Background: 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.
Results: 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.
Conclusions: As a heterogeneous catalyst VAlPO4 berlinite is both high active and strong stable for the selective oxidation of cyclohexane with molecular oxygen. We propose that KA oil is formed via a catalytic cycle, which involves activation of the cyclohexane by a key active intermediate species, formed from the nucleophilic addition of the lattice oxygen ion with the carbon in cyclohexane, as well as an oxygen vacancy formed at the VAlPO4 catalyst surface.
Keywords: Berlinite; Cyclohexane; Heterogeneous catalyst; Oxidation.