Infinite dilution activity coefficients of volatile organic compounds in two ionic liquids composed of the tris(pentafluoroethyl)trifluorophosphate ([FAP]) anion and a functionalized cation

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Highlights

  • Limiting activity coefficients and gas–liquid partition coefficients for 30 VOCs were determined by GLC.

  • Solution thermodynamic quantities were derived and analyzed.

  • [MO-EMPYR][FAP] and [HO-EMIM][FAP] were identified as ILs of very low and very high cohesivity, respectively.

  • [HO-EMIM][FAP] is an IL of extreme H-bond acidity exhibiting superior performance for petrochemical separations.

  • Both studied [FAP] ILs were indicated to separate some azeotropic mixtures of alcohols with aprotic oxygenates.

Abstract

Interactions of volatile organic compounds with two ionic liquids (ILs) containing tris(pentafluoroethyl)trifluorophosphate ([FAP]) anion and a functionalized cation, 1-(2-hydroxyethyl)-3-methylimidazolium ([HO-EMIM]) and 1-(2-methoxyethyl)-1-methylpyrrolidinium ([MO-EMPYR]), were explored through systematic GLC retention measurements. Infinite dilution activity coefficients γ1 and gas–liquid partition coefficients KL of 30 selected solutes in [HO-EMIM][FAP] and [MO-EMPYR][FAP] were determined at five temperatures in the range from (318.15 to 353.15) K. Partial molar excess enthalpies and entropies at infinite dilution were derived from the temperature dependence of the γ1 values. The Linear Free Energy Relationship (LFER) solvation model was used to correlate the KL values. The LFER correlation parameters and excess thermodynamic functions were analyzed to identify molecular interactions operating between the ILs and the individual solutes. By comparing the behaviors of the studied ILs and of their closely similar unfunctionalized analogs, net effects imparted by cation functionalization were also disclosed. The cohesivity of the two ILs was shown to differ dramatically: while [MO-EMPYR][FAP] ranks among ILs to the least cohesive, [HO-EMIM][FAP] belongs to the most cohesive ones. Both [HO-EMIM][FAP] and [MO-EMPYR][FAP] are capable of interacting with solutes specifically through dipolarity/polarizibility and hydrogen bonding, but apparently lack the ability to interact with solute lone electron pairs. The proton donating capability of [HO-EMIM][FAP], undoubtedly brought by the hydroxyl functionality, is enormous and imparts to this IL extraordinary potential for use in solvent-aided separations. As we have demonstrated on some model aliphatic/aromatic separation pairs, [HO-EMIM][FAP] as a solvent for the separation of aliphatic hydrocarbons from aromatics gives superior performance, its performance index (product of selectivity and capacity) surpassing distinctly that of conventional solvents and of most ILs studied so far. [HO-EMIM][FAP] and [MO-EMPYR][FAP] could also serve as efficient entrainers in separations of other non-petrochemical azeotropic systems of industrial importance by extractive distillation as exemplified for some mixtures of alcohols with aprotic oxygenates.

Introduction

During more than a decade, ionic liquids (ILs) have been promoting burgeoning research due to their special chemical and physicochemical properties offering prospects for great technological innovations. Since properties of these media can be controlled by almost countless structure variations of composing ions, still new ILs are synthetized and explored with the aim to improve their properties and tailor them for specific applications. In order to replace rather unstable hexafluorophosphate containing ILs, Merck developed and made commercially available a group of ILs based on tris(pentafluoroethyl)trifluorophosphate ([FAP]) anion [1]. The [FAP] based ILs exhibit excellent hydrolytic, thermal, and electrochemical stability as well as a lower water uptake and have been shown to provide advantages in various applications in electrochemistry [2], tribology [3], analytical chemistry [4], [5] and gas absorption [6], [7]. Studies on solvation properties of ILs containing [FAP] anion [8], [9] have indicated their great potential for use in solvent-aided separations, especially for ILs in which [FAP] is paired with a functionalized counter-cation. Measurements of infinite dilution activity coefficients γ1 for a number of test solutes in [FAP] based ILs have been reported confirming this potential [10], [11], [12], [13], [14]. Notable in this respect are systematic γ1 measurements recently carried out by Marciniak and Wlazlo on 1-(3-hydroxypropyl)pyridinium [HO-PPY] [12], 4-(2-methoxyethyl)-4-methylmorfolinium [MO-EMMOR] [13], and 1-(2-methoxyethyl)-1-methylpiperidinium [MO-EMPIP] [14] [FAP]s.

In this work, we examine interactions of selected VOCs with two [FAP] based ionic liquids containing functionalized counter-cations, namely 1-(2-hydroxyethyl)-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([HO-EMIM][FAP]) and 1-(2-methoxyethyl)-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate ([MO-EMPYR][FAP]).The methodology of the present investigation is closely parallel to that we used in our previous studies devoted to selected [EMIM] or [BMPYR] based ILs [15], [16], [17], [18], [19]. Here we thus report infinite dilution activity coefficients γ1 and gas–liquid partition coefficients KL measured by gas–liquid chromatography (GLC) for the same representative set of 30 selected hydrocarbons, alcohols, ketones, ethers, esters, haloalkanes, and nitrogen- or sulfur-containing compounds in [HO-EMIM][FAP] and [MO-EMPYR][FAP] as a function of temperature. The obtained thermodynamic properties are analyzed to disclose intermolecular interactions governing the observed behavior and identify the potential of these ILs for use as entrainers in solvent-aided separations. Through comparisons with available literature data for unfunctionalized analogs of similar structure also effects brought by the functionalization are disclosed.

Concurrently, infinite dilution activity coefficients for some of the examined solutes in [MO-EMPYR][FAP] have been measured by Marciniak and Wlazlo and presented just now [20]. A detailed comparison of our results to their values is also presented.

Section snippets

Theory

In gas–liquid chromatography (GLC), the infinite dilution activity coefficient γ1 and the gas–liquid partition coefficient KL=(c1L/c1G) for a solute (1) partitioning between a carrier gas (2) and a non-volatile liquid solvent (3) are calculated from the solute retention according to the following equations [21]lnγ1=lnRTm3VNp1sM3-B11-v1Lp1sRT+2B12-v¯1J34p0RT,lnKL=lnRTρ3γ1p1sM3-B11-v1Lp1sRT,where T is temperature of the column, m3, M3 and ρ3 mass, molar mass and density of the solvent,

Materials

The ionic liquids 1-(2-hydroxyethyl)-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([OH-EMIM][FAP], M = 572.17 g · mol−1) and 1-(2-methoxyethyl)-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate ([MO-EMPYR][FAP], M = 589.24 g · mol−1) were obtained in the high purity grade from Merck. Their purity of these ILs according to the producer’s specification was >0.99, the certified water content being ⩽100 · 10−6 (mass basis). The samples were handled with special precautions to avoid

Results

Infinite dilution activity coefficients and gas–liquid partition coefficients in [HO-EMIM][FAP] and [MO-EMPYR][FAP] were determined for a set of 30 selected solutes at (318.15, 323.15, 333.15, 343.15, and 353.15) K. The retention measurements were carried out on columns loaded with different amounts of the IL, namely 2.833 g (λ=0.25, L = 1.2 m) and 1.864 g (λ=0.35, L = 0.65 m) for [OH-EMIM][FAP], and 2.893 g (λ=0.25, L = 1.2 m) and 2.107 g (λ=0.40, L = 0.65 m) for [MO-EMPYR][FAP]. The values of γ1 and KL were

Comparison with existing data

Infinite dilution activity coefficients for some volatile organic compounds in [MO-EMPYR][FAP] have been reported by Marciniak and Wlazlo [20] during the final stages of preparation of this paper. Table 6 compares the present measurements with their data at the lowest and the highest temperature of our measurements. As seen, agreement between the two determinations is not entirely satisfactory. Considering that for both these determinations the declared relative standard uncertainty is 0.03 and

Conclusions

In this work, we have examined interactions of two ionic liquids containing [FAP] anion and a functionalized counter-cation, namely [HO-EMIM][FAP] and [MO-EMPYR][FAP], with various types of organic solutes and assessed the potential of these ILs to be used as solvents in separation processes. Through methodical GLC retention measurements, infinite dilution activity coefficients and gas–liquid partition coefficients of 30 hydrocarbons, alcohols, ketones, ethers, esters, haloalkanes, and

Acknowledgment

This work was funded from Ministry of Education of the Czech Republic through financial support for specific university research (MSMT No. 21/2012). The stay of E.F.O. at ICT, Prague was supported by EU Erasmus Scholarship and she thanks her teachers Dr. S. Pinho and Dr. O. Ferreira for preparing the way.

References (57)

  • N.V. Ignat’ev et al.

    J. Fluorine Chem.

    (2005)
  • N.V. Ignat’ev et al.

    J. Fluorine Chem.

    (2009)
  • P. Twu et al.

    J. Chromatogr. A

    (2011)
  • U. Domanska et al.

    Chem. Eng. J.

    (2012)
  • M. Wlazlo et al.

    J. Chem. Thermodyn.

    (2012)
  • A. Marciniak et al.

    J. Chem. Thermodyn.

    (2013)
  • A. Blahut et al.

    Fluid Phase Equilibr.

    (2010)
  • A. Blahut et al.

    J. Chem. Thermodyn.

    (2012)
  • A. Blahut et al.

    J. Chem. Thermodyn.

    (2013)
  • A. Marciniak et al.

    J. Chem. Thermodyn.

    (2013)
  • J. Víška et al.

    J. Chromatogr.

    (1974)
  • M. Souckova et al.

    Fluid Phase Equilibr.

    (2012)
  • A. Marciniak et al.

    J. Chem. Thermodyn.

    (2012)
  • U. Domanska et al.

    J. Chem. Thermodyn.

    (2011)
  • U. Domanska et al.

    J. Chem. Thermodyn.

    (2011)
  • M.H. Abraham et al.

    J. Chromatogr. A

    (2004)
  • S.K. Poole et al.

    J. Chromatogr. A

    (1995)
  • C.F. Poole et al.

    J. Chromatogr. A

    (2002)
  • C.F. Poole

    J. Chromatogr. A

    (2004)
  • L.M. Sprunger et al.

    Fluid Phase Equilibr.

    (2008)
  • M.H. Abraham et al.

    Fluid Phase Equilibr.

    (2007)
  • T.M. Letcher et al.

    Fluid Phase Equilibr.

    (2005)
  • S. Kossack et al.

    Chem. Eng. Res. Des.

    (2008)
  • F. Mutelet et al.

    J. Chromatogr. A

    (2006)
  • U. Domanska et al.

    J. Chem. Thermodyn.

    (2008)
  • A.L. Revelli et al.

    J. Chromatogr. A

    (2009)
  • E. Olivier et al.

    J. Chem. Thermodyn.

    (2010)
  • U. Domanska et al.

    Fluid Phase Equilibr.

    (2009)
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    Visiting Erasmus M.Sc. student from Escola Superior de Tecnologia e de Gestão, Instituto Politécnico de Bragança, Portugal.

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