The selective oxidation reaction of alcohols to aldehydes and ketones is a basic reaction in organic synthesis [1-2]. Most common inorganic compounds in order to oxidize alcohols are well known, such as KMnO4, MnO2 and K2Cr2O7 , but these oxidants are toxic. Due to beneficial properties of hydrogen peroxide, significant progresses have been achieved for oxidation of aliphatic alcohols with hydrogen peroxide, catalyzed by nanocomposite. Hydrogen peroxide is an environmentally safe and interesting oxidant for different organic oxidation reactions [4-5].
Polyoxometalates (POM) are categorized as one of the most important catalysts in inorganic metal oxide cluster group. Hetropolyacid (HPAs) compounds of keggin have attracted many attentions, because of their applications, such as significant catalytic properties and high activity [6-8]. The problem of HPAs is the low available surface area. Supporting compounds on nanoparticles is the best solution for improving their effective surface area and solubility in polar solvents. Mesoporous materials such as zeolites, silica, and titania have been used as supports for PTA [9-13]. Due to its chemical and physical stability, hydrophilic nature and being mesoporous, Silica (SiO2) has attracted growing attentions in diverse applications [14-15]. Using Polyoxometalates dissolved in polar solvents composed of homogeneous catalysts provides convenience in use due to the ease of their recovery. A utilized method is immobilizing POM species on the surface of nanoparticles which can be isolated and recovered from the reaction system by a simple filtration. Similar research has been done in recent years [16-17].
Magnesium aluminate has attracted many attentions as catalyst support because of its structure stability and high melting point. Magnesium aluminate has a good resistance against chemical changes. Also it has high resistance at very high temperatures, has a very high melting point (2135) and low density (3.58 g/cm3) . Different methods for production of magnesium aluminate nanoparticles include sol-gel [19-20] co-precipitation  and self- combustion. These methods are mainly expensive in large scale production. In general, smaller particle size along with higher surface area is suitable for various catalytic applications .
In this research, a new composite was prepared by a simple method for oxidation of aliphatic alcohols to aldehydes and ketones with hydrogen peroxide as oxidant and water as solvent. This nanocomposite was proved to be useful for immobilization of PTA on silica coated magnesium aluminate nanoparticles. Primary alcohols were converted to aldehydes with high selectivity and conversion ratios. Cyclohexanol was converted to cyclohexanone with a high selectivity [23-27]. At the end of the reaction, the products were isolated from the aqueous phase by a simple filtration (see Fig. 1).
Materials and methods
All chemicals were purchased from Sigma-Aldrich or Merck with analytical grade and used as received. H3PW12O40 (PTA) was purchased from Sigma-Aldrich. Fourier transform infrared (FT-IR) spectra were obtained using FT BOMEM MB102 IR spectrophotometer. X-ray diffraction (XRD) patterns of the synthesized samples were determined with a Philips X-ray diffractometer (model PW1730). The SEM images were obtained by Hitachi Japan S4160. The Scanning Electron Microscopy analysis was performed along with an Energy Dispersive Spectroscopy (EDX).
Synthesis of MgAl2O4 @SiO2–PTA nanocomposite
MgAl2O4 was prepared using the method reported by Mostafa Y. Nassar . Coating of MgAl2O4 nanoparticles with a layer of silica was performed by the following method. In this method, ethanol (1/5 ml) and H2O (10 ml) were mixed and put in ultrasonic bath for 1hour. After that, NH3 (1/5 ml) was added to this mixture. Then, 2g of MgAl2O4 particles were dispersed in it and the mixture was vigorously stirred at 40°C for 10 minutes. Then, TEOS (0/53ml) was added and mixing was continued for 1 hour and the mixture was stirred overnight. The coated particles were separated by filtration and washed with ethanol several times and dried at 80°C in the oven. In this process, for synthesis of the nanocomposite, ethanol/water (80 : 20 ratio) solution was prepared and then 1.0 g of PTA was added and the solution was stirred at room temperature for 1 hour. After that, 1.0 g of MgAl2O4 @SiO2 was dispersed in it and the mixture was stirred for another 3 hours. The resulting precipitation was separated by filtration. At last, this nanocomposite was washed with ethanol and dried in an oven at 100 ◦C for 2 hours. The typical preparation of composite (MgAl2O4@SiO2–PTA) is shown in Fig. 2.
A mixture of an alcohol (1mmol), catalyst (0.015 mmol), water (3 ml), and H2O2 (30% aq.) were stirred at 90 for the specified time. The reaction was characterized by TLC. CH3Cl was added to dilute the reaction mixture and the organic layer was isolated by filtration. At the end of the reaction, the isolated solution was purified on a silica-gel plate to obtain pure product. After separation of product, recycled catalyst was reused for the next oxidation cycle with alcohols (1 mmol), H2O2 (30% aq) and water.
RESULTS AND DISCUSSION
Characterization of MgAl2O4@SiO2–PTA composite
MgAl2O4@SiO2-PTA was synthesized by a simple method. MgAl2O4 was prepared via sol- gel combustion method as a support for nanocomposite. Then, it was coated by thin layer of silica. The FT-IR spectra of MgAl2O4 and MgAl2O4@SiO2-PTA, is presented in Fig. 3. In the IR spectra, the MgAl2O4 sample display two characteristic at 555 cm-1 and 700 cm-1 consecration to the [AlO6] groups, the lattice vibration of Mg–O stretching, and displaying the formation of MgAl2O4 spinel [29-30]. Two vibration bands at 3358 cm-1 and 1607 cm-1 appeared in the IR spectra of the products could be attributed to the stretching and bending vibrations of the molecular surface adsorbed water interacting with MgAl2O4 [31-34]. Two bands at 1080 cm-1 and 794 cm-1 were observed which are ascribed to the symmetrical and asymmetrical vibrations of the Si–O–Si bonds. Two other bands at 894 cm−1 and 981 cm−1 were attributed to the immobilized PTA .
The XRD patterns of MgAl2O4 and MgAl2O4@SiO2-PTA, are displayed in Fig. 4. The peaks are compatible with pure MgAl2O4 phase (JCPDS card no. 98-007-5897), indicating the retention of spherical spinel structure of MgAl2O4 during the functionalization and coating process. The crystallite size of MgAl2O4 nanoparticles was determined about 40 nm using Debye–Scherrer formula (Fig. 4A). The XRD patterns of MgAl2O4, after coating with silica and functionalization with PTA (Fig. 4B), are more or less the same as previous MgAl2O4 with the same spinel structure. Comparison to XRD patterns of MgAl2O4 (A) and MgAl2O4@SiO2–PTA (B). (see Table 1).
Scanning Electron Microscopy (SEM)
The morphology of MgAl2O4 and MgAl2O4@SiO2–PTA were investigated by SEM are shown in Fig. 5. SEM images display that MgAl2O4 and MgAl2O4@SiO2–PTA particles are well dissolved and most of the fabricated nanoparticles have spherical structures (Fig. 5 A and B). As it is shown in the Fig. 3, the maximum of particles size is 31nanometer.
EDX Microscopy and Brunauer-Emmett-Teller (BET) surface area analysis
The EDX spectrum is shown in Fig. 6. The spectrum clearly reveals the presence of all the expected main elements (O, Si, P, Al, Mg, and W) including the three component composite. The surface area of MgAl2O4 @SiO2–PTA was also estimated by using the N2 gas adsorption (BET) method and a value of 36.73m2 g−1 was found for this composite.
Aliphatic alcohols oxidation
As indicated in Table 2. The primary alcohols (amyl alcohol and ethanol) were converted to aldehydes after oxidation. The linear aliphatic alcohols (amyl alcohol, ethanol and 2-propanol) were more difficult to oxidize than cyclohexanol. Consequently, 5/1 molar ratio H2O2/alcohol and 6 hours reaction time was needed for oxidation of these alcohols (entries 2-4). About cyclohexanol (entry 1) the 2/1 molar ratio received of H2O2 /cyclohexanol was appropriate for this reaction (see Table 2).
The reaction of cyclohexanol, hydrogen peroxide and MgAl2O4@SiO2-PTA was chosen as a model for aliphatic alcohols oxidation reaction. In order to obtain the best results, different solvents such as water, acetonitrile, toluene, chloroform and solvent free condition were employed. The results are displayed in Table 3. The best results were obtained as soon as this reaction was terminated by H2O (see Table 3).
Although cyclohexanol is very active in this system, the control experiment displayed that no product was detected in cyclohexanol oxidation without the catalyst. MgAl2O4, MgAl2O4@SiO2, MgAl2O4@SiO2-PTA and H3PW12O40 were used to evaluate the effectiveness of catalyst on this reaction. As shown in Table 4. MgAl2O4@SiO2-PTA displayed the best catalytic activity for oxidation of cyclohexanol (see Table 4).
The oxidation of cyclohexanol to cyclohexanone by MgAl2O4@SiO2-PTA with various molar ratio of H2O2/ cyclohexanol was studied in details and results are listed in Table 5. When the amount of H2O2/ cyclohexanol was 2/1 (entry1), yield of product (cyclohexanone) was 93%, but while using equal ratio of H2O2/ cyclohexanol, the production yield decreased to 80 % at the same time. Increasing the ratio of H2O2/ cyclohexanol to 4/:1 resulted in a 60 % of decrease in the amount of cyclohexanone. The best molar ratio was 2 : 1 for H2O2/ cyclohexanol in this reaction. The amount of hydrogen peroxide in this reaction is very effective under these conditions. (see Table 5).
Recovering of the catalyst
The recovery process of MgAl2O4@SiO2-PTA in this system has the same process as the optimized conditions. The catalyst was isolated from final product by a simple filtration. After that, it washed twice with water and ethanol. After drying the isolated catalyst at 100 for 2 hours, it was reused in a new reaction. The results are listed in Table 6. The recovered catalyst could be reused for four times with no changes in activity (see Table 6).
In this study a novel nanocomposite as a recyclable catalyst was introduced for oxidation of aliphatic alcohols in water with hydrogen peroxide. The synthesized catalyst was analyzed by X-ray diffraction (XRD), FT-IR, SEM, EDX and Brunauer-Emmett-Teller (BET) surface area analysis techniques. Cyclohexanol was converted to cyclohexanone with a high yield by controlling the ratio of H2O2to cyclohexanol. The reaction of this nanocomposite is environmentally friendly. Also, the catalyst is recyclable in aliphatic alcohols oxidation process. This catalyst could be recovered by simple filtration and reused for four times without any loss of activity.
The authors wish to acknowledge the support of this work by the Department of Chemistry, Tehran North Branch, Islamic Azad University, Tehran.
CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this review article.