Influences of nitrogen species on the hydrodesulfurization reactivity of a gas oil over sulfide catalysts of variable activity

The inhibition effects of nitrogen species in gas oil on staged hydrodesulfurization (HDS) over commercial NiMoS/Al₂O₃ and CoMoS/Al₂O₃ catalysts of variable activity were investigated at 340 °C under initial 50 kg/cm² H₂ using as-distilled and nitrogen-free gas oils. Hydrogen renewal between stages was attempted to reveal an additional inhibition effects of the by-products such as H₂S and NH₃. All species in the gas oils were analyzed by GC-AED before and after hydrotreatment. NiMoS/Al₂O ₃ and CoMoS/Al₂O₃ catalysts showed contrasting activities in HDS and susceptibility to nitrogen species according to their catalytic natures. HDS over NiMoS-2/Al₂O₃ of higher activity was largely improved by removal of nitrogen species, while that over NiMoS-1 catalyst of limited activity looked insensitive. Basically, HDS of reactive sulfur species suffered less inhibitions by nitrogen species than that of refractory species. In contrast, two CoMoS catalysts showed significant influences of nitrogen species. One of the most refractory species, 4,6-DMDBT, was effectively hydrodesulfurized by the removal of nitrogen species and renewal of hydrogen in the staged HDS, showing synergy effects due to simultaneous removal of two inhibitors. It must be noted that 107 ppm level of nitrogen species, basically carbazoles, which remained after the first stage HDS, influenced still the HDS of refractory sulfur species which is most expected in the second stage for the deep desulfurization. The acidity of the catalysts is compared to their inhibitions by nitrogen species as well as H ₂S and NH₃ in the products. Additionally, the reactivity of nitrogen species in HDS is briefly discussed.