Preparation of Phthalocyanine Immobilized Bacterial Cellulose ... - MDPI

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Preparation of Phthalocyanine Immobilized Bacterial Cellulose Nanocomposites for Decoloration of Dye Wastewater: Key Role of Spacers Qiaoling Teng, Shiliang Chen * and Wenjie Xie Qianjiang College, Hangzhou Normal University, Hangzhou 310012, China; [email protected] (Q.T.); [email protected] (W.X.) * Correspondence: [email protected]; Tel.: +86-571-2886-1372

Received: 9 April 2018; Accepted: 20 June 2018; Published: 22 June 2018

Abstract: We report the preparation of a series of spacer-incorporated, tetra-amino cobalt (II) phthalocyanine (CoPc)-immobilized bacterial cellulose (BC) functional nanocomposites (CoPc@s-BC). Four kinds of flexible spacers with different lengths—diethylenetriamine (DT), triethylenetetramine (TT), tetraethylenepentamine (TP) and pentaethylenehexamine (PH)—were covalently attached onto pre-oxidized BC for the synthesis of the spacer-attached BC, and the attached spacers’ contents were carefully quantified. Using glutaraldehyde as a cross-linker, the CoPc catalyst was covalently immobilized onto the spacer-attached BC, and the immobilization steps were optimized by monitoring both the residual spacer contents and the resulting immobilized CoPc. All of the functionalization processes were characterized and confirmed by X-ray photoelectron spectroscopy (XPS). The series of spacer-incorporated, CoPc-immobilized BC nanocomposites, CoPc@s-BC, were used for the decoloration of dye wastewater. Both the adsorption capacity and adsorption rate were increased after the incorporation of spacers. When H2 O2 was employed as an oxidant, dye molecules were catalytically oxidized with these nanocomposites. Electron paramagnetic resonance (EPR) spin-trapping results showed that the highly reactive hydroxyl radical (·OH) was involved in the catalytic oxidation process. The spacer length had a direct effect on the catalytic efficiency of CoPc@s-BC—the decoloration rate for CoPc@TP-BC was as high as 41 µmol·min−1 ·g−1 , which was more than 50% higher than that without spacer. Keywords: bacterial cellulose; phthalocyanine; nanocomposite; spacer length; decoloration

1. Introduction Nanocellulose has spawned increasing interest from broad fields, because this naturally occurring nanomaterial combines the advantages of cellulose, such as being hydrophilic, environmentally benign and having an easily tunable surface, with prominent features of nanosized materials, such as having a very high surface area to volume ratio and considerable modification possibility [1]. The preparation of nanocellulose ranges from “top-down” processing by isolation of natural cellulose to “bottom-up” processing by culture medium with certain bacteria [2,3]. Thethe nanocellulose produced directly from bacteria is well known as bacterial cellulose (BC). Compared with “top-down” produced nanocellulose, BC is highly pure (hemicellulose- and lignin-free) and possesses unique and sophisticated three-dimensional, porous network structures which have been widely used in many fields. Potential applications of BC include mechanically reinforcement nanofillers [4,5], medical products [6], wound treatments [7], bio-inspired nanomaterials [8], and reaction templates [9]. In particular, its well-defined 3D network structures, high surface-area-to-volume-ratio, high accessibility, remarkable functionality, excellent mechanical properties and sustainability suggest

Appl. Sci. 2018, 8, 1021; doi:10.3390/app8071021

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that BC is highly suitable as a templates or substrate for the impregnation of a range of functional guest molecules, such as metal oxides, metal nanoparticles, mineral nanomaterials and carbonaceous nanomaterials [10,11]. Metal phthalocyanine complexes (MPcs), a class of versatile functional molecules [12], are promising catalysts for many processes because of their structural relations to naturally-occuring metal porphyrin complexes [13–16]. In practical applications, the immobilization of MPcs with solid substrates [17–22] is essential for convenient separation of catalysts from reaction media, long-term and continuous operation, overcoming the formation of inactive aggregates and preserving their activity. By carefully selecting appropriate substrate materials with specific microenvironment suitable for MPc reactions, their catalytic activity can be markedly enhanced. BC is undoubtedly a promising candidate for MPc immobilization, owing to the advantageous features mentioned above. Our group has been working on MPc immobilization using BC as a substrate [23–25], and we have comprehensively investigated the immobilization of MPcs onto BC. Despite the enormous strides made in this research field, immobilized MPcs catalysts often suffer from various disadvantages of heterogeneous reaction conditions when compared with their homogeneous counterparts [26]. Direct immobilization of MPcs onto substrate surfaces will inevitably lead to the mass transfer limitation of target molecules to the heterogeneous catalysts [27–29]. Steric hindrance between the substrates and the catalysts also disfavors the catalytic reaction [30,31]. To overcome these drawbacks, an appropriate long and flexible spacer arm can be employed to keep the MPc molecules at a reasonable distance from the substrate surface. The presence of the spacer may mitigate the unfavorable effect of diffusion limitation and steric hindrance, enhance the accessibility and mobility of the MPc catalysts and increase the reaction homogeneity, thus increasing the catalytic activity. Spacers with variable lengths have been widely employed by researchers to enhance different kinds of catalysts. An increase in spacer length can efficiently enhance the catalytic activities of catalysts, such as protoporphyrin photosensitizers [32], zirconium complexes [33], and enzymes [34,35]. A few examples of spacers that can influence the electron transfer activity of MPcs have been reported [36–38], However, little has been reported specifically concerning the effect of spacer length on the catalytic activity of MPcs. An investigation of how the spacer length influence the MPcs’ catalytic efficiency is, therefore, of timely importance. The present study aimed to combine the advantages of BC and the flexible spacer for the preparation of a series of spacer-incorporated, tetra-amino cobalt (II) phthalocyanine (CoPc)-immobilized bacterial cellulose (BC) functional nanocomposites (CoPc@s-BC). A range of flexible spacers with different lengths, namely diethylenetriamine (DT), triethylenetetramine (TT), tetraethylenepentamine (TP) and pentaethylenehexamine (PH) (Figure 1), were attached onto BC for the subsequent covalent immobilization of the CoPc catalyst. The resulting nanocomposites were employed for the decoloration of dye wastewater; the synergistic improvement of both the adsorption capacity and catalytic activity of CoPc was expected. The mechanism of the catalytic oxidation decoloration was investigated using the electron paramagnetic resonance (EPR) spin-trapping technique. The effect of the spacer length on the decoloration efficiency was carefully studied. This research illustrates the key roles of the spacer and the spacer length in the enhancement of catalytic activity of MPc and offers a new perspective into the design of a highly efficient catalytic system for the remediation of dye wastewater.

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3 of 14 3 of 14

NH CH2 CH2 CH2 NH2 a: diethylenetriamine (DT) CH2

NH2

NH

CH2

NH2

CH2

NH

CH2

NH2

CH2 CH2 NH2 CH2 NH CH2 b: triethylenetetramine (TT)

CH2

CH2

CH2

NH

CH2

NH CH2

CH2

CH2

NH2

c: tetraethylenepentamine (TP) NH

CH2

NH2

CH2

CH2

CH2

NH

CH2

NH CH2

CH2

CH2

NH

CH2

CH2

NH2

d: pentaethylenehexamine (PH)

Figure (a)(a) diethylenetriamine (DT); (b) (b) triethylenetetramine (TT);(TT); (c) Figure 1. 1. Chemical Chemicalstructures structuresof of diethylenetriamine (DT); triethylenetetramine tetraethylenepentamine (TP); and (d) pentaethylenehexamine (PH). (c) tetraethylenepentamine (TP); and (d) pentaethylenehexamine (PH).

2. Materials and Methods 2. Materials and Methods 2.1. Materials 2.1. Materials and and Reagents Reagents Acetobacter purchased from from BeNa BeNa Culture Culture Collection Collection Co., Co., Ltd. Ltd. (Category Acetobacter xylinum xylinum was was purchased (Category No. No. BNCC336985; Beijing, China). BC was produced by cultivating Acetobacter xylinum BNCC336985; Beijing, China). BC was produced by cultivating Acetobacter xylinum bacteria bacteria in in aa liquid culture culture medium medium containing containing8.0 8.0w/v w/v % % D-glucose, D-glucose, 1.0 1.0 w/v w/v % and 1.0 1.0 v/v v/v % liquid % yeast yeast extract extract and % ethanol, ethanol, as reported in the literature [39]. CoPc was synthesized from 4-nitrophthalic acid, urea and cobalt as reported in the literature [39]. CoPc was synthesized from 4-nitrophthalic acid, urea and cobalt chloride hexahydrate, according to a method described previously [40,41]. Reactive red X-3B chloride hexahydrate, according to a method described previously [40,41]. Reactive red X-3B (C.I. (C.I. Reative Red Red 2, 2, CC19 19H H10 10Cl 7S2S , M.W.: 615.33) was purchased from Shanghai Chemical Reagent Reative Cl22N N66Na Na2O O , M.W.: 615.33) was purchased from Shanghai Chemical Reagent 2 7 2 Factory. The The spin-trapping spin-trapping reagent, reagent, 5,5-dimethyl-1-pyrroline-N-oxide 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), (DMPO), was Factory. was purchased purchased from from Sigma Chemical Co. (Saint Louis, MO, USA). Ninhydrin reagent, DT, TT, TP, PH andchemicals all other Sigma Chemical Co. (Saint Louis, MO, USA). Ninhydrin reagent, DT, TT, TP, PH and all other chemicals were purchased from Sinopharm Reagent Co. Ltd. (Shanghai, China) used were purchased from Sinopharm Chemical Chemical Reagent Co. Ltd. (Shanghai, China) and usedand without without further purification. further purification. 2.2. Preparation and Characterization of CoPc@s-BC

To prepare CoPc@s-BC, CoPc@s-BC, the the following following steps steps were were carried carried out: out: (1) 20 mg of pure BC was incubated To prepare 30 mmol/L mmol/L NaIO with a 30 NaIO44 solution solution and and reacted reacted for for 8 h. The The resulting resulting oxidized oxidized BC was thoroughly ◦ C12 dried at 50 h. 12 Theh.content of aldehyde groups on oxidized washed with withultrapure ultrapurewater waterand and dried at °C 50for for The content of aldehyde groups on BC was determined accordingaccording to the literatures with minor [42,43].[42,43]. (2) The(2) dried, oxidized BC was determined to the literatures with modifications minor modifications The oxidized BC was into ainto spacer solution (DT (DT or TT or PH) and shaken onona dried, oxidized BCsubmerged was submerged a spacer solution or or TTTP or TP or PH) and shaken temperature-controlled shaker. a temperature-controlled shaker.The Thecontent contentofofthe theattached attachedspacer spacer was was indirectly indirectly determined determined by BC substrate substrate using using aa ninhydrin-based ninhydrin-based monitoring measuring the number of amino groups on the BC [30]. Ninhydrin reagent can can reactreact withwith primary amines to form system, as as described describedininthe theliterature literature [30]. Ninhydrin reagent primary amines to a colored complex which is soluble in C 2 H 5 OH/H 2 O mixed solvent and highly conjugated, with a form a colored complex which is soluble in C2 H5 OH/H2 O mixed solvent and highly conjugated, strong absorption at ca. 570 nm.570 Two hundred milliliters of ninhydrin reagentreagent and 100and μL 100 of HµL 2O with a strong absorption at ca. nm. Two hundred milliliters of ninhydrin were 1 mg oftospacer-attached BC and heated in a heated 100 °C water bath◦for 1 h. Five milliliters of H2added O weretoadded 1 mg of spacer-attached BC and in a 100 C water bath for 1 h. of C2H 5OH/H2O v/v) mixed solvent added, andwas theadded, mixture on aanalyzed UV-vis Five milliliters of(50:50, C2 H5 OH/H v/v)was mixed solvent andwas theanalyzed mixture was 2 O (50:50, absorption (UV-2450) at the wavelength of maximum absorbance: 570 nm. 570 (3) The on a UV-visspectrometer absorption spectrometer (UV-2450) at the wavelength of maximum absorbance: nm. spacer-attached BC was submerged into 12 mL of glutaraldehyde solution and shaken at 25 °C for (3) The spacer-attached BC was submerged into 12 mL of glutaraldehyde solution and shaken at2 ◦ C for h. The glutaraldehyde-activated BC was washed with ultrapure and then dried 50 25 2 h. The glutaraldehyde-activated BC3 times was washed 3 times water with ultrapure wateratand ◦ C for 12 °C fordried 12 h. The attached content after spacer glutaraldehyde activation was determined with then at 50residual h. Thespacer residual attached content after glutaraldehyde activation the above-mentioned ninhydrin method. ninhydrin (4) CoPc@s-BC was(4)harvested by was submerging was determined with the above-mentioned method. CoPc@s-BC harvested the by − 2 −2 glutaraldehyde-activated BC in a 2 × 10BCmol/L and reacted h at 25for °C.2 The submerging the glutaraldehyde-activated in a 2 CoPc × 10 solution mol/L CoPc solutionfor and2 reacted h at ◦ C. The product wasproduct washedwas 3 times with3 dimethylformamide, 3 times with3 ultrapure and then 25 washed times with dimethylformamide, times withwater ultrapure waterdried and ◦ at 50 dried °C forat1250h. The of immobilized CoPc was calculated the cobalttocontent on then C forcontent 12 h. The content of immobilized CoPc wasaccording calculatedtoaccording the cobalt CoPc@s-BC, measured by atomic by absorption spectrometry (Thermo (Thermo solar M6, Thermo Fisher, content on CoPc@s-BC, measured atomic absorption spectrometry solar M6, Thermo Waltham, MA, USA). All of the modification and functionalization processes are schematically shown in Figure 2. The elemental compositions and chemical bonding of BC, oxidized BC, TP-

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Fisher, Waltham, MA, USA). All of the modification and functionalization processes are schematically Appl. Sci. 2018, 8, x FOR PEER REVIEW 4 of 14 shown in Figure 2. The elemental compositions and chemical bonding of BC, oxidized BC, TP-attached BC, glutaraldehyde activated BC and CoPc@TP-BC were analyzed by analyzed X-ray photoelectron spectroscopy attached BC, glutaraldehyde activated BC and CoPc@TP-BC were by X-ray photoelectron (XPS). XPS spectra were recorded on a Kratos Axis Ultra XPS system with Al (mono) Kα spectroscopy (XPS). XPS spectra were recorded on a Kratos Axis Ultra XPS system with irradiation Al (mono) (hν 1486.6 eV).(hν The= binding energy of the XPS spectra by placing the principal Kα =irradiation 1486.6 eV). Thepeaks binding energy peaks were of thecalibrated XPS spectra were calibrated by C 1s binding energy peak 284.6 eV. placing the principal C 1s at binding energy peak at 284.6 eV.

BC

OH (Bacterial cellulose (BC))

H2N

BC

NaIO4

NH2

CHO (Oxidized BC)

OHC

BC

CHO

CH N NH2 (Spacer-attached BC) H2N

BC

CoPc

CH N N CH CHO (Glutaraldehyde activated BC) H2N

NH2 = DT or TT or TP or PH

BC CH

OHC

N N CH (CoPc@s-BC)

CH N

CHO = Glutaraldehyde

N N N N Co N N N N

NH2

NH2

Figure 2. 2. Synthesis Synthesis route route of of spacer-incorporated, spacer-incorporated, tetra-amino tetra-amino cobalt cobalt (II) (II) phthalocyanine phthalocyanine immobilized immobilized Figure bacterial cellulose nanocomposites (CoPc@s-BC). bacterial cellulose nanocomposites (CoPc@s-BC).

2.3. Adsorption and Catalytic Oxidation Decoloration 2.3. Adsorption and Catalytic Oxidation Decoloration The adsorption of reactive red X-3B was carried out in a glass flask sealed in a water bath at 50 The adsorption of reactive red X-3B was carried out in a glass flask sealed in a water bath at 50 ◦ C. °C. One milligram of CoPc@s-BC was added to 5 mL of reactive red X-3B dye wastewater (100 One milligram of CoPc@s-BC was added to 5 mL of reactive red X-3B dye wastewater (100 µmol/L, μmol/L, pH = 2). The catalytic oxidation was initiated by adding 8 mM H2O2 to the CoPc@s-BC pH = 2). The catalytic oxidation was initiated by adding 8 mM H2 O2 to the CoPc@s-BC containing containing the reactive red X-3B solution. At given time intervals, the samples were analyzed the reactive red X-3B solution. At given time intervals, the samples were analyzed immediately on immediately on a UV-vis absorption spectrometer (UV-2450) at the wavelength of maximum a UV-vis absorption spectrometer (UV-2450) at the wavelength of maximum absorbance: 539 nm. absorbance: 539 nm. The decoloration of reactive red X-3B was expressed as the change in the (C0The decoloration of reactive red X-3B was expressed as the change in the (C0 − C)/C0 value, where C)/C0 value, where C0 is the initial concentration of the dye, and C is the residual concentration of the C0 is the initial concentration of the dye, and C is the residual concentration of the dye. The amount of dye. The amount of adsorbed dye was calculated as follows: adsorbed dye was calculated as follows: C -C . Adsorbed dye (mol) = 100 mol / L  5  10-3 L  0 (1) −3 C0 C0 − C Adsorbed dye (µmol) = 100 µmol/L × 5 × 10 L × . (1) C0 The adsorption rate of CoPc@s-BC was derived from the slope of the adsorbed dye-adsorption adsorption rate of CoPc@s-BC was derived the slope ofcalculated the adsorbed dye-adsorption time The curves (within 90 min). The decoloration rate of from CoPc@s-BC was using Formula (2): time curves (within 90 min). The decoloration rate of CoPc@s-BC was calculated using Formula (2): 100 mol / L  5  10-3 L  90% , Decoloration rate (mol  min-1  g -1 ) = (2)  1 µmol/L t min100 10-3 g  CCoPc g− / mol 3 L × 90% × 5 630 × 10 −1 −1 Decoloration rate (µmol × min × g ) = , (2) −3 g × C min × 1red × 10X-3B, × 630g/mol where t is the time taken for decoloration of 90% oft reactive andCoPc CCoPc is the immobilized CoPc content of CoPc@s-BC. The EPR signal of radical spin-trapped by DMPO was detected with a where t is the time taken for decoloration of 90% of reactive red X-3B, and CCoPc is the immobilized Bruker-A300 X-band EPR spectrometer (Bruker, Karlsruhe, Germany). CoPc content of CoPc@s-BC. The EPR signal of radical spin-trapped by DMPO was detected with a Bruker-A300 X-band EPR spectrometer (Bruker, Karlsruhe, Germany). 3. Results and Discussion 3. Results and Discussion 3.1. Preparation and Characterization of CoPc@s-BC 3.1. Preparation Characterization of CoPc@s-BC CoPc@s-BCandnanocomposites were prepared by covalent immobilization of CoPc onto the nanofibers after BC had been oxidized NaIO4; by thiscovalent was spacer-attached withof a series flexible CoPc@s-BC nanocomposites werewith prepared immobilization CoPc of onto the spacers andafter activated with glutaraldehyde. the4 ;functionalization reactionswith werea monitored by Xnanofibers BC had been oxidized with All NaIO this was spacer-attached series of flexible ray photoelectron spectrum (XPS), with the preparation of CoPc@TP-BC as a typical spacers and activated with glutaraldehyde. All the functionalization reactions were representative monitored by (Figure 3), and the chemical compositions of the samples during the series of reactions were calculated and are shown in Table 1. For the as-prepared pure BC, the two characteristic peaks at 284.6 eV and 530.6 eV were ascribed to the binding energies of C 1s and O 1s, respectively (Figure 3a). No significant changes were found after NaIO4 oxidation (Figure 3b). Upon the TP attachment

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X-ray photoelectron spectrum (XPS), with the preparation of CoPc@TP-BC as a typical representative (Figure 3), and the chemical compositions of the samples during the series of reactions were calculated and are shown in Table 1. For the as-prepared pure BC, the two characteristic peaks at 284.6 eV and Appl. Sci. 8, ascribed x FOR PEER 5 of 14 530.6 eV2018, were toREVIEW the binding energies of C 1s and O 1s, respectively (Figure 3a). No significant changes were found after NaIO4 oxidation (Figure 3b). Upon the TP attachment process, an additional process, an 400 additional at ca.which 400 eVcorresponds was detected, corresponds to the characteristic peak at ca. eV waspeak detected, to which the characteristic peak of N 1s, implyingpeak the of N 1s, implying the of successful attachment the TP spacer onto3c). the The oxidized BC (Figureactivation 3c). The successful attachment the TP spacer onto theofoxidized BC (Figure glutaraldehyde glutaraldehyde activation step did obviously change fractions(Figure of C, O 3d andand N elements step did not obviously change the not relative fractions of C,the O relative and N elements Table 1). (Figure 3d and Table 1). After the CoPc functionalization process, a marked increase (from 6.91% to After the CoPc functionalization process, a marked increase (from 6.91% to 19%) of N 1s peak was 19%) of N 1s peak was found (Figure 3e and Table Furthermore, twoatnew at found (Figure 3e and Table 1); Furthermore, two new 1); characteristic peaks 779.6characteristic eV and 795.3 peaks eV were 779.6 eV and 795.3 eV were also detected (Figure 3e, inset). These two peaks were assigned to the also detected (Figure 3e, inset). These two peaks were assigned to the binding energies of Co 2p3/2 binding energies of Co 2p3/2 and Co 2p1/2, respectively. These results verify the success of all of the and Co 2p 1/2 , respectively. These results verify the success of all of the functionalization processes and functionalization processes the immobilization of CoPc on BC. the immobilization of CoPc and on BC. Co 2p 3/2 O 1s

C o 2p 1/2

C 1s

e

N 1s

808 800 792 784 776 Co 2p e d c b a

800

700

600

500

400

300

Binding Energy (eV) Figure 3. 3. XPS XPS spectra spectra of of (a): (a): bacterial bacterial cellulose cellulose (BC); (BC); (b): (b): oxidized oxidized BC; BC; (c): (c): tetraethylenepentamine tetraethylenepentamine Figure (TP)-attached BC; (d): glutaraldehyde-activated BC and (e): CoPc@TP-BC. window included included (TP)-attached BC; (d): glutaraldehyde-activated BC and (e): CoPc@TP-BC. The The window shows, in detail, the Co region of CoPc@TP-BC. shows, in detail, the Co region of CoPc@TP-BC. Table 1. Chemical Chemical compositions compositions of of BC BC during during the the functionalization functionalization processes. processes. Table 1.

BC BC Oxidized BC Oxidized BC TP-attached BC TP-attached BC Glutaraldehyde-activated Glutaraldehyde-activated BCBC CoPc@TP-BC CoPc@TP-BC

C (mol %) C (mol %) 63.13 63.13 65.51 65.51 63.55 63.55 64.53 64.53 72.44 72.44

O (mol %) O (mol %) 36.87 36.87 34.49 34.49 29.61 29.61 28.56 28.56 7.54 7.54

N (mol %) N (mol %) -6.84 6.84 6.91 6.91 19 19

Co (mol %) Co (mol %) ---1.01 1.01

3.2. Optimization of CoPc Immobilization Conditions

real applications, applications,the theoptimum optimumCoPc CoPcimmobilization immobilizationcontent contentonon the substrate is desired For real the BCBC substrate is desired to to enhance catalytic reaction efficiency withoutincreasing increasingthe thecost. cost.The Theimmobilized immobilized CoPc CoPc content enhance itsits catalytic reaction efficiency without directly depends dependsonon amount of attached hence, the condition reaction condition for spacer directly thethe amount of attached spacer;spacer; hence, the reaction for spacer attachment attachment should be carefullyprior monitored prior to CoPc immobilization. should be carefully monitored to CoPc immobilization. different spacers on the BC substrate as a function of reaction The attachment attachmentlevels levelsofof different spacers on oxidized the oxidized BC substrate as a function of time is illustrated in Figure 4. The attached amount of of allall spacers reaction time is illustrated in Figure 4. The attached amount spacersincreased increasedalmost almost directly proportionallywith withtime, time, from h, suggesting that the spacer attachment was constant. initially proportionally from 0 to0 4to h, 4suggesting that the spacer attachment rate wasrate initially constant. It is interesting that there appears be a directbetween correlation between the attachment level It is interesting that there appears to be a directtocorrelation the attachment level and the spacer and the spacer length, with longer spacers having lower amounts of spacer attached. The difference in attachment level increased with time. After reacting for 10 h, the attached amounts of TT, TP, and PH spacers were ~2.2 mmol/g, ~2 mmol/g and ~1.7 mmol/g, which were ca. 10%, 15% and 30% lower than that of the DT spacer (~2.4 mmol/g), respectively. Further prolongation of the reaction time did not consistently increase the attached spacer content on oxidized BC. Considering both the

Our previous work illustrated that under optimum oxidation conditions, oxidized BC contains 14.13% (wt./wt.) of aldehyde groups (ca. 5 mmol/g, slightly lower than the theoretical maximum content of 18.1% (wt./wt.)) [25], while the amount of attached spacer was lower than 2.5 mmol/g; that is, less than half of the aldehyde groups were effectively utilized for the attachment of spacers. The Appl. Sci. 2018, 8, 1021 6 of 14 relatively low-efficiency usage of aldehyde groups can be ascribed to two reasons. The first reason is the difficulty of producing heterogeneous reactions and the incompleteness characteristic of macromolecular reactions. The second may be attributed, least partly, intoattachment the steric length, with longer spacers having lower reason amounts of spacer attached. at The difference hindrance effect; that is, when amount spaceramounts increasedoftoTT, a certain the already level increased with time. Afterthe reacting for of 10attached h, the attached TP, andlevel, PH spacers were fixed spacers prevented further progress of the attachment reaction. It is reasonable to assume ~2.2 mmol/g, ~2 mmol/g and ~1.7 mmol/g, which were ca. 10%, 15% and 30% lower than that ofthat the the spacer most obvious steric hindrance effect arisesprolongation from PH, i.e., longest spacer. the above DT (~2.4 mmol/g), respectively. Further of the the reaction time didGiven not consistently factors, the levelscontent for all of spacersBC. were much smaller theoretical value, increase theattachment attached spacer onthe oxidized Considering boththan the the attachment level andand the the PH spacer had the lowest attached content. attachment efficiency, the optimum attachment time was set to 10 h.

Attached spacer (mmol/g)

2.5

DT TT TP PH

2.0 1.5 1.0 0.5 0.0 0

2

4

6 8 Reaction time (h)

10

12

4. Effect of the the reaction reaction time time on on the the amount amount of of attached attached spacer spacer on on the the oxidized oxidized BC, BC, TT == 30 30 ◦°C Figure 4. C ◦ (40 °CCfor forPH), PH), initial initial spacer spacer concentration concentration ==8% 8%((v/v). v/v).

The previous spacer-attached BC substrates wereoptimum further oxidation activated conditions, with glutaraldehyde, classic Our work illustrated that under oxidized BC acontains bifunctional chemical crosslinker, for the immobilization of the CoPc catalyst. The immobilized CoPc 14.13% (wt./wt.) of aldehyde groups (ca. 5 mmol/g, slightly lower than the theoretical maximum contentsof were affected by many as the contents of the attached and the content 18.1% (wt./wt.)) [25],factors, while such the amount of attached spacer wasspacers lower than 2.5activation mmol/g; levels. Figure 5a demonstrates the influence of initial glutaraldehyde concentration on the that is, less than half of the aldehyde groups were effectively utilized for the attachment of spacers. immobilized CoPc content for DT-attached BC. The immobilized CoPc content increased in The relatively low-efficiency usage of aldehyde groups can be ascribed to two reasons. The first association the increase of glutaraldehyde concentration. is reasonable to assume that with reason is thewith difficulty of producing heterogeneous reactions andItthe incompleteness characteristic of sufficient activation reagents, more binding sites were provided for CoPc immobilization. macromolecular reactions. The second reason may be attributed, at least partly, to the steric hindrance Meanwhile, amount of DT spacer decreased drastically, which statistically represents the effect; that is, the when the amount of attached spacer increased to a certain level, the already fixed spacers successfully activation of glutaraldehyde for CoPc immobilization. Two percent glutaraldehyde was prevented further progress of the attachment reaction. It is reasonable to assume that the most obvious required for the effect maximum CoPc; spacer. accordingly, of immobilized CoPc steric hindrance arises immobilization from PH, i.e., theoflongest Giventhe thecontent above factors, the attachment was asfor high μmol/g, and the residual DT spacer was 0.38 value, mmol/g. A the further increase in the the levels all as of ca. the550 spacers were much smaller than the theoretical and PH spacer had activation reagent did not lead to the disappearance of the DT spacer (even when the glutaraldehyde lowest attached content. concentration reached 5%),BC which may bewere ascribed to the incompleteness of the macromolecular The spacer-attached substrates further activated with glutaraldehyde, a classic reaction. Similar results were found for the immobilization of CoPc onto TT-attached BC (Figure 5b), bifunctional chemical crosslinker, for the immobilization of the CoPc catalyst. The immobilized TP-attached BC (Figure 5c) and PH-attached BC (Figure 5d). CoPc contents were affected by many factors, such as the contents of the attached spacers and the activation levels. Figure 5a demonstrates the influence of initial glutaraldehyde concentration on the immobilized CoPc content for DT-attached BC. The immobilized CoPc content increased in association with the increase of glutaraldehyde concentration. It is reasonable to assume that with sufficient activation reagents, more binding sites were provided for CoPc immobilization. Meanwhile, the amount of DT spacer decreased drastically, which statistically represents the successfully activation of glutaraldehyde for CoPc immobilization. Two percent glutaraldehyde was required for the maximum immobilization of CoPc; accordingly, the content of immobilized CoPc was as high as ca. 550 µmol/g, and the residual DT spacer was 0.38 mmol/g. A further increase in the activation reagent did not lead to the disappearance of the DT spacer (even when the glutaraldehyde concentration reached 5%), which may be ascribed to the incompleteness of the macromolecular reaction. Similar results were

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found the of CoPc Appl. Sci.for 2018, 8, immobilization x FOR PEER REVIEW

onto TT-attached BC (Figure 5b), TP-attached BC (Figure 5c) and 7 of 14

PH-attached BC (Figure 5d).

400 1.5

Immobilized CoPc

300

Residual DT content

1.0

200 100

0.5

Immobilized CoPc (mol/g)

2.0

Residual DT content (mmol/g)

0

500

2.0

400

Residual TT content

1.0

200 100

0.5

0

0

1

2 3 Concentration (v/v, %)

4

5

0.0 0

1

(a)

4

5

600

2.0

400 1.5

Immobilized CoPc Residual TP content

1.0

200 100

0.5


500

Residual TP content (mmol/g)

2.5

300


0.0

(b)

600


1.5

Immobilized CoPc

300

2.5

500

2.0

400 1.5

Immobilized CoPc

300

Residual PH content

1.0

200 100

0.5

Residual PH content (mmol/g)


500

2.5

600

Residual TT content (mmol/g)

2.5

600

0

0 0

1


(c)

4

5

0.0

0

1


4

5

0.0

(d)

Figure Figure 5. 5. Effect Effect of of the the glutaraldehyde glutaraldehyde concentration concentrationon on the the immobilized immobilized CoPc CoPc and and residual residual spacer spacer content ) DT-attached ) TT-attached BC; (c) (c) TP-attached BC;BC; andand (d) PH-attached BC, TBC, = contentfor for(a(a) DT-attachedBC; BC;(b(b) TT-attached BC; TP-attached (d) PH-attached ◦ 25 °C, reaction time = 2 h. T = 25 C, reaction time = 2 h.

3.3. Adsorption and Catalytic Oxidation Performance of CoPc@s-BC 3.3. Adsorption and Catalytic Oxidation Performance of CoPc@s-BC The prepared series of CoPc@s-BC were aimed at being functional nanocomposites for the The prepared series of CoPc@s-BC were aimed at being functional nanocomposites for the decoloration of dye wastewater. The total decoloration includes the adsorption of dye molecules and decoloration of dye wastewater. The total decoloration includes the adsorption of dye molecules the subsequent catalytic oxidation process [40]. The performances of the prepared CoPc@s-BC and the subsequent catalytic oxidation process [40]. The performances of the prepared CoPc@s-BC nanocomposites were firstly evaluated based on their adsorption behavior towards reactive red X-3B nanocomposites were firstly evaluated based on their adsorption behavior towards reactive red X-3B dye wastewater (Figure 6). For pure BC, only 0.05 μmol of the dye molecules were adsorbed after a dye wastewater (Figure 6). For pure BC, only 0.05 µmol of the dye molecules were adsorbed after a dynamic equilibrium, reached in 240 min. CoPc@BC had a much higher adsorption capacity than dynamic equilibrium, reached in 240 min. CoPc@BC had a much higher adsorption capacity than pure BC—0.2 μmol of dye was adsorbed by CoPc@BC under the same experimental conditions. The pure BC—0.2 µmol of dye was adsorbed by CoPc@BC under the same experimental conditions. adsorption capacities of all the spacer-attached nanocomposites were higher when compared with The adsorption capacities of all the spacer-attached nanocomposites were higher when compared with CoPc@BC. After dynamic equilibrium was reached, the amounts of the adsorbed dye by CoPc@DTCoPc@BC. After dynamic equilibrium was reached, the amounts of the adsorbed dye by CoPc@DT-BC, BC, CoPc@TT-BC, CoPc@TP-BC and CoPc@PH-BC were 37%, 54%, 67% and 17% higher than that of CoPc@TT-BC, CoPc@TP-BC and CoPc@PH-BC were 37%, 54%, 67% and 17% higher than that of CoPc@BC, respectively. As illustrated in Figure 5, the contents of immobilized CoPc for CoPc@DTCoPc@BC, respectively. As illustrated in Figure 5, the contents of immobilized CoPc for CoPc@DT-BC, BC, CoPc@TT-BC, CoPc@TP-BC and CoPc@PH-BC were 27%, 21%, 15% and 7% higher than that of CoPc@TT-BC, CoPc@TP-BC and CoPc@PH-BC were 27%, 21%, 15% and 7% higher than that of CoPc@BC, respectively. By comparing these statistics, it is easy to deduce that the enhancement of CoPc@BC, respectively. By comparing these statistics, it is easy to deduce that the enhancement of adsorption capacity was mainly caused by the incorporation of spacers. The incorporation of spacers adsorption capacity was mainly caused by the incorporation of spacers. The incorporation of spacers caused CoPc to immobilize onto BC in a more dispersed way, which effectively increased the number caused CoPc to immobilize onto BC in a more dispersed way, which effectively increased the number of contact opportunities between CoPc and the dye molecules; furthermore, the diffusion limitation of contact opportunities between CoPc and the dye molecules; furthermore, the diffusion limitation of of dye molecules to CoPc was effectively reduced. These effects were correlated with the spacer dye molecules to CoPc was effectively reduced. These effects were correlated with the spacer length. length. CoPc@TP-BC has a longer spacer than CoPc@DT-BC and CoPc@TT-BC, and its adsorption CoPc@TP-BC has a longer spacer than CoPc@DT-BC and CoPc@TT-BC, and its adsorption capacity was capacity was much higher. However, it is possible that when the spacer was too long, a CoPc might have attached onto BC through multiple-bonding, and the peripheral amino groups on CoPc (which was necessary for proton acceptance) [40] may have been excessively consumed; thus the adsorption capacity may have been reduced to some extent. The CoPc@PH-BC has the longest incorporated spacer, while its adsorption capacity was relatively lower than others.

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much higher. However, it is possible that when the spacer was too long, a CoPc might have attached onto BC through multiple-bonding, and the peripheral amino groups on CoPc (which was necessary for proton acceptance) [40] may have been excessively consumed; thus the adsorption capacity may have been reduced to some extent. The CoPc@PH-BC has the longest incorporated spacer, while its adsorption was REVIEW relatively lower than others. Appl. Sci. 2018,capacity 8, x FOR PEER 8 of 14 0.5

BC CoPc@BC CoPc@DT-BC CoPc@TT-BC CoPc@TP-BC CoPc@PH-BC

Adsorbed dye (mol)

0.4

0.3

0.2

0.1

0.0

0

50

100 150 Adsorption time (min)

200

250

◦ C) Figure 6. Adsorption concentration 100 100 µM, μM, pH pH == 2, T = 50 °C) Adsorption of reactive red X-3B wastewater (initial concentration with CoPc@s-BC (1 mg).

To determine the adsorption rate of CoPc@s-BC, the adsorption curves in Figure 6 were To determine the adsorption rate of CoPc@s-BC, the adsorption curves in Figure 6 were regressed regressed using a linear function (within 90 min); the parameters, including the slope and correlation using a linear function (within 90 min); the parameters, including the slope and correlation ratio, are ratio, are presented in Table 2. The amount of adsorbed dye as a function of adsorption time showed presented in Table 2. The amount of adsorbed dye as a function of adsorption time showed a linear a linear trend; all correlation ratios (r) for the CoPc@s-BC nanocomposites were >0.99. The trend; all correlation ratios (r) for the CoPc@s-BC nanocomposites were >0.99. The incorporation incorporation of spacers greatly facilitated the contact between the CoPc and the dye molecules—all of spacers greatly facilitated the contact between the CoPc and the dye molecules—all of the of the CoPc@s-BC had higher adsorption rates than CoPc@BC. The adsorption rate of CoPc@TP-BC CoPc@s-BC had higher adsorption rates than CoPc@BC. The adsorption rate of CoPc@TP-BC reached reached 0.385 μmol/min, which was more than two times higher than that of CoPc@BC (0.172 0.385 µmol/min, which was more than two times higher than that of CoPc@BC (0.172 µmol/min). μmol/min). Table 2. Slope and correlation ratios regressed from the adsorbed dye-adsorption time curves. Table 2. Slope and correlation ratios regressed from the adsorbed dye-adsorption time curves. Sample Sample BC BC CoPc@BC CoPc@BC CoPc@DT-BC CoPc@DT-BC CoPc@TT-BC CoPc@TT-BC CoPc@TP-BC CoPc@PH-BC CoPc@TP-BC

CoPc@PH-BC

Slope Correlation Correlation Ratio Slope Ratio 0.0563 0.9668 0.0563 0.9668 0.1723 0.9946 0.1723 0.9946 0.2453 0.9921 0.2453 0.9921 0.3466 0.9955 0.3466 0.9955 0.3852 0.9942 0.2240 0.9985 0.3852 0.9942 0.2240 0.9985

One of the main concerns for this study was to evaluate the effects of the spacers, especially the One of theonmain concernsactivity for thisofstudy was catalyst. to evaluate effects of the spacers, spacer length, the catalytic the CoPc All the series of CoPc@s-BC wereespecially employed the for spacer length, on the catalytic activity of the CoPc catalyst. All series of CoPc@s-BC were employed the catalytic oxidation of reactive red X-3B dye wastewater, with H2 O2 as an oxidant. The investigation for theinfluence catalyticofoxidation reactive X-3B dye withresults H2O2 to asthat an of oxidant. The of the spacers onofthe catalyticred oxidation ratewastewater, revealed similar adsorption investigation of the influence of spacers on the catalytic oxidation rate revealed similar results to that behavior. A control experiment was firstly conducted. H2 O2 alone showed almost no decoloration of adsorption behavior. experiment conducted. H2O2 alone showed almost activity—it was difficultAtocontrol decompose reactivewas redfirstly X-3B with H2 O2 without CoPc@s-BC (Figureno 7). decoloration activity—it was difficult to decompose reactive red X-3B with H 2O2 without CoPc@s-BC All of the prepared CoPc@s-BC nanocomposites are able to catalytic decolorize dye wastewater, and (Figure 7). All ofrate the was prepared CoPc@s-BC nanocomposites are able7,toinset). catalytic dye the decoloration correlation with the spacer length (Figure The decolorize typical optical wastewater, and the decoloration rate was correlation with the spacer length (Figure 7, inset). The image changes in reactive red X-3B solution during the decoloration process are shown in Figure 8. typical optical image changes in reactive red X-3B solution during the decoloration process are shown in Figure 8. For CoPc@BC, the decoloration rate was ca. 27 μmol·min−1·g−1. With the incorporation of the spacer, enhanced catalytic activity of CoPc catalytst was observed. Under the same reaction conditions, the decoloration rate of CoPc@DT-BC was ca. 8% higher than that of CoPc@BC. Much higher catalytic activity of CoPc was obtained with the incorporation of a longer spacer length. The

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1.0 1.0

CoPc@BC CoPc@BC CoPc@DT-BC CoPc@DT-BC CoPc@TT-BC CoPc@TT-BC CoPc@TP-BC CoPc@TP-BC CoPc@PH-BC CoPc@PH-BC H H22O O22

(C-0C)/C -C)/C0 (C 0 0

0.8 0.8 0.6 0.6

45 45

CoPc@TP-BC CoPc@TP-BC

40 40

0.4 0.4

35 35

CoPc@TT-BC CoPc@TT-BC

CoPc@PH-BC CoPc@PH-BC CoPc@DT-BC CoPc@DT-BC 25 25 CoPc@BC CoPc@BC 30 30

0.2 0.2 0.0 0.0

-1-1gg-1-1) ) Delorationrate rate ((mol molmin min Deloration

For CoPc@BC, the decoloration rate was ca. 27 µmol·min−1 ·g−1 . With the incorporation of the spacer, enhanced catalytic activity of CoPc catalytst was observed. Under the same reaction conditions, the decoloration rate of CoPc@DT-BC was ca. 8% higher than that of CoPc@BC. Much higher catalytic activity of CoPc was obtained with the incorporation of a longer spacer length. The decoloration ratesSci. of CoPc@TT-BC and CoPc@TP-BC were ca. 33 µmol·min−1 ·g−1 and 41 µmol·min−1 ·g−1 , which Appl. 99 of Appl. Sci. 2018, 2018, 8, 8, xx FOR FOR PEER PEER REVIEW REVIEW of 14 14 were more than 20% and 50% higher than that of CoPc@BC, respectively. This result is in good accordance with our principle idea—thatdistance the incorporation of a flexible spacer is ablesome to keep the to to keep keep the the CoPc CoPc catalyst catalyst at at aa preferred preferred distance from from the the solid solid substrate, substrate, which, which, to to some extent, extent, CoPc catalyst at a preferred distance from the solid substrate, which, to some extent, mitigates the mitigates mitigates the the adverse adverse effects effects of of diffusion diffusion limitation limitation and and steric steric hindrance, hindrance, increases increases the the reaction reaction adverse effects of enhances diffusion the limitation and steric hindrance, increases the reaction homogeneity and homogeneity and accessibility and mobility of the CoPc catalyst. These effects are more homogeneity and enhances the accessibility and mobility of the CoPc catalyst. These effects are more enhances the accessibility and mobility of the CoPc catalyst. Thesespacer effects are more likely to occur with likely likely to to occur occur with with longer longer spacer spacer lengths. lengths. However, However, when when the the spacer is is too too long, long, itit is is possible possible that that longer spacer lengths.covalently However, when the spacer is toomultiple-bonding, long, it is possible which that therestricts CoPc molecules the CoPc molecules immobilize through the CoPc molecules covalently immobilize through multiple-bonding, which restricts the the free free covalently immobilize through multiple-bonding, which restricts the freeextent. movementdecoloration of the CoPc and movement movement of of the the CoPc CoPc and and thus, thus, decreases decreases its its catalytic catalytic activity activity to to some some extent. The The decoloration rate rate −1 −1 thus,CoPc@PH-BC decreases its was catalytic activity to some extent. The decoloration rate for CoPc@PH-BC was ca. for for CoPc@PH-BC was ca. ca. 33 33 μmol·min μmol·min−1·g ·g−1,, which which was was higher higher than than CoPc@BC CoPc@BC but but lower lower than than − 1 − 1 33 µmol·min ·g , which was higher than CoPc@BC but lower than CoPc@TP-BC. CoPc@TP-BC. CoPc@TP-BC.

00

10 10

20 30 40 20 30 40 Reaction time (min) Reaction time (min)

50 50

60 60

Figure red X-3B (initial concentration 100 μM, pH == 2, TT =2, ◦ C) Figure 7. Catalytic Catalyticoxidation oxidationof reactive red X-3B (initial concentration µM, T °C) = 50with Figure 7. Catalytic oxidation ofofreactive reactive red X-3B (initial concentration 100100 μM, pHpH 2,= = 50 50 °C) with 2O2 (8 mM). Inset: effect of the spacer length on the decoloration rate of CoPc@s-BC (1 mg) and H with CoPc@s-BC (1 mg) O2mM). (8 mM). Inset: effect spacerlength lengthon onthe thedecoloration decoloration rate of Inset: effect of of thethe spacer of CoPc@s-BC (1 mg) andand H2OH2 2(8 reactive X-3B. reactive red reactive red red X-3B. X-3B.

Figure optical image changes of reactive red X-3B (initial concentration 100 μM, pH Figure 8. 8. Typical Typicaloptical opticalimage imagechanges changesof ofreactive reactivered redX-3B X-3B(initial (initialconcentration concentration 100 μM, pH = 2, 2, T Figure 8. Typical 100 µM, pH = =2, TT = 2O2 (8 mM). =50 50 °C) with CoPc@s-BC (1 mg) and H ◦ with CoPc@s-BC (1(1 mg) and HHO2O2(8(8mM). mM). = 50C) °C) with CoPc@s-BC mg) and 2

2

To sum up, the most preferable spacer length achieve the maximum decoloration efficiency To sum sumup, up,the themost mostpreferable preferablespacer spacerlength lengthtoto to achieve maximum decoloration efficiency To achieve thethe maximum decoloration efficiency of of CoPc@s-BC was derived from the TP spacer. Under optimum immobilization conditions, 2 mmol/g of CoPc@s-BC was derived from the TP spacer. Under optimum immobilization conditions, 2 mmol/g CoPc@s-BC was derived from the TP spacer. Under optimum immobilization conditions, 2 mmol/g of TP together with 495 μmol/g immobilized decoloration of attached attached TP TP together together with with 495 495 µmol/g μmol/g of of immobilized CoPc CoPc were were obtained, obtained, and and the the decoloration decoloration of attached of immobilized CoPc were obtained, and the −1·g−1. These results are comparable with rate for the resulting CoPc@TP-BC reached 41 μmol·min rate for the resulting CoPc@TP-BC reached 41 μmol·min−1·g−1. These results are comparable with related related literature. literature. Firstly, Firstly, flexible flexible spacer-attached spacer-attached BCs BCs were were employed employed to to support support the the CoPc CoPc catalyst, catalyst, which permitted a much higher amount of immobilized CoPc than other materials [44,45]. which permitted a much higher amount of immobilized CoPc than other materials [44,45]. Secondly, Secondly, the the incorporation incorporation of of the the spacers spacers increased increased both both the the adsorption adsorption rate rate and and the the adsorption adsorption capacity capacity of of the the nanocomposites [46]. Moreover, the incorporation of the spacers (especially for TP) effectively nanocomposites [46]. Moreover, the incorporation of the spacers (especially for TP) effectively increased the accessibility and reaction homogeneity of the CoPc catalyst, and thus, its catalytic

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rate for the resulting CoPc@TP-BC reached 41 µmol·min−1 ·g−1 . These results are comparable with related literature. Firstly, flexible spacer-attached BCs were employed to support the CoPc catalyst, which permitted a much higher amount of immobilized CoPc than other materials [44,45]. Secondly, the incorporation of the spacers increased both the adsorption rate and the adsorption capacity of the nanocomposites [46]. Moreover, the incorporation of the spacers (especially for TP) effectively increased the accessibility and reaction homogeneity of the CoPc catalyst, and thus, its catalytic activity was greatly improved compared to that without the spacer [23]. Appl. Sci. 2018, 8, xspin-trapping FOR PEER REVIEW of 14 The EPR technique is a powerful tool for the detection of short-lived, 10 active species. Herein, the 5,5-dimethyl-1-pyrroline-N-oxide (DMPO)-trapped EPR spectra were employed to to demonstrate demonstrate the the formation formation of of radicals radicals during during the the catalytic catalytic oxidation oxidation of of reactive reactive red red X-3B X-3B with with the the CoPc@s-BC (CoPc@BC)/H 2O2 reaction system (Figure 9). Without the existence of H2O2, no obvious CoPc@s-BC (CoPc@BC)/H2 O2 reaction system (Figure 9). Without the existence of H2 O2 , no obvious EPR detected. When both CoPc@BC CoPc@BC and H2O 2 were added, a strong, four-line spectrum EPR signal signal was was detected. When both and H 2 O2 were added, a strong, four-line spectrum with 1:2:2:1 was easily detected, which is theistypical characteristic spectrum of the with aapeak peakintensity intensityofof 1:2:2:1 was easily detected, which the typical characteristic spectrum DMPO-·OH adducts [47]. This result indicates that ·OH was generated during the reaction and was of the DMPO-·OH adducts [47]. This result indicates that ·OH was generated during the reaction responsible for the catalytic oxidation of reactive red X-3B. Similar results were found for the CoPc@sand was responsible for the catalytic oxidation of reactive red X-3B. Similar results were found for BC/H 2O2 reaction system; the only differences were the peak intensities, which were proportional to the CoPc@s-BC/H 2 O2 reaction system; the only differences were the peak intensities, which were the amount during the reaction. amount of ·OH was proportionalof tothe the formed amount ·OH of theradical formed ·OH radical duringAn the obvious reaction.higher An obvious higher amount observed for the CoPc@TP-BC/H 2O2 reaction system. Therefore, the CoPc@TP-BC had the highest of ·OH was observed for the CoPc@TP-BC/H2 O2 reaction system. Therefore, the CoPc@TP-BC had catalytic activity foractivity the decoloration of reactive red X-3B dye dye wastewater, which the highest catalytic for the decoloration of reactive red X-3B wastewater, whichisisinin good good accordance with the results illustrated in Figure 7. accordance with the results illustrated in Figure 7. CoPc@BC+H2O2 CoPc@DT-BC+H2O2 CoPc@TT-BC+H2O2 CoPc@PH-BC+H2O2 3501

3502

3503

CoPc@TP-BC+H2O2

without H2O2

3480

3495

3510 3525 Magnetic field (G)

3540

Figure 9. 5,5-dimethyl-1-pyrroline-N-oxide 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) (DMPO) spin-trapping spin-trapping electron paramagnetic resonance ◦ C). (EPR) spectra of reactive red X-3B dye wastewater (initial concentration concentration 100 100 μM, µM, pH == 2, 2, T T == 50 50 °C). The inset shows the magnification of the marked region. region.

Based Based on on the the above above experimental experimental results, results, aa possible possible catalytic catalytic reaction reaction mechanism mechanism is is proposed proposed in in Figure 10. Firstly, the reactive red X-3B dye molecules were adsorbed onto the nanocomposites from Figure 10. Firstly, the reactive red X-3B dye molecules were adsorbed onto the nanocomposites from solution. solution. Thanks Thanks to to the the incorporation incorporation of of flexible flexible spacers, spacers, the the contact contact between between dye dye molecules molecules and and the the CoPc pendants became easier; therefore, both the adsorption capacity and the adsorption rate were CoPc pendants became easier; therefore, both the adsorption capacity and the adsorption rate were enhanced catalytically decolorized enhanced by by the the spacers. spacers. Secondly, Secondly, the the adsorbed adsorbed dyes dyes were were catalytically decolorized in-situ in-situ by by the the ·OH radical. Due to the presence of spacers, the free movement and the reaction homogeneity of ·OH radical. Due to the presence of spacers, the free movement and the reaction homogeneity of CoPc CoPc on on CoPc@s-BC CoPc@s-BC were were improved; improved; thus, thus, more more ·OH ·OH is is produced produced during during the the oxidation oxidation of of dye dye molecules. molecules. These two processes cooperate in a synergistic manner and effective enhancement of the catalytic efficiency of CoPc can be reasonably expected. The decoloration was significantly associated with the spacer length of CoPc@s-BC. The most preferable spacer length arose from TP; thus, CoPc@TP-BC has the highest catalytic efficiency.

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These two processes cooperate in a synergistic manner and effective enhancement of the catalytic efficiency of CoPc can be reasonably expected. The decoloration was significantly associated with the spacer length of CoPc@s-BC. The most preferable spacer length arose from TP; thus, CoPc@TP-BC has Appl.highest Sci. 2018,catalytic 8, x FOR PEER REVIEW 11 of 14 the efficiency.

Figure Figure10. 10.Possible Possiblepathway pathwayfor forthe thecatalytic catalyticoxidation oxidationof ofreactive reactivered redX-3B X-3Bby byCoPc@s-BC/H CoPc@s-BC/H22O22..

4. Conclusions

BCs attached with varying lengths of flexible spacers were prepared for the the immobilization immobilization of CoPc and and the fabrication novel, spacer-incorporated, CoPc-immobilized CoPc@s-BC CoPc the fabrication of novel,of spacer-incorporated, CoPc-immobilized CoPc@s-BC nanocomposites. nanocomposites. These nanocomposites are promising materials for theof decoloration of reactive dye These nanocomposites are promising materials for the decoloration reactive dye wastewater. wastewater. The incorporation of spacers effectively the efficiency decoloration efficiency ofThe CoPc@sThe incorporation of spacers effectively increased theincreased decoloration of CoPc@s-BC. most BC. The most preferable length forboth optimizing both the capacity adsorption capacity and theoxidation catalytic preferable spacer length spacer for optimizing the adsorption and the catalytic oxidation was efficiency was derived from the TP spacer. With the incorporation of TP, the capacity adsorption efficiency derived from the TP spacer. With the incorporation of TP, the adsorption of capacity of CoPc@TP-BC was 67% that of and CoPc@BC, and its adsorption rateby increased by CoPc@TP-BC was 67% higher thanhigher that ofthan CoPc@BC, its adsorption rate increased more than moretimes. than two times. The CoPc@TP-BC able todecolorize efficiently reactive decolorize dye wastewater two The CoPc@TP-BC was able towas efficiently dyereactive wastewater with H2 O2 −1μmol·min −1·g−1, ca. 50% higher with 2O2 as an oxidant, and the decoloration high·min as 41 as an H oxidant, and the decoloration rate was as rate highwas as 41asµmol ·g−1 , ca. 50% higher than that than that of EPR CoPc@BC. EPR spin-trapping that highly ·OHfor is of CoPc@BC. spin-trapping experiments experiments revealed that revealed highly reactive ·OH isreactive responsible responsible the catalytic oxidation reaction. Theof incorporation of spacers boosts of the·OH formation of the catalyticfor oxidation reaction. The incorporation spacers boosts the formation and thus ·OH and thus increases the catalytic activity of CoPc@s-BC. increases the catalytic activity of CoPc@s-BC. Author Author Contributions: Contributions: S.C. conceived conceived the study, study, designed designed the the experiments, experiments, analyzed the data and wrote the paper. Q.T. performed the experiments. W.X. designed the experiments paper. Q.T. performed the experiments. W.X. designed the experimentsand andanalyzed analyzedthe thedata. data. Funding: This work was supported by the Zhejiang Provincial Natural Science Foundation of China (Grant no. Funding: This work was supported by the Zhejiang Provincial Natural Science Foundation of China (Grant no. LQ15E030005), Innovation Training Plan of Zhejiang Province for University Students (Xinmiao Talent Programme LQ15E030005), Innovation Training Plan of Zhejiang Province for University Students (Xinmiao Talent 2017R423028) and Research Programme of Qianjiang College (2018QJJL04). Programme 2017R423028) and Research Programme of Qianjiang College (2018QJJL04). Conflicts of Interest: The authors declare no conflict of interest. Conflicts of Interest: The authors declare no conflict of interest.

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