Selective Separation of Aromatics from Paraffins and Cycloalkanes

Article pubs.acs.org/jced

Selective Separation of Aromatics from Paraffins and Cycloalkanes Using Morpholinium-Based Ionic Liquid Fan Zhang, Yong Li, Lele Zhang, Wei Sun,* and Zhongqi Ren* Beijing Key Laboratory of Membrane Science and Technology, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China S Supporting Information *

ABSTRACT: Morpholinium-based ionic liquids (MILs) have attracted increasing interest because of their good extraction performance and low toxicity. In this study, two MILs, N-benzyl-N-methylmorpholinium bis(trifluoromethylsulfonyl)imide (MIL-a) and N-allyl-N-methylmorpholinium bis(trifluoromethylsulfonyl)imide (MIL-b), were synthesized. Six ternary systems, toluene−heptane−IL (MIL-a or MIL-b), benzene− hexane−MIL (IL-a or MIL-b) and benzene−cyclohexane−IL (IL-a or MIL-b), were studied in terms of both quantum chemical calculation and liquid−liquid extraction. The calculation results showed that both MILs had stronger interaction with aromatics than with alkanes. For both cations of MILs, stronger binding energy was obtained with toluene than with benzene. The difference between MIL-a-toluene and MIL-a-cyclohexane was larger than that of MIL-b. As a result, MIL-a showed higher selectivity on toluene than MIL-b. In other ternary systems, the interaction difference was larger than that between MIL-a-benzene and MIL-a-alkanes, which led to a better selectivity of benzene on MIL-b. The liquid−liquid extraction experiment was conducted at 298.2 K and atmospheric pressure. The distribution coefficients of aromatic compounds (benzene and toluene) were over 0.60 when MIL-a was used as extractant, and above 0.50 when MIL-b was used. The selectivity was more than 80, and the distribution coefficient of toluene was over 1.4, when MIL-b was used in the ternary system with benzene and hexane. Both MILs could be reused without significant loss of selectivity and distribution coefficients.

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INTRODUCTION Aromatic compounds are obtained almost exclusively from naphtha, which also contains paraffins and cycloalkanes. The separation of aromatics and aliphatic hydrocarbons is one of the most challenging and energy consuming operations. This is mainly because aromatics have very close boiling points with paraffins and cycloalkanes, and several of them can also form azeotropes, so the conventional distillation is not suitable to separate these compounds.1 The recommended process for these separations is liquid−liquid extraction with organic compounds as solvent. The most commonly used solvents include sulfolane (UOP/Shell Process), polyethylene glycols (UDEX Process), tetraethylene glycol (Union Carbide Process), dimethyl sulfoxide (DMSO Process), N-methylpyrrolidone (Arosolvan Process), and N-formylmorpholine (Uhde/Formex Process). All these processes are recognized for their high selectivity and yield, but the solvents used in these processes are toxic and harmful to the environment. There is an urgent need to develop new and environmentally benign solvents that can allow an effective separation and keep processes more environmentally friendly.2 Ionic liquids (ILs) are regarded as an alternative for typical organic solvents, as their applications offer several environmental advantages. ILs present a negligible vapor pressure at normal temperature and pressure conditions.3−10 This interesting © 2015 American Chemical Society

property would lead to a greener process because of the easier solvent regeneration by simple operations, such as flash distillation.11 One of the most valuable benefits of ionic liquids is that their properties can be designed by tuning the chemical structures of cations and anions. Both cations and anions can be selected among a huge diverse group according to the specific system in different applications. A significant number of papers have been published on the separation of aromatic from other paraffins and cycloalkanes using ionic liquids, while most of them are still focused on the accumulation of thermodynamic data in certain solvent systems.12−18 Ebrahimi studied the liquid−liquid equilibrium for {heptane + toluene or benzene + 1-(2-hydroxyethyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([hemim] [NTf2])} systems at 313.2 K and atmospheric pressure.19 The results indicated that the addition of a hydroxyl group to an imidazolium cation significantly enhanced the selectivity of the ionic liquid. ́ Dominguez studied the separation of nonane with different aromatic compounds with 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide and 1-methyl-3-propylimidazolium bis(trifluoromethylsulfonyl)imide.20 Their selectivities on Received: November 4, 2014 Accepted: April 28, 2015 Published: May 12, 2015 1634

DOI: 10.1021/je501008b J. Chem. Eng. Data 2015, 60, 1634−1641

Journal of Chemical & Engineering Data

Article

all aromatic compounds in ternary systems were high, which confirmed that the aromatic extraction for ternary systems considered in their work was feasible with both studied ionic liquids. Larriba investigated the performance of 1-ethyl-3methylimidazolium dicyanamide, 1-butyl-3-methylimidazolium dicyanamide, and 1-ethyl-3-methylimidazolium tricyanomethanide ILs as alternative solvents in the liquid−liquid extraction of toluene from heptanes at 313.2 K.21 They also studied the liquid−liquid extraction of toluene from n-heptane with mixed ILs.22,23 The six-membered heterocycle of morpholine provided it specific selectivity to aromatics, and N-formylmorpholine has been used in industrial processes (Uhde/Formex Process) already. Morpholinium-based ILs have attracted increasing interest for their extraction performance and low toxicity24 that is better than that of the commonly used imidazolium, pyridinium, or tetraalkylammonium-based ILs.25 Some applications of morpholinium ILs reported were in electrochemistry as electrolytes and gel electrolytes.26−28 They also can be used as ionic liquid crystals, reaction media,29 or corrosion inhibitors in some other areas. In this work, we had studied the liquid−liquid extraction of aromatics (benzene/toluene) from paraffin (heptane/hexane) and cycloalkanes (cyclohexane) using morpholinium-based ILs. For this purpose, liquid−liquid extraction for the ternary systems formed by (benzene + hexane + MIL, benzene + cyclohexane + MIL, toluene + heptane + MIL) was investigated at 298.2 K and atmospheric pressure. The quantum chemical calculation on the interactions of MILs with aromatics, paraffins, and cycloalkanes was also conducted. From these results, distribution ratios and selectivity for each MIL were calculated. The reliability of the LLE data had been checked by using the Othmer−Tobias correlation.30 The separation performance of all ternary systems was also tested at 313.2 K while the initial mole fraction of aromatic compounds was 0.1.

Table 1. Specifications for Chemicals Used chemical name benzene hexane

source

mole fraction purity

analysis method

0.99 0.99

none none

0.99

none

0.99

none

0.99

none

0.99

none

0.99

none

4-methylmorpholine LiNTF2 benzyl chloride allyl chloride MIL-aa

Energy Chemical Beijing Chemical Works Beijing Chemical Works Beijing Chemical Works Beijing Chemical Works Beijing Chemical Works Beijing Chemical Works Aladdin Aladdin Aladdin Aladdin synthesized

0.99 0.99 0.99 0.99 0.99

MIL-bb

synthesized

0.99

none none none none NMRc and FTIRd NMRc and FTIRd

heptane cyclohexane toluene ethanol acetone

a

N-Benzyl-N-methylmorpholinium bis(trifluoromethylsulfonyl)imide. N-Allyl-N-methylmorpholinium bis(trifluoromethylsulfonyl)imide. c Nuclear magnetic resonance spectroscopy. dFourier transform infrared spectrometer. b

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EXPERIMENTAL SECTION Materials. All reagents are listed in Table 1. The materials were of analytical grade and used as supplied by manufacturers, without further purification. In this study, both MILs were synthesized and their structures are shown in Figure 1. N-benzyl-N-methylmorpholinium bis(trifluoromethylsulfonyl)imide(MIL-a) and N-allyl-N-methylmorpholinium bis(trifluoromethylsulfonyl)imide(MIL-b) used in liquid−liquid extraction were synthesized in our laboratory following the procedure of Pernak.24 N-Benzyl(allyl)-N-methylmorpholinium chloride were synthesized by treating equimolar (0.1 mol) quantities of N-methylmorpholine and benzyl (allyl) chloride in deionized water. The solutions were vigorously stirred at room temperature for 3 h, and after that, a clear homogeneous solution was formed. Then 0.1 mol LiNTF2 was added in the water solution and stirred for another 3 h to ensure that the reaction was complete. The product was hydrophobic and could be separated with reactant and water easily. The MILs were subjected to vacuum and moderate temperature (T = 323.15 K) for several days to remove possible traces of solvents and moisture. The water content was less than 200 ppm determined by Karl Fischer Titrino. To ensure purity, the structures of final products were checked by nuclear magnetic resonance (NMR) spectroscopy and Fourier infrared spectrometer. 1H-NMR (600 MHz, DMSO): MIL-a 7.55 (m, 5 H), 4.67(m, 2 H), 3.96 (m, 4 H), 3.52 (m, 4 H), 3.05 (s, 3 H);

Figure 1. Structures of MIL-a and MIL-b.

MIL-b 6.06 (m, 1 H), 5.67 (m, 2 H), 4.12(d, 2 H), 3.94 (m, 4 H), 3.38 (m, 4 H), 3.10 (s, 3 H). The Fourier infrared spectrometer results are reported in the Supporting Information (Figure S1−S2). Liquid−liquid Extraction Process. Liquid−liquid extraction experiments were performed by placing 10 mL of both system solution phase and MILs phase in a 50 mL tapered bottle. The operation temperature was 298.2 K and its deviation was maintained within 0.1 K. The initial mole fraction of aromatic was varied from 0.1 to 1.0. The extractions were carried out under vigorously stirring long enough to obtain equilibrium. Then, the mixture was allowed to settle for about 2 h to guarantee a complete phase split. Samples from both layers were collected for analysis. The distribution coefficient (Di) and selectivity (S) of extraction processes can be calculated as follows: Di = 1635

xiIL xiraf

(1) DOI: 10.1021/je501008b J. Chem. Eng. Data 2015, 60, 1634−1641


Article

Figure 2. Optimized geometries between IL cations and aromatic (benzene/toluene), paraffin (hexane/heptane), and cycloalkanes (cyclohexane).

S=

Daromatic Dalkane

Table 2. Interaction Energies between MILs and Aromatic, Paraffin, and Cycloalkane

(2)

where xi is the mole fraction of component i, IL and raf represent the IL phase and raffinate phase, respectively. Analytical Methods. Sample concentrations were determined by gas chromatography (GC) analysis with an internal standard method. In different ternary systems, n-octane, n-hexane, and cyclohexane were added to samples as internal standards for GC. The concentrations of aromatic compounds and alkanes in the samples were analyzed by Agilent 7890A GC with a hydrogen flame ionization detector and 0.25 mm × 30 m DB-FFAP capillary column, programmed temperature; injection and detector temperatures were 493 and 523 K, respectively. Each analysis was carried out in duplicate to ensure enough accuracy.

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RESULTS AND DISCUSSION

Interactions of MILs with Aromatic, Paraffin, and Cycloalkanes. The optimized geometries in the gas phase were calculated and performed on isolated aromatic, paraffin, cycloalkane, ionic liquid cations and their complexes, using Gaussian 09 software.31 The hybrid density functional theory (DFT), which incorporates Becke’s three-parameter exchange with Lee, Yang, and Parr’s (B3LYP) correlation functional method, was employed together with the 6-311G++ basis set.32,33 The initial geometries of all compounds were first constructed and preoptimized at the semiempirical level with the Chem3D Ultra package. Then the obtained geometry with the lowest energy was further optimized with Gaussian 09 software. For each kind of interaction between cation and aromatic or alkane, more than three different initial geometries were optimized, which meant aromatics or alkanes were located at different positions around the cations of MILs. The optimized geometry with the lowest energy was used as the global minimum for the subsequent calculation. No imaginary frequency appeared in the calculated vibrational frequencies of the optimized structures, which ensured that the obtained structures were stable. The optimized geometries between MILs and aromatic, paraffin, and cycloalkanes are shown in Figure 2. The interaction energies between MIL cations and aromatic, paraffin, and cycloalkanes were summarized in Table 2. As the results listed in Table 2, the interactions between N-benzyl-N-methylmorpholinium and aromatic compounds were −22.32 kJ/mol for toluene and −15.29 kJ/mol for

a

system

E/hartreea

E/kJ/mol

ΔE/kJ/mol

benzene cyclohexane heptane hexane toluene MIL-a MIL-a-cyclohexane MIL-a-heptane MIL-a-hexane MIL-a-toluene MIL-a-benzene MIL-b MIL-b-cyclohexane MIL-b-heptane MIL-b-hexane MIL-b-toluene MIL-b-benzene

−232.3113027 −235.9448864 −276.4785575 −237.1542223 −271.6388766 −597.9921125 −833.9389281 −874.4725494 −835.1489966 −869.6394918 −830.3092392 −444.2968941 −680.2440648 −720.7786241 −681.4528819 −715.9452972 −676.6159779

−609933.32 −619473.29 −725894.45 −622648.41 −713187.87 −1570028.29 −2189506.65 −2295927.67 −2192683.69 −2283238.48 −2179976.90 −1166501.49 −1785980.79 −1892404.27 −1789154.54 −1879714.37 −1776455.25

−5.07 −4.93 −6.99 −22.32 −15.29 −6.00 −8.33 −4.64 −25.01 −20.43

1 Hartree = 27.211 eV = 627.509 kcal/mol = 2625.753 kJ/mol.

benzene. The interactions between the cation and alkane compounds were −5.07 kJ/mol for cyclohexane, −4.93 kJ/mol for heptane, and −6.99 kJ/mol for hexane. The results showed that MIL-a had stronger interaction with aromatic compounds than that with alkanes. The similar trend was observed when the cation was N-allyl-N-methylmorpholinium. The interactions between MIL-b cation and aromatics were −25.01 kJ/mol for toluene and −20.43 kJ/mol for benzene, while the values were −6.00 kJ/mol for cyclohexane, −8.33 kJ/mol for heptane and −4.64 kJ/mol for hexane between the cation and alkane compounds. The binding energies between cations and aromatics were much larger in magnitude than that between cations and alkanes. This was mainly because the six-member heterocycle of morpholinium modified with unsaturated substituent, such as benzyl group and allyl group, could attract aromatic compounds strongly. This also led to a high solubility of aromatic compounds in both MILs. Both cations of the MILs showed stronger interaction with toluene than with benzene. This was mainly because the benzyl group on the morpholinium skeleton had a similar structure with aromatic compounds, which would increase the solubility of toluene. The binding energy differences between aromatics and alkanes with N-allyl-N-methylmorpholinium were larger than that with N-benzyl-N-methylmorpholinium. As a result, 1636



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Table 3. Experimental LLE Data on Mole Fraction (x), Distribution Coefficient (Di), and Selectivity (S) of the Ternary Systems {Heptane(1) + Toluene(2) + MIL-a/MIL-b(3); Hexane(1) + Benzene(2) + MIL-a/MIL-b(3); Cyclohexane(1) + Benzene(2) + MIL-a/MIL-b(3)} at T = 298.2 K and Atmospheric Pressurea alkane-rich layer x1

x2

1.000 0.928 0.850 0.722 0.654 0.567 0.482 0.375 0.264 0.140 0.000

0.000 0.072 0.150 0.278 0.346 0.433 0.518 0.625 0.736 0.860 1.000

1.000 0.927 0.845 0.722 0.625 0.601 0.477 0.364 0.251 0.128 0.000

0.000 0.073 0.155 0.278 0.375 0.399 0.523 0.636 0.749 0.872 1.000

1.000 0.942 0.894 0.812 0.715 0.606 0.527 0.413 0.294 0.153 0.000

0.000 0.058 0.106 0.188 0.285 0.394 0.473 0.587 0.706 0.847 1.000

a

alkane-rich layer

MIL-rich layer x1

x2

x3

Dalkane

Heptane(1) + Toluene(2) + MIL-a(3) 0.061 0.000 0.939 0.061 0.022 0.060 0.919 0.024 0.025 0.124 0.851 0.029 0.023 0.188 0.788 0.032 0.024 0.239 0.738 0.036 0.023 0.296 0.681 0.040 0.025 0.336 0.639 0.052 0.027 0.427 0.546 0.072 0.020 0.484 0.495 0.077 0.014 0.563 0.422 0.103 0.000 0.661 0.339 Heptane(1) + Toluene(2) + MIL-b(3) 0.048 0.000 0.952 0.048 0.021 0.049 0.930 0.023 0.028 0.104 0.868 0.034 0.021 0.172 0.807 0.029 0.017 0.218 0.765 0.028 0.018 0.261 0.721 0.030 0.018 0.311 0.671 0.038 0.014 0.359 0.626 0.039 0.012 0.415 0.574 0.046 0.008 0.480 0.512 0.061 0.000 0.579 0.421 Hexane(1) + Benzene(2) + MIL-a(3) 0.038 0.000 0.962 0.038 0.035 0.092 0.872 0.037 0.048 0.163 0.790 0.054 0.047 0.250 0.703 0.058 0.030 0.327 0.643 0.042 0.030 0.403 0.568 0.049 0.031 0.435 0.534 0.058 0.028 0.506 0.465 0.068 0.023 0.565 0.412 0.080 0.016 0.661 0.323 0.105 0.000 0.787 0.213

Daromatic

x1

S

0.827 0.828 0.676 0.691 0.684 0.647 0.683 0.659 0.655 0.661

35.10 28.54 21.08 19.20 16.95 12.40 9.50 8.55 6.34

1.000 0.952 0.870 0.790 0.697 0.610 0.485 0.381 0.277 0.137 0.000

0.666 0.672 0.619 0.580 0.653 0.594 0.565 0.554 0.550 0.579

29.37 20.05 21.08 20.92 21.71 15.70 14.36 12.07 8.96

1.000 0.915 0.838 0.747 0.651 0.555 0.448 0.352 0.238 0.123 0.000

1.583 1.535 1.327 1.146 1.022 0.920 0.862 0.800 0.780 0.787

42.23 28.69 23.04 27.50 20.94 15.75 12.63 10.05 7.44

1.000 0.918 0.835 0.741 0.662 0.554 0.526 0.354 0.236 0.123 0.000

x2

MIL-rich layer x1

x2

x3

Dalkane

Daromatic

Hexane(1) + Benzene(2) + MIL-b(3) 0.000 0.058 0.000 0.942 0.058 0.048 0.016 0.067 0.917 0.017 1.405 0.130 0.018 0.185 0.797 0.021 1.423 0.210 0.020 0.253 0.727 0.025 1.205 0.303 0.019 0.317 0.664 0.027 1.046 0.390 0.017 0.363 0.620 0.028 0.930 0.515 0.012 0.435 0.553 0.025 0.845 0.619 0.013 0.485 0.502 0.034 0.785 0.723 0.010 0.544 0.446 0.036 0.752 0.863 0.007 0.622 0.372 0.048 0.720 0.989 0.000 0.738 0.262 0.747 Cyclohexane(1) + Benzene(2) + MIL-a(3) 0.000 0.006 0.000 0.994 0.006 0.085 0.064 0.110 0.826 0.070 1.282 0.162 0.054 0.208 0.738 0.065 1.282 0.253 0.061 0.286 0.653 0.082 1.129 0.349 0.061 0.354 0.585 0.094 1.013 0.445 0.056 0.430 0.514 0.102 0.966 0.552 0.033 0.512 0.455 0.074 0.928 0.648 0.045 0.546 0.409 0.129 0.843 0.762 0.038 0.635 0.327 0.160 0.833 0.877 0.025 0.703 0.273 0.202 0.801 1.000 0.000 0.787 0.213 0.787 Cyclohexane(1) + Benzene(2) + MIL-b(3) 0.000 0.030 0.000 0.970 0.030 0.082 0.043 0.097 0.860 0.046 1.177 0.165 0.037 0.168 0.794 0.045 1.022 0.259 0.041 0.266 0.692 0.056 1.029 0.338 0.037 0.311 0.652 0.055 0.920 0.446 0.033 0.373 0.594 0.059 0.836 0.474 0.032 0.394 0.575 0.060 0.831 0.646 0.026 0.491 0.483 0.073 0.761 0.764 0.023 0.574 0.403 0.096 0.752 0.877 0.015 0.641 0.343 0.125 0.731 0.989 0.000 0.738 0.262 0.747

S

82.72 67.85 47.72 38.17 33.30 34.40 23.05 20.73 15.03

18.30 19.74 13.84 10.80 9.52 12.57 6.55 5.20 3.97

25.37 22.94 18.38 16.58 14.11 13.84 10.41 7.82 5.85

The standard uncertainties u for the temperature and mole fraction are u(T) = 0.1 K and u(x) = 0.01.

where wII3 is the mass fraction of ionic liquid in the MIL-rich phase (lower layer), wI1 is the mass fraction of alkane (1) in the alkane-rich phase (upper layer), a and b are the fitting parameters of the Othmer−Tobias correlation. The regression coefficients (R2) are listed in Table 4 and Figure 4. The results are very close to unity and indicated a high degree of quality of the experimental LLE data. Distribution Coefficient and Selectivity. The calculated values of distribution coefficient and selectivity for the studied ternary systems at 298.2 K are listed in Table 3 and plotted in Figure 5. To all three systems, the distribution coefficient decreased with the increase of the aromatic contents in the ternary systems. Meanwhile, better selectivity was obtained when the concentration of aromatics was low. The same trend was obtained in the experimental results studied by Ebrahimi.19 As the results showed in Table 3 and Figure 5, the selectivity of aromatic compounds decreased with the increase of aromatic concentration. Both MIL-a and MIL-b could extract aromatic compounds selectively in all three ternary systems. This was

the selectivity of aromatic with N-allyl-N-methylmorpholinium was higher. Experimental LLE Data. Experimental compositions of alkanes-rich (raffinate) and MIL-rich (extract) phases in equilibrium for ternary systems {alkanes (1) + aromatics (2) + MILs (3)} at 298.2 K and atmospheric pressure are listed in Table 3 and plotted in Figure 3. According to the 1H NMR spectra of the samples from the heptane-rich phase (Supporting Information, Figure S3), no detectable signals associated with the MIL were observed. So the mole fraction of MILs in the alkanes-rich phase appeared to be negligible. Therefore, this would simplify the experimental procedure to purify the raffinate phase for solvent recovery. To verify the reliability of the experimental LLE data, the Othmer−Tobias equation was applied:30 ⎛ 1 − w II ⎞ ⎛ 1 − wI ⎞ 3 1 ⎟ = a + b ln⎜ ⎟ ln⎜ II I ⎝ w1 ⎠ ⎝ w3 ⎠

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Figure 3. Tie-lines for the ternary mixtures (a) {heptane + toluene + MIL-a}, (b) {hexane + benzene + MIL-a}, (c) {cyclohexene + benzene MIL-a}, (d) {heptane + toluene +MIL-b}, (e) {hexane + benzene + MIL-b}, (f) {cyclohexene + benzene + MIL-b} at T = 298.2 K.

MIL-b showed better separation performance than sulfolane.34,35 The distribution coefficient of aromatic compounds were obtained from 0.60 to 1.50 when MIL-a was used as extractant, and from 0.50 to 1.40 when MIL-b was used. On the other hand, the distribution coefficients of alkane compounds were obtained from 0.02 to 0.20 when MIL-a was used as extractant, and from 0.02 to 0.10 when MIL-b was used. More aromatic compounds could dissolve in MIL-a more than in MIL-b. The distribution coefficient of n-hexane, n-heptane, and cyclohexane remained at a low level. Compared with the structures of hexane and heptane, the six-membered ring structure of cyclohexane could improve the solubility in MILs, which would lead to a relatively high distribution coefficient. The best separation performance was observed when MIL-b was used in the ternary

mainly because both unsaturated substituents on the morpholinium skeleton, including the benzyl group and allyl group, could enhance the interactions between MIL and aromatics. And this could be demonstrated by the quantum chemical calculation in the section of interaction study. For the ternary mixtures containing toluene, MIL-a showed higher solubility and selectivity on aromatic compounds than MIL-b. This was mainly because the benzyl group on the morpholinium skeleton had a similar structure to toluene, which led to an increase of both aromatic solubility and selectivity. For the other two ternary mixtures, MIL-b showed higher selectivity on benzene than MIL-a. This was mainly because the allyl on the morpholinium skeleton could improve the extraction selectivity of benzene. For the ternary system of cyclohexane− benzene−MIL and n-heptane−toluene−MIL, both MIL-a and 1638



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Table 4. Constants of the Othmer−Tobias Correlation (a, b) and Regression Coefficients (R2) for the LLE Data of the Ternary Systems {Heptane(1) + Toluene(2) + MIL-a/ MIL-b(3); Hexane(1) + Benzene(2) + MIL-a/MIL-b(3); Cyclohexane(1) + Benzene(2) + MIL-a/MIL-b(3)} at T = 298.2 K, and Atmospheric Pressure a −2.2441 −2.5080 −1.7987 −1.9858 −1.8628 −2.0198

b Heptane(1) + Toluene(2) + MIL-a(3) 0.6650 Heptane(1) +Toluene(2) + MIL-b(3) 0.6046 Hexane(1) + Benzene(2) + MIL-a(3) 0.5737 Hexane(1) + Benzene(2) + MIL-b(3) 0.5454 Cyclohexane(1) + Benzene(2) + MIL-a(3) 0.7134 Cyclohexane(1) +benzene(2) + MIL-b(3) 0.67

MILs as the temperature increased. The distribution coefficient of alkane compounds increased while that of aromatic compounds kept steady. As a result, the selectivity of aromatic compounds decreased. It meant that both MILs could selectively separate aromatic compounds from alkane compounds at relatively low temperature. After each LLE experiment, the lower phase (MIL-rich) containing MIL, benzene or toluene, and some alkanes (hexane, heptane, and cyclohexane) was dissolved in 20 mL of dichloromethane. The solvents were then separated in a rotary evaporator at reduced pressure for more than 3 h. The purity of the recovered sample was checked by GC analysis. There was no detectable aromatic or alkane compounds residue in MILs. Both MILs could be reused for more than 10 times without detectable separation performance reduction.

R2 0.9762 0.9682 0.9872

■

0.9800

CONCLUSION Quantum chemical calculation and liquid−liquid extraction of aromatics from paraffin and cycloalkanes using two morpholinium-based MILs were investigated. Six ternary systems, that is, toluene−heptane−MIL-a, benzene−hexane−MIL-a, benzene− cyclohexane−MIL-a, toluene−heptane−MIL-b, benzene− hexane−MIL-b, and benzene−cyclohexane−MIL-b, were tested in this study. Both MILs showed stronger interaction with aromatics than with alkanes. The binding energy between MIL-b cation and toluene(benzene) was stronger than that between MIL-a and aromatic compounds. In the toluene−heptane system, MIL-a showed both a higher distribution coefficient and selectivity than MIL-b. In other ternary systems, MIL-a still showed a high distribution coefficient because of the similar structure between benzyl substituents and the benzene ring. But higher selectivity of benzene was observed when MIL-b

0.9556 0.9514

system consisting of benzene and n-hexane. The selectivity was more than 80 while the distribution coefficient of toluene was over 1.4. This would be attributed to the low hexane solubility in MIL-b. In this study, the separation performance of all ternary systems with MILs used as extractant at 313.2 K was also tested as the temperature had an industry application.1 The initial mole fraction of aromatic compounds was 0.1. The results are listed in Table 5. Compared with the experimental data at 298.2 K, the selectivity of aromatic compounds decreased in all ternary systems at 313.2 K. This was mainly because more alkanes dissolved in

Figure 4. Othmer−Tobias plots for the three ternary systems consisting of alkanes−aromatics−MILs.

Figure 5. Selectivity of both toluene and benzene in MIL-a and MIL-b. 1639



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Table 5. Comparison of Separation Performance of the Ternary Systems {Heptane(1) + Toluene(2) + MIL-a/MIL-b(3); Hexane(1) + Benzene(2) + MIL-a/MIL-b(3); Cyclohexane(1) + Benzene(2) + MIL-a/MIL-b(3)} at 298.2 K and 313.2 Ka alkane-rich layer T/K

a

x1

IL-rich layer x2

298.2 313.2

0.928 0.921

0.072 0.079

298.2 313.2

0.927 0.929

0.073 0.072

298.2 313.2

0.942 0.940

0.058 0.060

298.2 313.2

0.952 0.947

0.048 0.053

298.2 313.2

0.915 0.910

0.086 0.090

298.2 313.2

0.918 0.938

0.082 0.062

x1

x2

x3

Heptane(1) + Toluene(2) + MIL-a(3) 0.022 0.06 0.919 0.032 0.059 0.909 Heptane(1) + Toluene(2) + MIL-b(3) 0.021 0.049 0.930 0.039 0.045 0.916 Hexane(1) + Benzene(2) + MIL-a(3) 0.035 0.092 0.872 0.066 0.102 0.832 Hexane(1) + Benzene(2) + MIL-b(3) 0.016 0.067 0.917 0.030 0.077 0.894 Cyclohexane(1) + Benzene(2) + MIL-a(3) 0.064 0.110 0.826 0.071 0.118 0.812 Cyclohexane(1) + Benzene(2) + MIL-b(3) 0.043 0.097 0.860 0.061 0.068 0.871

Dalkane

Daromatic

S

0.024 0.035

0.827 0.749

35.10 21.35

0.023 0.042

0.666 0.630

29.37 14.95

0.038 0.071

1.583 1.702

42.23 24.14

0.017 0.031

1.405 1.439

82.72 46.11

0.070 0.078

1.282 1.303

18.30 16.79

0.046 0.065

1.177 1.098

25.37 16.84

The standard uncertainties u for the temperature and mole fraction are u(T) = 0.1 K and u(x) = 0.01. (2) Rydberg, J.; Cox, M.; Musikas, C.; Choppin, G. R. Solvent Extraction Principles and Practice, revised and expanded ed.; Dekker: New York, 2004. (3) Rogers, R. D. Materials scienceReflections on ionic liquids. Nature 2007, 447, 917−918. (4) Earle, M. J.; Esperanca, J. M.; Gilea, M. A.; Lopes, J. N.; Rebelo, L. P.; Magee, J. W.; Seddon, K. R.; Widegren, J. A. The distillation and volatility of ionic liquids. Nature 2006, 439, 831−834. (5) Maton, C.; De Vos, N.; Stevens, C. V. Ionic liquid thermal stabilities: Decomposition mechanisms and analysis tools. Chem. Soc. Rev. 2013, 42, 5963−5977. (6) Plaquevent, J. C.; Levillain, J.; Guillen, F.; Malhiac, C.; Gaumont, A. C. Ionic liquids: New targets and media for alpha-amino acid and peptide chemistry. Chem. Rev. 2008, 108, 5035−5060. (7) Parvulescu, V. I.; Hardacre, C. Catalysis in ionic liquids. Chem. Rev. 2007, 107, 2615−2665. (8) Burrell, A. K.; Del Sesto, R. E.; Baker, S. N.; McCleskey, T. M.; Baker, G. A. The large scale synthesis of pure imidazolium and pyrrolidinium ionic liquids. Green Chem. 2007, 9, 449−454. (9) Wang, H.; Xie, C.; Yu, S.; Liu, F. Denitrification of simulated oil by extraction with H2PO4-based ionic liquids. Chem. Eng. J. 2014, 237, 286−290. (10) Wang, L. S.; Wang, X. X.; Li, Y.; Jiang, K.; Shao, X. Z.; Du, C. J. Ionic liquids: Solubility parameters and selectivities for organic solutes. AIChE J. 2013, 59, 3034−3041. (11) Huddleston, J.; Rogers, R. Room temperature ionic liquids as novel media for ‘clean’ liquid−liquid extraction. Chem. Commun. 1998, 16, 1765−1766. (12) Pereiro, A. B.; Rodriguez, A. An ionic liquid proposed as solvent in aromatic hydrocarbon separation by liquid extraction. AIChE J. 2010, 56, 381−386. (13) Bedia, J.; Ruiz, E.; de Riva, J.; Ferro, V. R.; Palomar, J.; Rodriguez, J. J. Optimized ionic liquids for toluene absorption. AIChE J. 2013, 59, 1648−1656. (14) Garcia, J.; Garcia, S.; Torrecilla, J. S.; Oliet, M.; Rodriguez, F. Liquid−liquid equilibria for the ternary systems {heptane + toluene + N-butylpyridinium tetrafluoroborate or V-hexylpyridinium tetrafluoroborate} at T = 313.2 K. J. Chem. Eng. Data 2010, 55, 2862−2865. (15) Gomez, E.; Dominguez, I.; Gonzalez, B.; Dominguez, A. Liquid−liquid equilibria of the ternary systems of alkane plus aromatic +1-ethylpyridinium ethylsulfate ionic liquid at T = (283.15 and 298.15) K. J. Chem. Eng. Data 2010, 55, 5169−5175.

was used as extractant because of the low solubility of alkanes in MIL-b. The selectivity was more than 80 while the distribution coefficient of toluene was over 1.4, when MIL-b was used in the ternary system consisting of benzene and hexane. All experimental data were checked by the Othmer−Tobias equation to verify the reliability, and the regression coefficients are very close to 1. Compared with the results at 298.2 K, the selectivity of aromatic compounds decreased in all ternary systems at 313.2 K. Both MILs could be reused for more than 10 times without detectable separation performance reduction.

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ASSOCIATED CONTENT

S Supporting Information *

FTIR of MIL-a and MIL-b; 1H NMR spectra of the samples from the heptane-rich phase. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/je501008b.

■

AUTHOR INFORMATION

Corresponding Authors

* Tel: +86-10-6442-3628. Fax: +86-10-6442-3628. E-mail: [email protected]. *E-mail: [email protected]. Funding

This work was supported by the National Natural Science Foundation (21076011 and 21276012), Program for New Century Excellent Talents in University (NCET-10-0210). The authors gratefully acknowledge these grants. Notes

The authors declare no competing financial interest.

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REFERENCES

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