Synthesizing Vitamin E Molecularly Imprinted Polymers via

Article pubs.acs.org/jced

Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Synthesizing Vitamin E Molecularly Imprinted Polymers via Precipitation Polymerization Yinan Lu,† Yinpei Zhu,† Youhong Zhang,*,† and Kean Wang*,‡ †

School of Environmental Ecology and Bioengineering and Key Laboratory for Green Chemical Process of Ministry of Education, Wuhan Institute of Technology, Wuhan 430205, China ‡ Chemical Engineering Department and Center for Separation and Catalysis (CeCAS) Khalifa University of Science & Technology, Abu Dhabi, United Arab Emirates

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S Supporting Information *

ABSTRACT: Molecularly imprinted polymers (MIPs) were prepared by the precipitation polymerization process, with vitamin E as the template molecule, ethylene glycol dimethacrylate as the cross-linker, and three different functional monomers, respectively. Adsorption experiments were conducted to investigate the performance of each MIP and compared to the nonimprinted polymers (NIPs). It was found that MIPs are superior to NIPs in general, with acrylamide-based MIP showing the best adsorption capacity of 38.8 mg·g−1 and an imprinting factor of 2.6. The optimal imprinting ratio and cross-linking ratio to be 1:5 and 5:1, respectively. The Freundlich equation was found superior to Langmuir to describe the adsorption isotherm data on the MIP/NIP. The adsorption of vitamin E on the MIP was found to be a heterogeneous process and consists of two mechanisms, which were confirmed by the analysis of the thermodynamic properties of the adsorption process.

1. INTRODUCTION Vitamin E (or VE) is a fat-soluble vitamin vital to human health.1,2 It is commonly used to treat threatened/habitual abortion and to serve as an important antioxidant which prevents and treats cardiovascular disease, arteriosclerosis, and aging, etc.3 Due to its great demand and huge market potential, a few technologies have been developed for VE production in the past, while more efficient and economical technologies are emerging.1,4,5 VE can be produced from natural or industrial processes. A popular raw material to extract natural VE is deodorized distillate, a byproduct from the refining of edible oils which contains free fatty acids, neutral oils, natural VE, and phytosterols, etc.6 Because of the complex composition, similar physical and chemical properties of components, and easy oxidation, it is difficult to extract quality VE directly from the deodorant distillate. Therefore, pretreatments are commonly needed, which may include: esterification/transesterification, saponification, extraction, and urea complexation, etc.5,7 Such procedures improve the separation efficiency but also incur high cost. Therefore, the production of VE still faces the challenges today to improve the purity/productivity while reducing energy consumption and the cost of solvents.4 Molecularly imprinted polymers (MIPs) are polymeric adsorbents with complementary cavities/sites created by the imprinting of template molecules. A MIP presents good affinity/selectivity toward the template molecules and can effectively adsorb/separate the designated component from a solution of complex mixture.8,9 Bulk polymerization is the most © XXXX American Chemical Society

commonly used technology to prepare MIPs, of which the obtained products need to be grinded and crushed, leading to the particles of irregular shape and sizes. Furthermore, the grinding process may harm the internal structure/cavities and the separation performance as well.10,11 In contrary, the precipitation polymerization technology got such advantages of simplicity and low cost, capable of preparing spherical particles of uniform sizes.12,13 For example, Kitabatake et al. prepared curcumin MIP microspheres using methacrylamide as the monomer.14 The MIP showed good adsorption capacity and selectivity toward the target molecules and their analogues in aqueous mixture solutions. Zhang et al.15 used the precipitation polymerization method to fabricate MIP for protein separation. The MIP presented excellent capacity, selectivity, and reusability toward the large BSA molecules. Faizal et al.16 fabricated a α-tocopherol (a major component of VE) imprinted membranes (with the copolymer of methacrylate and acrylonitrile), which demonstrated a good selectivity to its analogs. Puoci et al. synthesized methacrylic acid (MAA) based MIPs for the HPLC analysis application17 and controlled release of α-tocopherol in gastrointestinal fluid.18 However, more comprehensive study of MIP technology for this application was rarely (if not at all) reported. In this study, VE molecularly imprinted polymers were prepared by the precipitation polymerization method (with the Received: October 19, 2018 Accepted: February 6, 2019

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DOI: 10.1021/acs.jced.8b00944 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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extra advantage of a “cleaner surface” for this application, because no emulsifier/stabilizer was added during synthesis). The MIPs were characterized and tested for VE adsorption. The effect of monomer, imprinting ratio, cross-linking ratio, adsorption temperature, and regeneration were investigated, while the mechanisms of VE adsorption on MIP were explored.

(C0 − Ce)V (1) m −1 −1 where C0 (mmol·L ) and Ce (mmol·L ) are the initial and equilibrium concentrations, respectively, V (L) is the volume, and m (g) is the mass of the adsorbent.The unit of Qe is converted to (mg·g−1) using the MW of VE. The adsorption kinetics were studied to ensure the adsorption equilibrium were reached. About 0.05 g of MIP was loaded to 10 mL of VE solution. The system was loaded into the oscillator under the same conditions. Samples were taken every 20 min, filtered, and analyzed by HPLC. To study the reusability of MIP at ambient temperature, about 3 g of MIPs was charged into a column of o.d. 2.5 × L 30 cm and pre-equilibrated with VE solution (C0 = 1 mmol· L−1) fed to the column at the flow rate of 0.3 mL·min−1. The effluent was analyzed with HPLC at certain time intervals to draw the breakthrough curve. After saturation, the column was eluted with n-hexane and isopropanol (a volumetric ratio of 9:1) at the flow rate of 0.4 mL·min−1. The sample solutions were also analyzed with HPLC. Qe =

2. EXPERIMENTS AND METHODS 2.1. Materials. VE (α-tocopherol = 99.01%; MW = 430.7 g/mol) was provided by Jiangsu Yuxin Vitamin Co. Ltd. (China). Acrylamide (AM), methacrylic acid (MAA), 4vinylpridine (4-VP), azodiisobutyronitrile (AIBN), ethylene glycol dimethyl acrylate (EGDMA), and methanol (HPLC grade) were purchased from Shanghai Macklin Biochemical Co. Ltd. (China). Other chemicals such as acetone, acetic acid, and so on were purchased from Sino Pharm Chemical Reagent Co. Ltd. (China). 2.2. Synthesis of VE-MIPs. A 0.11 g amount of VE and 0.088 g of AM were added to 40 mL of acetone solution. After ultrasonic treatment for 30 min, the mixture solution was placed in a low-temperature fridge chamber (4 °C) for 12 h. A 1.24 g amount of EGDMA and 0.03 g of AIBN were added to the above solution. After ultrasonic treatment and nitrogen purging for 10 min, the mixed solution was sealed under nitrogen and heated at 70 °C for 24 h. This optimal reaction temperature was selected after repeated experiments by considering the thermal stability of AIBN19 and the rate of reaction. After polymerization, the microspheres (d ∼ 2.1 μm) were washed with methanol−acetic acid (9:1, v:v) for 48 h in a Soxhlet extractor; then they were washed with methanol for 5 h and washed again with ultrapure water three times. Finally, the obtained MIP was dried in a vacuum oven at 50 °C for 24 h. The synthesis of nonimprinted polymers (NIPs) follows the same routes described above except that no template molecules were added. 2.3. Characterizations. The morphology of MIPs was observed by scanning electron microscopy (SEM). The infrared absorption spectra of the functional monomer AM, cross-linker EGDMA, and MIPs were determined by Fourier transform infrared spectroscopy (FTIR, Nicolet 6700; KBr; wavenumber range, 400−4000 cm−1; resolution, 4 cm−1). The surface properties of the samples were characterized by measuring the standard N2 adsorption isotherm on the samples using a Micromeritic ASAP 2020 pore and surface analyzer. The specific surface area (SBET) and pore size distribution (PSD) were derived using the standard software attached. 2.4. Adsorption Experiment. Batch adsorption experiments were conducted to measure the adsorption of VE on MIPs/NIPs in trichloromethane solutions, respectively. A 0.05 g amount of MIP/NIP was added to flasks with different concentrations of VE (from 0.3 to 1.8 mmol·L−1). Then the flasks were placed on a thermal oscillator for 120 min under a constant temperature. After equilibrium, the solution was withdrawn from a syringe filter (0.45 μm) and analyzed by an Agilent 1260 HPLC (mobile phase, methanol−water (99:1); C18 column (4.6 × 100 mm, 5 μm); column temperature, 20 °C; UV detector wavelength, 294 nm). The isotherms were measured at 25, 30, and 35 °C, respectively, for the MIP and NIP. The VE adsorption capacity, Qe (mol·g−1), is calculated using eq 1:

3. RESUTS AND DISCUSSION 3.1. Synthesis of MIPs. The schematic diagram for the synthesis of AM-based MIPs (or MIPAM) is illustrated in Figure 1 as an example. The principle of precipitation

Figure 1. Schematic diagram for the synthesis of MIPs.

polymerization is that all of the feed components can be dissolved in the solvent so that no emulsifier or stabilizer is needed. Upon the polymerization, the MIP was precipitated as homogeneous particles because of its insolubility and higher density in the solution.13 Specifically, the amino groups of AM interact with phenolic hydroxyl groups of VE to form intermediate via hydrogen bond. The intermediates then react with EGDMA to fix the space-stable structure under the action of the initiator of AIBN. The polymerization then sets in to form the complex of MIP with the template molecules. Finally, the template molecules (VE) are removed via elution. The cavities and amino groups are retained as the major binding sites for VE.13,20 3.2. Effect of Monomer. MIPs/NIPs were prepared with three monomers (MAA, AM, and 4VP), respectively, using the procedures described in section 2.2, and then subject to the adsorption experiments at 25 °C, respectively. Table 1 compares the adsorption capacities of each MIP/NIP (listed as QMIPs and QNIPS, respectively). It is seen that, in general, MIPs present higher capacity than NIPs, confirming the underlying mechanisms of the MIP technology. This capacity B



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Table 1. Adsorption Performance of the Synthesized MIP/NIP Samplesa entry

monomer

VE/monomer (IP ratio)

EGDMA/monomer (CL ratio)

QMIP (mg·g−1)

QNIP (mg·g−1)

IFQMIP/QNIP

M1 M2 M3 M4 M5 M6 M7 M8 M9

MAA 4-VP AM AM AM AM AM AM AM

1:4 1:4 1:4 1:2 1:3 1:5 1:6 1:5 1:5

5:1 5:1 5:1 5:1 5:1 5:1 5:1 3:1 4:1

26.86 32.88 34.46 13.78 32.73 38.76 41.68 29.50 35.87

13.28 13.51 13.42 8.98 12.87 14.58 16.41 13.76 16.21

2.02 2.43 2.57 1.53 2.54 2.66 2.54 2.14 2.21

a

Note: All the ratios are molar ratios.

sizes of ∼2.1 μm (M6). MIPs with other monomers were found to follow a similar trend. Therefore, CL = 5:1 was selected as the optimal ratio for the preparation of MIPs in the following sections. As shown in Table 1, M6 sample (MIPAM with CL = 5:1) presents the optimal adsorption capacity. 3.4. Effect of the Imprinting Ratio. The imprinting ratio (IP) of template molecules to functional monomers determines the density and distribution of adsorption sites in a MIP.25 In order to find the optimal IP for this application, a series of AM-imprinted MIPs (M3−M7) were prepared and tested for their VE adsorption capacities. Table 1 compared the five MIP samples with different IPs. It was found that as the ratio decreased (less VE templates, more monomer) from IP = 1:2 to 1:5, the adsorption capacity increased consistently for MIP/NIP, so that sample M6 (IP = 1:5) with the imprinting factor of 2.66 reached a high capacity QMIP = 38.8 mg·g−1. A further decrease in the ratio to IP = 1:6 (M7) would result in a slightly higher capacity but a lower imprinting factor. The reason of the above observations are explained as follows: a high IP ratio (more templates, less monomer) would lead to less specific binding sites and lower specific adsorption capacity, while excessive monomer (lower IP ratio) would increase the nonselective binding sites in the polymer, resulting in nonspecific adsorption capacity. So the molar ratio of VE to AM of IP = 1:5 was selected as the optimal imprinting ratio for the preparation of AM-based MIPs. 3.5. Surface Properties. The standard N2 adsorption isotherms on MIPs/NIPs were found largely linear (at P/P0 = 0−0.95) without obvious hysteresis. The surface areas, SBET, of the MIPs were found in the range of ∼55 m2·g−1, which varied slightly with different CL/IP ratios but without clear pattern. On average, the MIPs got slightly higher surface areas than NIPs (SBET ∼ 45−50 m2·g−1). The pore size analysis (BJH method) showed that the adsorbents are mostly macroporous (d = 20−300 nm), with the pore volume range of 0.15−0.2 cm3·g−1. This is in agreement with other research on similar polymeric adsorbents for adsorbing macromolecules of which the adsorption mechanism is predominantly chemical in nature. For example, Zhu et al.24 found the surface area of a MIP in the range of ∼200 m2·g−1, but the related NIP presented a higher surface area than the MIP. Xu et al. synthesized chitosan-based adsorbents for adsorption of dyes in aqueous solution with the surface area of ∼50 m2·g−1,26 The cross-linked chitosan−cyclodextrin adsorbents for the purification of whey proteins presented a surface area of ∼45 m2· g−1.27 3.6. FTIR. The FTIR spectra of EGDMA, AM, and MIPAM are shown in Figure 3, respectively. For EGDMA (a), peaks at 1150 cm−1 can be assigned to the vibration of C−O bond

ratio is defined as the imprinting factor (IF = QMIPs/QNIPS) and listed in the last column of Table 1, we can further see that the MIPs have capacities about twice these of NIPs. Among MIPs with the same imprinting ratio of 1:4 and cross-linking ratio of 5:1, AM-based MIP or MIPAM (M3) present the highest capacity of 34.5 mg/g and an IF of ∼2.6, which is superior to the MAA-based MIP or MIPMAA (M1) and marginally better than the 4VP-based MIP or MIP4VP(M2). The reason is discussed as follows: VE has a phenolic hydroxyl group which is weakly acidic. It would present some repulsive forces toward the negatively charged carboxylic groups of MAA. Therefore, MIPMAA presents a lower capacity.21 In contrast, AM and 4VP monomers possess amino groups, which are positively charged in aqueous solutions and therefore, M2 and M3 samples showed much higher VE capacities. Although the adsorption capacities are close to each other on M2 and M3 samples, the price of 4VP is much higher (more than doubled) than AM. Thus, MIPAM (M3) was selected for further experiments and investigations. 3.3. Effect of Cross-Linking Ratio (CL). Since MIPs are derived from the precipitation polymerization process, the morphology and structural stability of the polymers are strongly dependent on the amount of cross-linking agents used (the cross-linking ratio, or CL, is the molar ratio of EDGMA/monomer) in the entropic polymerization.13,22 A low cross-linking ratio reduces the structural stability, while the excessive high cross-linking degree would occupy too many active sites and reduce the capacity and selectivity of the MIP.23,24 To optimize this trade-off phenomenon, AM-based MIPs were synthesized with three different CLs of 3:1; 4:1, and 5:1, respectively. Figure 2 showed as the example the morphology of the MIPAM synthesized at different CLs. We see that, at CL = 3:1 (Figure 2a), the particle size of the MIP (M8) was small and failed to form stable spheres; As the CL increases from 4:1 (Figure 2b) and to 5:1 (Figure 2c), the MIP particles disperse gradually and form microspheres (M9) with relatively uniform

Figure 2. SEM images of MIPAM with various CLs: (a) 3:1; (b) 4:1; (c) 5:1. C



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Table 2. Isotherm and Fitting Parameters for VE on MIPAM/ NIPAMa Langmuir Qe = QmbCe/(1 + bCe) −1

2

adsorbents

b

Qm (mg·g )

R

MIPs NIPs

1.21 5.29

57.20 17.78

0.992 0.980

Freundlich Qe = kCe1/n k

n

R2

30.82 14.68

1.77 3.85

0.995 0.998

a Note: Qm (mg·g−1) = saturation capacity; b = adsorption affinity (L· mmol−1); Ce (mmol·L−1) = equilibrium concentration; k, n = characteristic constants.

Figure 3. FTIR spectra: (a) EGDMA; (b) AM; (c) MIPAM.

long hydrocarbon side chain of VE may play an important role in the “shape” selectivity. According to the schematic diagram of the synthesis in Figure 1, adsorption sites can be heterogeneous, depending on the shape and the number of amino groups in an adsorption site. Meanwhile, VE is a family of eight related molecules and more stereoisomers, with the dietary VE being predominantly α- and γ-tocopherol.3 This further complicated the adsorption process. For example, MIP imprinted by α-tocopherol may preferentially adsorb β- and γtocopherols as well, albeit with slightly lower affinity. As a result, the adsorption of V E presents strong surface heterogeneity on the MIP. Umpleby et al.32 studied the adsorption on a number of MIPs and concluded that the noncovalently imprinted polymer behaves similarly to the heterogeneous surface and can be well fitted by Freundlich isotherm, which is consistent with our current experimental results. The Scatchard analysis is a powerful tool to study the affinity of large molecules to ligands. In general, two straight lines imply heterogeneous affinities with two association constants related to the low- and high-affinity binding sites.16,28 The plots of (Qe/Ce ∼ Qe) were done for the isotherm data on each adsorbent, respectively, and the results were shown as insets in Figure 4a,b, respectively. It is seen that the plot for MIP consists of two distinct straight lines (inset of Figure 4a), which correspond to the adsorptions onto the imprinted specific sites and general polymer matrix, respectively. On the other hand, the plot for NIP (inset of Figure 4b) generated a nonstraight line, supporting the nonspecific adsorption of VE and its isomers on the polymer matrix of NIP. The isotherm data were also measured on the MIP/NIP at the other two temperatures (30 and 35 °C) and summarized in Table S1 of the Supporting Information. It can be seen that these data followed trends similar to those discussed above, while the adsorption capacity decreased consistently with the increase in temperature. The thermodynamic parameters were calculated for VE adsorption on the MIP/NIP using the

stretching. For AM (b), peaks at 3400 cm−1 correspond to amino groups, peaks at 1637 cm−1 to the stretching vibration of CC double bond, and the peak at 1720 cm−1 correspond to the stretching vibration of carbonyl groups, respectively. For MIPAM (c), the presence of two distinct peaks at 1720 cm−1 (CO stretching) and 1150 cm−1(C−O stretching) confirmed the existence of EGDMA in MIPs. The peaks at 3400 cm−1 (N−H stretching) of MIP indicated the successful polymerization of functional monomer AM with EGDMA.28 These amino groups will serve as the main binding sites for template molecules.8,29 3.7. Adsorption Studies. 3.7.1. Adsorption Isotherms. Panels a and b of Figure 4 show the adsorption isotherms of VE at 25 °C on M6 MIPAM and M6 NIPAM, respectively. It is seen that adsorption capacity of the MIP is significantly higher than that of NIP, supporting the underlying adsorption mechanism of MIP, of which the specific sites/cavities created by the molecular imprinting offer extra capacities. The adsorption isotherm data were fitted to Langmuir (line) and Freundlich (broken line) isotherm equations, respectively, with the fitting results listed in Table 2 and the data listed in Supporting Information. It is seen that the Langmuir model cannot adequately describe the isotherm data on either adsorbent, suggesting that the mechanisms of adsorption processes do not conform to the fundamental model assumptions (monolayer, no lateral interaction/steric hindrance between the adjacent sites, and so on).30 On the other hand, the Freundlich model gives excellent fittings to the data on either adsorbent, suggesting that the adsorption is largely chemical in nature and the adsorbent is heterogeneous toward the VE molecules.31 The reasons for the above observation are briefly discussed here: The binding affinity between the MIP and VE is mainly due to the hydrogen bonding between the amino groups of the polymer and the phenolic hydroxyl groups of VE, while the

Figure 4. Adsorption isotherms of VE on (a) MIPAM and (b) NIPAM. The insets are the Scatchard plot for each adsorbent. D



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Figure 5. Adsorption kinetics of VE on MIPAM: (a) batch experiments; (b) fixed bed elution experiments.

multiple-temperatured isotherm data and listed in Table S2. It can be seen that the adsorption processes on MIP/NIP are spontaneous (ΔG° < 0) and exothermic (ΔH° < 0), with the MIP being thermodynamically more favorable for V E adsorption (ΔG° and ΔH° are more negative), which is consistent with the observations in Scatchard plots of Figure 4. 3.7.2. Adsorption Kinetics. The rate of adsorption was also measured in a batch to evaluate the adsorption kinetics on MIPAM. The kinetic curve was shown in Figure 5a. It can be seen that the adsorption of VE gradually increases with time and tends to reach equilibrium at ∼120 min. The adsorbent is then saturated, and the curve levels off. The kinetic data were fitted to first and second order pseudokinetics, respectively, with the second order rate equation giving a slightly better goodness of fit.33,34 The regeneration/reusability results were shown in Figure 5b, from which we can see that the MIP can be effectively eluted up to 95% of the original capacity in the first cycle (in ∼90 min with the binary mixture solution of nhexane:isopropanol = 9:1) and thereafter be reused up to five cycles without further loss in capacity.

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AUTHOR INFORMATION

Corresponding Authors

*(Y. Zhang) E-mail: [email protected]. *(K. Wang) E-mail: [email protected]. ORCID

Youhong Zhang: 0000-0002-6846-8772 Kean Wang: 0000-0003-1111-1155 Funding

Financial and technical support from JiangSu YuXin Vitamin Co Ltd. is gratefully acknowledged. Notes

The authors declare no competing financial interest.

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REFERENCES

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4. CONCLUSIONS The Vitamin E molecularly imprinted polymer was synthesized successfully by the precipitation polymerization method with three precursor monomers and under various synthetic conditions. It was found that the AM-based MIP presents the best performance. SEM images showed that MIP particles were spherical and well dispersed at the cross-linking ratio of 5:1 and the imprinting ratio of 1:5. FTIR spectra indicated that the synthesis of the MIP is successful. Adsorption experiments found that the adsorption of VE is highly heterogeneous on the MIP/NIP, while the isotherm data were well described by Freundlich model. Scatchard plot and thermodynamic analysis confirmed two types of adsorption sites on the MIP. The regeneration experiments proved the technology developed in this research can serve as a good candidate for practical separation and purification of VE.

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tion on MIP/NIP derived from the above isotherm data (PDF)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.8b00944. Isotherm data of VE on the MIPAM and NIPAM at three different temperatures (25, 30, and 35 °C); the optimal fittings with Langmuir and Freundlich isotherms, respectively; thermodynamic parameters for the adsorpE



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NOTE ADDED AFTER ASAP PUBLICATION This article published February 20, 2019 with errors in the abstract graphic, Figure 4, errors to units in the text and Supporting Information. The corrected version published February 26, 2019.

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