Volatile Organic Compounds (VOCs) propagated from industrial reactions and automobile are not only detrimental to human health since they are malodorous, mutagenic and/or carcinogenic but also because they can create poisonous photochemical oxidants and suspended particulate substances through photochemical reactions. Many countries have already enacted rigorous rules to reduction VOC propagation . In order to effective elimination VOCs, different technologies have been developed, including adsorption, bio-destruction, membrane separation, thermal combustion and photo-catalytic oxidation . Among these technologies, catalytic oxidation is investigated as the most promising process for reduction VOCs’ propagation. Indeed, catalytic oxidation might be acted at temperatures much lower compared to thermal combustion. The advantage of the low temperature oxidation is the decrease of fuel consumption, especially for large volumes of diluted VOC contaminated air. The optimization of the catalyst formulation is not simple task due to the large diversity of VOCs and the complexity of VOC-containing mixtures. Primarily, the catalyst has to allow ignition temperatures as low as possible. In addition, it has to be very active, because of the low concentration of VOCs and the large volumes to be treated, and also highly selective in the sense that only desired product should be made. Eventually, the catalyst has to resist the reaction conditions over long periods of operation, i.e., has to be thermally stable.
Toluene is a typical aromatic hydrocarbon with three primary C-H bonds that is undesirable to the environment and can be oxidized to various useful chemical materials, such as benzyl alcohol, benzaldehyde, benzoic acid and benzoate . Among these materials, benzaldehyde is the most favorable and value-added product due to its large importance in human life . In the medicinal industries, it is utilized for the production of intermediates for ampiphilicin, ephedrine, chloramphenicol, etc. [5,6]. It is also a main intermediate for production the certain aniline dyes, processing the perfume and flavoring compounds . Traditionally, benzaldehyde is mainly manufactured by chlorination of the –CH3 group of toluene followed by saponification . This reaction strongly suffers from too much waste causing environmental difficulties due to the advantage of a remarkable amount of chlorine. In addition, it is associated with the equipment corrosion and manufacture of poisonous acidic waste . The Rhodia, Dowand Snia-Viscosa reactions were utilized as industrial realization of the liquid phase oxidation of toluene with homogeneous metal salt catalysts, but the halogen ions and acidic solvents were involved in these reactions [10–12].
Various oxidants such as TBHP and H2O2 are used in presence of transition metal compounds as homogeneous catalysts for this oxidation reaction. But, the environmental limitation and the catalyst separation is one of the important problems in the homogeneous catalysts. In order to avoid these problems, heterogeneous catalysts are under intensive investigation. In these catalysts, alumina, silica, zirconia, active carbon, zeolites and zinc oxide are commonly used as the supports [13–15]. Bulk oxides in general cannot be used in industrial reactions as they impart poor thermal stability that lead to fast decomposition of the catalyst . Hongpei Wang et al.  have investigated the catalytic oxidation of toluene with nano-sized γ-alumina supported CuO and MnOx as catalyst. In another report, Guan Huang and coworkers have used manganese tetraphenylporphyrin (Mn TPP) supported on chitosan (CTS) for catalytic oxidation of toluene with molecular oxygen as oxidant. Mn TPP/CTS containing 2 mg of Mn TPP as a catalyst showed excellent catalytic performance of up to 5.9% toluene conversion and 96% selectivity for benzaldehyde and benzyl alcohol in the toluene oxidation . A cost-effective method for the selective oxidation of toluene to benzaldehyde was developed based on immobilized CoOx on SiO2 catalyst with predominating cobaltous ions in the presence of N-hydroxyphthalimide (NHPI) and hexafluoropropan-2-ol (HFIP) using ambient molecular oxygen at room temperature. A toluene conversion of 91% and a selectivity to benzaldehyde of 68% were realized . CuMn mixed oxides catalysts doped with La were prepared following a co-precipitation method and used for the catalytic oxidation of toluene. These catalysts exhibit the best catalytic activity, with a 90% conversion temperature of 255°C . However, to the best of our knowledge, there is no report for application of Ag/ZnO nanocomposites for catalytic oxidation of toluene with H2O2 and TBHP. Here, we synthesized Ag/ZnO nanocomposites by the chemical precipitation method. In this strategy the size is control by arrested precipitation technique. The basic trick has been to synthesis and studies the nanomaterial in situ i.e. in the same liquid medium avoiding the physical changes and aggregation of tiny crystallites. Also studied their catalytic abilities in the oxidation of toluene with TBHP and H2O2 as the oxidant in the liquid phase.
MATERIALS AND METHODS
Chemicals and materials
Zinc nitrate hexahydrate (MERK), silver nitrate (Fluka; >99.0%), polyethylene glycol (PEG-4000), sodium borohydride (MERK) and ammonium hydroxide solution used in this work, all were of analytical grade.
Preparation of catalysts
Preparation of ZnO nanoparticles
ZnO nanoparticles were synthesized by the chemical precipitation method. Experimental details were as follows: firstly, 3 g polyethylene glycol (PEG-4000) as surfactant was dissolved in 30ml of distilled water. Secondly, 2.98 g of Zinc nitrate hexahydrate (Zn (NO3)2.6H2O) was added slowly to the above solution under the stirring, resulting in the formation of a clear solution. Thirdly, ammonium hydroxide solution was dropped into the mixing solution until the pH value reached 7, which immediately resulted in the formation of white precipitates in the beaker. Then kept at 60°C for 1 h under stirring, the as-produced precipitates were filtered, washed with alcohol and distilled water for several times. Lastly, the obtained white precipitate dried in an oven at 100°C overnight and calcined in air at 600°C for 2 h.
Preparation of Ag/ZnO nanocomposites
Ag/ZnO nanocomposites were prepared by a chemical reduction technique. Various amounts of Ag+ ions, prepared by dissolving silver nitrate salt in distilled water, then the solutions were added into the suspension of ZnO such that the Ag+ concentration was of 1 , 2.5 and 5 at.% in relation to ZnO. Sodium borohydride (NaBH4) solution (freshly prepared) was then added dropwise with vigorous stirring. The reduction of Ag from Ag+ to Ag0 was confirmed by the darkening of the solution. The suspensions were then filtered, washed and dried to give Ag deposited ZnO catalysts.
In a typical procedure, a mixture of 0.25 g catalyst, 20 ml solvent of acetonitrile and 32 mmol toluene was stirred under nitrogen in a 50 ml round bottom flask equipped with a condenser and a dropping funnel at room temperature for 30 min. Then 15 mmol of TBHP (tert-butylhydroperoxide) or H2O2 (30% in H2O) was added as oxidizing reagents. The resulting mixture was then refluxed for 8 h under N2 atmosphere. After filtration, the solid was washed with solvent and then the reaction mixture was analyzed by gas chromatograph (GC). Products identification was done with gas chromatography mass spectrometry (GC-MS) and confirmed by comparison of their retention times with authentic commercial samples of these compounds.
X-ray powder diffraction (XRD) patterns of the samples were recorded using a Bruker Advance D8 Diffractometer with Cu Kα radiation (λ=0.154 nm). Chemical analysis of the samples was done by energy dispersive X-ray (EDX) analysis joined to a Philips XL 30 scanning electron microscope. Transmission electron microscopy (TEM) measurements were performed on a LEO 912 AB TEM operated at 120 kV. Products of these oxidation reactions were analyzed by GC (Shimadzu 8A) and were identified by GC-MS (Finnigan TSQ-7000).
RESULTS AND DISCUSSION
Characterization of the catalysts
Fig. 1 shows XRD patterns of the Ag/ZnO nanocomposites with different Ag loadings. All the diffraction can be indexed as the typical hexagonal wurtzite structure of ZnO with lattice constants in agreement with the values in the standard card (JCPDS 36-1451). diffractions associated with Ag metal were also observed (38.1°, 44.3° and 64.4°) which were all consistent with the reported values (JCPDS 04-0783), indicating that the product was the composites of Ag and ZnO. diffractions associated with Ag metal were observed after loading of 1 % Ag and increased in intensity with increasing Ag loading. No diffractions were observed corresponding to silver oxide phases in the Ag/ZnO nanocomposites. Its average crystal size is determined to be 75 nm according to Scherrer equation (D = kλ / β cos θ).
The energy dispersive X-ray spectrum (EDS) analysis of the nanocomposites was performed in order to determine the loading amount of Ag.
Fig. 2 depicts the EDS spectrum of the products. It can be seen that the values of loaded Ag are near to 1%, 2.5% and 5%. Also, no other impurity peak is found in XRD and EDS spectra, indicating that the products are pure.
Fig. 3 shows the TEM images of Ag/ZnO nanocomposites. Based on the images, all the Ag nanoparticles were uniformly dispersed on the surface of ZnO, with a well-distributed spherical particles with the diameter ranging from 50 to about 80 nm. It can be seen that small spherical nanoparticles of Ag are found on all the three supports after Ag deposition.
Catalytic oxidation of toluene
Based on the earlier studies on the oxidation of toluene, TBHP was used as an oxidizing agent , this oxidizing agent was found to cause minimal destruction of Ag/ZnO nanocomposite, and to give better selectivity of the products. For comparative purposes, H2O2 was also used as an oxidizing agent. The solvent of acetonitrile was employed for the catalysis, since all the reagents dissolved and show the highest yields.
The performance of the synthesized nanocomposites as a heterogeneous catalysts for the oxidation of toluene was investigated using hydrogen peroxide and tert-butylhydroperoxide as oxidizing reagents (Table 1). In all the cases, the only products observed were benzaldehyde and benzoic acid. In the presence of 2.5 wt.% Ag/ZnO nanocomposite, conversion percentage of toluene was 76.3% with TBHP as an oxidant. Contrastive experiment result show that toluene oxidation with TBHP and H2O2 did not occur with removing of the catalyst under the same reaction condition. This indicated that Ag/ZnO nanocomposites acted catalysis during toluene oxidation. B. Du et al.  reported that with simple and efficient zeolite catalyst and in the presence of H2O2 oxidant, a conversion percent of around 32% of toluene with selectivity of 25% benzaldehyde, 20.8% benzyl alcohol and 27.5% benzoic acid was obtained. They also concluded that benzoic acid was obtained as the main product.
To investigate the catalytic power of reused Ag/ZnO nanocomposites, the nanocomposite used in each catalytic oxidation was isolated from the reaction mixture for reuse later. The experimental results showed that Ag/ZnO nanocomposites could be reused several times. These results proved that Ag/ZnO nanocomposites possessed good reusable catalysis.
Influences of reaction time on toluene oxidation reaction
In this experiment, the change in conversion (%) of toluene in the presence of TBHP oxidant and 2.5% Ag/ZnO nanocomposite was monitored and plotted with respect to time (Fig. 4). The reaction was carried out at reflux temperature for 8 h with 0.25 g catalyst and 32 mmol toluene and 15 mmol TBHP in a round bottom flask and some samples was drawn out at regular intervals and analyzed by GC. Fig. 4 shows that with increasing the time, the conversion of toluene was enhanced continuously until 76.1% and then remains constant after 7 h, therefore duration about 7-8 h is proper reaction time.
Influences of the loading amount of Ag on toluene oxidation reaction
For investigation of the loading effect Ag on the conversion and selectivity of the products three nanocomposites were tested. In Table 1, details of the conversion and selectivity of the products for each nanocomposite are shown. It is observed that maximum conversion occurs with the catalyst of 2.5% Ag/ZnO. It is known that Ag nanoparticles can be highly dispersed on ZnO nanoparticles at 2.5 wt.% loading. A drop of conversion of toluene of the nanocomposite with higher loadings than 2.5 wt.% is possibly due to a more reduction of the specific surface area of the nanocomposite. With these optimum conditions, the order of catalytic activities is as follows:
2.5% Ag/ZnO > 5% Ag/ZnO > 1% Ag/ZnO.
Influences of oxidant type on toluene oxidation reaction
Fig. 5 shows that the reactivity of the toluene toward oxidation with TBHP and H2O2 on Ag/ZnO nanocomposites depend on type of oxidant. tert-Butylhydroperoxide was found to be a more convenient oxidizing reagent due to weaker O-O bond than hydrogen peroxide. In this regard, it is worth noting that using H2O2 as reactant the complexes (both encapsulated and unsupported Ag) lose their characteristic color during the course of the reaction.
Oxidation of toluene with TBHP gave benzoic acid as the main product and benzaldehyde as by-product. When the oxidant was changed to hydrogen peroxide, benzaldehyde obtained as the main product. Although both TBHP and H2O2 oxidize toluene in the presence of Ag/ZnO nanocomposites, but TBHP give more oxidation of toluene under the similar conditions, which leads to the production of benzoic acid.
Comparison of catalytic power of ZnO-supported Ag with that unsupported Ag
The experimental data of toluene oxidation catalyzed by the ZnO-supported Ag or unsupported Ag in the presence of TBHP oxidant shows that the partial oxidation of toluene catalyzed by Ag supported on ZnO nanoparticles had better conversion than that of unsupported Ag nanoparticles. When 2.5% Ag/ZnO nanocomposite was used as catalyst, the toluene conversion was 76.3%. However, when the unsupported Ag nanoparticles was used as the catalyst, the toluene conversion was 35.9%. The result confirms the high stability of Ag nanoparticles on the ZnO surface or in the ZnO pores.
Ag/ZnO nanocomposites have been synthesized and were well characterized. Catalytic activities of these nanocomposites have been investigated for the selective oxidation of toluene to benzaldehyde and benzoic acid using TBHP and H2O2 as the oxidant, in the presence of acetonitrile solvent. Based on results, TBHP was found to be a more convenient oxidizing reagent. We found that 2.5 wt.% Ag/ZnO nanocomposite was a good heterogeneous catalyst for the oxidation of toluene with TBHP under our experimental conditions and could achieve about 76.3% conversion of toluene and selectivity of 29% benzaldehyde and 71% benzoic acid. According to the results, time has a crucial effect on the catalytic activity, so we were optimized the time and after 7 h the best results was shown in oxidation reaction of toluene. The recycling investigations exhibited that Ag/ZnO nanocomposites was recyclable, and could behave as a truly heterogeneous catalyst.
The authors are grateful to the Research Council of Kazerun and Shahrood branch, Islamic Azad University for financial assistance.
CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.