Hydroxyl Radical-Dominated Catalytic Oxidation in Neutral Condition

Feb 26, 2017 - ... of metal complexes dominated by hydroxyl radicals, where catalysts have poor stability and are self-destructive in repetitive cycli...
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Hydroxyl Radical-Dominated Catalytic Oxidation in Neutral Condition by Axially Coordinated Iron Phthalocyanine on MercaptoFunctionalized Carbon Nanotubes Dongjing Ni, Jinfei Zhang, Xiyi Wang, Dandan Qin, Nan Li, Wangyang Lu,* and Wenxing Chen* National Engineering Lab for Textile Fiber Materials & Processing Technology (Zhejiang), Zhejiang Sci-Tech University, Hangzhou 310018, China S Supporting Information *

ABSTRACT: The ligands and protein surroundings are important in peroxidase processes with iron porphyrins as catalysts. Similarly, two bioinspired composite catalysts made from iron phthalocyanine with axial ligands, 4-aminopyridine and 2-aminoethanethiol, were anchored on multiwalled carbon nanotubes to degrade some pollutants to the water environment, such as 4-chlorophenol, dyes, and so on. The effect of pH and sustained catalytic stability were investigated in the presence of two catalysts. Different axial ligands and carbon nanotubes that synergistically donated electrons to the central iron of iron phthalocyanine significantly improved the catalytic activity and stability during hydrogen peroxide activation. Electron paramagnetic resonance spin-trapping experiments indicated that catalytic oxidation is dominated by hydroxyl radicals in both catalytic systems, which is different from the high-valent metaloxo generated in common biomimetic catalytic systems with iron porphyrins in the presence of the fifth ligands. The high catalytic activity and strong durability are distinct from traditional peroxide-activating catalysts of metal complexes dominated by hydroxyl radicals, where catalysts have poor stability and are self-destructive in repetitive cyclic oxidation. In our catalytic system, the axial ligand and carbon nanotubes together affect the electronic structure of the central iron in which electron-donor substituents shift the FeIII/II potential to more negative values, which make the activation process of hydrogen peroxide occur at neutral pH, and increase the rate of the step from FeIII to FeII. However, the reaction takes place under acidic conditions, and FeIII/FeII cycling occurs slowly in the traditional Fenton system with hydrogen peroxide.

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INTRODUCTION

Fe3 + + H 2O2 → Fe2 + + HOO• + H+

•

Hydroxyl radical ( OH) is an effective species for rapidly oxidizing organic compounds unselectively.1,2 Various ironbased catalysts such as Fenton, UV-Fenton, and solar-Fenton have been used to degrade enormous stubborn and toxic contaminants because they generate strong radical oxidants (•OH or HOO•). These Fenton reactions have been investigated intensely for decades due to their efficiency, nontoxicity, low cost, and simplicity of control.3,4 However, there are some challenge in the widespread application of the Fenton reaction. These drawbacks mostly include two parts as follows: (1) The reaction takes place under a narrow working pH rage (2−3).3,5 (2) According to the apparent rate constant (k) for the reactions, the FeIII is slowly transformed to FeII by reduction (eq 2), compared with the rates of oxidation (eq 1), which make the reaction slow for various pollutants’ degradation.6 Fe2 + + H 2O2 → Fe3 + + OH− + HO•

M

s

k 2 = 0.001−0.01 (2)

At environmentally relevant pH values (4−9), large amounts of chemicals are spent for acidifying effluents at pH 2−3 before disposal, so it is significant to overcome the barrier.7 In addition, since the catalytic reaction efficiency largely depends upon the rate of FeIII to FeII, the rate-limiting step of the reduction of FeIII to FeII by oxidizing hydrogen peroxide (H2O2) is heavily retarded. Meanwhile, a large excess of H2O2 might be consumed due to the decomposition into O2 or superoxide radical (O2•−) in the reaction. Thus, some additional reductants and antioxidants could be used to accelerate FeII/FeIII redox cycles to boost the generation of • OH in the Fenton system.1,8 Although some workers have particular strategies to address these challenges and develop the catalysts with high activity and stability, such as Fe impregnated Received: Revised: Accepted: Published:

k1 = 76 M−1 s−1 (1)

© XXXX American Chemical Society

−1 −1

A

December 7, 2016 February 9, 2017 February 25, 2017 February 26, 2017 DOI: 10.1021/acs.iecr.6b04726 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Scheme 1. Schematic Diagram of the Preparation of FePc-CS-MWCNTs Composite

on prototypical mineral colloids,9,10 Fe-g-C3N4,11 magnetic nanoscaled perovskite-type mixed oxides,12 etc., these attempts are still far from meeting the practical environmental requirements. As we all know, iron as a micronutrient is harmless and even required by all living systems with important environmental and physiological significance, considered as relatively environmentally friendly matters. Among various kinds of iron-based catalysts, iron phthalocyanine (FePc) is an attractive catalyst because of its affordability, simple large-scale preparation, and its chemical and thermal stability.13,14 FePc derivatives have been considered to be promising catalysts as they are are used extensively in various catalytic fields, including allylic C−H amination, selective oxidation of phenols,15 CO2 reduction,16 fuel cells,17 selective oxidation and aldol reaction,18 and photocatalytic oxidation of thiols to disulfides.19 One of the most important applications of FePc is the catalytic activation of H2O2 to oxidize substances. Nevertheless, FePc derivatives as homogeneous catalysts are difficult to recover or remove from the reaction system. In nature, ferric heme-containing proteins as highly efficient catalysts have cyclic oxidation behaviors under neutral conditions to sustain life and evolution. The fifth iron ligand and proteins in ferric heme proteins play a very important role in peroxidase enzymatic reactions, and the effects of them are very significant on catalyzing the oxidation of organic substrates.20 With this inspiration, it is expected that ligand and support cooperating in a synergistic manner will help to change the ligand environment of central iron and then enhance catalytic efficiency. In our previous system, the homogeneous iron-based catalysts such as FePc and iron complex with aminopyridine ligand have been immobilized on diverse carbon supports by covalent bonding to obtain the heterogeneous catalysts.21,22 Meanwhile, the catalytic performance could be obviously influenced by the different binding form between the substituents of phthalocyanine ring and carbon materials.22

The difference of catalytic activities might be caused mainly by the change of the electron density in the central iron. However, the more direct change in the axial coordination environment that occurred at the central iron of FePc might provide a different channel for designing more effective catalysts. Furthermore, carbon nanotubes can be chosen as carrier materials due to their unique physical properties and structure, excellent electrochemical performance, and high stability under different conditions.23 Importantly, FePc is immobilized by carbon nanotubes as carrier which simulates the protein scaffold of the enzyme and might cause dramatic changes in the catalytic mechanism of the system. In our previous study, the introduction of fifth ligands can heighten the change of the ligand environment of the center atom and improve the catalytic performance.24 Alternatively, multiwalled carbon nanotubes (MWCNTs) play an important role in the electron transfer process in the catalytic oxidation of pollutants.25 Though the different ligands and carbon nanotubes could improve the reaction efficiency, there is a need to know what happens when the reaction control is mediated by different ligands and carbon nanotubes together. In this work, we have designed two catalysts using 4-aminopyridine (4-Py)-functionalized and 2-aminoethanethiol (CS)-functionalized MWCNTs to anchor FePc molecules and provide an axial ligand for the iron center, respectively. The electronic structure of FePc is affected significantly by the coordination of different axial ligands and carbon nanotubes to the iron site of the macrocycle, which influences their catalytic activity, durability, and working pH range. The oxidative degradation of 4-chlorophenol (4-CP) in the FePc-Py-MWCNTs/H2O2 and FePc-CS-MWCNTs/ H2O2 system was investigated by ultraperformance liquid chromatography (UPLC). A reaction mechanism has been proposed in the catalytic system using an electron paramagnetic resonance (EPR) spin-trap technique. X-ray photoelectron spectroscopy (XPS) was used for characterization. This research offers a new perspective into the design of highly efficient iron-involved reactions by imitating the peroxidase B

DOI: 10.1021/acs.iecr.6b04726 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research enzyme reaction with the fifth iron-porphyrin ligand and proteins around them. Furthermore, this work paves the way for developing a simple but highly efficient catalytic system, and can possess potential value in environmental protection.

analyzed by UPLC/Synapt G2-S HDMS (Waters) in negative ion mode, with lockmass correction of leucine enkephaline (LE, Tyr-Gly-Gly-Phe-Leu, m/z 554.2615). Oxidation Procedures and Analysis. The catalytic oxidation of 4-CP was conducted using FePc-CS-MWCNTs and H2O2 at 50 °C and pH 7. FePc-CS-MWCNTs (2 mg, containing 3.11 × 10−4 μmol FePc) were dispersed by sonication in distilled water with pH 7. The pH was adjusted by the addition of dilute aqueous H2SO4 or NaOH. After the sonication, 4-CP (50 μM, 20 mL) and H2O2 (5 mM) were added at 50 °C, and stirred by a magnetic stirrer in a container for given time intervals; samples were filtered through 0.22 μm pore size Teflon filters and then analyzed instantly by UPLC. The homogeneous contrast experiment maintained equal molar concentration of FePc and FePc in FePc-CS-MWCNTs. Continuous cycle experiments were performed 12 times. For each run, a known concentration of 4-CP was added to the reaction system to maintain an initial concentration of 50 μM, and H2O2 (5 mM) was added every cycle. At given time intervals, high-performance liquid chromatography was used to analyze the samples immediately at the corresponding maximum absorbance of the 4-CP.

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EXPERIMENTAL SECTION Materials and Reagents. FePc, 4-Py, pyridine (Py), CS, and MWCNTs (lot no. 7XHKH-PC, 10−20 nm diameter, 5− 15 μm length) were purchased from Tokyo Chemical Industry Co., Ltd. 4-CP (analytical reagent grade) and H2O2 (9.7 M, laboratory reagent grade) were purchased from Sinopharm Chemical Reagent Co., Ltd. Isopropanol (IPA) was obtained from Tianjin Wing Tai Chemical Co. Ltd. All were used without further purification. All other solvents used were of spectrometry grade. Aqueous solutions were prepared using doubly distilled water, which was used in the 4-CP decomposition process. Catalysts Preparation. The novel supported oxidation catalyst (FePc-Py-MWCNTs) was prepared by bonding FePc covalently on the MWCNTs (Scheme S1).26,27 CS-MWCNTs were synthesized using CS and MWCNT functionalization according to a previously reported method.25 FePc was coordinated to CS-MWCNTs through bond formation between the sulfur atom in 2-CS and the iron center in FePc. Briefly, the FePc-CS-MWCNTs composite was prepared by refluxing a mixture of CS-MWCNTs and FePc in tetrahydrofuran (THF) under nitrogen. The preparation of CSMWCNTs- and CS-MWCNTs-supported FePc is illustrated in Scheme 1. To verify the effect of MWCNTs, an FePc-loaded modified chloromenthylated bead composite was prepared by use of similarity method (Scheme S2). The Fe content of FePcPy-MWCNTs and FePc-CS-MWCNTs was measured by inductively coupled plasma (ICP) spectroscopy (Varian 720ES ICP-OES), and the FePc content in FePc-Py-MWCNTs, FePc-CS-MWCNTs, and FePc-loaded modified chloromenthylated bead were found to be 1.5 × 10−4, 1.6 × 10−4, and 1.45 × 10−4 mol g−1, respectively (Figure S1). Characterization. Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectra of FePc, MWCNTs, oxidized MWCNTs, CS-MWCNTs, Py-MWCNTs, FePc-PyMWCNTs, and FePc-CS-MWCNTs were recorded at room temperature with a Thermo Electron Nicolet 5700. XPS was used to investigate the chemical bonding of the prepared catalysts on a Thermo Scientific K-Alpha spectrometer (monochromatic Al Kα, 1486.6 eV). FePc, FePc-Py, and FePc-CS characterization was conducted with an ultraviolet− visible (UV−vis) spectrometer (Hitachi U-3010). The pchlorophenol concentration was determined by UPLC (Acquity BEH C18 column, 1.7 μm, 2.1 mm × 50 mm; Waters). EPR signals were detected on a Bruker A300 spectrometer for free radicals trapped by 5,5-dimethyl-pyrroline-oxide (DMPO) at room temperature. A conventional three-electrode electrochemical cell was used to collect electrochemical data using an IM6ex (Zahner, Germany) electrochemical workstation. The working electrode was a BAS glassy carbon electrode (3 mm), and platinum wire and a Hg/Hg2Cl2 electrode (0.22 V versus standard hydrogen electrode) served as a counter and pseudoreference electrode, respectively. The electrochemical properties of FePc, FePc-Py, and FePc-CS were investigated by linear sweep voltammogram at room temperature in dimethylformamide containing 0.01 mol L−1 tetrabutyl ammonium periodate. The final products during 4-CP oxidation by FePc-CS-MWCNTs/H2O2 were

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RESULTS AND DISCUSSION Catalyst Characterization. The absorption spectra of FePc in THF before and after CS addition are shown in Figure 1. The characteristic Q-band centered at 639 nm is visible in

Figure 1. UV−vis absorption spectra of FePc (FePc, 0.07 mM) and FePc-CS (FePc, 0.07 mM; CS, 0.07 mM) in THF solution.

the UV−vis spectrum of FePc.28,29 Regarding the absorption characteristics of FePc, a significant narrowing and increase in intensity dominates the spectrum because of the complexation of CS, which accompanies a red shift from 639 to 661 nm. The apparent bathochromic shift reflects the strong coordination interaction between FePc and CS. The absorption band had a red shift from 639 to 649 nm between FePc and Py (Figure S2), which is shorter than that for FePc-CS. Therefore, these results indicate that the N and S are both electron-donating substituents, by which the environment of the central iron of iron phthalocyanine was altered. The “push” electronic ability of CS to FePc is stronger than that for Py,30 and can improve and change the catalytic properties of FePc to a certain extent. XPS was used to study chemical bonding in MWCNTs, HOOC-MWCNTs, CS-MWCNTs, Py-MWCNTs, FePc-PyMWCNTs, and FePc-CS-MWCNTs. As shown in Figure 2, the oxygen content increased significantly in HOOC-MWCNTs, which implies that a large number of oxygen-containing groups exist on carboxyl-containing MWCNTs. When carboxylic-acidfunctionalized MWCNTs react with CS, the oxygen intensity C

DOI: 10.1021/acs.iecr.6b04726 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 2. (a) XPS spectra of MWCNTs, HOOC-MWCNTs, CS-MWCNTs, and FePc-CS-MWCNTs (spectral region from 50−1150 eV). Curve fit of N 1s peak of (b) FePc, (c) HOOC-MWCNTs, (d) CS-MWCNTs, and (e) FePc-CS-MWCNTs.

amide.25,34,35 In the S 2p spectrum (Figure S4), the binding energies of S 2p3/2 and S 2p1/2 (Figure S4c) increase by ∼1.2 eV, compared with that of CS-MWCNTs (Figure S4b). Such a binding energy shift indicates that the electronic density of the metal phthalocyanine (MPc) ring decreased, owing to the charge transfer from sulfur in CS to the iron center in FePc. This result is also proven by the N 1s spectrum (Figure 2e). The N 1s peak of FePc-CS-MWCNTs consists of two peaks separated by 1.43 eV, which are assigned to two groups of four N atoms in different chemical environments (in other words, the outermost four nitrogen atoms bond to carbon only, and the four innermost nitrogen atoms interact with the central iron cation).36 Therefore, we conclude that FePc was coordinated to CS-MWCNTs through the bond formed between the sulfur atom in CS and the iron center in FePc. As shown in Figure S5, the XPS spectra from a wide scan showed a clear increase in nitrogen atoms in MWCNTs. Since the iron ion is coordinated with the Py group in the axial direction in FePc-Py-MWCNTs, the extra ligand leads to a change in electron density on iron, which results in two groups of N 1s of FePc-Py-MWCNTs (Figure S6). CS and Py ligands produce essentially the same effect but not the same abilities because the lone pair orbital on

decreases, and new nitrogen and sulfur bands are detected, which indicates that CS may be supported on oxidized MWCNTs by the amide group (−NH−CO−). The immobilization of FePc on CS-MWCNTs can also be proposed from the new iron peaks and increasing nitrogen in the XPS spectrum of the FePc-CS-MWCNTs. For investigation of bonding modes, XPS spectra from a narrow scan of N 1s and S 2p were obtained. The binding energy is involved in the electron density around the nucleus, where the lower electronic density denotes a higher binding energy.31,32 An analysis of the N 1s peak area ratio and numbers is essential to develop a better understanding of the nature of the bonds between Fe and N. As shown in Figure 2b, the N 1s photoemission line from the FePc molecules consists of a single peak located at 398.52 eV, which is attributed to aza-bridging and pyrrole nitrogen atoms.33 Almost no N 1s signal was detected in HOOC-MWCNTs (Figure 2c), whereas the peak at 400.28 eV (Figure 2d) may be explained by the amide group (−NH−CO−) being used for bonding between carboxyl and amino (−NH2) groups in CS. The slight absorption peak at 1682 and 1545 cm−1 in the FTIR spectrum (Figure S3) also confirms that CS was immobilized on MWCNTs by D

DOI: 10.1021/acs.iecr.6b04726 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research the S and N atoms provides them with similar frontier orbital properties, whereas the “push” electronic capability of the S atom to the Fe iron is stronger than that for the N atom. On the other hand, MWCNTs’ pushing electron ability will affect the catalytic oxidation function of the catalysts. Catalytic Oxidation of 4-CP. The catalytic activity of the bioinspired system based on FePc-CS-MWCNTs has been investigated using 4-CP as the model. Changes in the concentration of 4-CP under different conditions are shown in Figure 3a. There was no obvious change in the concentration

Figure 4. Concentration changes of 4-CP (5 × 10−5 M) in the presence of different catalysts under different pH ([catalysts] = 0.1 g L−1 (containing 0.016 mM FePc), [H2O2] = 0.005 M, 50 °C, (a) pH 3, (b) pH 7).

catalytic system, whereas almost no catalytic activity was exhibited in the FePc/H2O2 and FePc-Py-MWCNTs/H2O2 catalyst systems. This occurs mainly because the oxidation capacity of H2O2 is different under different pH conditions (namely, the oxidation capacity in acidic pH is higher than that under neutral conditions).25,39 HOO− dissociated from H2O2 may favor coordination with FeII in FePc-CS-MWCNTs because of electron donation from the S atom in CS to the Fe atom center in FePc-CS-MWCNTs. The electron-donating ability from the S atom in CS is stronger than the N atom in Py, which results in the easier production of •OH radicals and makes the catalytic oxidation of 4-CP in neutral and acid conditions more rapid. In addition, compared with FePc-CSloaded modified chloromenthylated bead (Figure S7) and FePc-CS-MWCNTs, the carrier of MWCNTs plays some part in the catalytic degradation. Moreover, this indirectly demonstrates that the pushing electron ability of MWCNTs is also used to influence the iron coordination environment, which led to the reduction potential of FePc-CS-MWCNTs being shifted to more negative values, and enhancement of the reaction efficiency becomes easier. Reaction temperature is an essential contributor to catalytic activities and rate. By the ln κ − 1/T linear return, the activation energy of FePc-CSMWCNTs was 46.0 kJ/mol. It was found that activation energy of FePc-CS-MWCNTs was low compared with other ordinary catalysts,40−42 and the reaction was not strongly affected by the temperature dependence (Figure S8). The autocatalytic decomposition of some peroxide-activating catalysts of metal complexes must be avoided, which inevitably occurs during catalytic oxidation dominated by highly reactive • OH, especially in aqueous solution.43,44 Thus, stability is very important for complex catalysts. Here, the repetitive cyclic oxidation of 4-CP was sustained 12 times in the FePc-CSMWCNTs/H2O2 catalytic system. The first cycle finished within 120 min under experimental conditions. After the initial

Figure 3. (a) Concentration changes of 4-CP (5 × 10−5 M) under different catalysts. (b) Cyclic catalytic oxidation, with each cycle typically lasting 120 min ([FePc-CS-MWNTs] = 0.1 g/L (containing 0.016 mM FePc), [H2O2] = 0.005 M, 50 °C, pH 7).

of 4-CP in the presence of FePc and H2O2, which indicates that FePc had almost no catalytic activity for 4-CP oxidation by pH 7. There was also no adsorption with FePc-CS-MWCNTs or MWCNTs. The improved H2O2 catalysis activity of FePc/ MWCNTs may be attributed to the synergistic effect (strong interaction) between FePc and MWCNTs at acidic pH,37 but is not powerful enough at pH 7 to oxidize 4-CP. However, the concentration of 4-CP decreased rapidly with increasing reaction time in the presence of FePc-CS-MWCNTs, which indicates that the axial direction ligand S atom changes the electron density on the Fe center in FePc. This makes it easy for H2O2 to activate iron atoms in FePc. The catalytic reaction with MPcs depends on electron donation and acceptance after coordination between the central metal ions and H2O2, which is affected strongly by pH.38 Therefore, the effect of pH is important in the catalysis of MPcs. Figure 4 shows comparative experiments that have been investigated for different pH values. In the presence of FePc and H2O2 at pH = 3, only ∼22% of 4CP was eliminated in 90 min, whereas 97% of 4-CP was removed when FePc-Py-MWCNTs and H2O2 were present. However, the concentration of 4-CP decreased rapidly in the FePc-CS-MWCNTs and H2O2 system under the same conditions. The reaction also has a higher oxidation rate under neutral conditions in the FePc-CS-MWCNTs/H2O2 E

DOI: 10.1021/acs.iecr.6b04726 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research 4-CP had been removed, a certain amount of 4-CP and H2O2, the same as the first time, was added again into the system undergoing a catalytic reaction. The results in Figure 3b show that the catalytic system exhibited a high catalytic oxidation ability for H2O2 to oxidize 4-CP in each successive cycle, which indicates that FePc-CS-MWCNTs influence the activity and stability under neutral conditions. This result differs from the traditional catalytic oxidation reaction with •OH radicals, where oxidation has a poor stability and will self-destruct in reuse tests.45 We investigated the FePc-CS-MWCNTs/H2O2 catalytic system for oxidation degradation of other organic dye pollutants and pollutants in wastewater that are difficult to remove, as shown in Table 1. These pollutants can also be decomposed thoroughly and effectively at neutral pH and 50 °C in this system. Table 1. Catalytic Oxidation of Different Pollutantsa substrate

H2O2 (mM)

reaction time (min)

removal rate (%)

AR1 (5 × 10−5 M) BG1 (2.5 × 10−5 M) Rhb (2.5 × 10−5 M) 2,4-dichlorophenol (5 × 10−5 M) p-nitrophenol (5 × 10−5 M)

5 5 5 5

60 120 120 120

95.2 96.1 97.6 95.8

5

120

95.6

The dyes in the table include Acid Red 1 (AR1), Basic Green (BG1), and Rhodamine B (RhB). Also, two toxic substances are 2,4dichlorophenol and p-nitrophenol. Catalytic oxidation of different contaminants with 0.1 g/L FePc-CS-MWCNTs at 50 °C and pH 7.

Figure 5. DMPO spin-trapping EPR spectra in (a1) water (pH 7) and (a2) methanol solution in the presence of FePc-CS-MWNTs (0.1 g/ L) with H2O2 (5 mM) and DMPO (20 mM). (b) Linear sweep voltammogram of FePc, FePc-Py, and FePc-CS in dimethylformamide containing 0.01 mol L−1 TBAP with a scan rate of 50 mV/s ([FePc] = 1 × 10−3 mol L−1, [CS] = 1 × 10−3 mol L−1, [Py] = 1 × 10−3 mol L−1).

Mechanism. For investigation of intermediate active species that are generated on the basis of whether the O−O bond of H2O2 is cleaved heterolytically or homolytically, isopropyl alcohol, a typical hydroxyl radical scavenger, was used to determine whether hydroxyl radicals were the active species that are produced by homolytic cleavage in the catalytic decomposition of 4-CP.46,47 As shown in Figure S9, catalytic oxidation was inhibited in the presence of isopropyl, which indicates that •OH radical oxidation48 may dominate the catalytic mechanisms of the FePc-CS-MWCNTs catalytic system. We used EPR spin trapping to investigate detailed electron transfer in the FePc-CS-MWCNTs nanohybrid, where a common spin-trap reagent, DMPO, was used to detect DMPO-•OH in aqueous neutral conditions and DMPO-•OOH was used in methanol solution.44 As shown in Figure 5a, the EPR results exhibit an obvious DMPO-•OH signal (curve a) with a typical four-peak spectrum in an intensity ratio of 1:2:2:1, and a DMPO-•OOH (curve b) signal, which indicate that •OH and •OOH radicals were generated and participated in catalytic oxidation.49,50 According to literature studies, the Fe atom in FePc left two ligand fields except for the four ligand fields to N atoms in FePc.44 We conclude that the enhanced catalytic activity and excellent durability of the FePc-CS-MWCNTs composite catalyst may be attributed to the lone pair orbital on the S atom that provides a frontier orbital, which is expected to produce ligand field splitting. This leads to an electronic redistribution of d orbitals of the FePc iron ions, with similar changes in valence spin as a result. The N atom in Py can provide a similar effect to coordinate with the central iron ions in FePc, but is not stronger than the S atom.51,52 From an

electrochemical perspective, the electron transfer behavior between FePc/CS and FePc/Py may explain the promoted catalytic performance in FePc-CS-MWCNTs. Previous studies demonstrate that the nature of the Fe−S bond in the active cyt P450 and related synthetic model complexes is much more covalent relative to that of a neutral ligand.53 As shown in Figure 5b, the oxidation of FePc (1.28 V) became more rapid once ligands CS and Py were added. Obviously, the oxidation potential of FePc-CS (0.4 V) is much lower than that of FePcPy (1.18 V), which means that the catalyst FePc-CS-MWCNTs can be oxidized by H2O2 more easily. We can also conclude that the Fe−S bond is more covalent than the Fe−N bond.54 Moreover, the reduction potentials of H2O2 are 1.776 V under acidic conditions and 0.878 V in the neutral and alkaline ranges.55 Although the N atom in Py can change the electron density of iron ions in FePc, as the oxidizability of H2O2 is weak, H2O2 is not sufficiently powerful to oxidize FePc-PyMWCNTs under neutral conditions. Therefore, the excellent and promoted catalytic performance of FePc-CS-MWCNTs may arise from a stronger electronic interaction between CS and FePc. Degradation Pathway. On the basis of the experiments above, a mechanism for the catalytic oxidation of 4-CP by FePc-CS-MWCNTs/H2O2 was proposed as shown in Figure 6. The initiation step is Fe(II) in FePc of FePc-CS-MWCNTs reacting with H2O2 to form a complex, which is FeIIOOH in the FePc of FePc-CS-MWCNTs. In general, hemolytic and heterolytic cleavage of the peroxide OO bond occurs in competition.56 In this catalytic oxidation system, homolytic cleavage of the peroxide OO bond is the pathway that forms FeIII=O intermediates and •OH, and the •OH can destroy the

a

F

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time, and molecular mass can be profitably used in the highly accurate identification of structure.58 Structures of the compounds were determined by comparing their retention time with that of an authentic standard.59

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CONCLUSIONS We have synthesized CS as an axial ligand and MWCNTs as a carrier, which changes the coordination number of the central iron ions in FePc. The FePc-CS-MWCNTs have a stronger catalytic oxidation capability under neutral conditions and better stability than the composite FePc-Py-MWCNTs with Py as axial ligand and FePc-CS-loaded modified chloromenthylated bead with modified chloromenthylated bead as a carrier. H2O2 is not sufficiently powerful to oxidize FePc-Py-MWCNTs and the FePc-CS-loaded modified chloromenthylated bead under neutral conditions; one possible option is because of the lower electronic interaction between N atoms in Py to Fe in FePc, and the other option is possibly because of the chloromenthylated bead as a carrier. In contrast, the stronger ability of electron transfer from MWCNTs and CS to FePc accelerates the transition of FeIII=O to FeII under neutral conditions, which is critical in the traditional hydroxyl radical reaction system. Many •OH radicals were generated to destroy 4-CP into small biodegradable molecules that cannot damage the catalyst. This improved the inherently low stability of the commonly investigated FePc significantly because of the presence of ligand. This work offers new prospective coordination between metal phthalocyanine and its supports, on the basis of the appropriate selection of axial ligand for catalyst design and optimization.

Figure 6. Possible pathways for the formation of active species in the FePc-CS-MWCNTs activated system.

target 4-CP under neutral conditions. FeIII=O is easily transformed to the original FeII in the presence of H2O2, which is attributed to the electron donors CS and MWCNTs from FePc-CS-MWCNTs, and is considered a crucial step in the FeII, FeIIOOH, and FeIII=O cycle.57 For the electrondonor capability of axial ligand CS and carrier MWCNTs, a steady stream of •OH radicals will be generated as long as H2O2 is present to catalyze the oxidation of 4-CP substances, and the final products are biodegradable small molecules. As shown in Table 2, the six small molecular acids were obtained because of aromatic ring-opening oxidation during 4-CP oxidation by FePc-CS-MWCNTs/H2O2. Taken together, a combination of relevant experimental parameters, retention

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ASSOCIATED CONTENT

S Supporting Information *

Table 2. Oxidative Intermediates of 4-CP by FePc-CSMWCNTs in the Presence of H2O2 Examined by UPLC Synapt G2-S HDMS in Negative Ion Mode after 120 min of Reaction Time

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b04726. Additional schemes and figures, and ICP, XPS, and activation energy analysis (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +86 571 86843611. Fax: +86 571 86843611. *E-mail: [email protected]. Phone: +86 571 86843611. Fax: +86 571 86843611. ORCID

Wenxing Chen: 0000-0002-4554-1455 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51133006), 521 Talent Project of ZSTU, and Zhejiang Provincial Natural Science Foundation of China (LQ17E030003 and LY14E030013), and the Public Welfare Technology Application Research Project of Zhejiang Province (2015C33018). G

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DOI: 10.1021/acs.iecr.6b04726 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX