Synthesis and use of Silica materials as supports for the Fischer-Tropsch reaction

[摘要] The objective of the study was to prepare novel silica materials and then usethem as supports/binders for the Fisher-Tropsch (F-T) reaction. Hence the thesisis divided into two parts - (i) the synthesis of silica materials (ii) use of silicamaterials as supports.PART IThe studies that were carried out in this thesis evaluated the effect of templatesand synthesis conditions on the nano- and microstructure and properties of silicamaterials that are obtained by the sol-gel method.The studies with DL-tartaric acid and citric acid as templates revealed thatsynthesis conditions (temperature, NH4OH concentration, water/ethanolconcentration, time before NH4OH addition, static versus stirred conditions,stirring rate and solvent) all have an effect on the microstructure of the silica andinfluence the formation of particular silica morphologies.DL-tartaric acid produced longer and more uniform tubes when compared to citricacid. 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 butclosed tubes. Hollow spheres are exclusively formed when citric acid is usedunder certain conditions while only filled spheres are formed when DL-tartariciiiacid is used. The surface areas of the silicas formed from DL-tartaric acid arelower that the surface areas obtained for materials produced by citric acid. Thenitrogen adsorption-desorption isotherms of silica materials obtained from bothtemplates showed that the materials were mesoporous with some microporositypresent in them.Studies with mucic and tartronic acids as templates also showed that thetemplate as well as the synthesis conditions (such as solvent, temperature andstirring) affect the resulting silica morphology. Mucic acid produced silicamaterials with high surface areas, mesopores and a morphology that revealsfragmented tubes. Tartronic acid produced hollow tube materials with low surfaceareas and a combination of micro- and mesopores. The yield of the tubes washigher at lower temperatures for both templates.When sugars (e.g. glucose) were used only spherical particles were obtainedand some sugars gave particle sizes that are smaller than the ones that arenormally obtained by the sol-gel method.PART IICatalysts (Fe/Cu/K) supported on a range of silica materials with differentmorphologies (hollow nanotubes, hollow spheres, Stöber/closed spheres) wereevaluated in the Fischer-Tropsch reaction (8 bar, 250 °C, 400 h-1 GHSV). Thesupported iron catalysts modified the physico-chemical properties and activity ofivthe catalysts but not the catalyst selectivity. A Ruhrchemie catalyst (known F-Tcatalyst standard) was also evaluated under the same reaction conditions as thenew catalysts for comparison purposes.The Ruhrchemie catalyst was found to be the most active catalyst followed bythe catalyst supported on nanotubes, Stöber spheres and hollow spheresrespectively. Catalysts containing 18% silica showed the best activity comparedto the 9% and 27% silica catalysts.The product distribution and WGS activity were largely influenced by thepotassium 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 catalystsalso contained α – Fe and Fe3O4 in addition to χ' – Fe2.5C and somesuperparamagnetic iron oxides or carbides species. This finding supports thehypothesis that carbide formation is a requirement for active F-T catalysts. It alsosuggests that metallic iron is necessary for carbiding to occur, hence the need fora reduction pre-treatment.

[发布机构] University of the Witwatersrand