Fluid Phase Equilibria 226 (2004) 37–44

Activity coefficients at infinite dilution of various solutes in the ionicliquids [MMIM] +[CH3SO4]−, [MMIM] +[CH3OC2H4SO4]−,[MMIM] +[(CH3)2PO4]−, [C5H5NC2H5]+[(CF3SO2)2N]− and

[C5H5NH]+[C2H5OC2H4OSO3]−

Ryo Kato, Jurgen Gmehling∗

Carl von Ossietzky Universit¨at Oldenburg, Technische Chemie, D-26111 Oldenburg, Germany

Received 25 June 2004; received in revised form 30 August 2004; accepted 31 August 2004Available online 5 November 2004

Abstract

∞ romatics,a1 ate[ l-s d ionicl

nts©

K

1

bpaspanavss

(

nicum-irtu-icreac-ationap-

hav-mice re-oupsuiredthan

itym-tivity

0d

Infinite dilution activity coefficientsγi have been measured for various solutes (alkanes, alkenes, cycloalkanes, cycloalkenes, alcohols, ketones, esters, ethers, and water) in the five ionic liquids 1-methyl-3-methyl-imidazolium methylsulfate [MMIM]+[CH3SO4]−,-methyl-3-methyl-imidazolium methoxyethylsulfate [MMIM]+[CH3OC2H4SO4]−, 1-methyl-3-methyl-imidazolium dimethylphosph

MMIM] +[(CH3)2PO4]−,N-ethylpyridinium bis(trifluoromethylsulfonyl) imide [C5H5NC2H5]+[(CF3SO2)2N]−, and pyridiniumethoxyethyulfate [C5H5NH]+[C2H5OC2H4OSO3]− in the temperature range from 303.15 to 373.15 K. Additionally, densities of the investigateiquids were measured in the temperature range from 293.15 to 353.15 K.

Using all availableγ∞i -data, the selectivitiesS∞

ij = γ∞i /γ∞

j and the capacitiesk∞i = 1/γ∞

i at infinite dilution were determined for differeeparation problems not only for ionic liquids, but alsoN-methyl-2-pyrrolidone (NMP) and ethylene glycol.2004 Elsevier B.V. All rights reserved.

eywords: Ionic liquids; Infinite dilution activity coefficients; Selectivities; Measurement; Dilutor technique

. Introduction

In recent years, room-temperature ionic liquids haveecome very popular, since they show very interestingroperties, such as (1) low melting point (<373 K) and

wide liquid range (300 K), (2) suitable viscosity, (3)tability up to high temperature, (4) high solubility for botholar and non-polar organic and inorganic substances, (5)nd in particular negligible vapor pressure and thereforeon-flammability. This means that they can be applieds replacement for conventional toxic, flammable andolatile organic solvents. Therefore, they are very interestingolvents for industrial applications (chemical reactions,eparation processes, batteries, electrochemistry).

∗ Corresponding author. Tel.: +49 441 798 3831; fax: +49 441 798 3330.E-mail address:gmehling@tech.chem.uni-oldenburg.de

J. Gmehling).

Typically, ionic liquids are composed of a large orgacation and an inorganic polyatomic anion. Since a large nber of cations and anions can be combined, there is vally no limit in the number of ionic liquids. Therefore, ionliquids are discussed as designer solvents for biphasictions, selective solvents (entrainers) for various separprocesses[1–3], etc. Information about the synthesis andplication of ionic liquids is available in literature[4,5].

For a better understanding of their thermodynamic beior and with a regard to the development of thermodynamodels, reliable experimental phase equilibrium data arquired. Therefore, in the last years various research grstarted with the systematic measurements of the reqthermophysical properties. At the moment already more6250 data points for ionic liquids are available[6].

Up till now the available experimental data for activcoefficients at infinite dilution are limited only to a few cobinations of cations and anions. This paper presents ac

378-3812/$ – see front matter © 2004 Elsevier B.V. All rights reserved.oi:10.1016/j.fluid.2004.08.039

38 R. Kato, J. Gmehling / Fluid Phase Equilibria 226 (2004) 37–44

coefficients at infinite dilution for various solutes (alkanes,alkenes, cycloalkanes, cycloalkenes, aromatics, alcohols, ke-tones, esters, ethers, and water) in five ionic liquids mea-sured in the temperature range from 303.15 to 373.15 K. Inprevious papers, already infinite dilution activity coefficientsof 20 solutes in the ionic liquids [MMIM]+[(CF3SO2)2N]−,[EMIM] +[(CF3SO2)2N]−, [BMIM] +[(CF3SO2)2N]− and[EMIM] +[C2H5OSO3]− were reported[7]. Furthermore, apaper on binary vapor–liquid equilibria (VLE) and excessenthalpiesHE for hydrocarbons in ionic liquids is in press[8]. A publication of the VLE behavior of ternary systemswas already submitted[9].

Furthermore, the selectivities and capacities fordifferent separation problems (aliphatics–aromatics,alkanes–alkenes, oxygenated compounds (ethanol, 2-butanone, tetrahydrofuran)–water) derived from boundaryactivity coefficients in ionic liquids are compared with thevalues obtained for NMP and ethylene glycol (All solventsinvestigated are shown inTable 1).

2. Experimental

Two different techniques (gas–liquid chromatography,GLC, dilutor technique) were applied for the measurementso ∞l

sta-t -AW-D ne-t witht wasr ato-g wasc ad-i a-t meo f thema byt

γ

w a-t ftc e.Tt rubere

tA edf peaka

obtained using the following equation:

γ∞i = − nLRT

ϕsiP

Si ((F (1 + PS

L /P)/a) + Vg)(2)

wherenL =mL/ML is the number of moles of the solvent,Fthe flow rate of the carrier gas,PS

L the vapor pressure of thesolvent,P the total pressure andVg the gas phase volume inthe measuring cell.

For the measurement, the measuring cell is filled with ap-proximately 80 cm3 of the solvent. The composition of thesolute in the carrier gas leaving the measurement cells isanalyzed in fixed time intervals using gas chromatography(Hewlett-Packard; HP 6890). For the measurements a GCcolumn with a diameter of 3.175 mm and a length of 1.5 mfilled with Porapak P, 80/100 mesh was used.

The relative error for theγ∞i measurements carried out

using dilutor technique is approximately±2.5% [12]. Therequired pure component properties are obtained from theDortmund Data Bank[6].

The liquid densitiesρ of the ionic liquids investigated weremeasured using a vibrating tube densimeter DMA 4500 fromAnton Paar.

The solutes were used without furthermore purifications,because the impurities are separated in the GC column andnegligible amounts of impurities in the measuring cell haven sti-g eid.B ac-u om-p

3

-s , aro-m ionicl . Allγ

s es .(i

ea etherw P.A laa -z withb -a n-s P.

derR

f the infinite dilution activity coefficientsγi in the ioniciquids.

For the GLC method, the solid support used as theionary phase for all measurements was Chromosorb PMCS 60–80 mesh (acid-washed dimethyldichlorosila

reated Chromosorb). The carrier material was coatedhe ionic liquid dissolved in methanol. After the methanolemoved with the help of a rotary evaporator, the chromraphic column (length 250 mm, inner diameter 4.1 mm)arefully filled with the coated solid support. The liquid long (i.e. the amount of ionic liquid on the inert carrier merial) was determined gravimetrically. A detailed schef the gas–liquid chromatograph and the description oeasurement procedure was given by Knoop et al.[10]. Thectivity coefficients at infinite dilution can be calculated

he following equation:

∞i = RTmL

VNPsi MLϕs

i

(1)

hereR is the general gas constant,T the absolute temperure,mL the mass of the solvent,ML the molecular weight ohe solvent,Ps

i the saturation vapor pressure,ϕsi the fugacity

oefficient of the solute, andVN the net retention volumhe evaluation procedure for the measurement ofγ∞

i usinghe GLC apparatus was already discussed in detail by Gt al.[11].

A detailed description of the determination ofγ∞i using

he dilutor technique was published by Krummen et al.[12].highly diluted solute (<0.001 mole fraction) is stripp

rom the solvent by the carrier gas. From the decreasingrea with time measured by gas chromatography,γ∞

i can be

o effect on the quality of the results. The ionic liquids inveated in this paper were obtained from Prof. P. Wassersch1

efore measurement, all ionic liquids were purified by vum evaporation to remove the last traces of volatile counds.

. Results and discussion

Using GLC and the dilutor techniqueγ∞i data were mea

ured for alkanes, alkenes, cycloalkanes, cycloalkenesatics, alcohols, ketones, esters, ethers and water in five

iquids in the temperature range from 303.15 to 373.15 K∞i values measured are listed inTables 2–6. The requiredaturation vapor pressurePs

i [6], fugacity coefficients in thaturation stateϕs

i for the calculation of theγ∞i using Eqs

1) and (2)are listed inTable 7. The liquid densitiesρ of theonic liquids investigated are given inTable 8.

In Figs. 1 and 2the experimentalγ∞i values for benzen

nd cyclohexane measured in this work are shown togith the availableγ∞

i values in other ionic liquids and NMs can be seen fromFig. 1, for ionic liquids containing alkynd alkoxyl substituted sulfate [RSO4]−, [ROR SO4]−nd phosphate [RPO4]− anions theγ∞

i values for benene are much higher (3–7 times) than in ionic liquidsis(trifluoromethylsulfonyl) imide [(CF3SO2)2N]− and hexfluorophosphate [PF6]− as anion. In most ionic liquids coidered theγ∞

i values of benzene are higher than in NM

1 Institut fur Technische Chemie und Makromolekulare ChemieWTH Aachen.

R. Kato, J. Gmehling / Fluid Phase Equilibria 226 (2004) 37–44 39

Table 1Ionic liquids, NMP and ethylene glycol used for the discussion in this work

Reference no. Name Structure

Solvent Abbreviation Cation Anion

[7] 1-Methyl-3-methyl-imidazolium bis-trifluoromethylsulfonylimide

[MMIM] +[BTI] −

[7,15] 1-Ethyl-3-methyl-imidazoliumbis(trifluoromethylsulfonyl)imide

[EMIM] +[BTI] −

[15] 1-Ethyl-2,3-dimethyl-imidazoliumbis(trifluoromethylsulfonyl)imide

[EDMIM] +[BTI] −

[7] 1-Butyl-3-methyl-imidazoliumbis(trifluoromethylsulfonyl)imide

[BMIM] +[BTI] −

a N-Ethylpyridiniumbis(trifluoromethylsulfonyl)imide

[EPY]+[BTI] −

[16] 1-Hexyl-3-methylimidazoliumhexafluorophosphate

[HMIM] +[PF6]−

[17] 1-Hexyl-3-methylimidazoliumtetrafluoroborate

[HMIM] +[BF4]−

a 1-Methyl-3-methyl-imidazoliummethylsulfate

[MMIM] +[CH3SO4]−

a 1-Methyl-3-methyl-imidazoliummethoxyethylsulfate

[MMIM] +[CH3OC2H5SO4]−

[7] 1-Ethyl-3-methyl-imidazolium ethyl-sulfate

[EMIM] +[C2H5SO4]−

a Pyridinium ethoxyethylsulfate [PY]+[C2H5OC2H4SO4]−

a 1-Methyl-3-methyl-imidazoliumdimethylphosphate

[MMIM] +[(CH3)2PO4]−

[18] 1-Octyl-3-methylimidazolium chlo-ride

[OMIM] +[Cl]− Cl−

[19,6] N-Methyl-2-pyrrolidone NMP

[20,6] 1,2-Ethandiol Ethylene glycola Ionic liquids investigated in this work.

40 R. Kato, J. Gmehling / Fluid Phase Equilibria 226 (2004) 37–44

Table 2Infinite dilution activity coefficients in [MMIM]+[CH3SO4]− (2) measuredby dilutor technique

Solute (1) γ∞1 [–]

303.15 K 313.15 K 323.15 K 333.15 K

Pentane 46.1a 39.9a 34.5a 30.2a

Hexane 113 97.2 83.5 73.1Heptane 301 252 210 1791-Pentene 37.3 33.6 29.3 26.31-Hexene 98.3 84.6 73.1 63.81-Heptene 235 200 170 146Cyclopentane 56.9 45.3 36.9 30.7Cyclohexane 140 119 102 88.6Cyclopentene 27.6 23.6 20.4 17.6Cyclohexene 72.7 65.3 58.7 53.2Benzene 7.27 6.78 6.31 5.93Toluene 17.2a 15.4a 13.8a 12.6a

Methanol 0.07a 0.07a 0.05a 0.05a

1-Propanol 0.62a 0.44a 0.25a 0.12a

a glc.

This tendency is also observed for theγ∞i values of cyclohex-

ane in the ionic liquids investigated. [MMIM]+[CH3SO4]−and [MMIM]+[CH3OC2H5SO4]− show the highestγ∞

i val-ues for cyclohexane. For all ionic liquids shown theγ∞

i valuesfor cyclohexane are larger than for NMP. When the resultsfor the different imidazolium bis(trifluorosulfonylimides) arecompared, it can be seen that theγ∞

i values for benzeneand cyclohexane decrease with increasing chain length ofthe alkyl chain rest, this means from [MMIM]+[BTI] − to[BMIM] +[BTI] −. With the exception of [OMIM]+[Cl]− formost of the ionic liquids a similar temperature dependencyof theγ∞

i values of benzene and cyclohexane is observed.For the selection of an entrainer and to decide about the

capacity of the entrainer for a given separation problem, inmost cases the selectivity at infinite dilution (S∞

ij = γ∞i /γ∞

j )and the capacity (ki = 1/γ∞

i ) is used[13]. This means that

Table 3Infinite dilution activity coefficients in [MMIM]+[CH3OC2H4SO4]− (2)measured by dilutor technique

Solute (1) γ∞1 [–]

303.15K 313.15 K 323.15 K 333.15 K 343.15 K

Pentane 68.9 61.5Hexane 170 162Heptane 377 365111CCCCBTA22

Fig. 1. Experimental activity coefficients at infinite dilution lnγ∞1 for ben-

zene in the temperature range from 293 to 343 K together with a linearcorrelation of the data. (** ) Ionic liquids investigated in this work.

the selectivitiesS∞ij and capacitieski can directly be obtained

from the experimentalγ∞i values.

Krummen et al. [7,14] found that the ionic liq-uids ([MMIM] +, [EMIM] + and [BMIM]+[BTI] −,

Fhexane in the temperature range from 293 to 343 K together with a linearcorrelation of the data. (** ) Ionic liquids investigated in this work.

-Pentene 33.0 31.1 29.5 27.9-Hexene 83.0 73.6 66.9 58.7-Heptene 190 163 143 126yclopentane 46.5 43.9 41.7 39.5yclohexane 113 106 100 94.9yclopentene 25.5 24.5 23.4 22.5yclohexene 52.0 48.7 45.7 43.0enzene 4.59 4.44 4.30 4.17 4.05oluene 11.4 10.7 9.98 9.39 9.00cetone 2.19 2.2 2.21 2.23-Butanone 4.15 4.13 4.11 4.11-Pentanone 7.84 7.42 7.11 6.77

ig. 2. Experimental activity coefficient at infinite dilution lnγ∞1 for cyclo-

R. Kato, J. Gmehling / Fluid Phase Equilibria 226 (2004) 37–44 41

Table 4Infinite dilution activity coefficients inN-ethylpyridinium bis(trifluoromethylsulfonyl)imide (2) measured by dilutor technique

Solute (1) γ∞1 [–]

303.15 K 313.15 K 323.15 K 333.15 K

Pentane 21.6 20.4 19.4 18.6Hexane 34.2, 33.5a 31.7, 30.6a 29.7, 29.0a 27.9, 27.7a

Heptane 54.5, 52.4a 49.2, 47.5a 45.2, 44.5a 41.7, 41.2a

Octane 79.9a 73.3a 66.4a 61.0a

Dichloromethane 1.05a 1.03a 1.01a 0.98a

1-Pentene 9.10, 9.01a 8.69, 8.76a 8.31, 8.43a 8.041-Hexene 14.2, 14.6a 13.6, 13.5a 13.1, 13.1a 12.6, 12.7a

1-Heptene 23.6, 24.1a 22.2, 22.2a 21.2, 21.2a 20.2, 20.7a

1-Octene 37.7a 35.2a 33.0a 31.5a

Cyclopentane 11.5 10.8 10.2 9.60Cyclohexane 18.6, 18.5a 17.2, 17.2a 16.0, 16.0a 14.8, 14.7a

Cyclopentene 5.61 5.48 5.34 5.21Cyclohexene 8.89, 8.93a 8.30, 8.07a 7.88, 8.17a 7.48Benzene 1.26a 1.30a 1.35a 1.4a

Toluene 1.86a 1.92a 1.99a 2.06a

m-Xylene 2.67a 2.72a 2.74a 2.78a

p-Xylene 2.61a 2.65a 2.67a 2.69a

o-Xylene 2.36a 2.33a 2.34a 2.33a

Methanol 1.34a 1.23a 1.11a 1.00a

Ethanol 1.89a 1.72a 1.52a 1.37a

1-Propanol 2.59a 2.30a 1.98a 1.75a

2-Propanol 2.59a 2.33a 2.03a 1.78a

Chloroform 1.14a 1.14a 1.14a 1.15a

Acetone 0.460a 0.470a 0.479a 0.490a

2-Pentanone 0.93a 0.93a 0.94a 0.94a

Vinyl acetate 1.15a 1.16a 1.16a 1.15a

Methyl tert-butyl ether (MTBE) 2.86a 2.90a 2.89a 2.85a

Methyl tert-amyl ether (TAME) 4.61a 4.48a 4.42a 4.34a

Ethyl tert-butyl ether (ETBE) 6.84a 6.67a 6.52a 6.30a

Tetrahydrofuran 0.860a 0.860a 0.860a

Water 2.78 2.37 2.00a glc.

Table 5Infinite dilution activity coefficients in 1-methyl-3-methyl-imidazoliumdimethylphosphate (2) measured by dilutor technique

Solute (1) γ∞1 [–]

303.15 K 313.15 K 323.15 K 333.15 K

Octane 385 350 335 3081-Heptene 95.1 94.3 90.5 86.11-Octene 205 191 184 172Cyclohexane 48.5 46.9 44.5 44.0Cyclopentene 13.0 13.1 13.2 13.3Cyclohexene 22.2 23.7 25.2 25.5Benzene 3.55 3.57 3.61 3.66Toluene 7.26 7.29 7.30 7.32Acetone 2.26 2.27 2.28 2.292-Butanone 3.85 3.87 3.89 3.902-Pentanone 7.15 7.19 7.21 7.26Tetrahydrofuran 4.57 4.61 4.67 4.73

343.15 K 353.15 K 363.15 K 373.15 K 383.15 K

Ethanol 0.122 0.128 0.136 0.1401-Propanol 0.180 0.186 0.194 0.204Water 0.0518 0.0537 0.0575

[EMIM] +[C2H5SO4]−) show distinctly higher selec-tivities S∞

ij at infinite dilution for the separation of aliphaticfrom aromatic hydrocarbons by extractive distillation orextraction than the conventional entrainerN-methyl-2-

Table 6Infinite dilution activity coefficients in pyridinium ethoxyethylsulfate (2)measured by dilutor technique

Solute (1) γ∞1 [–]

303.15 K 313.15 K 323.15 K 333.15 K

Hexane 45.2 42.5 38.9 38.2Heptane 111 101 91.5 84.8Octane 232 195 161 1401-Pentene 7.80 7.27 6.09 5.731-Hexene 19.7 18.9 14.9 13.0Cyclopentane 16.0 14.3 13.8 13.3Cyclohexane 36.5 32.2 30.7 29.4Cyclopentene 10.8 10.1 9.78 8.71Cyclohexene 18.8 18.5 18.4 18.1Benzene 3.81 3.80 3.78 3.76Toluene 6.96 6.61 5.92 5.57Acetone 1.75 1.75 1.76 1.762-Butanone 3.09 3.11 3.16 3.192-Pentanone 5.15 5.21 5.27 5.30

42 R. Kato, J. Gmehling / Fluid Phase Equilibria 226 (2004) 37–44

Table 7Saturation fugacity coefficientsϕs

i , saturation vapor pressurePsi for the investigated solutes at temperatures from 303.15 to 383.15 K[6]

Solute ϕsi Ps

i

303.15 K 313.15 K 323.15 K 333.15 K 303.15 K 313.15 K 323.15 K 333.15 K

n-Pentane 0.962 0.953 0.943 0.931 82.03 115.7 159.4 214.8n-Hexane 0.981 0.976 0.969 0.961 24.94 37.25 54.03 76.36n-Heptane 0.991 0.987 0.983 0.978 7.805 12.36 18.91 28.07n-Octane 0.995 0.994 0.991 0.988 2.461 4.149 6.718 10.491-Pentene 0.958 0.948 0.937 0.925 101.4 141.6 193.1 257.81-Hexene 0.98 0.974 0.967 0.959 30.45 44.96 64.58 90.481-Heptene 0.99 0.987 0.982 0.977 9.533 14.92 22.6 33.221-Octene 0.995 0.993 0.991 0.987 3.036 5.045 8.065 12.45Cyclopentane 0.979 0.973 0.967 0.959 51.33 73.94 103.8 142.3Cyclohexane 0.989 0.985 0.981 0.976 16.22 24.62 36.24 51.91Cyclopentene 0.974 0.967 0.96 0.951 61.25 87.7 122.4 167.1Cyclohexene 0.989 0.986 0.982 0.977 14.82 22.61 33.44 48.11Benzene 0.991 0.987 0.984 0.979 15.9 24.37 36.18 52.21Toluene 0.994 0.992 0.99 0.987 4.886 7.885 12.28 18.52m-Xylene 0.996 0.995 0.993 0.991 1.463 2.497 4.099 6.499p-Xylene 0.996 0.995 0.993 0.991 1.549 2.644 4.335 6.854o-Xylene 0.997 0.996 0.995 0.993 1.16 2.045 3.439 5.547Methanol 0.984 0.98 0.975 0.969 21.83 35.38 55.47 84.4Ethanol 0.988 0.985 0.981 0.976 10.41 17.82 29.36 46.751-Propanol 0.996 0.994 0.991 0.988 3.774 6.926 12.1 20.262-Propanol 0.991 0.988 0.983 0.978 7.88 13.93 23.6 38.44Vinyl acetate 0.985 0.980 0.974 0.967 19.31 29.86 44.74 65.18Acetone 0.971 0.964 0.957 0.948 37.77 56.24 81.41 114.82-Butanone 0.986 0.982 0.977 0.971 15.22 23.63 35.53 51.892-Pentanone 0.991 0.988 0.984 0.98 6.086 9.877 15.45 23.4Methyl tert-butyl ether (MTBE) 0.976 0.969 0.961 0.952 40.85 59.64 84.86 117.9Ethyl tert-butyl ether (ETBE) 0.984 0.978 0.972 0.965 20.82 31.55 46.42 66.51Methyl tert-amyl ether (TAME) 0.988 0.984 0.979 0.974 12.64 19.53 29.22 42.46Di-isopropyl ether 0.981 0.975 0.968 0.959 24.74 37.2 54.29 77.12Tetrahydrofuran 0.988 0.984 0.98 0.975 26.8 40.22 58.6 83.17Dichloromethane 0.977 0.971 0.964 0.956 70.5 102 143.8 198Chloroform 0.985 0.98 0.975 0.968 32.32 48 69.25 97.4Water 0.998 0.997 0.996 0.994 4.231 7.358 12.3 19.87

343.15 K 353.15 K 363.15 K 373.15 K 383.15 K 343.15 K 353.15 K 363.15 K 373.15 K 383.15 K

Ethanol 0.97 0.96 0.96 0.95 72.2 108.3 158.4 226.51-Propanol 0.98 0.98 0.97 0.97 32.6 50.8 76.7 112.7Water 0.99 0.99 0.98 70.0 101.3 143.5

Table 8Densities for ionic liquids in the temperature range from 293 to 353 K (investigated with an Anton Paar DMA 4500 Densitometer)

Temperature (K) density,ρ (g cm−3)

[EPY]+[BTI] − [MMIM] +[CH3SO4]− [MMIM] +[CH3OC2H5SO4]− [PY]+[C2H5OC2H4SO4]− [MMIM] +[(CH3)2PO4]−

293.15 1.332 1.317 1.284298.15 1.536 1.328 1.314 1.281303.15 1.531 1.324 1.310 1.277 1.253308.15 1.526 1.320 1.307 1.273 1.249313.15 1.521 1.317 1.303 1.27 1.246318.15 1.516 1.313 1.300 1.266 1.243323.15 1.512 1.310 1.296 1.262 1.239328.15 1.507 1.306 1.293 1.259 1.236333.15 1.502 1.303 1.289 1.255 1.232338.15 1.300 1.286 1.251343.15 1.296 1.282 1.248348.15 1.293 1.279 1.244353.15 1.290 1.275

R. Kato, J. Gmehling / Fluid Phase Equilibria 226 (2004) 37–44 43

Fig. 3. Calculated selectivities at infinite dilutionS∞12 of various ionic liquids

for the separation problems listed inTable 9at 313.15 K. (** ) Ionic liquidsinvestigated in this work.

pyrrolidone [NMP] used in practice for the separation ofaliphatics from aromatics.

Fig. 3 shows the calculated infinite dilution selectivitiesS∞

ij at 313.15 K for seven separation problems of industrialinterest, such as benzene–cyclohexane (aliphatic–aromatic)(set 2), pentane–pentene (alkane–alkene) (set 3),ethanol–water (alcohol–water) (set 5) as listed inTable 9.For the comparison not only the ionic liquids, but alsotypical entrainers, such as NMP and ethylene glycol wereconsidered.

Most of the ionic liquids investigated show a higherselectivity than the conventional selective solvents NMPand ethylene glycol for the separation of aliphatics fromaromatics. For the separation of aliphatics from olefins(pentane–pentene, hexane–hexene) similar selectivitiesas for NMP are observed. For the separation of ethanol,2-butanone and tetrahydrofuran (THF) from water un-fortunately the number of availableγ∞

i values is stilllimited. For [MMIM] +[(CH3)2PO4]− the observed se-lectivities at infinite dilution are 2.7 for the system

Table 9List of separation problems investigated

Set no. System

1 Hexane (1)–benzene (2)2 Cyclohexane (1)–benzene (2)34567 2)

Fig. 4. Calculated selectivity at infinite dilutionS∞12 of various ionic liquids

for the separation of cyclohexane from benzene in the temperature rangefrom 303 to 333 K. (** ) Ionic liquids investigated in this work.

ethanol–water, 100 for 2-butanone–water and 120 forTHF–water. For ethylene glycol selectivities of 1.91 forethanol–water, 7.07 for butanone–water and 5.87 forTHF–water are obtained. This means that the values for[MMIM] +[(CH3)2PO4]− are notably higher than the selec-tivities obtained for ethylene glycol, the common selective

F het isw

Pentane (1)–pentene (2)Hexane (1)–hexene (2)Ethanol (1)–water (2)2-Butanone (1)–water (2)Tetrahydrofuran (1)–water (

ig. 5. Calculated capacityki of various ionic liquids for cyclohexane in temperature range from 303 to 333 K. (** ) Ionic liquids investigated in thork.

44 R. Kato, J. Gmehling / Fluid Phase Equilibria 226 (2004) 37–44

solvent for the separation of alcohols from water by extractivedistillation.

Fig. 4 shows the temperature dependency of the selec-tivities S∞

ij for the system cyclohexane–benzene in 14 ionicliquids and NMP in the temperature range from 303 to 333 K.It can be seen that with the exception of [OMIM]+[Cl]− theselectivity decrease with increasing temperature; this meansthat the partial molar excess enthalpies at infinite dilutionshould be positive, and because of the nearly parallel cur-vature (exceptions: [OMIM]+[Cl]−, [HMIM] +[PF6]−, etc.)the values of the partial molar excess enthalpies at infinitedilution should be similar for most of the systems consid-ered. It can be seen that [MMIM]+[CH3OC2H5SO4]− showsthe highest selectivity compared to the other ionic liquidsconsidered and NMP for the separation of aliphatics fromaromatics.

Fig. 5shows the capacitieski for cyclohexane in the vari-ous ionic liquids and NMP. From this figure it can be recog-nized that NMP shows the highest capacity in comparison toall ionic liquids.

4. Conclusion

Activity coefficients at infinite dilution for variouss id-amO l-pbS[ urer uidcv tial ofi trac-t ticsf tiest ionicls ria sl

ionicl acec ctived

LFHkmM

nL number of moles of the solventPS saturation vapor pressure of solutePS

L vapor pressure of the solventR general gas constantS∞

ij selectivity at infinite dilutionVg gas phase volumeVN net retention volume

Greek lettersγ∞ activity coefficient at infinite dilutionϕs fugacity coefficient in the saturation state

Subscripti component

Acknowledgments

The authors are grateful to Deutsche Forschungsgemein-schaft (DFG) for financial support of this study. The authorsalso would like to thank H. Stojek for performing a great partof the measurements using gas–liquid chromatography.

References

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olutes in the five ionic liquids 1-methyl-3-methyl-imzolium methylsulfate [MMIM]+[CH3SO4]−, 1-methyl-3-ethyl-imidazolium methoxyethylsulfate [MMIM]+[CH3-C2H4SO4]−, 1-methyl-3-methyl-imidazolium dimethyhosphate [MMIM]+[(CH3)2PO4]−, N-ethylpyridiniumis(trifluoromethylsulfonyl) imide [C5H5NC2H5]+[(CF3-O2)2N]−, and pyridiniumethoxyethylsulfate [C5H5NH]+

C2H5OC2H4OSO3]− were measured in the temperatange from 303.15 to 373.15 K with the help of gas–liqhromatography, respectively, dilutor technique. Theγ∞alues obtained can be used to decide about the potenonic liquids as selective solvents for extraction and exive distillation. For example, for the separation of alipharom aromatics most ionic liquids show higher selectivihan the conventional entrainers used, whereby foriquids with sulfate anions [RSO4]− and [RORSO4]− higherelectivities at infinite dilutionS∞

ij were obtained than foonic liquids with [(CF3SO2)2N]−, [BF4]− and [PF6]−nions. However, the capacityki of NMP for cyclohexane i

arger than for all other ionic liquids investigated.From the selectivities and capacities obtained for the

iquids it can be concluded that ionic liquids may replonventional entrainers applied for extraction and extraistillation processes.

ist of symbolsflow rate of carrier gas

E excess enthalpycapacity

L mass of solventL molecular weight of the solvent

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