The catalysed activation of n-octane over iron modified hydroxyapatites.

Abstract

This research effort investigated the activation of n-octane by oxidative dehydrogenation and dehydrogenation over iron substituted and iron supported on hydroxyapatite catalysts. Pure hydroxyapatite and iron substituted hydroxyapatite were prepared using a co-precipitation method and the iron supported on hydroxyapatite catalysts were prepared using a single wet impregnation technique. The weight percentages of iron in both the catalyst materials varied between 1 – 9 %. The catalysts were characterized by inductively coupled plasma optical emission spectroscopy, powder X-ray diffraction, Fourier transform infrared spectroscopy, Raman spectroscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, transmission electron microscopy and temperature programmed reduction and oxidation. The BET surface area and nitrogen adsorption and desorption isotherms were also determined. The iron supported on hydroxyapatite catalysts existed as bi-phasic materials of which the dominant phase was assigned to non-stoichiometric hydroxyapatite. The second phase was identified as hematite. The iron substituted hydroxyapatite catalysts were found to exist as a single phase non-stoichiometric hydroxyapatite. All the catalyst materials were hexagonally shaped, mesoporous nanoparticles and were stable under the conditions tested. The iron supported on hydroxyapatite catalysts showed the distribution of iron on the surface of the support with an aggregation of hematite clusters at certain locations. The distribution of iron in the iron substituted hydroxyapatite catalysts occurred throughout the hydroxyapatite-like materials. This suggested that calcium cations were substituted with iron cations within the apatite structure. Catalytic testing was performed in a continuous flow fixed-bed tubular stainless steel reactor, operated in a down-flow mode, using air as the oxidant and nitrogen as the diluent gas. Reactions were carried out over a temperature range of 350 – 550 °C with 50 °C increments. Gas flow rates were varied to attain gas hourly space velocities of 4000, 6000 and 8000 h-1. The carbon to oxygen ratios employed were 8:0, 8:2, 8:4 and 8:6, the first of which corresponded to anaerobic or dehydrogenation conditions. The catalyst bed was maintained at a volume of 1 cm3 with pellet sizes ranging between 600 – 1000 m and 300 – 600 m. The reaction product stream was analysed using two gas chromatographs, one fitted with a thermal conductivity detector, and the other with a flame ionization detector. A gas chromatograph coupled to a mass spectrometer was also used intermittently for qualitative assays. For blank runs, when the reactor tube was filled with carborundum only, conversions ranging between 1 – 6 % were obtained. Octene isomers, aromatic compounds, cracked products and carbon oxides were identified in the product stream. The product classes targeted in this study were octene isomers, C8 aromatic compounds and C8 oxygenated compounds. The use of the catalysts gave greater conversion and greater selectivity towards desired products when compared to the blank reactor runs. The catalytic performance of hydroxyapatite was improved upon modification with iron. The highest conversion was observed over the 9 % (by weight) iron supported on hydroxyapatite catalyst. However, this catalyst showed low selectivity towards the targeted value added products and showed high selectivity towards undesirable cracked oxygenated compounds (methanol, ethanol, 2-propanol and acetone) ranging between 18 – 46 mol% over the temperature range investigated. The highest selectivity towards C8 value added products was 83 mol% at a conversion of 7 mol% over the 3 wt% iron substituted catalyst.