Comparison of nitrogen tolerance of PdMo/Al2O3 and CoMo/Al2O3 catalysts in hydrodesulfurization of model compounds

• PdMo catalyst was more tolerant to inhibition by pyridine (quinoline) than CoMo.
• PdMo was more active than CoMo in thiophene HDS with added pyridine.
• PdMo was comparable to CoMo in benzothiophene HDS with added quinoline.
• N-tolerance depends on C–N bond breaking activity and inhibitor reactivity.

Pd promoted Mo/Al2O3 sulfide catalysts were tested in hydrodesulfurization (HDS) of thiophene and benzothiophene in the presence of different amounts of basic nitrogen compounds (pyridine, quinoline) and compared with CoMo/Al2O3. The Pd promoted catalysts were in both HDS reactions more nitrogen tolerant than CoMo/Al2O3. In thiophene HDS in the presence of pyridine, the PdMo catalysts were more active than CoMo. This was explained by the higher C–N bond hydrogenolytic activity of the adsorbed intermediate piperidine, allowing its removal by hydrodenitrogenation (HDN) and the recovery of the catalyst surface. In HDS of benzothiophene in the presence of quinoline, the PdMo catalysts were comparable to the CoMo catalysts. The HDN intermediate, tetrahydroquinoline, was formed easily on all catalysts. This compound is strong inhibitor as quinoline and much less reactive than piperidine. The higher C–N bond cleavage activity of PdMo catalysts observed in the reaction of pyridine was not sufficient to break this adsorbed intermediate. This impeded the recovery of catalytic sites needed for HDS and resulted in HDS activity comparable to CoMo/Al2O3.

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