J. Chem. Thermodynamics 42 (2010) 909–919

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J. Chem. Thermodynamics

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Ionic liquids as entrainers for water + ethanol, water + 2-propanol, and water + THFsystems: A quantum chemical approach

Vijay Kumar Verma, Tamal Banerjee *

Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati 781 039, Assam, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 19 October 2009Received in revised form 18 January 2010Accepted 2 March 2010Available online 6 March 2010

Keywords:Ionic liquidEntrainerEthanol2-PropanolTHFRelative volatilityCOSMO-RS

0021-9614/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.jct.2010.03.001

* Corresponding author. Tel.: +91 361 2582266; faxE-mail address: tamalb@iitg.ernet.in (T. Banerjee).

Ionic liquids (ILs) are used as entrainers in azeotropic systems such as water + ethanol, water + 2-propa-nol, and water + tetrahydrofuran (THF). Ionic liquids consisting of a cation and an anion has limitlesscombinations, thereby making experimentation expensive and time taking. For the prediction of theliquid phase nonidealities resulting from molecular interactions, ‘‘COnductor-like Screening MOdel forReal Solvents” (COSMO-RS) approach is used in this work for the screening of potential ionic liquids. Ini-tially benchmarking has been done on 12 reported isobaric IL based ternary systems with an absoluteaverage deviation of 4.63% in vapor phase mole fraction and 1.07% in temperature. After successfulbenchmarking, ternary vapor + liquid equilibria for the azeotropic mixture of (a) ethanol + water, (b) 2-propanol + water, and (c) THF + water with combinations involving 10 cations (imidazolium, pyridinium,quinolium) and 24 anions were predicted. The VLE prediction, which gave the relative volatility, showedthat the imidazolium based ionic liquid were the best entrainer for the separation of the three systems attheir azeotropic point. ILs with [MMIM] cation in combination with acetate [OAc], chloride [Cl], and bro-mide [Br] anion gave the highest relative volatility.

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1. Introduction

Ionic liquids (ILs) are gaining wide recognition as potentialenvironmental solvents due to their unique properties [1]. ILsconsisting of an organic cation and an inorganic/organic anionare liquid over a wide range of temperature and present excellentsolvation properties [2]. Low vapor pressure of ILs minimizes thechance of loss to atmosphere. This makes them an obvious choicefor entrainer because of its reusability, when compared withconventional entrainers like NMP, DMSO, and sulpholane. Excel-lent review of ionic liquids on separation techniques [3], thermo-dynamics of non-aqueous mixtures [4], analytical applications [5]and in the field of catalysis [6] are available in the literature. Thepossibility of changing cation + anion combination provides anexcellent opportunity to obtain task-specific ILs for specific appli-cations [7]. For azeotropic systems, a separation by ordinary dis-tillation becomes impossible. Extractive distillation is the widelyused technique to remove one of the components at its azeotropicpoint. This involves the addition of a new solvent (entrainer)which interacts with the components by altering their relativevolatilities.

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In this work, we have studied the commonly occurring azeotro-pic systems namely: ethanol + water, 2-propanol + water, and tet-rahydrofuran + water using ionic liquid as entrainer. There havebeen numerous experimental studies carried out on these systemsusing IL. Recently, Zhao et al. [8] measured the isobaric VLE for eth-anol + water system containing ionic liquids (ILs): 1-methyl-3-methylimidazolium dimethylphosphate, 1-ethyl-3-methylimida-zolium diethyl phosphate, 1-butyl-3-methylimidazolium bromide,1-butyl-3-methylimidazolium chloride, and 1-butyl-3-methylimi-dazolium hexafluorophosphate at atmospheric pressure (101.32kPa). Further Zhang et al. [9] measured the isobaric VLE data forthree ternary systems namely: water + 2-propanol + 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]), water + 1-propanol + [EMIM][BF4], and water + 1-propanol + 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) at 100 kPapressure by varying IL mass fractions. Calvar et al. [10] measuredthe isobaric VLE for the ternary system: ethanol + water + 1-bu-tyl-3-methylimidazolium chloride ([BMIM][Cl]) at 101.3 kPa.Additionally the ternary VLE of ethanol + water + 1-hexyl-3-meth-ylimidazolium chloride ([HMIM][Cl]) was measured at the sametotal pressure [11]. In another work published by the same authors[12], the VLE of ethanol + water + 1-ethyl-3-methylimidazoliumethylsulphate [EMIM][EtSO4] was obtained at 101.3 kPa. Li et al.[13] measured the isobaric VLE data for 2-propanol + water +

Nomenclature

[MMIM] 1-methyl-3-methylimidazolium[EMIM] 1-ethyl-3-methylimidazolium[BMIM] 1-butyl-3-methylimidazolium[HMIM] 1-hexyl-3-methylimidazolium[OMIM] 1-octyl-3-methylimidazolium[EPY] 1-ethylpyridinium[BPY] 1-butylpyridinium[HPY] 1-hexylpyridinium[OPY] 1-octylpyridinium[OQU] 1-octylquinoliumw1,3 weight fraction of ILx01;1 mole fraction of component (1) in liquid in IL free basisx01;2 mole fraction of component (2) in liquid in IL free basis

M1, M2, M3 molecular weights of component (1), component (2),and IL (3)

T temperatureP pressurew weight fractionx molar fraction in the liquid phasey molar fraction in the vapor phase

Greek lettersa relative volatilityc activity coefficient

910 V.K. Verma, T. Banerjee / J. Chem. Thermodynamics 42 (2010) 909–919

1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]) atatmospheric pressure.

For 2-propanol + water + IL systems, Zhang et al. [14] obtainedthe ternary VLE with the following ILs: 1-butyl-3-methylimidazo-lium tetrafluoroborate ([BMIM][BF4]), 1-ethyl-3-methylimidazo-lium tetrafluoroborate ([EMIM][BF4]), 1-butyl-3-methylimida-zolium dicyan-amide ([BMIM][N(CN)2]), 1-ethyl-3-methylimida-zolium dicyanamide ([EMIM][N(CN)2]), 1-butyl-3-methylimida-zolium acetate ([BMIM][OAc]), 1-ethyl-3-methylimidazolium ace-tate ([EMIM][OAc]), and 1-butyl-3-methylimidazolium chloride([BMIM][Cl]). In another study published by the same authors[15] an ebulliometer was used to obtain the T, x, y data for thesystem: water + 2-propanol + 1-butyl-3-methylimidazolium tetra-fluoroborate [BMIM][BF4] at 100 kPa with ionic liquid mass frac-tions of 0.30, 0.50, and 0.70. Jork et al. [16] have presented theternary VLE of (ethanol + water + IL) and (THF + water + IL) sys-tems with four commercially available ILs. The four ionic liquidsused were derived from imidazolium cations based on [BF4] anion.For investigating the influence of ionic liquids on the separationfactors, recently [17] isothermal VLE for propanol + water withternary systems involving ionic liquids: 1-butyl-3-methylimidazo-lium bis(trifluoromethylsulfonyl)imide [HMIM][(CF3SO2)2N] and1-butyl-1-methyl-pyrrolidinium bis (trifluoromethylsulfonyl)imide [BMPYR][(CF3SO2)2N] were measured.

Due to the great diversity of structure and property associatedwith the cations and anions, selection of ILs for the separation ofspecific mixture becomes difficult and time consuming. It is alsonecessary to understand the vapor + liquid behavior of a mixturecontaining IL since the relative volatility gets altered by the addi-tion of ILs. To date most of work has been done experimentallywith conventional Gibb’s free energy models like NRTL, UNIQUACetc. However, recently authors have applied the UNIFAC modelfor phase equilibria predictions in ionic liquids [18–20]. In a recentwork by Lei et al. [21] UNIFAC model have been applied for the pre-diction of VLE for IL based systems. Since there have been only alimited number of functional groups for ionic liquids included inthe UNIFAC model, the future development of this predictive mod-el will still require much more accurate experimental data. Thus aneed was felt to predict the activity coefficients of the volatile com-ponents without the need of a thermodynamic model in the liquidphase. Keeping this in mind, the COSMO-RS [22,23] model isadopted in this work, as it requires molecular structure informa-tion as the starting point and is independent of experimental data.The expressions and methodologies are demonstrated in our ear-lier work [24–27].

Till date most of the work has been focused on imidazoliumbased ionic liquids. Predictions involving phase equilibria of ionicliquids by COSMO-RS has been obtained earlier [28–31]. In this

work an attempt has been made to generate isobaric VLE data forimidazolium, pyridinium, and quinolium based cations in combi-nation with 24 anions. The 10 cations and 24 anions investigatedin this work are reported in table 1.

2. Computational details

2.1. COMSO file generation

The Quantum Chemistry Package of Gaussian 03 [32] has beenused to compute the COSMO files. For the isolated cation and an-ion, initial geometry optimization was done using Hartree Fock[33] level of theory. We obtained a planar structure for the imi-dazolium ring which agrees well with the previous studies [34].Frequency optimization was done at the same level of theory onthe optimal cation/anion geometry in order to detect the presenceof any imaginary or negative frequencies. The absence of negativevibrational frequencies verified the existence of a true minimum.Thereafter the final optimized structure was taken for the genera-tion of COSMO file using Gaussian 03 by P BV86 [35] density func-tional theory. The first step for COSMO-RS calculation is toestimate the sigma profile of each component which is obtainedfrom its respective COSMO file. The ideal screening charges (i.e.COSMO file) on the molecular surface are computed using [PBV86] [35] level of theory. The Triple Zeta Valence Potential (TZVP)[36] basis set has been used in combination with the density fittingbasis set of DGA1 [37] for generating the COSMO file. The radii ofthe elements are used to define the cavity for the molecule.

For the prediction, the complete dissociation of ionic liquid istaken to be equal to the dissociation of cation and anion [38].Thesigma profile (charge distribution of screening charge densities)[24–27] of ionic liquid is simply the algebraic sum of the sigmaprofile of cation and anion [Eq. (1)].

pionic liquidðrÞ ¼ pcationðrÞ þ panionðrÞ: ð1Þ

Here pcation(r) and panion(r) are the sigma profile for cation and an-ion, respectively. Initially, benchmarking of binary and ternary sys-tems using ionic liquid was performed on known systems. Theinfluence of the IL structure on the relative volatility and separationfactor of the low boiling component in extractive distillation arediscussed in further sections.

2.2. Vapor + liquid equilibria prediction

VLE data are usually regressed via excess Gibb’s Free EnergyModels such as Van Laar, Margules and Wilson’s equation. The to-

TABLE 1List of cations and anions.

S. no Name of cation(s) Acronym Structure

Cations1 1-alkyl-3-methylimidazolium [RMIM]

N+

N

R

R=methyl,ethyl,butyl,hexyl,octyl

2 1-alkylpyridinium [RPY]

N+R

R=ethyl,butyl,hexyl,octyl

3 1-Octylquinolium [OQU]

N+

Anions

01 Nitrate [NO3]

N+O-

O

-O

02 Thiocyanate [SCN] S-N

03 Acetate [OAc]

O

-O

04 Bisulphate [HSO4]

OH

SO O

O-

05 Tetrafluoroborate [BF4]

B-

F

F

F

F

06 Methylsulphonate [CH3SO3]

SO O

O-

07 Trifluoroacetate [CF3COO] O-O

F

F

F

08 Methylsulphate [CH3SO4]

SO

O

O

-O

09 Hexafluorophosphate [PF6]

P-F F

F

FF

F

(continued on next page)

V.K. Verma, T. Banerjee / J. Chem. Thermodynamics 42 (2010) 909–919 911

TABLE 1 (continued)

S. no Name of cation(s) Acronym Structure

10 Ethyl sulphate [C2H5SO4]

S

O

OO

O-

11 Dimethylphosphate [(CH3)2PO4]

P

O

OO

O-

12 Methylsulfonylacetamide [C3H7NO3S] O

H2NS

O

O

13 Tetracyanoborate [B(CN)4]

B-

N

N

N

N

14 Salicylate [C7H5O3] HO

O

-O

15 Bis(methylsulphonyl)amide [(CH3SO2)2N]N -

S

O

O

S

O

O16 Bis-oxaloborate [BHO4]

BO

OH

O-O

17 Diethyl phosphate [C4H10O4P]

P

O

OO

O-

18 p-Toulenesulphonate [C7H8O3S]

O-S

O

O19 2-(2-Methoxyethoxy)ethyl sulphate [C5H11SO6]

S OO

O

O-

OO

20 Decanoate [C10H19O2]O O -

912 V.K. Verma, T. Banerjee / J. Chem. Thermodynamics 42 (2010) 909–919

TABLE 1 (continued)

S. no Name of cation(s) Acronym Structure

21 Octylsulphate [C8H17 SO4]

S OO

O

O-22 Dibutylphosphate [(C4H9)2 HPO4]

PO

OO

OH

23 Bromine [Br] Br24 Chlorine [Cl] Cl

TABLE 2Isobaric vapor + liquid equilibrium benchmarking for ternary systems of 2-propa-nol + water + ILs (a: experimental data taken from Zhang et al. [14]).

Ionic liquid Temperature range/K RAADT RAADy

[BMIM][BF4] 355.29–362.43 0.21 2.20[EMIM][BF4] 354.65–358.56 0.20 2.34[BMIM][N(CN)2] 356.67–377.04 1.97 2.29[EMIM][N(CN)2] 357.08–367.98 1.44 2.46[BMIM][OAc] 355.15–374.04 1.15 0.44[EMIM][OAc] 355.3–372.46 0.94 0.59[BMIM][Cl] 356.32–381.69 0.73 1.40

TABLE 3Isobaric vapor + liquid equilibrium benchmarking for ternary systems of 2-propanol + wat

Ionic liquid Ref. weight fraction of ILs

[EMIM][BF4] [9] 0.30.40.60.8

[BMIM][BF4] [15] 0.30.50.7

TABLE 4Isobaric vapor + liquid equilibrium benchmarking for ternary systems of 1-propanol + wat

Ionic liquid Ref. Weight fraction of ILs

[BMIM][BF4] [9] 0.30.50.7

[EMIM][BF4] [9] 0.60.7

TABLE 5Isobaric vapor + liquid equilibrium benchmarking for ternary systems of water + ethanol +

Ionic liquid Ref. Weight fraction of ILs

[MMIM][DMP] [8] About 10%About 20%

[EMIM][DEP] [8] About 10%About 20%

[BMIM][Br] [8] About 10%About 20%

[BMIM][Cl] [8] About 20%About 30%

[BMIM][PF6] [8] About 10%About 20%

V.K. Verma, T. Banerjee / J. Chem. Thermodynamics 42 (2010) 909–919 913

tal pressure prediction, mole fraction in vapor phase and nonideali-ty of vapor phase are given by Eqs. (2)–(4), respectively.

P ¼ c1x1Psat1 þ c2x2Psat

2 ; ð2Þ

y1 ¼x1c1Psat

1

P; ð3Þ

Py1/1

/10¼ x1c1Psat

1 : ð4Þ

The vapor phase has been assumed to be ideal. This implies that thegas phase fugacity coefficient /1

/10

� �is equal to unity. For calculating

the effectiveness of the entrainer (here the ionic liquid) the relative

er + ILs.

Temperature range/K RAADT/% RAADy/%

355.37–357.37 0.68 4.37355.46–358.80 0.67 3.81355.6–361.29 0.71 3.82351.12–367.87 1.18 1.50

355.7–358.49 0.73 5.60356.29–360.29 0.82 5.12358.72–366.6 1.06 3.91

er + ILs.

Temperature range/K RAADT/% RAADy/%

370.39–363.7 0.84 6.91371.51–374.81 0.92 7.28374.81–370.56 1.16 6.61355.7–358.49 0.87 4.60356.29–360.29 1.08 3.77

ILs at P = 1 bar.

Temperature range/K AADT/% AADy/%

362.53–371.91 2.25 6.58363.58–373.54 2.15 7.43362.23–371.6 2.19 7.91363.21–371.79 2.2 6.36351.91–363.14 0.49 6.22352.92–360.99 0.55 5.18353.91–364.49 1.08 5.89356.52–364.46 1.8 6.55351.41–357.7 0.68 7.67352.01–354.53 0.49 5.46

FIGURE 1. Relative volatility prediction for imidazolium based ionic liquids for ethanol + water (w = 0.65) (list of anions as per table 1).

FIGURE 2. Relative volatility prediction for pyridinium and quinolium based ionic liquids for ethanol + water system (w = 0.65) (list of anions as per table 1).

FIGURE 3. Comparison of relative volatility for [MMIM], [EPY], and [OQU] based ionic liquids for ethanol + water system (w = 0.65) (list of anions as per table 1).

914 V.K. Verma, T. Banerjee / J. Chem. Thermodynamics 42 (2010) 909–919

FIGURE 4. Comparison of relative volatility for [MMIM] based ionic liquids for ionic liquid with weight factor for ethanol + water system (list of anions as per table 1).

FIGURE 5. Relative volatility prediction for imidazolium based ionic liquids for 2-propanol + water system (w = 0.65) (list of anions as per table 1).

FIGURE 6. Relative volatility prediction for pyridinium and quinolium based ionic liquids for 2-propanol + water system (w = 0.65) (list of anions as per table 1).

V.K. Verma, T. Banerjee / J. Chem. Thermodynamics 42 (2010) 909–919 915

volatility (a12) of the solution is calculated using the followingexpression:

a12 ¼Psat

1 c1

Psat2 c2

: ð5Þ

Isobaric vapor + liquid equilibria prediction for binary or ternarysystem requires the knowledge of non-ideal liquid phase activitycoefficient at some known liquid phase composition. In our casethe values of c1, c2 (activity coefficient of components 1 and 2,respectively) are predicted by COSMO-RS and saturated vapor

FIGURE 7. Comparison of relative volatility for [MMIM], [EPY], and [OQU] based ionic liquids for 2-propanol + water system (w = 0.65) (list of anions as per table 1).

FIGURE 8. Comparison of relative volatility for [MMIM] based ionic liquids with weight factor of 0.65 and 0.55 for 2-propanol + water system (list of anions as per table 1).

FIGURE 9. Relative volatility prediction for imidazolium based ionic liquids for THF + water system (w = 0.65) (list of anions as per table 1).

916 V.K. Verma, T. Banerjee / J. Chem. Thermodynamics 42 (2010) 909–919

pressures Psat1 ; Psat

2

� �are calculated using Antoine equation. In all

the VLE calculations, a vapor pressure of 0.001 mm of Hg has

been used for ILs. This value is comparable to that reported byPaulechka et al. [39] for [BMIM][(CF3SO2)2N].To compare our pre-

V.K. Verma, T. Banerjee / J. Chem. Thermodynamics 42 (2010) 909–919 917

dictions with the literature the relative average absolute devia-tion in temperature (RAADT) and average absolute deviation ofmole fraction in vapor phase (RAADy) are defined as follows(M: no of data points):

RAADT ¼ 1M

XM

j

Texp � Tcal

�� ��; ð6Þ

RAADy ¼ 1M

XM

j

yexpj � ycalc

j

������: ð7Þ

3. Results and discussion

3.1. Benchmarking of ternary ILs systems

Initially the benchmarking was done on ternary systems belonging to knownentrainer systems i.e. ethanol + water, 2-propanol + water, and THF + water. Tilldate no isobaric VLE data is available for THF + water systems, hence the bench-marking could not be done. In all the systems, the authors have used ionic liquidas entrainer to separate the two components. For the calculation of activity coeffi-cient, we require the liquid composition on IL-containing basis, which can be calcu-lated using the known formulae [15]:

x1;1 ¼x01;1

x01;1 þ x01;2 þw1;3

1�w1;3

x01;1 M1þx01;2 M2

M3

; ð8Þ

x1;2 ¼x01;2

x01;1 þ x01;2 þw1;3

1�w1;3

x01;1 M1þx01;2 M2

M3

; ð9Þ

where x01;1 ¼ 1� x01;2

3.1.1. Water + 2-propanol + ILsVLE for the system: water (1) + 2-propanol (2) + IL (3) were predicted for seven

different IL’s at a constant pressure of 100 kPa. This was compared with the mea-surements by Zhang et al. [14]. For the prediction of ternary isobaric VLE, seven bin-ary mixtures involving 49 points were involved. The predicted temperature andvapor phase composition are compared with the reported data and the relativeabsolute average error deviation in temperature T and vapor mole fraction y are gi-ven for different ionic liquids (table 2). An RAAD of 1.67% in vapor phase mole frac-tion and 0.95% in temperature was obtained for all the systems. Deviation fromexperimental values were found to be more for systems containing high weightfraction of ionic liquids. In another study on 2-propanol-water system, Li et al.[13] obtained the experimental data for water + 2-propanol + [EMIM][BF4],water + 2-propanol + [BMIM][BF4], water + 1-propanol + [EMIM][BF4], andwater + 1-propanol + [BMIM][BF4]. Using the reported experimental data, we pre-dicted and compared the percentage error in temperature and vapor phase molefraction. The results are tabulated in tables 3 and 4 for 2-propanol + water and 1-propanol + water system, respectively. The predictions are excellent consideringthe method to be a-priori. Further the isobaric VLE data for water + 2-propa-nol + [EMIM][BF4] was measured at atmospheric pressure (101.32 kPa) by Li et al.[13]. The results indicate an elimination of azeotropic phenomenon at a specific

FIGURE 10. Relative volatility prediction for pyridinium and quinolium based io

IL content. In the benchmarking of water + 2-propanol + [EMIM][BF4] systemsinvolving 35 points, we obtained a RAAD of 0.57% and 2.08% in temperature and va-por phase mole fraction, respectively.

3.1.2. Water + ethanol + ILsIsobaric VLE data for ethanol + water systems containing ILs: [MMIM][DMP],

[EMIM][DEP], [BMIM][Br], [BMIM][Cl], and [BMIM][PF6] were measured by Zhaoet al. [8] at atmospheric pressure. The results indicate that all the ILs studiedshowed a salting-out effect, which led to an enhancement of relative volatility ofethanol and even an elimination of the azeotropic phenomenon at specific IL con-tent. Using the experimental data, we predicted the temperature, vapor phase molefraction and relative volatility (table 5). This clearly indicates that the addition ofionic liquids enhances the relative volatility. Higher weight fraction of ionic liquidgave higher relative volatility as expected. An RAAD of 6.5% in vapor phase molefraction and 1.4% in temperature were obtained for all the systems.

The results showed that the VLE of ethanol + water systems in the presence ofdifferent ILs was obviously different from that of the IL free system. It was foundthat the salting-out effect followed the order of [BMIM][Cl] > [BMIM][Br] > [B-MIM][PF6] > [MMIM][DMP] > [EMIM][DEP], which was ascribed to the preferentialsolvation ability of the ions resulting from the dissociation of the IL.

3.2. Influence of ILs as an entrainer

The influence of ionic liquids on the phase behavior of the aqueous azeotropicsystems: ethanol + water, 2-propanol + water, and THF + water were investigatedusing COSMO–RS model. Ternary vapor + liquid equilibrium of the azeotropic mix-tures containing all possible combination of 10 cations and 24 anions were pre-dicted. For a quantitative prediction the relative volatility was predicted aroundthe azeotropic point (i.e. both temperature and mole fraction). A comparison wasmade by calculating the average of the relative volatility values within the range.For the calculation of activity coefficient, we obtained the liquid composition onIL-containing basis, which are calculated by Eqs. (8) and (9). Based on the reportedisobaric VLE and usual practice [40], a 0.65 weight fraction of ionic liquid have beenadopted. After analyzing the predicted results we performed predictions by varyingthe mass fraction of ILs.

3.2.1. Ethanol + water + ILsIsobaric VLE data of ternary system: water + ethanol + ILs were predicted using

240 possible combinations of 10 cations and 24 anions. With the help of predictedVLE data we obtained the relative volatility for each ionic liquid. Relative volatilitycorresponding to azeotrope composition (�0.90 mole fraction of ethanol) for etha-nol + water system are shown in figures 1 and 2 for imidazolium and pyridiniumbased cations, respectively. It is observed that the ionic liquids enhance the relativevolatility, thereby breaking the azeotrope behavior of the ethanol + water system.Large cations led to a noticeable decrease in the relative volatility. From figures 1and 2 it is evident that a smaller size cation gave rise to higher values of relativevolatility as compared to large cations. This is consistent with the findings reportedby Marsh et al. [41] where the partitioning coefficients of ethanol in an etha-nol + water + IL, were found to increase with the overall concentration of ethanol.It was observed that ethanol partitions less favourably into the ionic liquid as thealkyl chain length decreases and vice versa. Thus a smaller cation possesses stron-ger IL + water interaction as compared to a larger cation. Arlt and co-workers [16]working on ethanol + water + IL system observed that the IL + water interactionsin the liquid phase increases which forces ethanol into the vapor phase, therebyincreasing separation factors.

nic liquids for THF + water system (w = 0.65) (list of anions as per table 1).

918 V.K. Verma, T. Banerjee / J. Chem. Thermodynamics 42 (2010) 909–919

From the predicted data it is observed that the highest values of relative vola-tility in imidazolium, pyridinium, and quinolium based ionic liquid are [MMIM],[EPY], and [OQU], respectively. Comparisons based on the relative volatility of thesecations are shown in figure 3. The effect of cation on the relative volatility followedthe order: [MMIM] > [EPY] > [OQU]. For [MMIM] relative volatilities varied from 0.9to 29.22. After the isobaric VLE prediction ionic liquid with weight fraction of 0.55were computed and compared with our previous result. The variation of weightfraction with the [MMIM] cation is shown in figure 4. It is evident that a decreasein weight fraction of ionic liquid led to a decrease in relative volatility.

For the anions it is clear that three anions namely acetate [OAc], bromide [Br],and chloride [Cl] stands out with high values of separation factors. This is true sincethe studies carried out on ethanol + water system using [BMIM][Cl] [10] and[HMIM][Cl] [11] has yielded activity coefficients less than one with ionic liquids.This implies that ionic liquid consisting of [OAc], [Br], and [Cl] anions interactsstrongly with IL than with itself.

3.2.2. 2-Propanol + water + ILsIsobaric VLE data of the ternary system: water + 2-propanol + ILs were pre-

dicted using all possible combination of 10 cations and 24 anions. Relative volatilitycorresponding to azeotrope composition of 0.70 mole fraction of 2-propanol [42]are shown in figures 5 and 6 for imidazolium and pyridinium based cations, respec-tively. The figures clearly demonstrate that a large cation again leads to a noticeabledecrease in the relative volatility and the highest values of relative volatility in imi-dazolium, pyridinium, and quinolium based ionic liquid are [MMIM], [EPY], and[OQU] cation, respectively. This agrees well with the previous work of Rodriguezand Pereiro [43] which pointed out that imidazolium based ionic liquids having alonger alkyl group possess a hydrophobic steric effect which reduces the polar char-acter of the secondary OH group of 2-propanol.Additonally a smaller cation reduces

FIGURE 11. Comparison of relative volatility for [MMIM], [EPY], and [OQU] b

FIGURE 12. Comparison of relative volatility for [MMIM] based ionic liquids for ionic liq1).

the steric effect and increases the separation factor. Further, comparisons based onthe relative volatility of the three cations are given in figure 7. The effect of cationon the relative volatility followed the order: [MMIM] > [EPY] > [OQU]. For [MMIM]the relative volatility varied from 0.92 to 62.7. The variation of weight fraction withthe [MMIM] cation is shown in figure 8. It is again evident that a decrease in weightfraction of ionic liquid led to a decrease in relative volatility. Among the anions it isclear that the same three anions i.e. acetate [OAc], bromide [Br], and chloride [Cl]gave high separation factors. This is consistent with the work carried out by Zhanget al. [14] which gave the preferential order of anions as [Cl] > [OAc] > [BF4] onwater + 2-propanol system. This exactly matches with the order given in figure 5.

3.2.3. THF + water + ILsIsobaric VLE data of ternary system (water + THF + ILs) were predicted using

240 possible combinations. Relative volatility corresponding to azeotrope composi-tion (�0.68 mole fraction of THF) for THF + water system are shown in figures 9 and10 for imidazolium and pyridinium + quinolium cations, respectively. It is observedthat the ionic liquids enhance the relative volatility, thereby breaking the azeotro-pic behavior of the THF + water system. THF having a ringed structure is muchfavourable towards ionic liquid owing to its similar structure. We have reported asimilar trend for thiophene [27] which like THF, showed higher selectivity for smal-ler size cations.

The highest values of relative volatility and the order in imidazolium, pyridini-um and quinolium based cations were found to follow a similar trend (i.e. [MMI-M] > [EPY] > [OQU]) as seen for ethanol + water and 2-propanol + water systems.Comparisons based on the relative volatility of these cations are given in figure11. For [MMIM] the relative volatility varied from 0.9 to 20. The effect of weightfraction of ionic liquid with respect to [MMIM] cation is shown in figure 12. As ex-pected an increase in weight fraction of ionic liquid led to an increase in relative

ased ionic liquids for THF + water system (list of anions as per table 1).

uid weight factor of 0.65 and 0.55 for THF + water system (list of anions as per table

V.K. Verma, T. Banerjee / J. Chem. Thermodynamics 42 (2010) 909–919 919

volatility. Among the anions it is clear that the same three anions i.e. acetate [OAc],bromide [Br], and chloride [Cl] again gave high separation factors (figure 12). This isconsistent with the work of Jork et al. [16] in which they systematically varied theanion and found that [Cl] anion gave a high separation factor as compared to the[BF4] anion.

4. Conclusions

ILs represents a promising class of highly selective, nonvolatileentrainers for separation processes. The IL’s properties can be tai-lored by varying the cation and anion structures. The influence ofionic liquids on the phase behavior of the aqueous azeotropic sys-tem: ethanol + water, 2-propanol + water, and THF + water wereinvestigated. Ternary vapor + liquid equilibria of these azeotropicmixtures containing different ILs from 10 cations and 24 anionswere presented. It can be concluded that the addition of IL resultsin a remarkable increase of the relative volatility of the low boilingcomponent and eliminates the azeotropic system behavior. ILscomposed of small anions and cations exhibit the best entrainerproperties. The effect of IL on the cation in the relative volatilityfollowed the order: imidazolium > pyridinium > quinolium. For aparticular cation, shortening the length of the alkyl chain led toan increase in relative volatility. The isobaric VLE prediction resultsshowed that among all ILs studied, the ILs with [OAc], [Cl], or [Br]anions, and [MMIM] cation, has the most significant ability inenhancing the relative volatility. The entrainer properties of ILswere also found to be proportional to the weight fraction of ILs.

Acknowledgment

The modelling work reported in this article was financially sup-ported by a research Grant under the Fast Track Scheme (SR/FTP/08-08) from Department of Science and Technology (DST), Govern-ment of India.

Appendix A. Supplementary material

The VLE data for all the 240 possible combinations for etha-nol + water + IL, 2-propanol + IL, and water + THF + IL within theazeotropic range is given as a supplementary material. Supplemen-tary data associated with this article can be found, in the onlineversion, at doi:10.1016/j.jct.2010.03.001.

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JCT 09-341