Development of novel iron and cobalt-based Fischer Tropsch catalysts

Abstract

Due to economic and ecological challenges connected with the utilization of crude oil the Fischer Tropsch (FT) synthesis gains increasing attention as one promising process for the preparation of synthetic fuels from alternative feedstocks, such as coal, natural gas or biomass. In order to enhance the efficiency of the FT process, current FT research focuses on the development of new cobalt- (Co) and iron- (Fe) based catalysts to improve the catalytic performance. In the present work novel synthesis methods were applied for the preparation of Co- and Fe-based catalysts and their potential with respect to FT synthesis was tested. For every preparation method, the synthesis parameters were varied and the influence of each parameter on the catalyst structure was determined by means of extensive structural characterization before and after catalytic experiments. Finally, the results of the characterization of every catalyst were correlated to its FT performances obtained within this work. A focus of this work was the preparation of multi-component-FT-catalysts such as Co/Al2O3 and Pd-doped iron catalysts by the Flame Spray Pyrolysis (FSP) technique. FSP is a one step process providing multiple options to tune the properties of the resulting materials. Catalyst preparation by the conventional single-flame setup (SFSP) leads to an intensive intermixing of the different components. While this intensive intermixing had a positive effect on Fe-catalysts since a fine dispersion of Pd in the iron oxide matrix could be reached, it caused a pronounced Co-aluminate formation in case of the Co catalysts resulting in FT inactive catalysts. The use of a double-flame setup enabled the independent formation of cobalt oxide and alumina particles reducing the Co-aluminate formation and leading to FT active catalysts. By crossing the flames at an optimized distance, the two components were combined and the degree of metal-support interaction was decreased by changing of the intersection distance of the two flames. Another focus of this work was the preparation of novel Al2O3-stabilized Co- and Fe-Xerogels by the so-called Epoxide-Addition-Method (EAM) which is based on a sol-gel-approach. These xerogels exhibited FT activities comparable to conventional catalysts prepared by incipient wetness impregnation or precipitation. In contrast to these well-established preparation techniques however, the EAM provides potential for the design of materials with tunable composition and porosity. In addition, this method allows the preparation of catalytic materials in form of thin coatings. These coatings are of specific interest for potential applications in novel microreactor concepts. Using this method to prepare Co- and Fe-based catalysts, it was shown that for the latter the Fe/Al2O3-ratio could be varied over a wide range while the FT activity is maintained. However, if the Al2O3 content is too high the formation of irreducible Fe-aluminates is induced resulting in a loss of FT activity. Similarly, for the Co/Al2O3 catalysts a high amount of Co-aluminates was observed to result in diminished FT activities. Yet, these aluminates could be successfully reduced to metallic Co at high reduction temperatures (800 °C) without any significant sintering of the Co-particles. The activity increased significantly after the high temperature reduction up to values comparable to the reference catalyst prepared by the conventional impregnation method. This high stability against sintering depends however on the precursor employed for the catalyst preparation. While the application of nitrates resulted in catalysts that are highly stable, catalysts prepared from the corresponding chloride precursors showed significant sintering during the high temperature reduction accompanied by a decrease of the catalytic activity. The results summarized above reveal a strong relation between the synthesis method, the resulting catalyst structure and its performance as Fischer Tropsch (FT) catalysts. From the knowledge gained, guidelines were determined that may lead to further improved catalysts with respect to the application for FT synthesis.