Oxidative dehydrogenation of cyclohexane over alumina-supported vanadium oxide nanoliths

Featuring highly ordered one-dimensional nanopores, anodic aluminum oxide (AAO) makes an ideal substrate for fabrication of catalysts by atomic layer deposition (ALD). Vanadium oxide (VOx) catalysts supported on AAOs and prepared by ALD and incipient wetness impregnation are characterized by X-ray fluorescence and ultraviolet–visible (UV–Vis) spectroscopy. At low loadings (<4 V/nm2) the supported VOx are mostly isolated monomers; polyvanadate domains are gradually formed as the surface vanadium content increases. The catalytic performance at a series of loadings (<3–32 V/nm2), and hence different forms of VOx, for the oxidative dehydrogenation (ODH) of cyclohexane are investigated. Compared to the catalysts prepared by incipient wetness impregnation, the ALD VOx catalysts show specific activities that are between 2 and 7.5 times higher. This reflects a better dispersion of the catalytic species on the surface as synthesized by ALD. In the cyclohexane ODH reaction with the supported ALD VOx, the kinetic orders and activation energies are comparable to kinetics data reported previously for the supported VOx. The results indicate that the ALD technique can be applied as an alternative approach to synthesize the supported VOx catalysts and achieves very good dispersion even at loadings above one monolayer (8 V/nm2). In the ODH reaction, polyvanadate sites are shown to be more active, overall, than monovanadate sites. However, numerical modeling of the reaction pathways indicates that the olefin formation rate is ∼3 times faster on monomeric VOx sites than on polymeric VOx. By comparing the ODH of cyclohexane and the oxidations of cyclohexene and benzene, we find that both the sequential path and the direct path (the direct conversion from cyclohexane to benzene) are important in the oxidation process of cyclohexane.