Measurements of different thermodynamic properties of systems containing ionic liquids and correlation of these properties using modified UNIFAC (Dortmund)

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Abstract

Activity coefficients at infinite dilution (γ), vapor–liquid equilibria (VLE) and excess enthalpies (HE) for different alkanes and alkenes with the ionic liquids 1-butyl-3-methylimidazolium trifluoromethanesulfonate [BMIM]+[OTF], 1-hexyl-3-methylimidazolium trifluoromethanesulfonate [HMIM]+[OTF] and 1-octyl-3-methylimidazolium trifluoromethanesulfonate [OMIM]+[OTF] have been measured. The new data were used to revise and extend the group interaction parameter matrix of modified UNIFAC (Dortmund).

Introduction

Because of the unique material and solvent properties, ionic liquids have a substantial capability to replace volatile organic solvents in chemical processes [1], [2]. The scaleup of processes with ionic liquids from the laboratory to the industrial scale, requires a reliable knowledge of the phase equilibrium behavior of binary and higher systems. This knowledge is also required to choose the most suitable ionic liquid from the large variety of ionic liquids, which is due to the large number of possible anions, cations, alkyl rests and functional groups. For the problem-oriented selection of the most suitable ionic liquid, an easily applicable and reliable prediction method would be desirable. In previous publications it was shown that the group contribution method modified UNIFAC (Dortmund) can also be applied for ionic liquids by the introduction of the main groups for the cations (imidazolium, pyrrolidinium) and anions (bis(trifluoromethylsulfonyl)imid) [3], [4]. In this work the model is extended by the introduction of the anion trifluoromethanesulfonate (OTF).

A comprehensive database is a prerequisite for such a development. Therefore systematic measurements of activity coefficients at infinite dilution, vapor–liquid equilibria and excess enthalpies for systems with the ionic liquids [BMIM]+[OTF], [HMIM]+[OTF], [OMIM]+[OTF] and different alkanes and alkenes were performed. The measurements covered a temperature range from 303.15 to 413.15 K. The new data were used for fitting the group interaction parameters of modified UNIFAC (Dortmund) with alkanes and alkenes, this means between the groups CH2—OTF and Cdouble bondC—OTF.

The structures of the ionic liquids investigated are shown in Fig. 1.

Section snippets

Materials and purities

The purities of the alkanes and alkenes used and their suppliers are given in Table 1. For the VLE measurements all components were degassed and distilled at low pressure [5]. For the measurements of the excess enthalpies HE and activity coefficients at infinite dilution γi, the compounds were used without degassing. The purity was checked by gas chromatography. The water concentration (always <100 ppm) was measured by Karl Fischer titration. The investigated ionic liquids [BMIM]+[OTF], [HMIM]+

Densities

For the calculation of the different thermodynamic properties the density of all compounds is needed. The densities for [HMIM]+[OTF] and [OMIM]+[OTF] listed in Table 2 were measured with the help of a vibrating tube densimeter DMA 4500 from Anton Paar with an accuracy of 0.001 g/cm3. The densities of [BMIM]+[OTF] and the other compounds were taken from different sources [6], [7].

Activity coefficients at infinite dilution

The activity coefficients at infinite dilution are vitally important for the development and testing of

Modified UNIFAC (Dortmund)

The modified UNIFAC (Dortmund) method [15] belongs to the thermodynamic models which use the group contribution concept for the prediction of activity coefficients. In these models the molecules are fragmented in characteristic structural groups (increments) and instead of the interactions between molecules the interactions between these structural groups are considered.

The group interaction parameters of modified UNIFAC in this work were obtained by a simultaneous fit to the new measured

Results and discussion

Fig. 3 shows the correlated activity coefficients at infinite dilution using modified UNIFAC (Dortmund) together with the experimental data for alkanes and alkenes in different 1-alkyl-3-methylimidazolium trifluoromethanesulfonates [RMIM]+[OTF]. As can be seen modified UNIFAC allows not only the reliable description of the temperature dependency of γi, but also describes correctly the effect of the alkyl chain length of the ionic liquid and the chain length of the alkanes respectively alkenes.

Conclusion

Experimental vapor–liquid equilibria data at 363.15 K, excess enthalpy data at 363.15 and 413.15 K and activity coefficients at infinite dilution between 303.15 and 333.15 K were measured for the binary systems containing alkanes, alkenes and the ionic liquids 1-butyl-3-methylimidazolium trifluoromethanesulfonate [BMIM]+[OTF], 1-hexyl-3-methylimidazolium trifluoromethanesulfonate [HMIM]+[OTF] and 1-octyl-3-methylimidazolium trifluoromethanesulfonate [OMIM]+[OTF]. The new database was used for

Acknowledgements

The authors would like to thank R. Bölts for technical assistance and Deutsche Forschungsgemeinschaft (DFG) for partial financial support of this work. We also thank the Merck GmbH for supplying the ionic liquids.

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