Alkylsulfate-based ionic liquids in the liquid–liquid extraction of aromatic hydrocarbons

J. Chem. Thermodynamics 45 (2012) 68–74

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Alkylsulfate-based ionic liquids in the liquid–liquid extraction of aromatic hydrocarbons Silvia García, Marcos Larriba, Julián García ⇑, José S. Torrecilla, Francisco Rodríguez Department of Chemical Engineering, Complutense University of Madrid, E-28040 Madrid, Spain

a r t i c l e

i n f o

Article history: Received 31 May 2011 Received in revised form 6 September 2011 Accepted 13 September 2011 Available online 19 September 2011 Keywords: Aromatic/aliphatic separation (Liquid + liquid) equilibria Ionic liquids Sulfate anion

a b s t r a c t The (liquid + liquid) equilibrium data (LLE) for the extraction of toluene from heptane with different ionic liquids (ILs) based on the alkylsulfate anion (R-SO4) was determined at T = 313.2 K and atmospheric pressure. The effect of more complex R-SO4 anions on capacity of extraction and selectivity in the liquid–liquid extraction of toluene from heptane was studied. The ternary systems were formed by {heptane + toluene + 1,3-dimethylimidazolium methylsulfate ([mmim][CH3SO4]), 1-ethyl-3-methylimidazolium hydrogensulfate ([emim][HSO4]), 1-ethyl-3-methylimidazolium methylsulfate ([emim][CH3SO4]), or 1-ethyl-3-methylimidazolium ethylsulfate ([emim][C2H5SO4])}. The degree of quality of the experimental LLE data was ascertained by applying the Othmer–Tobias correlation. The phase diagrams for the ternary systems were plotted, and the tie lines correlated with the NRTL model compare satisfactorily with the experimental data. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction The use of ionic liquids (ILs) in separation processes has achieved much attention due to the environmental and operational problems related to conventional organic solvents, and the challenge of separating mixtures of several compounds that form azeotropes. The separation of aromatic hydrocarbons from aliphatic hydrocarbons combines these two aspects together with the relevance of the aromatics application in the petrochemical industry [1]. There are many researches focused on the determination of (liquid + liquid) equilibrium (LLE) data for systems containing {aromatic + aliphatic + IL} [2–34]. The aim of these works was to find suitable ILs which show values of selectivity and capacity of extraction of aromatics higher than those of sulfolane, which is the most common organic solvent used in the industry. Among all of them, the ILs based on the alkylsulfate anion (R-SO4) has reached a great interest because of they are easily synthesized in an atom-efficient and halide-free way at a reasonable cost. Moreover, the alkylsulfate-based ILs also show chemical and thermal stability and low melting points [35]. A few publications correspond to this anion R-SO4 combined with imidazolium or pyridinium cation [5,7,19–25]. Particularly, the 1-ethyl-3-methylimidazolium ethylsulfate ([emim][C2H5SO4]) IL has been considered as a possible solvent to extract aromatics from aliphatic mixtures [20]. ⇑ Corresponding author. Tel.: +34 91 394 51 19; fax: +34 91 394 42 43. E-mail address: [email protected] (J. García). 0021-9614/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jct.2011.09.009

Taking into account all these aspects, we have determined the (liquid + liquid) equilibria (LLE) for some different systems composed by {heptane + toluene + alkylsufate-based ILs} at T = 313.2 K and atmospheric pressure. The ILs used in this research were 1,3-dimethylimidazolium methylsulfate ([mmim][CH3SO4]), 1-ethyl-3-methylimidazolium hydrogensulfate ([emim][HSO4]), 1-ethyl-3-methylimidazolium methylsulfate ([emim][CH3SO4]), and 1-ethyl-3-methylimidazolium ethylsulfate ([emim][C2H5SO4]) in order to study the effect of more complex R-SO4 anions on selectivity and capacity of extraction. The selectivity and extractive capacity were calculated from the LLE data and were compared with those of sulfolane, and those of [mmim][CH3SO4] and [emim][C2H5SO4] ILs previously reported in literature [5]. The degree of quality of the experimental LLE data was ascertained by applying the Othmer–Tobias correlation. The phase diagrams for the ternary systems were plotted, and the tie lines correlated with the NRTL model compare satisfactorily with the experimental data. 2. Experimental 2.1. Chemicals Heptane and toluene over molecular sieves were supplied by Sigma–Aldrich with mass fraction purity greater than 0.995 and 0.997, respectively. Their water mass fractions were less than 0.00005. The ILs 1,3-dimethylimidazolium methylsulfate ([mmim][CH3SO4]), 1-ethyl-3-methylimidazolium hydrogensulfate ([emim][HSO4]), 1-ethyl-3-methylimidazolium methylsulfate ([emim][CH3SO4]), and 1-ethyl-3-methylimidazolium ethylsulfate

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([emim][C2H5SO4]) were provided by Iolitec GmbH with quoted mass fraction purities greater than 0.99, and halides and water mass fractions less than 0.0001. Water content and purity of the reagents were given by the manufacturer. All chemicals were used as received without further purification. To prevent water hydration, they were kept in their original tightly closed bottles in a desiccator. When any chemicals were used, they were always manipulated inside a glove box under a dry nitrogen atmosphere. 2.2. Experimental procedure and analysis The LLE experiments were performed in 8 mL vials with screw caps providing hermetic sealing. Mixtures of known masses of heptane/toluene feed were transferred to tared vials. After the vials were reweighed, the IL ([mmim][CH3SO4], [emim][HSO4],

[emim][CH3SO4], or [emim][C2H5SO4]) was gravimetrically added to the feed. The vials were then placed in a shaking incubator at 313.2 K with a shaking speed of 800 rpm for 5 h and were settled overnight. This was carried out according to the procedure previously reported [31]. Every weighing involved in the experimental work was carried out on a Mettler Toledo AB104 balance with a precision of ±0.0001 g. The uncertainties in the temperature measurements were ±0.1 K. The estimated error in the mole fraction in the prepared feed mixture was less than ±0.001. Compositions of the two insoluble phases were determined by 1 H NMR spectroscopy. Samples from the upper (heptane-rich phase) and lower layers (IL-rich phase) were analyzed using a Bruker Avance 500 MHz NMR spectrometer. The deuterated solvent was methanol, purchased by Sigma–Aldrich (methanol-d4 99.8 atom %D). For acquisition and processing the 1H NMR spectra,

TABLE 1 Experimental LLE data in mole fraction (x), distribution ratios (Di), and separation factors (a2,1) for the ternary systems {heptane (1) + toluene (2) + IL (3)} at T = 313.2 K.a

a

Feed (global composition)

Heptane-rich phase (upper layer)

IL-rich phase (lower layer)

x1

x2

xI1

xII1

0.5023 0.4873 0.4748 0.4406 0.4118 0.3568 0.3124 0.2803 0.2370 0.1805 0.1264 0.0000

0.0000 0.0267 0.0545 0.1135 0.1825 0.2891 0.3757 0.4423 0.5280 0.6402 0.7485 0.8509

1.0000 0.9528 0.9038 0.8066 0.7070 0.5500 0.4553 0.3918 0.3075 0.2106 0.1391 0.0000

{Heptane (1) + toluene (2) + [mmim][CH3SO4] (3)} 0.0000 0.0009 0.0472 0.0011 0.0962 0.0011 0.1934 0.0010 0.2930 0.0008 0.4500 0.0007 0.5447 0.0006 0.6082 0.0005 0.6925 0.0005 0.7894 0.0004 0.8609 0.0003 1.0000 0.0000

0.0000 0.0061 0.0110 0.0227 0.0314 0.0436 0.0522 0.0579 0.0660 0.0703 0.0773 0.0568

0.0009 0.0012 0.0012 0.0012 0.0011 0.0013 0.0013 0.0013 0.0016 0.0019 0.0022

0.4827 0.4877 0.4727 0.4087 0.3517 0.3127 0.2836 0.2173 0.1777 0.1255 0.0000

0.0000 0.0263 0.0540 0.1889 0.2906 0.3756 0.4344 0.5602 0.6459 0.7511 0.8508

1.0000 0.9505 0.8951 0.6849 0.5501 0.4550 0.3901 0.2761 0.2156 0.1434 0.0000

{Heptane (1) + toluene (2) + [emim][HSO4] (3)} 0.0000 0.0004 0.0495 0.0002 0.1049 0.0003 0.3151 0.0003 0.4499 0.0002 0.5450 0.0003 0.6099 0.0003 0.7239 0.0002 0.7844 0.0002 0.8566 0.0002 1.0000 0.0000

0.0000 0.0023 0.0040 0.0104 0.0151 0.0180 0.0203 0.0240 0.0253 0.0276 0.0194

0.0004 0.0002 0.0003 0.0004 0.0004 0.0007 0.0008 0.0007 0.0009 0.0014

0.5037 0.4872 0.4738 0.4457 0.4088 0.3537 0.3142 0.2797 0.2335 0.1775 0.1233 0.0000

0.0000 0.0273 0.0544 0.1147 0.1835 0.2964 0.3762 0.4459 0.5338 0.6467 0.7539 0.8513

1.0000 0.9614 0.9076 0.8101 0.7121 0.5623 0.4698 0.4012 0.3083 0.2165 0.1394 0.0000

{Heptane (1) + toluene (2) + [emim][CH3SO4] (3)} 0.0000 0.0018 0.0386 0.0019 0.0924 0.0018 0.1899 0.0017 0.2879 0.0016 0.4377 0.0016 0.5302 0.0015 0.5988 0.0013 0.6917 0.0012 0.7835 0.0010 0.8606 0.0006 1.0000 0.0000

0.0000 0.0065 0.0161 0.0314 0.0464 0.0708 0.0844 0.0946 0.1092 0.1246 0.1304 0.1542

0.002 0.002 0.002 0.002 0.002 0.003 0.003 0.003 0.004 0.005 0.004

0.4991 0.4882 0.4754 0.4441 0.4055 0.3554 0.3134 0.2792 0.2347 0.1767 0.1240 0.0000

0.0000 0.0268 0.0546 0.1154 0.1935 0.2910 0.3767 0.4438 0.5309 0.6488 0.7531 0.8508

1.0000 0.9570 0.9150 0.8239 0.7081 0.5777 0.4814 0.4086 0.3212 0.2131 0.1521 0.0000

{Heptane (1) + toluene (2) + [emim][C2H5SO4] (3)} 0.0000 0.0033 0.0430 0.0033 0.0850 0.0035 0.1761 0.0036 0.2919 0.0029 0.4223 0.0024 0.5186 0.0026 0.5914 0.0026 0.6788 0.0023 0.7869 0.0026 0.8479 0.0019 1.0000 0.0000

0.0000 0.0101 0.0219 0.0449 0.0628 0.0827 0.1142 0.1285 0.1333 0.1686 0.1908 0.2270

0.003 0.003 0.004 0.004 0.004 0.004 0.005 0.006 0.007 0.012 0.012

Standard uncertainties u are u(T) = 0.1 K, u(x) = 0.003.

xI2

D1

D2

a2,1

0.129 0.114 0.117 0.107 0.097 0.096 0.095 0.095 0.089 0.090 0.057

111.9 94.0 94.7 94.7 76.1 72.7 74.6 58.6 46.9 41.6

0.046 0.038 0.033 0.034 0.033 0.033 0.033 0.032 0.032 0.019

220.8 113.8 75.4 92.3 50.1 43.3 45.8 34.8 23.1

0.168 0.174 0.165 0.161 0.162 0.159 0.158 0.158 0.159 0.152 0.154

85.2 87.9 78.8 71.7 56.8 49.9 48.8 40.6 34.4 35.2

0.235 0.258 0.255 0.215 0.196 0.220 0.217 0.196 0.214 0.225 0.227

68.1 67.4 58.4 52.5 47.1 40.8 34.1 27.4 17.6 18.0

xII2

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the TopSpin software was used. The quantitative integration under certain peaks in the spectra is proportional to hydrogen moles associated to each peak. To select the peaks and estimate the uncertainty of the analytical technique, samples were gravimetrically prepared, and then their 1H NMR spectra were determined. Hydrogen atoms from the aromatic and imidazolium rings were not considered. The maximum standard deviation on mole compositions was estimated to be ±0.003. A detailed description of the calculation of the areas and the sample preparation can be found elsewhere [33].

standard deviation (r) presented in table 2 indicate the high degree of quality of the experimental LLE data.

3. Results and discussion

D1 ¼

3.2. Distribution ratios and separation factor The capacity of extraction and selectivity of these four ILs for the separation of toluene from heptane at different concentrations of toluene in the feed has been evaluated by the heptane and toluene distribution ratios D1 and D2, respectively, and the separation factor (a2,1), calculated from the experimental data as follows:

xII1 ; xI1

ð2Þ

3.1. Experimental LLE data 0.00 1.00

The mole fraction compositions for the toluene/heptane feed, heptane-rich phase (raffinate), and IL-rich phase (extract) for the ternary systems {toluene + heptane + [mmim][CH3SO4], [emim][HSO4], [emim][CH3SO4], or [emim][C2H5SO4]}, at T = 313.2 K and atmospheric pressure are shown in table 1 and plotted on the triangular diagrams in figures 1 to 4. The 1H NMR spectra of the samples from the heptane-rich phase showed no detectable signals arising from the IL, so the IL mole fractions in the heptane-rich phases appear to be negligible, as can be observed in the triangular diagrams from figures 1 to 4 This situation is desirable since it would eliminate the need of an extra unit to purify the raffinate phase for recovering the solvent. In order to ascertain the reliability of the experimental LLE data, the Othmer–Tobias correlation [36] was applied:

1

x

0.50

0.50

0.75

0.25

ð1Þ

where w3II is the mass fraction of IL (3) in the IL-rich phase (lower layer), w1I is the mass fraction of heptane (1) in the heptane-rich phase (upper layer), and a and b are the fitting parameters of the Othmer–Tobias correlation. The linearity of the plot, shown in figure 5, indicates the degree of quality of the data. The parameters of the Othmer–Tobias correlation are given in table 2. The regression coefficients (R2) very close to unity and the low values of the

1.00 0.00

0.25

0.50

FIGURE 2. Experimental and calculated LLE in mole fraction (x) for the ternary systems {heptane (1) + toluene (2) + [emim][HSO4] (3)} at T = 313.2 K with rx = 0.0036. Solid lines and full points indicate experimental tie-lines, and dashed lines and empty squares indicate calculated data by the NRTL model.

0.00

0.75

1

1

x

x

0.50

0.25

0.25

0.50

0.75

0.75

x2

0.50

0.75

1.00 0.00

1.00

0.25

x2

0.50

0.00 1.00

x3 FIGURE 1. Experimental and calculated LLE in mole fraction (x) for the ternary systems {heptane (1) + toluene (2) + [mmim][CH3SO4] (3)} at T = 313.2 K with rx = 0.0006. Solid lines and full points indicate experimental tie-lines, and dashed lines and empty squares indicate calculated data by the NRTL model.

0.00 1.00

0.75

x3

0.00 1.00

0.25

0.75

x2

    1  wII3 1  wI1 ¼ a þ b ln ; ln wII3 wI1

0.25

0.50

0.75

1.00 0.00

0.25

0.25

0.50

0.75

0.00 1.00

x3 FIGURE 3. Experimental and calculated LLE in mole fraction (x) for the ternary systems {heptane (1) + toluene (2) + [emim][CH3SO4] (3)} at T = 313.2 K with rx = 0.0011. Solid lines and full points indicate experimental tie-lines, and dashed lines and empty squares indicate calculated data by the NRTL model.

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0.00 1.00

TABLE 2 Fitting parameters of the Othmer–Tobias correlation (a, b), regression coefficients (R2), and standard deviations (r) for the ternary systems {heptane (1) + toluene (2) + IL (3)} at T = 313.2 K. a

0.25

0.75

–5.0018

x2

x

1

–3.8928

0.50

0.50

–3.4331 –3.1279

0.75

1.00 0.00

0.25

0.25

0.50

0.75

0.00 1.00

x3 FIGURE 4. Experimental and calculated LLE in mole fraction (x) for the ternary systems {heptane (1) + toluene (2) + [emim][C2H5SO4] (3)} at T = 313.2 K with rx = 0.0032. Solid lines and full points indicate experimental tie-lines, and dashed lines and empty squares indicate calculated data by the NRTL model.

D2 ¼

xII2 ; xI2

a2;1 ¼

ð3Þ

xII2 xI1 ; xI2 xII1

ð4Þ

where x is the mole fraction, superscripts I and II refer to the heptane-rich and IL-rich phases, respectively, and subscripts 1 and 2 to heptane and toluene, respectively. The values of D1, D2, and a2,1 are shown in table 1 together with the experimental LLE data. The distribution ratios and separation factors for the four ternary systems as functions of the toluene mole fractions in the heptane-rich phase (xI2 ) are plotted in figures 6 to 8. Comparisons with literature data for the ternary systems {heptane + toluene + [mmim][CH3SO4]}, {heptane + toluene + [emim][C2H5SO4]},

R2

b

{Heptane (1) + toluene (2) + [mmim][CH3SO4] (3)} 0.5161 0.9468 {Heptane (1) + toluene (2) + [emim][HSO4] (3)} 0.5344 0.9595 {Heptane (1) + toluene (2) + [emim][CH3SO4] (3)} 0.5868 0.9556 {Heptane (1) + toluene (2) + [emim][C2H5SO4] (3)} 0.5814 0.9718

r 0.2889 0.2075 0.3257 0.1939

and {heptane + toluene + sulfolane} at T = 313.2 K [5] were also made. As can be seen in figures 6 to 8, the literature data for the systems {heptane + toluene + [mmim][CH3SO4]} and {heptane + toluene + [emim][C2H5SO4]} are similar to our experimental data. The slight differences observed are probably due to the higher purity of our ILs. The distribution ratios of heptane, D1, for the four ILs studied in this work are below the corresponding of sulfolane for all values of xI2 and showed extremely low values, as can be observed in figure 6 and table 1, respectively. The high polarity of the R-SO4 anion could explain this behavior. On the other hand, as the alkyl chain length in the R-SO4 anion grows, the distribution ratios of heptane increase, which would mean lesser polarity of the IL and more affinity toward heptane. Between [mmim][CH3SO4] and [emim][CH3SO4] ILs, which differ in the alkyl chain length in the imidazolium cation, [emim][CH3SO4] IL shows higher distribution ratio of heptane. As figure 7 illustrated, the distribution ratios of toluene, D2, for the four ILs are lower than those of sulfolane for the whole range of compositions. Longer alkyl chain length in the R-SO4 anion corresponds to higher values of the distribution ratio of toluene. These facts would be consistent with the behavior previously reported for long alkyl chains in the cation with respect to the solubility of heptane and toluene [31]. Finally, the values of separation factors for the four ILs are higher than those of sulfolane for the whole range of compositions (figure 8). As a consequence of a higher solubility of toluene and heptane, [emim][C2H5SO4] IL shows the lowest values of separation factor. The practically no differences between [emim][HSO4]

-7.50

ln [(1-w3II)/w3II]

-6.50 -5.50 -4.50 -3.50 -2.50 -1.50 2.00

1.00

0.00

-1.00

-2.00

-3.00

-4.00

ln [(1-w1I)/w1I] FIGURE 5. Othmer–Tobias plot for the ternary systems at T = 313.2 K: {heptane (1) + toluene (2) + [mmim][CH3SO4] (3)} (h), {heptane (1) + toluene (2) + [emim][HSO4] (3)} (s), {heptane (1) + toluene (2) + [emim][CH3SO4] (3)} (e), {heptane (1) + toluene (2) + [emim][C2H5SO4] (3)} (4). Solid lines represent the linear Othmer–Tobias fit.

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0.020

D1

0.015

0.010

0.005

0.000 0.00

0.20

0.40

0.60

0.80

1.00

x2I FIGURE 6. Distribution ratio of heptane for the ternary systems {heptane (1) + toluene (2) + [mmim][CH3SO4] (3)} (h), {heptane (1) + toluene (2) + [emim][HSO4] (3)} (s), {heptane (1) + toluene (2) + [emim][CH3SO4] (3)} (e), {heptane (1) + toluene (2) + [emim][C2H5SO4] (3)} (4), {heptane (1) + toluene (2) + sulfolane (3)} ( ) from reference [5], {heptane (1) + toluene (2) + [mmim][CH3SO4] (3)} (j) from reference [5], {heptane (1) + toluene (2) + [emim][C2H5SO4] (3)} (N) from reference [5] at T = 313.2 K.

0.60

D2

0.45

0.30

0.15

0.00 0.00

0.20

0.40

0.60

x2

0.80

1.00

I

FIGURE 7. Distribution ratio of toluene for the ternary systems {heptane (1) + toluene (2) + [mmim][CH3SO4] (3)} (h), {heptane (1) + toluene (2) + [emim][HSO4] (3)} (s), {heptane (1) + toluene (2) + [emim][CH3SO4] (3)} (e), {heptane (1) + toluene (2) + [emim][C2H5SO4] (3)} (4), {heptane (1) + toluene (2) + sulfolane (3)} ( ) from reference [5], {heptane (1) + toluene (2) + [mmim][CH3SO4] (3)} (j) from reference [5], {heptane (1) + toluene (2) + [emim][C2H5SO4] (3)} (N) from reference [5] at T = 313.2 K.

and [mmim][CH3SO4] ILs separation factors is because of the corresponding values of D1 and D2 are compensated.

3.3. Correlation of LLE data The NRTL model [37] was used to correlate the LLE data in the present work, as it has proven to have adequate correlating capability with respect to ternary LLE data for systems containing ILs [38]. In this model, the two binary interaction parameters (Dgij/ R) and (Dgji/R) were calculated using an ASPEN Plus simulator. The regression method used in the ASPEN simulator was the generalized least-squares method based on maximum likelihood principles. The Britt–Luecke algorithm [39] was employed to obtain the model parameters with the Deming initialization method. The regression convergence tolerance was set to 0.0001. The value of the third nonrandomness parameter, aij, in the NRTL model was subject to optimization between 0 and 1.

Table 3 shows the values of the fitting parameters obtained using the NRTL model to correlate the experimental LLE data for the four ternary systems. The calculated tie lines from the correlation based on the NRTL model are plotted in figures 1 to 4. The values of the root mean square deviation (rx) for the four ternary systems are also listed in table 3. The rx is defined as:

rx ¼

8 2 91=2 PPP  > > calcd < i l m xexptl = ilm  xilm > :

6k

> ;

;

ð5Þ

where x is the experimental or the calculated mole fraction and the subscripts i, l, and m represent the component, phase, and tie-line, respectively. The value of k designates the number of tie lines. The low values for rx and the virtually identical experimental and calculated data observed in the triangular diagrams give an idea of the goodness of the NRTL model.

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250.0

200.0

α 2,1

150.0

100.0

50.0

0.0 0.00

0.20

0.40

0.60

0.80

1.00

x2I FIGURE 8. Separation factor for the ternary systems {heptane (1) + toluene (2) + [mmim][CH3SO4] (3)} (h), {heptane (1) + toluene (2) + [emim][HSO4] (3)} (s), {heptane (1) + toluene (2) + [emim][CH3SO4] (3)} (e), {heptane (1) + toluene (2) + [emim][C2H5SO4] (3)} (4), {heptane (1) + toluene (2) + sulfolane (3)} ( ) from reference [5], {heptane (1) + toluene (2) + [mmim][CH3SO4] (3)} (j) from reference [5], {heptane (1) + toluene (2) + [emim][C2H5SO4] (3)} (N) from reference [5] at T = 313.2 K.

TABLE 3 Values of the NRTL parameters regressed from LLE data for the ternary systems {heptane (1) + toluene (2) + IL (3)} at T = 313.2 K. Component

NRTL parameters

i–j

(Dgij/R)/K

(Dgji/R)/K

aij

1–2 1–3 2–3

{Heptane (1) + toluene (2) + [mmim][CH3SO4] (3)} 738.94 5139.9 –212.56 1200.4 556.96 468.47

0.2081 0.2291 0.0169

1–2 1–3 2–3

{Heptane (1) + toluene (2) + [emim][HSO4] (3)} 834.95 3876.5 –3479.7 9748.6 5421.3 1371.4

0.2447 0.1232 0.2849

1–2 1–3 2–3

{Heptane (1) + toluene (2) + [emim][CH3SO4] (3)} –1396.3 1404.9 –907.71 1986.9 5281.2 –458.75

0.0128 0.5136 0.0984

1–2 1–3 2–3

{Heptane (1) + toluene (2) + [emim][C2H5SO4] (3)} 750.54 91.048 –2073.3 3242.1 –69.194 853.87

0.6796 0.1195 0.0073

4. Conclusions Alkylsulfate-based ILs [mmim][CH3SO4], [emim][HSO4], [emim] [CH3SO4], and [emim][C2H5SO4] have been chosen for the determination of the LLE data in the liquid–liquid extraction of toluene from heptane at T = 313.2 K and atmospheric pressure. Comparisons with literature data for the ternary systems {heptane + toluene + sulfolane}, {heptane + toluene + [mmim][CH3SO4]}, and {heptane + toluene + [emim][C2H5SO4]} at T = 313.2 K were also made. The NRTL model was found to correlate satisfactorily the experimental LLE data for the four studied ternary systems. All the ILs separation factors were higher than those of sulfolane on the whole range of compositions. However, the distribution ratios of toluene were lower. The more complex R-SO4 anion in the IL seems to increase the solubility of toluene and heptane in the IL, therefore [emim][C2H5SO4] IL showed the highest distribution ratios of toluene, and consequently, the highest capacity of extraction of toluene.

Acknowledgements The authors are grateful to the Ministerio de Ciencia e Innovación of Spain (MICINN) and the Comunidad Autónoma de Madrid (CAM) for financial support of Projects CTQ2008-01591 and S2009/PPQ-1545, respectively. Silvia García also thanks MICINN for awarding her an FPI Grant (Reference BES-2009-014703) under the same project.

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JCT 11-226