Comparative study of the oxidative dehydrogenation of cyclohexane over vanadium isomorphic-substituted hydroxyapatite and hydroxyapatite-supported vanadium oxide

Vanadium oxide-modified hydroxyapatite (HAP) with varying vanadium content was synthesized by the coprecipitation and impregnation methods. The structures of the two series of catalysts were compared, and the effects of vanadium content on catalytic properties in cyclohexane oxidative dehydrogenation (ODH) are evaluated. The XRD, UV‒Vis and XPS analyses indicate that the V species in the two series of samples exist in different chemical environments. The samples prepared by the impregnation method are mainly composed of V2O5 species, while the samples prepared by the coprecipitation method show a replacement of a part of the P in the HAP lattice by VO4. The influence of the preparation method and vanadium content on the reaction rate and reaction route for the ODH of cyclohexane is investigated. VHAP shows higher cyclohexene selectivity. The effect of VOx species on the ODH reaction of cyclohexane was investigated and the reaction mechanism was proposed.