Phase composition, microstructure and resistance to attrition of alumina-based supports for Fischer-Tropsch catalysts
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The key element in gas-to-liquid technology is the Fischer-Tropsch (FT) synthesis in which liquid hydrocarbons are produced from syngas (CO and H2). During FT synthesis the catalyst and the catalyst support are subjected to considerable wearing. γ-alumina is considered as one of the most promising materials for catalyst support in a slurry bubble column reactor although the resistance to attrition is not satisfying. The γ-alumina is prepared by spray drying and is a highly porous aggregate. In order to improve the resistance to attrition, the porous support has been modified by impregnation of a metal oxide precursor followed by thermal treatment. The objective of the present thesis was to find the underlying mechanisms for the enhanced mechanical performance of the modified catalyst support based on γ-alumina. The first part of the work was devoted to the processing of the γ-alumina based support, the second part to the thermal evolution of the phase composition and the microstructure of the support and the last part was concerned with the characterization of the resistance to attrition. The distribution of the impregnated salt was investigated by electron probe micro analysis and the element mapping demonstrated that the impregnated salt was not homogeneously distributed in the γ-alumina aggregates. The inhomogeneous distribution of the metal oxide formed by decomposition of the salt reflected the inhomogeneous packing of the primary γ-alumina crystallites in the aggregates, and incorporation of the metal oxide in the initial step to produce γ-alumina would improve the distribution of the metal oxide. Previous investigations have shown that enhancement of the attrition resistance was achieved after calcination at high temperature. In-situ high temperature X-ray diffraction was carried out to investigate the thermal evolution of the phase composition and crystal structure of several impregnated catalyst supports. Refinement of the diffraction data revealed that the chemical reaction between γ-alumina and the impregnated metal oxide was controlled by the valence of the metal oxide and the site preference of the metal cation in the spinel phase formed in the reaction. The reaction with γ-alumina and impregnated NiO or MgO was initiated at around 500 °C. A metastable solid solution between γ-alumina and NiAl2O4 or MgAl2O4 spinel was formed. The reaction between γ-alumina and ZnO was less pronounced and the initiation of the reaction was more challenging to determine. Additional materials prepared by solid state reactions were investigated to elucidate the crystal structure and solid solubility of alumina in NiAl2O4 and similar spinels. The lattice parameter of the metastable solid solution between γ-alumina and the spinel revealed that α-alumina precipitated from the solid solution at around 1100 °C resulting in a reduction of Al2O3 content in the spinel. The formation of the metastable solid solution prevented the direct transformation of γ-alumina to θ- and α-alumina. Transmission electron microscopy demonstrated that the reaction between the impregnated oxide and γ-alumina had a pronounced effect on the final microstructure of the support. Non-impregnated supports transformed to α-alumina crystallites with an approximate size of 200 nm after thermal treatment, while mainly crystallites in the range 20 – 40 nm were observed in the impregnated supports. Dependent of the homogeneity of the γ-alumina aggregates and the amount of the impregnated oxide some large α-alumina crystallites were also formed. The intergranular interface or neck formation between the crystallites was shown to be significantly enhanced by the impregnation. Investigation of γ-alumina, impregnated with a Mn oxide precursor, revealed that the final phase composition and microstructure was strongly dependent on the oxidation state of Mn. By heat treatment in reducing atmosphere, where MnO is stable, a similar microstructure to those impregnated with precursors of MgO, NiO and ZnO was obtained. Heat treatment in air or inert atmosphere, where Mn is stable in a higher oxidation state, the formation of the metastable spinel solid solution did not occur. The influence of the phase composition and the microstructure on the resistance to attrition was investigated by Vickers hardness indentation on individual catalyst support aggregates and a standard attrition test. It was demonstrated that the Vickers hardness showed a very good correlation with the wear resistance. The supports with the modified microstructures exhibited the best mechanical performance, while the pure alumina based supports with larger crystallites showed poor performance. The hardness and wear resistance of the modified supports revealed that the increase in the performance appeared concurrently with the precipitation of α-alumina. It was demonstrated that the most wear resistant microstructure corresponded to the ones with small crystallites of α-alumina and spinel with considerable neck formation or intergranular interfaces between the crystallites. X-ray diffraction revealed that the spinel formed in the modified supports were strained, reflecting the strong interfacial bond between the phases and the thermal mismatch of α-alumina and spinel. Effect of possible epitaxial relationship between the two phases was therefore discussed. Based on the present findings the enhanced mechanical performance of the modified catalyst support reflects a considerable modification of the microstructure, which reduce the grain growth and coarsening of α-alumina and the formation of strong intergranular interfaces between nano-crystalline spinel and α-alumina.