A promising ionic liquid [BMIM][FeCl4] for the extractive separation of aromatic and aliphatic hydrocarbons

Article abstract of DOI:10.1021/je400076x

Liquid-liquid equilibria (LLE) of the following ternary systems were measured at atmospheric pressure, that is, {cyclohexane + benzene + [BMIM][FeCl₄]} at T = 298.15 K and 313.15 K, {n-hexane + benzene + [BMIM][FeCl₄]}, {n-heptane + toluene + [BMIM][FeCl₄]} at T = 298.15 K, and {cyclohexane + benzene + [BMIM]Cl} at T = 339.15 K. The extraction performance of ionic liquid (IL) [BMIM][FeCl₄] for the aromatic hydrocarbons (benzene or toluene) from their aliphatic hydrocarbons (cyclohexane, n-heptane, or n-hexane) was analyzed and compared with some imidazolium-based ILs. The results indicate that the IL [BMIM][FeCl₄] shows both higher extractive selectivity and distribution factor for the systems studied herein and thus is a promising solvent for the extractive separation of aromatic and aliphatic hydrocarbons. The LLE data were well-correlated by the nonrandom two-liquid (NRTL) model of nonelectrolyte solutions with the overall average absolute relative deviation (rmsd) being about 0.0188 in terms of the mole fraction based activity.

Full text of DOI:10.1021/je400076x

Article
pubs.acs.org/jced
A Promising Ionic Liquid [BMIM][FeCl4] for the Extractive Separation
of Aromatic and Aliphatic Hydrocarbons
Salem A. Sakal,†,‡ Ying-zhou Lu,‡ Xiao-chuan Jiang,‡ Chong Shen,§ and Chun-xi Li*,†,‡
†State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China
‡College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China
§Liming Research Institute of Chemical Industry, Luoyang, 471001, P. R. China
ABSTRACT: Liquid−liquid equilibria (LLE) of the following ternary systems were
measured at atmospheric pressure, that is, {cyclohexane + benzene + [BMIM][FeCl4]}
at T = 298.15 K and 313.15 K, {n-hexane + benzene + [BMIM][FeCl4]}, {n-heptane +
toluene + [BMIM][FeCl4]} at T = 298.15 K, and {cyclohexane + benzene + [BMIM]Cl}
at T = 339.15 K. The extraction performance of ionic liquid (IL) [BMIM][FeCl4] for the
aromatic hydrocarbons (benzene or toluene) from their aliphatic hydrocarbons
(cyclohexane, n-heptane, or n-hexane) was analyzed and compared with some
imidazolium-based ILs. The results indicate that the IL [BMIM][FeCl4] shows both
higher extractive selectivity and distribution factor for the systems studied herein and
thus is a promising solvent for the extractive separation of aromatic and aliphatic
hydrocarbons. The LLE data were well-correlated by the nonrandom two-liquid (NRTL)
model of nonelectrolyte solutions with the overall average absolute relative deviation
(rmsd) being about 0.0188 in terms of the mole fraction based activity.
performance and the structure of ILs, which in return can guide
the molecular design of task-specific ILs. In effect, some
preliminary conclusions have been achieved. First, the
preferential extractability of ILs for the aromatic components
is mainly arisen from the π−π interaction between the aromatic
cation of ILs, that is, imidazolium or pyridinium, and the
aromatic molecules.23 Second, the length of the alkyl substitute
to the imidazolium or pyridinium cation is vital to tuning the
physicochemical properties of ILs, for example, the solubility of
ILs for different hydrocarbons and thus their extractive
selectivity and capacity for the aromatic component. Third,
the anion of the ILs might be a secondary factor in comparison
with cation, although it has a deterministic role on the
hydrophobicity of the ILs. For example, ILs [BMIM]PF6 and
[BMIM]BF4 show a comparable extraction ability for benzene
from its cyclohexane mixture at ambient temperatures, but their
hydrophobicity is totally different, being hydrophobic and
hydrophilic ILs, respectively. Finally, an IL with a high
selectivity for the aromatic component is always accompanied
with a lower capacity and vice versa.
INTRODUCTION
■
Separation of aromatic and aliphatic mixtures via conventional
distillation is a costly process due to their close volatility and
even the presence of azeotropism, and thus their separation is
practically achieved through liquid extraction1 for the aromatic
content in the range of (20 to 65) wt % with some specific
solvents, for example, sulfolane,2−5 N-methyl pyrrolidone
(NMP),4 ethylene glycols,5−7 and propylene carbonate8 and
so forth, or through azeotropic distillation or extractive
distillation depending on the content of aromatics in the
stream. However, these organic extracting solvents are generally
toxic, volatile, flammable, and hard to be recycled facilely due to
their mutual solubility with the naphtha to be separated. To
overcome these demerits, ionic liquids (ILs) as novel and
greener solvents have attracted much attention in recent
years9−11 due to their unique characteristics and sharp contrast
to the molecular solvents, for example, less toxicity, non-
volatility, nonflammability, and negligible solubility in hydro-
carbon mixtures and thus easier to be recycled. Meanwhile, the
number of ILs is so huge that a satisfactory extractant can be
always expected or designed through appropriate combination
of cation and anion. Until now, many experimental data of
liquid−liquid equilibria (LLE) have been reported for the (IL +
aromatic + aliphatic) systems with varying ILs,12−22 where the
cations are mostly alkyl (R-) substituted imidazolium or
pyridinium, while the anions are diversified including halide,
PF6, BF4, dialkyl-phosphate, alkyl-sulfate, and bis-
{(trifluoromethyl)sulfonyl}amide, and so forth. These results
are helpful for screening a promising IL for the extractive
separation of an aromatic−aliphatic mixture, as well as for a
better insight into the relationship between the extraction
Based on the above cognition, a candidate IL for the
separation of aromatic/aliphatic mixture is better composed of
an aromatic cation, for example, R-substituted imidazolium or
pyridinium, and an appropriate anion that makes the ILs
suitable for practical use. More specifically, the ILs should have
a good extracting ability for aromatic component along with
high stability, low viscosity and toxicity, and ease of preparation
Received: March 8, 2012
Accepted: January 27, 2014
Published: February 13, 2014
© 2014 American Chemical Society
533
dx.doi.org/10.1021/je400076x | J. Chem. Eng. Data 2014, 59, 533−539

Journal of Chemical & Engineering Data
Article
and low cost. In this regard, the feasibility of ILs 1-butyl-3-
methylimidazolium chloride [BMIM]Cl24 and [BMIM]-
[FeCl4]25,26 is worthy of study. Besides, the LLE data for the
present ILs with aromatic (benzene, toluene) and aliphatic
(cyclohexane, n-hexane, n-heptane) binary mixtures are not
available. Therefore, the LLE data for the following ternary
systems at atmospheric pressure and different temperatures
were measured, namely, {cyclohexane + benzene + [BMIM]-
[FeCl4]}, {n-hexane + benzene + [BMIM][FeCl4]}, {n-heptane
+ toluene + [BMIM][FeCl4]}, and {cyclohexane + benzene +
[BMIM][Cl]}, and the experimental data were correlated by
the nonrandom two-liquid (NRTL) model.
hexane}, or {toluene + n-heptane}, is transferred into the upper
organic phase, while the IL component into the bottom water
phase completely. Samples from the organic phase are totally
free of IL, which is adopted by many authors and justified by
relevant analysis techniques.27,28
The relative composition of the binary mixtures, that is,
(benzene and cyclohexane), (benzene and n-hexane), and
(toluene and n-heptane), in the pretreated samples was
analyzed using gas chromatography (Shimadzu GC2010
equipped with a FID detector and FFAP capillary column, 30
m × 0.25 mm i.d. × 5 μm; carrier gas N2; temperature program:
(70 to 170) °C at 10 °C·min−1 and then maintained at 170 °C
for 30 min. The sample concentration was given by the GC
solution workstation according to the area of each chromato-
graph peak and the calibration curve made prior for the ternary
mixtures of (toluene + benzene + cyclohexane), (toluene +
benzene + n-hexane), (benzene + toluene + n-heptane). Once
the amount of each component for the binary system, namely,
(benzene and cyclohexane), (benzene and n-hexane), or
(toluene and n-heptane), had been determined, the mass
fraction of IL in corresponding both phases was calculated via
mass balance for the upper and bottom phase samples,
respectively. At least two samples were made for each phase,
and three injections were made for each sample in the GC
EXPERIMENTAL SECTION
■
Materials. The chemicals used in this study were all
purchased from Beijing Chemical Reagent Factory, of which
benzene, cyclohexane, n-hexane, toluene, and n-heptane are
with a nominal minimum mass fraction of 0.995. N-
Methylimidazole, chlorobutane, diethyl ether, and ethanol
anhydrous are of analytical reagent (AR) grade reagents and
were used as received.
Preparation of ILs [BMIM]Cl and [BMIM][FeCl4].
[BMIM]Cl was synthesized by refluxing the mixture of N-
methylimidazole and chlorobutane at 90 °C for 12 h with ca. 10
% excess stoichiometric chlorobutane. The excessive reactants
of the raw product were removed first by rotary evaporation
under reduced pressure and then washed three times with equal
volume of diethyl ether. The resulting white precipitate, that is,
[BMIM]Cl at room temperature, was filtrated and dried in a
vacuum oven. The melting point of [BMIM][Cl] is measured
by DSC being approximately 60.3 °C. The water content of
[BMIM][Cl] is analyzed by Karl Fischer method as 1415 ppm.
The purity of [BMIM][Cl] is confirmed qualitatively from both
1H NMR and 13C NMR spectra, and its purity is estimated
being about 0.986 in mole fraction. [BMIM][FeCl4] was
prepared by mixing equal moles of anhydrous iron chloride
dissolved in ethanol anhydrous and [BMIM]Cl. The mixture
was left stirring overnight at room temperature and then treated
by a rotary evaporator to remove ethanol and other residual
volatile impurities, and the water mass fraction was less than 4·
10−4 as measured by the Karl Fischer titrator (CBS-1A). The
purity of the final IL [BMIM][FeCl4] is above 0.99 in mole
fraction in terms of its elementary analysis results and 1H NMR
analysis for its precursor [BMIM]Cl.
Apparatus and Procedures. The LLE measurements were
conducted in a jacketed glass cell of about 150 mL sealed by a
silicon rubber cap. The equilibrium liquid temperature was
maintained by circulating water coming from a super
thermostat with temperature fluctuation within ( 0.1 °C).
First, known masses of ILs and liquid mixtures with known
composition were in turn added into the glass cell. The mixture
was stirred with a magnetic stirrer for at least 1.5 h at specified
temperature and then stopped stirring for 2 h to achieve a clear
phase separation. The time used here for equilibrium and phase
splitting was justified by some preliminary tests. Two samples
(about 1.0 mL for each) were taken out from both phases,
added immediately into two 10 mL test tubes, and weighed
their exact masses. Each tube was prefilled with about 3 mL of
internal standard and 3 mL of water with their exact masses
known in advance for the sake of mass balance. The tubes were
sealed with PTFE/silicone sheet, shaken for 10 min, and then
put aside overnight for settling. In this process, the organic
mixture, namely, {benzene + cyclohexane}, {benzene + n-
analysis. The reproducibility of the composition is within
%. The uncertainty of mole fraction for cyclohexane, benzene,
n-hexane, toluene, and n-heptane is 0.005 and for ILs is
1
within 0.002 as estimated from the water content of the IL
used. All components were prepared gravimetrically by an
electronic balance (type AR2130, Ohaus Corp., USA) with a
readability of 0.001 g.
RESULTS AND DISCUSSION
■
The experimental LLE data for the four ternary systems,
namely, {cyclohexane (1) + benzene (2) + [BMIM][FeCl4]
(3)}, {n-hexane (1) + benzene (2) + [BMIM][FeCl4] (3)}, {
n-heptane (1) + toluene (2) + [BMIM][FeCl4] (3)}, and
{cyclohexane + benzene + [BMIM][Cl]}, at different temper-
atures are presented in Tables 1 to 4, respectively. The resulting
phase diagrams for the first three systems above at 298.15 K
and for the last system at 339.15 K are shown in Figures 1 to 4,
respectively. As seen from Figures 1 to 4, the liquid
composition of the IL phase varies regularly with that of the
organic raffinate phase, and the data distribution in the figures
follows a definite pattern with low scattering, which justifies the
experimental method used herein and lays a foundation for the
data correlation with an appropriate thermodynamic model.
Further, the binodal curves for the ternary systems with IL
[BMIM][FeCl4], that is, Figures 1 to 3, are more symmetrical
than that with IL [BMIM]Cl, implying that [BMIM][FeCl4]
has a higher extracting capacity for the aromatic components
than [BMIM]Cl.
The selectivity (S) and distribution ratio (D) of the aromatic
component are two important parameters in assessing the
feasibility of an IL for the extractive separation of aromatics,
and thus they are also listed in Tables 1 to 4. The values of S
and D were calculated by virtue of the experimental LLE data
using eqs 1 and 2 below.
″
″
′
(x2/x1 )
S =
′
(x2/x1)
(1)
534
dx.doi.org/10.1021/je400076x | J. Chem. Eng. Data 2014, 59, 533−539

Journal of Chemical & Engineering Data
Article
Table 1. LLE Data in Mole Fraction for the {Cyclohexane
(1) + Benzene (2) + [BMIM][FeCl4] (3)} System at
Pressure p = 0.1 MPa and 298.15 K and 313.15 K along with
Table 3. LLE Data in Mole Fraction for the {n-Heptane (1)
+ Toluene (2) + [BMIM][FeCl4] (3)} System at Pressure p
= 0.1 MPa and 298.15 K along with the Selectivity (S) and
a
a
the Selectivity (S) and Distribution Factor of Benzene (D)
Distribution Factor of Toluene (D)
organic phase
IL phase
organic phase
IL phase
x′1
x′2
x″1
T = 298.15 K
0.181
x″2
S
D
x′1
x′2
x″1
x″2
S
D
0.950
0.920
0.856
0.784
0.710
0.634
0.567
0.478
0.389
0.262
0.050
0.080
0.144
0.216
0.290
0.366
0.433
0.522
0.611
0.738
0.159
0.175
0.170
0.165
0.158
0.152
0.144
0.139
0.133
0.120
0.089
0.126
0.194
0.248
0.305
0.361
0.406
0.454
0.499
0.568
10.69
8.17
6.76
5.43
4.71
4.11
3.67
2.99
2.39
1.67
1.79
1.56
1.35
1.14
1.05
0.99
0.94
0.87
0.82
0.77
0.970
0.929
0.875
0.813
0.763
0.666
0.599
0.524
0.420
0.357
0.030
0.071
0.125
0.187
0.237
0.334
0.401
0.476
0.580
0.643
0.064
0.128
0.213
0.285
0.357
0.426
0.477
0.530
0.583
0.636
11.55
10.14
9.34
8.31
7.80
6.51
5.38
4.73
3.73
3.58
2.16
1.79
1.70
1.52
1.51
1.28
1.19
1.11
1.01
0.99
0.164
0.160
0.149
0.147
0.131
0.132
0.123
0.113
0.099
a
The standard uncertainty u is u(T) = 0.3 K, and the combined
standard uncertainty uc is uc(x) = 0.0007.
T = 313.15 K
0.968
0.938
0.879
0.816
0.749
0.670
0.588
0.501
0.390
0.282
0.032
0.062
0.121
0.184
0.251
0.330
0.412
0.499
0.610
0.718
0.194
0.207
0.179
0.174
0.183
0.153
0.139
0.129
0.117
0.095
0.058
0.112
0.201
0.282
0.342
0.409
0.467
0.520
0.577
0.645
9.19
8.15
8.16
7.19
5.56
5.45
4.79
4.07
3.15
2.68
1.84
1.80
1.66
1.53
1.36
1.24
1.13
1.04
0.94
0.90
Table 4. LLE Data in Mole Fraction for the {Cyclohexane
(1) + Benzene (2) + [BMIM]Cl (3)} System at Pressure p =
0.1 MPa and 339.15 K along with the Selectivity (S) and
a
Distribution Factor of Benzene (D)
organic phase
IL phase
x′1
x′2
x″1
x″2
S
D
0.935
0.888
0.812
0.722
0.634
0.540
0.445
0.346
0.250
0.143
0.065
0.112
0.188
0.278
0.366
0.460
0.555
0.654
0.750
0.857
0.091
0.085
0.083
0.081
0.075
0.076
0.069
0.065
0.063
0.059
0.025
0.043
0.073
0.109
0.141
0.179
0.211
0.242
0.284
0.326
4.04
4.03
3.82
3.51
3.25
2.77
2.44
1.98
1.51
0.93
0.39
0.38
0.39
0.39
0.38
0.39
0.38
0.37
0.38
0.38
a
The standard uncertainty u is u(T) = 0.3 K, and the combined
standard uncertainty uc is uc(x) = 0.0013.
Table 2. LLE Data in Mole Fraction for the {n-Hexane (1) +
Benzene (2) + [BMIM][FeCl4] (3)} System at Pressure p =
0.1 MPa and 298.15 K along with the Selectivity (S) and
a
Distribution Factor of Benzene (D)
organic phase
IL phase
a
The standard uncertainty u is u(T) = 0.3 K, and the combined
x′1
x′2
x″1
x″2
S
D
standard uncertainty uc is uc(x) = 0.0008.
0.966
0.939
0.881
0.810
0.746
0.676
0.614
0.518
0.423
0.315
0.034
0.061
0.119
0.190
0.254
0.324
0.386
0.482
0.577
0.685
0.165
0.160
0.138
0.113
0.099
0.091
0.088
0.085
0.079
0.070
0.078
0.125
0.215
0.323
0.376
0.456
0.505
0.553
0.593
0.673
13.26
12.00
11.62
12.16
11.15
10.42
9.18
2.27
2.04
1.81
1.70
1.48
1.40
1.31
1.15
1.03
0.98
6.97
5.50
4.43
a
The standard uncertainty u is u(T) = 0.3 K, and the combined
standard uncertainty uc is uc(x) = 0.0012.
Figure 1. Binodal curves and tie lines for the ternary mixture
″
(x2)
{cyclohexane (1) + benzene (2) + [BMIM][FeCl4] (3)} at 298.15 K.
D =
○
′
, Predicted line by the NRTL equation; , experimental data.
(x2)
(2)
where x1 and x2 refer to the mole fraction of aliphatic and
aromatic, respectively, ′ represents the organic phase, and ″
represents the IL phase.
To compare the selectivity of different ILs for the aromatic
components from their aliphatic mixtures at varying conditions,
the selectivity for the four ternary systems studied here is
presented in Figure 5. By analyzing this figure, some
conclusions can be drawn. First, the selectivity values for all
of the systems studied are higher than unity except one point
for the {cyclohexane + benzene + [BMIM]Cl} system at a mole
fraction of benzene above 0.8. This indicates that both ILs
[BMIM][FeCl4] and [BMIM]Cl show a preferential extracting
ability for the aromatic components from their corresponding
aliphatic binary mixtures, which may be ascribed to the specific
π−π electron interaction between benzyl ring and the aromatic
535
dx.doi.org/10.1021/je400076x | J. Chem. Eng. Data 2014, 59, 533−539

Journal of Chemical & Engineering Data
Article
Figure 2. Binodal curves and tie-lines for the ternary mixture {n-
hexane (1) + benzene (2) + [BMIM][FeCl4] (3)} at 298.15 K. ,
○
Predicted line by the NRTL equation; , experimental data.
Figure 5. Selectivity of ILs for the aromatic component for ternary
systems, as a function of the mole fraction of the aromatic in the
◆
, {cyclohexane + benzene + [BMIM][FeCl4]}
at (298.15 and 313.15) K, respectively; , {n-hexane + benzene +
○
,
raffinate. Mixtures:
■
△
□
[BMIM][FeCl4]}, at 298.15 K;
, {n-heptane + toluene +
[BMIM][FeCl4]}, at 298.15 K; , {cyclohexane + benzene +
[BMIM]Cl} at 325.15 K.
γ2∞,aliphatic/γ2∞,IL as derived from eq 1. Fourth, the selectivity of
IL [BMIM][FeCl4] for the aromatic component from different
binary mixtures at fixed other conditions follows the order
(benzene + n-hexane) > (benzene + cyclohexane) > (toluene +
n-heptane). This observation is consistent with the increasing
similarity of the aromatic/aliphatic mixture in terms of their
molecular structure and intermolecular interaction and thus
may be universal for other ILs. Finally, the extraction selectivity
of IL [BMIM][FeCl4] is much higher than that of [BMIM]Cl
for the cyclohexane-benzene mixture, which is solely originated
from the influence of anion type. More specifically, the [FeCl4]
anion, as a complex of Cl− and Lewis acid FeCl3, shows a
stronger acidity than the chloride and thus introduces
additional electron donor−acceptor interaction between IL
and the benzyl ring, leading to a higher selectivity for the
aromatic components. Moreover, [BMIM][FeCl4] is a hydro-
phobic IL with much lower viscosity and melting temperature
in comparison with [BMIM]Cl and thus advantageous as a
promising candidate for the aromatic extraction process.
Figure 3. Binodal curves and tie-lines for the ternary mixture {n-
heptane (1) + toluene (2) + [BMIM][FeCl4] (3)} at 298.15 K. ,
○
Predicted line by the NRTL equation; , experimental data.
The distribution factor of aromatic component between IL
and the alkane phases for different ternary systems is presented
in Figure 6. By comparing Figures 5 and 6, it is obvious that the
variation extent of the distribution factor for all systems is much
lower than that of the selectivity for the influencing factors
studied, for example, temperature, and structural difference
between aromatic and aliphatic components. Second, IL
[BMIM][FeCl4] shows a much higher extracting capacity
than [BMIM]Cl since the distribution factor of the aromatic
component between [BMIM][FeCl4] and hydrocarbon is
about two to five times higher than that between [BMIM]Cl
and hydrocarbon. The higher extraction capacity of [BMIM]-
[FeCl4] coincides with the larger interionic interstice among
large cations and anions in comparison with [BMIM]Cl,
leading to a higher filling capacity for aromatic molecules.
Third, the extracting capacity of [BMIM][FeCl4] for different
aromatic−aliphatic mixtures follows the order (benzene/n-
hexane) > (benzene/cyclohexane) > (toluene/n-heptane),
being same as the selectivity order, which may be closely
related to the solubility of different pure components in
[BMIM][FeCl4]. To verify this assumption, the solubility of the
pure component in [BMIM][FeCl4] at 298.15 K was measured
via a gravimetric method by weighing the mass difference of the
Figure 4. Binodal curves and tie-lines for the ternary mixture
{cyclohexane (1) + benzene (2) + [BMIM]Cl (3)} at 339.15 K. ,
○
Predicted line by the NRTL equation; , experimental data.
imidazolium cation irrespective of the anions involved in the
ILs. Second, the effect of temperature on the selectivity of IL
for the aromatic component is relatively weak, as indicated by
the selectivity of [BMIM][FeCl4] for benzene from its
cyclohexane mixture at 298.15 K and 313.15 K, which may
be attributed to the weak response of the π−π electron
interaction to the temperature.11 Interestingly, a lower
temperature favors a higher selectivity of the IL for the
aromatic component, which is helpful for the practical use of IL
in an aromatic extraction process. Therefore, LLE data
measurements for other ternary systems were all conducted
at 298.15 K unless stated otherwise. Third, the selectivity
always decreases with the increase of the concentration of
aromatic component in the raffinate for all of the systems
studied, which is common for all aromatic extraction processes,
and the maximum selectivity corresponds to the infinite
dilution of the aromatic in its aliphatic mixture, being
536
dx.doi.org/10.1021/je400076x | J. Chem. Eng. Data 2014, 59, 533−539

Journal of Chemical & Engineering Data
Article
Table 5. Maximum Distribution Factor and Selectivity of
Different ILs for the Aromatic/Alkane Mixtures
aromatic + alkane
ionic liquids
mixture
T/K
Daromatic
S
ref
[BMIM]
[PF6]
benzene/
295.15
0.66
3.1
30
15
15
cyclohexane
[MMIM]
[DMP]
benzene/
cyclohexane
298.15
298.15
298.15
298.15
3.6
3.7
[EMIM]
[DEP]
benzene/
cyclohexane
[BMIM]
[FeCl4]
benzene/
cyclohexane
2.16
2.27
1.79
0.39
11.55 this
work
[BMIM]
[FeCl4]
benzene/n-hexane
13.26 this
work
[BMIM]
[FeCl4]
toluene/ n-heptane 298.15
10.69 this
work
Figure 6. Distribution factor of aromatic component between IL and
hydrocarbon raffinate for the ternary systems, as a function of the mole
[BMIM]Cl
benzene/
cyclohexane
339.15
4.04 this
work
◇
,
▲
,
fraction of the aromatic in the raffinate phase. Mixtures:
{cyclohexane + benzene + [BMIM][FeCl4]} at (298.15 and 313.15)
[OMIM]Cl
[OMIM]Cl
benzene/n-heptane 298.15
0.69
0.43
0.56
5.7
7.7
12
□
K, respectively; , {n-hexane + benzene + [BMIM][FeCl4]}, at 298.15
toluene/n-heptane
313.15
323.15
31
32
■
○
K; , {n-heptane + toluene + [BMIM][FeCl4]}, at 298.15 K;
,
[BMIM]
[BF4]
benzene/
15.9
{cyclohexane + benzene + [BMIM]Cl} at 339.15 K.
cyclohexane
[BMIM]
[BF4]
toluene/n-heptane
323.15
0.42
29.5
33
solute saturated IL samples before and after vacuum vapor-
ization. The solubility of benzene, toluene, cyclohexane, n-
hexane, and n-heptane in mole fraction basis was determined as
0.795, 0.714, 0.451, 0.402, and 0.073, respectively. The
solubility follows the order benzene > toluene > cyclohexane
> n-hexane > n-heptane, indicating that aromatic compounds
are more soluble in ILs than aliphatic compounds of a similar
size,23,29,30 and the lower solubility of alkyl benzene versus
benzene may be related to its weaker interaction with the
aromatic cation due to the steric hindrance of the alkyl
substitute present and also its lower filling ability into the
interstice of the IL phase.
By comparing the extraction performance of the two ILs,
namely, [BMIM][FeCl4] and [BMIM]Cl, for benzene from its
cyclohexane mixture, as presented in Figures 5 and 6, it is
obvious that [BMIM][FeCl4] is much superior to [BMIM]Cl
in terms of either selectivity or capacity. This conclusion may
be applicable for other aromatic−aliphatic mixtures, since the
stronger acidity and the larger size of the [FeCl4] anion versus
the Cl anion can result in a stronger interaction with aromatic
ring and a larger interstice among cations and anions of the IL,
and accordingly a higher extraction selectivity and capacity for
the aromatic component. Moreover, in comparison with
[BMIM]Cl, [BMIM][FeCl4] is a hydrophobic room temper-
ature ionic liquid with good fluidity and stability, ease of
preparation, and insensitivity to water. To show the excellent
performance of IL [BMIM][FeCl4] for aromatic extraction, the
selectivity and distribution factor values for several ILs reported
until now were collected and compared in Table 5. It is shown
that the imidazolium based ILs with [FeCl4] anion show an
excellent combination of both selectivity and distribution factor
in comparison with other ILs,15,12,30−33,36 for example,
[BMIM][BF4], and sulfolane has a little higher selectivity for
aromatic component, but its extraction capacity is much lower
than [BMIM][FeCl4]. Further, the higher aromatic selectivity is
only found for the ILs with [BF4] and [FeCl4] anions, which
may be associated with their stronger Lewis acidity than the
corresponding halide anions, that is, F and Cl, and other anions
like [PF6] and dialkylphosphates, for example, [DMP] and
[DEP].
sulfolane
sulfolane
benzene/n-hexane
toluene/n-heptane
303.15
313.15
0.35
0.31
29.1
33.0
36
36
solutions developed by Renon and Prausnitz34 was adopted for
the calculation of activity coefficients at specified temperature
and liquid compositions. This means that the ionic liquid
component is treated herein as a neutral molecule instead of
strong electrolyte dissociated completely, which is consistent
with the weak dielectric property of aliphatic and aromatic
hydrocarbons.
The experimental LLE data are fitted using the Levenberg−
Marquardt (LM) algorithm via minimization of the following
objective function (OF):
NP
2
2
′ ′
″ ″
′ ′
″ ″
2i
OF =
[(x γ − x γ ) + (x γ − x γ ) ]
∑
1i
1i
2i
2i
1i
1i
2i
(3)
i
where NP is the number of data points, 1 and 2 are
components 1 and 2 for a ternary system, respectively; the
superscripts ′ and ″ refer to top and bottom phases in
equilibrium, respectively; x and γ represent the mole fraction
and activity coefficients, respectively. The accuracy of the
correlation or prediction is represented in terms of the average
absolute relative deviation (rmsd), given by eq 4
m
n−1
1
6NP
rmsd =
(xαγα − xβγβ)2
ij ij
∑ ∑
ij
ij
i
j
(4)
Here, m and n are the numbers of tie lines and components for
a ternary system.
The regressed binary interaction parameters for the NRTL
model are presented in Table 6. It should be pointed out that
the pairwise interaction parameters for (aromatic + aliphatic) is
taken from literature.15,27,35
Figures 1 to 4 showed the experimental LLE data for studied
ternary systems {aliphatic (1) + aromatic (2) [BMIM][FeCl4]
(3)} compared with that predicted by the NRTL model. It is
seen that the experimental data in the triangle phase diagram
vary regularly with composition, and the calculated results go
smoothly through the experimental data points. This along with
the low correlation deviation in terms of an overall rmsd
Tie-Line Correlation. For correlating the experimental LLE
data of the ternary systems, the NRTL model of nonelectrolyte
537
dx.doi.org/10.1021/je400076x | J. Chem. Eng. Data 2014, 59, 533−539

Journal of Chemical & Engineering Data
Article
Table 6. NRTL Binary Parameters Regressed from LLE Data
and the Fitting Deviation in rmsd for Ternary Mixtures
{Aliphatic (1) + Aromatic (2) + ILs (3)}
REFERENCES
■
(1) Villaluenga, J. P. G.; Tabe-Mohammadi, A. A review on the
separation of benzene/cyclohexane mixtures by pervaporation
processes. J. Membr. Sci. 2000, 169, 159−174.
(2) Chen, J.; Duan, L.-P.; Mi, J.-G.; Fei, W.-Y.; Li, Z.-C. Liquid-liquid
equilibria of multi-component systems including n-hexane, n-octane,
benzene, toluene, xylene and sulfolane at 298.15 K and atmospheric
pressure. Fluid Phase Equilib. 2000, 173, 109−119.
(3) Chen, J.; Li, Z.; Duan, L. Liquid-liquid equilibria of ternary and
quaternary systems including cyclohexane, 1-heptane, benzene,
toluene, and sulfolane at 298.15 K. J. Chem. Eng. Data 2000, 45,
689−692.
(4) Krishna, R.; Goswami, A. N.; Nanoti, S. M.; Rawat, B. S.; Khana,
M. K.; Dobhal, J. Extraction of aromatics from 63−69 °C naphtha
fraction for food grade hexane production using sulpholane and NMP
as solvents. Ind. J. Technol. 1987, 25, 602−606.
(5) Yorulmaz, Y.; Karpuzcu, F. Sulpholane versus diethylene glycol in
recovery of aromatics. Chem. Eng. Res. Des. 1985, 63, 184−190.
(6) Wang, W.; Gou, Z. M.; Zhu, S. L. Liquid-liquid equilibria for
aromatics extraction systems with tetra ethylene glycol. J. Chem. Eng.
Data 1998, 43, 81−83.
(7) Al-Sahhaf, T. A.; Kapetanovic, E. Measurement and prediction of
phase equilibria in the extraction of aromatics from naphtha reformate
by tetra ethylene glycol. Fluid Phase Equilib. 1996, 118, 271−285.
(8) Ali, S. H.; Lababidi, H. M. S.; Merchant, S. Q.; Fahim, M. A.
Extraction of aromatics from naphtha reformate using propylene
carbonate. Fluid Phase Equilib. 2003, 214, 25−38.
NRTL parameters
component
(gij − gjj)
J·mol−1
5877.7
(gji − gii)
J·mol−1
178.05
(i) + (j)
αij
rmsd
cyclohexane (1) +
benzene (2)15
0.45
[BMIM][FeCl4]
7906.897
cyclohexane (1) + IL (3)
benzene (2) + IL (3)
0.1680
0.6047
837.375
0.011
0.045
18903.190
[BMIM][Cl]
8136.219
4968.146
cyclohexane (1) + IL (3)
benzene (2) + IL (3)
0.338
0.4528
0.4
5594.513
5693.221
12211.66
10513.72
n-hexane (1) +
6302.94
benzene (2)27
[BMIM][FeCl4]
2237.019
n-hexane (1) + IL (3)
0.2542
0.4035
0.3001
5653.548
4581.236
1143.8
0.011
0.008
benzene (2) + IL (3)
12870.150
1020.1
n-heptane (1) +
toluene (2)35
[BMIM][FeCl4]
7386.702
n-heptane (1) + IL (3)
0.3182
0.3720
3536.274
toluene (2) + IL (3)
15147.42
−846.141
overall RMSD = 0.0188
(9) Gmehling, J.; Krummen, M. Use of ionic liquids as entraining
agents and selective solvents for separation of aromatic hydrocarbons
in aromatic petroleum streams. DE Patent 101 54 052, 2003.
(10) Anjan, S. T. Ionic Liquids for Aromatic Extraction: Are They
Ready? Chem. Eng Prog. 2007, 38.
(11) Meindersma, G. W.; Podt, A.; de Haan, A. B. Selection of ionic
liquids for the extraction of aromatic hydrocarbons from aromatic/
aliphatic mixtures. Fuel Process. Technol. 2005, 87, 59−70.
(12) Letcher, T. M.; Deenadayalu, N. Liquid-liquid Equilibria for
Mixtures of 1-methyl-3-octyl-imidazolium Chloride + Benzene + an
Alkane at 298.15 K and 1 atm. J. Chem. Thermodyn. 2003, 35, 67−76.
(13) Letcher, T. M.; Reddy, P. Ternary (liquid + liquid) equilibria for
mixtures of 1-hexyl-3-methylimidazolium (tetrafluoroborate or hexa-
fluorophosphate) + benzene + an alkane at T = 298.2 K and p = 0.1
MPa. J. Chem. Thermodyn. 2005, 37, 415−421.
(14) Lu, Y.; Yang, X.; Luo, G. Liquid-Liquid Equilibria for Benzene +
Cyclohexane + 1-Butyl-3-methylimidazolium Hexafluorophosphate. J.
Chem. Eng. Data 2010, 55, 510−512.
(15) Wang, R.; Wang, J.; Meng, H.; Li, C.; Wang, Z. Liquid-liquid
equilibria for benzene + cyclohexane + 1-methyl-3- methylimidazolium
dimethylphosphate or + 1-ethyl-3-methylimidazolium diethylphos-
phate. J. Chem. Eng. Data 2008, 53, 1159−1162.
(16) Domanska, U.; Pobudkowska, A.; Eckert, F. Liquid−liquid
equilibria in the binary systems (1,3-dimethylimidazolium, or 1-butyl-
3-methylimidazolium methylsulfate + hydrocarbons). Green Chem.
2006, 8, 268−276.
(17) Deenadayalu, N.; Ngcongo, K. C.; Letcher, T. M.;
Ramjugernath, D. Liquid−liquid Equilibria for Ternary Mixtures (an
Ionic Liquid + Benzene + Heptane or Hexadecane) at T = 298.2 K and
Atmospheric Pressure. J. Chem. Eng. Data 2006, 51, 988−991.
(18) Meidersma, G. W.; Podt, A.; de Haan, A. B. Ternary liquid−
liquid equilibria for mixtures of toluene + n-heptane + an ionic liquid.
Fluid Phase Equilib. 2006, 247, 158−168.
justified the applicability of the NRTL model for the
representation of the LLE data for the IL-containing ternary
systems studied here.
CONCLUSIONS
■
A promising ionic liquid [BMIM][FeCl4] is found for the
extractive separation of aromatic−aliphatic mixture for its both
higher selectivity and capacity for the aromatic components
studied, that is, benzene and toluene, along with its favorable
attributes, for example, ease of preparation and cheapness, good
fluidity, and stability. The extractive selectivity of [BMIM]-
[FeCl4] for all of the studied systems shows the order of
(benzene/n-hexane) > (benzene/cyclohexane) > (toluene/n-
heptane), being consistent with other ILs reported with the
same cation but different anions. The higher extraction
selectivity of [BMIM][FeCl4] is ascribed to the additional
interaction between acidic anion and the aromatic ring besides
the π−π interaction between aromatic cation and the aromatic
solute, and the higher extracting capacity may be attributed to
its larger interionic interstice and accordingly a higher filling
capacity for aromatic molecules. The experimental LLE data
can be well-correlated through the NRTL model for non-
electrolyte solutions with an overall rmsd being about 0.0188.
AUTHOR INFORMATION
Corresponding Author
*Tel./fax: +86 10 64410308. E-mail: Licx@mail.buct.edu.cn.
■
(19) Abu-Eishah, S. I.; Dowaidar, A. M. Liquid-Liquid Equilibrium of
ternary systems of cyclohexane + (benzene, + toluene, + ethylbenzene,
or + o-xylene) + 4-Methyl-N-butyl pyridinium tetrafluoroborate ionic
liquid at 303.15 K. J. Chem. Eng. Data 2008, 53, 1708−1712.
(20) Arce, A.; Earle, M. J.; Rodriguez, H.; Seddon, K. R. Separation of
aromatic hydrocarbons from alkanes using the ionic liquid 1-ethyl-3-
methylimidazolium bis{(trifluoromethyl)sulfonyl}amide. Green Chem.
2007, 9, 70−74.
Funding
The authors are grateful to the financial support from National
Natural Science Foundation of China (21076005, 21376011)
and Research Fund for the Doctoral Program of Higher
Education of China (20090010110001).
Notes
The authors declare no competing financial interest.
538
dx.doi.org/10.1021/je400076x | J. Chem. Eng. Data 2014, 59, 533−539

Journal of Chemical & Engineering Data
Article
(21) Murugan, S. S.; Marvin, D. M.; Raymond, H. D.; Jerry, L. A.
Liquid−Liquid Equilibria for Toluene + Heptane + 1-Ethyl-3-
methylimidazolium Triiodide and Toluene + Heptane + 1-Butyl-3-
methylimidazolium Triiodide. J. Chem. Eng. Data 2000, 45, 841−845.
́
(22) Gonzaleza, J. E.; Domingueza, I.; Gonzaleza, B.; Canosa, J.
Liquid−liquid equilibria for ternary systems of {cyclohexane +
aromatic compounds + 1-ethyl-3-methylpyridinium ethylsulfate}.
Fluid Phase Equilib. 2010, 296, 213−218.
(23) Hanke, C. G.; Johansson, A.; Harper, J. B.; Lynden-Bell, R. M.
Why are aromatic compounds more soluble than aliphatic hydro-
carbons in dimethylimidazolium ionic liquids? A simulation study.
Chem. Phys. Lett. 2003, 374, 85−90.
(24) Zhu, S.; Wu, Y.; Chen, Q.; Yu, Z.; Wang, C.; Jin, S.; Ding, Y.;
Wu, G. Dissolution of cellulose with ionic liquids and its application: a
mini-review. Green Chem. 2006, 8, 325−327.
(25) Zhang, Q. G.; Yang, J. Z.; Lu, X. M.; Gui, J. S.; Huang, M.
Studies on an ionic liquid based on FeCl3 and its properties. Fluid
Phase Equilib. 2004, 226, 207−211.
(26) Chen, X.; Peng, Y. Chloroferrate (III) Ionic Liquid: Efficient
and Recyclable Catalyst for Solvent-free Synthesis of 3, 4-
Dihydropyrimidin-2 (1 H)-ones. Catal. Lett. 2008, 122, 310−313.
́
(27) Gonzalez, J. E.; Calvar, N.; Gonzalez, B.; Domınguez, A. Liquid-
Liquid Equilibrium for Ternary Mixtures of Hexane + Aromatic
Compounds + [EMpy][ESO4] at T = 298.15 K. J. Chem. Eng. Data
2010, 55, 633−638.
(28) Cassol, C. C.; Umpierre, P. A.; Ebeling, G.; Ferrera, B.; Chiaro,
S. X. S.; Dupont, J. On the Extraction of Aromatic Compounds from
Hydrocarbons by Imidazolium Ionic Liquids. Int. J. Mol. Sci. 2007, 8,
593−605.
(29) Seddon, K. R.; Stark, A.; Torres, M. J. Influence of chloride,
water, and organic solvents on the physical properties of ionic liquids.
Pure Appl. Chem. 2000, 72, 2275−2287.
(30) Blanchard, A. L.; Brennecke, F. J. Recovery of Organic Products
from Ionic Liquids Using Supercritical Carbon Dioxide. Ind. Eng.
Chem. Res. 2001, 40, 287−292.
(31) David, W.; Letcher, T. M.; Ramjugernath, D.; Raal, J. D. Activity
coefficients of hydrocarbon solutes at infinite dilution in the ionic
liquid, 1-methyl-3-octyl-imidazolium chloride from gas−liquid chro-
matography. J. Chem. Thermodyn. 2003, 35, 1335−1341.
(32) Zhou, Q.; Wang, L. S. Activity coefficients at infinite dilution of
alkanes, alkenes, and alkyl benzenes in 1-butyl-3-methylimidazolium
tetrafluoroborate using gas-liquid chromatography. J. Chem. Eng. Data
2006, 51, 1698−1701.
(33) Zhang, J.; Zhang, Q.; Qiao, B.; Deng, Y. Solubilities of the
gaseous and liquid solutes and their thermodynamics of solubilization
in the novel room-temperature ionic liquids at infinite dilution by gas
chromatography. J. Chem. Eng. Data 2007, 52, 2277−2283.
(34) Renon, H.; Prausnitz, J. M. Local Compositions in
Thermodynamic Excess Functions for Liquid Mixtures. AIChE J.
1968, 14, 135−144.
(35) García, J.; García, S.; Torrecilla, S. J.; Oliet, M.; Rodríguez, F.
Separation of toluene and heptane by liquid−liquid extraction using
N-methyl- N-butylpyridinium tetrafluoroborate isomers (z = 2, 3, or
4) at T = 313.2 K. J. Chem. Thermodyn. 2010, 42, 1004−1008.
(36) Meidersma, G. W.; Hansmeier, R. A.; de Haan, B. A. Ionic
Liquids for Aromatics Extraction. Present Status and Future Outlook.
Ind. Eng. Chem. Res. 2010, 49, 7530−7540.
539
dx.doi.org/10.1021/je400076x | J. Chem. Eng. Data 2014, 59, 533−539