Host (dealumination Y zeolite)-guest (substituted heteropolyacid) nanocomposites as an ecient catalyst in the removal of methyl orange

Substituted polyoxometal encapsulated into Y zeolite was synthesized with two ratios: H 5 PMo 10 W 1 Ti 1 O 40 .XH 2 O/Y (hereafter designated as SPY-1011, the numbers show the amount of Mo, W, Ti, respectively) and H 5 PMo 1 W 10 Ti 1 O 40 .XH 2 O/Y (hereafter designated as SPY-1101) was synthesized using the template synthesis method. The nanocomposite materials were characterized by XRD, FT-IR, UV-Vis, FESEM, and EDS techniques. The photocatalytic activity of systems was investigated within the photodegradation of methyl orange. The W, Mo, and Ti content of the catalyst was evaluated by the ICP method. The results showed that the photocatalyst performance depends on catalyst loading, pH effect, methyl orange concentration, the type, and the number of substituted species. The chemical oxygen demand (COD) experiment indicated mineralisation of the methyl orange with 83% after 120 min irradiation. The absences of hydrazine during degradation conrmed by the amperometric experiment, in turn, showed methyl orange converted to simple inorganic materials. Photocatalytic degradation of methyl orange follows a pseudo-rst-order kinetic.


Introduction
Some synthetic compounds used in various industries like textile, food, leather, paint, cosmetics, and plastic cause air contamination and wastewater and released hazardous materials [1][2][3][4][5]. Therefore, pollutant elimination is considered a good remedy or alternative to solve these disadvantages of these compounds. In this regard, there are a variety of conventional treatments such as chemical and biological oxidation, adsorption, electrochemical, coagulation, ion exchange, membrane separation, and advanced oxidation process [6][7][8][9]. Among them, photocatalytic degradation has attracted pronounced attention due to eco-friendly systems and compatibility with the environment [4,[10][11][12][13]. In these processes, semiconductors such as TiO 2 , ZnO, WO 3 , Fe 2 O 3 , CuO, ZrO 2 , CdS, In 2 O 3 , and SnO 2 was used as photocatalysts that produce non-toxic products [14,15]. However, one of the most effective photocatalysts is considered TiO 2 due to its high photocatalytic activity, biologically and chemically inert properties, and stability. Moreover, the TiO 2 possesses a large bandgap and low surface area. Also, the ltration of TiO 2 seems troublesome and costly because of its ne dispersion in the water.
Another method in the photodegradation process is using supported and supported heteropoly acids. These systems have high Bro'nsted acidity and electron acceptor properties due to their structure, therefore facilitating photocatalytic activity [25][26][27][28]. Thus, the hybridization of heteropoly acid/TiO2/support is another manner for the photodegradation process [29]. In this respect, several researchers applied mixed metal oxide and metal sul de to increase and improve the performance of the photocatalytic activity [23,[30][31][32][33][34][35].
On one hand, we entered TiO 2 in H 3 PMo 12 O 40 / Y composite for enhancing photocatalytic activity. It is worthy to note that in this case, the leaching of TiO 2 from the zeolite surface occurs. For this reason, we synthesized Ti-substituted H 3 PMo 12-x W x O 40 incorporated into the nanocage of zeolite for preventing Ti leaching and improving photocatalytic activity. On the other hand, we combined the advantages of photosensitivity and electron acceptor property of polyoxometal, a hollow structure of nanoporous and supporting characters of zeolite and photocatalytic activity of TiO 2 to improve contact surface, reduce bandgap and enhance photocatalytic behavior. Of course, the results were promising since rst, decorated Ti on polyoxometal acts the same as TiO 2 , and second, its photocatalytic reactivity becomes higher than H 3 PMo 12 O 40 /Y.
Although much work is done to date, more studies need to be conducted. In previous works, metal and metal oxide was doped on TiO 2 to improve photosensitivity. But in the present study, we report Ti-doped polyoxometal for studying photocatalytic behavior. For this purpose, the receptivity property of the electron, photosensitivity, and Bronsted acidity properties of polyoxometal have been combined with the semiconductor property of Ti and the feature of zeolite support to obtain optimum conditions for photocatalytic activity performance.

Materials and Characterizations
All materials were of the commercial reagent grade and were purchased from Sigma-Aldrich, Merck, and Fluka. NaY was synthesized using the template method. XRD patterns were achieved on an advanced Brucker-D8 diffractometer with Cu k a1 (l= 1.54 nm, 30 mA, 2q=5-60°). FT-IR spectra were achieved from KBr pellets using a Japan Shimadzu 8400s spectrometer. The morphology of the catalysts was achieved by FESEM on a Czech MIRA3 Tuscan microscope equipped with Energy Dispersive spectroscopy.
Preparation of zeolite-encapsulated substituted heteropoly acids In the typical procedure, (SPY-1011) was prepared by the mixture of MoO 3 (7.2 g), WO 3 (1.16 g), and TiO 2 (0.4 g) in the suspension of zeolite (2g) followed by stirring at 308 K for 24 h. Then, H 3 PO 4 was added and stirred at 95 °C for 3 h. The obtained materials were sintered and dried. Later, the solid materials were immersed in hot water, and after an hour of agitation, the samples were ltered and washed with hot water. Finally, the solid mass was treated with 0.1 M NaCl solution for the removal of unreacted metal cations.

Synthesis of H 5 PMo 1 W 10 Ti 1 O 40 /Y (SPY-1101)
The synthesis of SPY-1101 is similar to that of SPY-1011, except that the proportions change in this case.
In this method, to a suspension of 2 g of zeolite in 150 ml of water, 11.6 g of WO 3 , 0.4 g of TiO 2 , and 0.72 g of MoO 3 are added and were stirred at 308 K for 24 hours. The rest of the process is similar to method 2.2.1.
General procedure for photodegradation of methyl orange by zeolite-encapsulated substituted heteropoly acids In a typical procedure, a suspension containing catalyst and 50 mL aqueous solution of methyl orange (2.5 ×10 -5 g/L) was stirred rst in the dark to balance adsorption/desorption equilibrium and to remove the error of any initial adsorption effect. Then the homemade reactor was used for irradiation experiments at room temperature. The mixture was stirred magnetically during irradiation. To monitor reaction progress and degradation of methyl orange, at certain times, about 3 mL of the suspension was withdrawn and the photocatalysts were separated by ltering and using 0.45 µm disc. The photodegradation reaction progress was determined by using the band in the region of 465 nm is related to methyl orange.

Chemical oxygen demand
Chemical oxygen demand (COD) measurements were performed to determine the degradation amount of organic compounds. To this aim, COD was determined by applying standard methods including volumetric titration methodology at the beginning and the end of the experimental runs.

Results And Discussion
Preparation and characterization of zeolite-encapsulated substituted heteropoly acids SPY-1011 and SPY-1101were synthesized into nanocage of zeolite by the ship-in-a-bottle method. Based on the formation of polyoxometal into zeolite cage, the original white color of zeolite turned and remained unchanged after puri cation proved that HPA encaged into zeolite cage. Besides, due to the presence of Ti in the HPA structure, its solubility is reduced. Consequently, some substituted polyoxometal remained on the external zeolite surface. Due to the high loading of complexes on the zeolite surface, peaks related  (Fig. 1A). The peak in related of O-H is observed at 3430 cm -1 .
The broadening of peaks can be assigned to the increasing in OH amount and difference in their position. Also, the presence of peaks associated with zeolite, proved the structure maintained after encaging HPA on the zeolite cage. FESEM micrograph showed cubo-octahedral unit in accordance with zeolite. So it was proved that the morphology of zeolite is retained based on the formation of polyoxometal into zeolite structure, however, the presence of some ripples on the nanoscale on the external zeolite surface can be related to substituted polyoxometal which has not been removed even after puri cation (Fig.1C) FESEM-EDX spectrum of SPA-1011 and SPA-1101 con rmed the presence of zeolite and substituted heteropoly acid in the obtained photocatalyst. This, in turn, not only indicated the existence of Ti, Mo, W, P, Si, Al, Na, and O in the compound but also the ratio of elements (Fig. 1D).

Investigation of photocatalytic activity of SPY-1011
To detect the photocatalytic activity of SPY-1101 the degradation of methyl orange was tested as a function of different experimental parameters which all results are presented in Figs. 2 and 3.

The effect of catalyst amount
The effect of catalyst amount on the removal of methyl orange was studied by varying the catalyst amount from 0.03 to 0.09 g, and the results of absorbance changes in the region of 400-600 nm were brought in Fig.2A. In a low concentration of catalyst, the degradation of methyl orange does not occur completely, while in a high amount of catalyst, the adsorption of dye molecules increased. Optimum degradation was obtained with 0.05 g catalyst. However, at amounts higher than 0.05 g, photodegradation reduces because of turbidity of the solution in accordance with high amount catalyst [4,37].

The Effect of methyl orange concentration
The photodegradation of methyl orange was studied at the different initial concentrations of methyl orange in the range of (1.5-9) ´ 10 -5 mol/L. The results are shown in Fig. 2B. An increase in the concentration of methyl orange reduces the photodegradation process which can be related to the absorbance of light by using methyl orange rather than the catalyst, therefore, the photocatalytic e ciency reduces [38].

The effect of initial photocatalyst pH
The reaction was investigated at various solution pH. The pH of the solution is adjusted at the beginning of the reaction before irradiation. The results are shown in Fig.2C. As it is clear, the best result is obtained at pH=5. So pH plays important role in the photodegradation of methyl orange and catalytic activity affected by the strong electrostatic eld. Obviously, under the alkali condition, the adsorption and degradation of methyl orange reduce because of competition between the hydroxyl group and methyl orange ions. Therefore, it decreased the absorption of methyl orange on the catalyst surface. In acidic conditions, methyl orange converts to hydrazones due to resonance form [39].
Under the optimum conditions, decolorization of MO was investigated with SPY-1101 as a function of irradiation time. Fig.2D. shows the UV-Vis spectra of methyl orange before and after irradiation in the presence of SPY-1101. Methyl orange itself was photochemically inert in the absence of SPY-1101 which was observed without any change in the absorption spectrum. However, in the presence of photocatalyst, the absorption spectrum of methyl orange signi cantly reduced and after 3 hours, the degradation of MO is completed. Also, photodegradation of methyl orange was investigated in the presence of SPY-1011. The results show that SPY-1101 is better than SPY-1011 catalyst because of degradation yield (Fig. 3A).

Catalyst recovery and reusability
The reusability of the catalyst was tested in the photodegradation of methyl orange. At the end of the reaction, the catalyst was ltered, washed with water, and dried at 100°C for 3 h. The catalytic activity preserved until 4 consecutive runs and after that, was observed decreasing in photocatalytic activity (Fig.  3B).

kinetic of photodegradation
The photodegradation of MO in the presence of SPY-1101 obeys from pseudo-rst-order kinetic. At low MO concentration, the rate expression is: k΄ is the pseudo-rst-order rate constant MO adsorbed on the catalyst surface and after absorption/desorption equilibrium, the concentration of MO was determined at different times. In this case, C 0 and C t are the initial concentration at t=0 and at time t, respectively. By plotting lnC t /C 0 versus time for degradation MO, the plot is obtained linear which proved degradation of MO followed pseudorst-order (Fig. 4).

Chemical oxygen demand (COD)
Mineralization of MO was con rmed using COD experiments. To this end, the photodegradation product was studied using COD analysis. The obtained results showed a signi cant COD removal (83%) after 60 min irradiation. This, in turn, indicates the mineralization of MO followed by the formation of carbon dioxide and water. Hydrazine was produced by MO photodegradation. An amperometric experiment con rmed the presence of hydrazine during photodegradation. In this case, NaOH was selected as the electrolyte. The various concentration of standard hydrazine was used to detect hydrazine signal and ultimately, the unknown solution of hydrazine was injected and compared to that of standard hydrazine.
Decreasing the ow may be related to the increasing volume of the solution without increasing in hydrazine amount (Fig. 12). It, in turn, con rms the photodegradation of methyl orange. Another proof for this claim, the measurement of COD at the beginning and the end of the experiment run were determined by the standard method by volumetric titration methodology [40].

Conclusion
In this study, the preparation of SPY-1101 and SPY-1011catalyst is performed using template synthesis. The physicochemical properties of composites characterized by XRD, FT-IR, FESEM, ICP, and EDS technique. The catalysts display e cient photoactivity in the degradation of methyl orange under UV illumination. COD experiment proved mineralization of azo dye. The kinetics of the reaction obeyed pseudo-rst-order. Ti-substituted causes HPA that showed photocatalytic activity. Signi cantly, these catalytic systems are recoverable and can be used several times without signi cant loss in catalytic activity.   Plot of LnC0/Ct versus time and LnC0-Ct/C0 versus time