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1.2 Basics of Hydrogenation Reactions 1. What is the action of a heterogeneous catalyst? Heterogeneous catalytic reaction? Under what conditions can the reaction be in a state that is nearly ideal and efficient? Solid catalysts are widely used in the petroleum industry, and reactions often take place in gaseous, liquid, or gas-liquid mixed states; both the catalyst and the reaction involve distinct phase interfaces, a situation known as multiphase catalysis. Reactions that occur under heterogeneous catalysis are known as heterogeneous catalytic reactions. If the hydrocracking reaction catalyst is solid while the feedstocks are liquid and gaseous, the catalytic reaction that takes place is a heterogeneous catalytic reaction. In fixed-bed multiphase catalytic reactions, the chemical reaction process can reach an almost ideal and efficient state only when the operation is carried out in a regime close to plug flow. Only when the mass flow in a fixed-bed reactor is approximately plug flow and the radial temperature difference is very small, do the changes in the operating parameters of industrial plants have a typical and predictable impact on the degree of conversion, product distribution, and quality, thereby providing a better representation of the actual conditions in chemical processes. Conversely, if there are serious reaction engineering problems such as severe backmixing, channeling, and large radial temperature differences, the influence of operating parameters such as temperature, pressure, space velocity, and hydrogen-to-oil ratio on the reaction process will deviate from the ideal case. 2. Definition of hydrocracking: Hydrocracking is one of the key technologies for the deep processing of heavy oils. It is a hydrogenation process in which, in the presence of a catalyst and under high temperatures along with high hydrogen pressures, C-C bonds are broken, allowing large molecular hydrocarbons to be converted into smaller ones and thus lightening the oil products. It can process a wide range of raw materials, including straight-run naphtha, crude diesel, vacuum wax oil, as well as other materials obtained through secondary processing such as coker diesel, coker wax oil, and deasphalted oil. It is typically capable of producing high-quality liquefied gas, gasoline, diesel, jet fuel and other clean fuels, as well as high-grade petrochemical feedstocks such as light naphtha. For statistical purposes, the American Oil and Gas Magazine refers to hydrogenation processes with a conversion rate of over 50% as “hydrocracking”. In practical applications, people *tend to refer to those hydrogenation processes that reduce 10%–50% of the molecules in the feed oil through hydrogenation reactions as mild hydrocracking. The so-called “conventional (high-pressure) hydrocracking” refers to the hydrocracking process with a reaction pressure of 10.0 MPa or higher ； ““Medium-pressure hydrocracking” refers to a hydrocracking process at pressures below 10.0 MPa. In hydrocracking reactions, apart from cracking which is an endothermic reaction, most of the other reactions are exothermic. The overall heat effect is a strong exothermic reaction. 3. Characteristics of the desulfurization reaction: The C-S bonds in sulfur-containing compounds are relatively easy to break; their bond energy is much lower than that of C-C or C-N bonds (the bond energy of C-S bonds is 272 kJ/mol, that of C-C bonds is 348 kJ/mol, and that of C-N bonds is 305 kJ/mol). Therefore, during the hydrogenation process, the C-S bonds in sulfur-containing compounds break first, resulting in the formation of corresponding hydrocarbons and hydrogen sulfide. The hydrogen desulfurization reaction activity of various sulfides is related to their molecular size and structure. ①For molecules of the same size, desulfurization activity: thiol > disulfide > thioether > thiophene. ②If the type is the same, then: sulfides with a high molecular weight and complex structure, bifunctional rings > monocyclic rings ； 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 in this process; moreover, the hydrogenation of heterocycles is an exothermic reaction, and increasing temperature is unfavorable for the hydrogenation saturation of heterocycles ； However, it is favorable for the hydrolysis and denitration reactions of heterocyclic nitrides that are thermodynamically controlled in this temperature range. In short, operating at lower reaction temperatures favors the cyclohydrogenation reaction; however, the rate of hydrodecomposition is low at such temperatures, resulting in a lower overall hydrogenation-denitration rate ； As the reaction temperature increases, on the one hand, the hydrolysis 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 hydrodenitration rate. Taking pyridine as an example, as the reaction temperature increases, the reaction rate constant for the hydrodecomposition of the intermediate product obtained after the hydrogenation of pyridine increases. However, at sufficiently high temperatures, the effect of the decrease in the equilibrium concentration of this intermediate outweighs the increase in the hydrodecomposition reaction rate constant, resulting in a decrease in the overall hydrogenation-denitration reaction rate. 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 fairly 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 presence of other heteroatoms has little effect on hydrodenitration, as the adsorption equilibrium constant of nitrides at active sites is much larger than that of other heteroatoms; thus, their inhibitory effect 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.