Biobased Magnetic Microspheres Containing Aldehyde Groups

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Biobased Magnetic Microspheres Containing Aldehyde Groups: Constructed by Vanillin-derived Polymethacrylate/FeO and Recycled in Adsorbing Amine 3

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Huanyu Zhang, Jianping Deng, and Youping Wu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b02018 • Publication Date (Web): 23 Nov 2016 Downloaded from http://pubs.acs.org on December 4, 2016

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ACS Sustainable Chemistry & Engineering

Biobased Magnetic Microspheres Containing Aldehyde Groups: Constructed by Vanillin-derived Polymethacrylate/Fe3O4 and Recycled in Adsorbing Amine Huanyu Zhang,1,2,3 Jianping Deng,1,3 * and Youping Wu2,3 * 1

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical

Technology, Beisanhuan East Road 15#, Beijing 100029, China 2

State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical

Technology, Beisanhuan East Road 15#, Beijing 100029, China 3

College of Materials Science and Engineering, Beijing University of Chemical Technology,

Beisanhuan East Road 15#, Beijing 100029, China E-mail: [email protected] (Deng); [email protected] (Wu) KEYWORDS: Aldehyde, Biomass, Magnetic spheres, Polymers, Schiff base

ABSTRACT: The contribution reports a novel category of sustainable aldehyde-containing magnetic microspheres (ACMMs) prepared through suspension polymerization. For preparing

ACS Paragon Plus Environment

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the ACMMs, lignin-derived vanillin methacrylate (VMA) was used as biobased monomer, while methacrylated-Fe3O4 NPs were used as magnetic supplier. The resulting microspheres were proved to have remarkable magnetic property and adsorption capacity towards paraanisidine which was employed as a representative of amines. A maximum adsorption was found to be up to 433 mg/g (559 mg/g in theory). Also noticeably, the adsorption was realized by forming reversible Schiff base under mild conditions. Desorption processes were performed conveniently, proving that the ACMMS can be easily recycled. This work demonstrates the remarkable potentials of ACMMs to be used as scavenger resins in absorbing amines. Also worthy to be highlighted is that the abundant aldehyde groups enable the microspheres to be a promising platform for further preparing functional polymers by employing the Schiff base structure as linking parts, e.g. as biomaterials for immobilizing enzymes.

1. Introduction With the development of economy and technology, more functional polymers with special morphologies and functions are required and accordingly designed to satisfy diverse demands. For example, polymeric materials containing reactive aldehyde groups have constituted a unique class of polymeric materials.1–4 It is known that aldehyde groups can form Schiff base bond with amines under mild conditions. Moreover, the Schiff base bond structure is found to be a reversible linkage.5,6 Therefore, polymers containing aldehyde groups provide a promising platform for immobilizing biologically active compounds like drugs,7–9 enzymes,10,11 and DNA.12 They also provide a versatile platform for developing Schiff-bases with significant applications like antibacterial agents,13 adsorbents,14 and sensors.15,16 Nonetheless, despite the significant progress, the preparation of functional polymers of the kind is still majorly based on petroleum resources. The polymer materials containing aldehyde groups in literature are primarily limited


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to be derived from acrolein17 and glutaraldehyde.18,19 Unfortunately, both acrolein and glutaraldehyde show some intractable problems, including the exhaustion of petroleum/fossil resources and the environmental problems accompanying petroleum based materials. Additionally, acrolein polymerizes only under strict conditions.17 Especially, the low-molecular compounds (acrolein and glutaraldehyde) show toxicity as strong irritants for skin, eyes, and nasal passages.20 These problems restrict further development of the currently employed aldehyde-containing polymer materials. To solve the problems, the biomass substitute, vanillin, which is refined from lignin, shall be an ideal candidate due to the sustainability and low toxicity. Among the various biomass candidates for preparing bio-derived polymers,21–24 lignins may be one of the most ideal candidates.25 In particular deserving being highlighted is that, unlike other biomasses derived from crops,23 lignin can be isolated from wood and annual plants, and do not compete with human feeding.22,24 In spite of these advantages, there is still a long way to achieve wide practical applications of lignin due to the complex molecular structures. Fortunately, the rapidly developing refining technology provides us with interesting lignin-based building blocks,26 among which vanillin demonstrates the highest yield.27 Vanillin-based polymers have been widely explored for preparing renewable polymeric materials like polybenzoxazines,28 polyesters,29

and

polymethacrylates.30–32

In

spite

of

the

significant

advancements

aforementioned, to our knowledge, the construction of functional polymeric materials, in particular functional microspheres derived from vanillin, has not been exploited yet. To combine the two concepts of “biobased” and “functional polymeric spheres”, we have prepared vanillin-based polymer microspheres via suspension polymerization in the earlier work.33 We are convinced that the as-prepared polymeric microspheres containing reactive


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aldehyde groups will find various practical applications. So in the present work, a new type of hybrid polymeric microspheres composed of magnetic particles and vanillin-based polymers were prepared for conveniently recycling uses of the material. To further examine the magnetic polymer microspheres’ potentials as green adsorbent, paraanisidine was used as a representative of amino derivatives to investigate the adsorption capacity of the aldehyde groups in the microspheres. To our delight, the adsorption experiments clearly justify our hypothesis. Accordingly, the microspheres are expected to find uses as scavenger resins for amine sequestration and green adsorbent towards amines. They also provide a versatile platform for further preparing functional polymer materials through forming Schiff base bonds as bridge. 2. Experimental Section 2.1 Materials. FeCl3·6H2O, FeSO4·7H2O, NH3·H2O (28%), methylbenzene, anhydrous ethanol, petroleum ether, cyclohexanone, chloroform (CHCl3), tetrahydrofuran (THF), triethylamine (TEA), and oleic acid (OA) (all analytic grade) were purchased from Beijing Chemical Reagents Company (China) and used as received. Trimethylolpropane triacrylate (TMPTA) was purified by distillation under reduced pressure. 2,2-Azobis-(isobutyronitrile) (AIBN) was recrystallized from methanol, dried under vacuum at room temperature, and stored in an amber bottle. Paraanisidine and 3-(trimethoxysllyl) propyl methacrylate (TPM silane) were purchased from Aladdin and directly used. Poly(vinyl alcohol) (PVA, polymerization degree 1750±50) was obtained from Sinopharm Chemical Reagent Co. Monomer vanillin methacrylate (VMA) was synthesized according to a method reported previously.33 All the solvents were distilled by standard methods. Water was freshly deionized before use. 2.2 Measurements.


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Fourier transform infrared spectra (FT-IR spectra) were measured using a Nicolet NEXUS 670 spectrophotometer (Number of cumulative scans, 32; scan resolution, 3.857 cm-1; KBr pellet). The morphologies of the microspheres were observed by S-4800 electron microscope (SEM, Hitachi). Transmission electron microscopy (TEM) images were obtained on an H-800 (Hitachi) transmission electron microscope at an accelerating voltage of 200 kV. Magnetic characterization was carried out on a vibrating sample magnetometer (VSM, LakeShore 7410 VSM) at room temperature. X-ray photoelectron spectroscopy (XPS spectra) were performed in a ThermoFisher Scientific ESCALAB 250 spectrometer with an X-ray source of Monochromated Al Kalph 150W and the pass energy is 200eV for survey and 30eV for high resolution scans. Powder X-ray diffraction (XRD) patterns were obtained using a D/max2500 VB2+/PC X-ray diffractometer (Rigaku) using Cu Ka radiation. Paraanisidine content was determined by UV-vis absorption measurement on 756MC UV-visible spectrophotometer (Shanghai Jinghua Technology Instrument, λ = 305 nm). 2.3 Synthesis and modification of Fe3O4 nanoparticles. The methacrylate-modified Fe3O4 NPs (MethA-Fe3O4 NPs) were synthesized by a slight modification of the coprecipitation method in literature.34 The major processes are briefly described as follows. FeCl3·6H2O (2.05 g) and FeSO4·7H2O (1.18 g) were dissolved in 50 mL of deionized water (deoxygenated by ultrasonic treatment) in a three-necked flask. After stirring for 20 minutes under N2 atmosphere, 15 mL of ammonium hydroxide (25%, w/w) was quickly injected into the reaction mixture at room temperature. Magnetic NPs were immediately formed as black precipitate. Then, 1 mL of OA was dropped into the solution at 80 °C with intensely stirring (800 rpm) within 1 h. The reaction mixture remained under continuously stirring for another 3 h. After cooling down, the OA coating Fe3O4 NPs (OA-Fe3O4 NPs) were extracted


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from water into toluene phase (50 mL, with the help of sodium chloride). After the organic layer was collected and dried, a small amount of triethylamine (2.5 mL) was added. TPM silane (1.0 mL) was added dropwise in the dispersion. The reaction lasted for 48 h under N2 at room temperature. Finally, petroleum ether was added in the mixture, followed by magnetic separation. The product was re-dispersed in toluene and this procedure was repeated for three times. After the product was dried in a vacuum overnight, MethA-Fe3O4 NPs were obtained. 2.4 Preparation of aldehyde-containing magnetic microspheres (ACMMs). The preparation of ACMMs is briefly stated below. Firstly, 0.0044 g MethA-Fe3O4 NPs, 0.22 g VMA, 0.0066 g TMPTA, and 0.0088 g AIBN were completely dissolved in cyclohexanone (1 ml) via ultrasonic dispersion (20 min). A previously prepared PVA aqueous solution (5 wt%, 50 mL) was added in a 100 mL three-necked flask which was equipped with a mechanical stirrer (stirring speed, 250 rpm) and nitrogen inlet. Then the as-prepared cyclohexanone solution was added dropwise. After being stirred at a speed of 250 rpm for 30 min at 45 oC, the oil phase droplets were well dispersed. Afterwards, the reaction system was heated to 65 °C and retained at the temperature for 8 h. When the polymerization ended, the microspheres suspended in PVA aqueous solution were separated by the help of a magnet. After the microspheres were washed with hot deionized water three times and dried, ACMMs in brown were collected (78 wt%, yield). 2.5 Adsorption studies of ACMMs towards paraanisidine. Adsorption studies were performed towards paraanisidine in the present study. Each adsorption experiment was repeated 3 times to acquire an average value. In a typical adsorption test, 10 mg of the ACMMs was dispersed in a paraanisidine chloroform solution (10 mL) and then stirred for 24 h at about 30 °C. The adsorption capacity was measured at 12 concentrations (100, 200, 300,


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400, 500, 600, 800, 1000, 1200, 1400, 1600, and 2000 mg/L). The initial concentration of paraanisidine (C0, mg/L) and its concentration at equilibration state (Ceq, mg/L) were quantitatively determined by a UV-vis spectrophotometer at 305 nm with a calibration curve. The amount of paraanisidine adsorbed (qeq, mg/g) at equilibration state was calculated by the following equation:

qeq =

(C0 − Ceq )V m

(1)

where m is the mass of microspheres (g) and V the volume of chloroform solution (L). For adsorption kinetic studies, 10 mg of ACMMs was added in 10 mL of paraanisidine chloroform solution (1000 mg/L) and thoroughly mixed under stirring at approximately 25 °C. The paraanisidine concentrations of chloroform dispersion at preset time intervals were also measured by UV-visible spectrophotometer. The amount of adsorbed paraanisidine at time t, qt (mg/g), was calculated by the equation: qt =

(C0 − Ct )V m

(2)

where m is the mass of microspheres (0.01 g), C0 (mg/L) is the initial paraanisidine concentration, Ct (mg/L) is the concentration at time t, and V the volume of chloroform dispersion (L).

2.6 Desorption and recycling of ACMMs. We employed the mixed solvent composed of THF, H2O, and HCl as desorption solution to destroy the Schiff base bonds. The effects of varied solvent ratios (THF/H2O) and concentrations of HCl were separately explored. In a typical desorption test, after 0.01 g of ACMMs was saturated by paraanisidine (C0 = 1000 g/L), the qe,first of ACMMs was determined and calculated


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by equation (1). The microspheres after adsorption were separated by the help of a magnet, washed with chloroform three times, and dried. Desorption was performed using 10 mL of a series of mixture solvent containing THF, H2O, and HCl. The flasks were sealed and shaken for 6 h under room temperature for desorption. Two methods were used to investigate the efficiency of desorption. The first method was a weighting method, namely, by comparing the mass change of microspheres before and after desorption. The second method was a re-adsorption method, namely, by measuring the adsorption capacity of microspheres in the second cycle after desorption in the first cycle. The efficiency of desorption was firstly investigated by the weighing method; the amount of desorbed adsorbate, qd,m (mg/g), was calculated by the equation:

(3) where m is the mass of microspheres (0.01 g), m1 the mass of microspheres after adsorption, and m2 the mass of microsphere after desorption. The efficiency of desorption by this method, Eweight (%), was calculated by the equation:

(4) For the re-adsorption method, the adsorption capacity, qe,second, was calculated by equation (1), and the efficiency of desorption by this re-adsorption method, Ere-adsorption (%), was calculated by the equation:

(5)


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For adsorption cycles, the adsorption−desorption process was repeated for three cycles and the qe,second and qe,third of the ACMMs were also determined by equation (1).

3. Results and Discussion 3.1 Preparation and characterization of ACMMs. Combining multiple functions in one entity has become an effective method for extending applications of functional materials. Magnetic materials are widely employed as a component for preparing multi-functional materials such as catalysts,35 magnetic resonance imaging contrast agents,36 and targeted drug delivery media.37 In this work, we introduced the magnetic component for endowing the aldehyde-containing magnetic microspheres (ACMMs) with convenient recovery capacity. As shown in Figure 1, the procedure for preparing ACMMs involves two major steps. In the first step (Figure 1, Part A), coprecipitated Fe3O4 NPs were prepared and further coated by a layer of OA molecules in one-pot. Then, OA molecules were replaced by TPM silane with the help of TEA, making the Fe3O4 NPs coated with methacrylate groups (MethA-Fe3O4 NPs). The biobased monomer, vanillin methacrylate (VMA) was prepared via esterification between vanillin and methacryloyl chloride.33 The second step (Figure 1, Part B) is to prepare the crosslinked ACMMs via suspension polymerization route using VMA as monomer, MethA-Fe3O4 NPs as the magnetic source (the methacrylate moieties can take part in polymerization), TMPTA as crosslinking agent, AIBN as initiator, cyclohexanone as the dispersed phase, and PVA aqueous solution as the continuous phase. The anticipated ACMMs were successfully formed in this way. The resulting OA-Fe3O4 NPs, MethA-Fe3O4 NPs, and ACMMs were characterized by FT-IR, XRD, VSM, TEM, and SEM techniques, as to be reported below.


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Figure 1. Schematic strategy for (part A) preparing aldehyde-containing magnetic microspheres (ACMMs) starting from biobased monomer vanillin methacrylate (VMA) and MethA-Fe3O4 NPs and (part B) the suspension polymerization (Part B) for preparing ACMMs.

Figure 2. FT-IR spectra of OA-Fe3O4 NPs, MethA-Fe3O4 NPs, VMA and ACMMs. The spectra were measured in KBr tablet.


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In this study, a relatively simple chemical co-precipitation way was taken to prepare OA-Fe3O4 NPs and MethA-Fe3O4 NPs. The NPs were characterized by FT-IR, as shown in Figure 2. The sharp characteristic peaks of Fe−O at 577 cm-1 can be clearly observed in both OA-Fe3O4 and MethA-Fe3O4 NPs. Due to the coordination form of OA and Fe3O4, a double-peak structure of Fe–O appeared (577 and 619 cm-1). After OA molecules were replaced by TPM silane, the Fe–O characteristic peak changed to single peak. This, together with the characteristic peaks of 2940 (methyl and methylene), 1720 [–(C=O)−O−], 1640 [−C(CH3)=C], and 1160 cm-1 (Si−O), strongly demonstrates the successful obtainment of OA-Fe3O4 and MethA-Fe3O4 NPs. Furthermore, the XRD patterns also provided evidence for the consideration of successful synthesis of Fe3O4 NPs. As shown in Figure S1 (Supporting Information, SI), diffraction peaks (111), (200), (311), (222), (400), (422), (511), and (440) can be observed and indexed as face centered cubic Fe3O4 (the Joint Committee on Powder Diffraction Standards (JCPDS) reference (no. 19-0629)). The observations further showed us that both the two groups of Fe3O4 NPs were successfully prepared. The magnetic properties and morphology of the two groups of Fe3O4 NPs were investigated using VSM and TEM. The results are presented in Figure 3(A). From the hysteresis loops we find that the maximum saturation magnetization (MSM) of OA-Fe3O4 and MethA-Fe3O4 NPs were 51.0 and 61.8 emu/g, respectively. The difference in MSM is mainly related to the amount of nonmagnetic materials in the NPs. In more detail, the OA molecules in OA-Fe3O4 NPs have relatively longer molecular chains, resulting in a lower MSM. The TEM images inserted in Figure 3(A) show that both the two groups of Fe3O4 NPs had well dispersibility due to the barrier effect and lipophilicity of OA and methacrylate molecular structures.


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Figure 3. (A) The hysteresis loops of OA-Fe3O4 NPs and MethA-Fe3O4 NPs; The inset shows typical TEM images for (A-1) OA-Fe3O4 NPs, and (A-2) MethA-Fe3O4 NPs. (B) The hysteresis loop of ACMMs; the inset shows the responsivity of ACMMs to external magnetic field. The magnetic microspheres, i.e. ACMMs, were prepared via suspension polymerization route referring to our previous study dealing with microspheres derived from vanillin methacrylate (VMA).33 Following the strategy presented in Figure 1(Part B), we successfully prepared ACMMs with 78 wt % in yield.

Figure 4(A) presents the typical photograph of ACMMs. The color of ACMMs was brown. This color resulted from mixing the black Fe3O4 NPs into white PVMA, in which the low concentration of Fe3O4 (2 wt%, initial amount, relative to monomer amount) made the microspheres’ color deviate from black. The morphology of the prepared ACMMs was observed by SEM, as shown in Figure 4(B, C). The microspheres were 300~500 µm in diameter and contained pores on the surface. The porous structures were investigated further by crushing the microspheres and observing the fracture with SEM.


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Figure 4. (A) Typical photograph of ACMMs, (B) SEM image of ACMMs and (C) the enlarged image of one microsphere. Polymerization conditions: VMA, 0.22 g; PVA, 5.0 wt %; AIBN, 4.0 wt %; TMPTA, 2.7 wt %; MethA-Fe3O4 NPs, 2 wt%; cyclohexanone (1 mL); stirring speed, 250 rpm; time, 8 h; temperature, 65 oC.

The SEM image (Figure S2, Supporting Information, SI) showed that the pores were predominantly distributed on the surface of the microspheres. The formation process of the microspheres and the surface pores can be briefly described as follows. In the initial stage of polymerization, the organic solvent (cyclohexanone) and the dispersant (PVA) stabilized the dispersed liquid drops containing monomer and the others. With the polymerization proceeding, the morphology of the microspheres was cured by the polymerization of monomer and crosslinking agent. In this process, the MethA-Fe3O4 NPs were covalently linked to the polymer chains by their polymerizable methacrylate groups participating polymerization. In the late stage of polymerization, the incompatibility between the polymer chains and cyclohexanone drove the


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residual solvent inside the microspheres to undergo phase separation from the resulting polymer chains and gathered in the form of small droplets, finally leading to the pores after drying. FT-IR, VSM, and XRD techniques were also employed to characterize the magnetic ACMMs. In the FT-IR spectra (Figure 2), the major characteristic peaks, 2940 (methyl and methylene), 2750 [−(C=O)H], and 1700 cm−1 [−(C=O)H] were marked in the two FT-IR spectra of VMA and ACMMs. The disappearance of the peak around 1630 cm−1 in PVMA indicates the transformation of [−C(CH3)=C] groups in VMA to the saturated polymer main chains in PVMA. Also due to the transformation, the peak of ester group moved from 1720 to 1760 cm-1. Accordingly, FT-IR measurement strongly demonstrates the successful polymerization of VMA. Unfortunately, due to the much lower concentration of MethA-Fe3O4 NPs, the Fe–O characteristic peak merged with other peaks around 577 cm−1, so it could not be clearly observed. The microspheres’ magnetic property and magnetic responsivity were further investigated. The results are illustrated in Figure 3(B). The hysteresis loop shows that the maximum saturation magnetization (MSM) of ACMMs was 1.71 emu/g. Compared with the MSM of MethA-Fe3O4 NPs (61.8 emu/g), the ACMMs showed a very lower MSM. The difference in MSM was mainly due to the varied amount of nonmagnetic materials. The concentration of MethA-Fe3O4 NPs in the ACMMs after polymerization was only approximately 2.7 wt% (determined according to the yield of microspheres and the initial amount of MethA-Fe3O4 NPs). Nonetheless, such a low MSM was enough to ensure the magnetic microspheres to be quickly separated with an external magnetic field. This was demonstrated by the image of magnetic responsivity inserted in Figure

3(B) (the inset). The ACMMs can be easily separated under an external magnetic field, and the whole process was completed within 5 s, proving that the microspheres possessed sufficient


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magnetic property. The XRD pattern of the magnetic ACMMs was also measured (Figure S1). Different from the XRD pattern of PVMA (the polymer derived from monomer VMA) shown in Figure S1 (the inset), all the diffraction peaks of Fe3O4 NPs can be found in the pattern of ACMMs. Accordingly, ACMMs with the desired magnetic property were successfully prepared.

3.2 Recycled uses of ACMMs in adsorption towards amine.

Figure 5. Schematic strategy for adsorption and desorption of ACMMs towards paraanisidine. As mentioned above, VMA endows the magnetic ACMMs with abundant aldehyde groups. Due to the mild conditions for forming Schiff base and especially its reversible feature, the aldehyde-containing microspheres may be used as amine scavenger, to make the after-treatment procedure of organic syntheses simplified efficiently.17,38 Moreover, various functional structures can be potentially grafted on the ACMMs for preparing antibacterial,13 catalyst,39 and for biomacromolecules immobilization7–12, 19 by the connection of Schiff base bonds. The reversible characteristic of Schiff base also provides opportunities for preparing stimuli-responsive materials (e.g. drug controlled release and fragrance controlled release). Therefore, the ACMMs containing both aldehyde groups and magneticity are expected to find uses as scavenger resins for amine sequestration and even precursors for preparing multi-functional polymer.


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To justify our expectation, we took paraanisidine as a model of amines to examine the adsorption and the recycled uses’ capacity of the microspheres. The reason for using paraanisidine is because it is extensively used for preparing dyes, pharmaceuticals, etc.40 The processes are illustratively described in Figure 5. In the adsorption process, the amino groups of absorbate react with aldehyde groups in the microspheres to form Schiff base bonds via forming transition state through nucleophilic addition and then dehydration. Due to the specific reaction mechanism, a non-aqueous solvent, chloroform was selected as the solvent for performing the adsorption. In the desorption process, an acidic mixture solvent composed of polar solvent (THF), H2O, and HCl was employed. THF was used to moderately swell the microspheres, while H2O and HCl were used to break Schiff base bonds. The adsorbate combined form, adsorption isotherms, adsorption kinetics, and recycled uses were subsequently studied, as to be discussed next. The combination of adsorbate, paraanisidine, on ACMMs was investigated by FT-IR and XPS measurements. The results are illustrated in Figure 6. ACMMs before and after adsorbing paraanisidine with different initial concentrations (100, 500, and 1000 mg/L; adsorption time, 24 h) were characterized by FT-IR (Figure 6(A)). In the spectra of ACMMs after adsorption, the characteristic peak of C=N double bond appeared at 1630 cm-1, proving that Schiff-base was formed between aldehyde and paraanisidine. Nonetheless, the characteristic peaks of aldehyde groups (1700 cm-1) still can be clearly observed, showing that the aldehyde groups in the microspheres were not completely consumed. In addition, the strength of the aldehyde’s characteristic peaks weakened with increasing the initial concentration of paraanisidine, meaning that the saturated adsorption capacity of ACMMs also increased. This is consistent with the trend of the isothermal adsorption curve as to be discussed later.


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Figure 6. (A) FT-IR spectra of pure ACMMs and those after adsorbing paraanisidine with varied initial concentration (C0, 100, 500 and 1000 mg/L); the spectra were measured in KBr tablet. (B) X-ray photoelectron spectra (XPS) for ACMMs before and after adsorption; the C1s X-ray photoelectron spectra for ACMMs before (C) and after adsorption (D). The ACMMs before and after the adsorption process (initial concentration of paraanisidine, 1000 mg/L) were further analyzed by XPS. From the XPS in Figure 6(B), after adsorption, the peaks of C1s and N1s increased while the O1s peak weakened. This phenomenon further indicates the formation of Schiff base, as illustratively shown in Figure 5. Figure 6(C,D) presents the C1s element’s high resolution scan of the ACMMs before and after adsorption. The green lines mean the deconvoluted peaks (the related results summarized in Table S1). For C1s scans, C1s binding energy can be classified as follows: 284.6 eV (–C–C–, –C–H), 286.0 eV (–C– O–), 286.6 eV (–C=N–), 287.1 eV (–(C=O)H), and 288.8 eV (–(C=O)–O). The obvious shift in


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peak from 287.1 to 286.6 eV can be attributed to the aldehyde groups consumed and Schiff base bonds generated. According to the investigations and discussion above, together with adsorption isotherms and adsorption kinetics investigations (as to be discussed next), adsorption convincingly occurred on ACMMs through forming Schiff base bonds.

Figure 7. (A) Adsorption isotherm of paraanisidine by ACMMs and (B) fitting result using Langmuir model. (C) Time~adsorption quantity profiles of paraanisidine by the ACMMs and (D) fitting results using pseudo-first-order model and pseudo-second-order model. Adsorption isotherms and adsorption kinetics were further investigated and the results are illustrated in Figure 7 and Table S2. For adsorption isotherms, the relationships between the amount of paraanisidine adsorbed on the ACMMs (qeq, mg/g) vs. paraanisidine concentration at


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equilibrium in the solution (Ceq, mg/L) are illustrated in Figure 7(A). The trend of adsorption capacity increased rapidly at lower concentrations and then increased slowly at higher concentrations. We excitedly found that under the specific conditions, the maximum amount of paraanisidine adsorbed by the microspheres can be up to 433 mg/g. In order to evaluate the adsorption characteristics, Langmuir adsorption isotherm model was taken to fit in the experimental data. Figure 7(B) illustrates the Langmuir plot of Ceq/qeq vs. Ceq, enabling the calculation of Langmuir constants from the intercept and slope of the linear plot. The calculated parameters were listed in Table S2. By fitting experimental data to Langmuir isotherm model, we found that the qm of the ACMMs in theory can be 559 mg/g. According to literatures,41,42 the Langmuir isotherm model assumes the adsorption process as a monolayer adsorption on completely homogeneous surfaces where all sites have identical affinity to the adsorbate. As can be seen from the correlation coefficient value (RL2= 0.9968) (Figure 7B and Table S2), the experimental data exhibited an excellent matching for fitting to the Langmuir model, demonstrating that the adsorption to be a monolayer chemical adsorption. For conducting adsorption kinetic studies, the amount of paraanisidine adsorbed was examined as a function of time and the results were shown in Figure 7(C). The adsorption of paraanisidine exhibited an initially fast rate and then reached equilibrium within 6 h. Two kinetic models (the pseudo-first-order model and the pseudo-second-order model) were considered for examining the mechanism of adsorption process. The linear plots are illustrated in Figure 7(D), and the calculated parameters are listed in Table S2. The results show that the obtained regression coefficient (R22) value from the pseudo-second-order kinetic model is above 0.998. Hence, the adsorption kinetics could be approximated more favorably by pseudo-second-order kinetic model. This phenomenon further reflects that the adsorption is a kind of chemical adsorption.


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Table 1. Efficiency of desorption upon changing solvent ratio. Solvent ratio[a]

qd,m [b]

Eweight [c]

qe,second[d]

Ere-adsorption[e]

(THF:H2O) (V:V)

(mg/g)

(%)

(mg/g)

(%)

10:0

200

61

183

56

9:1

300

92

294

90

8:2

320

99

321

98

7:3

290

89

283

87

a

concentration of HCl, 0.01M. bmeasured by weighing method and equation (3). c measured by equation (4). dmeasured by equation (1). emeasured by readsorption method and equation (5).

Table 2. Efficiency of desorption upon changing concentration of HCl. Concentration

qd,m [b]

Eweight [c]

qe,second[d]

Ere-adsorption[e]

of HCl [a]

(mg/g)

(%)

(mg/g)

(%)

0

160

49

151

46

0.01

320

99

321

98

0.1

340

104

326

100

(H+) (mol/L)

a

solvent ratio, THF:H2O=8:2 (V:V). bmeasured by weighing method and equation (3). cmeasured by equation (4). dmeasured by equation (1). emeasured by readsorption method and equation (5).

According to the reaction mechanism presented in Figure 5, the reversible bonds of Schiff base facilitated the ACMMs to be recycled. Hence, the recycling capacity of ACMMs was subsequently investigated. Since the Schiff base bond can be destroyed by hydrolyzing under acid aqueous condition, we employed an acidic mixture solvent composed of polar solvent


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(THF), H2O and HCl, to break the Schiff base bond. Desorption was carried out separately under varied THF/H2O ratio and varied concentration of HCl. Both the results from weighing method and re-adsorption method are summarized in Table 1 and Table 2. The histograms for readsorption are presented in Figure S3. As shown in Table 1, four desorbing liquids (solvent ratio of THF:H2O, 10:0, 9:1, 8:2 and 7:3, in V:V, the same below; with the same HCl concentration, 0.01 mol/L) were used for desorption. Some interesting results were found, as to be reported below. (1) Without the help of water, even the completely swelling microspheres (THF:H2O=10:0) cannot be regenerated totally, with Eweight being 61% and Ere-adsorption being 56%. Both the two desorption efficiency increased with increasing the proportion of water in the mixed solvent. The two phenomena demonstrated that H2O promoted the reverse reaction of Schiff base. (2) The two desorption efficiency values (Eweight and Ere-adsorption) reached maximum, with qe,second being 321 mg/g, nearly equal to the firstadsorption (qe,first, 327 mg/g) when the proportion of H2O reached 8:2. Then the two desorption efficiency values decreased with further increasing H2O proportion. This phenomenon can be explained as follows: too much water blocked the swelling of microspheres such that the accessible Schiff base bonds decreased accordingly. For the influence of HCl concentration, three concentrations (0, 0.01, and 0.1 mol/L) were investigated, using the same solvent ratio (THF: H2O = 8:2, V/V) as desorbing liquid. As shown in Table 2, HCl can significantly improve the regeneration ability with Eweight and Ere-adsorption increasing from 49% and 46% both to almost 100%. A comparison between the two HCl concentrations, i.e. 0.1 and 0.01 mol/L, shows little difference in qe,second. However, a further higher acid concentration would make microspheres partly dissolved. Based on the investigations


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above, we choose the solvent ratio of THF/H2O being 8:2 (V/V) and HCl concentration being 0.01 mol/L as the best desorption liquid conditions for the subsequent investigations.

Figure 8. The adsorption capacity histograms of ACMMs in recycling adsorbing paraanisidine. Finally, with the help of magneticity, the ACMMs underwent three rounds of adsorption and desorption to investigate the recycling ability. As shown in Figure 8, no obvious decrease observed in the three rounds of adsorption. Moreover, FT-IR spectra were recorded to further prove the recycling process. The pure ACMMs and the ACMMs after 1st adsorption, 1st desorption, and 2nd adsorption, were investigated. The recorded spectra are presented in Figure

S4. Similar to the spectra shown in Figure 6(A), after desorption, the characteristic peak of Schiff base bond appeared at 1630 cm-1; it disappeared completely after desorption, with aldehyde peaks at 1700 [−(C=O)H] and 2750 cm-1 [(−(C=O)H)] intensified noticeably. Moreover, the almost overlapped spectra between the pure ACMMs vs. the ACMMs after 1st desorption and the ACMMs after 1st adsorption vs. those after 2nd adsorption further confirmed that the aldehyde groups in ACMMs have an excellent regeneration capacity. We hope to point out that, a further optimization of the ACMMs in structure and morphology may enable all the aldehyde groups to


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be accessible for forming Schiff base, for example, by preparing porous and core/shell structured microspheres. The as-prepared microspheres will be more favorable for practical applications. The studies aong the directions are ongoing currently in our lab.

4. Conclusions A novel class of sustainable, aldehyde-containing magnetic microspheres (ACMMs) was prepared via suspension polymerization using vanillin methacrylate (VMA) as monomer and MethA-Fe3O4 NPs as magnetic supplier. As a perfect combination of magneticity and aldehydecontaining functional materials, the obtained multi-functional ACMMs showed rapid magnetic responsivity; the unique molecular structure of VMA provided abundant, reactive aldehyde groups on the microspheres. The reaction capacity of the aldehyde groups was investigated by using paraanisidine as a representative of amino derivatives. The adsorption mechanism was demonstrated to be a chemical adsorption process, i.e. Schiff base formed between aldehyde on ACMMs and the amino groups in adsorbate. Remarkably, adsorption tests demonstrated the excellent adsorption ability of the ACMMs towards paraanisidine (433 mg/g as the experimental maximum; 559 mg/g as the theoretical maximum). Furthermore, with the help of magnetism, the ACMMs showed efficient regeneration capacity, which is related to the reversible Schiff base bonds. Hence, we highlight that such novel multi-functional microspheres have tremendous potentials to be used as amino scavenger resin in post-processing chemical reactions and as green adsorbents for amines. The preparation of this type of microspheres also provides a promising platform for preparing diverse functional polymer materials by forming the reversible Schiff bases, e.g. as biomaterials for immobilizing enzymes and as novel carriers for controlling release of drugs.


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ASSOCIATED CONTENT Supporting Information. The XRD patterns; Surface pore structures of ACMMs; The readsorption capacity histograms; FT-IR spectra of ACMMs after adsorption and desorption; Detail information for deconvoluted peaks of XPS; Parameters of paraanisidine adsorption. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Tel: +86-10-6443-5128. Fax: +86-10-6443-5128. E-mail: [email protected] (Deng); E-mail: [email protected] (Wu)

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21474007, 21274008) and the Funds for Creative Research Groups of China (51521062).

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For Table of Contents Use Only

Biobased Magnetic Microspheres Containing Aldehyde Groups: Constructed by Vanillin-derived Polymethacrylate/Fe3O4 and Recycled in Adsorbing Amine Huanyu Zhang, Jianping Deng,* and Youping Wu*

Synopsis: Biobased aldehyde-containing magnetic functional microspheres serve as green, highly efficient and reusable adsorbents for amines.


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Biobased Magnetic Microspheres Containing Aldehyde Groups

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