Magnetic CoFe2O4 nanocrystal: A novel and efficient heterogeneous catalyst for aerobic oxidation of cyclohexane

Magnetic nanocrystalline spinel-type cobalt ferrite complex oxides were prepared by a simple sol-gel auto-combustion method at five different molar ratios of Co-II to Fe-III (1:4.0, 1:2.8, 1:2.0, 1:1.5, and 1:1.1) and characterized by X-ray diffractometry (XRD), Fourier transform infrared spectrophotometry (FT-IR) and transmission electron microscopy (TEM). Their catalytic performance was evaluated in the oxidation of cyclohexane with oxygen as oxidant in the presence of neither solvents nor reducing agents. It has been revealed that the catalysts had high catalytic activities for heterogeneous oxidation of cyclohexane. Especially, when pure CoFe2O4 was used as catalyst; excellent results with turnover number of 288 and 92.4% selectivity for cyclohexanone and cyclohexanol was obtained under 1.6 MPa of initial oxygen pressure at 418 K after 6.0 h of reaction. The influences of initial oxygen pressure, reaction temperature, reaction time, and free-radical initiator on the oxidation of cyclohexane over CoFe2O4 were investigated. The catalyst can be easily separated by an external magnet and no obvious loss of activity was observed when reused in five consecutive runs. CoFe2O4 was proved to be also efficient for oxidation of linear alkanes. (C) 2009 Elsevier B.V. All rights reserved.