Synthesis of Functional Adsorption Resin and Its Adsorption

Article pubs.acs.org/IECR

Synthesis of Functional Adsorption Resin and Its Adsorption Properties in Purification of Flavonoids from Hippophae rhamnoides L. Leaves Song Lou,†,‡ Zhenbin Chen,†,§ Yongfeng Liu,†,‡ Helin Ye,†,‡ and Duolong Di*,† †

Key Laboratory of Chemistry of Northwestern Plant Resources and Key Laboratory for Natural Medicine of Gansu Province, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China ‡ Graduate University of the Chinese Academy of Sciences, Beijing 100049, PR China § State Key Laboratory of Gansu Advanced Non-ferrous Metal Materials, Lanzhou University of Technology, Lanzhou, 730050, PR China ABSTRACT: The efficient purification method of high purity flavonoids from Hippophae rhamnoides L. (sea buckthorn) was reported. A series of adsorption resins with novel structure were synthesized on the basis of the Friedel−Crafts catalyzed and amination reaction. Functional groups, such as the chloromethyl or the amino group, were introduced into the resin matrix, respectively, to raise the specific surface area, produce the hydrogen-bonding interaction, and enhance the adsorption selectivity toward flavone compounds. The results showed that the adsorbent DDM-1 with 0.44 mmol/g chloromethyl groups and the adsorbent DDM-2 with 0.68 mmol/g amino groups performed high purity and adsorption to flavone compounds in Hippophae rhamnoides L. leaves, respectively, obviously higher than that from commercial adsorbents due to their more pores, higher surface area and pore size, and stronger hydrogen-bonding interaction between flavonoids and surface functional groups of the adsorbents, which maintained the adsorption driving force. The isotherms and kinetics of synthetic resins were also studied systematically. In order to get the optimal sample, the No. 3 and No. H were selected and used in the developed two-separation-step protocol due to their high adsorption and purity capacities, respectively. The purity of flavonoids after the two steps was increased 6.62-fold from 5.55% to 36.75%. The method showed its universality via good effects on the purification of flavonoids from Hippophae rhamnoides L. leaves.

1. INTRODUCTION Hippophae rhamnoides L. (Sea buckthorn), a thorny nitrogen fixing deciduous shrub with very high nutraceutical and therapeutical values, is naturally distributed throughout Asia, Europe, and North America.1 In China it is found in the cold deserts region between 3000 and 5000 m above sea level. Its leaves have a long history of applications in Tibetan, Mongolian, Chinese, and Middle Asian medicines, and it is considered to be a good source of a large number of bioactive substances.2−4 Recently, the nutritional of H. rhamnoides L. has drawn more attention in North America and Europe.5−7 Modern medical and chemical studies have shown that flavonoids are the main active components in Hippophae rhamnoides L.8−10 According to modern pharmacology research, flavonoids have antiinflammatory, antitumor, antioxidant, and free-radical scavenging properties, and they also have been widely used in the research and development of natural medicine as well as in clinical application.11 Furthermore Hippophae rhamnoides L. has been thought to express their effects through multicomponents.12,13 Zhu et al. reported that the content of flavonoids in Hippophae rhamnoides L. leaves was much higher than any other domains.14,15 Due to the importance of flavonoids in Hippophae rhamnoides L. for humans, several methods of separation and purification, such as hot leaching, solvent extraction, supercritical fluid extraction (SFE), and ultrasound-assisted extraction were developed and extensively reported in many previous studies.16−20 However, these methods had some disadvantages, for example, © 2012 American Chemical Society

time-consuming, laborious, bulk amount of solvent wastage, poisonous residual solvents, high yielding cost, and low efficiency, and were not suitable for large-scale industrial production. It was of greater significance to develop a simpler and more environment-friendly technique with high efficiency, low toxicity, and high selectivity. The method of employing macroporous adsorption resins (MARs) to separate bioactive components from crude extracts of herbal raw materials has attracted great attention. It is known that MARs exhibit unique adsorption properties and many special advantages such as high mechanical strength, good acid and alkali resistance, variety of functional groups, long lifetime, simpler operation process, lower operation cost, less solvent consumption, higher efficiency, easier regeneration, and more friendly to environment.21,22 There had been some reports on the application of the commercial MARs in the enrichment process of flavonoids. Li and co-workers assessed the suitability of the use of macroporous adsorbent Amberlite XAD7HP in expanded bed adsorption processes for the isolation of flavonoids from crude extracts of Ginkgo biloba L.23,24 Geng reported an efficient purification method of flavonoids from Ginkgo biloba L., Radix puerariae, and Hypericum perforatum L.25 Zhang and co-workers combined Received: Revised: Accepted: Published: 2682

July 11, 2011 January 5, 2012 January 10, 2012 January 10, 2012 dx.doi.org/10.1021/ie201494k | Ind. Eng.Chem. Res. 2012, 51, 2682−2696

Industrial & Engineering Chemistry Research

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supercritical CO2 extraction, ultrasound assisted extraction, and macroporous resin adsorption technology together mainly for obtaining purified flavonoids from Houttuynia cordata Thunb.26 Fu evaluated the performance and adsorption characteristics of four widely used macroporous resins for the separation of deglycyrrhizinated, flavonoids enriched licorice.27 Kammerer employed model solutions containing isolated apple polyphenols to systermatically study their interaction with two strong anion exchangers differing in the structure of their polymeric matrix. 28 Bayraktar evaluated the adsorption isotherms of oleuropein and rutin at different temperatures, pH values, and solid/liquid ratios. The experiment data of adsorption isotherms were well fitted to a Langmuir model.29 Antonio studied the adsorption of flavonoid naringenin to enzymatically isolated cuticles of Solanum lycopersicum as a function of the temperature and naringenin concentration at two stages of fruit growth.30 We evaluated the adsorption properties of twenty-four kinds of macroporous adsorption resins in order to separate and purify lycopene from tomato skins extracts (lycopene oleoresin).31 The matrix of these MARs was usually of polyacrylic or styrene-divinyl benzene ester, and the absorption mechanism primarily relied on the hydrophobic force such as the van de Waals force in the aqueous solution, however, which may lead to the low adsorption selectivity.32 Certain adsorbates, such as flavonoids, have special structures which have multiphenolic groups and a hydrophobic skeleton, as shown in Figure 1. The structure indicates that if introducing some special functional groups onto current MARs matrix to activate the hydrogen bond interaction between adsorbents and flavonoids and improve the structure of the pores, higher purity flavonoids may be achieved from the complicated system of natural plants.33 In the present paper, a series of adsorption MARs with novel matrix and hydrogen-bonding groups were synthesized based on the investigation of the high selectivity separation and purification of flavonoids from Hippophae rhamnoides L. leaves. A styrene-co-divinylbenzene copolymer with chloromethyl groups (DDM-1) was prepared by Friedel−Crafts catalysis from initial beads which were nonpolar copolymer with no group (DDM-0), and then a novel adsorption resin with amino groups (DDM-2) was obtained by following the amination reaction via pendant chlorine groups from DDM-1, which has high surface area. Flavonoids were selected as the objective compound for the separation study, and the experiments were investigated compared to some commercial adsorption resins. The purity of flavonoids in the final products verified that the method for regulating MARs configurations was applicable to the purification of flavonoids. The relationships between pore sizes, functional groups, adsorption selectivity, and flavonoids were investigated. Two kinds of synthetic resins were used in a new two-step protocol to purify the flavonoids from Hippophae rhamnoides L. The results showed that the adsorption selectivity of the selected synthesized MARs was better, and the purity of flavonoids in the products was much higher than those obtained from the commercial adsorbents. Furthermore the results demonstrated that the selected synthesized MARs were suitable for the large-scale preparative separation and purification of flavonoids in industry.

Figure 1. Structures of flavonoids in sea buckthorn.

lenetetramine, zinc chloride, and sodium chloride were purchased from Tianjin Chemical Reagent Co., Inc. (Tianjin, China). Pharmaceutical grade chloromethyl methyl ether was purchased from Jinan Leqi Chemical Reagent Co., Inc. (Shandong province, China), and distilled water was used. The Hippophae rhamnoides L. leaves were obtained from Gansu Greenness Biotech Co., Ltd. (Gansu province, China), and they were employed after washing and drying. The structures of their main flavone compounds were shown in Figure 1. The standards were purchased from the National Institutes for Food and Drug Control (Beijing, China). 2.2. Pretreatment of Commercial Adsorbents. The initial beads, which were nonpolar styrene-co-divinylbenzene copolymer (DDM-0) with no functional groups, and commercial MARs (D101, LAS-21, AB-8) were purchased from Xi’an Sunresin Technology Co., Ltd. (Shanxi province, China). Before the adsorption experiments, the weighed resins were pretreated by soaking in ethanol overnight, subsequently washing thoroughly with distilled water to remove the monomers and porogenic agents trapped inside the pores during the synthesis process.34 2.3. Synthesis of Adsorbents with a Functional Group. 2.3.1. Preparation of MARs with Chloromethyl Groups. MARs with chloromethyl groups (DDM-1) were

2. EXPERIMENTAL SECTION 2.1. Chemical Reagents and Samples. Analytical grade ethanol, methanol, sodium chloride, dichloromethane, triothy2683

dx.doi.org/10.1021/ie201494k | Ind. Eng.Chem. Res. 2012, 51, 2682−2696


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Scheme 1. Schematic Illustration of the Preparing Process of DDM-1 from DDM-0

freeze-drying at 233 K for 24 h and outgas at 333 K for 24 h on the degas port of the analyzer. Infrared spectra of the adsorption resin were obtained from a Nexus-670 FT-IR spectrometer (Nicolet, USA) with a pellet of powdered potassium bromide and adsorbent in the range of 500−4000 cm−1. X-ray photoelectron spectroscopy (XPS) was used to evaluate the chlorine and nitrogen of modified resins. The spectra were recorded on an Escalab 210 Axis Ultra photoelectron spectrometer (VG Scientific, U.K.) using a Mg Kα excitation source. The topography of the particle was displayed by JSE-5600LV (JEOL, Japan) scanning electron microscopy. A Laser Light Scattering particle size analyzer, Malvern Instruments Mastersizer 2000 (U. K.), was used to measure the particle size and distribution of these three MARs. 2.4.2. Determination of the Number of Chloromethyl and Amino Groups in Adsorbents. The number of chloromethyl and amino groups before and after reactions were determined by an electrochemistry method. The accuracy of the weight of the synthetic beads was 0.2000 g, and heat digested a sample in a nickel crucible at 873 K for 8 h with 0.5 g of melting sodium hydroxide. The melted masses were diluted in distilled water to 250 mL, and then the electric potential of the sample aqueous solution was determined by an electrochemical workstation. The electrochemistry workstation was composed of a potentiometer (PHS-3D, Shanghai Precision & Scientific Instrument Co., Ltd., Shanghai, China) and chloride and calomel selectivity electrodes with double salt bridges consisting of a saturation aqueous solution of potassium chloride and an aqueous solution of potassium nitrate (0.1 mol/L). The standard curve of the amount of chloromethyl was drawn according to a standard solution of sodium chloride at different concentrations (10−1, 5 × 10−1, 10−2, 5 × 10−2, 10−3, 5 × 10−3, 10−4, 5 × 10−4 mol/L). The chloromethyl group content (N, mmol/g dry adsorbent) in the adsorbents was calculated from the following equation, with a range of 0.001−100 mmol/g and R2 = 0.9999

prepared using electrophilic substitution method from initial beads (Scheme 1). The initial beads were soaked in dichloromethane for 24 h. An organic solution composed of dichloromethane, initiator zinc chloride initiator, sodium chloride, and initial beads was mixed with chloromethyl methyl ether in a 1000 mL three-necked round-bottomed flask equipped with a mechanical stirrer, a reflux condenser, and a thermometer. The round-bottomed flask was heated with a programmed heater. The mixture was stirred to give a suspension of beads of a suitable size on the solution (100−120 rpm) and then held at 323 K for 20 h. The synthetic MARs were filtered out, packed in an extractor, and washed with a large amount of distilled water and then methanol until there was no white precipitate, while an aqueous solution of silver nitrate was added to the filtrate. The variation of the proportion between the initial beads and chloromethyl methyl ether in the synthesis process can produce MARs with different numbers of chloromethyl groups. The DDM-1 was named Nos. 1−12, corresponding to different contents of chloromethyl groups respectively. 2.3.2. Preparation of MARs with Amino Group. The MARs with amino group (DDM-2) were prepared using amination reaction from DDM-1. The DDM-1 was dried at 313 K under vacuum, swelled with methanol for 24 h, and mixed with excess triethylenetetramine in a 500 mL three-necked round-bottomed flask equipped with a mechanical stirrer, a reflux condenser, and a thermometer. The mixture was held at 303 K for 9 h, and beads with amino group were synthesized through an amination reaction. Then the beads were filtered out and washed with distilled water until there was no white precipitate, while an aqueous solution of silver nitrate was added to the filtrate. MARs with different numbers of amino groups (DDM-2) can be obtained by varying the reaction time and the proportion of DDM-1 beads with respect to aminating agent in the synthesis process. The DDM-2 was named Nos. A−L, obtained from Nos. 1−12. 2.4. Determination of the MARs’ Physical Parameters. 2.4.1. Analysis. The swelling ratio in water, defined as the ratio of the volume occupied by the resin when swollen in water to its volume in the dry state at room temperature, was determined using a small measuring cylinder. The specific surface area and the pore size distribution of the adsorbents were calculated by BET and BJH methods, respectively, via the nitrogen adsorption and desorption curves at 77 K using a Micromeritics ASAP 2020 automatic surface area and porosity analyzer (Micromeritics Instrument Corp., USA). Before the BET surface area measurement, the adsorbents were vacuum

C = exp N=

M − 3.654 52.185

0.25 × C × 1000 (1 − α) × 0.2

(1)

(2)

where C (mol/L) is the concentrations of the sample aqueous solution, M (mV) is the electric potential of the sample aqueous solution, and α is the moisture content imbedded in the adsorbent. The determination of α is described in section 2.4.3. 2684



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2.4.3. The Determination of Moisture Content of Resins. Two parallel samples of each kind of MAR were accurately weighed in weighing bottles on a Sartorius BT224S analytical balance (Sartorius Scientific Instruments Co., Ltd., Beijing, China) and then dried in a ZFD-5090 drying oven (Shanghai Zhicheng Analytical Instrument Manufacturing Co., Ltd., China) at 383 K until the mass was constant. The following equation was used to calculate the moisture content of the adsorbent

α=

Wwet − Wdry Wwet

City Yuhua Instrument Co., Ltd., China) at 323 K. The extract of Hippophae rhamnoides L. leaves was photophobically stored in a refrigerator at 253 K. Then the extract was thawed at ambient temperature before use and diluted to an appropriate concentration (160.34 g/L) with distilled water to produce sample solutions. 2.5. Determination of the Flavonoids. Flavonoids in the aqueous solution were determined by using a UV spectrophotometer (T-6, Beijing Purkinje General Instrument Co., Ltd., Beijing, China) at a wavelength of 500 nm. The method was in accordance with Chinese pharmacopoeia. The concentration of flavonoids was determined by the obtained standard plotted in the range of 0.007−0.04 mg/mL: A = 11.42C − 0.0065 (R2 =0.9998), where A and C are the absorbance and the concentrations of flavonoids (mg/mL), respectively. 2.6. Adsorption/Desorption Experiments. 2.6.1. Adsorption Experiment. An adsorption system was constructed with sample solution, pump, adsorption resins, and SHA-B incubator (Scheme 2). In the adsorption process, a 100 mL sample solution with a known concentration (initial concentration) was pumped into a 250 mL conical flask (MARs column) with 1.2 g of dry MARs. Subsequently, the flask was continually shaken in a SHA-B incubator (100 rpm, Jintan Zhengji Instrument Co., Ltd., Jiangsu province, China) at 308 K for 6 h. The concentration of flavonoids (equilibrium

× 100% (3)

where α (%) is the moisture content of the adsorbent, Wwet (g) is the weight of the hydrated adsorbent, and Wdry (g) is the weight of the dry adsorbent. 2.4.4. Preparation of Sample Solutions. Hippophae rhamnoides L. leaves (1 kg) were extracted with 12 L of distilled water in a bath at 353 K for 2 h; this extraction was repeated with 8 L of distilled water. The double extracts were mixed and negative pressure filtered, the filtrate was evaporated to yield the fluid extract 6 L of ethanol (95%, v/v) added with standing for 24 h, and the supernatant of the solution was evaporated to yield the fluid extract by removing the ethanol under reduced pressure in a rotary evaporator (RE-52C, Gongyi

Scheme 2. Schematic Diagram of the Adsorption/Desorption System of MARs

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deviation, and C represents the arithmetic mean of three parallel experiments. The values of P and RSD in the experiment were 93.46 and 0.35%, respectively, which indicated that the accuracy and precision could satisfy the needs of the experiment. 2.6.3. Adsorption Kinetics. Subsequently, the adsorption kinetics curves of flavonoids on the selected primal resins and modified ones with chloromethyl and amino groups were studied according to the following process: adding 3.16 g of pretreated resin (equal to 1.2 g of dry resin) and 1000 mL of the sample solution to each flask with a lid. Then the flavonoids concentrations in the adsorption process were determined with a UV spectrophotometer at different time intervals until equilibration. In order to obviate the influence of an over adsorbing driving force generated by high concentration, the sample solution with low concentration (5.75 g/L) was used in the adsorption kinetics experiment. 2.6.4. Adsorption Isotherms and Thermodynamics. After the 50 mL sample solutions with different concentrations were placed in contact with 3.16 g of pretreated resin (equal to 1.2 g of dry resin) in shakers at temperatures of 303, 308, 313, 318, and 323 K, the equilibrium adsorption isotherms of flavonoids on the selected resins were obtained at different temperatures.

concentration) in drain was analyzed with a UV spectrophotometer. The experiments of desorption were carried out as follows: 50 mL of ethanol (70%, v/v) were pumped into flask with adsorbate-laden MARs after the adsorption equilibrium reaching. The flask was shaken (100 rpm) at a constant temperature of 303 K for 6 h. Then corresponding concentration of flavonoids (concentration of desorbed solution) was analyzed with a UV spectrophotometer. The ratios and adsorption/ desorption capacities were calculated with the following equations. Adsorption ratio:

A(%) =

C0 − Ce × 100% C0

(4)

Adsorption capacity:

qe = (C0 − Ce) ×

Vi (1 − α)W

(5)

where A is the adsorption ratio (%), and qe is the adsorption capacity (mg/g of dry resin) at adsorption equilibrium. C0 and Ce are the initial and equilibrium concentrations of the sample solutions, respectively (mg/L). α is the moisture content of the resin (%). W is the weight of the resin used (g). Vi is the volume of sample solution used in the study (mL). Desorption ratio:

D(%) = Cd ×

Vd × 100% (C0 − Ce)Vi

3. RESULTS AND DISCUSSION 3.1. Characterization of the Synthetic Adsorption Resins. Infrared spectroscopy is one of the useful tools to identify the chemical modifications proposed in the reaction scheme. The grafting functional groups will have a significant effect on the diffuse reflectance infrared Fourier transformation spectra of the modified adsorbent. Figure 2 displays the FTIR spectra of DDM-0 and DDM-2 resins. In the spectrum of DDM-2, the N−H structures was disclosed by the occurrence of bands at 860 cm−1 as a result of out-of-plane bending vibrations of N−H and at 925 cm−1 as a result of the rocking vibration of the N−H. There were also some other new absorption bands at 1126, 1221, and 1382 cm−1. They were the stretching vibration of C−N.37,38 Meanwhile, the spectrum showed DDM-2 resins also had some chloromethyl groups due to the absorption bands at 1036, 1113, and 1246 cm−1.39 There was also an absorption band in the vicinity of 580 cm−1 and 2116 cm−1. The first band belonged to the stretching vibrations of the C−Cl bond, and the second one was the in-plane bending vibrations adsorption of the C−H bond of the benzene ring after binary substitution caused by the substitution of hydrogen atoms at the para position of benzene rings by −CH2Cl.39 Figure 3 shows XPS spectral changes of MARs before and after modification. The characteristic XPS substrate signal for unmodified MARs (left) at 285 eV was attributed to C 1s, and the peak at 200 eV for resins functionalized with chloromethylgroups (middle) was from Cl 2p and was attributed to the matrix of −CH2Cl. Chlorine (∼200 eV), carbon (∼285 eV), and nitrogen (∼399.5 eV) photoelectron peaks (in order from low to high binding energy) were observed for resins functionalized with amino groups (right). The existence of amino was confirmed by the signal at a binding of 399.5 eV, which is N 1s.39,40 Therefore, the signal evidently showed that chloromethyl- and amino groups had been grafted onto the MARs. Moreover, Figure 3 also showed DDM-2 resins had some chloromethyl groups, and the replacement ratio of amino- from chloromethylgroups was 68%. Direct observation of the MARs nanoparticle layers is provided by scanning electron microscopy (SEM). SEM micrographs of a

(6)

Desorption capacity:

qd = Cd ×

Vd (1 − α)W

(7)

where D is the desorption ratio (%), and qd is the desorption capacity (mg/g of dry resin) after desorption equilibrium. Cd is the sample concentration in the desorption solution (mg/L). Vd is the volume of the desorption solution (mL). C0, Ce, α, W, and Vi are the same as defined above. 2.6.2. Determination of Accuracy and Precision. In this experiment, the coefficient of recovery (P) and the relative standard deviation (RSD) were adopted to test the accuracy and precision, respectively, of our experiment methods. P was determined as follows:35,36 after adsorption was finished, the concentration of flavonoids (C1) in raffinate was first investigated as that in section 2.6.1; thereafter, 10.00 mL of the flavonoids (rutin) standard solution (0.047527 mg/mL) was added to a flask loaded with 50.00 mL of the raffinate, and the concentration of flavonoids (C2) was determined again. This process was conducted in six randomly selected adsorption experiments, and P was calculated as the following equations

P=

(50 + 10)C 2 − 50C1 × 100% 10 × 0.047527

(8)

As for RSD, it was obtained from three parallel experiments, and its value was calculated from the following equation

RSD =

S C

(9)

where C2 is the concentration of solution in the added sample, C1 is denoted the concentration of raffinate, S is the standard 2686



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Figure 2. IR spectra of adsorbent DDM-0 and DDM-2.

Figure 3. XPS characterization of DDM-0, DDM-1, and DDM-2.

modification of MARs significantly increased the particle size of the product. Table 1 lists some important characteristics of DDM-0, DDM-1, and DDM-2. The specific surface area of starting copolymer DDM-0 increased significantly after modification. The pore size distributions of the resin DDM-0, DDM-1, and DDM-2 as revealed by the BJH method are shown in Figure 6. Compared with DDM-0, DDM-1 and DDM-2 present more micropores and less mesopores after modification. The new pores of DDM-2 formed in postcross-linking reaction are mainly micropores. The swelling ratio in water of modified copolymer DDM-1 and DDM-2 are higher than starting copolymer DDM-0, indicating the specific area and pore volume of the resulting product increase after modified reaction. 3.2. Influence of Chloromethyl Groups Content in Adsorbent Matrix on Purification Effect. Influence of chloromethyl groups in the adsorbent matrix on adsorption and desorption ability toward flavonoids were investigated, and the

particle surface of MARs before and after prepared by the titled technique are shown in Figure 4. The dimension of modified MARs by amino groups was almost two times bigger than the original ones, and the surface of the synthetic material is much rougher than the unmodified resins. It is apparent that there are an amount of pores produced on modified MARs, and a uniform surface of resin particles is visualized. Moreover, the original MARs showed smaller average particle size and wide particle size distribution, DDM-1 showed bigger particle size with still wide particle size, and DDM-2 with amino groups showed biggest average particle size and narrower particle size distribution. This is further investigated by a laser granulometer, as demonstrated in Figure 5. The sample DDM-0 exhibited relatively wide size distribution, d0.1 = 240.17, d0.5 = 558.27, d0.9 = 1022.93 μm, while DDM-2 owns the narrow particle size distribution (d0.1 = 449.69, d0.5 = 938.15, d0.9 = 1834.25 μm). The result was in good agreement with the result of the SEM of the samples. It is evident that the chemical 2687



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Figure 4. SEM image of DDM-0, DDM-1, and DDM-2.

pore volume would be equilibrium and the adsorption capacity reached steady state. The desorption curves of the flavonoids laden on the DDM-1 were similar with the adsorption curves. In light of the considerations presented above, No. 3 was the appropriate one among No. 1−No. 12 for its optimal selectivity to flavones compounds with acceptable adsorption capacity. 3.3. Influence of Amino Groups Content in Adsorbent on Purification Effect. Adsorption capacities of the adsorbents with different contents of amino groups toward model compounds flavones were investigated, as shown in Figure 9 and Figure 10. The adsorption capacity improved with the increasing in the content and reached the apex when it was 0.68 mmol/g (No. H); the adsorption capacity would decrease with the continuously rising of content and reached steady state. While the content of amino group was low, specific surface area played the important factor, adsorption capacity would increase with the rising of surface area. On the other side, water molecule could hinder the hydrogen-bonding interaction between flavones and resins in aqueous solution, while the adsorbent had enough intensive hydrophobic force, it could help flavones overcome the interference from water molecules to approach the suitable location of adsorbent surface, which coincided with Schrader’s research.41 The adsorption capacity decreased with the continuously growing content of amino, the adsorbent with more amino groups has larger specific surface area, but its adsorption capacity was lower than No. H, it demonstrated that besides specific surface area and hydrogen-bonding, pore volume and polarity were also the main factors when the amino content was high. In light of the considerations presented above, No. H was the appropriate one among No. A−No. L for its optimal selectivity to flavones compounds with acceptable adsorption capacity. No. 3 and No. H were selected to represent DDM-1 and DDM-2 in kinetics and thermodynamics experiments. 3.4. Influence of Functional Groups on Adsorption Properties of Adsorbents. Commercial resins LSA-21, AB-8, D-101 and synthetic resins were selected to study the relationship between structures of adsorbents and adsorption performances. Their physical properties were listed in Table 2. Purification effects of these resins toward flavonoids in Hippophae rhamnoides L. leaves were studied, and the results were listed in Figure 11. As shown in Figure 10, the resin AB-8 and D-101 were polystyrene, and their selectivity to flavonoids were poor for their

Figure 5. Size distributions of DDM-0, DDM-1, and DDM-2.

adsorption/desorption curves were illustrated in Figure 7 and Figure 8. As shown in Figure 7, with the increase in the content, the adsorption capacity improved gradually and reached the apex when content was 0.44 mmol/g (No. 3), and then the adsorption capacity would decrease and reach steady state when the content reached 0.90 mmol/g. Generally speaking, the adsorption capacity of adsorbent from aqueous solution is dominated by many factors.41−43 In this solution system, the adsorbate− adsorbent interaction and specific surface area would play an important role. The effective interaction includes van der Waals force, hydrophobic affinity, and hydrogen bonding interaction, etc. At low content, the hydrophobic affinity and specific surface area played the main factors; the resin could hardly adsorb flavones owing to its weak hydrophobic affinity and low specific surface area. With the increasing in chloromethyl content, the adsorption capacity of the resin toward adsorbed increased obviously due to the increasing of hydrophobic affinity and specific surface area and then reached the apex, while the content was 0.44 mmol/g. With continuously rising of the content, the adsorption capacity descended. This phenomenon demonstrated that adsorption capacity would be also affected by physical structure. The pore volume grew down with continuously increasing of content, and the number of macromolecular flavones which can enter the resins would reduce. At last, the effect of hydrophobic affinity, specific surface area, and 2688



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Table 1. Important Characteristics of DDM-0, DDM-1, and DDM-2a adsorbent DDM-0 No. 1 No. 2 No. 3 No. 4 No. 5 No. 6 No. 7 No. 8 No. 9 No. 10 No. 11 No. 12 No. A No. B No. C No. D No. E No. F No. G No. H No. I No. J No. K No. L a

particle size (mm)

specific area (m2/g)

0.2−0.3

459 503 503−537

average pore size (nm) 6.77 12.82 11.89−12.82

537 537−603

11.89 9.90−11.89

603 603−659

9.90 9.15−9.90

659 586 586−642

9.15 13.32 11.89−13.32

642 642−659

11.89 11.32−11.89

659 659−714

11.32 10.72−11.32

714

10.72

main functional group

amount of functional group (mmol/g)

moisture proportion (%, w/ w)

chloromethyl chloromethyl chloromethyl chloromethyl chloromethyl chloromethyl chloromethyl chloromethyl chloromethyl chloromethyl chloromethyl chloromethyl amino amino amino amino amino amino amino amino amino amino amino amino

0.22 0.37 0.44 0.47 0.50 0.60 0.71 0.82 0.90 1.22 1.32 1.64 0.11 0.21 0.30 0.36 0.40 0.47 0.52 0.68 0.76 0.83 0.90 1.04

67.7 75−86

80−89

Nos. 1-12 DDM-1 with different contents of chloromethyl groups; NO A-L DDM-2 with different contents of amino groups.

Figure 7. Adsorption curves of adsorbents with different chloromethyl content (n = 3).

Figure 6. Pore size distribution curves of DDM-0, DDM-1, and DDM2 evaluated from the desorption isotherm of nitrogen.

the amino groups on the adsorbents and phenolic hydroxyls in flavone molecule. It improved the selectivity of the adsorbents toward flavone compound. However, the pore size of LSA-21 was 7.25 nm, which was still small. It was difficult for adsorbate to enter the interior space. The less entered adsorbate would make lower adsorption amount. Accordingly, the coefficient of recovery was very low although the sample purity was higher. Overall, the purification effects of LAS-21 had not greatly improved, and the sample purity of the flavonoids was still low due to their low specific surface and narrow pore structure. It is clear that the adsorbents without hydrogen group expressed poor selectivity to flavones compounds for their low

hydrophobic adsorption mechanisms. A large amount of hydrophobic impurities were adsorbed simultaneously with flavones compounds. Meanwhile the pore size of D-101 was low in spite of high surface area; therefore, the adsorption amount of D-101 was a little higher, but the sample purity of flavones compounds in the products was low. The pore size and surface area of AB-8 were all small, and the purity and adsorption amount were all very low. Generally speaking, the purification effect of LSA-21 was superior than the commercial resins without hydrogen groups. It was because there were hydrogen bonds interaction between 2689



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bond between phenolic hydroxyl and flavonoids, could increase their selectivity. However, the adsorption selectivity could not be fully expressed when the pore number and size was too small. Therefore, the pore number and size of adsorbents must remain large when groups were introduced. In this sense, triethylenetetramine was selected as copolymerization agent in this study. The results showed that synthetic resins had higher adsorption capacity than commercial resins, and DDM-1 and DDM-2 exhibited higher than starting copolymer DDM-0, which resulted from its high specific surface area and polarity. The specific surface area of starting copolymer was greatly increased after synthetic reaction. Meanwhile, the surface of synthetic resins contains polar groups, the polarity matching between the resin and flavones may partially account for the increase in the adsorption capacities. Moreover, adsorption ratio/capacity of the adsorbents with chloromethyl groups was higher than commercial resins and DDM-0 because of its better pore size and surface area, but the ratio/capacity was lower than the ones of (DDM-2) owing to their low hydrophobicities and weak hydrogen-bonding interaction with flavonoids. By comparison, the adsorption ratio/capacity of DDM-2 to flavonoids was obviously increased. It was due to the fact that there was stronger hydrogen-bonding interaction between the surface functional groups and flavonoids, which maintained the adsorption driving force. The resins with amino groups also had much bigger surface area and more pores than LSA-21 which would produce better adsorption and desorption ratio/capacity. The result showed that the selectivity of the adsorbent toward flavonoids was enhanced remarkably after hydrogen groups were introduced onto the matrix of resins. Desorption ratio/capacity of the adsorbents with different functional groups toward flavonoids was also investigated. Considering the stability of flavonoids, aqueous ethanol (70%, v/v) was used as desorption solutions. The desorption ratio/capacity of the flavonoids laden on the DDM-1, DDM-0, and commercial resins were rather lower than those laden on the DDM-2, owing to the stronger hydrogen-bonding interaction between amino groups and phenolic hydroxyl groups. Therefore, adsorbents with amino groups were most suitable for the purification on flavonoids among the synthesized adsorbents. 3.5. Adsorption/Desorption Kinetics. The kinetic adsorption data were investigated to understand the dynamics of the adsorption process in terms of the order of rate constant. The well-known kinetics models were adopted to describe adsorption process: pseudofirst-order and pseudosecond-order kinetics models. Equation of pseudofirst-order kinetics model:

Figure 8. Desorption curves of adsorbents with different chloromethyl content (n = 3).

Figure 9. Adsorption curves of adsorbents with different amino content (n = 3).

K1 log(qe − qt ) = − t + log qe 2.303

(10)

Equation of pseudosecond-order kinetics model:

t 1 1 = t+ qt qe K2qe2

(11)

where qe and qt are the adsorption capacity at equilibrium and at any time t (mg/g of dry resin), respectively. The parameters K1 (min−1) and K2 (g/(mg·min)) are the rate constants of the pseudofirst-order and pseudosecond-order for the adsorption process, respectively.

Figure 10. Desorption curves of adsorbents with different amino content (n = 3).

pertinences to the structural characteristics of flavonoids.25 Introducing hydrogen groups, which were able to form a hydrogen 2690



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Table 2. Physical Properties and Functional Group of Commercial Resins adsorbent

functional group

matrix

surface area (m2/g)

D101 AB-8 LSA-21

amino group

P(St-DVB) P(St-DVB) P(St-DVB)

700 480−520 600

particle diameter (mm) average pore size (nm) 0.3−0.8 0.3−0.8 0.3−1.2

5.18 6.44 7.25

moisture content (%)

true density (g/mL)

67.7 53.4 68.2

1.01−1.05 1.01−1.05 1.01−1.05

Figure 11. Purification results of commercial and synthetic resins toward flavonoids in Hippophae rhamnoides L. at 303 K (n = 3).

Plotting ln(qe − qt) against t for the pseudofirst-order equation and t/qt against t for the pseudosecond-order equation would get straight lines, respectively. Adsorption kinetics of flavonoids on DDM-0, DDM-1, and DDM-2 resins were studied at 308 K. Figure 12 presented the

model. This was deduced from the higher R2 values obtained with the pseudosecond-order model, and the calculated Qe (cal) values are closer to the experimental data than the calculated values of the pseudofirst-order model. Similar results were reported on adsorption of Fe(III) and phenolic compounds onto polymeric adsorbents.22,32,41 Furthermore, the principle of pseudosecondorder kinetics implied that concentrations of both adsorbate and adsorbent were involved in the rate determining step in the adsorption process, which may be chemisorptions. From Figure 12, it can be seen that all adsorption capacities increased rapidly in the first 150 min and then increased slowly. Finally, all the adsorption capacities reached equilibrium after 210 min. The reason of this phenomenon was that high intraparticle mass transfer resistance existed in the process of flavonoids diffusing into the micropores of the resins.42 Batch adsorption equilibrium tests were run for over 210 min. Figure 12 also showed that the equilibrium time for adsorption of flavonoids onto DDM-0 was about 200 min, while about 220 min to DDM-1 and 170 min to DDM-2. The k2 values in order DDM-1 > DDM-0 > DDM-2 also indicted that adsorption of flavonoids onto DDM-2 was more rapid than the other two resins. This may result from the different pore size distributions and hydrogen-bonding interaction of these three adsorbents. Compared with DDM-0 and DDM-1, DDM-2 has more micropores after the postcrosslink and introduction of amino groups. The diffusion resistance of flavones in micropores was larger than that in macropores and mesopores, so that the rates of flavones adsorbed onto DDM-0 and DDM-1 were lower than DDM-2. In general, adsorption rate of flavones from sample solution to adsorbent is controlled by intraparticle diffusion, especially in a rapidly stirred batch reactor.41 To prove if it was fit for adsorption in the current research, the intraparticle diffusion

Figure 12. Adsorption kinetic curves of flavones onto DDM-0, DDM-1, and DDM-2 (n = 3).

plot of flavonoids adsorption (qt) versus contact time (t). Kinetic parameters are determined and listed in Table 3. Results showed that flavonoids adsorption kinetics on DDM-0, DDM-1, and DDM-2 were better fitted by the pseudosecond-order kinetics 2691



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Table 3. Kinetic Parameters of Flavones Adsorption onto DDM-0, DDM-1, and DDM-2 at 303 Ka pseudofirst-order model

pesudosecond-order model

intraparticle diffusion

adsorbent

qe (exp) (mg/g)

K1 (min−1)

qe (cal)(mg/g)

R2

K2 (g/mg min)

qe (cal)(mg/g)

R2

Kp (mg/g min1/2)

R2

DDM-0 DDM-1 DDM-2

11.72 15.56 19.25

0.0166 ± 0.00141 0.0221 ± 0.00288 0.0244 ± 0.00297

10.11 ± 1.172 11.45 ± 1.749 22.89 ± 3.831

0.9476 0.7816 0.7703

0.00312 ± 0.000454 0.00413 ± 0.000948 0.00230 ± 0.000299

12.44 ± 1.547 16.21 ± 1.236 20.45 ± 2.277

0.9977 0.9985 0.9983

3.20 ± 0.5925 5.37 ± 0.7458 5.06 ± 0.8317

0.8622 0.9379 0.9704

a

Abbreviations: exp, experimental data; cal, calculated values.

model was used to validate44

Q t = k pt 1/2 + C

equilibrium adsorption isotherm of flavonoids on the No. H was consistent with II equilibrium adsorption isotherm classified by Brunauer, which was considered to be due to the multilayer reversible adsorption process taking place on the solid adsorbent. Point B is the first precipitous part in Figure 13 and represents the saturated adsorption capacity of the first monolayer of coverage on the resin, which is corresponding with our previous study.39 Presently, the most common theoretical models for modeling adsorption equilibrium data are the Langmuir and Freundlich isotherms. Langmuir equation:

(12)

where kp is the intraparticle diffusion rate constant (mg/g min1/2). C (mg/g), the constant, represents boundary layer thickness. The intraparticle diffusion rate constant is obtained from the slope of the straight line of Qt versus t1/2. If the rate-limiting step is intraparticle diffusion, a plot of solute sorbed against square root of contact time should yield a straight line. The satisfactory validating results for flavones adsorption on DDM-1 and DDM-2 were better than DDM-0 because the relative coefficients were larger than 0.9 (Table 3), and it implied that the intraparticle diffusion process was the rate-limiting step of the adsorption in this study.45−47 3.6. Adsorption Isotherms and Thermodynamics. To illustrate better the adsorption properties of the synthetic copolymer, equilibrium adsorption isotherm of flavonoids on synthetic resins No. H was investigated at five different temperatures of 303, 308, 313, 318, and 323 K, as shown in Figure 13.

qe =

qmCe KL + Ce

(13)

Freundlich equation:

qe = KFCe1/ n

(14)

where qm is the maximum adsorption capacity for forming a monolayer (mg/g), KL is the parameter relative to the adsorption energy (mg/L), KF reflects the adsorption capacity of the adsorbent ((mg/g)(L/mg)1/n), n represents the adsorption affinity of the adsorbent for the adsorbate, and qe (mmol/g) is the equilibrium concentration in the solid phase. The Langmuir and Freundlich equations are used to reveal the linearity fitting and to describe the equilibrated relationship between the concentrations of adsorbate in the fluid phase and the adsorbent at a given temperature (Figure 14). The correla-

Figure 13. Adsorption isotherm of flavones on No. H at different temperatures (n = 3).

The adsorption capacity increased with the increase in temperature. The high temperature would make the resins swells better. The larger the volume and aperture that the resins possess, the more molecules of flavonoids that would enter the resins. Meanwhile, stereospecific blockade of the adsorption site grew down with the increasing of the aperture size, thus the absolute number of adsorption site could increase in the adsorption process. Moreover, the molecular thermodynamic movement of flavonoids was intense under a higher temperature, and more molecules of flavonoids would get into the aperture of the resins. Therefore, the reaction temperature may play a dominant role in flavonoids adsorption on the spent adsorbents. Meanwhile, point B in Figure 13 indicated that

Figure 14. Adsorption capacities of flavones on DDM-0, DDM-1, and DDM-2 at 308 K (n = 3).

tion coefficients were calculated according to the Langmuir and Freundlich equations mentioned above, as listed in Table 4. It could be seen that the correlation coefficients (R2) estimated 2692



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Table 4. Isotherm Parameters of Flavones Adsorption on DDM-0, DDM-1, and DDM-2 at 308 K Freundlich

Langmuir

adsorbent

KF

n

R2

KL (L mmol−1)

qm (mmol g−1)

R2

DDM-0 DDM-1 DDM-2

1.513 ± 0.0417 2.186 ± 0.0313 1.278 ± 0.0635

1.663 ± 0.2346 1.348 ± 0.4556 1.348 ± 0.6519

0.926 0.972 0.970

6.903 ± 1.298 16.226 ± 3.381 81.189 ± 10.441

17.889 ± 2.638 24.272 ± 4.762 67.568 ± 8.327

0.886 0.907 0.970

from the Freundlich equation were high, the R2 values were larger than 0.9, and the exponent (n) was larger than 1 in all cases, which demonstrates that the adsorption of flavones onto adsorbents is favorable. The R2 values were much higher than those from the Langmuir equation, implying that the Freundlich isotherm could reasonably explain the adsorption process, suggesting the multilayer coverage of flavones onto the resin. In the same way, Freundlich adsorption model was also suitable to equilibrium adsorption at 303, 313, 318, and 323 K. Generally speaking, adsorbate−adsorbent interaction plays an important role in the adsorption capacity of adsorbent from aqueous solutions and the effective interaction includes van der Waals force and hydrogen-bonding interaction, etc.32,41,47 In this aqueous solution system, the van der Waals force was mainly hydrophobic interaction as well as the π−π interaction between the aromatic rings of the adsorbate and the matrix of the adsorbent. The adsorption capacity of DDM-2 was larger than DDM-0 and DDM-1. The specific surface area and micropore structure of starting copolymer were greatly increased after postcross-linking reaction, and more π−π sites were available to interact with the adsorbate. Thus, the DDM-2 possessed a larger number of π−π sites which could help the adsorption of flavones. In a word, adsorption capacity was affected by specific surface area, micropore structure, and adsorbent polarity. To better illuminate the adsorption mechanism of flavones onto adsorption resins, the adsorption thermodynamics parameters ΔH, ΔG, and ΔS were evaluated. The isosteric adsorption enthalpy change can be calculated with a derivative Van’t Hoff equation48

ln Kc =

−ΔH +c RT

Figure 15. Determination of the isosteric enthalpy changes of the adsorption of flavones onto DDM-0, DDM-1, and DDM-2 (n = 3).

the temperature subliming, suggesting the temperature was a controlling factor which influenced the adsorption process within the experimental temperature range.50 Compared to DDM-0 and DDM-1, the largest ΔG of DDM-2 suggested the strongest adsorptive affinity of flavones onto DDM-2. The result kept identical with the adsorption capacities. All the values of the enthalpy changes (ΔH0) were negative, indicating that the adsorption process was exothermic, and increasing temperature was theoretically unfavorable to the adsorption process. However, the experimental results showed that the adsorption capacity increased with the temperature increasing from 303 to 323 K, that is because the resin swelled in organic solvent, the pore diameter and surface area expanded with the increase in the temperature, which probably led to the shape of flavones molecular matching with the pore structure of the synthetic resin. Furthermore, the absolute value of ΔH0 for DDM-0 and DDM-1 less than 43 kJ/mol generally showed that the adsorption was a physical process while rising values were indicative of chemical adsorption.51 However, the ΔH0 on resin DDM-2 was larger than 43 kJ/mol, implying that the adsorption process of flavones on the synthetic resin was controlled by chemical rather than physical mechanism. The value of the entropy changes (ΔS0) was negative, demonstrating that the randomness decreased at the solid-solution interface during the adsorption of flavones on the resin. This was probably a reflection of the free movement of the flavones molecules in the solution being restricted to a two-dimensional movement on the sorption sites.50 3.7. Regeneration Test. Continuous adsorptionregeneration runs of an identical synthetic resin (No. H) were also performed to test its applicability. The superposition of flavonoids adsorption curves in the first and fourth cycles showed in

(15)

ΔG = − RT ln Kc (16) where Kc is the equilibrium constant, T is the absolute temperature (K), ΔH is the isosteric enthalpy change of adsorption (kJ/mol), R is the ideal gas constant (8.314 J/mol K), ΔG is the adsorption free energy (kJ/mol), and ΔH was determined by plotting log Kc versus 1/T, as shown in Figure 15 Kc =

Cp Cs

(17)

where Cp and Cs are the equilibrium concentrations of flavones on the resins and solution at different temperatures (303, 308, 313, 318, and 323 K). The adsorptive entropy change was calculated using the Gibbs−Helmholtz equation49

ΔH − ΔG (18) T Thermodynamic parameters of flavones adsorption onto the three adsorbents were listed in Table 5. The values of the free energy (ΔG0) of adsorption were negative at the five temperatures, implying that the adsorption process was spontaneous and feasible. Moreover, the values of ΔG0 increased with ΔS =

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Table 5. Thermodynamic Parameters of Flavones Adsorption onto the Three Adsorbents ΔG (kJ/mol)

ΔS (J/mol K)

adsorbent

ΔH (kJ/mol)

303 K

308 K

313 K

318 K

323 K

303 K

308 K

313 K

318 K

323 K

DDM-0 DDM-1 DDM-2

−24.52 −37.19 −52.65

−2.426 −3.945 −5.173

−3.183 −4.763 −7.657

−4.025 −5.758 −9.784

−5.884 −7.458 −11.546

−6.098 −9.620 −13.057

−39.89 −48.74 −61.69

−36.79 −45.92 −57.88

−33.51 −40.48 −53.57

−27.14 −35.32 −47.46

−26.06 −33.18 −45.24

purification effect were studied. The adsorbent DDM-1 modified by chloromethyl groups exhibited high adsorption capacity of flavonoids, which was attributed to its large specific surface area as well as the polar groups on the network. DDM-2 modified by amino groups performed high purity capacity of flavonoids owing to its large specific surface area and hydrogen-bonding between amino groups and flavonoids. The DDM-1 with 0.44 mmol/g chloromethyl group (No. 3) and the DDM-2 with 0.68 mmol/g amino group (No. H) performed optimal adsorption and purify ability. They were used in the developed twostep protocol for flavonoids purification. After treatment with DDM-1 and DDM-2 in order for one time, the purity of flavonoids was increased 6.62-fold from 5.55% to 36.75% with a recovery of 52.58%. The results showed that this method could be utilized in large-scale production of flavonoids purification from Hippophae rhamnoides L. in industry, due to the prominent advantages of synthetic MARs such as the simple procedure, low cost, and high universality, higher purification efficiency, and easier scale-up.

Figure 16 indicated that synthetic resins can be completely regenerated for repeated use without any significant capacity loss, which is significant for practical application.

■

AUTHOR INFORMATION

Corresponding Author

*Phone: +86-931-496-8248. Fax: +86-931-827-7088. E-mail: [email protected].

Figure 16. The breakthrough curves of flavones adsorption onto DDM-2 at 303 K (1st and 4th cycles).

■

ACKNOWLEDGMENTS This research project was financially supported by the Hundred Talents Program of the Chinese Academy of Sciences (CAS) and the National Natural Sciences Foundation of China (NSFC, no. 20974116).

3.8. Preparation of Flavonoids from Hippophae rhamnoides L. by Two-Step Resins. The aim of the present study, therefore, was to establish a green and efficient method for the large-scale preparation of flavonoids from Hippophae rhamnoides L. by synthetic resins. DDM-1 with 0.44 mmol/g chloromethyl groups and DDM-2 with 0.68 mmol/g amino groups were chosen and used in the developed two-step protocol. The sample solution (flavonoids concentration 0.735 mg/mL) was absorbed by DDM-1 with 0.44 mmol/g chloromethyl groups due to its high adsorption capacity, and then the sample solution used was the product of the first step process, which was then redissolved in water to afford a flavonoids concentration of 1.385 mg/mL. This solution was absorbed by DDM-2 with 0.68 mmol/g amino groups due to its high purity capacity for the second step. The purity of flavonoids after the first step was increased 5.25-fold from 5.55% to 29.11% with a recovery of 57.41%. The purity of flavonoids then increased to 36.75% with a recovery of 91.58% after the second purification. This method has high throughput, is green and effective, and is very suitable for the large-scale separation of flavonoids from Hippophae rhamnoides L. As far as we know, this is the first time that a new approach has been reported for the industrial-scale preparation of flavonoids for pharmaceutical use.

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REFERENCES

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4. CONCLUSIONS In this work, a series of adsorption resins with different functional groups and pore sizes were synthesized, and the influences of functional groups and physical structure with respect to the 2694



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