Electrostatic assembled of Keggin-type polyoxometalates onto poly(4-vinylpyridine)-grafted poly(vinylidene fluoride) membranes

Abstract

Polymer brushes have been used as supports for the immobilization of Keggin-type polyoxometalates (POMs) catalysts on poly(vinylidene fluoride) (PVDF) membranes by electrostatic-assembly. The poly(4-vinylpyridine) (P4VP) polymers are effectively grafted from PVDF membranes via surface-initiated atom transfer radical polymerization with activators generated by electron transfer (SI-AGET ATRP), and the P4VP-grafted PVDF membranes are characterized via ATR-FTIR, XPS, SEM, and AFM. Subsequently, the quaternization of the P4VP brushes process creates a membrane with a positive charge. The pendant pyridine and pyridinium groups facilitate the electrostatic assembly of POMs onto the P4VP-grafted membrane, thereby fabricating a well-designed membrane with potential catalytic applications. Therefore, such systems are interesting candidates for the heterogenization of POMs onto polymer-grafted membranes.

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Introduction

Polyoxometalates (POMs) are a class of discrete and anionic metal oxide clusters of early transition metals in their highest oxidation state. Keggin-type POMs (H₃PW₁₂O₄₀, HPW) have generated considerable interest as catalysts because their constitutional elements can be readily varied and tuned for acidic and redox catalytic applications.¹ However, POMs themselves are usually nonporous solids with the surface area less than 10 m² g⁻¹; therefore, they have limited surface sites for surface catalytic reactions.² To increase the surface sites for surface accessibility, POMs can be immobilized on weak-acidic or non basic supports with high surface area,^3–6 entrapped in positively charged polyelectrolyte through electrostatic attraction.^7–10 Supported POMs not only improve accessibility for substrates but also, in some cases, enhance their catalytic activity through support interactions.^11–13

Comparison among different heterogenization routes for the immobilization of POMs, entrapping them on membranes offers new ways to prepare catalytic membranes. Catalytic membranes (CMs) are very attractive because their advantages relate to the synergy between the chemical reaction and separation process in the same device. As a new type of contact reactors, they can be employed in the field of determining gas–liquid or liquid–liquid consecutive oxidation reactions. The separation of a product from the reaction mixture is one of the advantages of CMs; this feature will be beneficial for improving the yield and selectivity in equilibrium-limited reactions and consecutive catalytic reactions.¹⁴ Recently, a new generation of catalytically active membranes has been developed by coupling the low-temperature plasma-modification processes with the surface chemical immobilization reaction of POMs catalysts.^15,16 This technique demonstrated that small-sized catalysts may be easily surface immobilized, overcoming the normal leaching problems that occur with low-molecular-weight catalysts entrapped in membranes.¹⁷

The heterogenization of POMs in a polymeric matrix to create organic–inorganic hybrid catalysts is a recent trend in heterogeneous catalysis technology. There are several procedures for the entrapment of POMs into polymeric matrices and self-assembly methods.^18–20 This approach of heterogenization through electrostatic interactions represents a means to confer POMs avoiding deactivating the covalent bonding with the active sites and preventing unavoidable leaching via simple physical blending. Recently, Huang and Stang et al.^21–24 exploited the hierarchical assembly strategy to fabricate advanced supramolecular materials. This assembly process involved a stepwise procedure and a uniform increase to the architectural complexity of a substrate, starting from discrete precursors and growing in dimensionality by controlling the reactivity to obtain the final product. This strategy opens up a new opportunity for the fabrication of POMs-based supramolecular materials, which may find potential applications in hybrid catalysts.

Polymer brushes are ultrathin polymer coatings consisting of polymer chains that are tethered at one end to an interface of solid substrate through covalent attachment.²⁵ They represent versatile molecular construction for shaping surface properties. Their interest lies in the possibility of finely adjusting the composition of polymer brushes together with the possibility of post functionalizing the pendant lateral chains toward their desired use. The dynamic nature of the polymer brushes together with the particularly high local concentration engendered with such frameworks offer a unique opportunity in supported catalysts.^26–28 In some cases, the architecture of polymer brushes can significantly enhance the outcome of cooperative catalytic processes, which require the close proximity of synergetic active sites.

To introduce functional polymer brushes onto the membrane, surface modification via “grafting to” or “grafting from” methods have been established. Among the available different grafting from approaches, surface-initiated atom transfer radical polymerization (SI-ATRP) has been most extensively used to produce polymer brushes and it has the advantage of relatively fine controlling the grafted density and the degree of polymerization for polymer brushes.²⁹ However, to prepare surface-modified membranes with consistent performance, the preformed catalyst must be stored under an inert atmosphere, and experimental precautions are required to maintain an oxygen-free environment throughout the process. The present advancement overcomes these obstacles by activators generated by electron transfer for ATRP (AGET ATRP).³⁰ In this polymerization process, the activating catalyst species Cu(I) is regenerated from the oxidized Cu(II) form by electron transfer using environmentally acceptable reducing agents such as ascorbic acid. Most importantly, less than 500 ppm concentration of the catalyst needs to be used together with an appropriate reducing agent.³¹

In this work, we report the construction of Keggin-type POMs-decorated poly(4-vinylpyridine)-grafted PVDF membrane by electrostatic-assembly. First, poly(4-vinylpyridine) (P4VP) polymer brushes were effectively grafted from the surface of the PVDF membrane within a controllable manner by employing SI-AGET ATRP methodology. The function of the P4VP polymer brushes not only can be used to adjust the morphology, surface wettability, surface energy and surface charge of the PVDF membrane, but also make the membrane positively charged through the quaternization of the P4VP brushes. Subsequently, the pyridinium groups on the side-chain of the polymer brushes facilitated heterogeneous Keggin-type HPW catalysts onto the surface of P4VP-grafted membrane via electrostatic-assembly Scheme 1.^32–34

Scheme 1 Schematic illustration for the preparation of catalytically PVDF-g-QP4VP/HPW membranes.

Experimental section

Materials

PVDF membranes were prepared using the thermally induced phase separation (TIPS) method. LiOH·H₂O (98%), NaBH₄ (98%), diisobutylaluminium hydride (DIBAL-H, 1 M in hexane), 2-bromoisobutyryl bromide (BIBB, 98%), ethyl 2-bromobutyrate (99%) and CuBr₂ (99.95%) were purchased from Aladdin Chemical Co., Ltd., and used without further purification. 4-Vinylpyridine (96%) and tris[2-(dimethylamino)ethyl]amine (Me₆TREN, 99%) were purchased from Alfa Aesar. The monomer 4VP was distilled under reduced pressure over NaOH pellets and stored at −18 °C under Ar. Ascorbic acid (99%) and 1-bromohexadecane (97%) were purchased from Acros Organics. Analytical grade H₃PW₁₂O₄₀·xH₂O (HPW) was purchased from Sinopharm Chemical Reagent Co., Ltd., and used without further purification. All the other reagents and organic solvents, unless under specific illumination, were purchased from Sinopharm Chemical Reagent Co., Ltd. and used without any further purification. Deionized water (Millipore) was used for all the experiments.

Surface grafting of 4VP from PVDF membrane via SI-AGET ATRP

The surface grafting of 4VP from a PVDF membrane was accomplished as follows: PVDF membranes were first immersed into 60 mL LiOH·H₂O solution (1.8 mol L⁻¹) at 80 °C for 24 h. After that, the treated membranes were immersed into 50 mL NaBH₄ solution (0.078 mol L⁻¹) for 17 hand 50 mL DIBAL-H (0.104 mol L⁻¹) for 65 h to achieve hydroxylated PVDF. The hydroxylated membranes were placed into 50 mL of diethyl ether (containing 4.8 mL of triethylamine and 4.5 mL BIBB), and the mixture was stirred at 0 °C for 2 hand then at room temperature for 12 h. After the reaction, initiators were immobilized onto the surface of the PVDF membrane (named PVDF-Br membrane). The PVDF-Br membrane (1 × 1 cm) was weighed and placed into a flask containing 2.15 mL 4VP (20 mmol), 45.2 mg CuBr₂ (0.2 mmol), 92.2 mg Me₆TREN (0.4 mmol), 22.5 μL EBiB (0.2 mmol) and 5 mL solution (2-propanol and deionized water in 1 : 1 volume ratio). Then, 5 mL solution containing 35.2 mg ascorbic acid (0.2 mmol) was transferred into the flask under nitrogen protection. The graft polymerization was allowed to proceed for a predetermined period of time in an oil bath at 50 °C. The modified PVDF (named PVDF-g-P4VP) membrane was then taken out and washed thoroughly with excess anhydrous ethanol and water to ensure the complete removal of the adhered and physically adsorbed monomers or polymers, if any. The surface functionalized membranes were obtained after freeze drying overnight. The grafting yield (GY, mg cm⁻²) was calculated as follows:where W₀ and W₁ represent the weight of the dried membrane before and after P4VP grafting, respectively, and A is the membrane area.

Quaternization of P4VP and immobilization of HPW onto PVDF membranes

The PVDF-g-P4VP membranes were placed into 10 mL ethanol containing 0.6 g 1-bromohexadecane, and the reaction mixture was stirred at 75 °C for 24 h. After the reaction, the membranes were thoroughly rinsed with deionized water for 24 h and the water was changed every 4 h; thereafter, the modified membranes were dried under reduced pressure. The complete removal of unreacted 1-bromohexadecane from the modified membrane surface was confirmed by the absence of covalent bromide (∼70 eV) in the X-ray photoelectron spectroscopy Br 3d core level spectrum. The quaternization of P4VP was denoted as PVDF-g-QP4VP membrane. The PVDF-g-P4VP and PVDF-g-QP4VP membranes were immersed into an aqueous solution of 10 g L⁻¹ HPW for 24 h; then, they were washed three times with deionized water and finally dried under vacuum (hereafter, these membranes will be indicated as PVDF-g-P4VP/HPW and PVDF-g-QP4VP/HPW, respectively).

Characterization

The surface chemical compositions of the membranes were characterized through ATR-FTIR spectroscopy (Excalibur 3100, Varian): each spectrum was captured by 32 averaged scans at a resolution of 4 cm⁻¹. X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Fisher Scientific) was performed by using an Al Kα anode target, and the X-ray source was run at 250 W with an electron take-off angle of 45°. The C 1s neutral hydrocarbon peak at 284.6 eV was used as a reference for all the binding energies (BEs). The surface morphology of the membranes was observed using a scanning electron microscope (SEM, JSM7401, JEOL) under standard high-vacuum conditions. The surface characterization of the membrane was performed by an atomic force microscope (AFM, Nanoscope IIIa, Digital Instrument) equipped with a silicon cantilever using the tapping mode. The water contact angles at the membrane surfaces were measured using the Contact Angle System OCA 15EC (Dataphysics, Filderstadt, Germany) at room temperature by the sessile drop method. A water droplet of 1 μL was carefully dropped onto the membranes and the images were taken after 30 s. The contact angle value of each sample was measured at three different positions of the sample.

Results and discussion

Surface-initiated AGET ATRP of 4-vinylpyridine from PVDF membrane

The process and mechanism of SI-AGET ATRP of 4-vinylpyridine from the PVDF membrane surface was performed by a three-step process. The first stage was the hydroxylation of the PVDF (PVDF-OH) membrane with aqueous LiOH·H₂O.³⁵ At this stage, a hydrogen atom and a fluorine atom were eliminated and oxygen-containing functionality emerged. The second stage in the preparation process was subsequent to coupling the initiator of bromide onto the PVDF-OH membrane (PVDF-Br) via a dehydration reaction. The third stage was to grow the P4VP polymer brushes from the PVDF membrane surface via SI-AGET ATRP. The grafting yield was monitored by the weight increase of the membranes with polymerization times, which were adjusted from 4 h to 24 h.

As shown in Fig. 1a, the grafting yield increases linearly from 1.9 mg cm⁻² to 7.2 mg cm⁻² with increasing polymerization times from 4 h to 24 h, indicating that the chain growth of P4VP is consistent with a controlled/living process. Then, the ATR-FTIR spectra are used to determine the components of the grafted polymers at different polymerization times (Fig. 1b). In the ATR-FTIR spectrum of pristine PVDF, strong peaks are observed at 1400 cm⁻¹, 1172 cm⁻¹, and 875 cm⁻¹, which are attributed to –CH₂– in-plane blending, –CF₂– stretching, and skeletal vibration of the C–C bond, respectively. The successful grafting of the P4VP brushes onto the PVDF membrane surface can be deduced from the appearance of additional peaks at 1596 cm⁻¹, 1556 cm⁻¹, 1494 cm⁻¹, and 1413 cm⁻¹, which are attributed to νC=C and νC=N stretching of the pyridine ring of grafted 4VP.^36,37 The intensity of the –C=N absorption undergoes an increase with the reaction time, indicating that the grafting yield of the P4VP brushes from the membrane surface is dependent on the SI-AGET ATRP time. This result is consistent with the controllability of the ATRP reaction.

Fig. 1 (a) Growth of grafting yield as a function of polymerization time. (b) ATR-FTIR spectra of the pristine PVDF membrane and PVDF-g-P4VP membranes with different polymerization times.

Fig. 2 shows the respective wide-scan and C 1s core-level XPS spectra of pristine PVDF membrane and PVDF-g-P4VP membrane after undergoing SI-AGET ATRP reaction for 4 h. For the pristine PVDF membrane, only C 1s and F 1s signals are observed on the wide-scan spectrum (Fig. 2a). The C 1s core-level spectrum of the pristine PVDF membrane can be curve-fitted with four peak components (Fig. 2b). One of them at the BE of 285.8 eV is attributed to CH₂: another at BE of 290.5 eV is attributed to the CF₂.³⁸ The [CH₂] : [CF₂] area ratio of about 1.06 is in good agreement with the theoretical ratio of 1.0 dictated by the chemical structure of PVDF. In the case of the PVDF-g-P4VP membrane surface, the appearance of an additional N 1s signal at around 400 eV and the simultaneous clear increase in the relative intensity of the C 1s signal in the wide-scan spectrum (Fig. 2c) arising from the grafted P4VP chains indicate the successful grafting of 4VP from the PVDF membrane surface. The C 1s core-level spectrum of the PVDF-g-P4VP membrane can be curve-fitted with five peaks at the EB values of 285.8 eV and 290.5 eV that are assigned to the CH₂ and CF₂ species, respectively, of the PVDF main chains of the membrane (Fig. 2d). The peak components with BE at 284.6 eV for the C–H species and at 285.5 eV for C–N species are also associated with the grafted P4VP chains.³⁹ The peak component at 288.4 eV is assigned to the O=C–O species of the immobilized initiator. The persistence of the fluorine signal from the underlying PVDF membrane in the wide scan spectrum indicates that the thickness of the P4VP polymer layer is less than the probing depth of the XPS technique (about 10 nm in an organic matrix).⁴⁰

Fig. 2 Wide scan and C 1s core-level XPS spectra of the pristine PVDF membrane surface (a and b); (c and d) PVDF-g-P4VP membrane 1# for 4 h SI-AGET ATRP time.

Fig. 3 shows the respective C 1s core-level XPS spectra of PVDF-g-P4VP for different polymerization reaction times. The [N]/[C] ratios for the P4VP grafted surfaces increase from 0.055 to 0.106 corresponding to polymerization times from 4 h to 24 h. These results suggest that the amount of grafted P4VP chains increases gradually with an increase in the SI-AGET ATRP reaction times. This is in good agreement with the results that the GY values of the PVDF-g-P4VP surfaces increase with the grate polymerization time.

Fig. 3 C 1s core-level XPS spectra of the PVDF-g-P4VP membranes 1# (a), 2# (b), 3# (c), 4# (d), 5# (e), 6# (f).

The morphology of PVDF-g-P4VP membranes was associated with the surface coverage and chain conformation of grafted P4VP, which were characterized by SEM and AFM. Fig. 4 shows the representative SEM images of the PVDF-g-P4VP membrane surface from the reaction times of 4 h to 24 h. It is obvious that the pore size of the PVDF-g-P4VP membranes gradually decreases with an increase in the SI-AGET ATRP time, which is consistent with the GY results. A further increase in the SI-AGET ATRP time to more than 20 h, however, led to a plate-like aggregation structure of the grafted polymers and compact cover on the surface of the membrane. The decrease in the pore size and porosity with the increase in the polymerization time confirms that the membrane morphology is dependent on the P4VP brushes and is, therefore, controllable. The SEM images suggest that the SI-AGET ATRP can be an ideal surface modification tool for fine-tuning the morphology of the prepared membranes to achieve the desired catalytic performance of CMs.

Fig. 4 SEM of PVDF-g-P4VP 1# 4 h (a), PVDF-g-P4VP 2# 8 h (b), PVDF-g-P4VP 3# 12 h (c), PVDF-g-P4VP 4# 16 h (d), PVDF-g-P4VP 5# 20 h (e), PVDF-g-P4VP 6# 24 h (f).

AFM is applied by using the tapping mode over an area of 5 μm × 5 μm. The 3D images of the membranes for different polymerization times are shown in Fig. 5. The PVDF-g-P4VP membrane surfaces are rougher and with some cuspidal outshoot (attributable to the aggregation of the grafted P4VP chains) appearing throughout the surface, indicating that the surface-initiated P4VP brushes are indeed fabricated on the PVDF membrane surface. In addition, the surface roughness R_a values increase from 22.82 from 4 h of reaction time to 85.72 nm from 24 h of reaction time. This result is consistent with the previous findings that the grafting P4VP brushes can lead to a significant increase in the surface roughness value, as well as the surface morphology. It should be noted that the PVDF-g-P4VP (6#, 24 h) surface shows more uniformity with less number of cuspidal outshoots than those of the other membranes, suggesting that a higher P4VP polymer layer packing density results from the prolonged polymerization time.

Fig. 5 AFM of PVDF-g-P4VP 1# (a), PVDF-g-P4VP 2# (b), PVDF-g-P4VP 3# (c), PVDF-g-P4VP 4# (d), PVDF-g-P4VP 5# (e), PVDF-g-P4VP 6# (f).

Quaternization of PVDF-g-P4VP membrane and immobilization of HPW

In this study, the structure of the HPW catalysts after being loaded on the PVDF-g-P4VP and PVDF-g-QP4VP membrane surfaces are identified by ATR-FTIR and XPS. As shown in Fig. 6, the PVDF-g-P4VP/HPW membrane shows characteristic bands associated with the Keggin-type HPW unit, i.e. the P–O band at 1074 cm⁻¹, the W=O band at 975 cm⁻¹, the W–O–W_corner (the corner-sharing WO₆ octahedra) at 873 cm⁻¹, and the W–O–W_edge (the edge-sharing WO₆ octahedra) band at 813 cm⁻¹, indicating that the primary Keggin-type HPW is present on the PVDF-g-P4VP membrane surfaces.⁴¹ On the other hand, the W–O–W_edge band of [PW₁₂O₄₀]³⁻ on the PVDF-g-QP4VP/HPW membrane exhibits slightly red shifted positions (804 cm⁻¹) as compared to PVDF-g-P4VP/HPW, indicating the presence of an electrostatic interaction between the HPW anions and pyridinium groups of PVDF-g-QP4VP membrane.

Fig. 6 ATR-FTIR spectra of the PVDF-g-P4VP/HPW and PVDF-g-QP4VP/HPW membranes.

The evidence of the immobilization of HPW on PVDF-g-P4VP and PVDF-g-QP4VP membrane surfaces is obtained via XPS. The XPS atomic-composition results reveal the presence of tungsten after the immobilization procedure for both the modified membranes, as shown in Table 1.

Table 1 Surface chemical composition (in at.%) of modified membranes from the XPS data

Substrate	C 1s	O 1s	N 1s	F 1s	P 2p	W 4f	[W]/[C]
PVDF	52.30	1.37	1.35	44.97	—	—	—
PVDF-g-P4VP	85.91	3.43	7.25	3.23	—	—	—
PVDF-g-P4VP/HPW	28.36	8.64	2.42	0.93	0.39	9.27	0.33
PVDF-g-QP4VP/HPW	80.38	8.66	1.74	—	0.43	8.8	0.11

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Fig. 7 shows the XPS spectra of PVDF-g-P4VP/HPW and PVDF-g-QP4VP/HPW membranes. The XPS survey scans of these two samples clearly show the existence of assembled HPW on the membranes, as evidenced by the detection of W. In the case of the PVDF-g-P4VP/HPW membrane, the N 1s region can be deconvoluted into three components. The main peak component at about 398.8 eV is assigned to the imines (–N=) moiety of the pyridine rings; the peak component at about 399.6 eV is assigned to the hydrogen-bonded imines, and the peak component at 401.5 eV is assigned to the pyridinium ions.^42,43 The percentages of imines groups involved in the hydrogen bonding and protonation are about 46.8% and 12.9%, respectively. These results indicate the presence of electrostatic/ionic interactions and hydrogen bonding interactions between HPW and pyridine groups on the PVDF-g-P4VP membrane. Furthermore, Fig. 7 also shows the XPS spectra of the PVDF-g-QP4VP/HPW membrane: the corresponding N 1s core-level spectrum shows that the main form of interaction switches from hydrogen bonding to protonation or the formation of pyridinium ions. Based on the [N⁺]/[N] ratio, the degree of the alkylation of the pyridine rings is around 40.8%. The perfect fit observed for the W 4f_5/2 (35.4 eV) and W 4f_7/2 (37.5 eV) peaks in the W 4f level in the PVDF-g-P4VP/HPW and PVDF-g-QP4VP/HPW membranes establishes that the integrity of the Keggin-type HPW structure is retained, which is in good agreement with the results from the ATR-FTIR spectra.⁴⁴ These data show that the principally electrostatic/ionic interaction and hydrogen bonding link the catalyst on the P4VP-grafted PVDF membrane surface with no change in their oxidation states.

Fig. 7 XPS spectra of the PVDF-g-P4VP/HPW and PVDF-g-QP4VP/HPW membranes.

The morphology of the modified PVDF membranes was investigated by SEM, as shown in Fig. 8. The surface morphology of the PVDF membrane after grafting P4VP displays less porosity but a smoother surface than pristine PVDF membrane (Fig. 8a and b). This is due to the grafting polymers partially or completely filling the pores on the PVDF membrane surface. No obvious variation is observed on the surface of the PVDF-g-P4VP/HPW membrane (Fig. 8c); however, a uniform HPW small dense particles distribution is observed in the PVDF-g-QP4VP/HPW membrane (Fig. 8d). These results indicate that the quaternization of the pyridine groups of grafted P4VP chains into pyridinium groups can be used to effectively immobilize HPW onto the surface of the PVDF membrane. Water contact angles were employed to evaluate the effect of the membrane surface hydrophilicity and wetting characteristics. The water contact angle of non-modified hydrophobic PVDF membrane is 109.6°. After grafting P4VP, the contact angle values of PVDF-g-P4VP membranes significantly decrease and reach 37.2°. The static water contact angles further increase to 111.7° and 98.4° for the resulting PVDF-g-P4VP/HPW and PVDF-g-QP4VP/HPW, respectively. These results indicate that the grafted P4VP polymers on PVDF can effectively increase the hydrophilicity of the PVDF membrane, yet the hydrophilicity of catalytic membranes decreases due to the homogeneously immobilized hydrophobic HPW nanoparticles.

Fig. 8 SEM images of the pristine PVDF (a), PVDF-g-P4VP (b), PVDF-g-P4VP/HPW (c), and PVDF-g-QP4VP/HPW membranes (d).

Conclusions

In this work, P4VP polymers were successfully grafted from the surface of a PVDF membrane using the SI-AGET ATRP methodology. There is an approximately linear increase in the graft yield of the P4VP brushes with polymerization time, indicating that the chain growth from the membrane surface was consistent with a controlled process. Chemical and morphological changes for the modified membranes were confirmed by ATR-FTIR, XPS, SEM and AFM. Furthermore, the quaternization of the grafted P4VP brushes was carried out with 1-bromohexadecane to introduce the positively charged pyridinium nitrogen moieties. Both polar pyridine and pyridinium groups (PVDF-g-P4VP and PVDF-g-QP4VP, respectively) could act as binding sites for HPW, principally by electrostatic interaction and hydrogen bonds. These results indicate that such catalytic PVDF membranes should have considerable application potential in catalytic membrane reactors. Further work on their catalytic activity is currently in progress.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (no. 21306029), the Guangxi Natural Science Foundation of China (no. 2012GXNSFBA053021), the scientific research project of Guangxi Education Department Foundation (no. 201106LX064), the scientific research project of Guangxi University Foundation (no. 2013HZ001), the NSF of Guangxi Province (2014GXNSFFA 118003), the BAGUI scholar program (2014A001), and the State Key Laboratory Cultivation Base for the Chemistry and Molecular Engineering of Medicinal Resources, Ministry of Science and Technology of China (CMEMR2013-C09).

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