Influence of basicity in Fischer-Tropsch synthesis over supported iron-based catalysts

Master Thesis


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University of Cape Town

The Fischer-Tropsch synthesis catalyzed by iron is a well-established process for the production of synthetic fuels, waxes and high-value chemicals, such as α-olefins. A draw-back of the currently used iron-based catalysts is their short lifetime, caused by sintering and particle break-up. These disadvantages might be overcome by utilizing a supported iron-based catalyst. However, supported iron Fischer-Tropsch synthesis, which has been tested up to now, show a high methane selectivity. This might be caused by a lack of alkali near the catalytic site, which can be alleviated by using a basic support. Classical basic supports such as CaO and MgO will react with CO2 (a major by-product in iron-catalyzed Fischer-Tropsch synthesis) yielding carbonates and can therefore not be used, since the formation of carbonates will result in a large particle expansion. An alternative would be to generate a silica-based basic support by attaching basic groups to the silica. In this study iron Fischer-Tropsch catalysts supported on silica were tested for conversion of synthesis gas to hydrocarbon products. Silica was modified with aminopropyltriethoxysilane (APTeS) by impregnation followed by calcination to provide basic surface groups onto the silica surface. The CHN analysis and IR-analysis indicate the presence of amine groups in the APTeS-modified silica. The pore radius distribution of silica is slightly shifted towards higher pore radii in comparison to APTeS-modified silica. It might thus be stated that aminopropyltriethoxysilane covers the pore walls and does not seem to result in pore blockage. Thermal gravimetric analysis indicates that the thermal stability of APTeS-modified silica is low. A major difference between silica and APTeS-modified silica was their zeta-potential. Whereas the surface of silica is mainly negatively charged in the pH-range of interest during impregnation, the surface of APTeS-modified silica is mainly positively charged. This is attributed to the presence of amine groups on the surface. Iron was brought onto the support by impregnation. The surface modification of silica with APTeS seems to be destroyed upon calcination of the impregnated catalysts. The iron phase in the calcined iron catalyst supported on silica catalysts is mainly hematite (Fe203), whereas the iron phase in the calcined iron catalyst supported on APTeS-modified silica catalysts is mainly iron oxide hydroxide FeOOH. The presence of basic amine groups may favour the formation of FeOOH crystallites during the impregnation/calcination on the APTeS-modified silica. The FeOOH-crystallites on the APTeS-modified silica support are typically smaller than the Fe203 crystallites on silica. The maximum catalytic activity is obtained at 0.01 mol K I mol Fe for the iron catalyst supported on silica and at 0.02 mol K I mol Fe for the APTeS-modified catalyst, indicating the optimum potassium loading. The difference in the optimum potassium loading might be linked to the smaller crystallite sizes obtained with the APTeS-modified catalyst. All the potassium promoted catalysts show a lower methane selectivity compared to the 0 K iron catalyst supported on silica and the 0 K iron catalyst supported on APTeS-modified silica. The 1-olefin and n-olefin content in the fraction of linear hydrocarbons increase with increasing potassium loading over all the iron catalyst supported on silica promoted with potassium except for the catalysts 0.005 K and 0.01 K. Increasing potassium content on the catalyst resulted in higher 1-olefin content in the fraction of linear olefins. The trend suggests that potassium promotion suppresses secondary double bond isomerisation of 1-0lefin into internal olefins. The high degree of branching obtained with the 0.005 K catalyst and the 0.01 K catalyst, is characteristic of weak alkali promotion. The iron catalysts supported on APTeS-modified silica indicate an increase in the degree of branching with increasing potassium content.

Includes bibliographical references (leaves 115-124).