Degradation of Methylene Blue with a Cu(II)-Quinoline Complex Immobilized on a Silica Support as a Photo-Fenton-Like Catalyst.

A Cu(II)-quinoline complex immobilized on a silica support was prepared to enhance the degradation of dyes. Mesoporous silica functionalized with this Cu(II) complex was turned into a photo-Fenton-like catalyst. Various techniques were used to characterize the resulting material, and the catalytic activity was determined by the degradation of methylene blue (MB) under UV light irradiation. The Cu(II) ion was successfully coordinated to the quinoline ligand on a silica support. The dye degradation investigation has shown that 95% of the dye was degraded in 2.5 h. The active radical species involved in the reaction were OH• and O2 •-, suggesting that a peroxo complex intermediate might be formed during degradation processes.

Introduction

A dye is usually an organic or inorganic compound giving color to substrates such as food, fabric, textile cloth, paper, or plastic.[1] The worldwide production of dyes is over 700,000 tons per year.[2−4] About 1–20% of the total world production of dyes is lost during the dyeing process and is released in the textile effluents.[5−8] The washed-out dyes can potentially contaminate natural water and the ecosystem. In recent years, advanced oxidation processes (AOPs) have been used for wastewater treatment. The in situ generation of HO• is the basic principle of this process.[9,10] One promising AOP is the Fenton reaction. The Fenton reaction uses iron ions (Fe2+/Fe3+) as the catalysts in the presence of H2O2. It was reported that the Fenton reaction has a high performance and simple method of operation at room temperature.[11,12] However, a limitation of the Fenton reaction is the operational pH range of 2–4.[13] To overcome this disadvantage, an improvement of the Fenton reaction has been investigated. Copper ions have been reported as an alternative for iron ions in the Fenton reaction having the advantage of working over a broader pH range (3–7).[14] In addition, the coordination of copper ions with organic acids, pyridine, amino acids, and chelating agents in solution systems has been shown to enhance the generation of HO•.[15−17] Furthermore, extra reactions of the Cu-based Fenton catalyst can occur with photo assistance. For example, light can induce the decrease of the oxidation number of Cu(II)–Cu(I) through ligand to metal charge transfer (LMCT), which is followed by the generation of HO• for the decomposition of substrates.[18] The aim of this research is to investigate a new photo-Fenton-like catalyst. Inspired by copper complexes with organic ligands, quinoline was used as a ligand to form a complex with a Cu(II) ion and immobilized onto a modified silica for the decomposition of methylene blue (MB). The obtained catalyst was investigated by spectroscopic techniques and morphology methods. The catalytic performance of the prepared catalyst was determined from studies of MB degradation under the selected experimental conditions, including catalyst dosage, H2O2 concentration, time, and the UV light irradiation effect.

Results and Discussion

Catalyst Synthesis and Characterization

The SiO2 used in this work was amorphous silica, showing a broad peak at 22° (Figure S1) in its X-ray diffraction (XRD) spectra.[19] After the surface modifications, only characteristic peaks in the infrared (IR) region of silica were observed (Figure S2) in the FTIR spectra. This is due to the fact that the concentration of (3-aminopropyl) triethoxysilane (APTES) and the Cu(II)–quinoline complex was too low when compared with the amount of silica.[20] The SiO2 was spherical with different degrees of aggregation and dimensions smaller than 100 nm (Figure ) as the reaction was performed with a high concentration of water (14 mol L–1). In addition, the concentration of ammonia was relatively low (1.4 mol L–1) to inhibit the aggregation of the nascent silica particles.[21] No change in morphology of the silica particles was observed as a result of the modifications and only more extensive aggregation of the silica particles was observed.

Figure 1

SEM images of (a) SiO2, (b) SiO2–NH2, and (c) SiO2–Cu(II)–quinoline complexes.

The elemental mapping performed by energy dispersive X-ray spectroscopy (EDX) demonstrated that all elements in the catalyst were uniformly distributed on the silica surface (Figure ). Two peaks of Cu at 0.930 and 8.040 keV provide key evidence that there is Cu present in the silica (Figure ). The total Cu content was found to be 0.6 wt % in the selected area.

Figure 2

EDX mapping of the SiO2–Cu(II)–quinoline complex.

Figure 3

EDX spectra of the SiO2–Cu(II)–quinoline complex.

Adsorption–desorption isotherm for the different materials was measured and is shown in Figure . The specific surface area (SBET) was calculated using the Brunauer–Emmett–Teller (BET) equation. As shown in Table , the SBET of SiO2 (527.3 m2 g–1) decreased to 216.4 and 103.6 m2 g–1 in SiO2–NH2 and SiO2–Cu(II)–quinoline complexes, respectively. Pore volume (Vpmeso) and pore size distributions were obtained by the Barrett–Joyner–Halenda (BJH) model. The values of these parameters also decrease and remain in the characteristic ranges of mesoporous materials.[22]

Figure 4

N2 adsorption–desorption isotherms of SiO2, SiO2–NH2, and SiO2–Cu(II) complexes.

Table 1

Textural Properties of SiO2, SiO2–NH2, and SiO2–Cu(II) Complexes

sample	SBET (m2 g–1)	Vpmeso (cm3 g–1)	pore size distribution (nm)
SiO2	527.3	0.41	3.36
SiO2–NH2	216.4	0.16	3.23
SiO2–Cu(II)–quinoline complex	103.6	0.050	2.97

X-ray photoelectron spectroscopy (XPS) spectra of SiO2–quinoline before and after loading with Cu(II) are shown in Figure a. Peaks of Si 2p, C 1s, N 1s, and O 1s can be observed at bonding energy (BE) values of 103, 285, 399, and 532 eV, respectively. The comparison of the survey spectra of SiO2–quinoline before and after loading Cu(II) ions indicates the presence of the Cu element. A new peak of Cu(II) was observed at a binding energy of 935 eV. The value was assigned to the Cu 2p orbital. The strong Cu 2p1/2 and Cu 2p3/2 peaks at 952.1 and 932.3 eV were in an agreement with the oxidation state +2 of Cu 2p (Figure b). As shown in Figure c,d, the high-resolution of the N 1s spectrum fitted with deconvolution peaks of N element reveals peaks at 399.1 eV (primary amines) and 400.1 eV (imines).[23−25] These binding energies (BEs) of the nitrogen atoms were shifted to 399.2 and 400.4 eV, respectively, due to the electron density of N atoms being donated to form a shared bond with Cu(II) ions.[26]

Figure 5

(a) XPS survey spectra of SiO2–quinoline before and after loading of Cu(II), and the high-resolution XPS for (b) Cu 2p and N 1s (c) before and (d) after loading of Cu(II).

The band gap of SiO2 and SiO2–Cu(II) complexes was determined through the Kubelka–Munk function. Figure shows the extrapolation of the linear part of the plot on the energy axis. The observed band gaps were 5.8 and 2.8 eV, respectively. This result indicates that the addition of the Cu(II)–quinoline complex to the silica support decreases the band gap of SiO2.

Figure 6

Kubelka–Munk function of (a) SiO2 and (b) SiO2–Cu(II)–quinoline complexes.

Photo-Fenton-Like Degradation of MB

The catalytic performance of the catalyst was evaluated using the degradation of MB under UV light irradiation over 150 min as a model reaction. The control experiments included MB/cat. (UV) and MB/cat./H2O2 (dark), were performed under identical conditions. The time profile of MB degradation over the catalyst in different conditions is illustrated in Figure a. The amounts of MB removed after 150 min of experiments MB/cat. (UV) and MB/cat./H2O2 were 15.8% and 54, respectively. The best MB removal (95%) was obtained for MB/cat./H2O2 (UV). The photocatalytic activity was quantitatively investigated by the calculation of the apparent rate constant (k) of the catalyst using eq .where C is the concentration of the MB solution (mg L–1) at the reaction time t, C30 is the concentration of the MB solution at 30 min, and k is the apparent reaction rate constant. The slope (k value in min–1) was obtained from the linear plot of −ln(C/C30) vs t (min). The k value increased from −0.04 to 0.49 and 2.43 (10–2 min–1) (Figure b). This result also shows that the reaction rate was enhanced the most in the condition system MB/cat./H2O2 (UV).

Figure 7

(a) Catalytic efficiency and (b) pseudo-first-order kinetic plot for the degradation of MB in different reaction systems.

The changes in the UV–vis spectra of MB of the MB/cat./H2O2 (UV) system are shown in Figure . The characteristic band of MB (662 nm) decreased dramatically. Also, no new peak was observed, which suggests that the chromophore of MB was decomposed during the reaction.[27]

Figure 8

Changes in the spectra of MB.

Further experiments were conducted to determine the effect of H2O2 concentration. More extensive degradation with higher k values was observed when the H2O2 content was increased (Figure a). The degradation efficiency at 2.50 h increased from 15.8 to 95.0%. In addition, k values for MB degradation increased with increasing hydrogen peroxide concentration from −0.07 to 2.35 × 10–2 min–1 (Figure b). This finding suggests that H2O2 affects the MB degradation.

Figure 9

(a) Catalytic efficiency and (b) pseudo-first-order kinetic plot for degradation of MB using different concentrations of H2O2.

There are various radicals involved in the Fenton-based reaction. Important active radical species such as HO• and O2•–/HOO• were detected. The type of radical species in the MB degradation was determined using coumarin, isopropyl alcohol (IPA), and p-benzoquinone (BQ) as probes. Coumarin is a poorly fluorescent molecule that was used as a probe for the detection of the HO• radical. It produces the highly fluorescent compound 7-hydroxycoumarin upon the reaction with HO• radicals. The amount of 7-hydroxycoumarin reflects the amount of HO• generated in the system. The fluorescent spectra of the reaction mixture recorded at different time points (0–150 min) are shown in Figure . After exposing the reaction mixture to UV light for 30 min (60 min of total reaction time), coumarin was hydroxylated to 7-hydroxy-coumarin confirmed by the strong emission band at 460 nm. The band increased continuously with increasing exposure time. These results suggest that the HO• radical was generated in the conditions of cat./H2O2 under UV irradiation.

Figure 10

Changes in the fluorescence spectra of coumarin from 0 to 150 min.

Furthermore, results of radical trapping experiments are shown in Figure . The degradation of MB was not interrupted by the addition of HO• radical scavenger isopropyl alcohol (IPA). However, the HO• radical was still involved in MB degradation, as confirmed by the production of 7-hydroxycoumarin. On the other hand, O2•– trapping by p-benzoquinone (BQ) shows competitiveness with MB degradation.

Figure 11

(a) Catalytic efficiency and (b) pseudo-first-order kinetic plot of the catalyst in the presence of radical active species-trapping agents.

The results are in accordance with the radical generation by the Cu(II)/organic ligand/H2O2 system, which simultaneously generates HO• and O2•–/HOO• radicals via a complex mechanism forming a peroxo complex.[28] Therefore, the mechanism of the catalyst function in the photo-Fenton-like reaction is proposed in Scheme .[29] The catalysis takes place on the surface of the silica particles, where the active site of the [L–Cu2+] complex was immobilized. The unsaturated [L–Cu2+] complex could bind peroxide to form [L–Cu2+(−O2H)] (pathway (1)), which can subsequently transform into a peroxo complex [L–Cu+(O2H)•] as a result of charge transfer between a hydroperoxide anion (−O2H) and Cu2+ (pathway (2)).[30,31] The peroxo complex can then react with H2O2 to produce HO• radicals (pathway (4)).[30−34] The [L–Cu2+(−O2H)] can also be regarded as an oxidant based on a complex mechanism, directly oxidizing substrates through pathway (6).[35,36] In addition, the copper redox cycle with HO• and the production of dissolved O2 (pathway (4)) are likely to involve a catalytic cycle (pathways (7) and (8)).[20,37−41] During these processes, the substrate in the reaction can be excited (pathway (9)), transferring an electron to the metal-centered orbitals (pathway (10)) with this substrate to the metal charge transfer (SMCT) process contributing to the decolorization of the substrate via the loss of conjugation of the double bonds in the molecule.[40] With photo assistance, the photoreduction of Cu2+ to Cu+ occurs through charge transfer between the ligand and the metal. This is followed by the production of an oxidized ligand (L+) (pathway (11)), which could subsequently oxidize substrate molecules (pathway (13)).

Scheme 1

Proposed Mechanism of Action of the Heterogeneous Cu–L Complex Photo-Fenton-Like System

The stability of the catalyst was studied in consecutive photo-Fenton-like reactions for MB degradation. The results of this investigation are shown in Figure S3. A large capacity of adsorption of the catalyst was found after the first reaction run, and the degradation efficiency by the photo-Fenton-like reaction decreased. The apparent rate constants are 3.65 × 10–2 and 1.04 × 10–2 min–1 for the first and second reaction runs, respectively. These findings suggest that the catalysis performance could be affected by the leaching of active sites, which may then be decomposed by self-degradation reactions during the MB degradation.[37,42]

Conclusions

A new photo-Fenton-like catalyst was prepared, as evidenced by the reported characterizations. MB was degraded under UV light irradiation in the presence of this catalyst in 2.5 h. However, the catalyst was lost during the catalytic reaction. The development of this type of catalyst for future use remains a challenge, especially in terms of protection from self-degradation, which could be achieved by operating under milder conditions.

Materials and Experimental Section

Materials

All chemicals were used without further purification. Ammonium hydroxide (NH4OH, 28 wt %), (3-aminopropyl) triethoxysilane (APTES, 99%), 8-aminoquinoline (C9H8N2, 98%), ethanol (C2H5OH, 99.8%), tetraethyl orthosilicate (TEOS, 98%), isopropanol (IPA, C3H8O), glutaraldehyde 25 wt % (C5H8O2), glacial acetic acid (CH3COOH), and coumarin (C9H6O2, 99%) were purchased from Sigma-Aldrich. p-Benzoquinone (BQ, C6H4O2) and hydrogen peroxide 30 wt % (H2O2) were purchased from MERCK. Copper(Il) sulfate pentahydrate (CuSO4·5H2O, 98%) was purchased from Ajax Finechem. Deionized water (DI) was used throughout the entire experiment.

Synthesis

The synthesis of the Cu(II)–quinoline complex immobilized on silica is shown in Scheme .

Scheme 2

Synthesis of the Cu(II)–Quinoline Complex on Silica Support

Preparation of Silica Support

The silica support was prepared by the hydrolysis of TEOS.[43] A mixture of ethanol (170 mL) and DI water (511 mL) was sonicated for 10 min. This was followed by a dropwise addition of TEOS (7.0 mL) to the reaction mixture under ultrasonication. After 20 min, 28 wt % NH4OH (12 mL) was added to catalyze the condensation reaction. The reaction mixture was then stirred at 750 rpm for 60 min. Then, the formation of a white turbid suspension was observed. The solid was dried at 100 °C for 5 h and obtained as the silica support (1.8 g). This material was denoted SiO2.

Preparation of Amine-Functionalized Silica Support

The prepared silica support was dried at 100 °C for 30 min before further use. The dried solid (1.2 g) was dispersed in ethanol (10 mL) and continually stirred at 500 rpm. Then, APTES (3.6 mmol, 0.84 mL) was slowly added to the suspension. After 30 min, DI water (7.2 mmol, 130 μL) was added to generate the alkoxide groups of APTES. The amount of DI water was twice the amount for complete hydrolysis of APTES.[22] The reaction was continued for further 30 min. The solids (1.1 g) were obtained after drying at 100 °C for 2 h. The prepared amine-functionalized silica was denoted SiO2–NH2.

Preparation of the Cu(II)–Quinoline Complex Immobilized on Silica Support

A total of 1.0 g of amine-functionalized silica support (SiO2–NH2) was dispersed in DI water (4 mL). Then, the suspension was sonicated for 10 min. 8-Aminoquinoline (432 mg, 3 mmol) dissolved in 5 mL of CH3CN was slowly added into the suspension. After 30 min, the mixture of glutaraldehyde (2.4 mL, 6 mmol) and glacial acetic acid (0.69 mL, 12 mmol) was added dropwise. The reaction was carried out at room temperature for 12 h. Afterward, 20 mL of a 0.1 M copper(II) sulfate solution was added to the reaction mixture, which was then stirred at 60 °C for 5 h.[20] The product (0.8 g) was collected by centrifugation at 9000 rpm, washed by DI water, and dried at 80 °C for 4 h. The product was denoted the SiO2–Cu(II)–quinoline complex or catalyst.

Material Characterization

The surface of the samples was investigated by Fourier transform infrared spectroscopy-attenuated total reflectance (FTIR-ATR, Spectrum GX). The structure and composition were investigated by powder X-ray diffraction (XRD, Panalytical/Expert 2θ: 5–140°). Morphologies were observed by field emission scanning electron microscopy (FESEM, LEO1455VP) equipped with energy-dispersive X-ray spectroscopy (EDX). Specific surface areas of the samples were evaluated by Brunauer, Emmett, and Teller (BET) surface analysis (TriStar II 3020). The elemental composition and the surface chemical state were investigated by X-ray photoelectron spectroscopy (XPS, AXIS Ultra DLD). Characteristic reflectance spectra were obtained by diffuse reflectance spectroscopy (DRS, Agilent 8453). The dye concentration was measured by a UV–vis spectrophotometer (UV-6100 UV/VIS).

Photocatalytic Experiments

Photocatalytic performance was investigated in a closed chamber with vertical light irradiation. Ambient temperature inside of the chamber was maintained at 28 °C with air cooling. The light source was a 30 W UV lamp, which was placed so that the distance between the level of the solution and the light source was 13 cm. The reactions were carried out in a batch experiment setup to avoid the loss of catalyst dosage. In a typical experiment, 1.0 mg of the catalyst (0.2 g L–1) was added to a 5 ppm solution of MB (5.0 mL) and the mixture was ultrasonicated for 2 min. The reaction was continually stirred in the dark for 30 min to reach adsorption equilibrium (Figure S4). Then, one of the reactions was taken out and centrifuged to obtain a clear supernatant. The MB concentration determined at this point was labeled as C30. After that, 30 wt % H2O2 was added to the reaction mixture and the UV lamp was turned on. At predetermined reaction time points (30, 60, and 120 min), the reaction mixture was centrifuged and analyzed. The concentrations of MB were determined from its maximum absorption (664 nm) using a UV–vis spectrometer. Decolorization efficiency of MB was calculated using eq .where C is the concentration of the MB solution at the reaction time t, C0 is the initial concentration of the MB solution at 0 min, and A and A0 are the corresponding absorption values.