Separation of Phenols from Oil Using Imidazolium-Based Ionic Liquids

Nov 25, 2013 - Therefore, an alternative method to separate phenols from oil mixtures using a ... N-Methylimidazole was obtained from Aladdin Chemical...
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Separation of Phenols from Oil Using Imidazolium-Based Ionic Liquids Yucui Hou,*,† Yuehong Ren,† Wei Peng,‡ Shuhang Ren,‡ and Weize Wu‡ †

Department of Chemistry, Taiyuan Normal University, Taiyuan, Shanxi 030031, China State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China

‡

ABSTRACT: Phenolic compounds, which are important substances in industry, are mainly derived from coal liquefaction oil, coal tar, and petroleum, and also from biomass via pyrolysis. However, the traditional process of separation of phenols from oil using water, NaOH, and H2SO4 causes serious environmental problems. In this work, imidazolium-based ionic liquids (ILs) were used to extract phenol from a model oil of hexane and phenol in order to reduce environmental risk. It was found that these ILs could successfully separate phenols from hexane, and a small amount of [Bmim]Cl, equimolar to that of phenol in hexane, was enough to extract phenol with a high extraction efficiency of 99%. Anions of imidazolium-based ILs have a significant influence on the phenol extraction efficiency, which follows the order: Cl− > Br− > BF4− > PF6−. But the cation of these ILs shows a relatively small influence on the extraction. Particularly, [Bmim]Cl shows best performance in terms of phenol extraction efficiency. Furthermore, it is not sensitive to temperature changes and can be performed at ambient temperature. The extracted phenol in ILs could be recovered by evaporation, and the regenerated ILs could be reused for four cycles with no obvious loss in the phenol extraction efficiency. [Bmim]Cl was also used to extract phenols from real coal liquefaction oil, and the extraction efficiency of phenols could reach 90%. and have been used to extract organic sulfur from oil,8,9 which means that if ILs are used to extract phenol from oil, crosscontamination IL and oil can be avoided because of their immiscibility. Moreover, Villora et al.10 reported the ternary liquid−liquid phase equilibria of IL + n-hexane + an organic compound mixtures, which also indicates that ILs can be used to extract organic compounds, and ionic liquids and n-hexane are immiscible. On the other hand, ILs have been used to extract phenols from aqueous solution and show high extraction efficiency due to the strong interaction between ILs and phenols.11−13 Therefore, it is possible that ILs can be used to extract phenols from oil. In this work, we found that imidazolium-based ILs could extract phenols from oil at room temperature, the extraction efficiency is as high as from 90 to 99%, and the amount of ILs used is very low, almost equimolar to that of the extracted phenol in oil.

1. INTRODUCTION Phenolic compounds are important materials for the organic chemical industry and have a wide range of applications in producing phenolic resins, bisphenol A, caprolactam, alkylphenols, adipic acid, engineering plastics, pesticides, medicines, preservatives, explosives, and so on. They are mainly products derived from coal liquefaction, coal tar, and petroleum, and also from biomass via pyrolysis as documented in recent reports.1,2 Because of the mixture of oil containing phenols, it is necessary to separate phenols before further refining. The present industrial method to separate phenol compounds from oil mixtures is chemical extraction using aqueous alkaline solutions (such as aqueous NaOH solution) and then acidification of the extract by mineral acids (such as aqueous H2SO4 solution) to recover the phenols. The disadvantages of this process are the use of large amounts of both strong alkalis and acids and the production of excessive amounts of wastewater containing phenols that is difficult to deal with. Other ways to separate phenols from oils have been investigated, such as methanolmediated water extraction, aqueous Na2CO3 solution extraction, aqueous salt solution, and so on.3,4 However, these methods use water as a solvent and make it difficult to avoid producing phenol-containing wastewater. Therefore, an alternative method to separate phenols from oil mixtures using a nonaqueous method would be a better alternative. In our previous work,5,6 quaternary ammonium salts were found to efficiently separate phenols from oils by forming a deep eutectic solvent, which is a nonaqueous process that also avoids the use of mineral alkali and acids that produces phenolcontaining wastewater. Quaternary ammonium salts are similar to ionic liquids (ILs) in structure and properties, and some quaternary ammonium salts are ILs because they have room temperature melting points.7 Most ILs are immiscible with oil © 2013 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Materials. N-Methylimidazole was obtained from Aladdin Chemical Co., Ltd. (Shanghai, China) and distilled before use. Phenol (99%), 1-chlorobutane (98%), 1-bromobutane (98%), 1-chlorooctane (99%), 1-chlorohexane (98%), and 1-bromopropane (99%) were supplied by Aladdin Chemical Co., Ltd. (Shanghai, China). 1-Bromoethane (98%) was purchased from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China). Ethanol (99.7%) was supplied from Beijing Chemical Factory (Beijing, China). Received: September 12, 2013 Accepted: November 25, 2013 Published: November 25, 2013 18071

dx.doi.org/10.1021/ie403849g | Ind. Eng. Chem. Res. 2013, 52, 18071−18075

Industrial & Engineering Chemistry Research

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The real oil of 20.0 mL was charged in a test tube of 32 mL with a stirrer. The test tube was for the most part immersed into a water bath with a temperature of 30 ± 0.1 °C. After that, an amount of IL was added into the test tube and the mixture in the test tube was vigorously mixed by the magnetic stirrer. After mixing for 30 min, and settling down for another 30 min, the upper oil solution was sampled, and the phenol content in the oil phase was analyzed following the above standard method. Extraction efficiency could be calculated by the difference of contents of total phenolic compounds before and after extraction.

1-Butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]) and 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF6]) were obtained from Henan Lihua Pharmaceutical Co. Ltd. (Henan, China), with a purity of 99%. 1-Butyl-3-methylimidazolium chloride ([Bmim]Cl), 1-hexyl3-methylimidazolium chloride [Hmim]Cl, 1-octyl-3-methylimidazolium chloride [Omim]Cl, 1-butyl-3-methylimidazolium bromide ([Bmim]Br), 1-ethyl-3-methylimidazolium bromide ([Emim]Br), 1-butyl-3-methylimidazolium bromide ([Bmim]Br), and 1-propyl-3-methylimidazolium bromide ([Pmim]Br) were synthesized and characterized based on the procedures reported elsewhere.14,15 The synthesized ILs were dried under vacuum until they were a constant weight and stored in a desiccator due to their hydrophilic property. The mass fractions of water in the ILs were determined by Karl Fischer titration. 2.2. Extraction of Phenol in Hexane with ILs and Regeneration of ILs. The model oil of hexane and phenol with a phenol content of 5054 mg/L was prepared in a 1 L volumetric flask by mixing phenol and hexane via an electrical balance (BS224S Sartorius) with an accuracy of ±0.1 mg. In a typical experiment, 20.0 mL of the model oil was charged in a test tube of 32 mL with a stirrer. Then the test tube was mostly immersed into a water bath with a temperature of 30 ± 0.1 °C. After that, an amount of IL (according to the needed ratio of IL to phenol in hexane) was added into the test tube, and the mixture in the test tube was vigorously mixed by the magnetic stirrer. After mixing for 30 min and settling down for another 30 min, about 0.5 mL of upper hexane solution was sampled, and the phenol content in the hexane solution was analyzed by UV−vis spectroscopy. The process to determine phenol contents in hexane solution by UV−vis spectrophotometer (TU1901, Pgeneral Company, Ltd., China) is briefly shown as follows. n-Hexane was used as a solvent to dissolved phenol, and different phenol solutions were prepared and measured by UV−vis to obtain a standard curve. The maximum absorption of phenol in n-hexane solution occurred at 270.5 nm. The phenol-containing hexane samples were diluted with hexane and then were analyzed by UV−vis. Extraction efficiency could be calculated by the difference of phenol contents before and after extraction. As for the model oil of 20.0 mL, only small amount of IL of about 0.2 g was used, and it is difficult to regenerate accurately for many cycles. Hence, 100 mL of model oil was charged in a test tube. After extraction, the upper hexane phase was decanted and the lower phase of IL and phenol mixture was left in the test tube. Then, it was partly immersed in an oil bath with a temperature of 150 ± 0.1 °C. A pure nitrogen gas was bubbled through the mixture loaded in the test tube with a flow rate of about 30 mL min−1. After a desired time for removal of phenol, the weight of the test tube was measured by an electrical balance with an accuracy of 0.1 mg. The weight of regenerated IL could be calculated. Then, the regenerated IL was mixed with model oil for the next reuse cycle. 2.3. Extraction of Phenolic Compounds in Real Oil with ILs. The real oil was a distillate from 130 to 260 °C of coal liquefaction oil, which was supplied by China Coal Research Institute, Beijing, China. The content of total phenolic compounds in the real oil was measured following the standard method of “Determination of phenol and homologues contetns of crude phenol” (GB/T 24200−2009, a standard method of State Standard Administration of China, 1994).

3. RESULTS AND DISCUSSION 3.1. Time of Extraction and Settling Down. The compositions of coal liquefaction oil and coal tar oil are complicated, containing not only many kinds of phenolic compounds, including phenol, cresol, and xylenol, but also nonphenolic compounds, including hexane, benzene, and toluene, which are usually called neutral compounds. In this work, to simplify the extraction of phenolic compounds from the oil mixture, hexane was selected as a model substance for neutral compounds, and phenol for phenolic compounds. Because imidazolium-based ILs had been used to extract phenolic compounds from aqueous solution with high efficiency, imidazolium-based ILs were tested in this work. First, the effect of extraction stirring time on phenol content in hexane phase by different ILs was investigated. Phenol content in hexane as a function of time was determined, and the result is shown in Figure 1. Taking [Bmim]Cl as an example, when

Figure 1. Effect of extraction stirring time on phenol content in the hexane phase. The mixture was settled down for 30 min after extraction stirring. Conditions: mole ratio of added IL to phenol, 1; initial phenol content, 5048 mg/L; temperature, 30 °C.

equimolar amounts of [Bmim]Cl and phenol were added to a mixture of hexane and phenol with a phenol composition of 5048 mg/L, the phenol content was rapidly reduced with time, to 98.5 mg/L in 5 min and 58.8 mg/L in 10 min, and a new phase was observed at the bottom of the glass tube. The time taken to reach an approximate equilibrium composition in the extraction was less than 10 min from 5048 mg/L to 58.8 mg/L, which gave an extraction efficiency of 98.8%. This short equilibrium time means that there is a rapid mass transfer for phenol. It is noted that although [Bmim]Cl has a melting point of 73 °C and it is solid at the investigated temperatures, it became a liquid after extracting phenol from the hexane phase, which results from the formation of a eutectic solvent of [Bmim]Cl and phenol and lowering their melting point. In the 18072

dx.doi.org/10.1021/ie403849g | Ind. Eng. Chem. Res. 2013, 52, 18071−18075

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following work, the extraction time was fixed at 30 min, a contact time more than sufficient to establish equilibrium. As shown above, there were two phases observed during extraction stirring. After the extraction stirring was stopped, the two phases needed to settle down for clearing. If the densities of two phases are close, it may need a long time to clear. The effect of settling time on the composition of phenol in hexane is shown in Figure 2. Experimentally, after the stirring was

99.0%. When the mole ratio of [Bmim]Cl to phenol was further increased to 2 or 3, the phenol content and the corresponding extraction efficiency remained unchanged. The results indicate that [Bmim]Cl equal to phenol, in moles, was a sufficient amount to extract phenol from hexane. In our previous work,5,6 quaternary ammonium salts show the same phenomena as [Bmim]Cl. The halogen anion of quaternary ammonium salts plays a main role in extracting phenol from hexane by hydrogen bonding between the OH functional group of phenol and the halogen anion. Therefore, [Bmim]Cl has a chloride anion, and can form a hydrogen bond with phenol to extract phenol from hexane. [Bmim]Br also has a halogen anion bromide, and can extract phenol with a high efficiency of 95.4%. However, [Bmim]BF4 and [Bmim]PF6 do not have any one-atom halogen anions, but rather polyatomic anions like BF4− or PF6−. This results in a weaker interaction with phenol than that of [Bmim]Cl or [Bmim]Br. Hence, [Bmim]BF4 and [Bmim]PF6 exhibit phenol extraction efficiencies much lower than those of [Bmim]Cl and [Bmim]Br, which in our experiment are 73.4% and 49.3% (see Figure 4), respectively. It is also worth noting that there was no [Bmim]Cl detected in the upper hexane phase (determined by AgNO3 titration), also demonstrated by other reports.16

Figure 2. Effect of settling time on the phenol content in hexane phase and extraction efficiency after the mixture was stirred for 30 min by [Bmim]Cl. Conditions: IL, [Bmim]Cl; mole ratio of added IL to phenol, 1; initial phenol content, 5048 mg/L; temperature, 30 °C.

stopped, the two phases quickly become clear. After 5 min, the content of phenol in hexane was 84.3 mg/L with an extraction efficiency of 98.3%, and after 10 min, the content of phenol in hexane was 62.6 mg/L with an extraction efficiency of 98.7%, which reaches full extraction equilibrium. The density of the low phase of [Bmim]Cl and phenol was measured to be 1.12 g/ cm3, and the density of the oil phase of hexane was 0.659 g/ cm3. There is a large difference between the densities of the two phases, which resulted in it quickly settling down. In the following work, settling time was kept at 30 min for full clearing and reaching the equilibrium. 3.2. Effect of the Amount of Added IL on Extraction. Figure 3 shows the dependence of equilibrium composition of

Figure 4. Effect of anions of ILs with same cation [Bmim]+ on (a) the phenol content in the hexane phase in extraction equilibrium and (b) extraction efficiency. Conditions: mole ratio of added IL to phenol, 1; initial phenol content, 5048 mg/L; temperature, 30 °C. Figure 3. Effect of mole ratio of [Bmim]Cl to phenol on extraction efficiency and phenol content in hexane phase at 30 °C.

3.3. Effect of the Structure of ILs on Extraction. The effect of anion structure of imidazolium-based ILs on the phenol content in hexane in extraction equilibrium and extraction efficiency is shown in Figure 4. As ILs equimolar to phenol were used to extract phenol from mixtures of hexane and phenol with a phenol composition of 5048 mg/L, different anions of imidazolium-based ILs have a significant influence on the phenol extraction efficiency, which follows the order: [Bmim]Cl > [Bmim]Br > [Bmim]BF4 > [Bmim]PF6. This

phenol in hexane on [Bmim]Cl added. The phenol content in hexane was reduced as the amount of [Bmim]Cl was increased. When the mole ratio of [Bmim]Cl to phenol was 0.5, the phenol content decreased to 232 mg/L and the corresponding extraction efficiency was 95.4%. When the mole ratio of [Bmim]Cl to phenol was 1, the phenol content decreased to 48.0 mg/L and the corresponding extraction efficiency was 18073

dx.doi.org/10.1021/ie403849g | Ind. Eng. Chem. Res. 2013, 52, 18071−18075

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Table 1. Effect of the Cation Structure of Imidazolium-Based ILs on the Phenol Content in Hexane Phase in Extraction Equilibrium and Extraction Efficiencya

a

IL

[Bmim]Cl

[Hmim]Cl

[Omim]Cl

[Emim]Br

[Pmim]Br

[Bmim]Br

phenol content in hexane/mg L−1 extraction efficiency/%

45.0 99.1

57.1 98.9

125 97.5

168 96.7

234 95.4

255 94.9

Conditions: mole ratio of added IL to phenol, 1; initial phenol content, 5048 mg/L; temperature, 30 °C.

order apparently indicates that the smaller the size of anion of ILs is, the higher the phenol extraction efficiency is. According to the literature,11−13 there are two main interactions of imidazolium-based ILs with phenol: one is π−π conjugation of the two aromatic rings and the other is the hydrogen bonding of anion with phenol. The hydrogen bonding of anion with phenol has a relation with electronegativity of anion. Electronegativity of the anions has the following order Cl− > Br−. The chloride ion has larger electronegativity and a stronger interaction with the phenol hydroxyl group than that of the bromide ion. Hence, [Bmim]Cl shows greater phenol-removal efficiency than [Bmim]Br. Although anions of BF4− and PF6− have the same charge as the halogen ion, their volumes are larger than that of Cl− and Br−. Hence, BF4− and PF6− have a weaker interaction with phenol, consequently resulting in low phenol-removal efficiency. As for the cation structure, the length of alkyl on the methylimidazolium ion was investigated, and the effect on the phenol content in hexane phase in extraction equilibrium and extraction efficiency is shown in Table 1. For both the chloridebased ILs and bromide-based ILs, the longer the alkyl on methylimidazolium ion, the lower the extraction efficiency. For example, phenol extraction efficiencies are 99.1% for [Bmim]Cl and 97.5% for [Omim]Cl. The length of alkyl on the methylimidazolium ion of ILs may influence the π−π conjugation of the two aromatic rings of the methylimidazolium ion and phenol. An increase in the length of the alkyl on the methylimidazolium ion of ILs may decrease the interaction of imidazolium-based ILs with phenol, resulting in a decrease of the phenol extraction efficiency. 3.4. Effect of Temperature on Extraction. The effect of temperature on the phenol-extraction efficiency from hexane was investigated at temperatures from 10 to 40 °C, shown in Figure 5. As expected, when the separation temperature was increased, the phenol extraction efficiency decreased. For instance, when temperature was increased from to 10 to 40 °C, the extraction efficiency decreased from 99.3% to 98.7% for [Bmim]Cl, and the extraction efficiency decreased from 85.2% to 67.3% for [Bmim]PF6. Moreover, the extraction of phenol in hexane by [Bmim]Cl is less sensitive than that by [Bmim]PF6, possibly due to the stronger interaction between [Bmim]Cl and phenol in hexane phase than that between [Bmim]PF6 and phenol. Importantly, the extraction efficiency by [Bmim]Cl is not sensitive to temperature, and the extraction can be performed at around room temperature. The effect of temperature on the phenol-extraction efficiency is attributed to the increase of mutual solubility of hexane and phenol with an increase in temperature. 3.5. Reuse of ILs on Extraction. It is important to reuse ILs for efficient extraction of phenol from oil. Because of the extremely low vapor pressure of imidazolium-based ILs, we tested using the evaporation method to regenerate ILs. In this work, because of its high phenol extraction efficiency and easy synthesis, [Bmim]Cl was used to extract phenol in hexane and

Figure 5. Effect of temperature on the phenol content in hexane phase in extraction equilibrium and extraction efficiency. Conditions: mole ratio of added IL to phenol, 1; initial phenol content, 5048 mg/L.

reused. The phenol extraction efficiency using regenerated [Bmim]Cl is shown in Table 2, indicating that [Bmim]Cl can be reused several times and that the phenol extraction performance remains constant after four cycles. Moreover, [Bmim]Cl recovery is also shown in Table 2. As can be see from Table 2, the weight of recovered [Bmim]Cl is larger that of the original one to a small extent, which indicates a small amount of phenol may remain in the [Bmim]Cl solvent. However, the extraction efficiency is not influenced by the remaining phenol. 3.6. Extraction of Phenols in Real Oil with [Bmim]Cl. The real oil was a distillate from 130 to 260 °C of coal liquefaction oil, and it had a total phenol content of 34.6% in weight. Due to the complicated composition of the real oil, the mole of phenolic compounds was not calculated. For simplification, the mass of [Bmim]Cl added to the real oil was equal to the total mass of phenolic compounds. The contents of phenols in the real oil were measured in total phenolic compounds. The extraction efficiency was 92.5% at 30 °C. Water content in the real oil was decreased from 0.67% to 0.10%, which indicated that water was also extracted by the IL. Obviously, water can form hydrogen bonds with [Bmim]Cl and be extracted by the IL. Phenolic compounds in [Bmim]Cl can be recovered by evaporation with N2 at 150 °C, and [Bmim]Cl can be regenerated. The regenerated [Bmim]Cl was reused to 18074

dx.doi.org/10.1021/ie403849g | Ind. Eng. Chem. Res. 2013, 52, 18071−18075

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Table 2. Extraction Efficiency and Recovery of [Bmim]Cl versus Regeneration Cyclesa regeneration cycle number

1

2

3

4

5

extraction efficiency/% [Bmim]Cl recovery/%

99.15 102.0

99.20 101.6

99.05 101.8

99.26 102.2

99.07 102.9

Extraction conditions: mole ratio of added IL to phenol, 1; initial phenol content, 5048 mg/L; temperature, 30 °C; time, 30 min. Regeneration conditions: phenol in [Bmim]Cl was evaporated at 150 °C using 30 mL/min N2 for 6 h.

a

(4) Sato, S.; Matsumura, A.; Saito, I. Methanol-Mediated Extraction of Coal Liquid (5). Conceptual Design and Mass Balance of a Continuous Methanol-Mediated Extraction Process. Energy Fuels 2002, 16, 1337−1342. (5) Pang, K.; Hou, Y. C.; Wu, W. Z.; Guo, W. J.; Peng, W.; Marsh, K. N. Efficient Separation of Phenols from Oils via Forming Deep Eutectic Solvents. Green Chem. 2012, 14, 2398−2401. (6) Guo, W. J.; Hou, Y. C.; Wu, W. Z.; Ren, S. H.; Tian, S. D.; Marsh, K. N. Separation of Phenol from Model Oils with Quaternary Ammonium Salts via Forming Deep Eutectic Solvents. Green Chem. 2013, 15, 226−229. (7) Eike, D. M.; Brennecke, J. F.; Maginn, E. J. Predicting Melting Points of Quaternary Ammonium Ionic Liquids. Green Chem. 2003, 5, 323−328. (8) Zhang, S. G.; Zhang, Q. L.; Zhang, Z. C. Extractive Desulfurization and Denitrogenation of Fuels Using Ionic Liquids. Ind. Eng. Chem. Res. 2004, 43, 614−622. (9) Mochizuki, Y.; Sugawara, K. Removal of Organic Sulfur from Hydrocarbon Resources Using Ionic Liquids. Energy Fuels 2008, 22, 3303−3307. (10) Hernandez-Fernandez, F. J.; de los Rios, A. P.; Gomez, D.; Rubio, M.; Tomas-Alonso, F.; Villora, G. Ternary Liquid-Liquid Equilibria for Mixtures of an Ionic Liquid + n-Hexane + an Organic Compound Involved in the Kinetic Resolution of Rac-1-phenyl Ethanol (Rac-1-phenyl Ethanol, Vinyl Propionate, Rac-1-phenylethyl Propionate or Propionic Acid) at 298.2K and Atmospheric Pressure. Fluid Phase Equilib. 2008, 263, 190−198. (11) Vidal, S. T. M.; Correia, M. J. N.; Marques, M. M.; Ismael, M. R.; Reis, T. A. Studies on the Use of Ionic Liquids as Potential Extractants of Phenolic Compounds and Metal Ions. Sep. Sci. Technol. 2005, 39, 2155−2169. (12) Li, X.; Zhang, S. J.; Zhang, J. M. Extraction of Phenols with Hydrophobic ionic Liquids. Chin. J. Process Eng. 2005, 5, 148−151. (13) Poole, C. F.; Poole, S. K. Extraction of Organic Compounds with Room Temperature Ionic Liquids. J. Chromatogr. A 2010, 1217, 2268−2286. (14) Huddleston, J. G.; Willauer, H. D.; Swatloski, R. P.; Visser, A. E.; Rogers, R. D. Room-Temperature Ionic Liquids as Novel Media for ’Clean’ Liquid-Liquid Extraction. Chem. Commun. 1998, 1765−1766. (15) Dupont, J.; Consorti, C. S.; Suarez, P. A. Z.; de Souza, R. F. Preparation of 1-Butyl-3-Methyl Imidazolium-Based Room-Temperature Ionic Liquids. Org. Synth. 2002, 79, 236−241. (16) Eßer, J.; Wasserscheid, P.; Jess, A. Deep Desulfurization of Oil Refinery Streams by Extraction with Ionic Liquids. Green Chem. 2004, 6, 316−322.

extract phenolic compounds from the real oil at the same conditions. The extraction efficiencies were 90.5% and 90.0% for the second cycle and third cycle, which indicates that [Bmim]Cl can be used to extract phenolic compounds from the real oil.

4. CONCLUSION Imidazolium-based ionic liquids were investigated to extract phenol from a model oil of hexane and phenol with a phenol content of 5048 mg/L. The extraction could reach equilibrium in a short time and the two phases could easily settle down. The molar amount of IL used was equal or close to that of phenol in oil. Both the anion and cation of ILs influence the extraction efficiency of phenol in oil. For the same cation [Bmim]+, the extraction efficiency follows the order of [Bmim]Cl > [Bmim] Br > [Bmim]BF4 > [Bmim]PF6. The increase of length of alkyl on imidazolium would decrease the extraction efficiency. [Bmim]Cl shows the best extraction efficiency of about 99% and is not sensitive to temperature, and the extraction can be performed at room temperatures. The extracted phenol in IL could be recovered by evaporation at a high temperature of 150 °C under N2, and then the IL was regenerated. The regenerated IL could be reused four times with the same extraction efficiency. IL [Bmim]Cl could be used to extract phenolic compounds in real oil from distillated coal liquefaction liquid, and extraction efficiency was about 90%. Compared with the traditional methods to separate phenol compounds from oil, this proposed IL method could avoid the use of alkali and acid and the production of phenol containing wastewater.

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AUTHOR INFORMATION

Corresponding Author

*Y. Hou. E-mail: [email protected]. Tel./Fax: +86 351 2275148. Funding

The project is financially supported by Shanxi Scholarship Council of China (2011-086) and by the National Basic Research Program of China (2011CB201303). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Professors Zhenyu Liu, and Qingya Liu for their help. REFERENCES

(1) Li, J. H.; Wang, C.; Yang, Z. Y. Production and Separation of Phenols from Biomass-Derived Bio-Petroleum. J. Anal. Appl. Pyrolysis 2010, 89, 218−224. (2) Amen-Chen, C.; Pakdel, H.; Roy, C. Separation of Phenols from Eucalyptus Wood Tar. Biomass Bioenerg. 1997, 13, 25−37. (3) Matsumura, A.; Sato, S.; Kodera, Y. Methanol-Mediated Extraction for Coal Liquid (2) - the Effect of Phase Separation Caused by Methanol on Naphtha Fraction Derived from Wyoming Coal. Fuel Process. Technol. 2000, 68, 13−21. 18075

dx.doi.org/10.1021/ie403849g | Ind. Eng. Chem. Res. 2013, 52, 18071−18075