Hydroxyapatite (HAP) contains abundant defect sites and easily releases hydroxyl groups to produce new vacancies under calcination at high temperature. The highly dispersed VO_x/HAP catalyst was prepared by an impregnation method using these defects as inducement. VO_x species with different structures were analysed by XRD, XPS, H₂-TPR, Raman and UV–vis spectroscopy. At low calcination temperatures (500 °C and 600 °C), the V species are mainly V₂O₅ crystals. At high calcination temperatures (above 700 °C), VO_x on the HAP surface fills these defect sites and strongly interacts with HAP to form Ca−O−V or P−O−V bands. These scattered defects improved the dispersion of V species. An emphasis is given to the study of the catalytic performances in ODH of cyclohexane over the VHAP catalysts. The highly dispersed VO_x/HAP catalyst showed a high selectivity of cyclohexene, and the selectivity reached 48.2 % when the conversion of was 13.1 % at 410 °C. These improved selectivity is directly related to the chemical environment of highly dispersed VO_x species. In addition, the acidity reduction caused by high temperature calcination leads to the decrease of the adsorption capacity of VHAP to cyclohexene, which promotes the desorption of cyclohexene on the catalyst surface, inhibits the deep oxidation of cyclohexene and improves the selectivity.

Abstract

Hydroxyapatite (HAP) contains abundant defect sites and easily releases hydroxyl groups to produce new vacancies under calcination at high temperature. The highly dispersed VO_x/HAP catalyst was prepared by an impregnation method using these defects as inducement. VO_x species with different structures were analysed by XRD, XPS, H₂-TPR, Raman and UV–vis spectroscopy. At low calcination temperatures (500 °C and 600 °C), the V species are mainly V₂O5 crystals. At high calcination temperatures (above 700 °C), VO_x on the HAP surface fills these defect sites and strongly interacts with HAP to form Ca−O−V or P−O−V bands. These scattered defects improved the dispersion of V species. These highly dispersed VO_x/HAP catalysts were used for oxidative dehydrogenation (ODH) of cyclohexane to cyclohexene. The highly dispersed VOx/HAP catalyst showed a high selectivity for cyclohexene, and the selectivity reached 48.2 % when the conversion of cyclohexane was 13.1 % at 410 °C.