Deactivation of MoS2 catalysts during the HDS of thiophene

Catalytic properties of unsupported MoS2 catalysts in the thiophene hydrode sulfurization reaction were determined in the temperature range 623-653 K. The catalysts were prepared by ex situ decomposition of ammonium thiomolybd ate (ATM) crystals in a mixture of 15% H2S in H-2 at 673 K. Activity of cat alysts decreased very rapidly before reaching a steady state after 15 h on- stream. The thiophene conversion went down from 10-12 to 3-4% in that time. The surface area of the catalysts also decreased during the catalytic reac tion from 40-50 to 8-10 m(2)/g. Selectivity for hydrodesulfurization, hydro genation and isomerization reactions was affected distinctly by the deactiv ation process. By increasing the reaction time, double-bond isomerization i ncreased, hydrogenation of butenes decreased and hydrodesulfurization remai ned constant. Results indicate that the main cause of activity decay was su rface area loss that was due to sintering of MoS2 crystallites. Selectivity variation indicates that different active sites are involved for the three reactions. A deactivation model involving diminution of active sites locat ed in edge and rim sites of small MoS2 particles is proposed to explain the variation of product distribution.