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 35S-labelled dibenzothiophene was carried out over a series of sulÐded molybdenaÈalumina catalysts containing 6È20 wt.% molybdena at temperatures in the range 280È380 ¡C and at a pressure of 50 kg cm~2. The sulfur exchange data and hydrodesulfurization activity were simultaneously obtained and the e †ect of molybdenum content on the structure of sulÐded 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 o † with further loading of molybdenum. Thus, it was suggested that a monolayer dispersion of molybdenum sulÐde on alumina was maintained up to 2.89 atom nm~2 but some crystallites of molybdenum sulÐde 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.

Sulfidation State and Sulfur Behavior on Mo-based HDS Catalysts Supported on TiO₂ Using ³⁵S Tracer Methods

“…ous Mo contents were calculated from the Arrhenius plots (lnkHDS as a function of 1/T; not shown here). The activation energy for HDS of DBT over the Mo/ TiO2 catalysts was almost constant at 92±8 kJ/mol, irrespective of the Mo loading, which is close to the 84 ±8 kJ/mol generally observed for Mo/Al2O3 cata lysts 44) . For the same Mo loading, the activities ob served over the high SSA titania were more than twice higher than those observed over the 256 m 2 ・g -1 alumina.…”

“…The activities of Mo/Al2O3 catalysts described in a previous study are also shown in Fig. 8 44) . The value of kHDS for the Mo/TiO2 catalysts increased linearly with increasing Mo content up to about 16 wt％ MoO3, and then decreased with further increasing Mo content (i.e.…”

“…This portion of 35 S was approximately equal to shaded area A, and represented the total amount of labile sulfur on the catalyst under this reaction condition. 44) a t v a r i o u s t e m p e r a t u r e s a r e s h o w n i n F i g . 6.…”

“…8 44) . In addition, the percentage of labile sulfur So against the total amount of sulfur in MoS2 was 17.4％ on the 20 wt％ Mo catalyst but was almost constant at ca.…”

Investigation of Sulfur Behavior on Mo-based Hydrodesulfurization Catalysts Supported on High Surface Area TiO₂ by ³⁵S Radioisotope Tracer Method

“…The results are shown in Fig. 1, together with the activities of Mo/Al2O3 catalysts described in a previous study 29) . The value of kHDS for the Mo/TiO2 catalysts increased linearly with increasing Mo content up to about 16 wt% MoO3 and then decreased with further increasing Mo content (i.e.…”