SYNTHESIS AND USE OF SILICA MATERIALS AS SUPPORTS FOR THE FISCHER-TROPSCH REACTION

Abstract

The objective of the study was to prepare novel silica materials and then use them as supports/binders for the Fisher-Tropsch (F-T) reaction. Hence the thesis is divided into two parts - (i) the synthesis of silica materials (ii) use of silica materials as supports. PART I The studies that were carried out in this thesis evaluated the effect of templates and synthesis conditions on the nano- and microstructure and properties of silica materials that are obtained by the sol-gel method. The studies with DL-tartaric acid and citric acid as templates revealed that synthesis conditions (temperature, NH4OH concentration, water/ethanol concentration, time before NH4OH addition, static versus stirred conditions, stirring rate and solvent) all have an effect on the microstructure of the silica and influence the formation of particular silica morphologies. DL-tartaric acid produced longer and more uniform tubes when compared to citric acid. Tubes that are formed by DL-tartaric acid are hollow and open ended; however the ones formed in citric acid are a mixture of filled and hollow but closed tubes. Hollow spheres are exclusively formed when citric acid is used under certain conditions while only filled spheres are formed when DL-tartaric iii acid is used. The surface areas of the silicas formed from DL-tartaric acid are lower that the surface areas obtained for materials produced by citric acid. The nitrogen adsorption-desorption isotherms of silica materials obtained from both templates showed that the materials were mesoporous with some microporosity present in them. Studies with mucic and tartronic acids as templates also showed that the template as well as the synthesis conditions (such as solvent, temperature and stirring) affect the resulting silica morphology. Mucic acid produced silica materials with high surface areas, mesopores and a morphology that reveals fragmented tubes. Tartronic acid produced hollow tube materials with low surface areas and a combination of micro- and mesopores. The yield of the tubes was higher at lower temperatures for both templates. When sugars (e.g. glucose) were used only spherical particles were obtained and some sugars gave particle sizes that are smaller than the ones that are normally obtained by the sol-gel method. PART II Catalysts (Fe/Cu/K) supported on a range of silica materials with different morphologies (hollow nanotubes, hollow spheres, Stöber/closed spheres) were evaluated in the Fischer-Tropsch reaction (8 bar, 250 °C, 400 h-1 GHSV). The supported iron catalysts modified the physico-chemical properties and activity of iv the catalysts but not the catalyst selectivity. A Ruhrchemie catalyst (known F-T catalyst standard) was also evaluated under the same reaction conditions as the new catalysts for comparison purposes. The Ruhrchemie catalyst was found to be the most active catalyst followed by the catalyst supported on nanotubes, Stöber spheres and hollow spheres respectively. Catalysts containing 18% silica showed the best activity compared to the 9% and 27% silica catalysts. The product distribution and WGS activity were largely influenced by the potassium that is present in the samples and not the support type. Mössbauer spectroscopy showed that some active catalysts contained χ' – Fe2.5C and some superparamagnetic iron oxides or carbides while other catalysts also contained α – Fe and Fe3O4 in addition to χ' – Fe2.5C and some superparamagnetic iron oxides or carbides species. This finding supports the hypothesis that carbide formation is a requirement for active F-T catalysts. It also suggests that metallic iron is necessary for carbiding to occur, hence the need for a reduction pre-treatment.