Selective Separation of Tocopherol Homologues by Liquid−Liquid

Ind. Eng. Chem. Res. 2009, 48, 6417–6422

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Selective Separation of Tocopherol Homologues by Liquid-Liquid Extraction Using Ionic Liquids Qiwei Yang, Huabin Xing,* Yifeng Cao, Baogen Su, Yiwen Yang, and Qilong Ren National Laboratory of Secondary Resources Chemical Engineering, Zhejiang UniVersity, Hangzhou 310027, China

Selective separation of tocopherol homologues was performed by liquid-liquid extraction, using ionic liquids (ILs) as extractants in the presence of diluent. The distribution coefficients and selectivities of tocopherols in the biphasic system were determined. A selectivity of δ-tocopherol to R-tocopherol up to 21.3 was achieved when using [bmim]Cl as extractant diluted by methanol. Considering the structural differences of tocopherols, the separation mechanism based on the hydrogen-bonding interaction between IL’s anion and the -OH group on the tocopherols was proposed. The separation efficiency of IL was greatly affected by its anion, and followed the order [bmim]BF4 < [bmim]CF3SO3 < [bmim]Cl under the same conditions, which is consistent with the ascending order of IL’s hydrogen-bond basicity strengths. 1. Introduction In recent years, there is a growing interest in obtaining bioactive substances from natural resources. However, a common problem existing in obtaining the chemicals from natural resources is that the product of interest often appears in a mixture with various structurally related compounds,1 therefore resulting in the increasing necessity for the effective separation of these similar chemicals. A good case for this problem is the production of tocopherols. Tocopherols are the main compositions of natural Vitamin E and play an important role in human health due to their antioxidative capacity and ability to act as free radical scavenger.2 Tocopherols are widely contained in many vegetables and fruits and can be extracted as a mixture of four different homologues, R-, β-, γ-, and δ-tocopherol. These different tocopherols possess a same basic structure that consists of a chromanol head and an alkyl side chain, while differing in the number and position of methyl groups on the chromanol head (Figure 1). Because R-tocopherol has been reported to possess the highest biological activity, there is a strong need to separate it from the other forms of tocopherol.3 However, the separation of mixed tocopherol homologues has been very challenging because of their high structural similarity. The available methods for this problem mainly include various chromatographic technologies, such as high performance liquid chromatography,4,5 capillary electrochromatography,6 microemulsion electrokinetic chromatography,7 nanoliquid chromatography,8 supercritical fluid chromatography,9,10 and low pressure column chromatography.11 Although these technologies could give effective separation for the tocopherol homologues, they are almost only applied at a laboratory scale on account of the low capacity and the large consumption of absorbents and solvents. Therefore, novel methods for the effective separation of tocopherol homologues have been in great demand so far. Ionic liquids (ILs) have attracted much attention in the past decade, due to their unique physical and chemical properties,12,13 such as negligible vapor pressure, high thermal and chemical stability, and the feasibility of structural and functional tunability.14 Besides the applications in various chemical reactions,15,16 * To whom correspondence should be addressed. Tel.: 86 571 8795 1224. Fax: 86 571 8795 2773. E-mail: [email protected].

ILs have also been researched in different separation processes such as chromatography, membrane separation, extraction, etc.17,18 As for the liquid-liquid extraction processes for organic molecules, ILs have been explored in many applications since Rogers et al.19 reported the partition of substituted-benzene derivatives between water and the hydrophobic IL [bmim]PF6 initially. These studies include the removal of sulfides20,21 and nitrides22 from diesel and gasoline, the separation of aromatics from aliphatics,23,24 the removal of pollutants (e.g., phenols,25,26 dyes,27,28 organic acids29-31) from water, the isolation of biological substances (e.g., amino acids,32,33 erythromycin,34 penicillin,35 lysozyme,36 hemoglobin,37 cytochrome c,38 DNA39) from aqueous mixtures, the extraction of glycerol from biodiesel,40 the extractive essential oil terpenless,41 and so on. Considering the ILs’ high viscosities, they were used together with diluents sometimes.42 From these published results, ILs have been revealed to have the strong ability of interacting with organic molecules through various mechanisms (e.g., π-π, dispersion, ionic exchange, hydrogen bonding).22,43-46 Moreover, these interactions can be finely adjusted by the change of IL’s cation or anion taskspecifically, thus often bringing on elevated separation efficiency as compared to the traditional solvents.23 For example, several compounds with a hydrogen-bond donor group, such as phenols and other weak-acidic compounds, were separated from water solution with a high efficiency by using appropriate ILs as extractants.25,30 Therefore, because the tocopherol homologues have different hydrogen-bond acidities that stem from their

Figure 1. The structures of tocopherols.

10.1021/ie801847e CCC: $40.75  2009 American Chemical Society Published on Web 05/27/2009

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structural differences,47 it is reasonable to speculate that they are likely to be separated with each other effectively by some specific kinds of ILs via hydrogen-bonding interactions mainly. In the present work, we then investigated the application of ILs as extractants in the selective separation of tocopherol homologues from their mixtures for the first time. 2. Experimental Section 2.1. Materials. N-Methylimidazole (99.5%) and 1-chlorobutane (99.5%) were purchased from Shanghai Jingchun Reagent Co., Ltd. (China). 1-Butyl-3-methylimidazolium trifluoromethanesulfonate ([bmim]CF3SO3, 99%) and 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim]BF4, 99%) were purchased from Shanghai Chengjie Chemical Co., Ltd. (China). The mixed tocopherols (99%) were supplied by Heilongjiang Jiusan Oil and Fat Co. (China) that were produced from soybean deodorizer distillate, containing 45.3% δ-tocopherol, 44.5% β- and γ-tocopherol, and 9.1% R-tocopherol. All other chemicals (Analytical grade) were commercially available and used as received unless otherwise stated. The water mass fractions of [bmim]BF4 and methanol were 0.09% and 0.03%, respectively, determined by a Karl Fischer titrator. The [bmim]CF3SO3 was dried before use, and its water mass fraction was 0.05%. 2.2. Preparation of [bmim]Cl. The ionic liquid 1-butyl-3methylimidazolium chloride ([bmim]Cl) was synthesized according to previous literature.48 Typically, 1.5 mol of N-methylimidazole and 1.6 mol of 1-chlorobutane were mixed in a round-bottom flask, and then stirred mechanically at 70 °C for 72 h. The mixture was then washed repeatedly with ethyl acetate to remove any unreacted materials and dried in a high vacuum at 70 °C for 48 h. The resulting product was a viscous pale yellow liquid, which might solidify after several hours at room temperature, with a water mass fraction about 0.9%. 2.3. Preparation of Mixed Tocopherol Acetates. Four grams of mixed tocopherols was dissolved in 10 mL of hexane and added into a 25 mL round-bottom flask equipped with a condenser, followed by the addition of 3 mL of acetic anhydride, 0.01 mL of sulfuric acid, and 0.01 g of zinc chloride. After being magnetically stirred at room temperature for 2 h, the solution was filtered. The filtrate was washed by water and then dried under vacuum to get a viscous product. The purity of mixed tocopherol acetates was determined by HPLC with a value of about 99%. 2.4. Extraction Procedure. The extraction experiments were performed as follows: a known amount of the mixed tocopherols or the mixed tocopherol acetates was dissolved in hexane, and aliquots of this solution were contacted with an equal volume of an IL-methanol mixture in an Erlenmeyer flask. The flask was shaken for 3 h using a thermostatic rotary shaker with a speed of 200 r/min, and then allowed to settle for at least 3 h at the same temperature. Samples were taken from each of the two phases and diluted with the mixture of methanol and water (96/4, v/v) for the HPLC analysis. 2.5. HPLC Analysis. The HPLC systems consisted of an autosampler, an Atlantis C18 column (5 µm, 4.6 mm × 250 mm), a Waters 1525 binary pump, and a Waters 2487 dual λ absorbance detector. The mobile phase was a mixture of methanol and water (96/4, v/v). The detection of tocopherols was performed at 292 nm, and the detection of tocopherol acetates was performed at 285 nm, respectively. β-Tocopherol and γ-tocopherol were measured as a combined fraction because their structures are so similar that their HPLC peaks were overlapped under this analytical condition, and so were their acetates.

Figure 2. Distribution coefficients of tocopherols at different contact time. The initial concentration of tocopherol in hexane (mg/mL): δ 0.54, β and γ 0.53, R 0.11. Volume ratio of two phases: 1:1. Mole ratio of [bmim]Cl to methanol: 1:1.3. Temperature: 30 °C.

3. Results and Discussion Natural mixed tocopherols are composed of four different homologues, R-, β-, γ-, and δ-tocopherol. The four tocopherol homologues have different hydrogen-bond acidities resulting from their structural differences in the number and position of methyl groups on the chromanol head. We tried to separate the tocopherol homologues from each other by using some specific kinds of ILs via hydrogen-bonding interaction. 3.1. Extractive Separation of Mixed Tocopherols Using [bmim]Cl as Extractant. 1-Butyl-3-methylimidazolium chloride ([bmim]Cl) was selected as a model extractant for the separation of tocopherol homologues, because it had a relatively strong ability of interacting with hydrogen-bond donor molecules.22,49 It was diluted with methanol before use as it is highly viscous or even solid near room temperature. Nonpolar solvent, hexane, was selected as the solvent for mixed tocopherols to form a biphasic system with IL-methanol solution. Using hexane as solvent could also limit the interfering solvent-solute hydrogen-bonding interaction. The distribution coefficient of solute i (Di) and the selectivity of solute i to solute j (Si/j) are defined as eqs 1 and 2. Di ) Cei /Cri

(1)

Si/j ) Di/Dj

(2)

where Cei and Cri refer to the mass fractions of solute in the extract phase and in the raffinate phase, respectively. Extraction experiments were performed repeatedly. For most cases, the relative uncertainties of Di were less than 3%. When the concentration of tocopherol in the extract phase was very low, the HPLC peak area became very small and thus hard to be measured accurately. Therefore, while Di values were smaller than 0.01, the relative uncertainties of Di might be up to 9%. Preliminary experiments were carried out and showed that the extraction equilibrium was almost achieved within 10 min (Figure 2). Therefore, a phase contact time of 3 h was employed for subsequent extraction experiments to ensure the distribution equilibrium. As the data presented in both Figure 2 and Table 1 show, significant differences among the distribution coefficients of each tocopherol homologue were found, which decreased dramatically as the order δ-tocopherol > β- and

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Table 1. Extractive Separation of Mixed Tocopherols Using [bmim]Cl at 30 °Ca mole ratio of [bmim]Cl to methanol

distribution coefficient

1:1.3 1:2.7 1:5.4 pure methanolb pure [bmim]Clc

δ

β and γ

2.34 2.58 1.94 1.01 0.23

0.95 1.18 1.06 0.94 0.09

R

selectivity δ/R (β and γ)/R δ/(β and γ)

0.11 21.3 0.16 16.1 0.24 8.1 0.85 1.2 0.01 18.8

8.6 7.4 4.4 1.1 7.7

2.5 2.2 1.8 1.1 2.4

a The initial concentration of tocopherol in hexane (mg/mL): δ 0.54, β and γ 0.53, R 0.11. Volume ratio of two phases: 1:1. b Methanol amount was adjusted to reach an equilibrium phase volume ratio of about 1:1. c Carried out at 55 °C, mixed for 18 h.

Figure 3. The structures of tocopherol acetates.

Table 2. Extractive Separation of Mixed Tocopherol Acetates and Mixed Tocopherols Using [bmim]Cl at 30 °Ca

Table 3. Extractive Separation of Mixed Tocopherols Using Different ILs at 30 °Ca

distribution coefficient

selectivity

distribution coefficient

solutes

δ

β and γ

R

δ/R

(β and γ)/R

δ/(β and γ)

tocopherols tocopherol acetates

2.10 0.002

0.89 0.002

0.11 0.002

19.1 1.0

8.1 1.0

2.4 1.0

a The initial concentration of tocopherol in hexane (mg/mL): δ 2.31, β and γ 2.27, R 0.46. The initial concentration of tocopherol acetates in hexane (mg/mL): δ 2.30, β and γ 2.25, R 0.46. Volume ratio of two phases: 1:1. Mole ratio of [bmim]Cl to methanol: 1:1.3.

γ-tocopherol > R-tocopherol. Especially, R-tocopherol exhibited a much lower value of distribution coefficient than did the other forms of tocopherols. While most of the R-tocopherol remained in the hexane phase, the other forms were much easier to distribute into the extract phase, resulting in the high selectivities of R-tocopherol toward the other tocopherols. For example, while the mole ratio of [bmim]Cl to methanol was 1:1.3 and temperature was 30 °C, the selectivity of δ-tocopherol to R-tocopherol was as large as 21.3, and the selectivity of β- and γ-tocopherol to R-tocopherol was also higher than 8. Therefore, it is reasonable to consider that R-tocopherol could be selectively obtained from the mixture of various homologues in high purity by multistage extractions. Next, it was also shown in Table 1 that the selectivities of δ-tocopherol to R-tocopherol and β- and γ-tocopherol to R-tocopherol tended to increase along with the increase of IL’s content in the IL-methanol solution, thus suggesting a key role of [bmim]Cl in the extractive separation. Because the differences among the tocopherol homologues just lie in the different structural environment around the -OH group, it appears that the selective separation probably results from the hydrogenbonding interaction between the -OH groups and the chloride anion as expected. The methyl group that exists around the -OH group of tocopherol could reduce tocopherol’s hydrogen-bond acidity and weaken the affinity of tocopherol for [bmim]Cl, consequently resulting in a smaller distribution coefficient of R-tocopherol between the IL phase and hexane phase. To know more about the separation mechanism, further experiments were performed. 3.2. Extractive Separation of Mixed Tocopherol Acetates Using [bmim]Cl as Extractant. To explore the effect of the tocopherol’s phenolic -OH group on the extractive separation, distributions of mixed tocopherol acetates in the [bmim]Clmethanol-hexane biphasic system were determined, and the obtained results were compared to those of mixed tocopherols (Table 2). Because the -OH groups on the chromanol ring had been converted to ester groups (Figure 3), all of the tocopherol acetates were poor hydrogen-bond donors and thus were expected to show weak affinity for [bmim]Cl. Well, the experimental results shown in Table 2 accord with this expecta-

IL

δ

[bmim]BF4 0.02 [bmim]CF3SO3 0.31 [bmim]Cl 2.34

β and γ 0.01 0.15 0.95

R

selectivity δ/R (β and γ)/R δ/(β and γ)

0.003 6.7 0.04 7.8 0.11 21.3

3.3 3.8 8.6

2.0 2.1 2.5

a The initial concentration of tocopherol in hexane (mg/mL): δ 0.54, β and γ 0.53, R 0.11. Volume ratio of two phases: 1:1. Mole ratio of IL to methanol: 1:1.3.

Figure 4. Plot illustrating the hydrogen-bond basicity strength of different ILs according to Armstrong et al.50

tion. The distribution coefficients of tocopherol acetates are much smaller than those of tocopherols, as well as sharply decreased selectivities. The experimental results partly indicated that the high selectivities of tocopherol homologues in this biphasic system arose from the interaction of -OH groups and the chloride anion. 3.3. Effect of the IL’s Anion on the Extractive Separation. On the other hand, to make certain of the effect of the IL’s anion on the extractive separation, three kinds of ILs with different anions, which thus possessed different hydrogen-bond basicities, were investigated for the extractive separation of tocopherol homologues. As expected, the kind of IL’s anion had a significant impact on the extraction (Table 3). The distribution coefficients of tocopherols in biphasic system using different IL varied with the order [bmim]BF4 < [bmim]CF3SO3 < [bmim]Cl under the same conditions. The selectivities of tocopherol homologues had the same order. This order is consistent with the order of these ILs’ hydrogen-bond basicity strength (Figure 4).50 This phenomenon further disclosed that the selectivity was based on the hydrogen-bonding interaction between IL’s anion and the -OH group on the tocopherols.

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Table 4. Extractive Separation of Mixed Tocopherols Using [bmim]Cl at Different Temperaturesa distribution coefficient temperature (°C)

δ

β and γ

30 40 50

2.10 1.78 1.39

0.89 0.72 0.55

R

selectivity δ/R (β and γ)/R δ/(β and γ)

0.11 19.1 0.10 17.8 0.08 17.4

8.1 7.2 6.9

2.4 2.5 2.5

a The initial concentration of tocopherol in hexane (mg/mL): δ 2.31, β and γ 2.27, R 0.46. Volume ratio of two phases: 1:1. Mole ratio of [bmim]Cl to methanol: 1:1.3.

Figure 6. Distribution coefficients of tocopherols at different water contents of [bmim]Cl. The initial concentration of tocopherol in hexane (mg/mL): δ 0.45, β and γ 0.44, R 0.09. Temperature: 30 °C. Volume ratio of two phases: 1:1. Mole ratio of [bmim]Cl to methanol: 1:1.3.

Figure 5. Extraction of mixed tocopherols at different tocopherol concentrations. Temperature: 30 °C. Volume ratio of two phases: 1:1. Mole ratio of [bmim]Cl to methanol: 1:1.3. (a) Concentration in each phase at equilibrium. (b) Distribution coefficients versus concentration in raffinate phase plotted according to (a).

3.4. Effect of Temperature on the Extractive Separation of Mixed Tocopherols. The distribution data of tocopherols in the [bmim]Cl-methanol-hexane biphasic system at different temperatures were presented in Table 4. It is seen that both the distribution coefficients and the selectivities tend to decrease with the increase of temperature. The change of temperature could have multiple effects on the extraction, for instance, the liquid-liquid equilibrium of the [bmim]Cl-methanol-hexane system and the solubilities of

tocopherols in this biphasic system, and one of the effects might result from the reduced hydrogen-bond basicity of IL at a higher temperature according to Figure 4. 3.5. Effect of the Concentration of Tocopherol on the Distribution. Extraction experiments were also performed at different tocopherol concentrations. The equilibrium concentrations of tocopherols in the extract phase versus that in raffinate phase were plotted in Figure 5a, from which the existence of saturation concentrations of tocopherols (especially for δ and β and γ forms) in the extract phase was found. As a result, the distribution coefficients decreased notably over the saturation range, and decreased much more slowly while concentrations were lower (Figure 5b). 3.6. Regeneration of [bmim]Cl. Because the tocopherols are quite hydrophobic while [bmim]Cl is miscible with water, the water content of the IL phase should have a remarkable influence on the distribution of tocopherol between IL and hexane phase. The effect of water content of [bmim]Cl on the distribution of tocopherols was shown in Figure 6. It was revealed that the increase of water content could induce a decrease of the distribution coefficients of tocopherols. In fact, when aliquots of [bmim]Cl were diluted with equimolar water in the absence of methanol, the distribution coefficients of tocopherols between the IL phase and hexane phase were all lower than 0.02. So tocopherols probably could be removed by using a water-addition process, and then [bmim]Cl could be recovered after the distillation removal of water. More related work is underway. 4. Conclusion In this study, ILs were used as novel media for the extractive separation of tocopherol homologues in the presence of diluent, and they exhibited the strong ability of separating tocopherol homologues selectively. Especially, the experimental results indicated that the most bioactive component R-tocopherol could be isolated from the other tocopherol forms with great satisfaction using ILs as extractants, despite their high structural similarities. A selectivity of δ-tocopherol to R-tocopherol up to 21.3 was achieved when using [bmim]Cl as extractant diluted by methanol. The separation mechanism based on the hydrogenbonding interaction between IL’s anion and the -OH group on

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the tocopherols was proposed. The separation efficiency of ILs was greatly affected by the anion and followed the order [bmim]BF4 < [bmim]CF3SO3 < [bmim]Cl under the same conditions. This order is consistent with the ascending order of ILs’ hydrogen-bond basicity strengths. Further improvement research is in progress, as are the efforts to know more about the multicomponent phase equilibrium and the IL’s regeneration process. It is a common problem that the natural bioactive product of interest often appears in a mixture with various structurally related compounds, and there are theoretically numerous kinds of “designable” ILs that can be tuned to have specific properties. So the present work is not only useful for the separation of R-tocopherol, but also worthy of being considered as a reference to the production of other bioactive chemicals. Acknowledgment We are grateful for the financial support from the Ministry of Science and Technology of the People’s Republic of China (project nos. 2006BAD27B03 and 2007AA100404), the National Natural Science Foundation of China (20806066), and the China Postdoctoral Science Foundation (20070410398). Literature Cited (1) Chen, T. H.; Payne, G. F. Separation of Alpha- And DeltaTocopherols Due to an Attenuation of Hydrogen Bonding. Ind. Eng. Chem. Res. 2001, 40, 3413. (2) Packer, L. Protective Role of Vitamin E in Biological Systems. Am. J. Clin. Nutr. 1984, 53, 1050. (3) Hosomi, A.; Arita, M.; Sato, Y.; Kiyose, C.; Ueda, T.; Igarashi, O.; Arai, H.; Inoue, K. Affinity for Alpha-Tocopherol Transfer Protein as A Determinant of the Biological Activities of Vitamin E Analogs. FEBS Lett. 1997, 409, 105. (4) Pyka, A.; Sliwoik, J. Chromatographic Separation of Tocopherols. J. Chromatogr., A 2001, 935, 71. (5) Ng, M. H.; Choo, Y. M.; Ma, A. N.; Cheng, H. C.; Hashim, M. A. Separation of Vitamin E (Tocopherol, Tocotrienol, and Tocomonoenol) in Palm Oil. Lipids 2004, 39, 1031. (6) Chaisuwan, P.; Nacapricha, D.; Wilairat, P.; Jiang, Z.; Smith, N. W. Separation of Alpha-, Beta-, Gamma-, Delta-Tocopherols and AlphaTocopherol Acetate On a Pentaerythritol Diacrylate Monostearate-Ethylene Dimethacrylate Monolith by Capillary Electrochromatography. Electrophoresis 2008, 29, 2301. (7) Chang, L. C.; Chang, H. T.; Sun, S. W. Cyclodextrin-Modified Microemulsion Electrokinetic Chromatography for Separation of Alpha-, Gamma-, Delta-Tocopherol and Alpha-Tocopherol Acetate. J. Chromatogr., A 2006, 1110, 227. (8) Fanali, S.; Camera, E.; Chankvetadze, B.; D’Orazio, G.; Quaglia, M. G. Separation of Tocopherols by Nano-Liquid Chromatography. J. Pharm. Biomed. Anal. 2004, 35, 331. (9) Jiang, C. W.; Yang, Y. W.; Ren, Q. L.; Wu, P. D. Separation of Tocopherol Homologues by Supercritical Fluid Chromatography. Chin. J. Anal. Chem. 2003, 31, 1337. (10) Jiang, C. W.; Ren, Q. L.; Wu, P. D. Study On Retention Factor and Resolution of Tocopherols by Supercritical Fluid Chromatography. J. Chromatogr., A 2003, 1005, 155. (11) Wan, J. C.; Zhang, W. N.; Jiang, B.; Guo, Y. H.; Hu, C. R. Separation of Individual Tocopherols From Soybean Distillate by Low Pressure Column Chromatography. J. Am. Oil Chem. Soc. 2008, 85, 331. (12) Welton, T. Room-Temperature Ionic Liquids. Solvents for Synthesis and Catalysis. Chem. ReV. 1999, 99, 2071. (13) Plechkova, N. V.; Seddon, K. R. Applications of Ionic Liquids in the Chemical Industry. Chem. Soc. ReV. 2008, 37, 123. (14) Yang, Q. W.; Wei, Z. J.; Xing, H. B.; Ren, Q. L. Bronsted Acidic Ionic Liquids as Novel Catalysts for the Hydrolyzation of Soybean Isoflavone Glycosides. Catal. Commun. 2008, 9, 1307. (15) Parvulescu, V. I.; Hardacre, C. Catalysis in Ionic Liquids. Chem. ReV. 2007, 107, 2615. (16) Xing, H. B.; Wang, T.; Zhou, Z. H.; Dai, Y. Y. The Sulfonic AcidFunctionalized Ionic Liquids with Pyridinium Cations: Acidities and their Acidity-Catalytic Activity Relationships. J. Mol. Catal. A: Chem. 2007, 264, 53.

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ReceiVed for reView December 2, 2008 ReVised manuscript receiVed April 14, 2009 Accepted May 2, 2009 IE801847E