ARTICLE pubs.acs.org/IECR
Activity Coefficients of Organic Solutes at Infinite Dilution in Ionic Liquids. 1. 1-Hexyl-3-Methylimidazolium Hexafluorophosphate and 1-Octyl-3-Methylimidazolium Hexafluorophosphate and Their Application to Alkane/Aromatic and Aromatic/Aromatic Hydrocarbon Separation Yi Li, Li-Sheng Wang,* Yun-Xia Feng, and Chun-Yuan Zhang School of Chemical Engineering and the Environment, Beijing Institute of Technology, Beijing 100081, People’s Republic of China
bS Supporting Information ABSTRACT: Activity coefficients at infinite dilution, γ∞ i,3, for organic solutes (alkanes, alkenes, alkyl benzenes, alcohols, chloromethanes, acetonitrile, ethyl acetate, dioxane, tetrahydrofuran, and acetone) in 1-hexyl-3-methylimidazolium hexafluorophosphate [HMIM][PF6] and 1-octyl-3-methylimidazolium hexafluorophosphate [OMIM][PF6] have been determined using gasliquid chromatography at temperatures from 303.15 to 363.15 K with the ionic liquids as the stationary phase. The partial molar excess enthalpies at infinite dilution of the ionic liquids were calculated for the solutes from the temperature dependence of the experimental activity coefficients at infinite dilution. The solubility parameters of ionic liquids were also determined by the regular solution theory. The selectivities of various ionic liquids for the alkane/aromatic hydrocarbon and aromatic/aromatic were summarized from literature and calculated from γ∞ i,3.
’ INTRODUCTION Ionic liquids (ILs) have been studied intensively in recent years. The capability of ILs to separate organic mixtures is of great importance for the development of separation processes. Activity coefficient is a fundamental thermodynamic quantity which can provide a convenient measure of solution nonideality, as well as provide information on the intermolecular energy between solvent and solute. Selectivity of the ILs with the organic solutes can be calculated from the activity coefficients at infinite dilution of solutes in ILs. The properties of ILs can be changed by varying the structure of the cations and anions such as alkyl rests and functional groups. As a designable green solvent, physicochemical and thermophysical properties of ILs have been studied systematically by various research groups in the past years. The activity coefficients at infinite dilution of solutes in ionic liquids with an alkyl carbon chain in the cations have been studied when the anion is [BF4].18 The change of the infinite dilution activity coefficients of hydrocarbons and alcohols, with the number of carbon atoms contained in the cation aliphatic chain, has been discussed. The results indicate that the γ∞ i,3 values decrease as the length of the IL alkyl chain increases.4 It is necessary to estimate the effect of changing alkyl chain when the anion is not [BF4]. The prediction using thermodynamic theory such as linear solvation energy relationship (LSER)9 and universal functional activity coefficient method (UNIFAC) is helpful when the correlation parameters are given. The correlation of parameters requires sufficient experiment data of different solutes to reach the smaller relative deviations. The γ∞ i,3 values of solutes in 1-alkyl-3-methylimidazolium hexafluorophosphate ([CnMIM][PF6]) ILs: [BMIM][PF6],1012 [HMIM][PF6],13 and [OMIM][PF6],14 have been r 2011 American Chemical Society
studied so far, and the solutes are only 17 and 24 when the IL is [HMIM][PF6] and [OMIM][PF6], respectively. This work is a continution of a study on the determination of activity coefficients γ∞ i,3 at infinite dilution of various solutes (i) by the gasliquid chromatographic method (GC) for ionic liquids 1-hexyl-3-methylimidazolium hexafluorophosphate ([HMIM][PF6]) and 1-octyl-3-methylimidazolium hexafluorophosphate ([OMIM][PF6]). In this work, the γ∞ i,3 values of solutes with more and different functional groups have been determined using GC method. The selectivities of various ionic liquids for the alkane/ aromatic and aromatic/aromatic will be summarized from literature and calculated from the γ∞ i,3 data, including the data measured in this work. Currently in the petrochemical industry sulfolane has been widely used as aromatic extraction solvent. However, the sulfolane should be replaced because of its volatility and flammability. The selectivity of sulfolane for the alkane/aromatic hydrocarbons needs to be improved. The goal of this work is to search the possibility of using ionic liquid to replace sulfolane as aromatic extraction solvent. The structureproperty relationship of ILs and the variations of the selectivity with the change of molecular structure of the ILs, such as cation and anion species, the chain length, and polarity of chain of the functional group, need to be systematically studied. The results will be helpful not only for the theoretical improvement but also for the process design of the chemical engineering community. Received: December 9, 2010 Accepted: August 19, 2011 Revised: June 29, 2011 Published: August 19, 2011 10755
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Table 1. Experimental Activity Coefficients at Infinite Dilution of Organic Solutes in the Ionic Liquid [HMIM][PF6] as the Stationary Phase from 303.15 to 363.15 K T/ K = 303.15
T/K = 313.15
T/K = 323.15
T/K = 333.15
T/K = 343.15
T/K = 353.15
T/K = 363.15
nonane
93.092
77.042
62.683
53.293
48.407
42.662
37.883
methylcyclohexane cyclohexene
43.515 11.858
31.133 10.014
22.001 8.532
18.691 7.334
14.929 6.361
13.062 5.566
10.597 4.915
solute
styrene
2.146
2.100
2.057
2.017
1.976
1.948
1.916
ethylbenzene
4.400
4.199
3.957
3.813
3.669
3.518
3.380
o-xylene
4.093
3.876
3.686
3.533
3.385
3.236
3.113
m-xylene
5.089
4.796
4.424
4.185
3.928
3.746
3.604
p-xylene
4.682
4.409
4.171
3.953
3.752
3.618
3.426
1-propanol
2.479
2.411
2.340
2.282
2.228
2.183
2.133
1-pentanol 2-methylpropanol
4.380 3.423
4.080 3.285
3.824 3.164
3.592 3.051
3.411 2.934
3.213 2.858
3.056 2.772
2-butanol
4.148
4.041
3.942
3.852
3.771
3.695
3.623
acetonitrile
1.944
1.959
1.974
1.986
1.998
2.010
2.019
ethyl acetate
3.927
4.233
4.486
4.685
5.063
5.457
5.911
tetrahydrofuran
2.674
2.810
2.931
3.094
3.176
3.486
3.618
1,4-dioxane
2.254
2.334
2.412
2.504
2.605
2.717
2.840
dichloromethane
0.784
0.800
0.822
0.844
0.860
0.878
0.894
trichloromethane tetrachloromethane
0.528 4.582
0.565 4.908
0.620 5.304
0.672 5.723
0.710 6.289
0.775 6.902
0.837 7.637
’ EXPERIMENTAL SECTION Materials. The ionic liquids [HMIM][PF6] and [OMIM][PF6] were purchased from Shanghai Cheng Jie Chemical Co., Ltd. with a purity of 99.5%, according to manufacturer’s specifications with impurities, mass fractions of chlorine: w (Cl) < 5 104; water mass fractions analyzed by Karl Fischer analysis were less than 6 104. To confirm the purity of the ILs, the chemical shifts for the 1H NMR spectrum (parts per million, C3D6O) were determined and they appear as follows: [HMIM][PF6], δ: 9.07 (s, 1H, CHN), 7.737.66 (d, 2H, CHdCHN), 4.164.12 (t, 2H, CH2Me), 3.84 (s, 3H, CH3N), 1.791.74 (t, 2H, CH2N), 1.261.22 (m, 6H, (CH2)3), 0.870.84 (t, 3H, CH2CH3); [OMIM][PF6], δ: 9.07 (s, 1H, CH-N), 7.747.67 (d, 2H, CHdCH-N), 4.154.12 (t, 2H, CH2Me), 3.84 (s, 3H, CH3N), 1.801.76 (t, 2H, CH2N), 1.261.23 (m, 10H, (CH2)5), 0.860.84 (t, 3H, CH2CH3). The solutes provided by Beijing Chemical Reagents Company were all analytical reagents and were used without further purification. Dry helium was used as the carrier gas, and tetrahydrofuran was used as solvent to coat the ionic liquid onto the solid support, as the ILs could not dissolve well in alcohols. Chromosorb W/ AW-DMCS 100/120 mesh was used as solid support for the ionic liquid in the GC column. The purity of the ionic liquids was higher than 99%. Experimental Procedure. The GC method was used for the measurements of infinite dilution activity coefficients in ionic liquid. Helium was used as carrier gas. The retention time for the solute was obtained from the differences between the standard time and the detected time for solute. Coating the solid support material with the ionic liquid was performed by dispersing a certain portion of chromosorb in a solution of the ionic liquid in tetrahydrofuran followed by evaporation of the solvent using a rotating evaporator. The mass of the stationary phase was determined with a precision
of (0.0001 g. To avoid possible residual adsorption effects of the solutes on chromosorb, the amount of ionic liquid was 60% mass fraction related to the support material. The column was filled uniformly with the help of an ultrasound vibrator. Before experiment the column was conditioned by blowing carrier gas through at high flow rate at a high temperature (about 423 K) for about 24 h. The solute was injected as a liquid to the injector at temperature 443 K. The value of the dead time t G was determined with methane as the nonretainable pure component under the assumption that the effect of the solubility of methane in the ionic liquid was negligible. Each experiment was repeated at least twice at a given temperature to check the reproducibility. Retention times were generally reproducible within 0.011 min. The GC was equipped with a thermal conductivity detector (TCD). The flow rate was stabilized for at least 30 min before any measurement was made. The inlet column pressure was determined by inner manometer. Outlet pressure was kept equal to atmospheric pressure. To check the stability of the experimental condition, such as no occurrence of elution of the stationary phase by the helium stream, measurement of retention time was repeated every 60 h for hexane and benzene. No changes in the retention time were observed for the duration of 1 month. No measurements were taken for 1-pentanol at 303.15 K and 3-methylbutanol at (303.15, 313.15) K because of their long retention times. The measurements for other solutes were all carried out at temperatures from 303.15 to 363.15 K. Experimental Uncertainties. Retention time, dead time, column temperature, flow rate, input and output pressure, and the mass of the stationary phase all have experimental errors. The error limits of the flow rate U0 were estimated to be (0.5% at column temperature and (0.01% at the flow meter temperature. The error limits of the input pressure 10756
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Table 2. Experimental Activity Coefficients at Infinite Dilution of Organic Solutes in the Ionic Liquid [OMIM][PF6] as the Stationary Phase from 303.15 to 363.15 K solute methylcyclohexane
T/K = 303.15
T/K = 313.15
T/K = 323.15
T/K = 333.15
T/K = 343.15
T/K = 353.15
T/K = 363.15
9.086
8.781
8.457
8.294
8.125
7.861
7.658
19.045 4.784
17.731 4.672
16.218 4.568
15.262 4.477
14.224 4.378
13.483 4.302
12.750 4.227
styrene
1.817
1.837
1.863
1.885
1.912
1.929
1.950
o-xylene
2.882
2.839
2.801
2.771
2.736
2.708
2.672
m-xylene
3.428
3.352
3.289
3.228
3.171
3.118
3.070
p-xylene
3.651
3.581
3.517
3.456
3.394
3.353
3.302
1-propanol
1.582
1.470
1.378
1.274
1.154
1.086
1.014
1-butanol
1.560
1.474
1.410
1.332
1.256
1.192
1.136
1.350 1.582
1.316 1.496
1.292 1.410
1.266 1.368
2,2,4-trimethylpentane cyclohexene
1-pentanol 2-propanol
1.886
1.768
1.382 1.670
2-methylpropanol
1.497
1.447
1.398
1.354
1.312
1.278
1.244
2-butanol
1.681
1.630
1.586
1.546
1.504
1.472
1.440
1.602
1.524
1.456
1.384
1.324
1.282
3-methylbutanol acetonitrile
0.761
0.773
0.787
0.801
0.818
0.835
0.854
ethyl acetate
1.883
1.908
1.935
1.982
2.026
2.077
2.131
acetone
1.117
1.137
1.153
1.171
1.187
1.202
1.217
tetrahydrofuran 1,4-dioxane
1.840 3.268
1.907 3.505
1.989 3.784
2.102 4.098
2.218 4.481
2.325 4.836
2.438 5.109
dichloromethane
0.663
0.685
0.707
0.729
0.751
0.771
0.791
trichloromethane
0.468
0.508
0.557
0.609
0.658
0.708
0.761
tetrachloromethane
2.340
2.456
2.604
2.725
2.861
2.978
3.094
Table 3. Coefficients of Equation 4, a and b, van der Waals Volumes of Solvent (ri), Solubility Parameters of Solute (δi), Activity E,∞ Derived from eq 5, and Coefficients at Infinite Dilution (γ∞ i at 298.15 K) of Solutes in [HMIM][PF6] Using eq 4, Values of Hi Standard Deviation σ solute i
a
ria/cm3 3 mol1
δia/(J 3 cm3)0.5
a
b/K
γ∞ i at 298.15 K
HE,∞ /kJ 3 mol1 i
σ
nonane
98.95
15.527
0.891
1635.92
99.055
13.602
0.0626
methylcyclohexane
70.46
16.27
4.606
2518.29
46.549
20.938
0.1280
cyclohexene
88.69
14.051
2.867
1619.04
12.977
13.462
0.0019
styrene
66.25
19.127
0.073
209.22
2.171
1.740
0.0035
ethylbenzene
69.74
18.043
0.103
480.19
4.516
3.993
0.0131
o-xylene m-xylene
70.66 70.66
18.453 18.09
0.238 0.636
499.23 690.71
4.205 5.371
4.151 5.743
0.0092 0.0275
p-xylene
70.66
17.838
0.323
565.51
4.828
4.702
0.0184
1-propanol
42.17
24.557
0.016
266.03
2.509
2.212
0.0033
1-pentanol
62.63
22.576
0.774
686.38
4.608
5.707
0.0231
2-methylpropanol
52.39
23.751
0.051
390.77
3.496
3.249
0.0066
2-butanol
52.39
22.63
0.604
247.06
4.205
2.054
0.0006
acetonitrile
28.37
24.094
0.896
70.12
1.937
0.583
0.0010
ethyl acetate tetrahydrofuran
52.77 44.62
18.346 19.129
3.755 2.804
726.96 555.59
3.731 2.561
6.044 4.619
0.0187 0.0262 0.0015
1,4-dioxane
44.62
20.163
2.190
420.8
2.178
3.499
dichloromethane
34.71
20.378
0.567
245.43
0.772
2.041
0.0048
trichloromethane
43.5
19.028
2.137
845.02
0.498
7.026
0.0240
tetrachloromethane
52.3
17.577
4.573
933.54
4.229
7.762
0.0077
Values obtained from literature.22
Pi and the output pressure Po were estimated to be (0.6% and (0.02%, respectively. The uncertainty of determining n3 was estimated to be (0.5%. The uncertainty of the cross
virial coefficient estimation was 25%. Based on the error propagation law, γ∞ i,3 was estimated to have a relative uncertainty within (5%. 10757
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Table 4. Coefficients of Equation 4, a and b, van der Waals Volumes of Solvent (ri), Solubility Parameters of Solute (δi), Activity Coefficients E,∞ Derived from eq 5, and Standard Deviation σ at Infinite Dilution (γ∞ i at 298.15 K) of Solutes in [OMIM][PF6] Using eq 4, Values of Hi
a
γ∞ i at 298.15 K
ria/cm3 3 mol1
δia/(J 3 cm3)0.5
methylcyclohexane
70.46
16.27
1.199
305.00
9.222
2.536
0.0104
2,2,4-trimethylpentane cyclohexene
88.69 57.04
14.051 17.611
0.497 0.814
742.44 227.82
19.830 4.846
6.173 1.894
0.0125 0.0017
solute i
a
b/K
HE,∞ /kJ 3 mol1 i
σ
styrene
66.25
19.127
1.030
131.58
1.801
1.094
0.0026
o-xylene
70.66
18.453
0.609
136.28
2.904
1.133
0.0023
m-xylene
70.66
18.09
0.566
201.81
3.465
1.678
0.0008
p-xylene
70.66
17.838
0.686
184.59
3.688
1.535
0.0021
1-propanol
42.17
24.557
2.270
830.04
1.681
6.901
0.0282
1-butanol
52.4
23.289
1.481
586.17
1.624
4.874
0.0164
1-pentanol 2-propanol
62.63 62.62
22.576 21.671
0.541 1.347
279.50 600.50
1.493 1.949
2.324 4.993
0.0087 0.0476
2-methylpropanol
52.39
23.751
0.724
340.25
1.527
2.829
0.0039
2-butanol
52.39
22.63
0.419
285.33
1.707
2.372
0.0040
3-methylbutanol
62.62
22.322
1.177
516.43
1.741
4.294
0.0091
acetonitrile
28.37
24.094
0.417
211.02
0.748
1.754
0.0008 0.0071
ethyl acetate
52.77
18.346
1.378
229.19
1.839
1.906
acetone
39.04
19.774
0.626
156.06
1.108
1.298
0.0009
tetrahydrofuran 1,4-dioxane
44.62 44.62
19.129 20.163
2.339 3.962
528.94 846.45
1.759 3.074
4.398 7.038
0.0233 0.0318 0.0018
dichloromethane
34.71
20.378
0.660
324.99
0.651
2.702
trichloromethane
43.5
19.028
2.204
900.12
0.443
7.484
0.0086
tetrachloromethane
52.3
17.577
2.561
519.01
2.270
4.315
0.0063
Values are obtained from literature.22
Figure 1. Plot of ln γ∞ i of [HMIM][PF6] versus 1/T for the following solutes: 9, nonane; b, methylcyclohexane; 2, cyclohexene; 1, styrene; (, ethylbenzene; 0, o-xylene; O, m-xylene; Δ, p-xylene; — linear correlation.
’ THEORETICAL BASIS The equation proposed by Everett15 in 1965 and Cruickshank et al.16 in 1966 was used for obtaining the γ∞ i,3 for a volatile solute (i) in an involatile solvent (3) from GC results: n3 RT B11 Vi0 0 2B12 Vi∞ Pi þ JPout ¼ ln ln γ∞ i, 3 0 VN Pi RT RT ð1Þ
Figure 2. Plot of ln γ∞ i of [HMIM][PF6] versus 1/T for the following solutes: 9, 1-propanol; b, 1-pentanol; 2, 2-methylpropanol; 1, 2-butanol; 0, acetonitrile; O, ethyl acetate; Δ, tetrahydrofuran; 3, 1,4dioxane; (, dichloromethane; ), trichloromethane; , tetrachloromethane; — linear correlation.
where R (J 3 K1 3 mol1) is the gas constant, T (K) is the temperature, γ∞ i,3 is the activity coefficient of solute i at infinite dilution in the stationary phase 3, P0i (kPa) is the vapor pressure of the pure liquid solute i, and n3 is the number of moles of the stationary phase component on the column. The second and third terms in eq 1 are correction terms that arise from the nonideality of the mobile gaseous phase. B11 (cm3 3 mol1) is 10758
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Figure 3. Plot of ln γ∞ i [OMIM][PF6] versus 1/T for the followin solutes: 9, methylcyclohexane; b, 2,2,4-trimethylpentane; 2, cyclohexene; 1, styrene; (, o-xylene; 0, m-xylene; O, p-xylene; Δ, dichloromethane; 3, trichloromethane; , tetrachloromethane. — linear correlation.
Figure 5. Plot of ln γ∞ i of solutes in [CnMIM][PF6] as a function of carbon chain n: 9, cyclohexene; O, m-xylene; Δ, 1-propanol; 2, dichloromethane; 3, trichloromethane; 1, tetrachloromethane. The ln γ∞ i of solutes in [BMIM][PF6] are obtained from refs 1012.
Figure 4. Plot of ln γ∞ i [OMIM][PF6] versus 1/T for the following solutes: 9, 1-propanol; b, 1-butanol; 2, 1-pentanol; 1, 2-propanol; (, 2-methylpropanol; 0, 2-butanol; O, 3-methylbutanol; Δ acetonitrile; 3, ethyl acetate; ), acetone; , tetrahydrofuran; +, 1,4-dioxane; — linear correlation.
Figure 6. Residual function Y for [HMIM][PF6] and [OMIM][PF6] versus solute solubility parameters δi. Regression line: Y = 18.145 δi 221057 for [HMIM][PF6] and Y = 17.631 δi 208693 for [OMIM][PF6]. 9, for Y versus δi of [OMIM][PF6]; b, for Y versus δi of [HMIM][PF6].
3
1
the second virial coefficient of the solute, B12 (cm 3 mol ) is the cross second virial coefficient of the solute (1) with the carrier gas (2), V0i (cm3 3 mol1) is the liquid molar volume of 3 1 pure solute, and V∞ i (cm 3 mol ) is the partial molar volume of the solute in the ionic liquid at infinite dilution. VN (cm3 3 mol1) is the standardized retention volume defined by " # Tcol Pw0 1 VN ¼ JU0 ðtr tG Þ ð2Þ Tf Pout where tr (min) is the retention time, tG (min) is the dead time, U0 (mL 3 min1) is the flow rate, measured by a soap bubble flow meter, Tcol (K) is the column temperature, Tf (K) is the flow meter temperature, P0w (kPa) is the vapor pressure
of water at Tf (K), and Pout (kPa) is the pressure at the column outlet. The pressure correction term J appearing in eq 1 and eq 2 is given by20 # " 3 ðPin =Pout Þ2 1 J ¼ ð3Þ 2 ðPin =Pout Þ3 1 where Pin and Pout are the inlet and outlet pressures of the GC column, respectively. The data for calculating the correction terms have been obtained in the following way. For all solutes, values of P0i were calculated from the Antoine equation, with Antoine constants given by Boublik et al.17 Molar volumes of solutes V0i were estimated using experimental values of their densities; partial molar volumes of solute at infinite dilution V∞ i were assumed to be equal to V0i . The third term of eq 1 is much less 10759
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than the first and second terms, the γ∞ i,3 calculated from the partial molar volume is similar to the assumed V∞ i . Values of B11 and B12 were estimated based on the equations suitable for nonpolar liquids by Tsonopolous’ method18 with an uncertainty of < (10 cm3 3 mol1. The critical parameters needed for the calculations were available from the literature.18 The mixed critical properties Pcij, Tcij, Vcij, and Zcij and mixed acentric factor ωij were calculated by using equations given in the literature.18,19 The outlet pressure Pout was kept equal to the atmospheric pressure. The results of γ∞ i,3 were correlated with temperature by the following equation ln γ∞ i, 3 ¼ a þ
b ðT=KÞ
ð4Þ
) was The partial molar excess enthalpy at infinite dilution (HE,∞ i calculated from GibbsHelmholtz equation ∂ln γ∞ H E, ∞ i, 3 ¼ i ∂ð1=TÞ R
where the partial derivatives of ln γ∞ i with respect to 1/T were obtained from the slope of a straight line derived from eq 4. The selectivities, S∞ ij can be calculated from the equation given by ∞ ∞ S∞ ij ¼ γi3 =γj3
δs 104/(J 3 m3)1/2 a this work
ref 4
ref 24
ref 25
[EMIM][Tf2N]
1.93
2.75
[BMIM][Tf2N]
2.09
2.65
[HMIM][Tf2N] [BMIM][BF4]
2.43
[HMIM][BF4]
2.33
[OMIM][BF4] [BMIM][PF6]
2.25
[HMIM][PF6]
2.44
[OMIM][PF6]
2.37
1.95
2.52
2.48
2.65
2.66 2.27
2.66 2.95
ref 26
ref 27 2.27
2.13 2.06
2.66 2.51
2.53
ð6Þ
The activity coefficients at infinite dilution can be represented ) can be by a two-term eq 7: the combinatorial term (ln γ∞comb i represented by the modification21 to Flory’s equation, the ) is given by the regular solution theory: residual term (ln γ∞res i
Table 5. Solubility Parameters of the Ionic Liquids
IL
ð5Þ
∞comb ln γ∞ þ ln γ∞res i ¼ ln γi i
ð7Þ
¼ ln½ðri =rs Þ2=3 þ 1 ðri =rs Þ2=3 ln γ∞comb i
ð8Þ
¼ ðvi =RTÞðδi δs Þ2 ln γ∞res i
ð9Þ
3
1
where ri (cm 3 mol ) is the van der Waals volumes of solute; rs (cm3 3 mol1) is the van der Waals volumes of solvent; vi (cm3 3 mol1) is the solute molar volume; δi (J 3 cm3)0.5 and δs (J 3 cm3)0.5 are the solubility parameters of solute and solvent, respectively. Information on vi, δi, and ri was obtained from the literature.22 The van der Waals volumes of ILs were calculated by group contribution method.23 A residual function Y can be rearranged from eq 9:4
Values are at 25 °C, except those estimated in refs 25 and27, which are at 30 °C.
Yi ¼
a
ln γ∞res δi 2 2δs δs 2 i ¼ δi þ vi RT RT RT
ð10Þ
Table 6. Selected Ionic Liquids Used in the Calculations for Alkane/Aromatic and Aromatic (1)/Aromatic (2) Hydrocarbon Separation Problems ionic liquid
abbreviation
ref
1-ethyl-3-methylimidazolium tetrafluoroborate
[EMIM] [BF4]
1-butyl-3-methylimidazolium tetrafluoroborate
[BMIM][BF4]
33 34
1-hexadecyl-3-methylimidazolium tetrafluoroborate
[C16MIM] [BF4]
35
1-ethyl-3-methylimidazolium trifluoromethanesulfonate
[EMIM] [TfO]
36
1-butyl-3-methylimidazolium trifluoromethanesulfonate
[BMIM] [TfO]
37
1-hexyl-3-methylimidazolium trifluoromethanesulfonate
[HMIM] [TfO]
38
1-methyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide
[MMIM] [NTf2]
39
1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide
[EMIM] [NTf2] [BMIM] [NTf2]
39 39 40
n-ethylpyridinium bis(trifluoromethylsulfonyl)imide
[EPY] [NTf2]
1-methyl-3-methylimidazolium methylsulfate
[MMIM] [MeSO4]
40
1-ethyl-3-methylimidazolium methylsulfate
[EMIM] [MeSO4]
39
1-ethyl-3-methylimidazolium ethylsulfate
[EMIM] [ESO4]
39
1-methyl-3-methylimidazolium methoxyethylsulfate
[MMIM] [MASO4]
40
1-methyl-3-methylimidazolium dimethylphosphate
[MMIM] [(CH3)2PO4]
40
1-propyl-2,3-dimethylimidazolium tetrafluoroborate 1-(2-hydroxyethyl)-3-methylimidazolium tetrafluoroborate
[PDMIM][BF4] [HydEMIM][BF4]
28 29 30
1-butyl-3-methylimidazolium trifluoromethanesulfonate
[BMIM][CF3SO3]
1-(methyl)acryloyloxyalkyl-3-methylimidazolium bromide
[MAOOMIM]Br
31
1-hexyl-3-methylimidazolium hexafluorophosphate
[HMIM][PF6]
this work
1-octyl-3-methylimidazolium hexafluorophosphate
[OMIM][PF6]
this work
10760
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Table 7. Selectivities S∞ 12 of Alkanes (1)/Aromatic (2) Hydrocarbon for Ionic Liquids and Sulfolane at T = 323.15 K S∞ 12
ionic liquid
(2) benzene
toluene
ethylbenzene
o-xylene
m-xylene
p-xylene
n-pentane
38.58
22.52
10.80
15.34
11.80
12.49
n-hexane
59.88
34.95
16.76
23.81
18.32
19.39
n-heptane n-octane
98.56 152.13
57.52 88.79
27.58 42.57
39.19 60.48
30.15 46.53
31.91 49.26
(1) [PDMIM][BF4]a
[HydEMIM][BF4]b
[BMIM][CF3SO3]c,
[HMIM][PF6]d [OMIM][PF6]d sulfolanee
a
n-nonane
192.86
112.56
53.97
76.68
58.99
62.45
n-decane
1212.94
707.91
339.43
482.24
371.01
392.74
cyclohexane
26.67
15.57
7.46
10.60
8.16
8.64
methylcyclo- hexane
43.00
25.10
12.03
17.10
13.15
13.92
2,2,4-trimethylpentane
106.76
62.31
29.87
42.44
32.65
34.57
n-heptane
131.51
67.48
30.67
43.14
33.69
35.24
n-octane n-nonane
63.10 63.99
32.38 32.84
14.72 14.93
20.70 20.99
16.17 16.39
16.91 17.15
cyclohexane
53.65
27.53
12.52
17.60
13.75
14.38
methylcyclo- hexane
46.32
23.77
10.80
15.20
11.87
12.41
2,2,4-trimethyl-pentane
80.34
41.23
18.74
26.36
20.58
21.53
cyclopentane
12.89
8.65
5.54
6.59
5.59
5.77
cyclohexane
19.94
13.38
8.57
10.19
8.64
8.93
cycloheptane
27.42
18.40
11.78
14.02
11.88
12.28
cyclononane nonane
38.87 na
26.08 na
16.70 15.84
19.87 17.01
16.84 14.17
17.41 15.03
methylcyclo- hexane
na
na
5.56
5.97
4.97
5.28
methylcyclo- hexane
na
na
na
3.02
2.57
2.41
2,2,4-trimethylpentane
na
na
na
5.79
4.93
n-pentane
14.97
9.79
n-hexane
20.52
n-heptane
26.94
cyclohexane methylcyclo- hexane
12.52 17.36
4.61
6.50
na
na
na
13.41
8.90
na
na
na
17.61
11.69
na
na
na
8.18 11.35
5.43 7.53
na na
na na
na na
Ref 28. b Ref 29. c Ref 30 (328.15 K). d This work. e Ref 32; na: not analyzed.
The values of experimental γ∞ were obtained from this work, the values of ln γ∞comb were calculated by eq 8, the solubility parameters δi of different solutes are already known, then the values of Yi for each solvent were calculated by eq 10. Equation 10 shows that there is a linear relation between Yi and the solute solubility parameter δi for a given solvent and temperature T. The value of the solvent solubility parameter δs can be obtained from the slope of this line.
’ RESULTS AND DISCUSSION The values of γ∞ i,3 of solutes in [HMIM][PF6] and [OMIM][PF6] obtained over a temperature range from 303.15 to 363.15 K are listed in Tables 1 and 2. The coefficients a and b, γ∞ i,3 at the standard condition (T = 298.15 K) calculated using eq 4, the standard deviation σ of the fitted equation, and values of HE,∞ i derived from eq 4 are listed in Tables 3 and 4. The plots of measured ln γ∞ i,3 vs 1/T values are given in Figures 1, 2, 3, and 4. The γ∞ i,3 values for 1-pentanol and 3-methylbutanol were not measured at the temperature 303.15 K and 303.15, 313.15 K, respectively, because the retention times were far beyond the scope allowed by GC. Figure 5 shows the activity coefficients at infinite dilution of cyclohexene, m-xylene, 1-propanol,
dichloromethane, trichloromethane, and tetrachloromethane in [CnMIM]PF6 (n = 4, 6, 8) ionic liquids at 313.15 K. The γ∞ i,3 decreased with the increasing of carbon chain length on the was positive and increased with the chain length cations. HE,∞ i of the linear alkanes. The linear dependence between Yi and δi obtained from the γ∞ data on [HMIM][PF6] and [OMIM][PF6] is displayed in Figure 6. The solubility parameters of these two ILs calculated by this procedure, and the reported solubility parameters4,2427 of 1-alkyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide [CnMIM][Tf2N] and 1-alkyl-3-methylimidazolium tetrafluoroborate [CnMIM][BF4], are listed in Table 5. Besides the solubility parameters reported by ref 24, the other data show that the value of solubility parameter will decrease with the carbon chain length on the cation with same anion. The selectivities of various ionic liquids and sulfolane for the alkane/aromatic hydrocarbon, and various ionic liquids for aromatic/aromatic are summarized from literature and calculated from the γ∞ i,3 data. The candidate ILs included 1-alkyl-3-methyllimidazolium tetrafluoroborate ([CnMIM][BF4]),3335 1-alkyl-3-methyllimidazolium trifluoromethanesulfonate ([CnMIM][TfO]),3638 1-alkyl-3-methyllimidazolium bis(trifluoromethylsulfonyl)imide ([C nMIM][NTf2]), 39 and 10761
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Table 8. Selectivities S∞ 12 of Aromatic (1)/Aromatic (2) Hydrocarbon for Ionic Liquids at T = 323.15 K S∞ 12
ionic liquid
(2) (1) [MAOOMIM]Bra
[HydEMIM][BF4]b
[HMIM][PF6]c
[OMIM][PF6]c
a
benzene
benzene
toluene
ethylbenzene
0.457
o-xylene
m-xylene
p-xylene
0.193
0.279
0.210
na
0.422
0.613
0.461
na
1.447
1.089 0.756
na na
toluene
2.190
ethylbenzene o-xylene
5.190 3.581
2.370 1.632
0.691
m-xylene
4.762
2.170
0.918
1.323
0.513
0.233
0.328
0.256
0.268
0.455
0.639
0.499
0.522
1.406
1.099
1.149
0.781
0.817
benzene toluene
1.950
ethylbenzene
4.290
2.20
o-xylene
3.048
1.564
0.711
m-xylene
3.903
2.003
0.910
1.281
p-xylene ethylbenzene
3.732 na
1.915 na
0.870
1.224 1.074
o-xylene
na
na
0.932
m-xylene
na
na
1.118
1.200
p-xylene
na
na
1.054
1.132
o-xylene
na
na
na
m-xylene
na
na
na
1.174
p-xylene
na
na
na
1.256
na
1.046 0.956 0.894
0.949
0.833
0.884 1.061
0.943 0.852
0.796 0.935
1.069
Ref 31. b Ref 29. c This work; na: not analyzed.
1-alkyl-3-methyllimidazolium methylsulfate.40 Four ionic liquids: [PDMIM][BF4],28 [HydEMIM][BF4],29 [BMIM][CF3SO3],30 and [MAOOMIM]Br31 are found to have significant separation effect for the alkane/aromatic hydrocarbon and aromatic/ aromatic separation problems. They are listed in Table 6 together with the ionic liquids [HMIM][PF6 ] and [OMIM][PF6] investigated in this work. The selectivities of [CnMIM][BF4],3335 [CnMIM][TfO],38 and [CnMIM][NTf2]39 for the alkane/aromatic hydrocarbon mixtures separation show that the selectivity increases with increasing of number of organic groups on the cation. The values of selectivities decrease with increasing number of carbon atoms of aliphatic chain on cation for this separate problem. The comparison of [HMIM][PF6] and [OMIM][PF6] in Table 7 confirms this phenomenon. Table 7 shows that most of the investigated ionic liquids can be used as aromatic extraction solvents from alkane/aromatic hydrocarbon mixtures. The selectivities S∞ ij of [PDMIM][BF4] (with three organic groups on the cation) are higher than other ILs (with two organic groups on the cation). All ionic liquids listed in Table 7 show good separation ability, [PDMIM][BF4] and [HydEMIM][BF4] are much better than the experimental selectivities of sulfolane.32 Table 8 shows the selectivities S∞ 12 of ionic liquids for aromatic/aromatic hydrocarbon mixtures. The results of selectivity by using [HMIM][PF6] and [OMIM][PF6] to separate mxylene (1) and o-xylene (2), the results of S∞ 12 are slightly different from 1.200 to 1.174, and for p-xylene (1) and o-xylene (2) the results of S∞ 12 are different from 1.132 to 1.256. Compared with the [BMIM][BF4],34 by increasing the polarity of chain on cation, for example using [HydEMIM][BF4] to separate the mixture of xylenes, higher selectivities are obtained. The result of selectivity for the mixture of m-xylene (1) and o-xylene (2) is ∞ S∞ 12 = 1.281 and for p-xylene (1) and o-xylene (2) is S12 = 1.224.
From Table 8, it can be seen that [MAOOMIM]Br has the highest selectivity only for separation of m-xylene (1) and oxylene (2), for which the result is S∞ 12 = 1.323. For the comparison of type of anions, the IL with [BF4] will be better than that with [PF6] for the same kind of cations. This is probably because the hydrophobic properties of [PF6] are higher than [BF4]. Currently the experimental data are not enough; more activity coefficient data at infinite dilution for different ionic liquids are required to improve the understanding on the structures of ILs and their selectivity.
’ CONCLUSIONS Activity coefficients at infinite dilution of selected organic solutes in two ionic liquids [HMIM][PF6] and [OMIM][PF6] have been measured at the temperature range from 303.15 to 363.15 K. From the results of measurements for the two ILs, the γ∞ i,3 value decreases with increasing of carbon chain length on the cations. The partial molar excess enthalpies at infinite dilution of the ionic liquids were calculated for the solutes from the temperature dependence of the experimental activity coefficients was positive and increased at infinite dilution. The value of HE,∞ i with the chain length of the linear alkanes. The solubility parameters of ionic liquids were also determined by the regular solution theory. Solubility parameters decrease with the carbon chain length on the cation of ionic liquid with same anion. The selectivities of various ionic liquids for the alkane/aromatic and aromatic/aromatic were summarized from literature and calculated from γ∞ i,3 obtained in this study. The results show applicability of the ILs to separation processes, and the selectivity shows potential of ILs for application to petrochemical separation processes. Currently the experimental data are not enough, 10762
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’ ASSOCIATED CONTENT
bS
Supporting Information. Additional data. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Fax: +86-10-68911040. Tel: +86-10-68912660.
’ ACKNOWLEDGMENT Funding for this research was provided by the Beijing Institute of Technology. ’ NOMENCLATURE a = intercept of eq 4 b = slope of eq 4 B11 = second virial coefficient of pure solute B12 = cross virial coefficient of solute and carrier gas = heat of solution at infinite dilution for solute in ionic HE,∞ i liquid J = J factor n3 = amount of substance for ionic liquid P = pressure P0i = saturation pressure of solute P0w = saturation pressure of water at temperature of flow meter Pin = inlet pressure of the GC column Pout = outlet pressure of the GC column R = gas constant ri = van der Waals volume of solute rs = van der Waals volume of solvent S∞ 12 = selectivity at infinite dilution to separate component 1 and 2 tr = retention time tG = dead time Tf = flow meter temperature T = absolute temperature Tcol = column temperature U0 = flow rate vi = solute molar volume δi = solubility parameters of solute δs = solubility parameters of solvent VN = standardized retention volume V0i = liquid molar volume of pure solute Yi = residual function of solute i γ∞ i,3 = limiting activity coefficient of component i in stationary phase 3 ’ SUPERSCRIPTS ∞ = infinite dilution comb = combinatiorial term res = residual term ’ SUBSCRIPTS i, j = solutes s = solvents
ARTICLE
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