Vanadium oxide supported on non-stoichiometric strontium hydroxyapatite catalysts for the oxidative dehydrogenation of n-octane

• ODH of n-octane with V2O5 supported on strontium hydroxyapatite catalysts.
• In situ XRD under reduction–oxidation atmospheres showed the redox cycle of V2O5.
• Good selectivity towards octenes was shown by the 5 wt% V2O5 on Sr-HAp catalyst.
• Maximum selectivity for C8 aromatics was obtained using the 15 wt% V2O5 on Sr-HAp.
• Product selectivity is dependant on the phase of vanadium present on Sr-HAp catalyst.

A non-stoichiometric strontium hydroxyapatite (Sr-HAp) was prepared by the co-precipitation method. Vanadium oxide, with loadings of 5, 7.5 and 15 wt%, was supported on the strontium hydroxyapatite by the wet impregnation technique. The materials were characterized by techniques such as XRD, in situ XRD, ICP-OES, BET, FTIR, Raman Spectroscopy, SEM, TEM, TPR, TPO, TPD and oxygen chemisorption. From XRD and Raman analyses, vanadium oxide exists in the vanadium pentoxide phase for the 5 and 7.5 wt% loadings, whereas an additional pyrovanadate phase appears for the 15 wt% loading. Oxidative dehydrogenation reactions were carried out in a continuous flow fixed bed reactor. The catalytic results showed that selectivity towards desired products was dependent on the vanadium concentration and the phase composition of the catalyst. Good selectivity towards octenes was achieved using the 5 wt% V2O5 catalyst. At a conversion of 24% at 400 °C, there was a marked decrease in octene selectivity and a significant increase in the formation of C8 aromatics when V2O5 loading was 15 wt%. A maximum selectivity of 12% for C8 aromatics was obtained at a conversion of 32% at 500 °C, with the selectivity towards octenes decreasing to 52%. The 5 wt% V2O5 on Sr-HAp showed a selectivity of 73% towards octenes and 7% towards aromatics. The higher vanadium loaded catalysts showed preference towards C8 aromatics formation probably due to the pyrovanadate phase present on the surface of the Sr-HAp.

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