First principles study of heavy oil organonitrogen adsorption on NiMoS hydrotreating catalysts

The adsorption of quinoline, acridine, indole, and carbazole on the well-defined NiMoS hydrotreating catalyst edge surface has been studied by means of density-functional theory (DFT) using a periodic supercell model. Quinoline and acridine, the basic nitrogen-containing molecules present in heavy oils, are preferably adsorbed on the Ni-edge surface through the lone pair electrons of the nitrogen atom, which produces relatively high adsorption energies (−ΔE_a = 16–26 kcal mol⁻¹). Indole and carbazole, the non-basic nitrogen-containing molecules, primarily interact with the NiMoS catalyst edge surface through the π-electrons of the carbon atoms. While indole preferentially adsorbs on the NiMoS surface through the β-carbon of the pyrrolic ring (−ΔE_a = 19 kcal mol⁻¹), carbazole primarily interacts with the NiMoS surface through the phenyl rings (−ΔE_a = 13 kcal mol⁻¹). The relative adsorptivities and energetically preferred adsorption modes of the nitrogen-containing molecules in heavy oils can provide insights into experimental observations about hydrodenitrogenation (HDN) kinetics and reaction pathways.