Correlation of Catalyst Morphology with Attrition Resistance and Catalytic Activity of Fischer-Tropsch Catalysts
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Three alumina based support materials for the Fischer-Tropsch synthesis (FTS) catalyst has been prepared and investigated. The three support materials were prepared in order to obtain different mechanical strengths, henceforth denoted weak, medium and strong support. Magnesium modified γ-alumina support material calcined at 950°C and 1050°C were prepared as the medium and strong support respectively, whereas the unmodified alumina was used as the weak support. The modified support materials were both prepared by the incipient wetness impregnation method to obtain 10 wt .% Mg. Several batches were prepared to gain enough material for the planned experiments.Each of the support materials were subject to one crushing process in order to try to alter the particle morphology. The chosen method was determined based on the results from the specialization project. In this project, one method gave a larger change in morphology than other methods. Therefore a ball mill was the method of choice. Unprocessed samples and samples subjected to the ball mill for the three different materials were tested for attrition in an attrition rig, and characterized in terms of morphology using a particle analyzer. The different fractions were also analyzed with respect to particle morphology using an environmental scanning electron microscope (ESEM). FTS catalysts with 12 wt. % Co and 0,5 wt. % Re were prepared from the weak and the medium support materials, both unprocessed and milled fractions, using the incipient wetness impregnation. These four catalyst samples were tested for dispersion using a chemisorption experiment, and for activity and selectivity using a dedicated setup. Results from the particle analyzer showed that the morphology had not been altered as much as expected. Most change in shape occurred for the medium strength support material, but the observed alteration was much less than observed for the same experiment in the specialization project. For the weaker and stronger support, only a slight change was observed, and for the strong support material this change was in the direction of rounder particles. This is probably due to the excellent mechanical strength of the strong support, which leads to grinding of the kinks and corners of the particles and not the breakage of whole particles to smaller and more uneven fragments. With such relative small differences in morphology between the unprocessed and the milled materials, correlations of morphology with other parameters are difficult to obtain. The same correlation as from the specialization project, with rounder particles having a higher attrition resistance, was observed. A slight degree of correlation was also found between particle morphology and dispersion, where more uneven particles gave a slightly higher value of cobalt dispersion.