Activation of n-octane using vanadium oxide supported on alkaline earth hydroxyapatites

Vanadium pentoxide was supported on calcium, strontium, magnesium and barium hydroxyapatite with loadings of 2.5 and 10 wt% by the wet impregnation technique. The materials were characterized by XRD, ICP-OES, BET, FTIR, SEM, TEM, TPR and TPD. From XRD and IR analyses, vanadium is found in the vanadium pentoxide phase for the 2.5 wt% loadings, whereas an additional pyrovanadate phase exists for the 10 wt% loadings. Electron microscopy provides evidence of a homogenous distribution of the vanadium species on the hydroxyapatite. Oxidative dehydrogenation reactions carried out in a continuous flow fixed bed reactor showed that selectivity towards desired products was dependent on the phase composition of the catalyst and the n-octane to oxygen molar ratios. For the n-octane to oxygen molar ratio of 1:0.5, selectivity towards aromatics and octenes was obtained, whereas the n-octane to oxygen molar ratio of 1:2 showed selectivity towards aromatics and oxygenates. Good selectivity towards octenes was achieved using the 2.5 wt% V2O5 loaded hydroxyapatite catalysts. There was a marked decrease in octene selectivity and a significant increase in the formation of C8 aromatics when the 10 wt% loaded catalyst was used. In general, for the activation of n-octane, 3-octene in the octenes, o-xylene in the aromatics and 2-octanone in the oxygenates were the major products. At iso-conversion of 27% at 450 °C, V2O5 supported Ca-HAp showed highest selectivity towards octenes, V2O5 supported Sr-HAp showed highest selectivity towards aromatics, V2O5 supported Mg-HAp showed highest selectivity towards oxygenates and V2O5 supported Ba-HAp showed selectivity towards aromatics and oxygenates.

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► Oxidation of n-octane using vanadium oxide supported on hydroxyapatites.
► V2O5 loadings of 2.5 wt% and 10 wt% was supported on hydroxyapatites.
► Vanadium oxide supported on Sr-HAp showed best activity.
► The selectivity towards the products depends upon the oxidative environment strength.
► The cyclization mode is related to the dominant octene isomer formed.