Metal states in cobalt- and cobalt–vanadium-modified MCM-41 mesoporous silica catalysts and their activity in selective hydrocarbons oxidation

MCM-41 mesoporous molecular sieves with various V and Co loading were obtained by hydrothermal synthesis. The structural regularity and morphology of the resultant materials were characterized by XRD, TEM, SEM and N2 adsorption. The information about the nature, co-ordination and location of the metal species were obtained by temperature-programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS) and diffuse reflectance spectroscopy (DRS). These methods indicated different localization of the cations in extra-framework positions or in the skeleton of the molecular sieves. In the low loaded cobalt catalysts, Co2+ in tetrahedral position was observed. The increase in metal content led to the appearance of Co3+ in Oh symmetry. In both cases, the cobalt ions were placed outside of the silica framework. Vanadium in the bimetallic samples was incorporated inside the framework of the molecular sieves and on the channel walls. V5+ was in tetrahedral symmetry. Cobalt in the bimetallic samples was presented as Co2+ in Td symmetry. When Co and V were introduced together in the starting gel, a lower quantity of vanadium was incorporated into the mesoporous sieve. The modified MCM-41 molecular sieves obtained by hydrothermal synthesis were active in selective oxidation of styrene to benzaldehyde and of benzene to phenol.