Self-Assembly of Mn(II)-Amidoximated PAN Polymeric Beads Complex

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Energy, Environmental, and Catalysis Applications

Self-Assembly of Mn(II)-Amidoximated PAN Polymeric Beads Complex as Reusable Catalysts for Efficient and Stable Heterogeneous Electro-Fenton Oxidation Xiong Li, Lei Qin, Yufan Zhang, Zehai Xu, Lin Tian, Xinwen Guo, and Guoliang Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18704 • Publication Date (Web): 08 Jan 2019 Downloaded from http://pubs.acs.org on January 9, 2019

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ACS Applied Materials & Interfaces

Self-Assembly of Mn(II)-Amidoximated PAN Polymeric Beads Complex as Reusable Catalysts for Efficient and Stable Heterogeneous Electro-Fenton Oxidation Xiong Li,a § Lei Qin,a § Yufan Zhang,b § Zehai Xu,a Lin Tian,a Xinwen Guo,c Guoliang Zhang*a

a

Institute of Oceanic and Environmental Chemical Engineering, State Key Lab

Breeding Base of Green Chemical Synthesis Technology, Zhejiang University of Technology, Hangzhou 310014, China b

Department of Mechanical Engineering, College of Engineering, Carnegie Mellon

University, Pittsburgh, PA 15213, USA c

State Key Laboratory of Fine Chemicals, Department of Catalysis Chemistry and

Engineering, Dalian University of Technology, Dalian 116012, China

KEYWORDS: heterogeneous electro-Fenton; porous polymeric frameworks; phase inversion; post-synthetic grafting; strong metal-support interaction

ABSTRACT: A facile post-synthetic amidoxime modification method was reported on the preparation of transition metallic ions (Mn, Fe and Co)-PAN polymeric beads complex as reusable catalysts for efficient and stable heterogeneous electro-Fenton oxidation. Through one-step phase inversion, low-cost and chemically resistant 1

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polymeric PAN beads were fabricated on a large scale with controllable sizes and abundant porous structure. The post-functionalization strategy led more active sites to be uniformly distributed into modified PAN beads owing to the favorable channel confined effect and chelate coordination. Compared with pure PAN beads, the modified composite catalysts exhibited remarkably higher activity and stability in electro-Fenton oxidation over wide pH range of 3-10 without any addition of H2O2. By analysis, the grafted amidoxime group was extremely beneficial for improving metal loading and binding force between active sites and organic supports, which accelerated the active sites autocatalytic cycle to promote H2O2 activation by means of excited electron transfer from composites’ functional groups. The catalytic activity of Mn-AOPAN evaluated by the turn over frequency was 15 times more than traditional iron oxide, and very competitive compared with reported MOFs based composites. Moreover, strong metal and polymeric support interaction significantly enhanced stabilization of active sites dispersed in the porous matrix and solved the ever-present problem of metallic ions leaching at the greatest extent. The scalable introduction of functionalities into sophisticated structures after host framework synthesis will bring valuable insights to develop high efficient and stable heterogeneous catalysts for green electrochemical oxidation in practical application. 1. INTRODUCTION With the rapid development of world economy and industrialization, large numbers of toxic and refractory organic contaminants produced in the fields of textile, pesticide, pharmaceutical and petrochemical industries have been discharged, which have posed 2


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more significant hazards to ecological environment and human health.1-3 To deal with these emergent issues, conventional treatment including adsorption, filtration, coagulation and biodegradation has been attempted but is still difficult to meet the required discharge standards.4 Searching for new and effective methods in recalcitrant organic pollutants degradation has become of global concern.5 With the development of electrochemistry and catalysis theory, electrochemical advanced oxidation processes (EAOPs) such as electro-Fenton, photoelectron-Fenton and anodic oxidation are attracting considerable attention as the effective alternatives for non-degradable wastewater treatment.6 Since hydrogen peroxide can be generated in situ at the cathode and sludge production can be reduced in catalytic process, electro-Fenton technology is regarded as an economic, powerful and environmentally friendly process, which has been a topic of much interest.7-11 However, homogeneous electro-Fenton processes using metallic ions as catalysts often face some critical difficulties including narrow operating range, low recoverability and requirement of complicated subsequent treatment.12,13 Comparatively, immobilization of the dissolved metallic ions on the porous supports can be propelled as a prospective alternative.14,15 In this regard, fabrication of functional supports for anchoring highly-active sites is of great importance to promote heterogeneous electro-Fenton catalysts into the practical application. Immobilizing of metal or metal oxides on inorganic materials (such as silica, clays, zeolite and carbon materials) and organic materials (such as fibers, membranes, resins and gel materials) have been widely investigated as heterogeneous catalysts for 3



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Fenton oxidation over the past decades.16-19 The cost, stability and recycling of supports are the key points which should be taken into account for catalyst preparation. Due to the large surface area, uniform and adjustable pore diameter and well-defined channel, considerable attentions have been focused on developing the porous inorganic materials as the carriers of active sites.20,21 In comparison with the inorganic supports, organic supports take advantages of lower price, ease of processing, extraordinary variety and abundant surface organic groups. Therefore, immobilizing highly-active metallic sites into porous organic matrix for various catalytic reactions is attracting more and more interests in recent years. Although a variety of heterogeneous Fenton catalysts have been developed through different methods, there are still some bottlenecks existed in the design of catalytic material and Fenton chemistry. For example, the weak channel confined effect and chemical interaction of supports often causes the aggregation of introduced metallic sites on external surface and leaking of active species during the catalytic reaction, which may severely reduce the catalytic activity of supported catalysts. Obviously, the possible detrimental interaction between the catalytic sites and supports as a key issue will inevitably hinder the wide application of supported catalysts.22 Hence, how to facilitate the actives sites uniformly dispersed within suitable supports and improve metallic sites-support interaction still remains a big challenge. Compared with conventional granulated solids, heterogeneous catalysts on organic matrix by the construction of multi-dimensional structure and/or the surface functionalization may provide a promising option for improving the dispersion of 4


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active sites into host matrix and enhancing the binding force between them. Polymeric fibers with the convenience of textile technology and Nafion membranes in virtue of energy fields have been investigated as useful organic matrices for heterogeneous Fenton photocatalyst in the effective decomposition of dyes and refractory organic contaminants, although they seem to be too expensive to be applied as catalyst supports in an industrial scale.18,23-25 Similarly, commercial ion-exchange resins such as Purolite and Amberlite have also been selected as the supports of FeIII catalysts, which showed high activity on oxidizing high concentrations of organic compounds like dye aqueous solution.19,25 Spherical structures usually hold favorable complexity by manipulating their interior architecture, geometric morphology and chemical composition to boost their catalytic performance and have superiority in recyclability.17,26 Beside resin supports, due to their well-known biodegradability, biocompatibility, surface morphology, convenience of processing and wide abundance,27 the application of alginate gel beads as biopolymer matrix for Fe catalysts took big advantages in heterogeneous electro-Fenton process.28,29 The Fe alginate gel beads displayed high catalytic activities for the degradation of organic compounds like pesticides, and the sort of three-dimensional spherical structure facilitated the reutilization of iron very well. In view of the insufficient adhesion of Fe ions to alginate polymer, the alginate gel beads were further attempted to be modified by a complexing agent cyclohexane dinitrilo tetraacetic acid (CDTA) for heterogeneous oxidative degradation of antibiotics.30 With CDTA used as the iron binder on the surface of alginate gel beads, the synthesized catalysts were used for 5



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three successive runs with little alteration of activity toward the degradation of drugs, and the leached quantity reached as low as 0.8 % in the liquid medium. Considering the complexity in polymerization and post-production treatment due to the variant composition and sequential structure of alginate, which are widely varied depending on the species and tissues it is isolated from,27 we envisage that, if the three-dimensional spherical structure can be realized by another more commonly industrialized polymer and the polymeric supports can be introduced by suitable functionality through effective chemical modification, sufficient functionalized groups may be provided for anchoring the active sites, and therefore the efficiency and stability of heterogeneous electro-Fenton catalysts can be greatly improved. Based on these observations, herein, we have developed a facile post-synthetic amidoxime modification method to form active sites (such as Mn, Fe and Co)-PAN polymeric beads complex as reusable catalysts for efficient and stable heterogeneous electro-Fenton oxidation (Scheme 1). In comparison with in-situ surface modification, the strategy improves the controllability of modified location, promotes functional groups to uniformly disperse on the host matrix and strengthens their chemical stability. We selected the synthetic polyacrylonitrile (PAN) as very common and lower-cost raw material for the multi-dimensional polymeric matrix, which has been widely used in the preparation of diversified functional materials as result of good resistance to acids, bases and oxidizing agents.23 The polymeric PAN beads with controllable sizes and abundant porous structure were then successfully fabricated on a large scale via one-step phase inversion method. After treated with hydroxylamine 6


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hydrochloride, nitrile groups of PAN were converted to amidoxime groups, which were homogeneously distributed near opening of pore channels and/or on external surface of polymer wall. Compared with pure PAN beads, large amounts of surface functional groups on modified PAN beads led more active sites to be uniformly dispersed into channels of porous frameworks and significantly improved metal-support interaction. Immobilization of metallic ions inside modified PAN beads could not only take advantage of the channel confined effect, but also utilize the chelate coordination of amidoxime groups. Meanwhile, the prepared supported catalysts were easy to be separated for reutilization due to their large-sized spherical structure. To the best of our knowledge, self-assembly of heterogeneous organic beads catalysts produced by common and lower-cost industrialized polymer for Fenton oxidation were explored for the first time. The present work intended to investigate the possibility of promoting heterogeneous electro-Fenton technology by using transition metallic ions (Mn, Fe and Co)-PAN polymeric beads complex catalysts. As Mn3+ with higher standard reduction potential tends to accept electrons faster than Fe3+ and thus Mn2+ is more easily regenerated for catalytic reaction, the behavior of Mn based complex catalysts was paid more attention in electro-Fenton oxidation process.31 The catalytic mechanism of amidoximated PAN beads (AOPAN) was probed to understand the synergetic effect of metallic site and functional groups of support on enhancement of catalytic performance. The influence of porous structure and properties of spherical structured support on the uniform distribution of active 7



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sites and metal-support interaction was measured and evaluated. To utilize heterogeneous electro-Fenton oxidation, the optimum working conditions, stability and reusability of catalysts were examined. It is hoped that the strategy described here may provide a new opportunity for developing highly efficient and stable heterogeneous catalysts to promote green electrochemical oxidation in large scale application. 2. EXPERIMENTAL SETUP 2.1. Materials. The commercial dye Acid Red 73 was supplied by Shanghai Reagent Co., Ltd, China. Polyacrylonitrile (PAN, average molecular weight of 150 kDa) was purchased from Huitong Plastic Co., Ltd, Suzhou, China. All chemicals, such as hydroxylamine hydrochloride (NH2OH·HCl), Polyvinylpyrrolidone K30 (PVP K30), CoSO4·7H2O, MnSO4·H2O, NaOH, H2SO4, Na2SO4, Na2CO3 and FeSO4·7H2O were purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. Graphite sheets were provided by Jing Long Co., Beijing, China. Microporous membranes were purchased from Xin Ya Co. Ltd, Shanghai, China. All the reagents above were of analytical grade and used as received without further purification. All the solutions used in the experiment were prepared with deionized water made by a self-made RO-EDI system, whose ion concentration was monitored by IRIS Intrepid ICP and Metrohm 861 Compact IC to reach σ≤0.5 μS/cm. 2.2. Synthesis of porous PAN beads. PAN polymeric beads were synthesized by a simple one-step phase inversion method and the related equipment can be operated 8


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continuously at lab scale or on a large scale. Typically, a certain amount of PAN homopolymer (10 wt %) were added into DMF solvent under continuous stirring to form a uniform polymeric solution. Subsequently, the resultant homogeneous solution was slowly dripped into an ethanol/water mixture (1:4, v/v) through itself natural gravity under constant stirring. The size of PAN beads could be precisely controlled by altering the infusion speed and volume of polymer droplet. The obtained PAN beads were washed by using deionized water to remove residual ethanol and soaked into deionized water at room temperature. 2.3. Amidoxime modification of PAN beads. The as-synthesized PAN beads were added to 0.5 M NH2OH·HCl solution, and the pH of which was adjusted to 6.0 by sodium carbonate. Next, the mixture was heated at 343 K in a water bath. Then the beads were washed by using deionized water to wipe off residual reactants. Through the treatment of NH2OH·HCl, the C≡N groups on the polyacrylonitrile were transformed into C(NOH)NH2 groups with strong chelating ability. 2.4. Synthesis of transition metal (Fe, Co and Mn)-ammoximated PAN polymeric beads catalyst. Active sites were immobilized into modified PAN beads by liquid-impregnating method. As-prepared ammoximation PAN beads (AOPAN) were immersed into metallic precursor under constant stirring at room temperature. The products were washed by using deionized water to remove residual precursor on the surface of AOPAN. The obtained amidoximation PAN beads catalysts were termed as Fe-AOPAN, Co-AOPAN and Mn-AOPAN. 2.5. Characterization. Scanning electron microscopy (SEM, SU8010, Hitachi) 9



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was used to observe the surface morphology of prepared samples. The elemental spectrum was obtained by energy-dispersive X-ray spectroscopy (EDX) to acquire the elemental information. Fourier transform infrared spectra (FT-IR) of samples were performed by Nicole 6700 spectrometer (Thermo, USA). X-ray diffraction (XRD) patterns of samples were determined by X-ray diffractometer (X'Pert Pro). X-ray photoelectron spectroscopy (XPS) experiments were conducted by RBD upgraded PHI-5000C ESCA system (Perkin Elmer) with Mg Kα radiation (hv=1253.6eV). TriStar II 3020 (USA) specific surface and porosity analyzer was used to determine nitrogen adsorption-desorption isotherms and specific surface areas under condition of 77 K. The concentration of metallic species was measured by ICP-AES in an Optima 2000 instrument (PerkinElmer, USA) after digestion in acid solution. 2.6. Electro-Fenton degradation experiments. All experiments were carried out in an open, undivided and cylindrical glass cell of 500 mL capacity at room temperature. In the bottom of reactor, there was an aeration device which provided continuous air flow for the reactor near the cathode and a stirring device in the center of the bottom position to avoid concentration distribution (Figure S1). In a typical run, a certain amount of catalysts (3 g·L-1) and 0.35 L of 100 mg·L-1 acid red 73 aqueous solution were added to the reactor. The initial pH value of the dye solution was adjusted by using NaOH and H2SO4 solution. A steady flow of air (600 mL·min-1) was injected to the reactor by an air pump for 10 min to reach a stable distribution of O2 (around 8.0 mg·L-1) in the aqueous solution which was stirred continuously at room temperature (298K) before starting electric Fenton reaction. The voltage drop 10


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was supplied by a DC power (YG-1502JJ). The anode plate and cathode plate which were made of graphite sheets with an effective surface area of 20 cm2 were connected to the DC power though copper wire. The gap of two opposed electrode was 3 cm, and the concentration of Na2SO4 electrolyte which enhanced solution conductivity was 0.02 M. In all experiments, 2 mL samples were taken every 20 min by a syringe from the reactor and filtered through the microporous membrane with membrane pore size of 0.45 μm. The treated samples were preserved by sealed tubes in the dark and stored at 277 K until the analysis. The content of dyes taken from the reactor was detected by U-2910 digital spectrophotometer. Total organic carbon (TOC) of the solution was measured by N/C 3100 TOC analyzer (Analytik Jena AG, Germany). The gas chromatography (GC) coupled with tandem mass spectrometry (MS, Thermo ISQ) was used to analyze the degradation products. The turnover frequency (TOF) of catalyst Fe-AOPAN, Co-AOPAN and Mn-AOPAN were calculated from the electro-Fenton degradation per hour on one active site. 3. RESULTS AND DISCUSSION 3.1. Catalysts characterization. The structure and chemical composition of the developed Mn-based PAN beads were characterized by different technologies. Figure 1 shows the SEM and EDS mapping images of PAN and AOPAN loaded with Mn (II) ions. Compared with pure PAN beads (Figure S2 ESI†), the prepared Mn-based PAN bead exhibited a good spherical-like morphology with relatively smooth surface. Note that this one-step phase inversion strategy is much more efficient for controllable 11



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preparation of PAN beads in a wide range of particles diameter from 1.5 to 3.1 mm (Figure S3 ESI†). In a sharp contrast, the self-prepared beads with narrow diameter distribution in range of 2-2.5 mm were selected as the supports (Figure S4 ESI†). Cross-sectional SEM images (Figure 1a, b, e and f) show that the interior cavity of self-assembly polymeric beads possessed a loose porous framework structure. As illustrated from the surface magnification diagram, it was found that large numbers of micropores were uniformly distributed on the external surface of PAN beads (Figure 1c). After treated with hydroxylamine hydrochloride, the surface pore size of AOPAN beads increased while the porosity decreased dramatically (Figure 1g). Meanwhile, EDS spectrum (Figure S5) and mapping images give the obvious evidences that most of Mn (II) active sites were uniformly dispersed in the channel surface (2.08 %, Figure 1p) of Mn-AOPAN internal framework, not aggregated on the surface (0.93 %, Figure 1h). Similarly, Fe and Co active sites were also uniformly dispersed on the surface and internal framework of Fe-AOPAN and Co-AOPAN (Figure S6 and Figure S7 ESI†). Compared with Mn-AOPAN beads, it is clearly displayed that much less Mn (II) ions were anchored on the external surface (0.29 %, Figure 1d) and internal cavities (0.11 %, Figure 1l) of PAN beads matrix. The findings suggested that the immobilization of Mn (II) ions into support could be not only through the adsorption of pore channels, but also more dependent on the surface functional groups of support. As a result, PAN beads post-functionalized by hydroxylamine hydrochloride can strongly chelate the Mn (II) ions and promote their uniform dispersion inside porous support. As shown in Table S1, ICP results clearly 12


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demonstrate that the loading ratio of Mn on AOPAN increased to 4 times higher than that of Mn on the unmodified PAN bead. According to N2 adsorption/desorption isotherms (Figure S8 ESI†), it is confirmed that the occurrence of porous structure was shown in the self-assembly PAN beads, in agreement with SEM images. As shown in Table S2, the BET surface area and specific mesopore volume of PAN beads are 78.19 m2g-1 and 0.12 cm3g-1, respectively.

The wide distribution of pore size for Mn-AOPAN indicated that the

synthesized polymeric beads simultaneously exhibited micropore and mesopores inside host matrix (Figure S9 ESI†). After amidoxime modification, the BET surface area and specific mesopore volume of AOPAN beads are 20.11 m2g-1 and 0.05 cm3g-1, respectively. The average pore diameter of polymeric beads increased from 6.25 nm to 9.24 nm for AOPAN. In comparison with AOPAN, the Mn loading of Mn-AOPAN was accompanied by the slight increase in BET, pore size and mesopore volume of sample. FTIR spectra of PAN, AOPAN, Mn-PAN and Mn-AOPAN are shown in Figure 2a. As apparent from the figure, the characteristic peaks at 1360, 1450 and 2940 cm-1 are attribute to the bend vibration of methine C-H, bend vibration of methylene C-H and stretching vibration of methylene/methine C-H.32 According to previous studies, the absorption peaks at 2244 and 1730 cm-1 are corresponded to C≡N stretching vibration and carbonyl groups of PAN beads.33,34 In comparison with pure PAN bead, surface amidoximation modified with hydroxylamine hydrochloride led the intensity of characteristic absorption peak of C≡N at 2240 cm-1 to be weakened. As noted, the 13



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new absorption peaks appear at 922 and 1650 cm-1 for sample AOPAN, which should be attributed to the N-O and C=N bond vibration.35,36 Though the characteristic peak within the scope of 1600-1700 cm-1 appears in four samples, the peak intensity of AOPAN and Mn-AOPAN was much higher than that of PAN and Mn-PAN, which was mainly due to the new and strong adsorption peak of C=N stretching at 1650 cm-1 overlaping with the C=C stretching (1660 cm-1) and –OH bending vibration peak.37,38 Moreover, the absorption peaks for AOPAN between 3500 and 3000 cm-1 are observed to be broader and stronger than that for PAN, which was assigned to N-H and O-H stretching vibration. As reported by the previous studies, the characteristic peak at 3200 cm-1 is attributed to the N-H stretching vibration39,40 In comparison with sample PAN and Mn-PAN, it is clear that the stronger peak around 3200 cm-1 appears in the spectra of both AOPAN and Mn-AOPAN, which well indicates that the vibration of -NH2 occurred on the amidoxime modified PAN. When Mn (II) ions were immobilized on PAN beads, no obvious variation on the position and intensity of the characteristic peaks were observed compared with pure PAN bead, indicating no chemical bond formation between Mn (II) ions and PAN. The cross-linking reaction between Mn (II) sites and amidoxime group of AOPAN is evidenced by the FTIR data. After Mn loading, the adsorption peak in sample AOPAN at 922 cm-1 is shifted to the higher wavenumber (928 cm-1). Moreover, the appearance of a new peak at 1100 cm−1 for sample Mn-AOPAN is attributed to the vibration of O-Mn-O species. A similar phenomenon can be observed in FTIR spectra of Fe-AOPAN and

14


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Co-AOPAN (Figure S10 ESI†). The results confirm that the adhesion between metal ions and functional groups of support was enhanced by coordination.37 Crystalline structures of the prepared different samples are shown in Figure 2b and Figure S11. In the XRD diffraction patterns, the self-assembly PAN beads exhibits a sharp and broad characteristic peak at 2θ=17 and 26.6°, which corresponds to the (100) and (110) crystallographic planes (JCPDS No. 48-2119).41,42 Compared with original PAN beads, the shift of (100) crystal face was found with higher diffraction angles and obvious reduction in the intensity of characteristic peaks were observed for AOPAN beads, indicating that the surface amidoxime modification had significant effect on the crystal structure of polymeric beads. After being chelated with Mn (II) ion, the intensity of the diffraction peaks was lowered for both sample Mn-PAN and Mn-AOPAN. However, it is worth to note that the more weakening of the reflection peak at 2θ=17° was observed for sample (Mn, Fe, Co)-AOPAN, which demonstrated the stronger interaction between functional groups of AOPAN and metallic ions compared to the unmodified PAN. XPS characterizations were carried out to further analyze the chemical states of mental active site on the surface of the as-prepared catalysts. The full XPS spectra of different samples are displayed in Figure 3a and Figure S12a. It is clear that the elements of C 1s, N 1s, O 1s, Mn 2p, Fe 2p and Co 2p are detected on the surface of the samples. As illustrated from the figure, the binding energy of Mn 2p for Mn-AOPAN is 641.3 and 653.0 eV, which is contributed to Mn 2p3/2 and 2p1/2 of Mn-O species, respectively.43 Similarly, the binding energy of Fe 2p for Fe-AOPAN 15



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is 713.6 and 727.1 eV, which belongs to Fe 2p 3/2 and 2p1/2 of Fe-O species (Figure S12b). The Co 2p3/2 peaks deconvolute to the peaks at 781.3 and 779.6 eV, which might be related to the formation of Co-N and Co-O species (Figure S12c). However, the intensity of the characteristic peaks at Mn 2p for sample Mn-PAN is very weak (Figure 3b). This reveals that the introduction of amidoxime groups led more metallic ion to be dispersed on the surface of porous support. The N 1s spectra of Mn-PAN and Mn-AOPAN are observed at 398.6 and 399.0 eV, respectively (Figure 3c). Figure 3d represents that the N 1s spectrum of Mn-AOPAN is divided into four characteristic peaks at 400.4, 399.5, 398.9 and 398.2 eV, counterparting to the -NH2, -NOH, -C≡N and -NH-, respectively.44-46 Compared with Mn-PAN, the peak of N 1s for sample Mn-AOPAN is observed with a 0.4 eV shift to higher energy, which could be ascribe to the strong interaction between nitrogen species and Mn (II) sites by the electrostatic adsorption and coordinating interaction, in consistent with the results of FTIR. 3.2. Catalytic Activity. Electro-Fenton process plays a significant role in the degradation of toxic and recalcitrant organic contaminants. Figure 4a and 4b depict the decolorization and TOC removal efficiency of acid red 73 by using the different catalytic systems. The electrochemical oxidation was firstly performed to compare the activity of transition metal ions-mediated electro-Fenton process. The results showed that the low decolorization (86.4%) and TOC removal (49.5%) performance was observed in electro-Fenton system by using Mn-PAN as the catalyst. The poor catalytic activity of Mn-PAN catalyst was mainly ascribed to the low Mn loading and the weak interaction between the metallic sites and support. In order to enhance the 16


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electrocatalytic activity of metal-organic composites, we attempted to immobilize the MnII sites on the functional spherical PAN frameworks through liquid impregnation method. In the electro-Fenton process, sample Mn-AOPAN exhibited higher degradation efficiency of acid red 73 (95.77 % decolorization and 70.5% TOC removal) than pure Mn-PAN, which could be attributed to strong interaction between metallic ions and functional groups of supports. This well verified that the surface grafted amidoxime groups of PAN beads promoted the electron transfer of MnIII/MnII species and subsequently accelerated the ·OH radicals production.47 For the better understanding of the influence of active site species on performance of the composited catalysts, we also fabricated Fe-AOPAN and Co-AOPAN and investigated their electrocatalytic activity. Results showed that higher removal rate of acid red was observed for Mn-AOPAN catalytic system in comparison to electro-Fenton with other two metallic supported catalysts, which may due to the fact that Mn possess more valency state, thus benefiting to the decomposition of H2O2.48 In addition, as reported, the standard reduction potential of Mn3+ is higher, therefore, Mn3+ can accept electrons to convert into Mn2+ faster than Fe3+ and Co3+ (Eq. 1-3).49 Mn 3+ +e-  Mn 2 , E   1.50 V/NHE

(1)

Fe3+ +e-  Fe 2 , E   0.771 V/NHE

(2)

Co3+ +e-  Co 2 , E   0.17 V/NHE

(3)

UV-vis spectrum was applied to keep track of electro-Fenton degradation of acid red 73 by using Mn-AOPAN (Figure 4c). It was clearly found that three well-resolved absorption peaks of original dye wastewater were shown at 220, 338 and 510 nm. The 17



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main absorption peak at 510 nm in visible region was assigned to chromophore groups of dyes. As the catalytic reaction proceeded, the intensity of characteristic absorption peak at 510 nm decreased significantly, demonstrating that the chromophore groups of dyes were destructed by the generated hydroxyl radicals. With the operation time increasing, the intensity of all three absorption peaks significantly decreased until they disappeared. The result indicated that most of organic contaminants were decomposed into carbon dioxide and water during electro-Fenton oxidation.50 By GC-MS analysis, some micromolecular organics with the amino, carbonyl and carboxyl groups such as product A existed in the reaction solution after the electro-Fenton oxidation (Table S3 and Figure S13). The degradation of acid red 73 with different catalysts in electro-Fenton is represented by pseudo first-order kinetics (Eq. 4) and combined first-order kinetics model (Eq. 5). The equations are as follows: C  C0 exp(kt )

(4)

C  C1 exp(k1t )  C2 exp(k2t )

(5)

where C represents dye concentration at time t, C0 and k are dye concentration at initial time and reaction rate coefficient of first-order kinetics, respectively. C1, C2, k1 and k2 represent initial dye concentration and reaction rate coefficient of two independent first-order reactions, respectively. Figure 5a and b depict the comparison of pseudo first-order kinetics and combined first-order kinetics model for degradation of acid red 73 in various reaction systems. Obviously from the figure and Table S4, the combined first-order kinetic model

fits 18


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the results better than pseudo first-order kinetic model in heterogeneous electro-Fenton degradation due to the higher regression coefficient (>0.999). The apparent rate constants fitted by pseudo-first-order kinetics of AOPAN supported catalysts (>0.026 min-1) are significantly higher than those of electrochemistry (0.009 min-1) and unmodified PAN supported catalysts (0.018 min-1). Moreover, Mn-AOPAN (0.040 min-1) exhibits the highest rate constant among all the AOPAN supported catalysts. These results further confirm that the cooperative effect of Mn active sites and AOPAN support endowed Mn-AOPAN with excellent catalytic activity. In heterogeneous catalysis, the turn over frequency (TOF) value is proposed to evaluate catalytic efficiency of each active site. The formula of the turn over frequencies for all the three kinds of samples can be calculated as following (Eq. 6): TOF=

mole of Acid Red reacted mole of catalyst  reaction time

(6)

By calculation, the catalytic activity in view of TOF values varied in the order of Mn-AOPAN (1.49 h−1) > Co-AOPAN (0.18 h−1) >Fe-AOPAN (0.12 h−1). The Mn-AOPAN catalyst presented the highest TOF value, which was much higher (approximately 15 times) than traditional iron oxide and very competitive compared with the metal-organic frameworks (MOFs) based composites reported elswhere.51 This well demonstrated that the introduced Mn(II) active sites into porous amidoximated PAN beads favored oxidation ability of electro-Fenton reaction. 3.3. Effect of metal loading on electrochemical behavior. As is well known, the catalytic activity of electrocatalysts often closely related to the loading amount of 19



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active sites. Figure 5c represents the performance of Mn-AOPAN bead loaded with different Mn concentrations on the degradation of acid red 73. As apparent from the figure, the removal efficiency increased from 90.44 to 95.77 % when the immersion Mn concentration varied from 0.1 to 0.4 M. The reason of which might be that more active sites were provided for the activation of in-situ generated hydrogen peroxide at the cathode to produce large numbers of ·OH hydroxyl radicals. However, when the Mn loaded concentration further increased to 0.5 M, the lower electrochemical oxidation efficiency of acid red 73 (91.47 %) was observed, which was attributed to that excess Mn (II) ions can react with hydroxyl radicals to form low active species Mn3+ and OH- through the following equation (Eq. 7) and result in a decrease of the removal rate.52 These observations revealed that the Mn active sites bonding with the groups of porous PAN beads played more significant role in improving the catalytic activity.  M II +  OH  M III +OH -

(7)

3.4. Effects of solution pH on electrochemical oxidation. In general, the initial pH of reaction system has a vital influence on the performance of electro-Fenton oxidation. As reported, the narrow pH working range has been considered as a significant problem hindering the wide industrial application of homogeneous electro-Fenton oxidation.53 In the present study, immobilizing MnII site on self-assembly porous polymeric frameworks can effectively widen the pH working scope of electro-Fenton oxidation to strong alkaline condition. As can be seen from Figure 5d, the highest decoloration efficiency was observed at pH 3.0 owing to the 20


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larger amount of hydroxyl radicals generated, in consistent with the previous studies.54,55 With the pH value going up to neutral or alkaline condition, a gradual reduction in decoloration rate happened. It was attributed to the conversion of Mn (II) ion to hydrous manganese dioxide under the condition of high pH value, which could directly catalyze the decomposition of H2O2 to O2 and H2O.56 However, it was observed that the decoloration rate still remained more than 90.0 %, even if the electro-Fenton oxidation happened at high pH value of 10.0. This demonstrated that the surface functional groups of AOPAN beads were used to stabilize Mn (II) ions for relieving the generation of hydrous manganese dioxide, which can effectively overcome the problem of MnO2 precipitation at a neutral or strong alkaline pH. Only when the solution pH further increased to 11.0, the catalytic decoloration efficiency of acid red 73 by using Mn-AOPAN as catalyst decreased somewhat to 83.82 %. The finding of pH effect over the electro-Fenton degradation further exhibited that Mn (II) active sites and functional support possessed strong anti-alkaline properties and had an important influence on broadening the pH working range. 3.5. Catalyst stability. As a view of cost efficiency, the reusability of prepared catalyst is of great significance in the large scale application of electro-Fenton oxidation process. In order to test the stability of catalyst Mn-AOPAN, recycling experiments were conducted under the optimum working conditions. After each cycle, the Mn-AOPAN beads were recovered from the final effluent and cleaned with deionized water. As shown from Figure 6a, it was clearly found that sample Mn-AOPAN still remained high catalytic activity (more than 91.29 % decoloration) 21



Page 22 of 42

for all the runs. Compared with the first run, no obvious loss (only 4.48 %) of catalytic performance was observed for last two runs. The reduction of removal rate was mainly attributed to the adsorption of organic molecules on the surface of manganese active sites. Moreover, the leached manganese ions had negligible effects on the decolorization performance during the electro-Fenton oxidation (Figure 6b), since the leaching was kept less than 0.5 mgL-1 in each run. Especially in Run 3, the concentration of leached manganese was as low as 0.27 mgL-1, which was much lower than traditional catalysts supported with metallic ions.25 Beside sample Mn-AOPAN, the Fe-AOPAN and Co-AOPAN also exhibited the high stability since no significant decrease of the degradation rate was observed after three cycles and the concentrations of leached metallic ions were less than 0.6 mg/L (Figure S14 ESI†). As noted, all the metallic ions (Mn, Fe and Co)-PAN polymeric beads could maintain the spherical structures without damage after the continuous operations. Compared with pure PAN supported catalysts (leaching of Mn was higher than 1.2 mgL-1), it was obvious that the grafting of amidoxime groups on self-assembly PAN beads provided strong interaction between Mn (II) sites and polymeric host frameworks through forming strong covalent or coordinate bond (inset of Figure 6b), which enhanced stabilization of active sites highly dispersed in the porous matrix and solved the metallic ions leaching problem at the greatest extent. 3.6. Possible mechanism of metal-organic composite for electro-catalytic oxidation. In heterogeneous electro-Fenton process, the structure and properties of supported catalyst often plays a significant role in the production of ·OH from the 22


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decomposition of H2O2.29 As mentioned above, the results of catalytic experimental have clearly demonstrated that the strong metal-support interaction effectively improved the electro-Fenton oxidation performance. In the present study, the possible mechanism for activation of H2O2 on prepared Mn-AOPAN was proposed to clarify the effect of MnII and functional groups of supports on enhancing catalytic activity (as shown in Scheme 2). The possible reactions in the catalytic process are expressed as following (Eq. 8-11): O 2  2H   2e-  H 2 O 2

 Mn II /PAN+ H 2 O 2   Mn III / PAN  OH + OH  Mn III /PAN+ H 2 O 2   Mn II /PAN+ HO 2  + H +  Mn III / PAN  e-   Mn II / PAN

(8) (9) (10) (11)

Firstly, the dissolved oxygen molecules in the reaction system diffused to the surface of graphite cathode and adsorbed on it through electrostatic interaction. The reactive oxygen molecules then obtained two electrons and produce hydrogen peroxide in situ at the cathode (Eq. 8). After H2O2 molecules were generated around the graphite cathode, the surface ≡MnII/PAN active sites of catalyst Mn-AOPAN would quickly react with H2O2 to form ≡MnIII/PAN and ·OH (Eq. 9).57,58 Subsequently, the generated ≡MnIII/PAN sites further interacted with H2O2 to form HO2· and ≡MnII/PAN ions (Eq. 10). It is worth to note that the rapid conversion of ≡MnIII/PAN to ≡MnII/PAN was of great importance for the catalytic oxidation reaction proceeding. In heterogeneous electro-Fenton, an in-situ recycling of manganese species (MnIII/PAN→MnII/PAN) would appear on the surface of cathode 23



Page 24 of 42

through Eq 11. Compared with original PAN supports, it should be pointed out that the conversion efficiency of the MnII/MnIII cycle reaction occurring on the amidoxime functionalized PANs was much higher due to the variety of groups on the surface. According to previous studies, it is reasonable to deduce that the excitation of -NH2 groups for modified PAN beads followed by the generated electrons shifted to Mn center to accelerate ≡MnIII/PAN to produce the highly-active ≡MnII/PAN, which had a positive effect on reacting with H2O2 to produce ·OH and ≡MnIII on the surface.59,60 The strong metal-support interaction accelerated active species MnII/MnIII autocatalytic cycle during the activation of H2O2 decomposition. As a result, the unique properties and structure of amidoximated PAN porous beads made the ≡MnII/PAN sites highly dispersed into host matrix and synthetic effect of Mn sites and functional frameworks played vital roles in enhancement of electrocatalytic splitting. 4. CONCLUSIONS In conclusion, we have developed a facile and effective approach via post-synthetic amidoxime modification to self-assembly of transition metallic ions (Mn, Fe and Co)-PAN polymeric beads complex as reusable catalysts on a large scale for efficient and stable heterogeneous electro-Fenton oxidation. Compared with traditional in-situ surface modification, the strategy improves the controllability of modified location, promotes functional groups to uniformly disperse on the host matrix and strengthen their chemical stability significantly. Immobilization of metallic ions inside modified 24


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PAN beads can not only take advantage of the favorable channel confined effect, but also utilize the chelate coordination of amidoxime groups. The electro-Fenton experiments showed that transition metal-AOPAN catalytic system, especially Mn-AOPAN, held excellent removal rate of organic compounds in a wide pH range of 3-10 under low current density. The catalytic activity in view of the TOF values varied in the order of Mn-AOPAN > Co-AOPAN > Fe-AOPAN. The Mn-AOPAN catalyst presented the highest TOF value, which was 15 times more than traditional iron oxide and very competitive compared with reported MOFs based composites. The strong metal-support interaction through grafted amidoxime groups had positive effects on improving the stabilization of easily leaked metallic ions into porous polymeric frameworks and accelerating the active sites MnII/MnIII autocatalytic cycle, which offered a valuable insight for understanding the relationship between functional groups of structured support and electrochemical characteristics of catalysts. ASSOCIATED CONTENT Supporting Information Schematic diagram of electro-Fenton system, SEM images of pure PAN , Fe-AOPAN and Co-AOPAN beads, photographs of self-assembly PAN beads with different sizes and metal-amidoximated PAN polymeric beads complexes, surface area and textural data, kinetic parameters for electrochemistry and electro-Fenton oxidation, EDS elemental analysis, adsorption/desorption isotherms, BJH pore size distribution, FTIR, XRD, XPS spectra of Fe-AOPAN and Co-AOPAN, mass spectra of intermediate products, and reusability and metal leaching of Fe-AOPAN and Co-AOAPAN. This 25



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material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Address: Institute of Oceanic and Environmental Chemical Engineering, State Key Lab Breeding Base of Green Chemical Synthesis Technology, Zhejiang University of Technology, Hangzhou 310014, China. Email: [email protected]. Phone/Fax: +86-571-8832 0863 § These

authors contributed equally.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS We thank for financial support the National Natural Science Foundation of China (Grant No. 21236008, 21476206, 21506193 and 21736009), the Zhejiang Provincial Natural Science Foundation of China (Grant No. LY18B060010) and the Minjiang Scholarship from Fujian Provincial Government. REFERENCES (1) Rodrigo, M. A.; Oturan, N.; Oturan, M. A. Electrochemically Assisted Remediation of Pesticides in Soils and Water: A review. Chem. Rev. 2014, 114, 8720-8745. (2) Cheng, N.; Tian, J.; Liu, Q.; Ge, C.; Qusti, A. H.; Asiri, A. M.; Al-Youbi, A. O.; Sun, X. Au-Nanoparticle-Loaded Graphitic Carbon Nitride Nanosheets: Green

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Scheme 1. Fabrication and modification of PAN porous skeleton beads and immobilization of transition metal ions on the modified AOPAN supports.

34


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Figure 1. SEM images of the cross section, internal channel structure, surface morphology and manganese element distribution for sample Mn-PAN (a-d) and Mn-AOPAN (e-h). Energy dispersive spectrometric (EDS) mapping images of internal frameworks for sample Mn-PAN (i-l) and Mn-AOPAN (m-p).

35



3500

2500

2000

928

1100 1450

1730

PAN Mn-PAN AOPAN Mn-AOPAN

-NH2

3000

-CN

O-Mn-O

1650

3200

2243

922

Transmittance

2940

1660

1360

C=C

(a)

-C=N

1500

1000

Wavenumbers (cm-1) PAN Mn-PAN AOPAN Mn-AOPAN

(b)

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 42

10

15

20

25

30

35

2 thera (Degree)

Figure 2. (a) FTIR spectra of PAN, Mn-PAN, AOPAN and Mn-AOPAN; (b) XRD spectra of PAN, Mn-PAN, AOPAN and Mn-AOPAN.

36


1000

800

600

(a)

400

200

635

0

395.0

640

(c)

Mn-AOPAN N 1s

Intensity 400.0

402.5

Binding Energy (eV)

645

650

655

660

665

Binding Energy (eV)

Mn-PAN Mn-AOPAN

397.5

Mn 2p1/2 653.0 eV

Mn-PAN

Binding Energy (eV) N 1s

(b)

Mn-AOPAN Mn 2p3/2 641.3 eV

Intensity

N 1s

O 1s

Mn 2p

Intensity

Mn-PAN Mn-AOPAN

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


C 1s

Page 37 of 42

405.0 395.0

(d) 399.5 eV

398.9 eV 398.2 eV 400.4 eV

397.5

400.0

402.5

405.0

Binding Energy (eV)

Figure 3. XPS spectra of different samples: full spectra of Mn-PAN and Mn-AOPAN (a); and fine XPS spectra of Mn 2p (b), N 1s (c) and N 1s peak splitting of Mn-AOPAN (d).

37



EC (a) EF-PAN EF-AOPAN EF-Mn-PAN EF-Mn-AOPAN EF-Fe-AOPAN EF-Co-AOPAN

0.8 0.6 0.4

(b)

EF-PAN EF-AOPAN EF-Mn-PAN EF-Mn-AOPAN

60

40

20

0.2 0.0

80

TOC Removal (%)

1.0

C/C0

0

20

40

60

80

100

120

0

140

0

20

40

60

T (min)

80

100

120

140

T (min)

2.0

(c)

510nm 0

20

60 100 140

1.5 Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 42

0min 20min

338nm

1.0

40min 60min 80min

0.5

0.0 200

100min 120min 140min

300

400

500

600

700

800

Wavelength (nm)

Figure 4. (a) Degradation behavior of acid red 73 under different EAOPs. (T = 298 K, CMn-AOPAN = 3g L-1, j = 7.5 mA cm-2, Vair = 0.6 L min-1, CNa2SO4 = 0.02 mol L-1, Cdye = 100 mg L-1, pH=3.0). (b) TOC removal of acid red 73 under different EAOPs. (c) UV-vis spectral change of acid red 73 solution catalyzed by Mn-AOPAN in electron-Fenton oxidation.

38


Page 39 of 42

1.0

(a)

0.6 0.4

0

20

40

60

80

100

120

140

T (min)

1.0

(c)

0.6 0.4

0.0

0

0.4

40

60

80

100

(d)

120

140

pH=3 pH=7 pH=9 pH=10 pH=11

0.8

C/C0

0.6

20

T (min)

1.0

0.1M Mn-AOPAN 0.2M Mn-AOPAN 0.4M Mn-AOPAN 0.5M Mn-AOPAN

0.8

0.6 0.4 0.2

0.2 0.0

EC Mn-PAN Mn-AOPAN Fe-AOPAN Co-AOPAN

0.2

0.2 0.0

(b)

0.8

C/C0

C/C0

1.0

EC Mn-PAN Mn-AOPAN Fe-AOPAN Co-AOPAN

0.8

C/C0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

20

40

60

80

100

120

140

0.0

0

20

40

60

80

100

120

140

T (min)

T (min)

Figure 5. Comparison of kinetic simulation of different reaction systems: pseudo first-order kinetics (a) and combined first-order kinetics (b). Effect of immersion Mn concentration for Mn-AOPAN catalysts (c) and initial solution pH (d) on the acid red 73 degradation by electro-Fenton process (T = 298 K, CMn-AOPAN = 3g L-1, j = 7.5 mA cm-2, Vair = 0.6 L min-1, CNa2SO4 = 0.02 mol L-1, Cdye = 100 mg L-1).

39



Page 40 of 42

Figure 6. (a) Reusability of of Mn-AOPAN within electro-Fenton reaction; (b) Manganese leaching in the solution of different cycles.

40


Page 41 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Scheme 2. Possible mechanism of prepared metal-organic composites for activation of hydrogen peroxide in electro-Fenton system.

41



Self-assembly of Mn(II)-amidoximated PAN Polymeric Beads Complex as Reusable Catalysts for Efficient and Stable Heterogeneous Electro-Fenton Oxidation 374x159mm (150 x 150 DPI)


Page 42 of 42

Self-Assembly of Mn(II)-Amidoximated PAN Polymeric Beads Complex

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