Development of high surface area attrition-resistant spray-dried iron catalysts for Fischer-Tropsch Synthesis

Abstract

Iron is used as catalyst in the industrial process Fischer-Tropsch Synthesis (F-TS), which is a catalytic chemical reaction that transforms synthesis gas (CO + H2) to create paraffins and olefins for fuels and chemicals. This study aimed to design iron catalysts with a high surface area and attrition resistance for F-TS. The following catalysts: α-Fe2O3, K/Cu/Fe and 0.367 M K/Cu/Fe spray-dried at 200 °C were prepared using the co-precipitation, impregnation and spray-drying methods. The catalysts were then characterized using various characterization techniques including thermogravimetric analysis (TGA), Brunauer-Emmett-Teller (BET) analysis, scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray fluorescence (XRF). Attrition resistance comprised physical tests with the accredited standard method of testing materials (ASTM). The results showed that the 0.367 M K/Cu/Fe spray-dried at 200 °C catalyst has a large surface area of 39 m2 /g and this could be attributed to the small particle size and the catalyst being obtained in a powder form. The α- Fe2O3 catalyst was found to have more physical attrition resistance with a value of 2.2 wt%/h. This was attributed to the small fines produced during the physical attrition test, meaning that the catalyst has a high mechanical strength as compared to other catalysts. The α-Fe2O3 catalyst was also found to have more chemical attrition resistance. This was attributed to the minimum phase change that occurred during the reduction or activation with CO as compared to other catalysts. All the catalysts including α-Fe2O3, K/Cu/Fe and 0.367 M K/Cu/Fe spray-dried at 200 °C demonstrated good selectivity characteristics (low methane and high C5+) hydrocarbons.This was ascribed to the iron carbide (χ‐Fe5C2) active phase or site, which increased the chain growth and favoured the production of C5+ hydrocarbons while decreasing methane selectivity. The α-Fe2O3 catalyst showed high activity and stability, as there was minimal loss of catalytic activity as compared to other catalysts. This was as a result of less CO2 selectivity produced by the catalyst, meaning that there was low water-gas shift activity (WGS) and the active sites were less affected by the presence of water, which causes high loss of catalytic activity. The activity and selectivity of the catalysts need to be improved before the industrial application.