Mechanism and kinetics of hydrodenitrogenation

Removal of the nitrogen atom from aromatic nitrogen-containing molecules takes place via hydrogenation of the aromatic rings, C--N bond cleavage of the resulting saturated amines by NH3 elimination, and alkene (de)hydrogenation. The separate reaction steps cannot be combined into one kinetic equation, because every reaction takes place on a different catalytic site which differs in its ability to bind reactants, intermediates and products. A multi-site Langmuir-Hinshelwood rate model has to be used in the kinetic modeling. Depending on the type of molecule, aniline or pyridine-like, the rate determining step is phenyl hydrogenation or ring opening elimination, respectively. The HDN behaviour of polycyclic aromatic N-containing molecules is based on a combination of the HDN reactions of aniline and pyridine-like molecules. The catalyst components influence the rate as well as the adsorption constants. Ni has a positive effect on all reactions steps for all reactant molecules, while the effect of phosphorus depends on the type of rate determining reaction, and thus on the reactant. Hydrogenation reactions are promoted by P (aniline and alkene), and elimination reactions are negatively influenced (piperidine and decahydroquinoline). The reverse is true for H2S, suggesting that phosphorus increases the number of sulfur vacancies which are considered to be the catalytic sites for (de)hydrotreating reactions. (C) 1997 Elsevier Science Ltd.