Abstract
Model hydrodesulfurization catalysts (Co2.75Mo11, Ni2.75Mo11 and Ni0.9Co1.85Mo11 wt%/γ-Al2O3) prepared by impregnation of similar supports with similar starting materials were used in the following test reactions: hydrodesulfurization (HDS) of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene, with and without addition of H2S or NH3. The presence of H2S or NH3 has a profound effect on the catalyst ranking and the activities of various reaction pathways, e.g., the hydrogenation (HYD) route and the direct desulfurization (DDS) route. In the case of the HDS of DBT and 4,6-DMDBT, hydrogen sulfide only weakly poisoned CoMo and NiCoMo catalyst, whereas a NiMo catalyst is poisoned more strongly. On the other hand, ammonia strongly poisoned HYD route of CoMo catalyst, whereas DDS route of NiMo catalyst is not at all poisoned in the case of the HDS of DBT. The trimetallic NiCoMo catalyst exhibited the best performance in the conversion of DBT. A comparison of the HYD and DDS rate constants among CoMo, NiMo and NiCoMo catalysts shows that in the presence of rich H2S, the activity of HYD and DDS of the NiCoMo catalyst equals the sum of the activity of the HYD of NiMo catalyst and of the DDS of CoMo catalyst. On the other hand, in the presence of rich NH3, the activity of the NiCoMo catalyst equals the sum of the HYD and DDS of NiMo catalyst, which each in turn are higher than those of CoMo catalyst. This dual promoting effect of NiCoMo catalyst causes higher activity than either NiMo or CoMo catalyst in the presence of either rich H2S or rich NH3.
