Nature of vanadium species in vanadium-containing silicalite and their behavior in oxidative dehydrogenation of propane

V-contg. silicalite samples prepd. hydrothermally were characterized using a combination of physicochem. techniques and the activity of these silicalites in the oxidative dehydrogenation of propane (I) using O and N2O was studied. FT-IR data on the mechanism of I transformation in the presence of gaseous O on these samples were also reported. The results indicate the presence of various types of V species, and in particular, the formation of small amts. of a tetrahedral V5+ species stabilized in its configuration by interaction with the framework. This species probably forms at defect sites and is the more active and selective species in the oxidative dehydrogenation of I. Activated O species generated by the interaction of O or N2O with reduced V sites are suggested to be responsible for the selective transformation of I to propylene (II). However, the good selectivity of this catalyst in II formation from I is also connected to the type of chemisorption of the intermediate II on these tetrahedral V5+ sites and to the relative inertness towards its further transformation due to the specific coordination environment of V. In particular, the data indicate that II is coordinatively adsorbed as a π-complex with the CH3 group interacting with a near-lying weakly basic silanol. IR data suggest that the rate of formation of II from I in the presence of O is higher than the rate of its consecutive transformation to surface oligomers or acrolein and acrylic acid, probably intermediates to aroms. and COx, resp. The model of the coordination environment and the peculiar characteristics of these tetrahedral V5+ sites in the silicalite in relation to their selective behavior in I oxidative dehydrogenation are discussed. [on SciFinder(R)]