Structure-activity relationships in hydroprocessing of aromatic and heteroaromatic model compounds over sulphided NiO-MoO 3/γ-Al 2O 3 and Nio-WO 3/ γ -Al 2O 3 catalysts; chemical evidence for the existence of two types of catalytic sites

Abstract

The hydroprocessing of nitrogen-containing model compounds such as pyridine, quinoline, acridine, pyrrole and carbazole was studied using a batch method at 340°C and 70 bar H 2 over sulphided NiO-MoO 3/Al 2O 3 and NiO-WO 3/Al 2O 3 catalysts, where it was found that saturation of the heteroaromatic rings always occurs prior to any cleavage of C-N bonds, generally C sp3-N bonds. The rates of hydrogenation of the heteroaromatic rings were shown to be mostly influenced by the aromaticity of these rings (π-electron delocalization) and not by the basicity of the nitrogen atom. On the other hand, the basicity of nitrogen atoms considerably influences the cleavage of C sp3-N bonds in the absence of steric effects. The hydroprocessing of substrates of general formula C 6H 5X, where X is an electronegative atom (OR, NHR, SR, Br, Cl, F) was also studied under the same experimental conditions. The rates of hydrogenation of the phenyl moiety and the rates of hydrogenolysis of the C sp2-X bond are both correlated with the same substituent constant, σ o R, which represents the delocalization of π-electrons in the molecule by resonance. Hydrogenation is favoured over hydrogenolysis for highly electron-donating substituents such as NHR and OR (R=H, C 6H 5); conversely, hydrogenolysis is favoured over hydrogenation for slightly electron-donating substituents such as SR (R=H, C 6H 5) or halogens (Cl, Br). The presence of such correlations constitutes chemical evidence for the existence of two distinct catalytic sites, one responsible for hydrogenation associated with an electron-withdrawing character and the other responsible for the hydrogenolysis of C sp2-X bonds associated with an electron-donating character.