Synthesis, characterization and catalytic properties in oxidation of styrene over cobalt-substituted microporous nickel phosphate CoVSB-5

Cobalt-substituted microporous nickel phosphates VSB-5 (CoVSB-5s) employing triethylamine as an organic template have been synthesized under hydrothermal conditions. The incorporation of cobalt (II) into the framework of VSB-5 has been characterized by powder X-ray diffraction pattern, diffuse reflectance UV-vis spectroscopy, ICP-AES elemental analysis and IR spectroscopy methods. The molar ratio of Co/(Co + Ni) incorporated into the framework of CoVSB-5 is up to 20.01 mol% without changing the structure of the VSB-5 and no other phases produced. The cobalt ions coordinated in the architecture of the as-synthesized CoVSB-5 are located in octahedral and tetrahedral sites. After calcination in air at a temperature above 300 °C, the related absorption bands from UV-vis spectrum which are ascribed to Co(II) in a tetrahedral environment shift to high wavelengths at 510 and 586 nm and the octahedral Co(II) turns into tetrahedral Co(II) due to dehydration under calcination conditions. CoVSB-5 powders have a little lower crystallinity than that of pure VSB-5. The crystallinity reduces from 82% for 5CoVSB-5 to 53% for 20CoVSB-5 using VSB-5 as standard. The catalytic properties of those CoVSB-5s are demonstrated by the heterogeneous catalytic oxidation of styrene with dilute hydrogen peroxide. The conversion of styrene over CoVSB-5s is higher than that of VSB-5 and the primary product of the oxidation reaction is benzaldehyde. The influence factors of the catalyst's calcined condition, reaction temperature and cobalt contents on the catalytic activity have been investigated.