The highly selective oxidation of cyclohexane to cyclohexanone and cyclohexanol over VAlPO4 berlinite by oxygen under atmospheric pressure
© The Author(s) 2018
Received: 19 December 2017
Accepted: 21 March 2018
Published: 4 April 2018
The oxidation of cyclohexane under mild conditions occupies an important position in the chemical industry. A few soluble transition metals were widely used as homogeneous catalysts in the industrial oxidation of cyclohexane. Because heterogeneous catalysts are more manageable than homogeneous catalysts as regards separation and recycling, in our study, we hydrothermally synthesized and used pure berlinite (AlPO4) and vanadium-incorporated berlinite (VAlPO4) as heterogeneous catalysts in the selective oxidation of cyclohexane with molecular oxygen under atmospheric pressure. The catalysts were characterized by means of by XRD, FT-IR, XPS and SEM. Various influencing factors, such as the kind of solvents, reaction temperature, and reaction time were investigated systematically.
The XRD characterization identified a berlinite structure associated with both the AlPO4 and VAlPO4 catalysts. The FT-IR result confirmed the incorporation of vanadium into the berlinite framework for VAlPO4. The XPS measurement revealed that the oxygen ions in the VAlPO4 structure possessed a higher binding energy than those in V2O5, and as a result, the lattice oxygen was existed on the surface of the VAlPO4 catalyst. It was found that VAlPO4 catalyzed the selective oxidation of cyclohexane with molecular oxygen under atmospheric pressure, while no activity was detected on using AlPO4. Under optimum reaction conditions (i.e. a 100 mL cyclohexane, 0.1 MPa O2, 353 K, 4 h, 5 mg VAlPO4 and 20 mL acetic acid solvent), a selectivity of KA oil (both cyclohexanol and cyclohexanone) up to 97.2% (with almost 6.8% conversion of cyclohexane) was obtained. Based on these results, a possible mechanism for the selective oxidation of cyclohexane over VAlPO4 was also proposed.
With the development of petrochemical industry, the oxidation of cyclohexane under mild conditions, with molecular oxygen or air, is of great interest [1, 2]. In the autoxidation of cyclohexane, most industrial processes are involved with the usage of soluble transition metal catalysts, including vanadium oxide, at 423 ~ 453 K and afford the mixture of cyclohexanol, cyclohexanone and dicarboxylic acids, which is formed by further oxidation of cyclohexanone and cyclohexanol [2, 3]. However, the use of soluble metal catalysts in these systems often requires a tedious catalyst separation step . Thus, it is necessary to develop effective recyclable heterogeneous catalysts for selective oxidation of cyclohexane by O2.
The AlPO-n families are divided into two groups: dense-phase berlinite or tridymite and porous aluminophosphate molecular sieve . Berlinite is the nonporous and stable phase of polymorphous aluminophosphates  and potentially mainly used in functional material fields, such as acoustic wave device, memory glass  and piezoelectric material , as well as, high-performance sealants for corrosion- and wear-resistant coatings . Porous aluminophosphates and their derivates (MeAPO-n) incorporated with transition metals were widely used as catalysts, including VAPO-5 molecular sieves . For example, they have been frequently used as catalysts for the selective oxidation of cyclohexane to produce cyclohexanol and cyclohexanone [10, 11]. At the same time, the heterogeneous MeAPO-n molecular sieve as catalysts is a very controversial issue and it is generally recognized that metals are leached into the polar solvents, such as acetic acid .
Berlinite is more stable than MeAPO-n molecular sieve [5, 6]. But they had seldom been applied in catalytic cyclohexane oxidation. Accordingly, we report the application for the first time as well as the preparation, characterization and catalytic performance in cyclohexane oxidation of a new VAlPO4 berlinite, in which vanadium was incorporated. It is found to be an active recyclable heterogeneous catalyst for the selective oxidation of cyclohexane with molecular oxygen under mild conditions.
Al(CH3COO)3·2H2O, H3PO4 (85% sol in water), and V2O5 were used as the sources of aluminum, phosphorus, vanadium, and triethyl amine (Et3N) was used as template. VAlPO4 berlinite was synthesized from the gel according to the following molar ratio: 0.02 V:0.92 Al:1.0 P:0.81 Et3N:30 H2O. During typical synthesis, Al(OAc)3 was hydrolyzed firstly at room temperature for 2 h, and aqueous solution of V2O5 and H3PO4 was added into the obtained solution. The formed mixture was stirred at room temperature for 2 h and Et3N were then added into the homogeneous gel at 273 K under vigorous stirring. Finally, the mixture was stirred at 273 K for another 3 h. The final gel was charged in a Teflon-lined autoclave and allowed to crystallize at 453 K for 48 h. The VAlPO4 berlinite was filtered and washed several times with deionized water until the pH value was 7. The crystals were dried at 373 K for 6 h and then calcined at 823 K for 10 h to remove the Et3N template.
VAlPO-5 molecular sieve was also synthesized according the method reported by Concepción et al. .
XRD was performed on a Brucker D8 Advance diffractometer with Cu Kα1 radiation according to the scanning range of 2θ = 6–80° at a rate of 1°/min. Fourier transform infrared (FT-IR) spectroscopy was conducted on a Varian 3100 spectrometer in transmission mode with the resolution of 4 cm−1. The VAlPO4 specimen was mixed with KBr according to the weight ratio of 1:200 and pressed into pellets for measurement. The spectra were recorded as the accumulated results of 125 scans and the spectra of dry KBr were selected for background subtraction. X-ray photoelectron spectroscopy (XPS) was carried out on a Phi Quantum 2000 Scanning ESCA Microprobe with Al Kα radiation. A C1s binding energy of 284.6 eV was used as the reference. Microphotography and EDAX analyses were performed on a Philips SEM 505 instrument equipped with an EDAX detecting unit. Chemical analyses of V content were performed by atomic absorption spectroscopy (AAS) with a Varian AA240 spectrometer. The chemical compositions determined with EDAX were compared with the results obtained by XPS and the content of vanadium obtained by AAS analyses of the solutions prepared by thermal acid digestion of the sample.
The catalytic performance of VAlPO4 berlinite was tested through cyclohexane (≥ 99.5%, without further purification, Beijing Chem. Corp.) oxidation as model reaction with molecular oxygen under atmospheric pressure. The reaction was carried out at 348 K in a 250 mL three-neck flask equipped with a condenser. Typically, 80 g cyclohexane, 40 g acetic acid (used as solvent), 0.5 g cyclohexanone (used as initiator) and 0.5 g catalyst were added into the three-neck flask at room temperature. Then, the reactor was heated to the reaction temperature and the reaction solution was stirred with an external magnetic stirrer. At the reaction temperature, the reactor was charged with a flow of O2. The flow rate of the O2 was controlled in the way that bubbles of oxygen appeared in the solution and that no oxygen could be detected in the outlet of the condenser to ensure that oxygen was totally consumed by the oxidation of cyclohexane. After 6 h, the reaction stopped. After cooling down to room temperature, the reaction mixture was diluted with 20 g ethanol to produce a homogeneous solution and then the catalyst was separated through filtration. The filtration solution was used for composition analysis.
To examine the stability of the catalyst, the solution of product mixtures obtained from the oxidation of cyclohexane as mentioned above was filtered to remove the catalyst. The obtained solution was used directly as the reactant without the addition of catalyst, cyclohexanone and acetic acid and subjected to the oxidative reaction in the same condition: reaction temperature of 348 K, the oxidant of molecular oxygen and atmospheric pressure. After 10 h, the reaction stopped. The product mixture was sampled and analyzed.
The reaction products were analyzed by GC–MS and HPLC for identification (Additional files 1 and 2) . The quantitative analyses of cyclohexanol and cyclohexanone were carried out by Agilent 4890D gas chromatography with OV-1701 column (30 m × 0.25 mm × 0.3 µm) and the internal standard of methylbenzene. The carboxylic acids were analyzed on Agilent 1100 Series HPLC instrument with a 250 × 4.6 mm Microsorb-MV (C18) column and an ultraviolet detector. The analysis conditions were provided as follows: flow phase of water/methanol (10 ~ 30%)/KH3PO4 (5 mM), pH value (3 ~ 4) of flow phase adjusted with H3PO4 (25%), flow rate of 1.0 mL min−1, column temperature of 298 K and ultraviolet wavelength of 212 nm. The contents of by-products acid were determined according to external standard method and calculated according to the equation W sp = W st ·A sp /A st × 100%, where sp and st indicated specimen and standard, respectively. The conversion rate of cyclohexane and the yield of cyclohexanol and cyclohexanone were calculated according to the converted cyclohexane.
The solid catalyst was separated by filtration and washed with 20 mL of acetone, and then dried at 373 K for 2 h after each reaction.
Results and discussion
Catalytic oxidation of cyclohexane over VAlPO4 berlinite and VAPO-5 molecular sieve
For comparison, under the same reaction conditions for the oxidation of cyclohexane, we studied the catalyst of AlPO4 berlinite without the incorporation of V and the catalyst of VAlPO4 berlinite. AlPO4 berlinite did not exhibit any significant activity. The higher activity of VAlPO4 berlinite may be attributed to that V(V) ions are incorporated into the berlinite framework, resulting in oxygen vacancies in close vicinity to V(V), and possessed a higher tendency to draw electrons as compared to those in V2O5. In order to check the reusability of the catalyst, it was recycled for five times without activity loss. Thus, in the oxidation of cyclohexane with molecular oxygen under mild conditions, compared with other berlinite catalysts, such as AlPO4, CoAlPO4 and MnAlPO4, VAlPO4 berlinite showed higher catalytic activity. Then, Factors influencing the reaction using VAlPO4 berlinite as catalyst were studied systematically, with a possible reaction mechanism also proposed.
Effect of solvents
Conversions of cyclohexane and selectivities to products in different solvents
Effect of reaction temperature
Effect of reaction time
Mechanistic consideration to the oxidation of cyclohexane with molecular oxygen over the VAlPO4 catalyst
A new material, VAlPO4 berlinite, has been prepared and characterized. It is proved that the vanadium is incorporated into the framework of AlPO4 berlinite. The catalytic activity of VAlPO4 berlinite in cyclohexane oxidation is higher than that of CoAPO4 or MnAPO4 under the same conditions and similar loads of cobalt and manganese. Furthermore, AlPO4 berlinite without the incorporation of any metal is not active in the oxidation of cyclohexane with molecular oxygen under mild conditions. Although the catalytic activity of VAPO4-5 molecular sieve is similar to that of VAlPO4 berlinite under the same conditions, high leaching ratio of vanadium into the solution is observed when VAPO4-5 molecular sieve is used as catalyst. Meanwhile, the mechanism for the oxidation of cyclohexane with molecular oxygen over the VAlPO4 catalyst may have resulted from a catalytic cycle involving a key active intermediate species-formed from the nucleophilic addition of the lattice oxygen ion with the carbon in cyclohexane—that leaves an oxygen vacancy at the VAlPO4 catalyst surface, which further splits oxygen molecules into atoms and then acts as a reservoir that can take up these atoms and then release them to form molecules. In conclusion, VAlPO4 berlinite is an efficient recyclable heterogeneous catalyst for the selective oxidation of cyclohexane with molecular oxygen under mild conditions.
This study was conceived as a result of discussion between DLS and YXF. The synthesis and characterization of the VAlPO4 catalyst and its catalytic performance evaluation were carried out by YH. The spectroscopic analysis was performed by DLS, who proposed also the reaction mechanism of the selective oxidation of cyclohexane with oxygen over the VAlPO4 catalyst. The manuscript was wrote by DLS. All authors read and approved the final manuscript.
We are grateful for the financial support provided by the Science and Technology Program of Guangzhou (No. 201607010166), China.
The authors declare that they have no competing interests.
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