Article pubs.acs.org/jced
Phenol Distribution Behavior in Aqueous Biphasic Systems Composed of Ionic Liquids−Carbohydrate−Water Yuhuan Chen,*,† Yanshan Meng,‡ Jin Yang,† Huanrong Li,† and Xiuwu Liu† †
School of Chemical Engineering, Hebei University of Technology, Tianjin 300130, China No. 4 Company, China Petroleum Pipeline Bureau, Langfang 065000, China
‡
ABSTRACT: Aqueous biphasic systems (ABSs) composed of 1-alkyl-3-methylimidazolium tetrafluoroborate ([Cnmim]BF4, n = 3 to 8) ionic liquids (ILs), 6-(hydroxymethyl)oxane-2,3,4,5tetrol, and water for phenol extraction are reported in this work. The effects of the phase-forming components’ concentrations, the temperature, the initial phenol concentration, and the length of the alkyl chain linked to the imidazolium ring on phenol partitioning behavior were investigated. The results showed that increasing the phase-forming components’ concentrations, especially the glucose concentration, and the length of the alkyl chain linked to the imidazolium ring are quite favorable for phenol partitioning to the IL-rich phase. The distribution ratio (D) of phenol is as high as 78. The results are compared with previously reported work, and the effects on phenol extraction are explained from the point view of molecular structure and thermodynamic modeling.
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INTRODUCTION Phenol and its derivatives are frequently found in waterways. Most phenolic compounds are listed as priority pollutants by the U.S. Environmental Protection Agency (EPA)1 because of the potential harm to human health and the environment, and EPA calls for lowering the phenol content in wastewaters to less than 1 mg·L−1.2 Therefore, removal of phenols from waters and wastewaters is an important issue in order to protect public health and the environment. Most of the technologies for the treatment of phenols, such as liquid membrane, cloud-point extraction, and micelle extraction, as well as promising alternative approaches, are liquid−liquid extraction methods. However, the traditional liquid−liquid extraction methods for phenol usually involve volatile organic compounds (VOCs) as extractants and solvents, such as 1-hexanol, 1-heptanol, 1octanol, n-hexane, amines, cyclohexane, benzene, toluene, ethylbenzene, cumene, acetate esters, diisopropyl ether, kerosene, and more complex molecules such as n-octylpyrrolidone (OPOD).3−7 Therefore, new extraction systems for phenol without VOCs are urgently needed. Ionic liquids (ILs) as novel compounds that open the window to new applications have been paid great attention in green chemistry because of their unique properties,8−10 and ILs have been applied in various aspects, such as biocatalysis, electrochemistry, and separation. Fan et al.11 reported the use of hydrophobic 1-methyl-3-alkylimidazolium hexafluorophosphate ([Cnmim]PF6, n = 4, 6, 8) and 1-methyl-3-alkylimidazolium tetrafluoroborate ([Cnmim]BF4, n = 6, 8), to replace VOCs for phenol extraction and obtained good results. Khachatryan et al.12 and others13−15 also reported the use of the hydrophobic ILs 1-butyl-3-methylimidazolium hexafluorophosphate ([C4mim]PF6) and 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([Rmim][NTf2]) for extraction of phenolic compounds. In this work, we have investigated new green extraction systems based on [Cnmim]© 2012 American Chemical Society
BF4 (n = 3 to 8), carbohydrate, and water aqueous biphasic systems (ABSs) for phenol extraction without any extractants. Compared with the published extraction methods and systems for phenol, this kind of system is much greener because it avoids VOCs, and it shows great promise in extraction in comparison with the reported hydrophobic IL−water systems. In this research work, the phenol distribution in ABSs composed of different ILs [Cnmim]BF4 (n = 3 to 8), 6(hydroxymethyl)oxane-2,3,4,5-tetrol (glucose), and water has been systematically investigated. The effects of the phaseforming components’ concentrations, the temperature, the initial phenol concentration, and the length of the alkyl chain linked to the imidazolium ring on the phenol distribution have also been studied. All of these effects are explained from the point view of interactions between the phenol hydroxyl group and the IL and thermodynamic modeling. The results confirm the suitability of IL-based ABSs for partitioning and offer indispensable fundamental data for industrial applications.
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EXPERIMENTAL SECTION Materials. The ILs [Cnmim]BF4 (n = 3 to 8) were synthesized according to the procedure described in the literature.11 The synthesized ILs were dried under vacuum at 353 K for 48 h before use. The water content of the ILs after drying was measured by Karl Fischer titration (Metrohm KF 787) and found to be less than 0.0005 mass fraction. The residual chloride in the ILs was 0.002 mol·L−1, as determined by the method reported by Seddon et al.16 Glucose (w > 0.99) and phenol (w > 0.99) were analytical-grade and purchased from Beijing Chemistry Reagent Company. Glucose was dried under vacuum at 353 K for 48 h before use without other Received: December 8, 2011 Accepted: June 20, 2012 Published: June 28, 2012 1910
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Table 1. Phenol Distribution Ratios (D) in [Cnmim]BF4−Glucose−H2O Systems at 298.15 K with Different Concentrations of Phase-Forming Components and Different Lengths of the Alkyl Chain on the Imidazolium Ringa D 100 wIL
100 wG
n=3
n=4
n=5
n=6
n=7
n=8
35.00
0.0010 5.0010 9.9910 15.0110 20.0010 24.9810 30.05 35.00 15.00
ha ha ha ha ha 2.51 7.72 13.04
ha ha ha 9.34 14.65 21.11 26.65 35.57 8.81 9.34 10.12 11.03 12.18 14.14
9.10 14.40 17.76 23.11 28.99 34.03 39.92 47.2
2.99 10.11 22.04 32.50 44.14 54.93 64.30 74.00
1.95 9.31 24.90 36.77 51.47 68.14
1.58 10.07 27.56 45.92 60.30 78.17
29.99 35.03 39.98 44.98 50.06 54.97 a a
h means homogeneous.
purification. Water was twice-distilled and had an electrical resistance of less than 2 × 10−7 S·cm−1. Procedures. The experiments were performed according to literature procedures.10 Feed samples were prepared by mixing 2 g of a glucose solution with fixed concentration, 2 g of the IL solution, and 1 g of the phenol solution in a sealed glass vessel. Another mixture with the same amount of glucose solution and IL solution and water was prepared as a blank for the determination of phenol by UV−vis spectroscopy. The mixture was set in the thermostat with a desired temperature for 24 h to separate thoroughly after being vigorously stirred for 3 h. Previously reported work17 showed that these times are enough to achieve equilibrium, which was in agreement with the experimental results in this work. The system separated into two liquid phases that became transparent with a well-defined interface. After separation of the two phases, samples of both phases were collected with a long pinhead syringe and analyzed after dilution with water. The concentrations of phenol in the top and bottom phases were determined using a TU-1901 UV−vis spectrometer (Bejing Purkinje General Instrument Co., Ltd.) at λmax = 270 nm. The distribution ratio (D), defined as the ratio of the concentration of phenol in the IL-rich phase to that in the glucose-rich phase, was calculated as as D = cphenol‑upper/ cphenol‑lower, where cphenol‑upper and cphenol‑lower are the molar concentrations of phenol in the upper and lower phases, respectively. All of the experiments were run in duplicate, and the accuracy was about ± 0.001.
Figure 1. Effect of phase-forming components’ concentrations on the phenol distribution in the [C4mim]BF4−glucose−H2O system at T = 298.15 K: ■, wIL; ●, wG.
glucose concentrations is favorable for increasing D, which can also be seen from Figure 2 and Table 1. Compared with the effect of the IL concentration, the influence of glucose content is more dramatic, that is, increasing glucose content is much more favorable for phenol partitioning in the IL-rich phase. However, when the viscosity of the extraction system is considered, the [C4mim]BF4−glucose−H2O system with wIL = 0.3500 and wG = 0.1500 is preferable. In the [C4mim]BF4-based system, D increases from 9 to 36 as the glucose concentration increases from wG = 0.1501 to 0.3500. These data can be compared with results for [C4mim]PF6, a hydrophobic IL with the same length of the alkyl chain linked to the imidazolium ring: D is 20 with a 1:1 volume ratio of phenol solution to IL,14 11.3 with a 3:1 volume ratio,12 and 11.2 with a 5:1 volume ratio11 at about pH 7 and 298 K. From these results, it is clear that IL-based ABSs are suitable for phenol extraction. Effect of Initial Phenol Concentration on Phenol Distribution. The effect of the initial phenol concentration on phenol distribution was studied at T = 298.15 K by adding phenol solutions with different initial concentrations to equal
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RESULTS Effect of IL and Glucose Concentrations on Phenol Distribution. Phenol distribution results for the [C4mim]BF4−glucose−H2O system with different IL and glucose concentrations at T = 298.15 K and an initial phenol concentration of c = 0.010 mol·L−1 are clearly shown in Table 1 and Figure 1. The experiments were conducted by mixing a wG = 0.1500 glucose solution with [C4mim]BF4 solutions having wIL = 0.2999, 0.3503, 0.39.98, 0.44.98, 0.5006, and 0.5497 and by mixing a wIL = 0.3500 [C4mim]BF4 solution with glucose solutions having wG = 0.0000, 0.0500, 0.0999, 0.1501, 0.2000, 0.2498, 0.3005, and 0.3500. From Figure 1, it can be clearly seen that increasing the IL and 1911
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mol·L−1. This is quite different from conventional organic solvent extraction, such as the kerosene−H2O system with OPOD as an extractant.7 In these systems, the initial phenol concentration has little effect on the phenol distribution. In the following experiment, an initial phenol concentration of c = 0.010 mol·L−1 was preferable for considering the analysis of phenol. Effect of Temperature on Phenol Distribution. The effect of temperature on phenol distribution is shown in Figure 4 and Table 3. The experiments were carried out in
Figure 2. Effect of the length of the alkyl chain linked to the imidazolium ring on the phenol distribution in the [Cnmim]BF4 (n = 3 to 8)−glucose−H2O systems at T = 298.15 K with wIL = 0.3500: ■, wG = 0.000010; ●, wG = 0.050010; ▲, wG = 0.099910; ▼, wG = 0.150110; ◆, wG = 0.0200010; ◀, wG = 0.249810; ▶, wG = 0.3005; ★, wG = 0.3500.
masses of [C4mim]BF4−glucose−H2O with wG = 0.1500 and wIL = 0.3500, as shown in Figure 3 and Table 2. Figure 3 shows Figure 4. Effect of temperature on the phenol distribution in [C4mim]BF4−glucose−H2O systems with wIL = 0.3500: ■, wG = 0.1500; ●, wG = 0.2000.
Table 3. Effect of Temperature on D in [C4mim]BF4− Glucose−H2O Systems with wIL = 0.3500 at cphenol = 0.010 mol·L−1 D
Table 2. Effect of the Initial Phenol Concentration on D in the [C4mim]BF4−Glucose−H2O System with wIL = 0.3500 and wG = 0.1500 at 298.15 K 0.50 10.99
1.00 9.34
2.00 8.19
4.00 7.72
6.10 7.51
wG = 0.1500
wG = 0.2000
21.75 16.97 13.06 11.27 9.34 7.88 7.05
26.59 21.36 17.92 14.56 12.28 10.57 9.12
[C4mim]BF4−glucose−water ABSs with wIL = 0.3500 and wG = 0.1500 or 0.2000 using cphenol = 0.010 mol·L−1 at T = (278.15, 283.15, 288.15, 293.15, 298.15, 303.15, and 308.15) K. Figure 4 shows that D is higher at lower temperature for both systems, indicating that partitioning at low temperature is desirable. For example, decreasing the temperature from 298.15 K to 278.15 K caused D to increase from 9.34 to 21.70. This phenomenon is consistent with published work on the kerosene−H2O system with OPOD as an extractant7 and the water−methyl isobutyl ketone (MIBK) system,18 whereas it differs from hydrophobic IL−H2O systems,11 for which temperature has little influence on D. Published work shows that phenol extraction is mostly due to the formation of new compounds between phenol and the extractant. From the experimental results on the temperature effect, the same conclusion can be drawn, as can be confirmed
Figure 3. Effect of initial phenol concentration on the phenol distribution in the [C4mim]BF4−glucose−H2O system at T = 298.15 K.
102·cphenol/mol·L−1 D
T/K 278.15 283.15 288.15 293.15 298.15 303.15 308.15
8.00 7.24
that the higher phenol initial concentration, the smaller is D. That is, IL-based ABSs are promising alternatives for extraction from dilute phenol solutions. For example, D in the ABS was 10.99 with an initial phenol concentration of 0.0050 mol·L−1, while it was 7.24 with an initial phenol concentration of 0.0800 1912
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thus cannot form ABSs without glucose or at lower glucose concentrations. For [C7mim]BF4 and [C8mim]BF4 ABSs, the viscosity is too large, making D hard to detect. Figure 2 shows that in the different systems, the effects of the length of the alkyl chain linked to the imidazolium ring on the phenol distribution have the same trend. The longer the alkyl chain linked to the imidazolium ring, the larger is D, which reaches a maximum of 78 in the [C8mim]BF4-based system. This is consistent with the results for hydrophobic IL−water systems.11 Other work3 has also reported phenol extraction by amines, alcohols, and acids and found that the extractive efficiency using amines, alcohols, and acids with longer hydrocarbon chains is higher than those with shorter ones. Figure 2 also shows that the increase in D is more dramatic in longer-alkyl-chain IL systems. The increasing rate follows the sequence [C8mim]BF4 > [C7mim]BF4 > [C6mim]BF4 > [C5mim]BF4 ≈ [C4mim]BF4 ≈ [C3mim]BF4.
from the following thermodynamic modeling. The van’t Hoff equation (eq 1) expresses the temperature variation of the equilibrium constant (K) of a reaction given the standard enthalpy change (ΔrHθm) for the process: Δ H θ (T ) d ln K (T ) = r m2 dT RT
(1)
This can also be written as ln K (T ) = −
Δr Hmθ +C RT
(2)
Therefore, a plot of the natural logarithm of the equilibrium constant versus the reciprocal temperature should give a straight line. The slope of the line is equal to −1 times the standard enthalpy change divided by the gas constant, −ΔrHθm/ R. From the plot of the natural logarithm of the distribution ratio (ln D) versus the reciprocal temperature in the experiment, as shown in Figure 5, it is evident that ln D has
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DISCUSSION
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CONCLUSIONS
Phenol distribution into the IL-rich phase can be attributed to the interaction between the hydroxyl group of phenol and the hydrogen attached to the C2 carbon of the imidazolium ring of the IL. Previous studies19−21 have indicated that the hydrogen attached to the C2 carbon on the imidazolium cation is “acidic” (i.e., it has a relatively large positive charge), adding significantly to the hydrogen-bonding ability of the C2 hydrogen with hydroxyl group of phenol. This can also be confirmed by thermodynamic modeling. As a result, increasing the IL content is advantageous for phenol distribution. It is known from our previous work10 that addition of glucose, increasing the length of the alkyl chain linked to the imidazolium ring, and decreasing the temperature can lower the mutual solubility of the IL with water, which reduces the chances of hydrogen bonding between the IL and water. Thus, all of the above-mentioned are favorable for phenol distribution into the IL-rich phase. Additionally, the interaction of hydrogen bonds is much stronger at lower temperature, so extraction at low temperature is preferred. When the initial phenol concentration is low, phenol can be almost totally extracted into the IL-rich phase. Therefore, IL-based ABSs are suitable for the disposal of diluted phenol wastewater.
Figure 5. Relationship between ln D and 1/T: ■, wG = 0.1500; ●, wG = 0.2000. Solid lines are linear fits to eq 2.
a linear relationship with 1/T, with correlation coefficients of R2 = 0.9931 and 0.9980 for the [C4mim]BF4−glucose−water ABSs with wIL = 0.3500 and wG = 0.1500 and 0.2000, respectively. That is, D and K have a relationship that can be expressed as (3) D = aK
ABSs composed of ILs have been used for phenol extraction in our research work. The effects of the phase-forming components’ concentrations, the temperature, the initial phenol concentration, and the length of the alkyl chain linked to the imidazolium ring on the phenol distribution have been discussed. The results show that increases in the phase-forming components’ concentrations, especially the glucose concentration, and in the length of the alkyl chain linked to the imidazolium ring are quite favorable for phenol partitioning. Compared with conventional liquid−liquid extraction and hydrophobic IL−water systems, IL-based ABSs show great promise. All of the effects on phenol extraction have been explained from the point of view of molecular structure and thermodynamic modeling. All of the information confirms the suitability of IL-based ABSs for partitioning and offers indispensable fundamental data for industrial applications.
where a is a coefficient that is independent of temperature. For the ABSs, the relationship between D and K is more complex than for organic solvent extraction systems because of the volume changes, so a is used in eq 3. Thus, the slope in Figure 5 is related to the standard enthalpy ΔrHθm as described above, and ΔrHθm was calculated to be −26.4361 J·mol−1. The negative enthalpy demonstrates that the extraction process is exothermic, so increasing the temperature is unfavorable for phenol extraction. Effect of the Length of the Alkyl Chain Linked to the Imidazolium Ring on Phenol Distribution. The effect of the length of the alkyl chain linked to the imidazolium ring on the phenol distribution was measured at T = 298.15 K with cphenol = 0.010 mol·L−1 in [Cnmim]BF4−glucose−H2O (n = 3 to 8) systems with wIL = 0.3500, as shown in Figure 2 and Table 1. [C3mim]BF4 and [C4mim]BF4 are hydrophilic and 1913
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(17) Haddoua, B.; Canselier, J. P.; Gourdon, C. Cloud point extraction of phenol and benzyl alcohol from aqueous stream. Sep. Purif. Technol. 2006, 50, 114−121. (18) Greminger, D. C.; Burns, G. P.; Lynn, S.; Hanson, D. N.; King, C. J. Solvent extraction of phenols from water. Ind. Eng. Chem. Process Des. Dev. 1982, 21, 51−54. (19) Cadena, C.; Anthony, J. L.; Shah, J.; Morrow, T. I.; Brennecke, J. F.; Maginn, E. J. Why is CO2 so soluble in imidazolium-based ionic liquids? J. Am. Chem. Soc. 2004, 126, 5300−5308. (20) Huang, X. H.; Margulis, C. J.; Li, Y.; Berne, B. J. Why is the partial molar volume of CO2 so small when dissolved in a room temperature ionic liquid? Structure and dynamics of CO2 dissolved in [Bmim+][PF6−]. J. Am. Chem. Soc. 2005, 127, 17842−17851. (21) Camper, D.; Scovazzo, P.; Koval, C.; Noble, R. Gas solubilities in room-temperature ionic liquids. Ind. Eng. Chem. Res. 2004, 43, 3049−3054.
AUTHOR INFORMATION
Corresponding Author
*Tel.: +86 22 60204294. Fax: +86 22 60204294. E-mail:
[email protected]. Funding
This work was supported by the Natural Science Foundation of Hebei Province, China (B2011202031), the Science and Technology Research and Development Program of Hebei Province (10215657), and the Innovation Foundation for Outstanding Youth of Hebei University of Technology (2011006). Notes
The authors declare no competing financial interest.
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