The oxidative activation of n-octane over titania supported cobalt catalysts.
This research effort explored routes to valorise alkanes via heterogeneous catalysis. Dehydrogenation and oxidative dehydrogenation are the two routes that were probed in this research effort to activate n-octane by means of titania supported cobalt catalysts. No literature existed on the oxidative activation of higher linear alkanes over titania supported cobalt catalysts, despite the catalyst not being novel, that prompted probing this system in this application. Five titania supported cobalt catalysts with cobalt loadings between 5-25 wt% were synthesised via the wet impregnation technique. These catalysts were characterised to assess their structural morphology as well as their chemical and physical properties. Several techniques were used in the characterisation studies, including microscopic, spectroscopic, diffraction and surface techniques including XRD, XPS, BET, SEM-EDX, TEM, ICP, TGA, Raman and TPR analysis. The results from these studies provided insight into the structurereactivity correlation. The catalysts were found to comprise of spherical shaped, high surface area mesoporous material that was thermally stable at high temperatures with minimal decomposition and mass loss. The support was confirmed to be the anatase phase of titania. The surface was found to be enriched in Co3O4 species that were well dispersed on the surface, but with some aggregation when higher cobalt loadings were used. The catalysts were more reducible at lower temperatures for higher cobalt loadings, with the onset of reduction occurring at 350 °C. Catalytic testing was carried out in a fixed bed reactor with air as the oxidant and nitrogen as the diluent, spanning the temperature range 350-550 °C. GHSV’s of 4000 and 6000 h-1 were used, while the n-octane to oxygen ratios were varied between 1 and 2. The catalyst bed volume was held at 1 cm3, with pellet sizes of between 500-1000 μm being used. The role of the support and the effect of cobalt content and varying n-octane to oxygen ratios were investigated, to establish the impact on n-octane conversion and the product profile. The product distribution was qualitatively and quantitatively monitored by GC and GC-MS analysis. The dominant products were found to be the octenes, aromatics and carbon oxides, obtained in varying amounts with a variation in temperature. Comparative studies based on the different metal loadings and varied n-octane to oxygen ratios indicated that higher cobalt content enhanced the selectivities to octenes while oxidatively richer environments enhanced conversions as well as aromatics formation. The ideal reaction conditions were oxidatively richer environments (8C:4O) and higher cobalt contents in the catalysts, in order to obtain high yields of octenes and aromatics, which were the value added products in this study. COx formation was found to decrease significantly with an increase in temperature. This research effort showed that higher linear alkanes can and do dehydrogenate, maintaining the chain length without cracking, to undergo dehydrocyclisation to form aromatics. This titania supported cobalt catalyst system, together with the reaction parameters employed, successfully activates n-octane oxidatively to yield valorised octenes and C8 aromatics products.