(Liquid + liquid) equilibrium for the ternary systems {heptane + toluene + 1-allyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide} and {heptane + toluene + 1-methyl-3-propylimidazolium bis(trifluoromethylsulfonyl)imide} ionic liquids

J. Chem. Thermodynamics 43 (2011) 1641–1645

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(Liquid + liquid) equilibrium for the ternary systems {heptane + toluene + 1-allyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide} and {heptane + toluene + 1-methyl-3-propylimidazolium bis(trifluoromethylsulfonyl)imide} ionic liquids 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 16 December 2010 Received in revised form 17 May 2011 Accepted 19 May 2011 Available online 27 May 2011 Keywords: Aromatic/aliphatic separation (Liquid + liquid) equilibria Ionic liquids 1-Methyl-3-propylimidazolium bis(trifluoromethylsulfonyl)imide 1-Allyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide

a b s t r a c t The (liquid + liquid) equilibrium (LLE) data for two systems containing heptane, toluene, and 1-methyl-3propylimidazolium bis(trifluoromethylsulfonyl)imide ([mpim][Tf2N]) or 1-allyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([amim][Tf2N]) ionic liquids (ILs) were determined at T = 313.2 K and atmospheric pressure. The effect of a double bond in an alkyl side chain in the imidazolium cation was evaluated in terms of selectivity and extractive capacity. The results show a decrease of the amount of toluene and heptane dissolved in the IL with the allyl group. Thus, the distribution ratios of toluene and heptane of [mpim][Tf2N] IL are higher than those of [amim][Tf2N] IL. On the other hand, the separation factor of the [amim][Tf2N] IL increases comparing to [mpim][Tf2N] IL. The NRTL model was used to correlate satisfactorily the experimental LLE data for the two studied ternary systems. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction The removal of aromatic hydrocarbons from gasoline to comply with stricter legislative limits in the early 1990s has resulted in a continuous attempt to improve current industrial processes for the separation of aromatic compounds [1]. The liquid–liquid extraction Sulfolane Shell UOP Process mixes high yield in the recovery of aromatic compounds and a good balance of solvent properties. However, when the aromatic content is below 20 wt.% the process is less efficient because it requires additional stages to purify the extract and raffinate phases. Thus, the improvement of liquid–liquid extraction of aromatic hydrocarbons has meant the necessity of a thorough search for new solvents to replace the existing ones. In this context, ionic liquids (ILs) have emerged as a novel alternative to traditional organic solvents due to their nonvolatile nature [2]. Researches in this field have caused a considerable number of publications of (liquid + liquid) equilibrium (LLE) data for systems composed by an ionic liquid, an aromatic and an aliphatic hydrocarbon [3–32]. Among all the ILs thus far investigated only few posses both a higher selectivity and a higher extractive capacity than those of sulfolane [33]. Moreover, some of them are greatly affected by ⇑ 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.2011.05.025

water and other impurities which modify their physical properties [34]. The current trend toward more stable and pure ILs justifies the use of bis(trifluoromethylsulfonyl)imide as the anion for this solvents in liquid–liquid extraction of aromatics [11,12,30–32]. Hence, and as a continuation of our previous work [21,22,28–32], we have focus our research on the LLE for the ternary systems formed by {toluene + heptane + 1-methyl-3-propylimidazolium bis(trifluoromethylsulfonyl)imide ([mpim][Tf2N])} and {toluene + heptane + 1-allyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([amim][Tf2N])} at 313.2 K and atmospheric pressure. The selectivity and extractive capacity were calculated from the LLE data and were compared with those of sulfolane [6]. Moreover, the influence of a double bond in an alkyl side chain in the imidazolium cation was analyzed. The degree of consistency 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 were compared satisfactorily with the experimental data. 2. Experimental 2.1. Chemicals Heptane and toluene over molecular sieves, with water mass fractions less than 5  105, were provided by Sigma–Aldrich with

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mass fraction purity greater than 0.995 and 0.997, respectively. The two ILs 1-methyl-3-propylimidazolium bis(trifluoromethylsulfonyl)imide ([mpim][Tf2N]) and 1-allyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([amim][Tf2N]) were supplied by Iolitec GmbH with quoted mass fraction purities greater than 0.99, halide mass fractions less than 1  104, and water mass fractions less than 1  104. 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 each chemical was used, it was always manipulated inside a glovebox under a dry nitrogen atmosphere.

were obtained through weighing with an electronic balance having a precision of ±0.0001 g. Toluene in the mixture was chosen as the standard, and its response factor was set to 1.0. The response factor for heptane was then calculated using the renormalization method before every run of samples to ensure measurement accuracy. Samples were taken in triplicate and each of them injected six times in the GC. The average compositions are the ones reported here. The estimated uncertainties in the compositions, calculated as the standard deviation of the measurements, were less than 0.001. 3. Results and discussion

2.2. Experimental procedure and analysis The LLE experiments with the systems {heptane + toluene + [mpim][Tf2N]} and {heptane + toluene + [amim][Tf2N]} were performed in 8 mL vials with screw caps providing hermetic sealing. The gravimetrically prepared feed mixtures of heptane/toluene/IL were shaken at T = 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 [32]. The error in the mole fraction in the prepared feed mixture was less than 0.001. Samples from the heptane-rich phase were analyzed by 1H NMR. The spectra showed no detectable signals arising from the ILs, so the IL mole fractions in the heptane-rich phases appear to be negligible. Thus, gas chromatographic analyses of each layer plus an overall mass balance on hydrocarbons in the mixture were done to determine the phase compositions. Because of the nonvolatile nature of the IL, a precolumn is needed in the gas chromatograph to collect the IL present in the lower layer in order not to disrupt the analysis. A detailed description of the equipments and the analysis conditions can be found elsewhere [32]. An area normalization method with response factors was carried out to determine the hydrocarbon molar ratio in each layer. The gas chromatography response factors for the hydrocarbons were calculated by using standard mixture samples of pure heptane and toluene. The compositions of these standard samples

3.1. Experimental LLE data The experimental LLE data for the ternary systems {heptane + toluene + [mpim][Tf2N] or [amim][Tf2N]} at T = 313.2 K and atmospheric pressure are given in table 1 and plotted on triangular diagrams in figures 1 and 2, respectively. As can be seen, the upper layers (heptane-rich phases) for the two ternary systems are free of ILs. On the other hand, the amount of the heptane dissolved in the ILs is small. Finally, it can be seen that toluene has a higher affinity toward heptane than toward the ILs, as shown in the negative slopes of the tie lines on the triangular diagrams. The reliability of the experimentally measured LLE data can be ascertained by applying the Othmer–Tobias correlation [35]:

ln

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

ð1Þ

where w3II is the mass fraction of IL (3) in the lower layer (IL-rich phase), w1I is the mass fraction of heptane (1) in the upper layer (heptane-rich phase), and a and b are the fitting parameters of the Othmer–Tobias correlation. The linearity of the plot indicates the degree of consistency 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 small values of the standard

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. Feed (global composition)

Heptane-rich phase (upper layer)

IL-rich phase (lower layer)

x1

x2

xI1

xII1

0.5012 0.4873 0.4734 0.4437 0.4152 0.3589 0.3164 0.2778 0.2297 0.1758 0.1222 0.0000

0.0000 0.0279 0.0556 0.1171 0.1748 0.2862 0.3690 0.4468 0.5418 0.6492 0.7562 0.8510

1.0000 0.9723 0.9430 0.8789 0.8159 0.6883 0.5997 0.5058 0.3891 0.2686 0.1666 0.0000

{Heptane (1) + toluene (2) + [mpim][Tf2N] (3)} 0.0000 0.0409 0.0277 0.0421 0.0570 0.0365 0.1211 0.0400 0.1841 0.0386 0.3117 0.0387 0.4003 0.0377 0.4942 0.0356 0.6109 0.0320 0.7314 0.0279 0.8334 0.0218 1.0000 0.0000

0.0000 0.0281 0.0542 0.1133 0.1661 0.2615 0.3382 0.3965 0.4561 0.5182 0.5819 0.6750

0.041 0.043 0.039 0.046 0.047 0.056 0.063 0.070 0.082 0.104 0.131

0.5028 0.4764 0.4736 0.4438 0.4088 0.3557 0.3116 0.2796 0.2324 0.1782 0.1258 0.0000

0.0000 0.0266 0.0556 0.1161 0.1854 0.2932 0.3811 0.4445 0.5368 0.6398 0.7498 0.8510

1.0000 0.9711 0.9397 0.8726 0.7961 0.6656 0.5634 0.4888 0.3858 0.2768 0.1703 0.0000

{Heptane (1) + toluene (2) + [amim][Tf2N] (3)} 0.0000 0.0323 0.0289 0.0269 0.0603 0.0275 0.1274 0.0271 0.2039 0.0289 0.3344 0.0274 0.4366 0.0269 0.5112 0.0250 0.6142 0.0255 0.7232 0.0224 0.8297 0.0182 1.0000 0.0000

0.0000 0.0246 0.0511 0.1052 0.1673 0.2496 0.3184 0.3633 0.4325 0.5080 0.5564 0.6315

0.032 0.028 0.029 0.031 0.036 0.041 0.048 0.051 0.066 0.081 0.107

xI2

D1

D2

a2,1

1.014 0.951 0.936 0.902 0.839 0.845 0.802 0.747 0.709 0.698 0.675

23.4 24.6 20.6 19.1 14.9 13.4 11.4 9.1 6.8 5.3

0.851 0.847 0.826 0.821 0.746 0.729 0.711 0.704 0.702 0.671 0.632

30.7 29.0 26.6 22.6 18.1 15.3 13.9 10.7 8.7 6.3

xII2

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0.00 1.00

0.25

0.00 1.00

0.25

0.75

1.00 0.00

1

x

x

0.50

0.50

0.50

0.75

0.50

0.75

0.25

0.25

0.75

0.00 1.00

1.00 0.00

x2

0.50

x2

1

0.75

0.25

0.25

0.50

0.00 1.00

0.75

x3

x3

FIGURE 1. Experimental and calculated LLE in mole fraction (x) of the ternary system {heptane (1) + toluene (2) + [mpim][Tf2N] (3)} at T = 313.2 K with rx = 0.0024. Solid lines and full points indicate experimental tie lines, and dashed lines and empty squares indicate calculated data by the NRTL model.

FIGURE 2. Experimental and calculated LLE in mole fraction (x) of the ternary system {heptane (1) + toluene (2) + [amim][Tf2N] (3)} at T = 313.2 K with rx = 0.0027. Solid lines and full points indicate experimental tie lines, and dashed lines and empty squares indicate calculated data by the NRTL model.

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

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.

3.2. Distribution ratios and separation factor

a

The feasibility of using the ILs as solvents to perform the liquid– liquid extraction of toluene from its mixtures with heptane was evaluated by the distribution ratios (Di) and the separation factor (a2,1), calculated from the experimental LLE data as follows:

D2 ¼

a2;1 ¼

;

ð2Þ

;

ð3Þ

xII2 xI1 ; xI2 xII1

r

1.8220

{Heptane (1) + toluene (2) + [mpim][Tf2N] (3)} 0.6175 0.9870

0.1107

1.9639

{Heptane (1) + toluene (2) + [amim][Tf2N] (3)} 0.6589 0.9927

0.0687

0.30 0.25

ð4Þ

where x is the mole fraction, superscripts I and II refer to the heptane-rich and IL-rich phases, and subscripts 1 and 2 to heptane and toluene, respectively. The values of Di and a2,1 are shown in table 1, together with the experimental LLE data. The distribution ratios of toluene and heptane and the separation factor for the two ternary systems versus the toluene molar fraction in the heptane-rich phase (x2I) are plotted in figures 3 to 5, together with the distribution ratios and separation factor of sulfolane [6]. Comparisons with the distribution ratios and the separation factor of [emim][Tf2N] and [bmim][Tf2N] ILs [32] were also made. The plotted results of figures 3 to 5 for [mpim][Tf2N] IL are intermediate between those for [emim][Tf2N] and [bmim][Tf2N] ILs. This is consistent with the effect of the alkyl chain length on extractive capacity and selectivity previously reported [32]. As can be seen in figure 3, the distribution ratio of heptane for [mpim][Tf2N] IL is slightly higher than that of [amim][Tf2N] IL in the whole range of composition. The double bond of the allyl group in the imidazolium cation appears to reduce the affinity of [amim][Tf2N] IL toward heptane compared to the propyl group in the [mpim][Tf2N] IL. Thus, the values of distribution ratio of heptane for [amim][Tf2N] IL are similar to that of [emim][Tf2N]. On the other hand, both ILs show a distribution ratio of heptane

0.20

D1

D1 ¼

xII1 xI1 xII2 xI2

R2

b

0.15 0.10 0.05 0.00 0.00

0.20

0.40

0.60

0.80

1.00

x2I FIGURE 3. Distribution ratio of heptane for the ternary systems {heptane (1) + toluene (2) + [mpim][Tf2N] (3)} (4), {heptane (1) + toluene (2) + [amim][Tf2N] (3)} (h), {heptane (1) + toluene (2) + [emim][Tf2N] (3)} (}) from reference [32], {heptane (1) + toluene (2) + [bmim][Tf2N] (3)} (s) from reference [32], and {heptane (1) + toluene (2) + sulfolane (3)} (⁄) from reference [6] at T = 313.2 K.

higher than that of sulfolane only at compositions below 0.60. This trend seems to be the same when comparing the distribution ratio of toluene plotted in figure 4. [mpim][Tf2N] IL shows values larger than those of [amim][Tf2N] IL in the whole range of compositions. Similarly, the values of distribution ratio of toluene for [amim][Tf2N] IL are similar to those of [emim][Tf2N] IL. Moreover,

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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.

D2

1.20

0.80

Component

NRTL parameters

i–j

((gij  gjj)/R)/K

0.20

0.40

0.60

0.80

aij

1–2 1–3 2–3

0.2038 0.4608 0.2850

1–2 1–3 2–3

{Heptane (1) + toluene (2) + [amim][Tf2N] (3)} 333.78 395.58 433.17 692.34 4390.0 619.47

0.1994 0.4626 0.1806

0.40

0.00 0.00

((gji  gjj)/R)/K

{Heptane (1) + toluene (2) + [mpim][Tf2N] (3)} 110.41 197.40 178.50 673.39 1500.1 -288.01

1.00

x2I FIGURE 4. Distribution ratio of toluene for the ternary systems {heptane (1) + toluene (2) + [mpim][Tf2N] (3)} (4), {heptane (1) + toluene (2) + [amim][Tf2N] (3)} (h), {heptane (1) + toluene (2) + [emim][Tf2N] (3)} (}) from reference [32], {heptane (1) + toluene (2) + [bmim][Tf2N] (3)} (s) from reference [32], and {heptane (1) + toluene (2) + sulfolane (3)} (⁄) from reference [6] at T = 313.2 K.

50.0

40.0

α2,1

30.0

20.0

10.0

0.0 0.00

0.20

0.40

0.60

0.80

1.00

cation. With a double bond, the hydrophobic character of the alkyl side chain decreases and makes difficult the accommodation of heptane in the IL and therefore reducing the solubility as well. On the other hand, the interactions between the imidazolium cation and toluene, especially H–H bonds [36], can be lower because of the inclusion of a double bond and thus the amount of dissolved toluene in the IL decreases. Hence, [mpim][Tf2N] IL shows a higher distribution ratio of toluene and heptane, whereas [amim][Tf2N] IL improves the selectivity of the extraction process of toluene from heptane, reaching similar values to those of [emim][Tf2N] IL. Both ILs could be considered an alternative to sulfolane in the liquid extraction of aromatic from aliphatic mixtures. Although [amim][Tf2N] IL could be more suitable because of its higher values of separation factor than those of sulfolane, the differences between the values of both ternary systems with [amim][Tf2N] and [mpim][Tf2N] ILs, and with sulfolane are very small. 3.3. Correlation of LLE data

x2I FIGURE 5. Separation factor for the ternary systems {heptane (1) + toluene (2) + [mpim][Tf2N] (3)} (4), {heptane (1) + toluene (2) + [amim][Tf2N] (3)} (h), {heptane (1) + toluene (2) + [emim][Tf2N] (3)} (}) from reference [32], {heptane (1) + toluene (2) + [bmim][Tf2N] (3)} (s) from reference [32], and {heptane (1) + toluene (2) + sulfolane (3)} (⁄) from reference [6] at T = 313.2 K.

the distribution ratio of toluene in these two ILs is higher than that in sulfolane at any value of x2I. The values of the separation factor of [mpim][Tf2N] IL is lower than that of [amim][Tf2N] IL (figure 5) on the whole range of compositions. Thus, the inclusion of a double bond in the alkyl chain in the imidazolium cation seems to improve the selectivity in the extraction of toluene from heptane by decreasing proportionally more the amount of dissolved heptane than the amount of dissolved toluene in the IL-rich phase. Hence, the separation factor of [amim][Tf2N] IL reaches values similar to that of [emim][Tf2N] IL. On the other hand, the values of separation factor of [amim][Tf2N] IL are similar to those of sulfolane at low composition. However, differences between both solvents were observed as the toluene mole fraction in the heptane-rich phase grows. [mpim][Tf2N] IL only shows higher separation factor than that of sulfolane at composition above 0.40. At compositions below 0.40, the separation factor for [mpim][Tf2N] IL is slightly lower than that of sulfolane. In summary, the inclusion of a double bond in an alkyl side chain in the imidazolium cation seems to reduce the amount of dissolved toluene and heptane in the IL. According to Arce et al. [12], the aliphatic hydrocarbons can be accommodated in the hydrophobic region of the alkyl side chains in the imidazolium

The NRTL model [37] was used to correlate the LLE data in the present work, as it has proven an adequate correlating capability with respect to ternary LLE data for systems containing ILs [38]. In this model, the two binary interaction parameters (gij  gjj)/R and (gji  gii)/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 two ternary systems. The calculated tie lines from the correlation based on the NRTL model are plotted in figures 1 and 2. The values of the root mean square deviation (rx) for the two ternary systems are listed in the captions of figures 1 and 2. The rx is defined as:

rx ¼

91=2 8P P P  exptl > > xilm  xcalc < ilm = i

> :

l

m

6k

> ;

ð5Þ

where x is the mole fraction and the subscripts i, l, and m provide a designation for the component, phase and the tie lines, respectively. The value k designates the number of tie lines. It can be conclude from these figures and the rx that the NRTL model correlates satisfactorily the experimental results for the LLE of the two ternary systems.

S. García et al. / J. Chem. Thermodynamics 43 (2011) 1641–1645

4. Conclusions LLE data for the ternary systems {heptane + toluene + [mpim][Tf2N] or [amim][Tf2N]} were determined experimentally at T = 313.2 K and atmospheric pressure. The degree of consistency of the LLE data was ascertained by applying the Othmer–Tobias correlation. The NRTL model was used to correlate satisfactorily the experimental LLE data for the two studied ternary systems. The effect of the inclusion a double bond in an alkyl side group in the imidazolium cation has been studied. An allyl side group seems to reduce the amount of toluene and heptane dissolved in the IL comparing with an alkyl side group. A double bond can change the hydrophobic character of the alkyl side chain make difficult the accommodation of heptane in the IL and the interactions between the cation and toluene. Thus, the distribution ratios of toluene and heptane of [mpim][Tf2N] IL are higher than those of [amim][Tf2N] IL. On the other hand, the separation factor of [amim][Tf2N] IL increases comparing to [mpim][Tf2N] IL. The values of distribution ratios and separation factor obtained for [amim][Tf2N] IL are similar to those of [emim][Tf2N] IL. Finally, the values of the separation factor for [amim][Tf2N] IL was higher than those of sulfolane on the whole range of composition. Although [pmim][Tf2N] shows values of the separation factor higher than those of sulfolane only at mole fractions of toluene in the heptane-rich phase higher than 0.40, the differences with sulfolane are small. Hence, both [amim][Tf2N] and [mpim][Tf2N] IL could be considered as an alternative to sulfolane for aromatics extraction. Acknowledgements The authors are grateful to the Ministerio de Ciencia e Innovación of Spain (MICINN) and the Comunidad 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. References [1] A. Arce, M.J. Early, H. Rodríguez, K.R. Seddon, A. Soto, Green Chem. 10 (2008) 1294–1300. [2] G.W. Meindersma, A.J.G. Podt, A.B. de Haan, Fuel Process. Technol. 87 (2005) 59–70. [3] M.S. Selvan, M.D. McKinley, R.H. Dubois, J.L. Atwood, J. Chem. Eng. Data 45 (2000) 841–845. [4] T.M. Letcher, N. Deenadayalu, J. Chem. Thermodyn. 35 (2003) 67–76. [5] T.M. Letcher, P. Reddy, J. Chem. Thermodyn. 37 (2005) 415–421. [6] G.W. Meindersma, A.J.G. Podt, A.B. de Haan, Fluid Phase Equilib. 247 (2006) 158–168.

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JCT 10/491