Effect of fluorine addition on the poisoning of NiMo/Al2O3 catalysts by nitrogen compounds during the hydrodesulfurization of dibenzothiophene compounds

The effect of fluorine added to NiMo/Al2O3 catalysts on their poisoning by basic (quinoline) and non-basic (carbazole) nitrogen compounds during the hydrodesulfurization (HDS) of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) was investigated. Fluorinated NiMo catalysts had a higher activity than fluorine-free catalysts, and this superior activity of fluorinated catalysts was maintained even after poisoning by nitrogen compounds, as confirmed by NO chemisorption and FTIR spectroscopy. The HDS rate was retarded to a greater extent by quinoline than by carbazole. In the HDS of DBT, the difference between the activities of the two types of catalysts remained constant even when the poisoning was extensive. This is in contrast to the case of 4,6-DMDBT HDS, in which the difference in activity decreased when the catalysts were extensively poisoned. The product distribution changed with poisoning showing a characteristic trend that was dependant on the combination of reactants and nitrogen compounds. In DBT HDS, the hydrogenation (HYD) route was poisoned to a more significant extent than the direct desulfurization (DDS) route by both quinoline and carbazole, and was independent of catalyst fluorination. In the HDS of 4,6-DMDBT, quinoline retarded HYD more than DDS but carbazole retarded DDS more than HYD.