Characterization and catalytic performance of Co-MCM-41 for styrene epoxidation

The cobalt-containing MCM-41 can be used as the catalyst for the liquid-phase epoxidation of styrene with molecular oxygen. The correlations between its structures, especially the states of Co in it, and its catalytic performance were studied. It may be helpful to make clear the nature of active sites for the epoxidation. Co-MCM-41 samples synthesized by a template-ion exchange (TIE) method, a direct hydrothermal (DHT) method and a conventional impregnation method have been characterized in detail. The X-ray diffraction and N-2-adsorption results indicate that the hexagonal array of mesopores in these samples is sustained as the cobalt contents are lower than 2.2 %. The results of diffuse reflectance UV-Vis spectroscopy and H-2 temperature-programmed- reduction suggest that the cobalt introduced into Co-MCM-41 by the TIE method exists mainly as single-site Co(II) ions, whereas the cobalt in the sample synthesized by the DHT method exists mainly as Co2SiO4 in a microcrystalline form and the samples prepared by the impregnation method contain a large proportion of cobalt oxide. With a similar cobalt content (ca 1.0%), the sample prepared by the TIE method gave a conversion of 45 % in the epoxidation of styrene with molecular oxygen, however, the sample prepared by the DHT method or impregnation method gave a conversion lower than 30 %. Since cobalt oxide and Co2SiO4 show a lower activity for the epoxidation reaction and there may also exist a small part of single-site Co(II) ions in the samples prepared by the DHT method or impregnation method, we thus suggest that the single-site Co(II) ions in MCM-41 are the real active sites for the epoxidation with molecular oxygen