Introduction

Oxidation reactions are among the most important transformations in synthetic chemistry and offer an important methodology for the introduction and modification of functional groups. Therefore, the oxidation of alcohols by metal oxides to the corresponding carbonyl compounds is a worthwhile goal^1,2. In this way, chemists have used different kinds of metal salts and oxides in the form of homogeneous catalysts ^3,4 or supported metal ions and supporting oxometal catalysts as heterogeneous systems^5,6.

Many different oxidants were used for the oxidation of alcohols such as pyridinium chlorochromate (PCC), pyridinium dichromate (PDC), KMnO₄, MnO₂, CrO₃ and so on. Most of these oxidizing reagents which can be used in a research laboratory in stoichiometric amounts are expensive or toxic^1,7. Thus, the advantages of environmentally friendly oxidizing agents, such as H₂O₂ and O₂, have been extensively studied. H₂O₂ is attractive for producing active oxidation species in aqueous solution, with H₂O as a by-product.Supported metal oxide catalysts are frequently used as catalysts in partial oxidation reactions^8–10. In these catalysts, Al₂O₃, TiO₂, SiO₂ and ZrO₂ are commonly used as the supports. Bulk oxides in general cannot be used in industrial processes as they impart poor thermal stability that lead to fast deactivation of the catalyst. Furthermore, it is also known that bulk Co₃O₄ leads to high combustion of organic molecules to carbon oxides¹¹. Santra et al.¹² have studied the oxidation of benzyl alcohol with molecular oxygen in the presence of gold nanoparticles on mesoporous cerium-tin mixed oxide as catalyst. In another report, Tian and coworkers have employed RuO₂/TiO₂ nanobelt heterostructures for gas-phase selective oxidation of benzyl alcohol. The RuO₂/TiO₂samples exhibited good activity for oxidation of benzyl alcohol¹³. However, to the best of our knowledge, there is no report for application of Co₃O₄ nanoparticles supported on γ-alumina for partial oxidation of alcohols with H₂O₂ and TBHP. In this work we report Co₃O₄ nanoparticles supported on γ-alumina as catalysts for partial oxidation of alcohols to the corresponding carbonyl compounds in the liquid phase.

Experimental

Instrument and Reagents

Powder X-ray diffractions were performed by a Siemens D 5000. Specific surface area was measured by BET techniques in liquid N₂ temperature by a Strohlien. The surface morphology of the samples was obtained using a Jeol-JSM-5610 LV scanning electron microscopes (SEM). The reaction products of oxidation were identified by GC-MS (Finnigan TSQ-7000) and were analyzed by GC (Shimadzu 8A). All the reagents were commercial grade obtained from Merck. None of the oxidation products were found in the alcohols before the oxidation reaction.

Preparation of the Catalysts

Aluminum nitrate {Al(NO₃)₃.9H₂O}, aqueous ammonia {NH₃.H₂O} and deionized water were used as starting chemicals. Two hundred milliliters of deionized water was taken in a 2 l capacity round-bottom flask and stirred well using magnetic stirrer. Then, aluminum nitrate (1.5 M) solution and (10.4 M) solution of aqueous ammonia were added to 200 ml of deionized water drop by drop to precipitate Al cations in the form of hydroxides. The temperature was maintained ~55 °C during precipitation/digestion experiment. The pH after precipitation was found to be in the range of 5.4-6.4. The precipitates were further digested at 55 °C for 1 h. After the alumina gel was formed, it was filtered and washed by distilled water. Then, (0.5 M) cobalt nitrate Co(NO₃)₂ aqueous solution was added to the alumina-gel. This gel was stirred and homogenized and was placed in an oven under temperature of 100 °C for 24 h. The mixture was then heated 2 °C/min till the temperature reached 600 °C and the mixture was kept at this temperature for 4 h.

Experimental Procedure

In a typical procedure, a mixture of 0.2 g catalyst, 15 ml solvent and 20 mmol cyclohexanol was stirred under nitrogen in a 100 ml round bottom flask equipped with a condenser and a dropping funnel at room temperature for 30 min. Then 15 mmol of TBHP (solution 80% in di-tert-butylperoxide) or H₂O₂ (30% in H₂O) was added as oxidizing reagents. The resulting mixture was then refluxed for 6 h under N₂ atmosphere. After filtration, the solid was washed with solvent and then the reaction mixture was analyzed by GC. Products identification was done with GC-MS and confirmed by comparison of their retention times with authentic commercial samples of these compounds.

Results and Discussion

Characterization of the Catalysts

The XRD pattern presented in Figure 1 indicates that γ-alumina nanoparticles are formed. There is no significant change in the XRD pattern with 10 wt.% Co₃O₄ nanoparticles supported on γ-alumina which confirms that Co₃O₄ dispersed through pores does not change the γ-alumina structure.

Figure 1: XRD patterns of 10 wt.% Co3O4/nano-γ-alumina.   Click here to View figure

Scanning electron micrograph (SEM) of a typical sample of 10% Co₃O₄/nano-γ-alumina is shown in Figure 2. It is clarified from Figure 2 that the sizes of the particles are in the ranges of 40-80 nm. This result was coincident with the particle sizes calculated from the Scherrer equation. Similar images were obtained for the other catalysts. Specific surface area measured with BET method was 186 m²/g for γ-alumina and 164 m²/g for 10% Co₃O₄/nano-γ-alumina. This reduction in specific surface area for the supported Co₃O₄ may be an indication of encapsulation of Co₃O₄ in the nano-γ-alumina pores.

Figure 2: SEM photograph of 10% Co3O4/nano-γ-alumina.   Click here to View figure

Catalytic Oxidation of Cyclohexanol

The use of TBHP as an oxidant was based on the earlier studies on the oxidation of alcohols¹⁴, this oxidant was found to cause minimal destruction of the cobalt oxide catalysts, and to give better activity of the catalysts. The performance of the set of samples prepared as catalysts for the oxidation of alcohols was tested with TBHP. The solvent of acetonitrile was employed for the catalysis, since all the reagents dissolved and gave the highest yields of the products. At first, the reactivity of a model compound, cyclohexanol, was examined under a variety of experimental condition (Table 1). In all reactions were produced only one product (cyclohexanone) therefore, selectivity (%) is 100 with respect to it. The research results showed that three kinds of catalysts could catalyze cyclohexanol oxidation with TBHP. The activity of the catalysts was as follows: 10% Co₃O₄/nano-γ-alumina > 15% Co₃O₄/nano-γ-alumina > 5% Co₃O₄/nano-γ-alumina. In the presence of 10% Co₃O₄/nano-γ-alumina, conversion percentage of cyclohexanol was 82.3% with TBHP as an oxidant and acetonitrile as the solvent. Contrastive experiment results show that cyclohexanol oxidation with TBHP did not occur in the absence of the catalyst under the same reaction condition. In addition, unsupported γ-alumina has shown lower catalytic activity than the supported catalyst.

Table 1: Results of the cyclohexanol oxidation using Co₃O₄/nano-γ-alumina as catalysts.

Reaction condition: 0.2 g catalyst, cyclohexanol 20 mmol, oxidant 15 mmol, solvent of acetonitrile, reflux temperature, reaction time 6 h.

Influences of Reaction Time on Cyclohexanol Oxidation Reaction

In this experiment, the change in conversion percentage of cyclohexanol in the presence of tert-butylhydroperoxide oxidant and 10% Co₃O₄/nano-γ-alumina catalyst was monitored and plotted with respect to time (Figure 3). The reaction was carried out at reflux temperature for 6 h with 0.2 g catalyst and 20 mmol cyclohexanol and 15 mmol TBHP in a round bottom flask and some samples was drawn out at regular intervals and analyzed by GC. Figure 3 shows that the conversion of cyclohexanol increases continuously until 82.1% as time increases and then remains constant after 5 h, therefore duration about 5-6 h is proper reaction time.

Figure 3: The effect of reaction time on cyclohexanol conversion.   Click here to View figure

Influences of the Loading Amount of Cobalt Oxide on Cyclohexanol Oxidation Reaction

For investigation of the loading effect Co₃O₄ on the conversion and selectivity of the products three catalysts were tested. In Table 1, details of the conversion and selectivity of the products for each catalyst are shown. It is observed that maximum conversion occurs with the catalyst of 10% Co₃O₄/nano-γ-alumina. It is known that cobalt oxide can be highly dispersed on γ-alumina at 10 wt.% loading. A drop of conversion of cyclohexanol of the catalyst with higher loadings than 10 wt.% is possibly due to a more reduction of the specific surface area of the catalyst. Under these reaction conditions, the order of catalytic activities is as follows:

10% Co₃O₄/nano-γ-alumina > 15% Co₃O₄/nano-γ-alumina > 5% Co₃O₄/nano-γ-alumina.

Influences of Substrates and Oxidant Type on Cyclohexanol Oxidation Reaction

Figure 4 shows that the reactivity of the cyclohexanol toward oxidation with TBHP and H₂O₂ on Co₃O₄ nanoparticles supported on γ-alumina catalysts depend on type of oxidant. tert-butylhydroperoxide (TBHP) was found to be a more convenient oxidizing reagent due to weaker O^_O bond than hydrogen peroxide (H₂O₂).

Figure 4: The effect of oxidant type on cyclohexanol conversion in the presence of acetonitrile as the solvent.   Click here to View figure

In this study, experiments on various selected alcohols were performed and the comparisons with respect to conversion and product selectivity are represented in Table 2. Higher conversion was obtained for cyclohexanol on 10% Co₃O₄/nano-γ-alumina catalyst using tert-butylhydroperoxide oxidant. Table 2 shows that the reactivity of the alcohols toward oxidation with TBHP and H₂O₂ on 10% Co₃O₄/nano-γ-alumina catalyst depends on the particular structure of the substrate.

Table 2: Effect of the 10% Co₃O₄/nano-γ-alumina catalyst in the oxidation of different alcohols.

Conclusion                                      

In this study, a γ-alumina was prepared, and then its role in the catalytic activity of the cobalt oxide nanoparticle in the oxidation reaction of cyclohexanol in the liquid phase was investigated. The characteristics of catalysts that were prepared using the XRD, SEM and BET were investigated. In addition, the catalytic activity of the cobalt oxide nanoparticles supported on γ-alumina in oxidation of cyclohexanol with tert-butylhydroperoxide (TBHP) as well as hydrogen peroxide (H₂O₂) as the oxidant and in the liquid phase were studied which revealed that TBHP oxidant was better than H₂O₂. In addition, methanol, ethanol and acetonitrile were used as the solvent. Due to its high polarity and the creation of reactive oxygen species in the reaction medium, acetonitrile was far better than other solvents. Finally, the oxidation of different alcohols was analyzed using optimum conditions that became clear that various alcohols showed different percentage of conversion that was based on the type of alcohols structure.

Acknowledgement

We gratefully acknowledge financial support from the Research Council of kazerun Branch, Islamic Azad University.

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