Poisoning effect of nitrogen compounds on the performance of CoMoS/Al2O3 catalyst in the hydrodesulfurization of dibenzothiophene, 4-methyldibenzothiophene, and 4,6-dimethyldibenzothiophene

The poisoning effect of carbazole and quinoline on the performance of sulfi ded CoMo/Al2O3 in the hydrodesulfurization (HDS) of dibenzothiophene (DBT), 4-methyidibenzothiophene (4-MDBT), and 4,6-dimethyldibenzothiophene (4,6-D MDBT) has been examined. The HDS of 4-MDBT or 4,6-DMDBT is retarded by the presence of the nitrogen compounds in the reaction stream even at low conce ntrations, e.g. 50 ppm, while the HDS of DBT remains nearly unaffected by t hese compounds unless the concentrations are high, in excess of 500 ppm. Th e nitrogen compounds suppress the ring-hydrogenation step (HYD) more than t he direct desulfurization step (DDS) in the HDS of DBT, while an opposite t rend is observed in the HDS of 4-MDBT or 4,6-DMDBT, i.e. DDS is suppressed to a greater extent than HYD. The nitrogen compounds poison acidic sites on the catalyst, as confirmed by the reduced amounts of pyridine adsorbed and by the suppressed isomerization of 2,2 ' -dimethylbiphenyl on the poisoned catalyst. Quinoline exhibits a stronger poisoning effect than carbazole. T he HDS of the three DBT compounds studied in this work are suppressed to di fferent extents by the nitrogen compounds because the extent of steric hind rance to the C-S-C bond by methyl groups attached to the ring structure is different depending on the DBT compounds. (C) 2001 Elsevier Science B.V. Al l rights reserved.