Hydrocarbon synthesis from carbon monoxide and hydrogen on impregnated cobalt catalysts II: Activity of 10% Co/Al 2O 3 and 10% Co/SiO 2 catalysts in Fischer-Tropsch synthesis

Abstract

The results obtained in the investigation of the influence of the nature of the support and the temperature of pretreatment on the activity of cobalt catalysts in Fischer-Tropsch synthesis at atmospheric pressure are presented. Pretreatment by calcination in air was found to decrease cobalt reducibility and cobalt dispersion. The degree of reduction was higher with Co/SiO 2 than with Co/Al 2O 3 at all calcination temperatures. Except for the dried samples, the cobalt dispersions were found to be higher with Co/Al 2O 3. The variations in the catalytic properties of Co/Al 2O 3 and Co/SiO 2 in Fischer-Tropsch synthesis with the pretreatment temperature differ for both supports. When the calcination temperature was increased, the total hydrocarbon yield decreased over Co/SiO 2 catalysts and increased over Co/Al 2O 3 catalysts. The C 5+ fraction selectivities over Co/Al 2O 3 catalysts increased with the pretreatment temperature, while over Co/SiO 2 catalysts they remained practically constant. The resistance of cobalt catalysts to the action of air was higher with samples already used in Fischer-Tropsch synthesis than with freshly reduced ones. For freshly reduced Co/Al 2O 3 catalysts an increase of catalytic activity was found up to a certain level after repeated doses of air were passed through the catalyst bed. The nature of the oxide phases are thought to be responsible for the variation of catalytic properties. On the Co/SiO 2 catalyst surface, Co 3O 4 supposedly reduces the total hydrocarbon yield. In spite of the decrease of cobalt dispersion, the cobalt-support surface compounds cause an increase in the total hydrocarbon yield and the C 5+ fraction selectivity on the Co/Al 2O 3 catalysts.