Oxidation and oxidative dehyrogenation of n-octane using V₂O₅ supported on hydroxyapatites.

Abstract

Vanadium pentoxide with loadings varying from 2.5-15 wt% was supported on hydroxyapatite (HAp) by the wet impregnation technique. The materials were characterized by techniques such as X-ray powder diffraction (XRD), Inductively coupled plasma-optical emission spectroscopy (ICP-OES), Brunauer Emmett Teller (BET) surface area measurement, Fourier Transformation- Infrared Spectroscopy (FT-IR), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Temperature Programmed Desorption (TPD) and Temperature Programmed Reduction (TPR). 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 2.5 wt% 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 15 wt% V₂O₅ on HAp showed a selectivity of 72% towards octenes. A maximum selectivity of 10 % for C8 aromatics was obtained using the 15 wt% V₂O₅ on HAp catalyst at a conversion of 36 % at 550 ˚C. Vanadium pentoxide with 2.5 wt% and 10 wt% loading supported on Ca-HAp, Sr-HAp, Mg- HAp, Ba-HAp was tested at different temperatures with varying n-octane to oxygen ratios. The selectivity towards products depended on the hydroxyapatite support. Ca-HAp showed preference towards octenes, Sr-HAp towards aromatics, Mg-HAp towards oxygenates and Ba- HAp towards aromatics and oxygenates. In the development of a more detailed mechanistic study for the oxidative dehydrogenation noctane, the role of the intermediates such as 1-octene, 2-octene, 3-octene, 4-octene and 1, 7- octadiene was investigated. The influence of hydrocarbon to oxygen ratios was also considered. Ethyl benzene and styrene were produced with high selectivities using 1-octene and 1, 7- octadiene as feeds, whereas o-xylene was the main product when 2-octene, 3-octene and 4- octene were used as feedstocks.