HYDROCARBON SYNTHESIS FROM CARBON-MONOXIDE AND HYDROGEN ON IMPREGNATED COBALT CATALYSTS .2. ACTIVITY OF 10-PERCENT CO/AL2O3 AND 10-PERCENT CO/SIO2 CATALYSTS IN FISCHER-TROPSCH SYNTHESIS

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/SiO2 than with Co/Al2O3 at all calcination temperatures. Except for the dried samples, the cobalt dispersions were found to be higher with Co/Al2O3. The variations in the catalytic properties of Co/Al2O3 and Co/SiO2 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/SiO2 catalysts and increased over Co/Al2O3 catalysts. The C5+ fraction selectivities over Co/Al2O3 catalysts increased with the pretreatment temperature, while over Co/SiO2 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/Al2O3 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/SiO2 catalyst surface, Co3O4 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 C5+ fraction selectivity on the Co/Al2O3 catalysts.