Among the four types of hydrogenation reactions, why is denitration the slowest in reaction rate?

Copied from elsewhere; still, let’s share the characteristics of the denitration reaction. The function of hydrodenitration is to: 1. Remove nitrogen from the feedstock to a level that meets the process requirements, so as to fully utilize the functionality of the hydrocracking catalyst； II. Produce products that meet specification requirements (performance characteristics such as oil stability are related to nitrogen content). The rate constants for the denitration reaction of basic nitrides vary by only one order of magnitude; quinoline exhibits the highest denitration rate, while the rate decreases as the number of aromatic rings increases. Different nitrides are affected by steric hindrance to roughly the same extent. During the denitration reaction, nitrides are not adsorbed onto the catalyst surface via the terminal nitrogen atoms, but rather through the π-bonds of the aromatic rings. Hydrogenation of the heterocyclic rings occurs first, prior to the hydrogenolysis of the C-N bonds. Therefore, the denitration reaction should first involve the hydrogenation and saturation of the aromatic rings, followed by further ring-opening denitration. Therefore, hydrogenation denitration requires more energy than hydrogenation desulfurization. The hydrogenation activity of nitrogen-containing compounds is characterized as follows for monocyclic nitrides: pyridine > pyrrole ≈ aniline > benzene ring ； Polycyclic nitrides: polycyclic > bicyclic > monocyclic ； Heterocycles > Aromatic rings. The hydrogenation of nitrogen-containing compounds is thermodynamically characterized by low equilibrium constants for the hydrogenation reaction within the typical temperature ranges used for this process; moreover, the hydrogenation of heterocycles is an exothermic reaction, and increasing temperature is unfavorable for the hydrogenation saturation of heterocycles ； However, the hydrogenolysis and denitration reactions of heterocyclic nitrides are thermodynamically favorable in this temperature range. In summary, operating at lower reaction temperatures favors the cyclohydrogenation reaction; however, the hydrogenolysis rate is low at such temperatures, resulting in a lower overall hydrodenitration rate ； As the reaction temperature increases, on the one hand, the nitrolysis rate increases, which facilitates an increase in the denitration rate; on the other hand, the equilibrium constant of the hydrogenation reaction decreases, resulting in a lower concentration of heterocyclic hydrogenation products, and thus a decrease in the overall denitration rate. Therefore, as the temperature increases, the overall hydrogenation denitration rate reaches a maximum; before this point, the reaction is controlled by kinetics, while after that it is controlled by thermodynamics. In some cases, the thermodynamic equilibrium between heterocyclic nitrides and their hydrogenated products can limit and influence the overall rate of hydrodenitration. Taking pyridine as an example, as the reaction temperature increases, the reaction rate constant for the hydrogenolysis of the intermediate product formed upon the hydrogenation saturation of pyridine increases. However, once a certain high temperature is reached, the decrease in the equilibrium concentration of this intermediate has a greater impact than the increase in the reaction rate constant for hydrogenolysis; as a result, the overall rate of hydrogenation denitration decreases. The temperature at which the highest conversion is achieved is related to the operating pressure; the higher the pressure, the higher the temperature required for maximum conversion. This characteristic is very similar to that of the hydrogenation of polycyclic aromatic hydrocarbons. Only under quite high pressures can the equilibrium constraint between pyridine and ** be ignored. Low temperature and high pressure are favorable for the denitration reaction of heterocyclic nitrides. The influence of the presence of other heteroatoms on hydrodenitration is due to the fact that the adsorption equilibrium constant of nitrides at active sites is much larger than that of other heteroatoms; thus, the inhibitory effect of these other heteroatoms on the hydrodenitration reaction is minimal. On the contrary, the presence of thiophene and hydrogen sulfide can even promote the hydrolysis of C-N bonds under high-temperature conditions. Taking the effect of thiophene on the denitration of pyridine as an example: at low temperatures, the hydrogenation reaction of pyridine is moderately inhibited due to competitive adsorption ； At high temperatures, the formation of hydrogen sulfide due to the HDS reaction accelerates the breakdown of C-N bonds, thereby increasing the overall rate of the HDN reaction. However, the self-blocking and mutual blocking effects between nitrides are much more significant.

From fast to slow, it should be demetallization, diene saturation, desulfurization, deoxygenation, monene saturation, denitrogenation, and aromatic saturation

In short, heteroatom nitrogen is generally located inside compounds such as cyclic aromatics; hydrogenation is a process of gradually opening these rings, which is why nitrides are difficult to remove