Elucidation of molybdenum-based catalysts using a radioisotope tracer method Part 1.—Hydrodesulfurization activity and structure of sulfided molybdena–alumina catalysts

Abstract

Hydrodesulfurization of radioactive ³⁵ S-labelled dibenzothiophene was carried out over a series of sulfided molybdena–alumina catalysts containing 6–20 wt.% molybdena at temperatures in the range 280–380°C and at a pressure of 50 kg cm ^-2 . The sulfur exchange data and hydrodesulfurization activity were simultaneously obtained and the effect of molybdenum content on the structure of sulfided catalysts was estimated. The sulfur exchange rates were approximately the same for all samples at the same temperature and apparent activation energies of hydrodesulfurization reactions were 20±2 kcal mol ^-1 for all catalysts. These results suggest that the mechanism of hydrodesulfurization and the nature of active sites did not vary with molybdenum content. On the other hand, the amount of labile sulfur increased linearly with the molybdenum content up to 2.89 atom nm ^-2 but then levelled off with further loading of molybdenum. Thus, it was suggested that a monolayer dispersion of molybdenum sulfide on alumina was maintained up to 2.89 atom nm ^-2 but some crystallites of molybdenum sulfide would be formed when molybdenum was added beyond 2.89 atom nm ^-2 . Moreover, the sulfur exchange rate hardly varied with the molybdenum content and the hydrodesulfurization rates of dibenzothiophene on the catalysts paralleled the amount of labile sulfur.