Studies towards a mechanistic insight into the activation of n-octane using vanadium supported on alkaline earth metal hydroxyapatites

• ODH of n-octenes with V2O5 supported on alkaline earth hydroxyapatite catalysts.
• In situ XRD under n-octane showed the redox cycle of V2O5.
• All the octene isomers showed good selectivity towards aromatics.
• Selectivity towards products depends on the strength of the oxidative environment.
• The oxidation–reduction pathway follows the Mars–van Krevelen mechanism.

To gain insight into the mechanism for the formation of products in the activation of n-octane over alkaline earth hydroxyapatites, the role of the octene isomers, i.e. 1-octene, 2-octene, 3-octene and 4-octene, together with 1,7-octadiene were investigated using 2.5 and 10 wt% V2O5 supported on calcium, strontium, magnesium and barium hydroxyapatites as catalysts. The fresh catalysts were characterized by DR-UV–Vis spectroscopy, XPS, in situ X-ray diffraction and oxygen chemisorption. The redox natures of the fresh catalysts are explained by TPR–TPO–TPR analysis and used catalysts were characterized by XRD, ICP-OES, BET, FTIR, SEM, TEM and TPD. Oxidative dehydrogenation reactions were carried out in a continuous flow fixed bed reactor and hydrocarbon to oxygen molar ratios were varied. The selectivity towards the desired products was dependent on the phase composition of the catalyst and on the hydrocarbon to oxygen molar ratios. All the octene isomers showed good selectivity towards aromatics. Oxygenates and carbon oxides (COx) were also formed. Highest selectivity towards aromatics was shown by the 2-octene and 3-octene isomers with both vanadium loadings at all hydrocarbon to oxygen molar ratios. However, as the oxygen molar ratios were increased, a decrease in the selectivity towards aromatics and an increase in selectivity towards COx was observed. The terminal octene and 1,7-octadiene preferred the 1,6-cyclization mode towards aromatics formation, whereas for 2-octene, 3-octene and 4-octene the 2,7 cyclization mode was preferred. The oxidation–reduction pathway follows the Mars–van Krevelen mechanism. Initially, n-octane is oxidized to octenes, followed by oxidation and oxidative dehydrogenation to aromatics and oxygenates. Combustion to give carbon oxides is a side reaction, at all stages, though at times quite minor.

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