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A high level of hydrodenitrogenation (HDN) is required to achieve a desirable conversion of other hydroprocessing reactions. This results from a strong adsorption of nitrogen-containing compounds on catalytic sites that slows down the hydrogen activation process and hinders the adsorption of other reactants. Studies on model compounds and real feeds indicate that less than 50 ppm of nitrogen in the feed can poison catalytic sites. Kinetic studies determined adsorption constants of various nitrogen-containing compounds and concluded that at least four different catalytic sites are required to interpret experimental observations in contrast with a dual site site ffi concept, which only considered two sites. The advancements in experimental techniques allowed identification of products formed during very early stages of hydrodenitrogenation. These results confirmed that the removal of the amino group from saturated amines, as the last step in hydrodenitrogenation, is governed by the type of carbon to which the amino group is attached rather than the number of hydrogen atoms attached to carbon in a and b position to nitrogen Performance of conventional Co(Ni)Mo(W)/Al 2 O 3 catalysts during hydrodenitrogenation was enhanced by combination with various additives and by replacing the traditionally used g-Al 2 O 3 support with novel supports. Catalytic functionalities could be modified by using different precursors of active metals and varying conditions during preparation. Progress has been made in the development of catalysts possessing a high selectivity for hydrodenitrogenation. In this case, the nonconventional catalysts based on the carbides and nitrides of transition metals exhibited high activity and selectivity. Noble metal sulfides alone or supported on different supports were active for HDN as well. Feedstocks used for catalyst evaluation included model compounds and mixtures of model compounds as well as real feeds. The challenges in the development of catalysts for hydrodenitrogenation of heavy feeds containing asphaltenes and metals have been identified.In general, compared to Q, conversions were little affected by a methyl (Me) on the aromatic ring, while being lower for Me on the N-ring, except for 2-MeQ. It is evident that Me substitution in the 2-position had little effect on HDN over the NiMoP/Al 2 O 3 catalyst, whereas it actually enhanced the HDN rate over the CoMo/Al 2 O 3 catalyst. For both catalysts the HDN rate was suppressed in 6-MeQ and particularly in 3-MeQ and 4-MeQ. The distribution of N-containing intermediates was influenced by the ring substitution and the type of catalyst. This suggests that several factors influence the overall HDN mechanism of Qs. This should be kept in mind while analyzing differences in the networks proposed by different authors.The preceding discussion on the HDN of Q mostly referred to conventional catalysts. A number of studies on the HDN of Q were conducted over unconventional catalysts. For example, 3-and 4-MeQs were more reactive than Q over the unsupported Zr,...

The hydrodeoxygenation of methyl-substituted phenols was carried out in a flow microreactor at 300°C and 2.85 MPa hydrogen pressure over a sulfided CoMo/Al 2 O 3 catalyst. The primary reaction products were methylsubstituted benzene, cyclohexene, cyclohexane, and H 2 O. Analysis of the results suggests that two independent reaction paths are operative, one leading to aromatics and the other to partially or completely hydrogenated cyclohexanes. The reaction data were analyzed using Langmuir-Hinshelwood kinetics to extract the values of the reactant-to-catalyst adsorption constant and of the rate constants characterizing the two reaction paths. The adsorption constant was found to be the same for both reactions, suggesting that a single catalytic site center is operative in both reactions. Ab initio electronic structure calculations were used to evaluate the electrostatic potentials and valence orbital ionization potentials for all of the substituted phenol reactants. Correlations were observed between (a) the adsorption constant and the two reaction rate constants measured for various methyl-substitutions and (b) certain moments of the electrostatic potentials and certain orbitals' ionization potentials of the isolated phenol molecules. On the basis of these correlations to intrinsic reactantmolecule properties, a reaction mechanism is proposed for each pathway, and it is suggested that the dependencies of adsorption and reaction rates upon methyl-group substitution are a result of the substituents' effects on the electrostatic potential and orbitals rather than geometric (steric) effects.

The effect of the ratio of solute molecular diameter to the substrate pore diameter on the diffusion of polyaromatic compounds in four different pore sizes of amorphous gamma-aluminas was studied at atmospheric pressure and ambient temperature. Diffusion rates of the polyaromatic compounds in the aluminas decreased as the ratio of solute molecular diameter to pore diameter increased. Empirical correlations between the apparent reduction in solute effective diffusivity and the ratio of molecular diameter to pore diameter were obtained.< coronene < octaethylporphyrin = tetra-phenylporphyrin for a given alumina.Effective diffusivities at ambient temperature and pressure were determined by applying a pore diffusion model with a Freundlich isotherm to diffusion run data.

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The effect of the ratio of reactant molecular diameter to catalyst pore diameter on the restrictive diffusion of reactants in catalysts under catalytic hydroprocessing conditions was studied. Hydrogenation of four nitrogen-containing heterocyclic model compounds dissolved in decalin was carried out in a batch-mode operation at 623 K and 5.27 MPa of hydrogen partial pressure over three Ni-Mo/alumina catalysts encompassing a range of molecular to pore sizes of 0.03 to 0.35. An empirical correlation for restrictive diffusion under realistic processing conditions was developed. The correlation shows that the effective diffusivity is strongly influenced by the ratio of the reactant molecular size to the average micropore size, taking into account changes in pore sizes due to coke deposition. The restrictive effect under reaction conditions was found to be larger than the theoretical prediction for nonreactive conditions.

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