Article pubs.acs.org/IECR
Polyethylenimine-Assisted Extraction of α‑Tocopherol from Tocopherol Homologues and CO2‑Triggered Fast Recovery of the Extractant Guoqiang Yu,† Yangyang Lu,† Xianxian Liu,‡ Wen-Jun Wang,*,†,‡ Qiwei Yang,‡ Huabin Xing,‡ Qilong Ren,‡ Bo-Geng Li,† and Shiping Zhu*,§ †
State Key Laboratory of Chemical Engineering and ‡Key Laboratory of Biomass Chemical Engineering of Ministry of Education, Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, People’s Republic of China § Department of Chemical Engineering, McMaster University, Hamilton, Ontario L8S 4L7, Canada ABSTRACT: The separation of natural homologues provides a great challenge due to high similarities of their structures and properties. In this work, the use of the polymeric extractant polyethylenimine (PEI) for the separation of α-tocopherol from the tocopherol homologues was investigated. Various PEI−cosolvent solutions were used to extract tocopherols from their hexane solutions. The effects of PEI molecular weight, cosolvent type, and PEI−cosolvent composition were systematically studied. High distribution and selectivity coefficients of tocopherols were obtained with PEI−acrylonitrile (ACN) as the extractant. A clear advantage of PEI extractant is its stimuli-responsive property triggered by CO2. PEI−extracted tocopherols could be replaced and released from PEI chains with CO2 bubbling, with PEI−CO2 precipitated out from the extract phase, which greatly facilitates the back extraction of tocopherols and the recovery of PEI for reuse. With CO2 bubbling, it took only three back extractions to recover over 90% tocopherols, compared with 12 in theory without CO2 treatment. Precipitated PEI−CO2 could be redissolved in cosolvent for reuse by N2 bubbling and heating. The distribution coefficients decreased by approximately 10% after being recycled three times.
1. INTRODUCTION Nowadays approximately 40% of drugs are derived from natural products.1 Extraction of active ingredients from raw natural materials has become an important industrial sector. Separation of natural homologues is always challenging due to the similarities in their structures and properties. An example of this is the separation of α-tocopherol from tocopherol homologues. Tocopherols, also known as vitamin E, represent a popular product in the vitamin market. Tocopherols are widely used in medicine, health care products, food, cosmetics, animal feed, plastics processing additives, etc.2,3 Commercially, vitamin E is derived mainly from vegetable oil deodorizer distillate, which contains α-, β-, γ-, and δ-tocopherols as shown in Figure 1. As biological activities of the tocopherols differ significantly from each other (in the order α- > β- > γ- > δtocopherol),2,4 further separation of α-tocopherol from its tocopherol homologues can add significant value to vitamin E products. Techniques used to separate α-tocopherol from its tocopherol homologues includes derivatization/extraction,5 adsorption,6 anion exchange,7 liquid−liquid extractions using organic solvents and/or ionic liquids (ILs),8,9 and a number of chromatographic separation methods.10−22 In view of the first principles involved, most of the above methods could be divided into two categories: one is based on solid−liquid mass transfer and the other is based on liquid−liquid mass transfer. Adsorption, ion exchange, and chromatography belong to the former category, while extraction belongs to the latter. The relative history of the former methods is longer, and these approaches are more widely used in industry. The solid−liquid © 2014 American Chemical Society
methods usually have high separation selectivities. However, the mass transfer rate is generally low because it is governed by the efficiency of mixing between the solid and liquid phases. This results in decreased production efficiencies and increased solvent consumption. Liquid−liquid methods have clear advantages in this regard due to the greater ease of mixing. Moreover, Yang et al.8,9 recently obtained high separation efficiencies and selectivities in the liquid−liquid extraction of αtocopherol from tocopherol homologues using ILs. Although promising, there are still some major problems with the liquid− liquid extraction method, which need to be addressed prior to commercialization, such as difficulties with extractant recovery.23 Recently, a research program to develop polymeric extractants for the separation of natural homologue products was launched by our group. Use of polymeric extractants provides several advantages compared to small molecule ionic liquids (ILs) including that they are easier to recovery, have lower toxicity, and have a reduced impact on the environment. It is well understood that the separation of components in a mixture is based on differences in their physical and/or chemical properties. Figure 1 shows the molecular structures of α-, β-, γ-, and δ-tocopherols. It can be seen that the numbers and positions of the methyl substituents on the benzene ring among the tocopherols are different. Steric effects in the Received: Revised: Accepted: Published: 16025
June 26, 2014 September 24, 2014 September 25, 2014 September 25, 2014 dx.doi.org/10.1021/ie502568h | Ind. Eng. Chem. Res. 2014, 53, 16025−16032
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Figure 1. Molecular structures of α-, β-, γ-, and δ-tocopherols.
Tocopherol homologues (99%) were supplied by Heilongjiang Jiusan Oil and Fat Co., containing 45.3% δ-tocopherol, 44.5% β- and γ-tocopherols, and 9.1% α-tocopherol. Methanol (99.9%, liquid chromatography grade) was purchased from Merck Chemicals. N2 (99.999%) and CO2 (dry ice grade) were supplied by Jingong Air Co. Hexane, acrylonitrile (ACN, >99%), dimethyl sulfoxide (DMSO, >99%), and ethanol (EtOH, 99.7%) were all of analytical grade and were commercially obtained. 2.2. Extractions. A tocopherol−hexane solution with a tocopherol homologue concentration of 2.5 mg/mL and PEI− cosolvent solutions having various PEI concentrations as shown in Table 1 were prepared. The two solutions of 30 mL were added into a 250 mL Erlenmeyer flask. The mixture was shaken using a thermostatic rotary shaker at a speed of 250 rpm for 20 min at 30 °C. It was then maintained at the same temperature with no mixing for 30 min to allow for phase separation. Samples (1 mL) were taken from both the raffinate phase (hexane phase) and the extract phase (PEI−cosolvent phase), diluted with methanol in separate 5 mL flasks for highperformance liquid chromatographic (HPLC) analysis. 2.3. Extractant Recovery. For recovering the tocopherols, the raffinate phase was removed first. The extract phase was slowly bubbled with CO2 for approximately 10 min. Two extractant recovery processes, back extraction and centrifugation, are shown in Scheme 1. 2.3.1. Back Extraction. Hexane (30 mL) was added to the CO2-bubbled extract phase, followed by stirring for 60 min. Stirring was stopped and the phase separation was allowed to proceed for 60 min. The back extraction solvent phase (hexane phase) was then removed, collected, and analyzed via HPLC. The CO2-bubbled extract phase was further back extracted an additional two times, each with 30 mL of fresh hexane. The back extraction solvent phase for each back extraction was collected and analyzed using HPLC. Finally, the CO2-bubbled extract phase was heated to 100 °C with N2 bubbling and refluxed for approximately 3 h to release CO2 from the PEI, and then it could be used again as the extractant. 2.3.2. Centrifugation. The CO2-bubbled extract phase was stirred for 60 min, followed by centrifugation at 6000 rpm for 20 min. The centrifugal supernatant (cosolvent solution) was removed, collected, and analyzed with HPLC. The PEI−CO2 precipitant was washed twice, each with 30 mL of cosolvent. The cosolvent solution (centrifugal supernatant) after each washing was collected by centrifugation and analyzed with HPLC. Finally, 30 mL of fresh cosolvent was added to the PEI−CO2 precipitant, and then the mixture was heated to 100 °C with N2 bubbling and refluxed for 3 h to release CO2 from
tocopherols might also play a role in the separation. Since the methyl substituents on the benzene ring are electron-donating and their numbers and positions have an effect on the acidity of the phenolic hydroxyl group, the acidities of tocopherols differ from each other in the order α- < β-, γ- < δ-tocopherol, which was verified by Yang at el.24 through computational chemistry. Moreover, it was also shown that the phenolic hydroxyl group was the major site to interact with other molecules via specific interactions.24 Therefore, the differences in acidity among the tocopherols could be used for the separation of α-tocopherol from its tocopherol homologues. The polymeric extractants must be alkaline. The different abilities of tocopherols to bind to the alkaline polymer should facilitate the separation of the tocopherol homologues. The β-, γ-, and δ-tocopherols with stronger acidities are expected to be extracted by the polymeric extractant, while α-tocopherol remains in the original solution. In addition to presenting different binding opportunities, the polymeric extractant should be easy to recover; i.e., its interactions with the extracted molecules should be easily broken. A general approach is to introduce an acid to the extract phase to replace the tocopherols in interacting with the polymer to release the latter. The recent advent in employing CO2 as a trigger for the development of smart materials and processes provides another opportunity for such applications. Quite a few CO2-triggered stimuli-responsive molecules have been reported, such as alkylamine, amidine, and guanidine.25 Moreover, these molecules have been applied to many different areas, including reversible surfactants,26−32 ILs,33,34 gels,35 catalysts,36 molecular sensors,37 nanostructured materials,38 etc. Gaseous CO2 is acidic, and it becomes a perfect choice matching the alkaline polymer because it is nontoxic and inexpensive. In this work, polyethylenimine (PEI) samples, which contain a large number of amine groups and are commercially available, were selected as the polymeric extractant in the separation of αtocopherol from its homologue−hexane solution by liquid− liquid extraction. The effects of PEI molecular weight, cosolvent type, and PEI−cosolvent composition on the separation efficiency and selectivity were systematically studied. The CO2-triggered PEI recoverability and reusability as the extractant were also investigated.
2. EXPERIMENTAL SECTION 2.1. Materials. PEIs (99%; branched; primary, secondary, and tertiary amine groups in approximately 1/2/1 molar ratio) were purchased from Polysciences, Inc. The PEI1 sample has molecular weight (MW) 600 g/mol and polydispersity index (PDI) 1.08, while PEI2 has MW 1800 g/mol and PDI 1.14. 16026
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where Di is the distribution coefficient of solute i, and Dj is that of solute j. The recovery efficiency of solute i (Ei) is defined as
Table 1. Distribution and Selectivity Coefficients of Tocopherols in the PEI−Cosolvent/Hexane Biphasic Systema distribution coeff extractant PEI1− ACN
PEI2− ACN
PEI1− DMSO
PEI2− DMSO
PEI1− EtOH
PEI2− EtOH
ϰ(N)b
Cc
Ei = Miu /Mi
selectivity coeff
Dδ
Dβ and γ
Dα
Sδ/α
Sβ and γ/α
0
0
0.49
0.29
0.17
2.88
1.71
0.162 0.323 0.485 0.162
0.133 0.267 0.400 0.133
3.75 5.70 7.05 3.25
2.07 3.21 3.92 1.70
0.59 0.93 1.19 0.45
6.36 6.13 5.92 7.22
3.51 3.45 3.29 3.78
0.323 0.485 0
0.267 0.400 0
5.15 6.35 0.50
2.79 3.41 0.28
0.74 0.93 0.12
6.96 6.83 4.17
3.77 3.67 2.33
0.162 0.323 0.485 0.162
0.098 0.196 0.294 0.098
0.77 1.02 1.21 0.72
0.45 0.58 0.69 0.41
0.18 0.23 0.26 0.16
4.28 4.43 4.65 4.50
2.50 2.52 2.65 2.56
0.323 0.485 0.485
0.196 0.294 0.358
0.92 1.06 1.00
0.53 0.60 0.99
0.19 0.21 0.99
4.84 5.05 1.01
2.79 2.86 1.00
0.728 0.970 0.485
0.537 0.716 0.358
2.79 4.07 1.00
2.39 3.31 0.99
1.72 2.29 0.99
1.62 1.78 1.01
1.39 1.45 1.00
0.728 0.970
0.537 0.716
3.14 4.42
2.62 3.57
1.76 2.36
1.78 1.87
1.48 1.51
where Mui is the mass of solute i in the back extraction solvent phase or centrifugal supernatant, and Mi is that in the extract phase after extraction.
3. RESULTS AND DISCUSSION 3.1. Extractions. Two PEIs (PEI1 MW 600 and PEI2 MW 1800) in combination with three types of cosolvents (ACN, DMSO, and EtOH) were evaluated as the extractant system. The calculated distribution and selectivity coefficients based on the HPLC integrations are presented in Table 1. The extraction results in Table 1 confirm that PEI is a good candidate for use as the polymeric extractant for the tocopherols. Compared with the pure cosolvents, adding the PEI greatly improved the distribution and selectivity coefficients. The Dδ value of PEI1−ACN (ϰ(N) = 0.485) was 14.4 times higher than that of pure ACN. The corresponding Sδ/α value was 2.1 times higher. Higher distribution coefficients for the β-, γ-, and δ-tocopherols than Dα are preferred and expected since α-tocopherol is more inclined to remain in the raffinate phase. Moreover, both the distribution and selectivity coefficients increased with the PEI concentration except for the case when ACN was used as the cosolvent, in which case the selectivity coefficients slightly decreased. The distribution coefficients seemed more susceptible to the PEI concentration than the selectivity coefficients. The Dδ value increased by 32.5%, while the Sδ/α value increased only by 3.5% with ϰ(N) increasing from 0.162 to 0.323 in the PEI1−DMSO solution. It can be seen that changing the PEI concentrations to adjust the distribution coefficients for the extraction processes has only a minor influence on the selectivity coefficients, leading to better control of extraction capacities. 3.1.1. Effects of Cosolvent Type on Distribution and Selectivity. The same PEI concentration in the different cosolvents was used for comparison. The results suggest a great influence of the cosolvent type on the extraction. In general, ACN gave higher distribution and selectivity coefficients than other cosolvents. The Dδ and Sδ/α values of PEI1−ACN (ϰ(N) = 0.485) were 7.05 and 5.92, which were 5.8 and 1.3 times higher than those of PEI1−DMSO (ϰ(N) = 0.485), and 7.1 and 5.9 times higher than those of PEI1−EtOH (ϰ(N) = 0.485). It was found that PEI2 (MW 1800) did not have a high solubility in ACN, and the PEI2−ACN solution was somewhat turbid. However, the solution became clear upon addition of tocopherol−hexane for extraction. Compared to pure DMSO as extractant, PEI−DMSO had slightly higher selectivity coefficients. This might be attributed to the high polarity of DMSO restraining less polar tocopherols from being extracted to the extract phase.9 The clear advantage of EtOH over the other cosolvents was its low toxicity, which would contribute to greener industrial operations. However, EtOH and hexane were normally miscible. They gradually became immiscible with the PEI addition. Thus, higher PEI concentrations were required in EtOH than in ACN and DMSO for the phase separation with hexane. Although both the distribution and selectivity coefficients gradually increased with the PEI concentration, the separations of α-tocopherol from the tocopherol homologues using PEI−EtOH were less
a
The initial concentrations of the tocopherol homologues in hexane (mg·mL−1) were δ, 1.14; β and γ, 1.13; and α, 0.23. bϰ(N) is the ratio of the overall moles of amine groups of the PEI, including R3N, R2NH, and RNH2, to moles of cosolvent. cC is the weight of PEI per milliliter of cosolvent (g/mL).
the PEI. The resulting solution could be reused as the extractant. 2.4. HPLC Analysis. The HPLC system consisted of a Waters 1525 binary pump, an autosampler, a column heater, and a Waters 2487 dual λ absorbance detector. An Atlantis T3 column (5 μm, 4.6 × 250 mm) was used. The mobile phase was a mixed solution of methanol and water (95/5 v/v), and the wavelength for detection was 292 nm. It should be pointed out that the β- and γ-tocopherols were regarded as a combined fraction in the experiments because the HPLC peaks overlapped due to their similar structures. Figure 2 gives typical HPLC chromatograms of the extract phase and raffinate phase after extraction using PEI1−ACN (ϰ(N) = 0.162) as the extractant. 2.5. Determination of Distribution, Selectivity, and Recovery Efficiencies. The distribution coefficient of solute i (Di) is the mass fraction ratio of the solute i in the extract (PEI−cosolvent) and raffinate (hexane) phases at the equal volume of extractant and tocopherol−hexane solution: Di = Cie/Cir
(1)
Cei
where is the mass fraction of solute i in the extract phase, and Cri is that in the raffinate phase. The selectivity coefficient of solute i to solute j (Si/j) is the ratio of their distribution coefficients: Si / j = Di /Dj
(3)
(2) 16027
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Scheme 1. Experimental Approaches Used To Separate α-Tocopherol from Its Tocopherol Homologues and Recovery of the Extractant
Figure 2. HPLC chromatograms of the extract phase and raffinate phase after extraction using PEI1−ACN (ϰ(N) = 0.162) as the extractant. ϰ(N), molar ratio of amine groups (R3N, R2NH, and RNH2) on the PEI to ACN. Initial concentrations of tocopherol homologues in hexane (mg·mL−1): δ, 1.14; β and γ, 1.13; α, 0.23.
well as the PEI solubility in the cosolvents may have resulted in these changes. Steric effects are expected to increase with the increase of the PEI molecular weight, which could reduce the extraction capacities and change the interactions between the PEI chains and different tocopherol molecules. The distribution coefficients thus decreased and the selectivity coefficients increased. An opposite change in the distribution coefficients of PEI−EtOH might be attributable to better phase separations with hexane when using PEIs of higher molecular weight. Table 2 summarizes some of the best extraction data from the literature using IL−cosolvent as the extractant. The distribution and selectivity coefficients from this work are
efficient than those with the PEI−ACN and PEI−DMSO systems. 3.1.2. Effects of PEI Molecular Weight on Distribution and Selectivity. The effect of PEI molecular weight on the distribution and selectivity coefficients can be observed in Table 1. The Dδ value decreased by 13.3% and the Sδ/α value increased by 13.5% comparing PEI1−ACN (ϰ(N) = 0.162) to PEI2−ACN (ϰ(N) = 0.162). The selectivity coefficients generally increased with the PEI molecular weight from 600 to 1800 g/mol. The distribution coefficients decreased with the PEI molecular weight in ACN and DMSO, but increased in EtOH. The steric effects in the PEI chains and tocopherols as 16028
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Table 2. Distribution and Selectivity Coefficients of Tocopherols in the IL−Cosolvent/Hexane Biphasic System at 30 °Ca distribution coeff extractant [bmim]Cl−ACN
9
[bmim]Cl−DMSO9 [emim]Gly−ACN24 [emim]Ala−ACN24 [emim]Ac−ACN24 [hmim]Cl−ACN9 [omim]Cl−ACN9
selectivity coeff
ϰ(IL)b
Dδ
Dβ and γ
Dα
Sδ/α
Sβ and γ/α
0.02 0.05 0.02 0.05 0.02 0.02 0.02 0.05 0.05
4.07 5.37 0.53 0.55 19.2 20.9 19.3 6.50 9.98
2.02 2.77 0.29 0.29 9.31 10.5 9.40 3.44 5.35
0.36 0.48 0.10 0.09 1.51 1.55 1.39 0.59 0.90
11.3 11.2 5.3 6.1 12.7 13.5 13.9 11.0 11.1
5.6 5.8 2.9 3.2 6.2 6.8 6.8 5.8 5.9
a The initial concentrations of the tocopherol homologues in hexane (mg·mL−1) were δ, 1.00; β and γ, 0.98; and α, 0.20. bϰ(IL) was the mole fraction of IL in the IL−cosolvent solution.
Scheme 2. Interactions and Release of CO2 with Various Amine Groups in the PEI Chains
Table 3. Distribution, Selectivity, and Recovery Efficiencies of Tocopherols in Three Consecutive Recyclesa distribution coeff
recovery efficiencyc (%)
selectivity coeff
recycle
Dδ
Dβ and γ
Dα
Sδ/α
Sβ and γ/α
Eδ
Eβ and γ
Eα
1 2 3 1b
3.75 3.68 3.40 3.75
2.07 2.01 1.84 2.07
0.59 0.57 0.53 0.59
6.36 6.46 6.42 6.36
3.51 3.53 3.47 3.51
89.9 90.1 89.4 42.3
93.3 93.2 92.7 61.1
96.1 96.0 95.9 90.4
PEI1−ACN (ϰ(N) = 0.162) was used as the extractant, and the initial concentrations of the tocopherol homologues in hexane (mg·mL−1) were δ, 1.14; β and γ, 1.13; and α, 0.23. bNo CO2 bubbling prior to the back extraction. cThe Eδ, Eβ and γ, and Eα values referred to the recovery efficiencies after three times of back extraction. a
systems were heated with N2 bubbling. CO2 was released from the PEI to re-form a homogeneous PEI−cosolvent solution, which was used for further extractions. 3.2.1. Back Extraction. As indicated by the results listed in Table 1, the best cosolvent with regard to the distribution and selectivity coefficients was ACN. PEI1−ACN (ϰ(N) = 0.162) was selected to study the recovery and reuse of the extractant. The Dδ, Dβ and γ, and Dα values of 3.75, 2.07, and 0.59, respectively, meant most δ-tocopherol and β- and γ-tocopherols were extracted to the PEI1−ACN phase, while more than half of the α-tocopherol remained in the hexane phase. The tocopherols in the extract phase must be recovered, and the extract phase could be reused as the extractant. Scheme 1
comparable; in fact, the differences are actually smaller than they appear. The Dδ value of [emim]Ac−ACN (ϰ(IL) = 0.02) appeared to be much higher than that of PEI1−ACN (ϰ(N) = 0.485).24 However, the amount of δ-tocopherol extracted to the [emim]Ac−ACN phase in practice was not much more than that extracted to the PEI1−ACN phase. It was less than 10%. 3.2. Extractant Recovery. PEI has a high solubility in cosolvents such as ACN, DMSO, EtOH, and N,N′dimethylformamide (DMF). As shown in Scheme 2, CO2 can interact with the primary and secondary amine moieties in the PEI chains to form zwitterionic groups.39,40 This zwitterionic nature makes the PEI chains insoluble in the cosolvents at room temperature. The PEI−CO2 precipitant−cosolvent 16029
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required to reach an Eα over 92%. The recovery efficiencies were lower than those using the PEI−cosolvent systems. 3.2.2. Centrifugation. Besides the above back extraction method, the recoverability of tocopherols from the extract phase by centrifugation was also investigated. The experimental process is shown in Scheme 1. With PEI1−ACN (ϰ(N) = 0.162) as the extractant, the recovery efficiencies of δtocopherol, β- and γ-tocopherols, and α-tocopherol were 90.8, 94.0, and 96.7%, respectively. These values suggested that the tocopherols could also be effectively recovered from the extract phase by centrifugation. However, a drawback of this centrifugation method is its large energy consumption due to the small difference in density between the cosolvent and PEI− CO2. The PEI−cosolvent systems are believed to be an efficient extractant for industrial tocopherol separations. Take the separation conducted through a countercurrent extraction tower with 12 theoretical stages as an example.42 For a target of 90% purity of α-tocopherol with pure ACN as extractant, a total ACN having a volume equal to 5 times the tocopherol− hexane solution volume is required, and the yield of αtocopherol is just 17%. When PEI1−ACN (ϰ(N) = 0.162) is used as the extractant to obtain the same 90% purity of αtocopherol under the same operating conditions, the volume of extractant required is only 60% of that of the tocopherol− hexane solution volume. The yield of α-tocopherol reaches 65%. The advantages of PEI−ACN (ϰ(N) = 0.162) to pure ACN are clear. After the extraction, the recovery of tocopherols from the extract phase can be facilitated greatly by the back extraction of tocopherols and the recovery of PEI for reuse with the aid of CO2. The PEI−cosolvent systems have clear advantages in tocopherol separations.
reviews the experimental approach. The tocopherols in the extract phase were recovered by back extraction, and the extractant was recycled three times. Table 3 summarizes the corresponding distribution, selectivity, and recovery efficiencies of the tocopherols for the three consecutive recycles. It is believed that CO2 replaces the tocopherols interacting with the PEI to facilitate their recovery from the extract phase. To confirm the contribution of CO2 bubbling, a control experiment, without CO2 bubbling, was carried out prior to the back extraction. These results are included in Table 3. It is clear that CO2 bubbling makes a significant difference. Almost all the Eδ, Eβ and γ, and Eα values after the three back extractions reached 90% in the CO2 bubbling experiments, while only the Eα value reached 90% (Eδ and Eβ and γ values were just 42.3 and 61.1% without CO2 bubbling). The recovery efficiencies without CO2 bubbling can be estimated from the distribution coefficients using the following equations:23 Mi , n + 1 =
Mi ,0 =
MPEIDiMi , n MPEIDi + MHex
(4)
MPEIDiMi ,Ini MPEIDi + MHex
Ei , n = 1 −
(5)
Mi , n Mi ,0
(6)
where Mi,n is the mass of i-tocopherol (i is δ, β and γ, or α) in the PEI−cosolvent phase after back extraction for n times, MPEI is the mass of the PEI−cosolvent phase, MHex is the mass of the back extraction solvent phase (hexane phase), Di is the distribution coefficient of i-tocopherol, Mi,0 is the mass of itocopherol in the PEI−cosolvent phase after extraction, Mi,Ini is the initial mass of i-tocopherol in hexane before extraction, and Ei,n is the recovery efficiency of i-tocopherol after back extraction for n times. Using these eqs 4−6, it was estimated that about 12 back extractions using fresh hexane each time would be required to recover 90% of δ-tocopherol without CO2 bubbling, compared to three back extractions with CO2 bubbling. Thus, the CO2 bubbling plays an important role in the recovery of tocopherols. Moreover, the theoretical estimates of the Eδ, Eβ and γ, and Eα values were about 44, 63, and 92% after three back extractions, agreeing well with the experimental results (42.3, 61.1, and 90.4%), suggesting the estimates are reliable. The leftover PEI1−ACN phase could be used again as the extractant after the back extraction, as shown in Scheme 2. The distribution coefficients dropped only about 10% after being reused for three cycles, from 3.75, 2.07, and 0.59 to 3.40, 1.84, and 0.53 for δ-tocopherol, β- and γ-tocopherols, and αtocopherol, respectively. Given that the PEI1−ACN solution gradually turned slightly yellow during the process and PEI can be oxidized by air,41 it is speculated the minor drop in the distribution coefficients was due to some oxidation of the PEI. There were almost no changes in the selectivity coefficients and recovery efficiencies. The extractant system had a great recyclability with good performance of the extraction. It is worth comparing the recovery efficiencies of tocopherols from this study with those reported for an IL−cosolvent system reported in the literature.25 When [emim]Ala−DMF (ϰ(IL) = 0.05) was used as the extractant, the Dα value was 2.46. After five back extractions, the Eα value was just 73.1%, which agreed well with its theoretical estimate. Ten back extractions were
4. CONCLUSION In summary, the separation of α-tocopherol from its tocopherol homologues using PEI−cosolvent as extractant by liquid−liquid extraction was systematically studied. Two different molecular weight PEIs (PEI1 MW 600 and PEI2 MW 1800) and three types of cosolvents (ACN, DMSO, and EtOH) were used to investigate their effects on the distribution and selectivity coefficients of tocopherols. These coefficient values were much higher than those using the pure cosolvents, demonstrating the effectiveness of PEI as extractant. Moreover, CO2 was employed to facilitate the recovery of tocopherols from the extract phase. A theoretical estimation showed that 12 back extractions were required for recovery efficiencies over 90% without CO2 bubbling, but only three back extractions were needed to reach the same efficiency with the aid of CO2. The distribution coefficients in the recycling experiments decreased by only about 10% after being reused three times, and there were almost no changes in the selectivity coefficients. The recovery of tocopherols could also be facilitated by centrifuging PEI−CO2 precipitants from the extract phase. Compared to ILs reported in the literature, the extraction efficiency and selectivity with PEI was slightly lower, but the reclaiming and recycling of the PEI extractant was greatly facilitated by its CO2-responsive feature. This work demonstrates that CO2responsive polymeric extractants are useful in the separation of natural products.
■
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
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*E-mail:
[email protected].
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China (Grants 21376211 and 20976153, Key Grant 20936006), the Chinese State Key Laboratory of Chemical Engineering at Zhejiang University (Grants SKL-ChE-11D02 and SKL-ChE12T05), and the Program for Changjiang Scholars and Innovative Research Team in University in China (IRT0942) for financial support.
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