Aliphatic Mixtures

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Energy & Fuels 2007, 21, 1724-1730

Extraction of Aromatic Hydrocarbons from Aromatic/Aliphatic Mixtures Using Chloroaluminate Room-Temperature Ionic Liquids as Extractants Jie Zhang, Chongpin Huang,* Biaohua Chen, Pengju Ren, and Zhigang Lei State Key Laboratory of Chemical Resource Engineering, Beijing UniVersity of Chemical Technology, Beijing 100029, People’s Republic of China ReceiVed NoVember 30, 2006. ReVised Manuscript ReceiVed February 14, 2007

The extraction of aromatic hydrocarbons from aromatic/aliphatic mixtures was investigated using chloroaluminate ionic liquids as extractants. Three types of chloroaluminate ionic liquids, i.e., 1-butyl-3methylimidazolium chloride-aluminum chloride (BMIC/AlCl3), trimethylamine hydrochloride- aluminum chloride (Me3NHCl/AlCl3), and triethylamine hydrochloride-aluminum chloride (Et3NHCl/AlCl3), were prepared and used to extract aromatic hydrocarbons. Chloroaluminate ionic liquids have strong aromatic hydrocarbon solvent capacities, small solvent capacities for n-heptane, and good extractive performances. BMIC-2.0AlCl3 exhibits better extractive performance than Me3NHCl-2.0AlCl3 and Et3NHCl-2.0AlCl3. Both the benzene distribution coefficient and aromatic/n-heptane selectivity increase with an increasing ratio of AlCl3/organic salt (Et3NHCl) in ionic liquids. The steric effect of substituent groups on the benzene ring lowers the aromatic extractive performance. The π complextion between aromatic molecules with highly delocalized π electron and Lewis acid species (Al2Cl7- or AlCl3) facilitates the aromatic absorption of chloroaluminate ionic liquids. A lower temperature is favorable for aromatic extraction of the ionic liquids. The regeneration tests show that the used ionic liquids can be recovered through vacuum distillation effectively.

1. Introduction Separation of aromatic hydrocarbons (benzene, toluene, and xylene) from C4 to C10 aliphatics is an important step in the production of the fuels, as well as in the production of the basic chemical feedstock. Aromatics are premium blending stocks for motor fuels, because the high-octane-number hydrocarbons in the gasoline boiling range are primarily aromatic hydrocarbons, which come from catalytic reformates. Moreover, some petroleum processes, such as naphtha steam cracking, need the removal of aromatic hydrocarbons to purify products and lower operation costs. The separation of aromatics from C4 to C10 aliphatic alkane mixtures is challenging because these hydrocarbons have close boiling points and sometimes form azeotropes.1 The liquid-liquid extraction has been widely used in industrial aromatic hydrocarbon separation and purification because of mild operation conditions and simple processes.2 The extractant is a key factor for the extraction process. Some conventional polar organic chemicals, such as glycol, sulfolane, tetraethylene glycol, and N-methylpyrrolidone, are used in “Volex”, “Shell”, “Tetra”, and “Arsolvan” processes, which have been extensively applied to commercial extraction of aromatic hydrocarbons from aromatic/aliphatic mixtures.3 However, the high volatility of these chemicals results in the loss of * To whom correspondence should be addressed: Postbox 35, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China. Telephone: +86-10-64412054. Fax: +86-10-64419619. E-mail: [email protected]. (1) Meindersma, G. W.; de Haan, A. B. Desalination 2002, 149, 29. (2) Agulyansky, A.; Agulyansky, L.; Travkin, V. F. Chem. Eng. Process. 2004, 43, 1231. (3) Ali, S. H.; Lababidi, H. M. S.; Merchant, S. Q.; Fahim, M. A. Fluid Phase Equilib. 2003, 214, 25.

extractants. Moreover, these organic solvents are generally toxic and flammable. It is of particular interest to develop new extractants with a high distribution coefficient of aromatic hydrocarbons, high selectivity of aromatics to alkane, and little solvent loss. Ionic liquids that consist of organic cations and inorganic anions exist in the liquid state at ambient temperatures. Ionic liquids have received great attention as new kinds of safe solvents with applications in electrochemistry, catalysis, biocatalysis, and polymerization in the past decade.4-8 Ionic liquids are typically nonvolatile and nonflammable and have high thermal stability and strong polarity compared with conventional solvents. In general, ionic liquids have a higher density than organic liquids. Therefore, when ionic liquids are contacted with saturated aliphatic hydrocarbons, they exist as a separate phase and show very low solubility of saturated aliphatic hydrocarbons. Nonvolatility and thermal stability of ionic liquids minimize the loss of solvents and facilitate the regeneration of used ionic liquids by distillation following the extraction process. Hence, the extraction of aromatic hydrocarbons from mixed aromatic/ aliphatic streams with ionic liquids is expected to require less procedures and energy consumption than conventional solvents. In recent years, significant progress has been made in the applications of room-temperature ionic liquids in some extraction processes, such as desulfurization, denitrogenation, and (4) Holbrey, J. D.; Seddon, K. R. Clean Prod. Processes 1999, 1, 223. (5) Fischer, T.; Sethi, A.; Welton, T.; Woolf, J. Tetrahedron Lett. 1999, 40, 793. (6) Dyson, P. J.; Ellis, D. J.; Parker, D. G.; Welton, T. Chem. Commun. 1999, 25. (7) Kubisa, P. Prog. Polym. Sci. 2004, 29, 3. (8) Zulfiqar, F.; Kitazume, T. Green Chem. 2000, 2, 296.

10.1021/ef060604+ CCC: $37.00 © 2007 American Chemical Society Published on Web 04/04/2007

Extraction of Aromatic Hydrocarbons

Energy & Fuels, Vol. 21, No. 3, 2007 1725

2. Experimental Section

Figure 1. Structure of 1-butyl-3-methylimidazolium, trimethylquaternary ammonium, and triethyl-quaternary ammonium cations.

separation of lactic acids and metal ions from water.9-12 The extractions of aromatic hydrocarbons from aromatic/aliphatic mixtures by neutral ionic liquids have been investigated.13 These ionic liquids consist of large organic cations based on one- and three-site-substituted methylimidazolium and anions, such as hexafluorophosphate, tetrafluoroborate, alkylsulfates, alkylsulfonates, chloride, bromide, nitrate, sulfate, triflate, bis(trifyl)imide, etc. Nevertheless, the solvent capacities for aromatic hydrocarbons of neutral ionic liquids are restricted because the interaction between neutral ionic liquids and aromatic hydrocarbons is weaker than the chemical interaction, such as π complexation, between aromatic hydrocarbons and Lewis acid liquids. The aromatic molecules have a highly delocalized π electron, which can interact with Lewis acid sites strongly. Thus, Lewis acid ionic liquids would have higher solvent capacities for aromatics. According to the paper of Zhang et al.,10 the solvent capacity of BMIMPF6 (neutral ionic liquid) for thiophene is about half of AlCl3-based ionic liquids. The separation of aromatics from aromatic/aliphatic mixtures using Lewis acid ionic liquids can be classified into complexation extraction with a high capacity and selectivity. Chloroaluminate room-temperature ionic liquids are typical Lewis acid ionic liquids and easy to prepare. Moreover, they have a low cost and low melting points.14 The representative organic cations are 1-butyl-3-methylimidazolium, trimethylquaternary ammonium, and triethyl-quaternary ammonium, as shown in Figure 1. The anions are AlCl4- and Al2Cl7-. The viscosities of AlCl3-based ionic liquids decrease with an increasing AlCl3 concentration in the liquids. Therefore, the chloroaluminate ionic liquids with a high AlCl3 concentration are more attractive to serve as extractive solvents. However, because the data about the extractive performance of chloroaluminate ionic liquids used as extractants to separate aromatics from aromatic/aliphatic mixtures are lacking, some experiments of chloroaluminate ionic liquids should be carried out to evaluate their suitability for separating applications. In this study, we focused on the separation of aromatics in the range below 15% (molar fraction) aromatics in the aromatic/ aliphatic mixtures, which are approximate to the concentration of the feeding stream from naphtha crackers. We studied in detail the solvent properties of several chloroaluminate ionic liquids for aliphatic hydrocarbon and various aromatic hydrocarbons. The distribution coefficient of aromatics in the liquidliquid equilibrium and selectivity of aromatics to aliphatic hydrocarbon were also investigated. The regeneration properties of used ionic liquids were evaluated. The effects of the structure of ionic liquids and aromatics on the extractive performance were discussed. (9) Bo¨smann, A.; Datsevich, L.; Jess, A.; Lauter, A.; Schmitz, C.; Wasserscheid, P. Chem. Commun. 2001, 2494. (10) Zhang, S.; Zhang, Q.; Zhang, Z. C. Ind. Eng. Chem. Res. 2004, 43, 614. (11) Matsumoto, M.; Mochiduki, K.; Fukunishi, K.; Kondo, K. Sep. Purif. Technol. 2004, 40, 97. (12) Wei, G.; Yang, Z.; Chen, C. Anal. Chim. Acta 2003, 488, 183. (13) Meindersma, G. W.; Podt, A.; de Haan, A. B. Fuel Process. Technol. 2005, 87, 59. (14) Qiao, C. Z.; Zhang, Y. F.; Zhang, J. C.; Li, C. Y. Appl. Catal., A 2004, 276, 61.

2.1. Preparation of Ionic Liquids and Modeling Oils. The 1-methylimidazole and 1-chloro-butyl were obtained from Aldrich Chemicals. All of the other analytical reagents, such as acetonitrile, ethyl acetate, methylene chloride, trimethylamine hydrochloride (Me3NHCl), triethylamine hydrochloride (Et3NHCl), aluminum chloride (AlCl3), benzene, toluene, ethyl benzene, n-heptane, n-octane, etc., were purchased from Beijing Chemical Reagents Company. 1-Butyl-3-methylimidazolium chloride (BMIC) was synthesized by the methods reported in the literature.14 Aluminum chloride was purified by sublimation in a sealed tube. Trimethylamine hydrochloride and triethylamine hydrochloride were purified by recrystallization from dry, redistilled ethyl acetate-acetonitrile mixtures. A decolorizing charcoal treatment was used in the first two recrystallizations, and the product finally dried in vacuum (80 °C). BMIC-AlCl3, Me3NHCl-AlCl3, and Et3NHCl-AlCl3 were prepared by slow addition of the desired amount of waterless aluminum chloride to the imidazolium salt, trimethylamine hydrochloride, and triethylamine hydrochloride, respectively. The reaction was left stirring overnight at 60 °C, to allow for a perfect homogenization of the product. The whole process was kept under a dry nitrogen atmosphere in a glove box to avoid the hydrolysis of AlCl3. Finally, light brownish liquids were obtained. It has a density of 1.4-1.5 g/cm-3 at room temperature.10 The ionic liquids, once prepared, could be stored for a long time in a dry inert atmosphere. The molar ratios of AlCl3/triethylamine hydrochloride were 2.0, 1.5, and 1.2, respectively. For 1-butyl-3-methylimidazolium chloride and trimethylamine hydrochloride, only the ionic liquids with a molar ratio (AlCl3/organic salts) of 2.0 were prepared. The extractions of benzene, toluene, or ethyl benzene from mixtures of aromatics and n-heptane were used as representatives for the aromatic/aliphatic separation. For benzene/n-heptane, toluene/ n-heptane, and ethyl benzene/n-heptane, five kinds of modeling oils with different molar fractions of aromatics (2.5, 5.0, 7.5, 10.0, and 12.5%) were prepared, respectively. 2.2. Experimental Procedure. The solvent capacities of an ionic liquid for aromatic hydrocarbons and n-heptane were measured at room temperature by adding an excess amount of hydrocarbons dropwise to a glass vial containing the ionic liquid to form a twophase system. After solvent equilibrium was achieved, an excess of hydrocarbons in the upper phase was removed carefully. The amount of absorbed hydrocarbons in ionic liquid was measured by the weight gain. The whole process was carried out under dry nitrogen in a glove box. Because the chloroaluminate room-temperature ionic liquids are sensitive to moisture, liquid-liquid extraction experiments were carried out in a flask with a volume of approximately 75 mL under protection of dry N2 in a glove box equipped with a magnetic agitator and a glass thermometer. The operation temperature was controlled by a thermostatic water bath with a temperature controller (temperature range of 5-95 °C and temperature accuracy of (0.05 °C). For each experiment, 20 mL of ionic liquid and 20 mL of the aromatic/n-heptane mixture were placed into the flask. The operation temperature and the aromatic molar fraction in modeling oil were varied. The extractions were carried out under the same stirring speed for 30 min to obtain equilibrium.13 Then, the system was allowed to settle for about 2 h, and the samples were collected for analysis. The used ionic liquids were placed in a flask with a magnetic agitator for regeneration through vacuum distillation at 0.02 atm. Then, the extraction performances of regenerated ionic liquids were investigated. 2.3. Analysis and Characterization. The 0.1 g samples were collected from both phases. A total of 0.1 g of n-octane was added to samples as an internal standard for gas chromatography (GC) analysis. The mass fractions of aromatic hydrocarbons and nheptane in the samples were analyzed by a Varian CP 3800 gas

1726 Energy & Fuels, Vol. 21, No. 3, 2007

Figure 2. Solvent capacities of chloroaluminate ionic liquids for aromatic hydrocarbons and n-heptane.

chromatograph with a hydrogen flame ionization detector (FID) and 0.25 mm × 50 m PONA capillary column. The temperatures of the detector, injector, and column oven were 250, 250, and 130 °C, respectively. The ionic liquid in the sample was collected in a precolumn because the ionic liquid cannot be analyzed by a FID. Then, the mass fractions were converted to molar fractions. Each analysis was carried out in triplicate to ensure enough accuracy. In a ternary mixture, once the fractions of two components were obtained, the third one, i.e., the ionic liquid, can be determined by subtracting the sum of the measured molar fractions of the aromatic hydrocarbons and n-heptane from unity.13 The effects of ionic liquids on aromatic hydrocarbons were investigated by infrared spectra at room temperature on a Bruker Tensor 27 Fourier transform infrared (FT-IR) spectrometer. All spectra were acquired at 1 cm-1 resolution with a total of 64 scans per spectrum. The samples were prepared by mixing benzene and ionic liquids in a volume ratio of 1:1 and then smeared into liquid films on KBr windows.15 The results were repeatedly confirmed by a Nicolet Nexus-470 spectrometer.

Zhang et al.

Figure 3. Effects of cations on benzene distribution coefficients using ionic liquids with different cations as extractants. The operation temperature is 20 °C.

is powerless.10 Therefore, the solvent capacities of ionic liquids for n-heptane are very low. On the contrary, for the delocalized π cloud of aromatic hydrocarbons, the molecules can be easily polarized in the ionic liquids. Meanwhile, aromatic molecules with a delocalized π electron can interact with a Lewis acid, Al2Cl7- or AlCl3, through π complexation. Therefore, the solvent capacities of chloroaluminate ionic liquids for aromatic hydrocarbons are much higher than those for n-heptane. Such characteristics mentioned above might result in a good performance of chloroaluminate ionic liquids in extractive separation of aromatic hydrocarbons from aromatic/n-heptane mixtures. 3.2. Effect of the Cation. Bo¨smann et al.9 reported that the structures of both the cation and anion in the ionic liquids have strong effects on their extractive performances, including the distribution coefficient, Di, and the selectivity, Si. The former is calculated from the ratio of the molar fractions in the extract and raffinate phases at equilibrium. The ratio of the molar fractions of aromatic hydrocarbons or n-heptane in the ionic liquid (IL) and raffinate phases (i.e., distribution coefficients) is defined as

3. Results and Discussion 3.1. Solvent Capacities of Ionic Liquids for Aromatic Hydrocarbons and n-Heptane. The solvent capacities of chloroaluminate ionic liquids for aromatic hydrocarbons and n-heptane are shown in Figure 2. Apparently, the solvent capacities of different ionic liquids for n-heptane are much lower than those for the aromatic hydrocarbons. The solvent capacities of BMIC-2.0AlCl3 for aromatic hydrocarbons are the highest, followed by Et3NHCl-2.0AlCl3 and Me3NHCl-2.0AlCl3. The ionic liquids with a lower ratio of AlCl3/organic salt, Et3NHCl1.5AlCl3 and Et3NHCl-1.2AlCl3, have lower capacities for both aromatic hydrocarbons and n-heptane than Et3NHCl-2.0AlCl3. The substituent groups on the benzene ring, methyl and ethyl, reduce the capacities of ionic liquids for aromatic hydrocarbons. It is concluded by a comparison of the results of Et3NHCl1.5AlCl3, Et3NHCl-1.2AlCl3, and Et3NHCl-2.0AlCl3 that ionic liquids with a lower AlCl3 molar fraction are more sensitive to substituent groups on the benzene ring than ionic liquids with a higher AlCl3 molar fraction. It is consistent with the paper of Zhang et al.10 The aliphatic hydrocarbons are weakly polarizable for the saturated structure of the molecules. The driving force for entering n-hepane molecules into highly ionized ionic liquids (15) Yang, Y.; Kou, Y. Chem. Commun. 2004, 226.

raf IL raf Daro ) CIL aro/Caro and Dhep ) Chep/Chep

The selectivity, Saro/hep, of aromatic/n-heptane is defined as the ratio of the distribution coefficients of aromatic hydrocarbons and n-heptane

Saro/hep ) Daro/Dhep The extraction experiments were carried out using chloroaluminate ionic liquids with different cations as extractants. The ratio of AlCl3/organic salts is 2.0, and the experimental temperature is 20 °C. The molar fractions of benzene in the initial mixture are 2.5, 5.0, 7.5, 10.0, and 12.5%, respectively. As shown in Figure 3, it is obvious that all of the distribution coefficients of benzene, using three kinds of choroaluminate ionic liquids with different cations as extractants, are above 1.0. BMIC-2.0AlCl3 has the highest distribution coefficient of benzene, followed by Et3NHCl-2.0AlCl3. Me3NHCl-2.0AlCl3 has the lowest distribution coefficient of benzene among the three. The benzene distribution coefficients with the three ionic liquids decrease with an increasing benzene content, and BMIC-2.0AlCl3 is the most sensitive. Haan et al.13 reported the same trend of benzene distribution coefficients in the experiments for using neutral ionic liquid, mebupy-BF4, as the


Figure 4. Effects of cations on n-heptane distribution coefficients using ionic liquids with different cations as extractants. The operation temperature is 20 °C.

Figure 5. Effects of cations on benzene/n-heptane selectivities using ionic liquids with different cations as extractants. The operation temperature is 20 °C.

extractant. However, the value of benzene distribution coefficients of mebupy-BF4 is lower than that for BMIC-2.0AlCl3. The n-heptane distribution coefficients of ionic liquids with different cations are summarized in Figure 4. It can be seen that the n-heptane distribution coefficient with BMIC-2.0AlCl3 is somewhat low. The distribution coefficients with Et3NHCl2.0AlCl3 and Me3NHCl-2.0AlCl3 are almost the same and slightly higher than that of BMIC-2.0AlCl3. The molar concentrations of n-heptane in ionic liquid phase are extremely low. The same results have been observed in solvent capacity experiments (Figure 2). In comparison to Figure 3, the same trend can be seen in Figure 5; that the selectivities of benzene/n-heptane decrease with an increasing benzene molar fraction in the organic phase. The order of the selectivities of benzene/n-heptane with the same anion ionic liquids is BMIC-2.0AlCl3 > Et3NHCl-2.0AlCl3 ≈ Me3NHCl-2.0AlCl3. The distribution coefficients of nheptane are relatively stable, but the distribution coefficients of benzene decrease with an increasing molar fraction in the organic phase. Therefore, the benzene/n-heptane selectivities exhibit the same trend with the distribution coefficients of benzene. The infrared spectrum was performed to investigate the reason why the extractive performance of BMIC-2.0AlCl3 was better than Me3NHCl-2.0AlCl3 and Et3NHCl-2.0AlCl3. Figure 6 shows the effects of ionic liquids with various cations on the infrared spectrum of benzene. Pure benzene shows two char-


Figure 6. FT-IR spectra of benzene with different ionic liquids.

acteristic bands at 1959 cm-1 (band 1) and 1814 cm-1 (band 2), originating from the stretching vibrations of the benzene ring structure. Under the effects of ionic liquids, bands 1 and 2 would shift to a higher wavenumber. When we mixed in BMIC2.0AlCl3, band 1 shifts 11 cm-1 and the shift of band 2 is 10 cm-1, while Me3NHCl-2.0AlCl3 makes band 1 shift 9 cm-1 and band 2 shift 8 cm-1. For Et3NHCl-2.0AlCl3, the shift of band 1 is 9 cm-1 and the shift of band 2 is 8 cm-1. The effects of Me3NHCl-2.0AlCl3 and Et3NHCl-2.0AlCl3 on the infrared spectrum of benzene are almost same but less apparent than that of BMIC-2.0AlCl3, which indicates that the interaction between benzene and BMIC-2.0AlCl3 is stronger than those of the other two ionic liquids. For ionic liquids with the same anion, the structure and size of the cations are important to determine the extractive performance. In the presence of the aromatic cation (1-butyl-3-methylimidazolium), BMIC-2.0AlCl3 has an aromatic character, and thus, a higher benzene distribution coefficient and benzene/n-heptane selectivity than the ionic liquids with quaternary ammonium cations are observed, according to the principle of similitude-compatible. Et3NHCl2.0AlCl3 shows a little better extractive performance than Me3NHCl-2.0AlCl3, because Et3NH+ is a larger cation than Me3NH+, by extending the alkyl group chain length on the nitrogen atom. For the similar reason, the same order was observed in the solvent capacity experiments (Figure 2). 3.3. Effect of the Anion. Although BMIC-2.0AlCl3 has a better extractive performance, the cost of ionic liquids synthesized by 1-alkyl-3-alkylimidazolium chloride is much higher than that of quaternary ammonium ionic liquids. Moreover, quaternary ammonium ionic liquids also exhibit a satisfying extractive performance. Therefore, the investigation of anion effects on the extractive performance of chloroaluminate ionic liquids was carried out using Et3NHCl/AlCl3 ionic liquids with different ratios of AlCl3/Et3NHCl. The mixtures of benzene and n-heptane are the modeling oil. The operation temperature is 20 °C. The experimental results are shown in Figures 7-9. As shown in Figure 7, it is apparent that the distribution coefficients of benzene increase with an increasing ratio of AlCl3/Et3NHCl. Among the three ionic liquids with the same cation, Et3NHCl-2.0AlCl3 has the highest distribution coefficient for benzene, followed by Et3NHCl-1.5AlCl3. Et3NHCl1.2AlCl3 has the lowest distribution coefficient for benzene. The similar results can be seen in experiments of solvent capacities (Figure 2). Et3NHCl-2.0AlCl3 also exhibits the highest distribution coefficient for n-heptane, and the difference between Et3NHCl-


Zhang et al.

Figure 7. Effects of anions on benzene distribution coefficients using ionic liquids with different anions as extractants. The operation temperature is 20 °C.

Figure 9. Effects of anions on benzene/n-heptane selectivities using ionic liquids with different anions as extractants. The operation temperature is 20 °C.

Figure 8. Effects of anions on n-heptane distribution coefficients using ionic liquids with different anions as extractants. The operation temperature is 20 °C.

Figure 10. Effects of various aromatic hydrocarbons on aromatics distribution coefficients using Et3NHCl-2.0AlCl3 as the extractant. The operation temperature is 20 °C.

1.2AlCl3 and Et3NHCl-1.5AlCl3 on the n-heptane distribution coefficient is slight (Figure 8). This trend is consistent with the results of solvent capacity experiments shown in Figure 2. Figure 9 shows that the order in the benzene/n-heptane selectivity with Et3NHCl/AlCl3 ionic liquids is Et3NHCl-2.0AlCl3 > Et3NHCl-1.5AlCl3 > Et3NHCl-1.2AlCl3. The aromatic extractive efficiency of Et3NHCl/AlCl3 ionic liquid varies with the molar ratio of AlCl3/Et3NHCl in synthesis. With the increase of the ratio, the extractive performances of ionic liquids improve distinctly. It is believed that, at the ratio of 1.0, AlCl4- is the dominant anion and the ionic liquid is neutral. With the increase of the molar ratio of AlCl3/Et3NHCl in synthesis, the bigger anion, i.e., Al2Cl7-, is formed and its amount gradually increases, thereby the ionic liquids exhibit strong Lewis acidity.16 Moreover, with the content increase of Al2Cl7-, it is easier for n-heptane to dissolve into ionic liquids, by reason that the bigger anion probably facilitates the insertion of the n-heptane molecule into ions, resulting in little higher distribution coefficients and solvent capacity of Et3NHCl2.0AlCl3 than those of Et3NHCl-1.5AlCl3 and Et3NHCl1.2AlCl3. However, the distribution of benzene was mainly influenced by the chemical property of the ionic liquids. Consequently, because Lewis acid species can interact with aromatics strongly through π complextion, the higher the concentration of Al2Cl7- in ionic liquids, the stronger the

interaction between aromatics and ionic liquids. Therefore, Et3NHCl-2.0AlCl3 shows higher benzene distribution coefficients than Et3NHCl-1.5AlCl3 and Et3NHCl-1.2AlCl3. In this case, the benzene/n-heptane selectivity exhibits the same trend with benzene distribution coefficients. It is also consistent with the results of the solvent capacity experiment (Figure 2). 3.4. Effects of Various Aromatic Hydrocarbons. From the results of the solvent capacity experiments, it is evident that the substituting groups, methyl and ethyl, can influence the solvent capacities of chloroaluminate ionic liquids for aromatic hydrocarbons. Accordingly, the investigation of the extractive performance with various aromatic hydrocarbons is attractive and important. The experiments were carried out at 20 °C using Et3NHCl-2.0AlCl3 as the extractant. The benzene/n-heptane, toluene/n-heptane, and ethyl benzene/n-heptane are modeling oils. The results are shown in Figures 10-11. Figure 10 shows that the order in the aromatic distribution coefficient with Et3NHCl-2.0AlCl3 is benzene > toluene > ethyl benzene. In this case, the effects of various aromatic hydrocarbons on the n-heptane distribution coefficient can be deduced. Therefore, the same order in the aromatic/n-heptane selectivities is also observed as shown in Figure 11. There are two effects, i.e., the electron-donating effect and steric effect, of the alkyl group influencing the extractive performance of Et3NHCl-2.0AlCl3 for aromatics. The electron-donating groups, such as methyl and ethyl, on the benzene ring enhance π complextion between alkyl benzene and the Lewis acid species

(16) Zhao, D.; Wu, M.; Kou, Y.; Min, E. Catal. Today 2002, 74, 157.



Figure 11. Effects of various aromatic hydrocarbons on aromatic/nheptane selectivities using Et3NHCl-2.0AlCl3 as the extractant. The operation temperature is 20 °C.

Figure 12. Effects of the temperature on benzene distribution coefficients using Et3NHCl-2.0AlCl3 as the extractant.

in chloroaluminate ionic liquids, whereas the steric effect hinders the insertion of aromatic molecules into ionic liquids, resulting in the decrease of aromatic solvent capacity. The experimental results show that the extractive performances of Et3NHCl2.0AlCl3 decrease with an increasing alkyl group length on the benzene ring. It is indicated that the effect of steric hindrance is much stronger than the electron-donating effect for the same alkyl group. The steric hindrance can also be indicated in the solvent capability experiments obviously. 3.5. Effect of the Temperature. The effect of the operation temperature on the extractive performance was investigated using Et3NHCl-2.0AlCl3 as the extractant and the benzene/nheptane mixture as the modeling oil. The operation temperatures were 20, 40, and 60 °C, respectively. The experimental results are shown in Figures 12-14. As shown in Figures 12-14, the distribution coefficient for benzene decreases, while for n-heptane, it increases with an increasing operation temperature. Consequently, the benzene/

Figure 13. Effects of the temperature on n-heptane distribution coefficients using Et3NHCl-2.0AlCl3 as the extractant.

Figure 14. Effects of the temperature on benzene/n-heptane selectivities using Et3NHCl-2.0AlCl3 as the extractant. Table 1. Boiling Temperatures of Aromatics in Et3NHCl-2.0AlCl3 at 0.02 atm aromatic hydrocarbons

gasification temperature in Et3NHCl-2.0AlCl3 (°C)

natural boiling temperature (°C)a

benzene toluene ethyl benzene

45 68 84

-9 10 35

a

Obtained by the calculation according to the equation of Antoine.

n-heptane selectivity decreases at a higher operation temperature, at least with benzene molar fractions in the benzene/n-heptane mixture in the range of 2.5-12.5%. Similar results can be seen in the paper of Haan et al.13 It is likely that the temperature has an effect on the acting force between the charged ion pairs of ionic liquids, resulting in change of the insertion of aliphatic and aromatic molecules into ionic liquids. 3.6. Regeneration of Used Ionic Liquids. Vacuum distillation was used to regenerate the used Et3NHCl-2.0AlCl3, saturated by benzene, toluene, and ethyl benzene, respectively.

Table 2. Aromatic Hydrocarbon Extraction Using Regenerated Et3NHCl-2.0AlCl3 as the Extractanta aromatics distribution coefficient

aromatic/n-heptane selectivity

aromatics

fresh Et3NHCl-2.0AlCl3

regenerated Et3NHCl-2.0AlCl3

fresh Et3NHCl-2.0AlCl3

regenerated Et3NHCl-2.0AlCl3

benzene toluene ethyl benzene

1.65 1.06 0.73

1.64 1.03 0.71

40.2 25.9 17.8

39.1 25.0 16.5

a The modeling oils were benzene/n-heptane, toluene/n-heptane, and ethyl benzene/n-heptane, respecitively. The molar fraction of the aromatic hydrocarbon was 7.5% for each mixture, and the operation temperature was 20 °C.


Zhang et al.

The initial operation temperature was 5 °C and then increased at the rate of 1.0 °C/min. At a pressure of 0.02 atm, once the first bubble appeared, the temperature was recorded and maintained. The regeneration process was accomplished when no more bubbles appeared. The results are shown in Table 1. As the nonvolatility of ionic liquids, the bubble point temperature of the system could be considered as the gasification temperature of aromatics in ionic liquids. The gasification temperatures of benzene, toluene, and ethyl benzene in Et3NHCl-2.0AlCl3 were higher than their natural boiling point temperatures in the experimental degree of vacuum. Such an increase of the gasification temperatures of aromatic hydrocarbons could be attributed to the chemical force, such as π complextion, between aromatic hydrocarbons and chloroaluminate ionic liquids. The extractive performances for aromatics by regenerated Et3NHCl-2.0AlCl3 are listed in Table 2. The aromatic extractive performance of the regenerated ionic liquid was almost recovered.

aromatic/n-heptane selectivity can be obtained using chloroaluminate ionic liquids as extractants to separate aromatic hydrocarbons from aromatic/aliphatic mixtures. The ionic liquid BMIC-2.0AlCl3 with the aromatic character of the imidazolium cation exhibits a better extractive performance than Me3NHCl2.0AlCl3 and Et3NHCl-2.0AlCl3. Both the benzene distribution coefficient and aromatic/n-heptane selectivity increase with an increasing ratio of AlCl3/organic salt (Et3NHCl) in ionic liquids. The structure and size of the cation and anion as well as the steric effect of the substituent group on the benzene ring can affect extractive performances and solvent capacities of chloroaluminate ionic liquids. The π complextion between aromatic molecules with highly delocalized π electron and Lewis acid species (Al2Cl7- or AlCl3) facilitates the aromatic dissolution. At a lower operation temperature, the ionic liquids show a better extractive performance. The used ionic liquids can be regenerated through vacuum distillation effectively.

4. Conclusion

Acknowledgment. The authors are grateful for the financial support from the National Natural Science Foundation of China (20490209) and the Project 20625621 supported by NSFC.

Chloroaluminate ionic liquids have strong aromatic hydrocarbon solvent capacities and small solvent capacities for n-heptane. A remarkable aromatic distribution coefficient and

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Aliphatic Mixtures

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