Oxidative dehydrogenation of n-octane using vanadium pentoxide-supported hydroxyapatite catalysts

Abstract

Vanadium pentoxide, with loadings varying from 2.5 to 15wt%, was supported on hydroxyapatite (HAp) by the wet impregnation technique. The materials were characterised by techniques such as X-ray diffraction (XRD), inductively coupled plasma-optical emission spectroscopy (ICP-OES), BET, FTIR, SEM, transmission electron microscopy (TEM), temperature programmed reduction (TPR) and temperature programmed desorption (TPD). From XRD and IR analyses, vanadium is found in the vanadium pentoxide phase for the lower loadings, whereas for weight loadings in excess of 10%, an additional pyrovanadate phase exists. 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 vanadium concentration and the phase composition of the catalyst. Good selectivity towards octenes was achieved using the 2.5wt% V2O5 on HAp loaded catalyst. There was a marked decrease in octene selectivity and a significant increase in the formation of C8 aromatics when higher loadings of vanadium were used. At a conversion of 24% at 450°C, the 15wt% V2O5 on HAp showed a selectivity of 72% towards octenes. A maximum selectivity of 10% for C8 aromatics was obtained using the 15wt% V2O5 on HAp catalyst at a conversion of 36% at 550°C.