Synthesis of an efficient photocatalyst by incorporation of … · 2020. 5. 8. · Mesoporous silica KIT-6 was synthesized adopting the method reported by Kleitz et˝al. [43]First,

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SN Applied Sciences (2020) 2:1037 | https://doi.org/10.1007/s42452-020-2845-6

Research Article

Synthesis of an efficient photocatalyst by incorporation of phthalocyanine into KIT‑6

Zahra Navaei1 · Mohammad Ali Zanjanchi1

Received: 25 January 2020 / Accepted: 28 April 2020 / Published online: 8 May 2020 © Springer Nature Switzerland AG 2020

AbstractThe three dimensional KIT-6 with interconnected channels was synthesized and was used for uploading sulphonated copper phthalocyanine (CuPcS). Grafting of CuPcS to the walls of KIT-6 was performed by anchorage of 3-(aminopropyl)-triethoxysilane onto KIT-6 via post-synthesis method. Exploitation of dimethyl sulfoxide as solvent caused a high amount of CuPcS to be loaded onto KIT-6. The CuPcS/NH2-KIT-6 was comprehensively characterized using small-angle X-ray diffraction, nitrogen BET and BJH physisorption, diffuse reflectance spectroscopy, FTIR, scanning electron microscopy, energy-dispersive X-ray and transmission electron microscopy methods. The photocatalyst was tested for degradation of 2,4-dichlorophenol (DCP) in aqueous solutions under visible and UV-A light irradiation. The results obtained reveal that a very low amount of the photocatalyst can progressively and efficiently proceed degradation of DCP. The decomposition process is completed within about 3 h using a dose of 0.2 g/L of the catalyst under visible light. The reusability of the catalyst is fine and there is not any obvious reduction in its activity following four cycles of repeated use. The kinetics of photodegradation and the intermediates products were studied and the mechanism of the photocatalyst was clarified upon the obtained results.

Keywords Phthalocyanine · KIT-6 · Mesoporous · Dichlorophenol · Photocatalyst

1 Introduction

Photodegradation is one of the very popular methods for elimination of pollutants from the environment which could be performed with the aid of heterogeneous cata-lysts [1, 2]. Heterogeneous catalysis is an advanced oxi-dation process which is widely used for removing the contaminants from water and wastewaters [3]. There are some valuable and convenient catalysts such as TiO2 which are very suitable for this purpose [4]. However, one of the serious problems for their application is their inactivity under solar (visible) light. For the degradation process to be economical and widespread application, this would be a great disadvantage. Therefore, these types of catalysts must suffer some structural modifications to be active under solar light.

Phthalocyanines are aromatic macrocyclic compounds and their metal complexes are conventionally used as dyes and pigments owing to high stability, negligible toxicity and distinctive color characteristics [5]. Another important property of metallo-Phthalocyanines and their derivative is that they could intensively absorb green–blue part of visible light and produce highly active oxidizing species upon irradiation with solar lights [6]. Therefore, they pos-sess capability to accomplish a photo-induced reaction by generating highly reactive species. Photo-reactivity of phthalocyanine could arose either from singlet oxygens (1O2) formed from excitation of ground state molecular oxygen (3O2) or in the form of reactive oxygen species such as superoxide [7, 8]. The long life of singlet oxygen atoms and the formation of superoxide radicals are the two main factors for photo-reactivity of metallo-phthalocyanines.

* Mohammad Ali Zanjanchi, [email protected] | 1Department of Chemistry, University of Guilan, P.O. Box 41335-1914, Rasht, Iran.

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These highly active species interact with most contami-nants and degrade them effectively. Next step to get more benefit from exploitation of metallo-phthalocyanine is to distribute them onto an appropriate solid support. Immo-bilization of phthalocyanine onto a high surface area sup-port leads to many improvements. Generally, heteroge-neous photocatalytic systems have several advantages compared to that of homogeneous ones. Long life-time, easy and simple recovery-recycling and more effective catalytic centers are among the advantages [9, 10].

There are scientific reports concerning photocatalytic activity of certain loaded metallo-phthalocyanines into various supports including zeolites [11, 12], silica [13], titanium oxide [14, 15], polymer fibers [16–18], carbon nanotubes [19], graphene [20–22], resin [23, 24], and mesopores silica [25–27]. Mesoporous solid materials are preferred to microporous ones for loading phthalo-cyanine complexes. This is because of larger pores, wider pore openings and higher surface area of mesoporous materials which facilitate diffusion and immobilization of phthalocyanine supra-molecular species into their struc-tures [28]. Recently, the newly emerged inorganic–organic hybrid materials of metal–organic frameworks (MOF) were also introduced as the base for preparation of many active photocatalysts. The comprehensive review articles con-cerning the fast growing and promising future for these materials are available [29–31]. There are several different approaches including sensitization by dyes for tuning the optical properties of MOF materials and to convert them to visible-light active photocatalyst [32, 33].

Another important point for using dye-containing supports is to control the leaching of active species dur-ing photocatalytic reaction [25]. This will affect the reus-ability of the photocatalyst. Also, there are reports which relate reduction of the photoactivity to the alteration of the phthalocyanine species due to interaction with silicon network surroundings of the mesoporous type [27]. The orientation of phthalocyanine on the surface which can provide different accessibility for the substrate molecules also may affect the activity of a heterogeneous catalyst. Sirotin et al. [34] reported that the orientation of phth-alocyanine onto MCM-41 and SBA-15 used as oxidation catalysts is depended to the preparation details including use of extra ligand.

Massive planar aromatic π-system of phthalocyanines provide a high affinity for aggregation. General believes is that aggregation is not in the favor of photocatalytic activity and monomeric species of phthalocaynine are more active [23]. However, Sorokin and co-workers pre-sented evidences that in several catalysis or photoca-talysis reactions using immobilized phthalocyanines, dimeric forms are more active than monomeric [6, 35]. Therefore, complementary studies are required to verify

this matter with more clearly. Nature of the solvent, type of structure and size of pores and channels of the sup-port could affect the ratio of monomeric and dimeric forms of the loaded phthalocyanine.

One of the approaches used to immobilize metal-phthalocyanines is via electrostatic interaction between contrary charged phthalocyanine and surface. In our previous studies, we loaded sulfonated cobalt phthalo-cyanine into channels of MCM-41 [36]. For this purpose, first we functionalized the phthalocyanine with 3-(ami-nopropyl)triethoxysilan. This body acquires a positive charge in a moderate acidic medium and therefore adsorb the negative CoPc(SO3H)4 species. In the pre-sent study, we extended our studies for selecting a dif-ferent mesoporous type, KIT-6. This mesoporous material displays improved pore interconnectivity and provides superior pore accessibility related to better diffusion within the interconnected channels [37]. KIT-6 is even more active than another interconnected silica support SBA-15 regarding the concept of per-site catalytic activ-ity [38]. The 2-D hexagonal structure of SBA-15 can be modified to a 3-D cubical KIT-6 by incorporation of addi-tional co-template (n-butanol) in the synthesis solution [39]. KIT-6 with larger pores, thicker walls and higher thermal, hydrothermal and structural stability consid-ered to be excellent support in heterogeneous cataly-sis [40]. These structural features of KIT-6 will justify the reason for preparation and study of our phthalocyanine-KIT-6 photocatalyst. We also showed that exploitation of dimethyl sulfoxide (DMSO) as solvent increases the extent of loading and aggregation of phthalocyanine onto KIT-6. We used 2,4-dichlorophenol (DCP) as a probe toxic species for testing our photocatalyst. Degradation of chlorophenols by advance oxidation processes is a very serious research matter [41, 42].

2 Experimental

2.1 Materials and reagents

Copper(II) phthalocyanine-tetrasulfonic acid tetraso-dium salt)CuPcS), 3-Aminopropyltriethoxy-silane (APTES) and Pluronic P123 (EO20PO70EO20) were purchased from Sigma-Aldrich. Tetraethyl orthosilicate (TEOS), hydrochlo-ric acid (HCl, 37%), 1-butanol (BuOH), dimethyl sulfoxide (DMSO), toluene, dichloro-methane, 4-chlorophenol (CP) and 2,4-dichlorophenol (DCP) in high-purity grade were purchased from Merck, Germany. All reagents employed were used without any purification. Bis(trimethylsilyl) trif-luoroacetamide + 1% trimethylchlorosilane (BSTFA + TMCS, Merck) was used as trimethylsilyl (TMS) derivatization.

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SN Applied Sciences (2020) 2:1037 | https://doi.org/10.1007/s42452-020-2845-6 Research Article

2.2 Preparation of photocatalyst

Mesoporous silica KIT-6 was synthesized adopting the method reported by Kleitz et al. [43]. First, 4 g of P123 was dissolved in a mixture consisted of 6.5 mL HCl (37%) and 144 g of deionized water. The mixture was stirred vigor-ously at 35 °C for 3 h. Subsequently, 4.0 g of butanol was added with continuous stirring for 1 h. Later, 8.52 g TEOS was added to the mixture. The mixture was stirred for 24 h while keeping the temperature around 35 °C and eventu-ally it was transferred to a stainless steel autoclave. It was heated in an oven at 100 °C for 24 h under static condition. Then the autoclave was taken out from the oven and it was cooled to room temperature. The solid product was collected following its washing with deionized water for several times. Finally, it was dried at 100 °C for 12 h. The product was calcined at 550 °C for 5 h in an electrical fur-nace. At this stage, the structure-directing template mol-ecules (surfactant) were burned out and eliminated from the structure of KIT-6.

The mesoporous KIT-6 was functionalized through APTES grafting [44]. For this purpose, 1 g of the calcined KIT-6 was dispersed in 50 mL of dry toluene. It was stirred for few minutes at room temperature. 10 mL of APTES was added dropwise to the suspension and the mixture was heated under reflux at 100 °C for 24 h. Afterwards, the sam-ple was filtered, washed with dichloromethane and dried at 70 °C for 3 h. This compound is called NH2-KIT-6.

For loading of CuPcS to the functionalized KIT-6 either an aqueous or organic media were used. For preparing the photocatalyst using aqueous medium, 25 mL of CuPcS at a concentration of 0.001 mol/L was taken and its pH was tuned to 3. Later, 0.1 g of NH2-KIT-6 was added to this solution. The suspension was stirred for 16 h at room tem-perature. Lastly, the pale blue solid product was filtered, washed with deionized water for several times and finally dried at 70 °C for 3 h. This compound is designated CuPcS/NH2-KIT-6(A) which A stands for aqueous. For preparing the photocatalyst in a non-aqueous condition, 25 mL of CuPcS solution in DMSO at a concentration of 0.001 mol/L was taken and its pH was modified to 3. Similar to the aqueous procedure, 0.1 g of NH2-KIT-6 was added to this solution. The suspension was stirred for 16 h at 90 °C. A very dark blue solid product was formed. The solid was fil-tered, washed with deionized water and dried at 70 °C for 3 h. The designation CuPcS/NH2-KIT-6(D) assumes that the sample was prepared using DMSO as an organic solvent.

2.3 Characterization

Small angle powder XRD patterns were collected with a Panalytical X’Pert Philips diffractometer with copper tube (λ = 1.5406 Å). The diffractions were recorded in the 2θ

range of 0.7°–10° and in steps of 2° min−1. The Pore volume and surface area of the samples was measured by Sibata apparatus 1100. Prior to the measurement, the samples were degassed at 250 °C for 5 h. Pore size distribution and mesoporous adsorption–desorption loop characteristics were obtained by BJH method using a Belsorb Mini 2 apparatus. A UV-2100 Shimadzu spectrophotometer with an optional DRS attachment were used for recording the diffuse reflectance spectra of the samples. Barium sulfate was used as a white reference standard. Fourier-transform infrared (FTIR) KBr spectra were recorded in the range of 400–4000 cm−1 with Shimadzu FT-IR spectrometer (Model 8400). A ratio of 1:100 sample-to-KBr weights were used for preparation of the analyzing disc. The appearance and morphology of the particles was recorded by JEM-2000FXII electron microscope. Also, TEM images were obtained using Philips CM30 electron microscope. Energy-disper-sive X-ray analysis (EDX) linked to TESCAN MIRA ∏ field emission scanning electron microscope (FESEM) was used for compositional analysis of the samples.

2.4 Photocatalytic experiments

The photocatalytic experiments were carried out in a 150 mL vessel. The suspensions were maintained stirring by use of a magnetic stirrer. All the degradation reactions were carried out in aqueous media. The visible light source was A 100 W tungsten lamp (manufactured by Pars Shahab Lamp Co., Iran) was used as the visible light source. The typical reaction mixture contained catalyst (10 mg) and 50 mL aqueous solution of DCP (40 mg/L). A 10 min stir-ring of the suspension in dark was proposed to account for the establishment of adsorption/desorption equilibrium. A small volume (2 mL) sampling out of the degradation reactor were taken at a number of times during irradiation. The spectrophotometer Shimadzu UV-2100 was used to read and record the absorbances which were used to cal-culate DCP concentration by means of a calibration plot. The degradation progress could be monitored by this way.

2.5 Recognition of intermediate species

The recognition of the intermediate species was done using GC–MS analysis. The samples were prepared as described below. The samples were first filtered to remove the solid catalyst. The filtrate was treated with dichlo-romethane. Two phases were formed. The dichlorometh-ane phase (organic phase) was first dried with the aid of anhydrous sodium sulfate and then its derivatization was done using BSTFA. Aqueous phase was evaporated to dryness and then it was dissolved in 200 μL anhydrous acetonitrile. It was subsequently derivatized with 200 μL

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of BSTFA for 60 min at 70 °C. This compound is suitable for GC–MS analysis.

The intermediates were recognized by GC–MS. The derivatized samples of the degradation processes were inspected using an Agilent 6890 GC (Agilent, Santa Clara, CA). The column of 5% phenyl and 95% dimethylpolysi-loxane (J&W Scientific) with dimensions of 30 m, 0.25 mm was used. The carrier gas was extra pure helium. The flow of the gas was maintained at about 2.0 mL min−1. The injection port was set to split/splitless and it was kept at a temperature of 250 °C. The injection volume was 1 μL. The data collection was done in the range of m/z 35–500. The free software AMDIS (Automated Mass Spectral Deconvo-lution and Identification System, version 2.64, NIST, US) were used for the evaluation and analysis. The identifica-tion of the components was done by matching the spectra against Wiley version 08 library.

3 Results and discussion

3.1 XRD studies

The structural characteristics of KIT-6 and its features fol-lowing functionalization with APTES and CoPcS loading were recorded using XRD measurements. The low angle powder XRD patterns of KIT-6, NH2-KIT-6, CuPcS/NH2-KIT-6(A) and CuPcS/NH2-KIT-6(D) are shown in Fig. 1. All of the XRD patterns show a sharp reflection around 2θ of 1 degree corresponding to the (211) plane in the cubic la3d symmetry [43, 45]. This shows that the degree of long-range ordering in our KIT-6 based mesoporous materials has been preserved. The reduction in the intensity of the main peak for the treated samples may be an indication of

incorporation of APTES and CuPcS into the structure of KIT-6. However, this cannot be reorganized for CuPcS/NH2-KIT-6(D). It is a general believe that measurement of the reflec-tion angles and their intensities at very low angles cannot be done accurately. The little shift of 2θ in the XRD for the treated samples could be attributed to the expansion of the unit cell after loading APTES and CuPcS into KIT-6. This is in agreement with the observation of Song et al. [46] for their functionalized MCM-41. More shift of CuPcS/NH2-KIT-6(D) relative to CuPcS/NH2-KIT-6(A) is caused by the presence of more copper phthalocyanine in the former.

3.2 Textural studies

The textural features of KIT-6 were checked using BJH method and BET surface area measurements. Figure 2a shows the H1 hysteresis loop for a type IV adsorption–des-orption isotherm which clearly indicates presence of chan-nels with mesopore aspect in KIT-6. Figure 2b shows the narrow range pore-size distribution obtained from BJH data. An average pore size of 7.06 nm was found for KIT-6 synthesized at our exploited conditions. This is in close agreement with the reported size by Kleitz and co-work-ers [39]. The surface area and pore volume for KIT-6, using conventional single-point BET method, are 1041 m2/g and 0.37 cm3/g, respectively.

3.3 Diffuse reflectance spectroscopy

Diffuse reflectance spectra of CuPcS/NH2-KIT-6(A) and CuPcS/NH2-KIT-6(D) are shown in Fig. 3. The CuPcS/NH2-KIT-6(A) is pale blue and therefore shows little absorb-ances for its main bands. However, CuPcS/NH2-KIT-6(D) is dark blue and shows intense absorption of visible light. The spectrum of CuPcS/NH2-KIT-6(A) shows absorption bands at about 660 and 600 nm which correspond to the monomer and aggregated (dimeric) forms of copper phth-alocyanine, respectively [14, 47]. There is higher degree of aggregation for CuPcS/NH2-KIT-6(D) as it shows absorp-tion of light at the wavelengths lower than 610 nm. The mesoporous KIT-6 and NH2/KIT-6 do not show any absorp-tion of visible light because they do not contain phthalo-cyanine groups (Fig. 3a, b).

We inspected the extent of monomeric and aggregated forms of sulfonated copper phthalocyanine in water and DMSO to find out the conceivable reason for higher load-ing of CuPcS in CuPcS/NH2-KIT-6(D). Figure 4 shows that the ratio of monomeric to aggregated CuPcs in DMSO is much higher than the ratio in water. Therefore, possibly monomeric forms of CuPcS are appropriate species to diffuse inside the pores of NH2/KIT-6 and be adsorbed as aggregated species in the structure. The chance for the aggregated CuPcS to diffuse inside the pores of NH2/KIT-6

Fig. 1 XRD patterns of (a) KIT-6, (b) NH2-KIT-6, (c) CuPcS/NH2-KIT-6(A) and (d) CuPcS/NH2-KIT-6(D)

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would be much less due to their sizes. This may explain the reason for more phthalocyanine uptake by CuPcS/NH2-KIT-6(D) which was prepared using DMSO.

We also noticed that the loading of CuPcS into NH2-KIT-6 in addition to pH and the type of solvent, depends to the temperature and concentration of CuPcS in the solu-tion too. Figure 5 Shows that a concentration of 1 × 10−3 M of CuPcS in DMSO at temperature of 90 °C resulted in a high loading of the dye into KIT-6 structure.

3.4 FTIR spectroscopy

The FTIR spectra of KIT-6, NH2-KIT-6, CuPcS/NH2-KIT-6(A) and CuPcS/NH2-KIT-6(D) were shown in Fig. 6. All the spectra of the samples exhibited the characteristics vibra-tional bands of Si–O–Si asymmetric stretching at 1080 and 1230 cm−1, Si–O–Si symmetric stretching at 799 and 806 cm−1, and Si–O–Si bending vibration at 462 cm−1 related to silica frameworks. The bands around 3426 and 1636 cm−1 are ascribed to –OH stretching vibration of silanol groups and physically adsorbed water molecules. Following the post-grafting of APTES to KIT-6, the intensity of the vibrational bands weakened due to the interaction between silanol groups and APTES. The presence of –NH2 bending vibration at 693 cm−1, –CH2– asymmetric stretch-ing at 2933 cm−1 confirms the successful grafting of APTES on NH2-KIT-6. By comparing the spectra of NH2-KIT-6 with those of CuPcS/NH2-KIT-6(A) and CuPcS/NH2-KIT-6(D), it is observed that the 799 cm−1 band is shifted to 806 cm−1 and also the intensity of this band has decreased due to immobilization of CuPcS in the structure. This confirms that CuPcS is successfully anchored into NH2-KIT-6. The

band around 1422 in CuPcS/NH2-KIT-6(A) and CuPcS/NH2-KIT-6(D) are related to the anchored complex copper phthalocyanine.

3.5 Morphological and EDX analysis

The morphologies of the prepared samples were studied by FESEM. The FESEM micrographs of KIT-6 and CuPcS/NH2-KIT-6(D) were shown Fig. 7a, b, respectively. Figure 7b reveals that the CuPcS particles have almost uniformly dis-tributed onto KIT-6 with an average size of about 30 nm.

The results of EDX analysis of KIT-6, NH2-KIT-6, CuPcS/NH2-KIT-6(A) and CuPcS/NH2-KIT-6(D) are shown in Fig. 8. It is obvious that by functionalization with APTES nitrogen signal will appear in the EDX pictorial presentation. Also, by loading CuPcS into NH2-KIT-6, the signals of copper and sulfur will be detected. However, the exact copper to sulfur ratio of 4 (in atomic ratio) cannot be verified from the EDX calculated data. This may be partly due to uncertainty in complete sulfunating of copper phthalocyanine and partly because of local analysis nature of EDX and the errors asso-ciated with it. The EDX results shows higher amount of copper and sulfur for CuPcS/NH2-KIT-6(D) which confirms higher loading of the dye in KIT-6.

The highly ordered pore structure of our prepared KIT-6 could be observed from the TEM imaging of the samples obtained before and after modifications (Fig. 9). The nano size CuPCS/NH2-KIT-6(D) containing the phthalocyanine species can be recognized in the figure.

Fig. 2 BJH analysis of KIT-6, a N2 adsorption–desorption isotherm, b pore-size distribution

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3.6 Photocatalytic degradation of DCP

The efficiencies of the photocatalytic degradation of DCP were studied following the optimization of the dif-ferent experimental parameters so that a capable and cost-effective photocatalytic reaction conditions can be designed. We decided to find out the reaction conditions that allow the maximum degradation of DCP using little amount of the catalyst and within a short time.

3.7 Effect of catalyst dose

We found an optimal concentration of CuPcS/NH2-KIT-6(D) as 10 mg in 50 mL for the degradation of DCP. The effect of various amounts of 2, 5, 10, 20 and 40 mg of the photocatalyst on the degradation of DCP is depicted in Fig. 10. The conditions of the photocatalytic test is also shown at the caption to that figure. Using low amount of photocatalyst in a photodegradation reac-tion is an enormous advantage including lower cost of the material and higher efficiency of the process due to the reduced scattering of the exposed light. Generally, using high amount of photocatalyst prevent effective diffusion of light through solution. We cannot have this optimal value (10 mg) for CuPcS/NH2-KIT-6(A) because of

Fig. 3 Diffuse reflectance spectra in visible region. (a) KIT-6, (b) NH2-KIT-6, (c) CuPcS/NH2-KIT-6(A) and (d) CuPcS/NH2-KIT-6(D)

Fig. 4 UV–Vis spectra of CuPcS solutions: (a) in water and (b) in DMSO (concentration: 5 × 10−6 M)

Fig. 5 Diffuse reflectance spectra of CuPcS/NH2-KIT-6(D) prepared with the use of DMSO as solvent at different concentration of CuPcS and temperature of loading, respectively: (a) 2.8 × 10−6 M, 90° C, (b) 1.4 × 10−5 M, 90°C, (c) 1.0 × 10−3 M, room temperature, (d) 1.0 × 10−3 M, 90°C

Fig. 6 FTIR spectra of (a) KIT-6, (b) NH2-KIT-6, (c) CuPcS/NH2-KIT-6(A), and (d) CuPcS/NH2-KIT-6(D)

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very low concentration of CuPcS species in this sample. In fact, we found that we should use five times more (50 mg) of CuPcS/NH2-KIT-6(A) to gain similar result.

3.8 Effect of pH

The effect of different pH on the degradiation rate of DCP was also examined. It was observed that the optimal pH for the degradation of DCP was 5.5 which is the pH of the DCP at its initial concentration. The effect of different pH values in the general view of degradation is evident in the next sections.

3.9 Degradation of DCP

Figure 11 shows the concentration changes of DCP during photodegradation experiments using CuPcS/NH2-KIT-6(D) as photocatalyst under visible or UV-A lights. The degrada-tion is nearly completed within about 3 h time. Using as low as 10 mg photocatalyst for a 40 mg/L concentration of DCP shows that a huge concentration of active sites pre-sents in CuPcS/NH2-KIT-6(D). These results show that the efficiency for CuPcS/NH2-KIT-6 compared to that of CoPcS/NH2-MCM-41 (See Ref. [36]) is several times higher. Pos-sibly, the specific channels of KIT-6 and the use of DMSO provide an extraordinary condition that many CuPCS spe-cies were loaded into the mesoporous support which are responsible for the photodegradation reaction. The pho-todegradation process initiates with the photogeneration

of superoxide radicals (O2·−) and/or singlet oxygen (1O2)

arose from CuPcS species attached to the channel walls of KIT-6. These processes have been generally agreed due to the previous studies [43, 44]. The main cause of the degradation of DCP is most probably related to the gen-erated superoxide radicals. These species will oxidize DCP molecules that are in close contact with them inside the pores of CuPcS/NH2-KIT-6(D). There are previous reports that leaded us to this point [8].

3.10 Kinetic studies

The kinetics of DCP degradation by CuPcS/NH2-KIT-6(D) was also studied for inclusiveness of the work. It was found that it is fitted into pseudo first-order reaction kinetics. The first-order kinetics for our data could be expressed based on the equation ln(C0/Ct) = kobst as indicated in Fig. 12. The apparent rate constants (kobs) for the different initial con-centrations of DCP are calculated from the slopes of the lines obtained by plotting ln(C0/Ct) versus irradiation time. Table 1 shows the kinetics parameters including reaction rates, rate constants and half-lifes (t1/2) at various initial concentrations of DCP. Our results show that the degrada-tion rate constant increases to some extent upon the initial concentration increase.

Fig. 7 FESEM images of a KIT-6 and b CuPcS/NH2-KIT-6(D)

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3.11 Degradation products and possible mechanism

During photocatalytic degradation of DCP several inter-mediate compounds may be formed. The analysis of the produced compounds is very important for evaluating the comprehensive achievement of the catalytic processes. It is not far from the fact that any or some of the degradation products may harmfully influence the environment. This may have harmful effects equal to or greater than those of the pollutant itself. In the present work, GC–MS techniques

were used for identification of the organic compounds in the reaction mixture. We found that the two main inter-mediate products were butanediol and butanedioic acid based on the recorded chromatograms (Fig. 13). Accord-ing to the related sources [48–50] two types of the path-way may be involved in DCP degradation by CuPcS/NH2-KIT-6(D). In the first degradation way, DCP is oxidized by CuPcS/NH2-KIT-6(D) to form 2-chlorine-1,4-quinone and then yield methylated products and finally to unsatu-rated dicarboxylic acids. In the second approach, the

Fig. 8 EDX analysis of KIT-6 (a), NH2-KIT-6 (b), CuPcS/NH2-KIT-6(A) (c) and CuPcS/NH2-KIT-6(D) (d)

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dechlorination and opening the loop of DCP occurs and ethanediol is formed (Fig. 14).

Fig. 9 TEM images showing KIT-6 (a, b) and CuPcS/NH2-KIT-6(D) (c, d)

Fig. 10 Effect of the amount of CuPcS/NH2-KIT-6(D) on DCP degra-dation. Irradiation time: 60 min, irradiation source: 100 W tungsten lamp, solution volume: 50 mL, concentration of DCP: 40 mgL−1

Fig. 11 Degradation progress of DCP using CuPcS/NH2-KIT-6(D) under visible light (a) and UV-A (b). Volume of DCP solution: 50 mL, concentration of DCP: 40 mg/L, catalyst weight: 10 mg, solution pH: 5.5, visible source: 100 W tungsten lamp and UV-A source: Krypton lamp

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3.12 Catalyst stability and reusability

It is very important that the heterogeneous catalyst holds enough stability especially when it is to be used in envi-ronmental applications. Therefore, the catalyst stability and reuse must be studied. Figure 15 shows the activity of CoPC after four successive uses for degradation of DCP under the same condition. The residue from the photocat-alytic uses was separated, washed by deionized water for several times and then the solid was dried and were used again. The figure shows that there is no obvious reduction of the activity of CuPcS/NH2-KIT-6(D) for degradation of DCP after four consecutive recycles of the catalyst.

3.13 Comparison with similar works

Table 2 introduces and compares several recent photo-catalytic systems based on silica or MOF as porous sup-port materials which are modified with phthalocyanine or another dye in order to act under visible light. This work is also mentioned for comparing some operational param-eters presented in the table.

4 Conclusion

The incorporation of CuPcS complex in the intercon-nected channels of mesoporous KIT-6 using dime-thyl sulfoxide as solvent leads to achieve remarkable

Fig. 12 Kinetics plot for degradation of DCP with different initial concentration using CuPcS/NH2-KIT-6(D). The initial concentrations are (a) 50, (b) 40, (c) 30, (d) 20 ppm, (e) 10 ppm in water. Irradiation source: A 100 W tungsten lamp, DCP solution volume: 50 mL, cata-lyst weight: 10 mg and pH 5.5

Table 1 Kinetic data for degradation of DCP by CuPcS/NH2-KIT-6(D)

Concentra-tion (mg/L)

R2 Reaction rate (mg/(L min−1))

kobs (min−1) t1/2 (min)

10 0.9847 0.062 0.0062 11120 0.9955 0.140 0.0070 99.030 0.9993 0.243 0.0081 85.540 0.9925 0.376 0.0094 73.750 0.9879 0.540 0.0108 64.2

Fig. 13 Gas chromatogram of DCP photodegradation products in dichloromethane (a) and aqueous (b) phases

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Fig. 14 Schematic repre-sentation of the proposed mechanism for photocatalytic degradation of DCP according to the results of this study and in agreement with the previ-ous reports [48, 49]

Fig. 15 Recyclability tests for CuPcS/NH2-KIT-6(D) for degradation of DCP. Reaction conditions: DCP concentration 40 mg/L, solution volume 50 mL, catalyst weight 10 mg, pH 5.5

concentration of copper phthalocyanine species onto KIT-6. The photocatalyst displays a very high activity for degradation and mineralization of 2,4-dichlorophenol under visible or UV-A lights. The photodegradation is nearly completed within 3 h. This is a great achievement when a relatively high concentration of 40 mg/L of DCP and a relatively low dose of 0.2 g/L of the catalyst under visible light are involved. The degradation of DCP occurs relatively fast via a first order reaction kinetics. Two types of pathway is presented for the involved mechanism according to the identified intermediate compounds.

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Acknowledgements The authors thank the University of Guilan for supporting this work.

Compliance with ethical standards

Conflict of interest The authors declare that they have no conflict of interest and no software applicability in this work.

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Table 2 Comparison of some visible-driven photocatalysts based on silica or MOF porous materials modified with phthalocyanine or other dyes

NA not available

Porous support photoactive dye Application Photocatalyst weight (mg)

Reaction Time (min) Refs.

Al-MCM-41(silica) ZnPC Degradation of pesticides 25 180-300 [26]HMS (silica) CuPC Degradation of pesticide 25 30 [25]Ti-MCM-41(silica) FePC Oxidative desulfurization 20 120 [51]MCM-41(silica) CoPC Degradation of methyl orange 40 120 [52]NH2-MCM-41 (silica) CoPCS Degradation of dichlorophenol 30 180 [36]MOF-5 8-hydroxyquinoline Degradation of phenol 200 70 [33]UiO-66(Zr) (MOF) Rhodamine B Hydrogen production NA 180-300 [53]UIO-66 (NH2) MOF) ZnPC Degradation of methylene blue 20 120 [54]UiO-66–NH2 (MOF) Amino substituted ligands Degradation of methylene blue NA NA [55]Zr-UIO-66 (MOF) Erythrosin B Hydrogen production 10 NA [56]KIT-6 (silica) CuPCS Degradation of dichlorophenol 10 180 This work

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Synthesis of an efficient photocatalyst by incorporation of phthalocyanine into KIT-6Abstract1 Introduction2 Experimental2.1 Materials and reagents2.2 Preparation of photocatalyst2.3 Characterization2.4 Photocatalytic experiments2.5 Recognition of intermediate species

3 Results and discussion3.1 XRD studies3.2 Textural studies3.3 Diffuse reflectance spectroscopy3.4 FTIR spectroscopy3.5 Morphological and EDX analysis3.6 Photocatalytic degradation of DCP3.7 Effect of catalyst dose3.8 Effect of pH3.9 Degradation of DCP3.10 Kinetic studies3.11 Degradation products and possible mechanism3.12 Catalyst stability and reusability3.13 Comparison with similar works

4 ConclusionAcknowledgements References

Documents

Synthesis of an efficient photocatalyst by incorporation of … · 2020. 5. 8. · Mesoporous silica KIT-6 was synthesized adopting the method reported by Kleitz et˝al. [43]First,