Selection of Imidazolium-Based Ionic Liquids for Vitamin E Extraction

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Selection of imidazolium-based ionic liquids for vitamin E extraction from deodorizer distillate Lei Qin, Jianan Zhang, Hongye Cheng, Lifang Chen, Zhiwen Qi, and Weikang Yuan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01330 • Publication Date (Web): 22 Nov 2015 Downloaded from http://pubs.acs.org on November 28, 2015

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ACS Sustainable Chemistry & Engineering

Selection of imidazolium-based ionic liquids for vitamin E extraction from deodorizer distillate Lei Qin, Jianan Zhang, Hongye Cheng, Lifang Chen, Zhiwen Qi*, Weikang Yuan Max Planck Partner Group at the State Key Laboratory of Chemical Engineering, School of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China *

Corresponding author. Fax: (+) 86-21-64253528. E-mail: [email protected].

Abstract Deodorizer distillate as a by-product of vegetable oil refining is an important source of natural vitamin E. Simple and green extraction can be employed to recover vitamin E from deodorizer distillate, and its performance strongly relies on the solvent used in the process. In order to screen an imidazolium-based ionic liquid (IL) as entrainer, a methodology integrating theoretical and experimental evaluations was applied. ILs composed of 47 types of anions were first investigated by COSMO-RS to figure out the most potential one, and the effect of the alkyl chain on the imidazolium ring was addressed by experiment. The IL [C6MIm]Ac was finally determined as the most promising solvent with the selectivity of 108.23 and the partition coefficient of 8.182 for α-tocopherol. The liquid-liquid equilibrium data and the extraction result for corn oil deodorizer distillate indicate its superior performance for the extraction process dealing with low concentration of vitamin E. Key words: vitamin E; deodorizer distillate; solvent selection; ionic liquid; extraction; hydrogen bond; interaction forces

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Introduction Deodorizer distillate is a valuable waste in vegetable oil refining processes, which contains several high value-added products, such as sterol, squalene, and natural vitamin E. Because natural vitamin E is superior to the synthetic one in biological function, dietary safety and public acceptance,1 it is extensively utilized in health foods, cosmetics and other high value-added products. Therefore, the recovery of vitamin E from deodorizer distillate is widely studied. Molecular distillation3 is a current industrial method for recovering tocopherols from the deodorizer distillate. However, it suffers from problems of unit capacity limit and low separation selectivity and energy efficiency. Meanwhile, other techniques, such as solvent extraction,4-5 supercritical fluid extraction,6 chemical treatment,7 adsorption8 and ion-exchange,9 have been developed. Among these processes, extraction is regarded as the most promising option because of its moderate operation conditions and simple separation equipment. However, when applying to the deodorizer distillate system with organic solvents, it suffers from low selectivity towards tocopherols and large consumption of solvent. For instance, Vicente et al.4 adopted ethyl lactate to recover α-tocopherol from olive oil, and the highest partition coefficient of and selectivity to tocopherol at 288.2 K were only 1.30 and 10.6, respectively. Hence, highly effective entrainer is strongly demanded for practical application for extraction operation for this system. In recent years, ionic liquids (ILs) have been intensively studied for extraction due to their unique physical and chemical properties such as non-flammability, very


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low volatility, high thermal and chemical stabilities, wide range of liquid and flexible design strategy.10-11 Considering all these features, ILs can potentially become alternative to replace volatile organic solvents in extraction processes,12 especially for mixtures which are difficult to separate (e.g. recovery of bitumen from oil or sands,13-14 separation of benzene from cyclohexane,15 thiophenic sulfur compounds from hydrocarbons,16-17 ethyl acetate from ethanol,18 and high value-added compounds from biomass19-22). For the recovery of vitamin E, ILs as extractant have also been reported. Ni et al.23 investigated amino acid based ILs for the selective extraction of α-tocopherol from methyllinoleate, and found that the selectivity reached up to 29 with [C2MIm]Ala or [C2MIm]Lys diluted by DMF. However, for these functionalized ILs, it is more difficult and complex to synthesize compared with common ILs. Meanwhile, the ILs extraction performance for this system in open literature needs to be improved from the perspective of practical application. Moreover, considering the huge number of cation-anion combinations, the ILs that have been previously reported for recovering vitamin E (tocopherol) are very limited and there are still large amount of ILs that not been studied. Therefore, further screening of suitable ILs in a wide range for extraction of vitamin E is highly desirable. For systems containing compound with phenolic structure or aromatic ring, such as vitamin E in this work, the imidazoloum- and pyridinium- based ILs usually display great extraction performance,24-25 which is attributed to the strong π-π interaction between the aromatic ring and the nitrogen-based heterocycle cations. In



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addition, among the numerous ILs, imidazolium-based ILs have been proved to be highly attractive and versatile because of their facile accessibility, high stability and easy recycling.26 Therefore, the imidazolium-based ILs were concerned in this work when selecting the proper cation candidate. The aim of this work is to select imidazolium-based ionic liquids for tocopherol recovery from the oil deodorizer distillate taking α-tocopherol and methyllinoleate as the model component of tocopherol and fatty acid methyl ester, respectively. The selectivity and solubility of ILs (47 anions with [C4MIm]+ as cation, one of the most frequently-used and widely researched types of imidazolium cation) in the model oil deodorizer distillate was predicted by COSMO-RS, and the effect of the number and the length of the alkyl chains on the imidazolium ring on the extraction equilibrium were optimized experimentally. Moreover, the extraction performance of the screened IL was confirmed by liquid-liquid equilibrium data and oil deodorizer distillate extraction result.

Experimental and theoretical methods Experimental method Methyllinoleate (≥ 99%) and α-tocopherol (≥ 97%) were purchased from Acros (Belgium) and Alfa Aesar (UK), respectively. The ILs [C4MIm]BF4, [C4MIm]PF6, [C4MIm]NO3, [C4MIm]C3H5O3, [C4MIm]Ac, [C2MIm]Ac, [C5MIm]Ac, [C6MIm]Ac, [C7MIm]Ac, [C8MIm]Ac, [C2Im]Ac, [C2MMIm]Ac, [C4Im]Ac and [C4MMIm]Ac with purity of > 98% were obtained from Lanzhou Institute of Chemical Physics,


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Chinese Academy of Sciences. All ionic liquids were dried under vacuum condition at 60 °C by rotary evaporator for at least 24 h to make sure the water content below 1000 ppm (determined by Karl-Fischer titration). Other chemicals were purchased from Aladdin Chemical Co., Ltd. with purity above 99.0 wt% without further purification. In general, although there are various kinds of components in the deodorizer distillate, a mixture with two main components (i.e. tocopherols and fatty acid methyl ester) can be obtained after acid-catalyzed methyl esterification and crystallization separation.1 In view of this, the oil deodorizer distillate was modeled by the mixture of α-tocopherol and methyllinoleate (mass ratio of 0.08:1, which is a representative content in the deodorizer distillate of industrial vegetable oils). In a typical run, 1 g model oil was first prepared in a 5 mL screw-capped vial and a certain amount of ILs (molar ratio of IL to model oil is 2.5:1) was then charged into the model oil. The sealed vial was immersed in an oil bath to control the liquid temperature at 25 °C, 80 °C or 100 °C with a temperature fluctuation of ±0.1 °C (Huber Ministat 230, Germany), and vigorously agitated with magnetic stirrer (1000 r/min). After stirring for 3 h, the system was allowed to settle for 8 h at same temperature (25 °C, 80 °C or 100 °C) to ensure complete extraction equilibrium. Samples of the extract and the raffinate were carefully taken by syringe without destroying the liquid phase boundary, and the concentration of α-tocopherol and methyllinoleate in the two phases were measured by HPLC (SHIMADZU LC-2010AHT) with methanol as the mobile phase and equipped with ODS-BP (5µm) sinochrom column at a wavelength of 200 nm. A



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nitrogen analyzer (Antek 9000, USA) was applied to determine the possible solubility of IL in the raffinate phase. The experiments were repeated three times to ensure the reproducibility of the results (the standard deviation of liquid-liquid extraction experimental < 0.5%). Theoretical methods COnductor-like Screening MOdel for Real Solvents (COSMO-RS) is a statistical thermodynamics method based on the quantum chemical COSMO calculations.27-28 The COSMO-RS calculations were performed by COSMOthermX (COSMOlogic GmbH & Co. KG, Version C3.0, Release 13.01) at the BP86/TZVP level with the parameter file BP_TZVP_C30_1301.ctd. The COSMO-files of all compounds involved in this work were taken from the database. In this work, the selectivity to α-tocopherol with different ILs and the solubility of imidazolium-based (i.e. [C4MIm]+) ILs in methyllinoleate were calculated to preselect proper anion candidates. In the calculation of COSMO-RS, the molar ratio of α-tocopherol to methyllinoleate was the same as the model oil (i.e. 0.08:1), meanwhile the cation and anion were treated as dissociated parts with their molar ratio corresponding to the stoichiometry of the IL (Detailed information about COSMO-RS for ionic liquids can be found elsewhere.29). Solubility parameter is a characteristic of substance, and the difference of the solubility parameters of two substances can offer a very intuitive judgement about the mutual solubility between them. In this work, to explain the influence of structure of imidazolium cations on extraction performance, the solubility parameters of ILs,


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α-tocopherol and methyllinoleate were calculated as following.30 Firstly, full geometry optimizations were performed using Dmol3 in Materials Studio (Accelrys Inc.). Then electrostatic potential-derived charges were directly calculated and assigned to all atoms (see Figure S1a, exemplified with the cation of [C2MIm]+). Afterwards, Amorphous Cell Module was employed for bulk ILs or other compounds. Their densities were then computed through molecular simulations in NPT using Forcite Module and force field. A sample model of the amorphous cell (20 cations and 20 anions) was shown in Figure S1b. By using the density, the same work for bulk ILs was repeated. After that, NVT simulations were run by using the same module and force field as NPT simulations. For IL, another step was needed to add the intra-molecular term to the cohesive energy density by using Blends Module with the same force field.31

Results and discussion Selection of anions In general, the structure of anion has a great influence on the extractive efficiency of ILs. Here, in order to select the suitable anion candidate, two important parameters were calculated to evaluate the extractive properties of ILs with the fixed cation of [C4MIm]+. The first one is the selectivity to α-tocopherol, which is the ratio of partition coefficients of α-tocopherol and methyllinoleate in two phases,

Sij = βi / β j where

βi

(1)

represents the partition coefficient of solute i defined by



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β i = xiE / xiR , E

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(2)

R

with xi and xi referring to the concentration of solute i in the extract and raffinate, respectively. A high selectivity means, to some extent, the specificity of extractants is high and less non-target product will be moved into the extract. The other parameter is the solubility of ILs in the raffinate. A low solubility of ILs in the raffinate indicates less loss of ILs in the extraction process. Since the main component in the raffinate of the system is methyllinoleate, the solubility of IL in methyllinoleate is used as a standard of evaluation when screening the anions. Therefore, for a proper anion and the corresponded IL, the selectivity should be high and the solubility of IL in methyllinoleate should be low enough. The selectivity and the solubility of ILs in methyllinoleate were calculated by COSMO-RS. The calculated 47 anions cover the common (e.g. PF6-, BF4-, Tf2N- and Ac-) and uncommon (e.g. BClF3-, ClO4-, I3- and N(CN)2-) groups. The detailed information of anions are given in Tables S1 (Supporting Information). For most ILs with [C4MIm]+ as cation, the selectivity to α-tocopherol varies over a wide range (from 10-3 to 10) with different anions. A more intuitive change can be obtained when the simulated result is plotted with the HB_acc3 of anions, which is one of most useful descriptors that quantifies the hydrogen-bonding (HB) acceptor strength (basicity).32 As seen in Figure 1a, with the enhanced ability of the hydrogen bond acceptor (HB_acc3), the selectivity increases exponentially. For example, for anions with very low HB acceptor strength (e.g. 0 and 2.4871 for PF6- and BF4-, respectively), they have very poor selectivity (0.0079 and 0.0218, respectively). When


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increasing the HB acceptor strength (e.g. 19.0409 for NO3-), a medium selectivity (0.2692) can be reached. When the anions have high HB acceptor strength (e.g. 31.2026 and 38.9278 for C3H5O3- and Ac-, respectively), very attractive selectivity (1.4029 and 7.0219, respectively) can be obtained. Therefore, it can be concluded that the stronger HB acceptor ability of the anion is favorable for the extraction process. 9

(a)

8

Prediction by COSMO-RS Prediction by COSMO-RS for selected anions Experimental result for selected anions

7

Selectivity

6 5 4 3 2 1 0 0

5

10

15

20

25

30

35

40

45

HB_acc3 of the anion 0.0

Solubility of ILs in methyllinoleate (lg(X))

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(b) Prediction by COSMO-RS Prediction by COSMO-RS for selected anions

-0.5 -1.0 -1.5 -2.0 -2.5 -3.0 -3.5 -4.0 0

5

10

15

20

25

30

35

40

45

HB_acc3 of the anion

Fig. 1 Influence of HB_acc3 of anions on extractive properties. (a) Selectivity towards α-tocopherol with ILs composed of [C4MIm]+ and different anions. (b) Solubility of ILs in methyllinoleate.

Figure 1b demonstrates the solubility of ILs in methyllinoleate versus the HB_acc3 of anions. As seen, the value of (lg(X)) ranges from -0.7 to -3.5, which is



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located between the two boundary lines, and it shows a tendency of decreased solubility with increasing the HB_acc3 of anions. For example, for the HB_acc3 of PF6- (0.0000) < BF4- (2.4871) < NO3- (19.0419) < C3H5O3- (31.2026) < Ac- (38.9278), the corresponding solubility is PF6- (-1.3075) > BF4- (-1.9877) > NO3- (-2.7319) > C3H5O3- (-2.8904) > Ac- (-3.3773). The tendency could be attributed to the increased difference in polarity between methyllinoleate and ILs with stronger polar anions. Moreover, it indicated the anion with stronger HB acceptor ability can result in a low solubility of IL in methyllinoleate, which is favorable for the extraction process. Taking into account the influence on both the selectivity to α-tocopherol and the solubility of IL in the raffinate, anion with stronger HB accepter ability is desirable.

CCl4 α-tocopherol α-tocopherol+[C4MIm]PF6 α-tocopherol+[C4MIm]BF4 α-tocopherol+[C4MIm]NO3 α-tocopherol+[C4MIm]C3H5O3 α-tocopherol+[C4MIm]Ac

2600

2800

3000

3200

3400

3600

3800

4000

Wavenumber /cm-1 Fig. 2 IR spectra for α-tocopherol with different ILs with different anions

In order to verify the accuracy of the prediction, experiments were carried out with five representative imidazolium-based ILs ([C4MIm]PF6, [C4MIm]BF4, [C4MIm]NO3, [C4MIm]C3H5O3 and [C4MIm]Ac), as shown in Figure 1a (marked as hollow circle). As seen, the COSMO-RS predictions are in good agreement with


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experimental results. Hence, the selectivity towards α-tocopherol predicted by COSMO-RS in this work is reliable. In order to interpret the relationship between the selectivity and the HB acceptor ability of the anion, the infrared (IR) spectrometry was employed, as illustrated in Figure 2. As seen, α-tocopherol has two big absorbance peaks at 3500 cm-1 and 2900 cm-1, respectively. The former is related to the stretch of unbonded phenolic hydroxyl in α-tocopherol, and the latter is the characteristic peak related to three groups of CH3-, CH2- and CH- on its molecular structure. The absorbance spectra of α-tocopherol with an equimolar amount of ILs is also illustrated in Figure 2. For the case of α-tocopherol dissolved in [C4MIm]PF6 or [C4MIm]BF4, no change in the absorbance peak at 3500 cm-1 is observed, which indicates the absence of hydrogen bond formed by the phenolic hydroxyl. However, it enlarges and shifts to lower wavenumber when dissolved in [C4MIm]NO3, [C4MIm]C3H5O3 or [C4MIm]Ac. As suggested by Günzler and Gremlich,

33

this is due to the formation of hydrogen bond which leads to

equalized electron cloud and decreased energy of the system. Moreover, the shifted absorbance peak ranked the ability of forming the hydrogen bond with α-tocopherol as follows: [C4MIm]Ac > [C4MIm]C3H5O3 > [C4MIm]NO3. In addition, the H-bond energies between α-tocopherol and ILs were calculated by COSMO-RS and shown in Table S2. The absolute value of H-bond energies increase with the order of PF6- (-0.50) < BF4- (-1.04) < NO3- (-2.48) < C3H5O3- (-3.42) < Ac- (-4.21), which is consistent with the IR spectra result. Based on the IR measurement and the calculated H-bond energies, it can be concluded that the enhancement of selectivity is attributed to the



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HB acceptor ability of anion. Considering its relatively high selectivity to α-tocopherol (7.0219) and low logarithmic solubility in methyllinoleate (-3.3773) of [C4MIm]Ac, the anion of Ac- is chosen as the most promising anion candidate for further study. Selection of imidazolium based cations As mentioned, the imidazoloum-based ILs are generally suitable solvent candidate for extraction of systems containing compound with phenolic structure or aromatic ring. The imidazolium-based cation can be diversified by changing the number of the substituent group and the length of its alkyl chain in the imidazolium ring (especially at the 1- and 3- positions). Therefore, the influence of the number and the length of alkyl chain of the substituent group in the imidazolium ring on the extraction efficiency were investigated combined while fixing the selected anion of Ac-. Influence of the number of substituent group (alkyl chain) in the imidazolium ring Six imidazoloum-based cations, namely [C2Im]+, [C2MIm]+, [C2MMIm]+, [C4Im]+, [C4MIm]+ and [C4MMIm]+, were introduced to investigate the effect of the number of substituent groups. The selectivity to α-tocopherol and the partition coefficients of α-tocopherol and methyllinoleate with ILs with six cations and Acanion are listed in Table 1. For [C2MMIm]Ac and [C4MMIm]Ac, the extraction temperature was 80 °C and 100 °C, respectively, because it is solid at room temperature (their melting points is nearly 65 °C and 95 °C, respectively). As seen, with more substituent groups in the imidazolium ring, the selectivity gradually arises,


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e.g. [C2Im]+ (1.12) < [C2MIm]+ (2.56) < [C2MMIm]+ (2.94) and [C4Im]+ (1.97) < [C4MIm]+ (5.86) < [C4MMIm]+ (10.75), meanwhile both partition coefficients decrease (C2: 0.107 to 0.038 for β(α), and 0.095 to 0.013 for β(Me); C4: 0.358 to 0.043 for β(α), and 0.182 to 0.004 for β(Me)).

Table 1. Partition coefficients and selectivities to α-tocopherol using different ILs determined by experiment

Experimental ILs

T (°C) β(α)

β(Me)

S

[C2Im]Ac

25.0

0.107

0.095

1.12

[C2MIm]Ac

25.0

0.039

0.015

2.56

[C2MMIm]Ac

80.0

0.038

0.013

2.94

[C4Im]Ac

25.0

0.358

0.182

1.97

[C4MIm]Ac

25.0

0.123

0.021

5.86

[C4MMIm]Ac

100.0

0.043

0.004

10.75

Fig. 3 σ-Profiles and σ-surfaces of [C4Im]+, [C4MIm]+ and [C4MMIm]+



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For a cation, a strong ability of HB donor will strengthen the interaction between the anion and solute, especially for the solute with an electron-accepting group (e.g. methyllinoleate in this work), which can enhance the solubility of solute in ILs and increase its partition coefficient. Therefore, from the polarity of the structured imidazolium cation, the effect of the alkyl chain can be figured out. The polarity of cations can be described with the help of their σ-profiles, which is the most important molecule-specific properties of the COSMO-RS theory.28 Taking [C4Im]+, [C4MIm]+ and [C4MMIm]+ as examples and as illustrated in Figure 3, for the three cations, one large shouldered peak is centered close to the boundary (-0.0082 e/Å2) between the non-polar and the HB donor regions, which indicates a slight ability of HB donor. Moreover, a minor peak can be observed in the HB donor region only for [C4Im]+ (-0.02 e/Å2), corresponding to the hydrogen directly connected with nitrogen at the 3- position in imidazolium ring, which causes a stronger polarity than other two cations. In addition, the HB_don3 (HB donor strength) of the three cations are 8.5965, 1.9353 and 0.3651, respectively, which again confirms that the polarity of the cations decreased with increasing the number of the alkyl chain. As a result, the IL with [C4Im]+, which has the strongest ability of HB donor among the three cations, shows the highest partition coefficient of α-tocopherol and methyllinoleate. However, the selectivity to α-tocopherol for [C4Im]Ac is the lowest. This is attributed to the stronger influence of the polarity for cations with enhanced ability of HB donor on the partition coefficient of methyllinoleate, which has an


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electron-accepting group, than α-tocopherol. Hence, ILs composed of cation with less substituent groups has a better partition coefficient but a poor selectivity. Considering both the partition coefficient and the selectivity, the di-substitutional imidazolium-based ILs have satisfying performance. Meanwhile, in most cases, tri-substitutional imidazolium-based ILs (e.g. [C4MMIm]Ac or [C2MMIm]Ac) have higher melting point and viscosity,34 which is unfavorable for practical application and easily increases the probability of vitamin E oxidation at high temperature. Thus, the di-substitutional imidazolium cation, i.e. [C4MIm]+, is selected for further study.

Influence of the alkyl side chain length in the imidazolium cation The influence of the alkyl chain length of the di-substitutional imidazolium cation on the extraction efficiency were investigated by varying the length of substituent groups at 1- position on the imidazolium ring. As seen in Figure 4 (also see the detailed data in Table S3, Supporting Information), the partition coefficients of both α-tocopherol and methyllionleate raise when increasing the alkyl chain. However, the two trends are different. For methyllinoleate, the partition coefficient (β(Me)) grows slightly with the alkyl substituent increasing from C2 to C8 (e.g. 0.015 for [C2MIm]Ac and 0.171 for [C8MIm]Ac). In contrast, for α-tocopherol, it rapidly increases from C2 to C6 (e.g. 0.039 for [C2MIm]Ac and 8.182 for [C6MIm]Ac), and then slows down from C6 to C8 (e.g. 9.387 for [C7MIm]Ac and 9.713 for [C8MIm]Ac). As a result, the selectivity reaches its maximum at 108.23 for [C6MIm]Ac, which is about 40 times higher than the lowest with [C2MIm]Ac.



11

120

10

110

β (α) β (Me)

9

100

Selectivity

90

8

80

7

70 6

60

5

50

4

40

3

30

2

20

1

10

0

0 0

1

2

3

4

5

6

7

8

Selectivity

Partition coefficient

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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9

Number of carbon atoms in alkyl chain

Fig. 4 Partition coefficient and selectivity of extraction of ILs with different length of chain alkyl

Taking into account of both the high partition coefficient of α-tocopherol (8.182) and high selectivity to α-tocopherol (108.23), the ionic liquid [C6MIm]Ac is demonstrated to be a suitable solvent for the separation of α-tocopherol and methyllinoleate.

Analysis of the influence of alkyl chain length on partition coefficient To understand the enhanced partition coefficient of both α-tocopherol and methyllinoleate from C2 to C8, the solubility parameter δ are calculated by Materials Studio. As seen in Figure 5 (a) and Table S4, from C2 to C8, δ decreases as the chain alkyl length varies (33.88 for [C2MIm]Ac and 26.93 for [C8MIm]Ac). The same is true for the difference of δ (∆δ) between ILs and α-tocopherol or methyllinoleate, which indicates the dissolution of α-tocopherol or methyllinoleate will be more effective in ILs composed of cation with longer chain.35 As a result, the partition coefficients of α-tocopherol and methyllionleate are enhanced with increasing the alkyl chain length. Moreover, although ∆δ(Me) is smaller than ∆δ(α), the partition coefficient of α-tocopherol is higher than methyllinoleate because of strong hydrogen


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bonds between ILs and α-tocopherol, which cannot intuitively reflect through ∆δ. 12

[C4MIm]Ac

30

10

[C5MIm]Ac

[C6MIm]Ac

[C7MIm]Ac [C MIm]Ac 8

25 20

5

∆δ(Me)

15 10

14

Molecular interation energy (Kcal/mol)

35

(b) [C2MIm]Ac

0

HB_don3 Misfit HB vdW

[C2MIm]Ac [C4MIm]Ac [C5MIm]Ac [C MIm]Ac [C7MIm]Ac 6

[C8MIm]Ac

8 6

14 12 10 8

Methyllinoleate

6

HB_don3

40

∆δ(α)

(a) 3 0.5 Solubility parameter(J/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4

4

2

2

0

0

-2

-2

-4

-4

-6

-6

-26

-26

δ(α) = 17.41, δ(Me) = 18.08 (calculated by the MS in this work). ∆δ(solute) = δ(ionic liquid) – δ(solute). Fig. 5 Different ILs’ s Solubility parameters (a) and their molecular interaction energies with α-tocopherol (b) calculated by Materials Studio and COSMO-RS, respectively

The influence of the interaction energies between ionic liquids and solutes on the partition coefficient are determined by 3 energy descriptors in COSMO-RS theory, i.e. van der Waals (vdW) interaction, HB interaction and misfit interaction.36 The first two are common type forces and the last is defined as electrostatic interaction of the two contacting segments in COSMO-RS which could be regarded as another indirect form of polarity.28 The 3 descriptor parameters together with the corresponding HB_don3 of the cations of the ILs with varying alkyl chain length are clearly denoted in Figure 5 (b). As illustrated, with the length of alkyl substituent from C2 to C8, the vdW interaction energies, HB interaction energies and corresponding HB_don3 almost keep unchanged, which implies the vdW interaction and the hydrogen bond are not the dominant factor on the rapid increasing of partition coefficient of α-tocopherol for different imidazolium cations.



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α-tocopherol α-tocopherol+[C2MIm]Ac α-tocopherol+[C4MIm]Ac α-tocopherol+[C5MIm]Ac α-tocopherol+[C6MIm]Ac α-tocopherol+[C7MIm]Ac α-tocopherol+[C8MIm]Ac

2600

2800

3000

3200

3400

3600

3800

4000

-1

Wavenumber /cm

Fig. 6 IR spectra for α-tocopherol with different ILs with different anions

To verify the HB interaction calculated by COSMO-RS theory, the IR spectra of α-tocopherol in ILs with different length of cation alkyl chain was recorded. As seen in Figure 6, the absorption peak of α-tocopherol at 3500 cm-1 enlarges and shifts to lower wavenumber (3400 cm-1), when equimolar amount of ILs is added into α-tocopherol. This is evident that the hydrogen bond is formed between α-tocopherol and the ILs. Moreover, the absorption peaks of α-tocopherol dissolved in ILs with varying chain length of alkyl group are almost overlapped. It means the ability to form hydrogen bond between α-tocopherol and these ILs is roughly equal, which is consistent with COSMO-RS calculated results. This suggests that there exists other vital interaction besides the hydrogen bond between ILs and vitamin E that plays more important role on the extraction performance. For misfit interaction energies (Figure 5b), an apparent decrease tendency of the misfit interaction energies with increasing the length of chain alkyl is observed (e.g. 12.31 Kcal/mol for [C2MIm]Ac and 8.74 Kcal/mol for [C8MIm]Ac). It can be


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attributed to the drop of average charge density of imidazolium cations, which make the mismatching property between the IL and α-tocopherol become smaller. And then, the gradually drop of misfit interaction will weaken the contradictive interaction between α-tocopherol and cations. From the above analysis, it can be concluded that the increasing of partition coefficients of α-tocopherol with varying the length of alkyl chain is mainly dominated by the misfit interaction or electrostatic interaction between α-tocopherol and ionic liquids.

Evaluation of the extraction performance of [C6MIm]Ac In order to further evaluate the performance of the above determined ionic liquid [C6MIm]Ac by experiment, the liquid-liquid equilibrium data for the ternary system (α-tocopherol + methyllinoleate + [C6MIm]Ac) at atmospheric pressure and 25 °C are displayed in Table S5 and illustrated in Figure 7. From the phase diagram, α-tocopherol has a much higher solubility in the IL compared with methyllinoleate, which can be seen from the partition coefficient (2.399 to 11.031 for β(α)). Moreover, the content of IL in the raffinate phase is very low (in the range of 0.39% - 1.45%, Table S4), which it becomes easier to recover the ILs in the raffinate in the subsequent process. The measured solubility of IL in methyllinoleate and methyllinoleate in IL are 0.37g/100g and 4.21g/100g, respectively. In the region of low α-tocopherol concentration, the miscibility gap is very wide, and the residue content of α-tocopherol in the raffinate is less than 2% after only one-pass extraction process.



0.0

1.0

0.1

0.9

0.2

0.8 0.7

0.4

0.6

me th

0.5

l ero ph oco

yll ino lea te

0.3

α -t

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0.5

0.6

0.4

0.7

0.3

0.8

0.2

0.9

0.1

1.0 0.0

0.0 0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

[C6MIm]Ac Fig. 7 Phase diagram for the experimental data (mass based) of the ternary system (α-tocopherol + methyllinoleate + [C6MIm]Ac) at 25 ºC

Table 2. Extraction performance of [C6MIm]Ac for corn oil deodorizer distillate (initial concentration [C6MIm]Ac:deodorizer distillate=1:1, wt/wt)

Real oil

α

β+γ

δ

Content, %

4.09

1.27

0.93

β(α)

17.25

10.50

39.19

η, %

96.02

93.62

98.21

Extraction In addition, the corn oil deodorizer distillate was also used to examine the extraction efficiency of [C6MIm]Ac, and the result is demonstrated in Table 2. As seen, for deodorizer distillate, the total vitamin E content is 6.29% (α: 4.09%, β and γ: 1.27%, δ: 0.93%), and the rest is mainly fatty acid methyl ester which is determined by GC-MS. With [C6MIm]Ac as an entrainer, the partition coefficient of four isomers for deodorizer distillate (α: 17.25, β and γ: 10.50, δ: 39.19) are higher than the


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partition coefficient for model oil. Meanwhile, after one step extraction, the recovery rate is already up 93%, which means through two-pass extraction process, the loss of vitamin E is very small and acceptable. At last, because vitamin E is fat-soluble, and [C6MIm]Ac is hydrophilic ionic liquid, the crude vitamin E (contains 45.2% of natural vitamin E and the rest is mainly fatty acid methyl ester) is simply obtained in upper layer after a certain water is added into the extract phase. Compared with other solvents for the studied system in literature, such as ethyl lactate4 and amino acid ILs23, [C6MIm]Ac has outstanding extraction performance. Its high extraction performance, low mutual solubility with methyllinoleate and simple subsequent separation with water make the operation easier and the equipment simpler, which benefits its commercial application. Moreover, from the industrial point of view, the stability of the potential ILs is also essential. For this purpose, the thermogravimetric analysis of [C6MIm]Ac was carried out. As seen from Figure S2, the weight of [C6MIm]Ac is unimpaired from room temperature to 150 °C, which indicates its good stability for room temperature extraction process. From the above, [C6MIm]Ac can be considered as high potential solvent candidate for the extraction of vitamin E from deodorizer distillate.

Conclusions Imidazolium-based ILs were evaluated as solvent for potential application in the separation of vitamin E from deodorizer distillate. For anions, the formation of



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hydrogen bond between the anion and α-tocopherol is in favour of the improvement of the selectivity of α-tocopherol. For cations, with lengthening the alkyl chain in the di-imidazolium ring, the misfit interaction or electrostatic interaction between α-tocopherol and ionic liquids plays the major role on the extraction. [C6MIm]Ac was identified as the most promising solvent, which has the highest selectivity of 108.23 and high partition coefficient of α-tocopherol for separation of α- tocopherol from methyllionleate. Based on experimental liquid- liquid equilibrium data and the extraction performance for deodorizer distillate, [C6MIm]Ac is confirmed to be suitable for extraction process for mixture with low α-tocopherol content.

Acknowledgements This research is supported by the National Sciences Foundation of China (NSFC U1462123), Major State Basic Research Development Program of China (973 Program 2012CB720502), and 111 Project (B08021). The financial support by the Max Planck Partner Group Funding from the Max Planck Society, Germany, is greatly acknowledged.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. Some calculated results about ILs by COSMO-RS, solubility parameters of different ILs by Material Studios; determined selectivity to α-tocopherol and partition coefficient of α-tocopherol and tie lines data for (α-tocopherol + methyllinoleate +


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[C6MIm]Ac) system at 25 ºC by experiment; thermogravimetric analysis of [C6MIm]Ac.

Notes The authors declare no competing financial interest.

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For Table of Contents Use Only

Title: Selection of imidazolium-based ionic liquids for vitamin E extraction from deodorizer distillate Authors：Lei Qin, Jianan Zhang, Hongye Cheng, Lifang Chen, Zhiwen Qi*, Weikang Yuan Synopsis：Cation and anion of ionic liquid for vitamin E extraction from deodorizer distillate is screened by molecular simulation and experiment.


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Selection of Imidazolium-Based Ionic Liquids for Vitamin E Extraction

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