Diffusion effects and direct C-N cleavage in the HDN of o-toluidine and methylcyclohexylamine over sulphided NiMo/γ-Al2O3 and Mo(P)/γ-Al2O3 catalysts

Abstract The mechanism of the C-N bond cleavage in hydrodenitrogenation over NiMo/γ-Al 2 O 3 and Mo(P)/γ-Al 2 O 3 catalysts was studied. The major part of C-N cleavage of o-toluidine takes place via ring hydrogenation and β-Hofmann elimination. Direct conversions from o-toluidine to toluene and from methylcyclohexylamine to methylcyclohexane were observed as well. Some of these direct conversions were, however, only apparent. They were caused by diffusion limitation, because the intermediates could not diffuse out of the pores fast enough to be detected. The influence of diffusion was greater for the more active NiMo/γ-Al 2 O 3 catalyst. Experiments with small catalyst particles at low conversions demonstrated that, even in the absence of diffusion limitation, direct C-N bond cleavage takes place. This is ascribed to C-N bond hydrogenolysis or to nucleophilic substitution of the NH 2 group by a SH group, followed by C-S bond hydrogenolysis.