ELUCIDATION OF MOLYBDENUM-BASED CATALYSTS USING A RADIOISOTOPE TRACERMETHOD - PART 2 - PROMOTION EFFECT OF COBALT ON MOLYBDENA ALUMINA CATALYST/

Hydrodesulfurization (HDS) reactions of radioactive S-35-labelled dibe nzothiophene were carried out over a series of cobalt-promoted molybde na/alumina catalysts at temperatures between 240 and 320 degrees C and under 50 kg cm(-2) pressure. HDS and hydrogenation (HYD) activities o f Mo/Al2O3 catalysts were remarkably enhanced with the addition of cob alt, 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 sulf ur exchange increased significantly with the addition of cobalt, indic ating that cobalt makes the sulfur more mobile and that the active pha ses in the promoted catalysts are different from that in the unpromote d catalyst. The promoting effect of cobalt was thus attributed to the formation of more active sites. On the other hand, the amount of labil e sulfur increased linearly with the Co/Mo ratio up to ca. 0.5 whereas the rate constant of sulfur exchange remained almost constant. Moreov er, 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 t he same active sites.