Nature of Vanadium Species in Vanadium-Containing Silicalite and Their Behavior in Oxidative Dehydrogenation of Propane

Abstract

V-containing silicalite samples prepared hydrothermally were characterized using a combination of physicochemical techniques and the activity of these silicalites in the oxidative dehydrogenation of propane using O2 and N2O was studied. FT-IR data on the mechanism of propane transformation in the presence of gaseous oxygen on these samples were also reported. The results indicate the presence of various types of vanadium species, and in particular, the formation of small amounts 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 ot propane. Activated oxygen species generated by the interaction of O2 or N2O with reduced vanadium sites are suggested to be responsible for the selective transformation of propane to propylene. However, the good selectivity of this catalyst in propylene formation from propane is also connected to the type of chemisorption of the intermediate propylene on these tetrahedral V5+ sites and to the relative inertness towards its further transformation due to the specific coordination environment of vanadium. In particular, the data indicate that propylene is coordinatively adsorbed as a π-complex with the CH3 group interacting with a nearlying weakly basic silanol. Infrared darta suggest that the rate of formation of propylene from propane in the presence of O2 is higher than the rate of its consecutive transformation to surface oligomers or acrolein and acrylic acid, probably intermediates to aromatics and COx, respectively. 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 propane oxidative dehydrogenation are discussed.