An investigation of iron modified hydroxyapatites used in the activation of n-octane

Abstract Iron (9 wt.%) modified hydroxyapatites were synthesised using two chemical techniques, namely wet impregnation and co-precipitation. SEM-EDX showed that the material prepared using a wet impregnation technique (9-WET) contained iron distributed on the surface of hydroxyapatite, identified as small particles of Fe2O3, using PXRD and Mossbauer spectroscopy. In contrast, PXRD, Mossbauer spectroscopy and SEM-EDX of the material prepared using a co-precipitation technique (9-COP) revealed that both iron and calcium had been incorporated into the hydroxyapatite structure. The formation of an iron pyrophosphate phase upon reduction of 9-COP was identified using in situ XRD. The material was then oxidised back to form a single phase hydroxyapatite-like material. NH3-TPD showed that the surface of the material prepared by co-precipitation was dominated by both weak acidic and basic sites. The material prepared using the wet impregnation technique, showed a greater degree of reduction. Hydroxyapatite and the iron modified hydroxyapatite materials were tested in the activation of n-octane, using a continuous flow, fixed-bed reactor operated in a down flow mode, and gave a range of octene isomers, aromatic compounds, cracked products and carbon oxides. The product distribution was in agreement with the acid-base properties of the materials. The material prepared using the co-precipitation technique favoured the formation of octene isomers. The conversion of n-octane was in accordance with the reducible nature of the materials. The selectivity toward octene isomers was relatively high over the material prepared using the co-precipitation technique (49 mol% at 550 °C), and may have been due to the formation of the iron pyrophosphate phase formed at higher temperatures. The material prepared using the wet impregnation technique showed a maximum selectivity of 27 mol% towards octene isomers at 350 °C. Selectivity towards carbon oxides was lower over the material prepared using the co-precipitation technique compared to the material prepared using the wet impregnation technique over the entire temperature range tested.