Magnetically recyclable core–shell Fe3O4@chitosan-Schiff base complexes as efficient catalysts for aerobic oxidation of cyclohexene under mild conditions

Abstract

Five magnetic core–shell type Fe3O4@chitosan-Schiff base Co(II), Cu(II) and Mn(II) complexes were prepared in a simple way and well characterized by Fourier transform infrared spectrophotometry (FT-IR), X-ray photoelectron spectroscopy (XPS), X-ray diffractometry (XRD), transmission electron microscopy (TEM), dynamic light scattering (DLS) and thermogravimetry (TG). Their abilities to catalyze oxidation of cyclohexene with molecular oxygen in the absence of any solvents or reducing agents were investigated. It has been revealed that the as-prepared samples have high catalytic activities for heterogeneously aerobic oxidation of cyclohexene and 2-cyclohexene-1-one is the main product in all cases. Especially, when the Schiff base Co(II) complex derived from 5-nitrosalicyaldehyde was employed as catalyst, 46.8% of cyclohexene conversion, 77.2% of selectivity for 2-cyclohexene-1-one and as high as 4.7×103 of turnover number were obtained under ambient pressure at 70°C for 12h of reaction. The catalyst can be magnetically separated easily for reuse and no obvious loss of activity was observed when reused in five consecutive runs.