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Influence of anion structure on the liquid–liquid equilibria of 1-(3-hydroxypropyl)pyridinium cation based ionic liquid–hydrocarbon binary systems
Fluid Phase Equilibria 304 (2011) 121–124
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Influence of anion structure on the liquid–liquid equilibria of 1-(3-hydroxypropyl)pyridinium cation based ionic liquid–hydrocarbon binary systems Andrzej Marciniak ∗ , Ewa Karczemna Department of Physical Chemistry, Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland
a r t i c l e
i n f o
Article history: Received 18 November 2010 Received in revised form 24 January 2011 Accepted 26 January 2011 Available online 2 February 2011 Keywords: Liquid–liquid equilibria Ionic liquid 1-(3-Hydroxypropyl)pyridinium bis(trifluoromethylsulfonyl)-amide 1-(3-Hydroxypropyl)pyridinium trifluorotris(perfluoroethyl)phosphate [N-C3 OHPY][NTf2 ] [N-C3 OHPY][FAP]
a b s t r a c t Binary liquid–liquid equilibria for 10 systems containing an ionic liquid (1-(3hydroxypropyl)pyridinium bis(trifluoromethylsulfonyl)-amide and 1-(3-hydroxypropyl)pyridinium trifluorotris(perfluoroethyl)phosphate) with a hydrocarbon (n-hexane, n-heptane, cyclohexane, benzene, toluene) were measured by dynamic method. The influence of anion structure of 1(3-hydroxypropyl)pyridinium cation based ionic liquids on solubility of aliphatic and aromatic hydrocarbons is discussed. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Dearomatization processes play key role in chemical industry and are used, inter alia, to produce kerosene with a better smoke point, diesel oil with a higher cetane number or a mineral oil with a better viscosity index. In industrial processes different types of entrainers are used, namely: di-, tri-, and tetra-ethyleneglycols (abbreviations: DEG, TEG and TETRA, respectively), N-methylpyrrolidone (NMP), N-formylmorpholine (NFM), dimethylsulfoxide (DMSO), tetramethylenesulfone (sulfolane) [1]. Although the processes based on these solvents are well known, there is still need to find better environmental friendly solvents with better selectivity and capacity (solvent power). Ionic liquids (ILs) are relatively new class of salts whose melting temperature is below 100 ◦ C [2]. In general ILs are composed of organic cations with either inorganic or organic anions. This class of compounds has important properties required for entrainers, namely negligible vapor pressure, a wide liquid range and stability at high temperatures. From activity coefficients at infinite dilution measurements it was shown that a large number of ionic liquids have
∗ Corresponding author. Tel.: +48 22 234 58 16; fax: +48 22 628 27 41. E-mail address: [email protected] (A. Marciniak). 0378-3812/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.fluid.2011.01.025
better selectivity and capacity in extraction of aromatics from aromatic/aliphatic mixtures than typical solvents such as sulfolane and NMP [3]. To design an ionic liquid to specific separation problem the knowledge of influence of cation and anion structure on solubility is required. In this paper, the influence of anion structure of 1-(3-hydroxypropyl)pyridinium cation based ionic liquids on solubility of aliphatic and aromatic hydrocarbons is presented. Binary liquid–liquid equilibria (LLE) for 10 systems containing an ionic liquid (1-(3-hydroxypropyl)pyridinium bis(trifluoromethylsulfonyl)amide, [N-C3 OHPY][NTf2 ], 1-(3-hydroxypropyl)pyridinium trifluorotris(perfluoroethyl)phosphate, [N-C3 OHPY][FAP]) with a hydrocarbon (n-hexane, n-heptane, cyclohexane, benzene, toluene) were measured by the dynamic method. The potentials of [N-C3 OHPY][FAP] as extractant for the separation of aromatic hydrocarbons from aromatic/aliphatic mixtures have already been studied by measurements of activity coefficients at infinite dilution [4]. It was found that the investigated [N-C3 OHPY][FAP] ionic liquid shows much higher selectivity and capacity at infinite dilution than the generally used organic solvents such as NMP, sulfolane, and other ionic liquids especially based on [FAP]− anion for the separation of aliphatic hydrocarbons from aromatic hydrocarbons. Ionic liquids are considered as extractants in separation of sulfur compounds from fuels [5,6] therefore the solubility of aromatic and
122
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2. Materials and methods
of temperature measurements was ±0.05 K, and that of the mole fraction did not exceed ±0.0002. The reproducibility of the LLE experimental points was ±0.1 K. The experimental results are listed in Table 1.
2.1. Materials
3. Modeling
The ionic liquid [N-C3 OHPY][NTf2 ] had a purity of >0.999 mass fraction and was supplied by Merck. The ionic liquid [N-C3 OHPY][FAP] had a purity of >0.999 mass fraction and was supplied by Merck. The ionic liquids were further purified by subjecting the liquid to a very low pressure of about 5 × 10−3 Pa at a temperature about 95 ◦ C for ca. 5 h. This procedure removed any volatile chemicals and water from the ionic liquid. The list of hydrocarbons used in this study including source and grade is as follows: n-hexane, Fluka, grade >0.997 mass fraction; n-heptane, Sigma–Aldrich, grade >0.995 mass fraction; cyclohexane, Sigma–Aldrich, grade >0.997 mass fraction; benzene, Sigma–Aldrich, grade ≥0.999 mass fraction; toluene, Sigma–Aldrich, grade ≥0.999 mass fraction. While the purity of hydrocarbons was high all hydrocarbons were used without further purification. The structures of investigated ionic liquids [NC3 OHPY][NTf2 ] and [N-C3 OHPY][FAP] are presented below:
The LLE was correlated with the NRTL model describing the excess Gibbs energy [9]. The equations were described by us earlier [10]. For correlation the temperature-dependent model adjustable parameters g12 − g22 = a12 T + b12 and g21 − g11 = a21 T + b21 were calculated. The NRTL nonrandom parameter ˛ was set to a value of ˛ = 0.1, which has given the best results of the correlation. By analogy to the previous experiments (spectroscopic measurements) [11], it was assumed in this work that the solubility at the solvent-rich phase was in the range of x1 = 10−5 in n-hexane, nheptane, and cyclohexane and x1 = 5 × 10−5 in toluene. The results of the correlation, values of the model parameters, and the corresponding standard deviations are given in Table 2. For the systems presented in this work the average root-mean-square deviation x is less than 0.0008. The results of the correlation are presented in Figs. 1 and 2.
aliphatic hydrocarbons in ionic liquids for this problem are very useful and important.
N+
O OH
F
O
N S
S O
F
F
O
F
F
F F F
F N+
OH
F
F
F P-
F F
F F
F
F F F F
F
F F
2.2. Water content The water content was analyzed by Karl-Fischer titration technique (method TitroLine KF). A sample of IL was dissolved in methanol and titrated with steps of 2.5 L. The results obtained have shown the water content to be less than 100 ppm. 2.3. Liquid–liquid phase equilibria apparatus and measurements Two phases disappearance observed with an increasing temperature have been determined using a dynamic (synthetic) method described previously [7,8]. The compound was kept under the nitrogen in a drybox. Mixtures of solute and solvent were prepared by weighing the pure components to within 10−4 g. The sample of solute and solvent was heated very slowly (at less than 2 K h−1 near the equilibrium temperature) with continuous stirring inside a Pyrex glass cell, placed in a thermostat. The foggy solution disappearance temperature detected visually was measured with a calibrated electronic thermometer P 550 (DOSTMANN electronic GmbH). The measurements were carried out over a wide range of solute mole fraction ranging from 5 × 10−5 to 1. The uncertainty
4. Results and discussion For this study, the liquid-phase behavior for a 10 binary ionic liquid–hydrocarbon systems was determined. Results are presented in Table 1 and in Figs. 1 and 2 and 1S to 5S. The increase of solubility with the increase of temperature is observed for all systems, therefore the liquid–liquid equilibria diagrams have shapes typical for diagrams with upper critical solution temperature (UCST). These types of diagrams for systems ionic liquid + hydrocarbon were reported in the literature [12,13]. With the increase of the length of the alkane chain the increase of miscibility gap is observed, accordingly the solubility of n-heptane is lower than for n-hexane. In comparison, cyclohexane shows better solubility in the investigated ionic liquids than n-hexane. This stands that aliphatic hydrocarbons with cyclic structure, analogous to pyridinium structure, reveal better solubility than linear alkanes (similia similibus solvuntur). Moreover the molar volume of cycloalkanes (1.089 × 10−4 m3 mol−1 for cyclohexane at 298.15 K) [14] is smaller than for linear alkanes (1.314 × 10−4 m3 mol−1 for n-hexane at 298.15 K) [14], therefore the packing effect addition-
A. Marciniak, E. Karczemna / Fluid Phase Equilibria 304 (2011) 121–124
123
Table 1 Experimental binary liquid–liquid equilibria for systems {ionic liquid (1) + hydrocarbon (2)}. x1
T (K)
x1
T (K)
x1
T (K)
[N-C3 OHPY][NTf2 ] + n-Hexane 0.9763 0.9708 0.9613
300.5 304.4 310.3
0.9543 0.9485 0.9385
316.4 320.9 328.8
0.9303
335.3
[N-C3 OHPY][NTf2 ] + n-heptane 0.9906 300.1 0.9864 304.9 0.9800 313.3
0.9737 0.9702 0.9660
322.7 329.1 336.0
0.9624 0.9570
342.3 352.0
[N-C3 OHPY][NTf2 ] + cyclohexane 0.9680 294.5 0.9612 299.9 0.9553 305.4
0.9469 0.9385 0.9327
314.2 323.8 330.9
0.9270 0.9212 0.9139
337.0 343.0 351.0
[N-C3 OHPY][NTf2 ] + benzene 0.3470 0.3459 0.3447
298.9 308.2 318.3
0.3431 0.3420 0.3409
326.6 334.7 343.9
0.000141a 0.000147a 0.000151a
340.8 347.0 352.5
[N-C3 OHPY][NTf2 ] + toluene 0.5087 0.5073 0.5049
297.1 303.7 310.2
0.5020 0.4996 0.4971
318.6 330.3 340.4
0.4945 0.4918
350.7 359.9
[N-C3 OHPY][FAP] + n-hexane 0.9541 0.9472 0.9414
303.5 307.3 311.2
0.9334 0.9201 0.9062
316.3 323.7 330.4
0.8925
336.7
[N-C3 OHPY][FAP] + n-heptane 0.9707 0.9653 0.9626
308.8 312.1 315.4
0.9569 0.9515 0.9455
320.7 325.6 330.3
0.9401 0.9353 0.9269
336.7 342.2 351.2
[N-C3 OHPY][FAP] + cyclohexane 0.9427 303.2 0.9359 309.0 0.9267 316.9
0.9215 0.9169 0.9085
320.8 323.8 329.0
0.8993 0.8816 0.8655
334.3 343.5 350.9
[N-C3 OHPY][FAP] + benzene 0.2177 0.2172 0.2160 0.2137 0.2109
292.0 294.3 296.8 301.1 305.2
0.2074 0.2041 0.2010 0.1977 0.1943
311.8 318.5 325.7 332.3 337.4
0.1896 0.1851 0.000053a 0.000116a 0.000175a
344.3 352.6 298.4 327.9 352.0
[N-C3 OHPY][FAP] + toluene 0.2840 0.2817 0.2799 0.2766
296.3 301.2 306.9 313.4
0.2739 0.2713 0.2691 0.2672
320.3 328.5 335.7 342.9
0.2655 0.2638
350.0 358.3
a
Hydrocarbon rich phase.
Table 2 Correlation of the LLE data by means of the NRTL equationa , b , and the mole fraction deviations x . System
g12 − g22 (J mol−1 ) a12
[N-C3 OHPY][NTf2 ] + hexane [N-C3 OHPY][NTf2 ] + heptane [N-C3 OHPY][NTf2 ] + cyclohexane [N-C3 OHPY][NTf2 ] + benzene [N-C3 OHPY][NTf2 ] + toluene [N-C3 OHPY][FAP] + hexane [N-C3 OHPY][FAP] + heptane [N-C3 OHPY][FAP] + cyclohexane [N-C3 OHPY][FAP] + benzene [N-C3 OHPY][FAP] + toluene a b
−81.00 −70.06 −50.22 −48.83 −40.15 −76.67 −61.39 −62.11 −78.16 −63.81
Parameters g12 − g22 = a12 T + b12 , g21 − g11 = a21 T + b21 . Parameter ˛ = 0.1.
g21 − g11 (J mol−1 ) b12 24,575 23,114 14,325 831.6 1653 22,080 18,671 17,257 5130 3410
a21 160.6 144.7 133.9 121.7 119.5 161.1 134.7 144.4 109.4 150.5
x b21 −18,945 −15,791 −10,015 4206 813.3 −17,894 −10,977 −12,356 16,675 −770.4
0.0014 0.0017 0.0011 0.0002 0.0003 0.0003 0.0013 0.0015 0.0006 0.0003
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370
aromatic ones. In case of aliphatic hydrocarbons it indicates that the van der Waals interactions and packing effects play the main role in solubility. More extended structure of [FAP]− anion has higher surface than [NTf2 ]− one, for that reason the hydrocarbons can interact stronger via the dispersion forces. The aromatic hydrocarbons as opposed to the aliphatic ones have polarizable delocalized bonds in structure, consequently in this case the main role on solubility, apart from van der Waals interactions and packing effects, plays induced dipole interactions. In the [FAP]− structure more strongly electronegative fluoride atoms are presented than in the [NTf2 ]− , therefore the interaction between polarizable hydrocarbon and [FAP]− anion is much stronger.
T/K
350
330
310
5. Conclusions
290
0
0.2
0.4
x1
0.6
0.8
1
Fig. 1. Liquid–liquid phase equilibria of {[N-C3 OHPY][NTf2 ] (1) + hydrocarbon (2)} binary systems: (䊉) n-hexane; () n-heptane; () cyclohexane; () benzene; () toluene. Solid lines are calculated by means of the NRTL equation.
T/K
370
The knowledge of the impact of anion structure on the liquid phase behavior of ionic liquids with hydrocarbons is useful for developing ionic liquids as “designer solvents” for the extraction of aromatic hydrocarbons from aromatic/aliphatic mixtures. The solubility of investigated hydrocarbons is lower for [N-C3 OHPY][NTf2 ] ionic liquid. From the liquid–liquid equilibria measurements it was shown that ionic liquids stand good alternative as “green solvents” in aliphatic/aromatic hydrocarbons extraction processes. All systems were modeled by using the NRTL equation, with linear temperature-dependent adjustable parameters providing an excellent fit of the experimental data.
350
Acknowledgment
330
Funding for this research was provided by the Ministry of Sciences and Higher Education in years 2008–2011 (Grant No. N209 096435). Appendix A. Supplementary data
310
290
Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.fluid.2011.01.025. References 0
0.2
0.4
x1
0.6
0.8
1
Fig. 2. Liquid–liquid phase equilibria of {[N-C3 OHPY][FAP] (1) + hydrocarbon (2)} binary systems: (䊉) n-hexane; () n-heptane; () cyclohexane; () benzene; () toluene. Solid lines are calculated by means of the NRTL equation.
ally increases solubility of cycloalkanes. Benzene and toluene have much better solubility in the ionic liquids than aliphatic hydrocarbons due to aromatic nature. Strong interaction between six -delocalized electrons in the aromatic structure with the polar ionic liquids, especially with pyridinium cation, causes better solubility. The solubility is lower for toluene than for benzene because of presence of a methyl group in the toluene structure. More aliphatic character of toluene reduces solubility in ionic liquids. Figs. 1S to 5S show the influence of the anion structure of investigated ionic liquids on the solubility of the hydrocarbons. The solubility of investigated hydrocarbons is higher in the ionic liquid based on [FAP]− anion than for the [NTf2 ]− one. For aliphatic hydrocarbons the differences in solubility are not so high as for the
[1] J.P. Wauquier, Petroleum Refining, vol. 2, Separation Processes: Editions Technip, Paris, 1995. [2] J.M. Crosthwaite, M.J. Muldoon, S.N.V.K. Aki, E.J. Maginn, J.F. Brennecke, J. Phys. Chem. B 110 (2006) 9354–9361. [3] A. Marciniak, Fluid Phase Equilib. 294 (2010) 213–233. [4] A. Marciniak, M. Wlazło, J. Phys. Chem. B 114 (2010) 6990–6994. [5] J.D. Holbrey, I. López-Martin, G. Rothenberg, K.R. Seddon, G. Silvero, X. Zheng, Green Chem. 10 (2008) 87–92. [6] L. Alonso, A. Arce, M. Francisco, O. Rodríguez, A. Soto, AICHE J. 53 (2007) 3108–3115. ´ [7] U. Domanska, A. Marciniak, J. Phys. Chem. B 108 (2004) 2376–2382. ´ [8] U. Domanska, A. Marciniak, M. Królikowski, J. Phys. Chem. B 112 (2008) 1218–1225. [9] H. Renon, J.M. Prausnitz, AIChE J. 14 (1986) 135–144. ´ [10] U. Domanska, A. Marciniak, Fluid Phase Equilib. 238 (2005) 137–141. ´ [11] U. Domanska, M. Laskowska, A. Pobudkowska, J. Phys. Chem. B 113 (2009) 6397–6404. [12] A. Marciniak, E. Karczemna, J. Phys. Chem. B 114 (2010) 5470–5474. ´ [13] U. Domanska, A. Marciniak, J. Chem. Thermodyn. 37 (2005) 577–585. [14] Design Institute for Physical Properties, Sponsored by AIChE (2005, 2008, 2009, 2010), DIPPR Project 801 – Full Version. Design Institute for Physical Property Research/AIChE. Online version available from: http://knovel.com/web/portal/browse/display? EXT KNOVEL DISPLAY bookid=1187&VerticalI.D=0.
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Fluid Phase Equilibria journal homepage: www.elsevier.com/locate/fluid
Influence of anion structure on the liquid–liquid equilibria of 1-(3-hydroxypropyl)pyridinium cation based ionic liquid–hydrocarbon binary systems Andrzej Marciniak ∗ , Ewa Karczemna Department of Physical Chemistry, Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland
a r t i c l e
i n f o
Article history: Received 18 November 2010 Received in revised form 24 January 2011 Accepted 26 January 2011 Available online 2 February 2011 Keywords: Liquid–liquid equilibria Ionic liquid 1-(3-Hydroxypropyl)pyridinium bis(trifluoromethylsulfonyl)-amide 1-(3-Hydroxypropyl)pyridinium trifluorotris(perfluoroethyl)phosphate [N-C3 OHPY][NTf2 ] [N-C3 OHPY][FAP]
a b s t r a c t Binary liquid–liquid equilibria for 10 systems containing an ionic liquid (1-(3hydroxypropyl)pyridinium bis(trifluoromethylsulfonyl)-amide and 1-(3-hydroxypropyl)pyridinium trifluorotris(perfluoroethyl)phosphate) with a hydrocarbon (n-hexane, n-heptane, cyclohexane, benzene, toluene) were measured by dynamic method. The influence of anion structure of 1(3-hydroxypropyl)pyridinium cation based ionic liquids on solubility of aliphatic and aromatic hydrocarbons is discussed. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Dearomatization processes play key role in chemical industry and are used, inter alia, to produce kerosene with a better smoke point, diesel oil with a higher cetane number or a mineral oil with a better viscosity index. In industrial processes different types of entrainers are used, namely: di-, tri-, and tetra-ethyleneglycols (abbreviations: DEG, TEG and TETRA, respectively), N-methylpyrrolidone (NMP), N-formylmorpholine (NFM), dimethylsulfoxide (DMSO), tetramethylenesulfone (sulfolane) [1]. Although the processes based on these solvents are well known, there is still need to find better environmental friendly solvents with better selectivity and capacity (solvent power). Ionic liquids (ILs) are relatively new class of salts whose melting temperature is below 100 ◦ C [2]. In general ILs are composed of organic cations with either inorganic or organic anions. This class of compounds has important properties required for entrainers, namely negligible vapor pressure, a wide liquid range and stability at high temperatures. From activity coefficients at infinite dilution measurements it was shown that a large number of ionic liquids have
∗ Corresponding author. Tel.: +48 22 234 58 16; fax: +48 22 628 27 41. E-mail address: [email protected] (A. Marciniak). 0378-3812/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.fluid.2011.01.025
better selectivity and capacity in extraction of aromatics from aromatic/aliphatic mixtures than typical solvents such as sulfolane and NMP [3]. To design an ionic liquid to specific separation problem the knowledge of influence of cation and anion structure on solubility is required. In this paper, the influence of anion structure of 1-(3-hydroxypropyl)pyridinium cation based ionic liquids on solubility of aliphatic and aromatic hydrocarbons is presented. Binary liquid–liquid equilibria (LLE) for 10 systems containing an ionic liquid (1-(3-hydroxypropyl)pyridinium bis(trifluoromethylsulfonyl)amide, [N-C3 OHPY][NTf2 ], 1-(3-hydroxypropyl)pyridinium trifluorotris(perfluoroethyl)phosphate, [N-C3 OHPY][FAP]) with a hydrocarbon (n-hexane, n-heptane, cyclohexane, benzene, toluene) were measured by the dynamic method. The potentials of [N-C3 OHPY][FAP] as extractant for the separation of aromatic hydrocarbons from aromatic/aliphatic mixtures have already been studied by measurements of activity coefficients at infinite dilution [4]. It was found that the investigated [N-C3 OHPY][FAP] ionic liquid shows much higher selectivity and capacity at infinite dilution than the generally used organic solvents such as NMP, sulfolane, and other ionic liquids especially based on [FAP]− anion for the separation of aliphatic hydrocarbons from aromatic hydrocarbons. Ionic liquids are considered as extractants in separation of sulfur compounds from fuels [5,6] therefore the solubility of aromatic and
122
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2. Materials and methods
of temperature measurements was ±0.05 K, and that of the mole fraction did not exceed ±0.0002. The reproducibility of the LLE experimental points was ±0.1 K. The experimental results are listed in Table 1.
2.1. Materials
3. Modeling
The ionic liquid [N-C3 OHPY][NTf2 ] had a purity of >0.999 mass fraction and was supplied by Merck. The ionic liquid [N-C3 OHPY][FAP] had a purity of >0.999 mass fraction and was supplied by Merck. The ionic liquids were further purified by subjecting the liquid to a very low pressure of about 5 × 10−3 Pa at a temperature about 95 ◦ C for ca. 5 h. This procedure removed any volatile chemicals and water from the ionic liquid. The list of hydrocarbons used in this study including source and grade is as follows: n-hexane, Fluka, grade >0.997 mass fraction; n-heptane, Sigma–Aldrich, grade >0.995 mass fraction; cyclohexane, Sigma–Aldrich, grade >0.997 mass fraction; benzene, Sigma–Aldrich, grade ≥0.999 mass fraction; toluene, Sigma–Aldrich, grade ≥0.999 mass fraction. While the purity of hydrocarbons was high all hydrocarbons were used without further purification. The structures of investigated ionic liquids [NC3 OHPY][NTf2 ] and [N-C3 OHPY][FAP] are presented below:
The LLE was correlated with the NRTL model describing the excess Gibbs energy [9]. The equations were described by us earlier [10]. For correlation the temperature-dependent model adjustable parameters g12 − g22 = a12 T + b12 and g21 − g11 = a21 T + b21 were calculated. The NRTL nonrandom parameter ˛ was set to a value of ˛ = 0.1, which has given the best results of the correlation. By analogy to the previous experiments (spectroscopic measurements) [11], it was assumed in this work that the solubility at the solvent-rich phase was in the range of x1 = 10−5 in n-hexane, nheptane, and cyclohexane and x1 = 5 × 10−5 in toluene. The results of the correlation, values of the model parameters, and the corresponding standard deviations are given in Table 2. For the systems presented in this work the average root-mean-square deviation x is less than 0.0008. The results of the correlation are presented in Figs. 1 and 2.
aliphatic hydrocarbons in ionic liquids for this problem are very useful and important.
N+
O OH
F
O
N S
S O
F
F
O
F
F
F F F
F N+
OH
F
F
F P-
F F
F F
F
F F F F
F
F F
2.2. Water content The water content was analyzed by Karl-Fischer titration technique (method TitroLine KF). A sample of IL was dissolved in methanol and titrated with steps of 2.5 L. The results obtained have shown the water content to be less than 100 ppm. 2.3. Liquid–liquid phase equilibria apparatus and measurements Two phases disappearance observed with an increasing temperature have been determined using a dynamic (synthetic) method described previously [7,8]. The compound was kept under the nitrogen in a drybox. Mixtures of solute and solvent were prepared by weighing the pure components to within 10−4 g. The sample of solute and solvent was heated very slowly (at less than 2 K h−1 near the equilibrium temperature) with continuous stirring inside a Pyrex glass cell, placed in a thermostat. The foggy solution disappearance temperature detected visually was measured with a calibrated electronic thermometer P 550 (DOSTMANN electronic GmbH). The measurements were carried out over a wide range of solute mole fraction ranging from 5 × 10−5 to 1. The uncertainty
4. Results and discussion For this study, the liquid-phase behavior for a 10 binary ionic liquid–hydrocarbon systems was determined. Results are presented in Table 1 and in Figs. 1 and 2 and 1S to 5S. The increase of solubility with the increase of temperature is observed for all systems, therefore the liquid–liquid equilibria diagrams have shapes typical for diagrams with upper critical solution temperature (UCST). These types of diagrams for systems ionic liquid + hydrocarbon were reported in the literature [12,13]. With the increase of the length of the alkane chain the increase of miscibility gap is observed, accordingly the solubility of n-heptane is lower than for n-hexane. In comparison, cyclohexane shows better solubility in the investigated ionic liquids than n-hexane. This stands that aliphatic hydrocarbons with cyclic structure, analogous to pyridinium structure, reveal better solubility than linear alkanes (similia similibus solvuntur). Moreover the molar volume of cycloalkanes (1.089 × 10−4 m3 mol−1 for cyclohexane at 298.15 K) [14] is smaller than for linear alkanes (1.314 × 10−4 m3 mol−1 for n-hexane at 298.15 K) [14], therefore the packing effect addition-
A. Marciniak, E. Karczemna / Fluid Phase Equilibria 304 (2011) 121–124
123
Table 1 Experimental binary liquid–liquid equilibria for systems {ionic liquid (1) + hydrocarbon (2)}. x1
T (K)
x1
T (K)
x1
T (K)
[N-C3 OHPY][NTf2 ] + n-Hexane 0.9763 0.9708 0.9613
300.5 304.4 310.3
0.9543 0.9485 0.9385
316.4 320.9 328.8
0.9303
335.3
[N-C3 OHPY][NTf2 ] + n-heptane 0.9906 300.1 0.9864 304.9 0.9800 313.3
0.9737 0.9702 0.9660
322.7 329.1 336.0
0.9624 0.9570
342.3 352.0
[N-C3 OHPY][NTf2 ] + cyclohexane 0.9680 294.5 0.9612 299.9 0.9553 305.4
0.9469 0.9385 0.9327
314.2 323.8 330.9
0.9270 0.9212 0.9139
337.0 343.0 351.0
[N-C3 OHPY][NTf2 ] + benzene 0.3470 0.3459 0.3447
298.9 308.2 318.3
0.3431 0.3420 0.3409
326.6 334.7 343.9
0.000141a 0.000147a 0.000151a
340.8 347.0 352.5
[N-C3 OHPY][NTf2 ] + toluene 0.5087 0.5073 0.5049
297.1 303.7 310.2
0.5020 0.4996 0.4971
318.6 330.3 340.4
0.4945 0.4918
350.7 359.9
[N-C3 OHPY][FAP] + n-hexane 0.9541 0.9472 0.9414
303.5 307.3 311.2
0.9334 0.9201 0.9062
316.3 323.7 330.4
0.8925
336.7
[N-C3 OHPY][FAP] + n-heptane 0.9707 0.9653 0.9626
308.8 312.1 315.4
0.9569 0.9515 0.9455
320.7 325.6 330.3
0.9401 0.9353 0.9269
336.7 342.2 351.2
[N-C3 OHPY][FAP] + cyclohexane 0.9427 303.2 0.9359 309.0 0.9267 316.9
0.9215 0.9169 0.9085
320.8 323.8 329.0
0.8993 0.8816 0.8655
334.3 343.5 350.9
[N-C3 OHPY][FAP] + benzene 0.2177 0.2172 0.2160 0.2137 0.2109
292.0 294.3 296.8 301.1 305.2
0.2074 0.2041 0.2010 0.1977 0.1943
311.8 318.5 325.7 332.3 337.4
0.1896 0.1851 0.000053a 0.000116a 0.000175a
344.3 352.6 298.4 327.9 352.0
[N-C3 OHPY][FAP] + toluene 0.2840 0.2817 0.2799 0.2766
296.3 301.2 306.9 313.4
0.2739 0.2713 0.2691 0.2672
320.3 328.5 335.7 342.9
0.2655 0.2638
350.0 358.3
a
Hydrocarbon rich phase.
Table 2 Correlation of the LLE data by means of the NRTL equationa , b , and the mole fraction deviations x . System
g12 − g22 (J mol−1 ) a12
[N-C3 OHPY][NTf2 ] + hexane [N-C3 OHPY][NTf2 ] + heptane [N-C3 OHPY][NTf2 ] + cyclohexane [N-C3 OHPY][NTf2 ] + benzene [N-C3 OHPY][NTf2 ] + toluene [N-C3 OHPY][FAP] + hexane [N-C3 OHPY][FAP] + heptane [N-C3 OHPY][FAP] + cyclohexane [N-C3 OHPY][FAP] + benzene [N-C3 OHPY][FAP] + toluene a b
−81.00 −70.06 −50.22 −48.83 −40.15 −76.67 −61.39 −62.11 −78.16 −63.81
Parameters g12 − g22 = a12 T + b12 , g21 − g11 = a21 T + b21 . Parameter ˛ = 0.1.
g21 − g11 (J mol−1 ) b12 24,575 23,114 14,325 831.6 1653 22,080 18,671 17,257 5130 3410
a21 160.6 144.7 133.9 121.7 119.5 161.1 134.7 144.4 109.4 150.5
x b21 −18,945 −15,791 −10,015 4206 813.3 −17,894 −10,977 −12,356 16,675 −770.4
0.0014 0.0017 0.0011 0.0002 0.0003 0.0003 0.0013 0.0015 0.0006 0.0003
124
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370
aromatic ones. In case of aliphatic hydrocarbons it indicates that the van der Waals interactions and packing effects play the main role in solubility. More extended structure of [FAP]− anion has higher surface than [NTf2 ]− one, for that reason the hydrocarbons can interact stronger via the dispersion forces. The aromatic hydrocarbons as opposed to the aliphatic ones have polarizable delocalized bonds in structure, consequently in this case the main role on solubility, apart from van der Waals interactions and packing effects, plays induced dipole interactions. In the [FAP]− structure more strongly electronegative fluoride atoms are presented than in the [NTf2 ]− , therefore the interaction between polarizable hydrocarbon and [FAP]− anion is much stronger.
T/K
350
330
310
5. Conclusions
290
0
0.2
0.4
x1
0.6
0.8
1
Fig. 1. Liquid–liquid phase equilibria of {[N-C3 OHPY][NTf2 ] (1) + hydrocarbon (2)} binary systems: (䊉) n-hexane; () n-heptane; () cyclohexane; () benzene; () toluene. Solid lines are calculated by means of the NRTL equation.
T/K
370
The knowledge of the impact of anion structure on the liquid phase behavior of ionic liquids with hydrocarbons is useful for developing ionic liquids as “designer solvents” for the extraction of aromatic hydrocarbons from aromatic/aliphatic mixtures. The solubility of investigated hydrocarbons is lower for [N-C3 OHPY][NTf2 ] ionic liquid. From the liquid–liquid equilibria measurements it was shown that ionic liquids stand good alternative as “green solvents” in aliphatic/aromatic hydrocarbons extraction processes. All systems were modeled by using the NRTL equation, with linear temperature-dependent adjustable parameters providing an excellent fit of the experimental data.
350
Acknowledgment
330
Funding for this research was provided by the Ministry of Sciences and Higher Education in years 2008–2011 (Grant No. N209 096435). Appendix A. Supplementary data
310
290
Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.fluid.2011.01.025. References 0
0.2
0.4
x1
0.6
0.8
1
Fig. 2. Liquid–liquid phase equilibria of {[N-C3 OHPY][FAP] (1) + hydrocarbon (2)} binary systems: (䊉) n-hexane; () n-heptane; () cyclohexane; () benzene; () toluene. Solid lines are calculated by means of the NRTL equation.
ally increases solubility of cycloalkanes. Benzene and toluene have much better solubility in the ionic liquids than aliphatic hydrocarbons due to aromatic nature. Strong interaction between six -delocalized electrons in the aromatic structure with the polar ionic liquids, especially with pyridinium cation, causes better solubility. The solubility is lower for toluene than for benzene because of presence of a methyl group in the toluene structure. More aliphatic character of toluene reduces solubility in ionic liquids. Figs. 1S to 5S show the influence of the anion structure of investigated ionic liquids on the solubility of the hydrocarbons. The solubility of investigated hydrocarbons is higher in the ionic liquid based on [FAP]− anion than for the [NTf2 ]− one. For aliphatic hydrocarbons the differences in solubility are not so high as for the
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