High catalytic activity of magnetic Fe3O4@SiO2-Schiff base-Co(II) nanocatalyst for aerobic oxidation of alkenes and alcohols and DFT study

A new heterogeneous nanocatalyst (Fe3O4@SiO2-Schiff base-Co(II)) was successfully fabricated applying silica-coated magnetite nanoparticles as a suitable and efficient support for covalent anchoring of a cobalt(II) Schiff base complex. This catalyst was characterized by FT-IR, XRD, SEM, TEM, EDX, ICP-OES, CHN, DRS, TGA, VSM techniques, and QM study. According to theoretical calculations, the Schiff base-Co(II) complex at high spin state with square planar geometry is more stable than low spin state. Also, Schiff base-Co(llyimidazole complex has distorted square pyramidal geometry. Evaluating the activity of the nanocatalyst was performed for the oxidizing of different olefins (styrene, norbornene, alpha-methyl styrene, cyclohexene, and cyclooctene) and alcohols (benzyl alcohol, benzhydrol, n-butanol, n-octanol and n-heptanol). In these reactions, the influence of two various oxidants, molecular oxygen and tert-butyl hydroperoxide (TBHP), were examined. The results exhibited that the nanocatalyst was considerably effective for the aerobic epoxidation of norbornene, cyclohexene, and cyclooctene with 100% conversion and selectivity within 1 h. Also, by oxidizing of benzyl alcohol and benzhydrol, benzoic acid (conversion: 94%, selectivity: 91%) and benzophenone (conversion and selectivity: 100%) were achieved, respectively. The results suggested that the nanocatalyst has a remarkable role in oxidation reaction of olefins and alcohols. Other promising advantages of the catalyst were easy magnetic separation and recyclability for several times without significant loss of catalytic efficiency. (C) 2018 Elsevier B.V. All rights reserved.