Elucidation of hydrodesulfurization mechanism using [formula omitted] radioisotope pulse tracer methods

Abstract

The sulfidation state in a series of Co-promoted Mo/Al 2O 3 catalysts was investigated using a 35 S pulse tracer method. 35 S -labeled H 2S ([ 35 S ]H 2S) pulses were introduced into catalysts in a nitrogen stream until the radioactivity in the recovered pulse approached the radioactivity of the introduced pulse. From the amount of introduced [ 35 S ]H 2S, the amount of sulfur accumulated on the catalyst was estimated. The result indicated that the amounts of sulfur accumulated on the catalysts increased with increasing temperature for all catalysts. Only molybdenum was sulfided in both Co–Mo/Al 2O 3 and Mo/Al 2O 3 catalysts below 300°C, but the sulfided states of the catalysts at 400°C were very close to the stoichiometric states where Co and Mo are present as Co 9S 8 and MoS 2. Further, hydrodesulfurization (HDS) reactions of radioactive 35 S labeled dibenzothiophene were carried out over the series of Co-promoted Mo/Al 2O 3 catalysts. The amount of labile sulfur and the release rate constant of H 2S were determined. The promotion effect of cobalt on activity of the molybdenum catalyst was attributed to the formation of more active sites. Moreover, the increase in the catalytic activity with Co/Mo ratio among the promoted catalysts was due to increase in the number of the sites with the same activity.