Biobased Microspheres Consisting of Poly(trans-anethole-co-maleic

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Biobased microspheres consisting of poly(transanethole-co-maleic anhydride) prepared by precipitation polymerization and adsorption performance Yunbin Yuan, Xueyong Yong, Huanyu Zhang, and Jianping Deng ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01438 • Publication Date (Web): 03 Feb 2016 Downloaded from http://pubs.acs.org on February 3, 2016

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ACS Sustainable Chemistry & Engineering

Biobased Microspheres Consisting of Poly(transanethole-co-maleic anhydride) Prepared by Precipitation Polymerization and Adsorption Performance Yunbin Yuan, Xueyong Yong, Huanyu Zhang and Jianping Deng* State Key Laboratory of Chemical Resource Engineering; College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China Corresponding Author: * E-mail: [email protected]. ABSTRACT This article reports the first biobased microspheres derived from phenylpropenic resources. To explore the potentials of biomass derived trans-anethole (ANE) and to develop new biobased polymeric materials, ANE was used for preparing an unprecedented kind of polymeric microspheres constructed by poly(trans-ANE-co-maleic anhydride) [poly(ANE-co-MAH)] through free radical precipitation polymerization in methyl ethyl ketone/n-heptane mixed solvent with 2,2’azobisisobutyronitrile as initiator. Microspheres (about 1 µm in size) with spherical morphology and narrow size distribution were obtained under appropriate conditions. Following the same preparative strategy, crosslinked microspheres were further prepared with divinylbenzene as crosslinking agent and then subjected to hydrolyzation of the surface anhydride groups into carboxyl groups, aiming at developing microsphere adsorbents. The hydrolyzed microspheres exhibited considerable adsorption ability towards trivalent chromic ion [Cr(III)] and an organic dye (methyl red), with maximum adsorption quantity of 10.8 and 17.6 mg/g, respectively. The established preparative strategy can be potentially extended to other renewable phenylpropenes and ACS Paragon Plus Environment


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MAH derivatives, and also can be taken as a versatile platform for fabricating biobased polymeric microspheres. Apart from being potentially used as adsorbents towards diverse adsorbates (metal ions, organic compounds, etc.), the microspheres also can be further explored as biomaterials, emulsifiers, among others. Keywords: microspheres, trans-anethole, precipitation polymerization, adsorption performance

INTRODUCTION Much attention has recently been paid towards renewable bio-resources to substitute unrenewable fossil derivatives. As far as polymers are concerned, continuous efforts have been made to replace unsustainable commercially available polymers with sustainable ones.1–7 Accordingly, typical biobased compounds including naturally bio-polymers like cellulose,8,9 starch,10 and chitosan,11–13 and small organic molecules like plant oils,14,15 terpenes,16 and furfurals17 have been intensively investigated for the purpose. However, for an important class of essential oils, i.e. phenylpropenes, the studies dealing with them for constructing polymers have been only scarcely reported.18,19 Phenylpropenes are organic compounds naturally synthesized in plants, with transanethole (ANE) as a typical example. ANE is rich in anise,20 fennel,21 and Chinese star anise22 in over 80% proportion in the form of essential oil. It has been widely used in food functional ingredients for seasonings, confectionary products, alcoholic beverages, and a fixative in perfume industry.23 Phenylpropenes contain vinyl functional groups, and however they undergo free radical polymerization with much difficulty. There are two reasons for this situation. Firstly, the phenol groups (e.g. in isoeugenol and its precursor eugenol) are not favorable for radical polymerization. Secondly, the 1,2-disubstituted vinyl structures (e.g. in ANE and isosafrole) lead to high steric hindrance.24–27 To explore the potentials of phenylpropenes and to overcome the problems in polymerizing phenylpropenes as mentioned above, two methods have been elegantly established: (1) cationic

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polymerization;28 and (2) radical copolymerization of phenylpropenes with other monomers, e.g. maleic anhydride (MAH).24–27 These excellent studies provided interesting (co)polymers with unique structures and properties. Nonetheless, so far all the related studies were performed in solution polymerizations, just yielding bulk (co)polymers (namely, non-spherical products). Taking into consideration the advantages of polymeric spheres like large specific surface area and high surface reaction activity,29–31 we in the present study established the first precipitation copolymerization method of ANE and MAH, by which we sufficiently prepared copolymer microscaled spheres. Another driving force for us to accomplish the present study is the copolymers of styrene and MAH. Poly(styrene-co-maleic anhydride)s (SMAs) are one class of the most important commercialized industrial polymers. The reactive anhydride moieties in SMAs make them readily functionalized for diverse targets. SMAs have found enormous applications in practice for instance as absorbents,32 scale inhibitors,33 emulsifiers,34,35 carriers,36 and as distinctive building blocks for constructing new architectures and materials.37–41 Unfortunately, SMAs are not renewable polymers. Moreover, styrene is a hazardous compound to the environment.42 In the above context, we endeavored to prepare the biobased copolymeric microspheres as reported below, which may be taken as renewable substitutes for SMAs. Herein, it is important to point out that, besides ANE, MAH can also be regarded as a “green” compound; nowadays it can be obtained by environmentally friendly method.43 In a preceding study, we used another phenylpropene, i.e. eugenol, to successfully construct polymeric microspheres and investigated their oil-absorbing ability.44 In the present study, we obtained mono-dispersed microspheres by copolymerizing ANE and MAH through free radical precipitation copolymerization route (Scheme 1). We further prepared crosslinked microspheres


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following the same strategy. The resulting crosslinked copolymer microspheres were subjected to surface hydrolyzation, followed by a preliminary exploration on their adsorption properties. Exciting results were obtained and the powerful preparative strategy can be taken for further designing and developing more (co)polymer spheres by using other phenylpropenes and MAH and even its derivatives (e.g. maleimides).

Scheme 1. A schematic for preparing poly(ANE-co-MAH) microspheres by precipitation polymerization in MEK (methyl ethyl ketone, good solvent) and n-heptane (poor solvent).

EXPERIMENTAL SECTION Materials Trans-Anethole (ANE) and maleic anhydride (MAH) were purchased from Tokyo Chemical Industry (TCI) and used without further purification. 2,2’-Azobisisobutyronitrile (AIBN) was purchased from Beijing J&K Scientific Co. and recrystallized from ethanol before use. Divinylbenzene (DVB) was obtained from Alfa Aesar. Chromic nitrate nonahydrate [Cr(NO3)3•9H2O] and methyl red were obtained from Sinopharm Chemical Reagent Co. and used as received. Methyl ethyl ketone (MEK), n-heptane, and ethanol were purchased from Beijing Modern Eastern Fine Chemical Co. All the aqueous solutions were prepared by using deionized water. Measurements


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FT-IR spectra were measured with a Nicolet NEXUS 870 infrared spectrometer (KBr tablet). Molecular weights of the copolymers were recorded by gel permeation chromatography (GPC, Waters 150C) calibrated by using polystyrenes as standard and tetrahydrofuran (THF) as eluent (Waters Styragel HT6E-HT5-HT3, in series at 35 °C, flow rate of 1.0 ml/min). The surface morphology of the microspheres was observed with a Hitachi S-4800 scanning electron microscope (SEM). 1H and

13

C NMR spectra were recorded in DMSO-d at room temperature. Elemental

analysis was performed on a Vario EL cube (Elementar Analysensysteme GmbH). Thermal gravimetric analysis (TGA) was performed with a STA 449C thermal analyzer (Netzsch) under N2 at a heating rate of 10 °C/min. Differential scanning calorimetry (DSC) measurements were conducted using a Netzsch DSC204F1 instrument under a flow of nitrogen at a heating rate of 20 °C/min. Atomic absorption spectrophotometer (AAS) (Varian SpectraAA55B) fitted with chromium hollow cathode lamps was used for determining the metal ions concentration. UV-vis absorption measurements were carried out by a UV-vis spectrophotometer (Shanghai Jinghua 756MC). Synthesis of microspheres A typical procedure for preparing poly(ANE-co-MAH) microspheres is illustrated in Scheme 1 and will be briefly stated as follows. First, monomer MAH was added in a 100 mL three-necked flask under slowly stirring in a solvent mixture of MEK and n-heptane. Then, the mixture of monomer ANE, crosslinker DVB (for crosslinking microspheres only), and initiator AIBN was dropped into the flask under gentle stirring. The flask containing the mixture was connected with a condenser and a nitrogen inlet, and placed in a constant temperature water bath. The polymerization lasted for a certain time under N2 atmosphere. The amount of each reagent was predetermined and the data are presented in the corresponding tables (see below). After polymerization, the product


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was filtered, collected, washed with ethanol for 3~5 times, and dried. Microspheres yield was determined gravimetrically. Surface hydrolyzation of microspheres The resulting crosslinked poly(ANE-co-MAH) microspheres were immersed into 0.1 M NH3•H2O for 6 h after shaken in a thermostat oscillator for 30 min, then the pH was adjusted to 5~6 by adding a suitable amount of 0.1 M HCl aqueous solution. Subsequently the microspheres were filtered, washed by deionized water for five times, and then dried. Adsorption of Cr(III) We took Cr(III) as a model for toxic metal ions since it is a typical toxic metal causing serious environmental problems, e.g. pollution water. Adsorption of Cr(III) was performed by mixing the hydrolyzed microspheres (0.025 g) and 50 mg/L Cr(III) aqueous solution (10 mL) in a glass bottle. The dispersion was then stirred in a thermostat oscillator at room temperature for 1 h. Thereafter, the dispersion was placed for a determined time. After completing the adsorption, the supernatant was extracted and the Cr(III) concentration was determined by AAS. The removal percentage of adsorbate is calculated by the following equation: − × 100 1 %CrIII removal = Where C0 and Ce are the initial and equilibrium concentration (mg/L) of the metal ions. Effects of adsorbent amount Adsorption experiments were conducted by using a batch of microspheres with varied mass (0.005–0.04 g) for adsorbing Cr(III) from aqueous solutions (10 mL, 50 mg/L) at pH = 5. The supernatant was extracted and the Cr(III) concentration was analyzed by AAS. Effects of Cr(III) concentration


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The effects of incremental Cr(III) concentration on the adsorption behavior were investigated. The microspheres of constant weight (0.025 g) were immersed into Cr(III) solution with varied concentrations (10 mL, 10–100 mg/L) at pH = 5. After adsorption, the supernatant was extracted and the Cr(III) concentration was measured by AAS. Effects of pH The effects of pH on the adsorption behavior were investigated. The microspheres of constant mass (0.025 g) were immersed into Cr(III) solution with the same concentration at varied pH (3~6). After adsorption, the supernatant was extracted and the Cr(III) concentration was measured by AAS. Equilibrium isotherm The equilibrium isotherm model examined in the study is Langmuir isotherm, which has been widely applied to metal ion adsorption tests. The study was performed by using a certain amount of microspheres (0.025 g) in solutions of Cr(III) at varied concentrations for 12 h. After the adsorption reached equilibrium, AAS was used to determine the Cr(III) concentration of the residual supernatant. The amount of Cr(III) adsorbed by the microspheres, qe (mg/g), was calculated according to the following equation: =

− 2

Where V is the volume of the solution, and m is the mass of the absorbent. The Langmuir isotherm applied in this study is expressed by the following equation:45 1 = + 3


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Where Ce is the equilibrium concentration (mg/L) of Cr(III), qe is the amount of Cr(III) adsorbed at equilibrium (mg/g), qm is the maximum adsorption capacity corresponding to the complete monolayer coverage (mg/g), and KL is the Langmuir constant related to the adsorption energy. Kinetic study For the kinetic studies, 10 mL Cr(III) solution in concentration of 50 mg/L was mixed with 0.025 g hydrolyzed microspheres for a certain time. By measuring the Cr(III) concentrations by the AAS method in predetermined time intervals, the removal rate of Cr(III) was determined. For time t, the amount of adsorbed Cr(III), qt (mg/g), was calculated by the following equation: " =

− " 4

Where Ct is the concentration of Cr(III) in solution at specified time (mg/L). The pseudo-first-order kinetic and pseudo-second-order kinetic fitting were followed to investigate the adsorption mechanism. The pseudo-first-order kinetic was used for calculating by linear form of the equation, following:45 log − " = %&' −

() + 5 2.303

Where qe and qt are the amounts of Cr(III) adsorbed (mg/g) at equilibrium and at time t (min), respectively, and k1 (1/min) is the rate constant of first-order adsorption. The pseudo-second-order kinetic is used in the linear form of the equation:46 + 1 + = + 6 " (- - Where k2 (g/mg min) is the rate constant of second-order adsorption, and the other parameters keep the meaning as stated above. Adsorption of methyl red


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Methyl red is a widely used organic dye. We used it as an example to examine the microspheres’ adsorption ability towards organic dye. Adsorption experiments of methyl red were performed by mixing the hydrolyzed microspheres (0.005 g) and 50 mg/L methyl red ethanol solution (10 mL) in a glass bottle. The dispersion was then shaken in a thermostat oscillator at room temperature for 30 min. Thereafter, the dispersion was placed for a determined time. After completing the adsorption, the supernatant was extracted and the methyl red concentration was determined by UV-vis spectrophotometer. Other adsorption formulae were kept the same as in the case of Cr(III) adsorption, as stated above.

RESULTS AND DISCUSSION Effects of solvent mixture on the formation of microspheres

Figure 1. SEM images of poly(ANE-co-MAH) microspheres prepared at varied MEK/n-heptane ratio: (A) 6:4; (B) 5:5; (C) 4:6; (D) 3:7; (E) 2:8 (v/v). Polymerization conditions: [ANE] = [MAH], 0.4 mol/L; [AIBN], 0.02 mol/L; 75 °C, 6 h. For more information, see Table 1.


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When precipitation polymerization method is utilized, a suitable solvent or solvent mixture might be the most important factor for preparing desirable microspheres. To determine the most suitable solvent, we tried several kinds of reaction media including butyl acetate, ethyl acetate, methyl ethyl ketone (MEK), and MEK/n-heptane mixture. Finally, we found the MEK/n-heptane mixture could serve as a suitable solvent mixture to provide mono-dispersed microspheres. Meanwhile, we further found that the MEK/n-heptane ratio exerted large effects on the formation of microspheres. The resulting polymeric microspheres were observed by SEM, as presented in Figure 1. The related data are presented in Table 1. The five tested polymerizations provided microspheres in a yield over 50 wt%. The polymer chains possessed Mn of 24000 to 35000 g/mol. Nonetheless the data of Mw/Mn seem to be rather large (3~4), due to the specific precipitation polymerization approach. Table 1. Eﬀects of solvent mixture on the microspheres.a Average Diameterc

MEK/n-heptane b

Mn (g/mol)

Mw/Mn

(v:v)

a

b

CVc (%)

Yieldd (wt%)

(µm)

6:4

24000

3.4

1.48

20.27

62.6

5:5

35000

4.5

1.40

10.83

68.3

4:6

30000

3.2

1.47

6.12

62.2

3:7

27000

3.8

0.86

8.14

68.2

2:8

24000

3.8

0.88

4.54

50.1

Polymerization conditions: [ANE] and [MAH], 0.4 mol/L; [AIBN]; 0.02 mol/L; 75 °C, 6 h.

b

Measured by GPC. cAverage diameters and CV (coefficient of variation) determined according to

SEM images, only spherical particles were calculated. dDetermined by gravimetric method.


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The microspheres (Figure 1) showed an average size of around 900 nm to over 1.4 µm (Table 1), depending on the composition of the solvent mixture. According to the SEM images, the solvent mixture exerted a significant impact on the microspheres morphology. In the polymerization systems under investigation, MEK served as a “good” solvent, whereas n-heptane served as a “poor” one. Figure 1 shows us that the microspheres with optimal morphology appeared in the case of MEK/n-heptane ratio being 4:6 (v/v, Figure 1-C). The morphology of all the five sets of microspheres is essentially acceptable, however apparent difference still can be observed. When the unfavorable solvent n-heptane was in an excess amount (over 60% in the system), as Figure 1-D and 1-E show, the microspheres decreased in size to smaller than 1 µm, and the molecular weight of the polymer chains also decreased gradually depending on the n-heptane usage (Table 1). Moreover, coagulation of the microspheres became more obvious and their morphology became more irregular than the optimal ones. We explain this phenomenon as follows. When the unfavorable solvent was in an excessive amount, the polymer chains precipitated at a relatively earlier stage and the polymerization stopped; the more the unfavorable solvent added, the quicker the polymer chains precipitated out and further the lower the molecule weight of the resulting polymer. In addition, when monomer feed kept same, more nucleation sites formed for the microspheres to be subsequently fabricated.47 Thus, the chance of coagulation among the microspheres increased, eventually leading to microspheres in a smaller size and more obvious coagulation than the optimal ones (Figure 1-D and 1-E). When the favorable solvent MEK was in an excess amount (over 40% in the system), the microspheres became more and more irregular in morphology. Especially in the solvent mixture consisting of MEK/n-heptane = 6:4 (v/v), only a few spherical particles could be found in the SEM image (Figure 1-A). In the polymerization systems containing excessive favorable solvent, the


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solvent mixture could not offer a proper condition to form regular microspheres. Owing to the relatively higher solubility of the polymer chains in the favorable solvent MEK, swelling and even re-dissolution of the previously formed nucleation sites led to high CV (coefficient of variation, the ratio of the standard variation to the average particle size, Table 1) in the microspheres; and meanwhile non-spherical microspheres were formed with appreciable coagulation among them. In brief, an excessive amount of the favorable solvent is either undesirable for the spheres to grow regularly. According to the morphology of the microspheres, a solvent mixture consisting of MEK/n-heptane = 4:6 (v/v) seems to be the most suitable solvent medium and was used for preparing microspheres in the subsequent experiments. Effects of monomer concentration on microspheres Another key factor determining the formation of regular polymer particles when prepared by precipitation polymerization is the concentration of monomers. We observed the critical effects of monomer concentration on the microspheres, as demonstrated by the data in Table 2 and the SEM images in Figure 2. For the polymer chains forming the microspheres, the molecular weight increased from 15000 to 38000 g/mol when the total monomer concentration increased from 0.4 to 1.2 mol/L. Meanwhile, the average diameter of the microspheres increased from 1.2 to around 2 µm, and the yield of them increased from 28.5 to 67.1 wt%. Nonetheless, only the group with each monomer concentration being 0.4 mol/L could provide microspheres with low CV (6.12%). For other groups, all the values of CV were over 15%. Furthermore, no matter how to change the monomer concentration from the optimal formula, the microspheres showed obvious irregularity in morphology. A possible explanation is proposed as follows. When the total monomer concentration was below the optimal one, the copolymerization rate was correspondingly slower. The precipitated nuclei had a larger chance to coagulate with each other


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during the growing period. Also, the growth of the nuclei also became slower. As a result, nonspherical particles with smaller particle size and lower yield were fabricated, as can be seen in Figure 2-A and 2-B. The CV and yield data in Table 2 also showed the same result. When the monomer concentrations were increased and exceeded the optimal case, as shown in Figure 2-D and 2-E, the copolymerization rate became too fast, thus increasing the coagulation probability of the previously formed large particles. Additionally, small particles continued to be formed in the course

Figure 2. SEM images of poly(ANE-co-MAH) microspheres prepared at varied monomer concentration: [MAH] = [ANE] =(A) 0.2; (B) 0.3; (C) 0.4; (D) 0.5; (E) 0.6 mol/L. The molar ratio of total monomer to initiator kept at 40/1. For more details, see Table 2. Table 2. Effects of monomer concentration.a [ANE] = [MAH]

Mnb

Average Mw/Mn

b

CVc (%)

Yieldd(wt%)

Diameterc(µm)

(mol/L)

(g/mol)

0.2

15000

3.9

1.2

17.50

28.5

0.3

23000

2.4

1.28

18.75

49.9

0.4

30000

3.2

1.47

6.12

62.2

0.5

25000

3.7

1.98

9.19

61.4


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0.6 a

38000

2.6

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1.92

27.60

67.1

Polymerization conditions: [AIBN], 0.02 mol/L; MEK/n-heptane, 4:6 (v/v); 75 °C, 6 h. The molar

ratio of total monomer to initiator kept in 40/1. bMeasured by GPC. cDetermined according to SEM images. dDetermined by gravimetric method. of polymerization. So larger CV data were found in these batches of microspheres (Table 2). The results indicate that moderate monomer concentrations are required for forming poly(ANE-coMAH) microspheres with regular spherical morphology. Apart from the influencing factors discussed in detail above, we also investigated the effects of other factors, including monomer/initiator molar ratio (Figure S1 and Table S1 in Supporting Information, SI, the same below) and polymerization temperature (Figure S2 and Table S2). For the minor affecting factors, detailed information and a brief discussion are presented in Supporting Information. Structure and thermal properties of microspheres We next characterized the microspheres by FT-IR spectroscopy, for the purpose of acquiring more information about the microspheres structure. The typical FT-IR spectra of the two monomers, ANE and MAH, and the poly(ANE-co-MAH) microspheres are shown in Figure 3. For poly(ANE-co-MAH),the clear signals at 1857 and 1777 cm−1 indicate the anhydride groups, which exist only in the monomer MAH. The peaks at 1513 and 1247 cm-1 mean the benzene ring and methoxyl group in ANE, respectively. From the observations in Figure 3, we conclude that the microspheres contain two monomer units, ANE and MAH. 1H and

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C NMR spectra were also

recorded on the poly(ANE-co-MAH) microspheres; however, because of the complexity and obvious overlapping of the signals, quantitative information could not be obtained. The relevant NMR spectra are shown in Figure S3.


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With the copolymer microspheres in hand, we further investigated their thermal stability. For this purpose, three groups of microspheres were prepared and examined as examples and the related TGA thermograms are presented in Figure 4. The copolymers with different monomer ratio did not show pronounced difference in thermostability. All the microspheres exhibited two decomposing

Figure 3. Typical FT-IR spectra of trans-anethole (ANE), maleic anhydride (MAH), and copolymer poly(ANE-co-MAH) microspheres (KBr tablet).

Figure 4. TGA thermograms of poly(ANE-co-MAH) microspheres measured in N2 at a heating rate of 10 °C/min. Three samples indicate three groups of copolymer microspheres (6:2, 4:4, and 2:6).


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processes. When the microspheres were heated over 120 °C, the small molecules and oligomers adhered on the microspheres began to decompose at this low temperature. Subsequently, the copolymer chains began to decompose at about 270 °C, and the fastest decomposing speed occurred at about 350 °C. Additionally, the microspheres were also subjected to DSC analysis (Figure S4 in SI), and the Tg of the copolymer chains was found to be around 200 °C. The thermostability of the microspheres enables them to be used under normal conditions and even at a moderately high temperature. Crosslinkedpoly(ANE-co-MAH) microspheres

Figure 5. SEM images of (A) crosslinked poly(MAH-co-ANE) microspheres and (B) the hydrolyzed microspheres. Table 3. Comparison of crosslinked and non-crosslinked microspheres.a Average Diameterb (µm)

a

CVb (%)

Yieldc(wt%)

Crosslinked microspheres

0.97

3.09

74.1

Non-crosslinked microspheres

1.47

6.12

62.2

Polymerization conditions: For crosslinked microspheres, [ANE], 0.32 mol/L; [MAH], 0.4 mol/L;

[DVB], 0.04 mol/L; [AIBN], 0.02 mol/L; MEK/n-heptane, 4:6 (v/v); 75 oC, 6 h. For non-


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crosslinked microspheres: [ANE] = [MAH], 0.4 mol/L; [AIBN], 0.02 mol/L; MEK/n-heptane; 4:6 (v/v); 75 oC, 6 h. bDetermined according to SEM images. cDetermined by gravimetric method.

From the view point of practical applications, especially the fact that non-crosslinked microspheres can dissolve in alkaline solution, crosslinked polymer microspheres rather than noncrosslinked may be more desirable. Based on the successful preparation of copolymer microspheres reported above, we further prepared crosslinked poly(ANE-co-MAH) microspheres in the same way by using DVB as crosslinker, and then investigated their adsorption properties. As shown in Figure 5, the crosslinked microspheres showed more desirable spherical morphology than the non-crosslinked ones. Compared to the non-crosslinked microspheres, the crosslinked ones showed a higher yield and smoother surface, due to the better copolymerization ability of DVB. Moreover, the crosslinking effect of DVB decreased the distance between polymer chains inside the microspheres, so the average diameter and CV decreased remarkably in the crosslinked microspheres (Table 3). The crosslinked microspheres did not dissolve in alkali, acid, and organic solvents like tetrahydrofuran. They even hardly swelled in these solvents. To improve the adsorption property of the microspheres, we hydrolyzed the anhydride groups on the crosslinked microspheres surface into carboxyl groups. When the microspheres were hydrolyzed by ammonia, the morphology of the hydrolyzed microspheres changed little from the ones before hydrolyzation. In addition, after hydrolyzation, the average diameter of the microspheres remained at approximately 1.00 µm, CV increased from 3.09% to 4%, and coagulation between the microspheres only slightly increased. The surface carboxyl groups had a better hydrophilic property than anhydride groups, so the surface copolymer chains on the hydrolyzed spheres stretched to a certain degree in basic solution.


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Adsorption property of crosslinked poly(ANE-co-MAH) microspheres After hydrolyzation, the carboxyl groups on the surface of the copolymer microspheres hopefully enable them to be ideal absorbents. To justify this hypothesis, adsorption tests were carried out by using the hydrolyzed copolymer microspheres towards Cr(III) and methyl red. The former was taken as a model for metal ions, while the latter represents organic compounds. Adsorption quantity vs. time (qe-t) curves for Cr(III) and methyl red are shown in Figure 6. The adsorption quantity (qe) was found to be 10.8 mg/L for Cr(III) adsorption in water and 17.6 mg/L for methyl red adsorption in ethanol.

Figure 6. Time-adsorption quantity profiles of (A) Cr(III) and (B) methyl red on the microspheres. Concentration of microspheres, 50 mg/L. For (A) microspheres, 2.5 g/L; pH, 5, in water; for (B) microspheres, 1.0 g/L, in ethanol. For detailed process, see Experimental section.

Besides the qe-t curve, the effects of microspheres amount (Figure S5), the effects of Cr(III) concentration and the qe-ce curve (Figure S6 and S7), and the effects of pH (Figure S8) were also investigated. The detailed data and analysis are presented in Supporting Information. Based on the investigations, we fitted the result into Langmuir isotherm and kinetic equations to examine the adsorption mechanism, as to be reported next.


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Langmuir Isotherm. Langmuir model has been widely applied to study isothermal equilibrium adsorption processes. The basic hypothesis of the Langmuir theory is that immobilization of adsorbate occurs on a homogenous surface by monolayer adsorption; the adsorbed adsorbates do not have any interaction among them; all the adsorption sites have identical affinity to adsorbates and the adsorption at one site does not influence the adjacent sites. Thus, the Langmuir isotherm is valid for monolayer adsorption in which the adsorption behavior completes on surfaces containing identical active sites.

Figure 7. Langmuir isotherm for Cr(III) adsorption onto hydrolyzed microspheres pH, 5; time,12 h; microspheres, 2.5 g/L.

The adsorption of Cr(III) by hydrolyzed microspheres matched very well the Langmuir isotherm, as confirmed in Figure 7. The linear correlation coefficient (R2, calculated by Origin software, the same below) is 0.9922, meaning that the adsorption of Cr(III) by hydrolyzed microspheres fits the Langmuir isotherm very well. The values of qm and KL for Cr(III) were calculated from the slope and the intercept of the linear plot illustrated in Figure 7. qm was found to be 9.8 mg/g, and KL 1.33 L/mg.


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Pseudo-first-order kinetic. Kinetic studies are important for performing adsorption tests, because they can be used to predict the adsorbing rate and supply data for us to deeply understand the adsorption mechanism. For Cr(III), the fitting equations applied here are pseudo-first-order and pseudo-second-order kinetic models. For pseudo-first-order kinetics, values of k1 are calculated from the plots of log(qe-qt) versus t (see Figure 8-A) for sample Cr(III) ions. The obtained R2 value is relatively smaller and the experimental qe value did not agree with the value calculated from the linear plot. The data are listed in Table 4. Pseudo-second-order kinetic model. This model is widely applied to adsorption systems to express the chemical adsorption which implies a strong electrostatic interaction between the negatively charged surface and metal ions. The linear plot of t/qt versus t is shown in Figure 8-B and the obtained R2 value in this case is quite close to 1. It also shows a good agreement between the experimental and the calculated qe values listed in Table 4, indicating the satisfactory applicability of this model to describe the adsorption process of Cr(III) ions on the microspheres.

Figure 8. Kinetic model for adsorption of Cr(III) by the microspheres: (A) pseudo-first-order model; (B) pseudo-second-order model. Table 4. Kinetic models for Cr(III) adsorption by hydrolyzed microspheres. Parameters of pseudo-first-order kinetics

k1(1/min)

qe(mg/g)

R2


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Parameters of pseudo-second-order kinetics

0.290

6.73

0.8591

k2 (g/mg min)

qe (mg/g)

R2

0.108

11.23

0.9899

FT-IR spectroscopy was used to verify the adsorption mechanism on the hydrolyzed poly(ANEco-MAH) microspheres. The FT-IR spectra of Cr(III) are shown in Figure S9, providing further support for the adsorption accomplished through chelating bonds between carboxyl and Cr(III) ions. For methyl red, the corresponding experiment and fitting results are presented in Supporting Information. The microspheres also showed relatively high adsorption towards this organic compound. For a further improvement in adsorption ability, surface modification or morphology optimization of the microspheres are needed. Probable methods include the preparation of porous microspheres and a further transformation of the anhydride groups into other functional structures. Encouraged by the remarkable adsorption ability of the microspheres towards Cr3+ ions, next we will explore the microspheres’ adsorption towards other metal ions, e.g. Pb, U, and As. Studies along the direction are currently ongoing in our lab.

CONCLUSION We successfully prepared poly(ANE-co-MAH) microspheres by precipitation polymerization using the biomass-derived trans-anethole, and maleic anhydride. Microspheres of satisfactory spherical morphology and narrow size distribution could be obtained under the optimized conditions. The optimal microspheres in spherical morphology were obtained with [ANE] = [MAH] = 0.4 mol/L, [AIBN] = 0.02 mol/L in mixed reaction medium of MEK and n-heptane [4:6 (v/v)] at 75 °C for 6 h. To develop a novel type of absorbents, we further fabricated crosslinked


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microspheres in the same manner and then transformed their surface anhydride groups into carboxyl groups through hydrolyzation. The adsorption ability of the hydrolyzed crosslinked poly(ANE-coMAH) microspheres was explored; the qe was found to be respectively 10.8 mg/L for Cr(III) adsorption in water and 17.6 mg/L for methyl red adsorption in ethanol. We found that the adsorption of Cr(III) was accomplished by chelation effects, while the adsorption towards methyl red by electrostatic attraction. The adsorption tests demonstrate that the biobased microspheres have potentials to be applied as new absorbents in wastewater treatment and even in other fields. ASSOCIATED CONTENT Supporting Information. Figures and Tables of the effect on microspheres of monomer/initiator molar ratio, polymerization temperature and monomer ratio, NMR spectrum, DSC of the microspheres, adsorption curves relevant to Cr(III) and methyl red. This information is available free of charge via the Internet at http://pubs.acs.org/ AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21474007, 21274008, 21174010) and the Funds for Creative Research Groups of China (51221002). REFERENCES (1) Iwata, T. Biodegradable and Bio-Based Polymers: Future Prospects of Eco-Friendly Plastics. Angew. Chem. Int. Ed. 2015, 54, 3210–3215.


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Biobased Microspheres Consisting of Poly(transanethole-co-maleic anhydride) Prepared by Precipitation Polymerization and Adsorption Performance Yunbin Yuan, Xueyong Yong, Huanyu Zhang and Jianping Deng* State Key Laboratory of Chemical Resource Engineering and College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China E-mail: [email protected]

TOC

For Table of Contents Use Only


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Trans-anethole is used as a representative of biobased phenylpropenes to construct polymer microspheres by copolymerization with maleic anhydride. The crosslinked, hydrolyzed microspheres show remarkable adsorption ability towards Cr(III) and methyl red.


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Biobased Microspheres Consisting of Poly(trans-anethole-co-maleic

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