Elucidation of molybdenum-based catalysts using a radioisotope tracer method Part 2 Promotion effect of cobalt on molybdena/alumina catalyst

Abstract

Hydrodesulfurization (HDS) reactions of radioactive 35S-labelled dibenzothiophene were carried out over a series of cobalt-promoted molybdena/alumina catalysts at temperatures between 240 and 320°C and under 50 kg cm-2 pressure. HDS and hydrogenation (HYD) activities of Mo/Al2O3 catalysts were remarkably enhanced with the addition of cobalt, the maximum promotional effect occurring with a Co/Mo molar ratio of ca. 0.5. The amount of labile sulfur and the rate constant of sulfur exchange increased significantly with the addition of cobalt, indicating that cobalt makes the sulfur more mobile and that the active phases in the promoted catalysts are different from that in the unpromoted catalyst. The promoting effect of cobalt was thus attributed to the formation of more active sites. On the other hand, the amount of labile sulfur increased linearly with the Co/Mo ratio up to ca. 0.5 whereas the rate constant of sulfur exchange remained almost constant. Moreover, the enhancement in catalytic activity did not change significantly with the cobalt content. The increase in the catalytic activity with the cobalt content was thus ascribed to an increase in the number of the same active sites.