Synthesis and use of Silica materials as supports for the Fischer-Tropsch reaction
Date
2006-11-17T09:30:54Z
Authors
Mokoena, Emma Magdeline
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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
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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
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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.
Description
Faculty of Science
School of Chemistry
9911467t
EMMA.MOKOENA@sasol.com
Keywords
Silica, Sol-Gel-Synthesis, Fischer-Tropsch, Reaction, Supports