔS1E, at infinite dilution for 62 solutes: alkanes, alkenes, alkynes, cycloalkanes, aromatic hydrocarbons, alcohols, thiophene, ethers, ketones, esters, butanal, acetonitrile, 1-nitropropane, and water in mentioned above IL were determined using inverse gas chromatography (IGC) at 10 K intervals from T = 318.15 K to T = 368.15 K. The selectivity Sij and the capacity kj were calculated for selected separation problems to analyze [COC2mMOR][FAP] as potential entrainer in extraction processes and desulphurization of fuels.

Section snippets

Materials

The ionic liquid [COC2mMOR][FAP] had a purity of >0.995 mass fraction and was supplied by Merck. The ionic liquid was further purified by subjecting the liquid to a very low pressure of about 5 · 10−3 Pa at temperature of about 363 K for ca. 5 h. This procedure removed any volatile chemicals and water from the ionic liquid. The water content was analyzed by Karl–Fischer titration technique (method TitroLine KF). The sample of IL was dissolved in methanol and titrated with steps of 0.0025 cm3. The

Results and discussion

Table 1 lists determined activity coefficients at infinite dilution for 62 selected solutes at six temperatures from 318.15 K to 368.15 K in [COC2mMOR][FAP]. The values of γ13 increase with an increase of alkyl chain length in series of homologous and take following order: n-alkanes > branched alkanes > cycloalkanes > alkenes > alkynes > aromatic hydrocarbons for hydrocarbons with the same carbon number in structure. This is a typical sequence for other ILs and the reasons for this behavior were explained

Conclusions

Activity coefficients at infinite dilution and the gas–liquid partition coefficients for 62 solutes in the ionic liquid 4-(2-methoxyethyl)-4-methylmorpholinium trifluorotris(perfluoroethyl)phosphate were measured by inverse gas chromatography at the temperatures from (318.15 to 368.15) K.

From all currently investigated ionic liquids based on [FAP] anion the [N-C3OHPY][FAP] one reveals the highest selectivities for separation of aliphatic/aromatic hydrocarbons and for separation of

Acknowledgments

This work has been supported by the project 2011/01/B/ST5/00800. Michał Wlazło wishes to thank the support by the European Union in the framework of European Social Fund through the Warsaw University of Technology Development Programme, realized by Center for Advanced Studies.

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