【Q&A Question 221】2018.08.15

(1) The catalyst reacts with the reactants to form intermediate compounds, thereby altering the reaction pathway. (2) The role of a catalyst is to shorten the time required to reach equilibrium, but it cannot change the equilibrium state. (3) Catalytic reactions are selective.

(1) The catalyst reacts with the reactants to form intermediate compounds, thereby altering the reaction pathway. (2) The role of a catalyst is to shorten the time required to reach equilibrium, but it cannot change the equilibrium state. (3) Catalytic reactions are selective.

(1) The catalyst reacts with the reactants to form intermediate compounds, thereby altering the reaction pathway. (2) The role of a catalyst is to shorten the time required to reach equilibrium, but it cannot change the equilibrium state. (3) Catalytic reactions are selective.

During use, due to carbon deposition, metal accumulation, or changes in the state of the active components, the catalyst’s activity gradually decreases to the point where it can no longer meet the production requirements; at this time, the catalyst must be regenerated to restore its activity.

Most catalysts for hydrogenation reactions are transition metals, and these catalyst powders are often impregnated onto activated carbon and alumina particles； Different catalysts require different reaction conditions; some can react at atmospheric pressure, while others need to be carried out under pressure. Porous skeletal nickel (also known as Raney nickel) is commonly used as a catalyst in industry. The role of a catalyst is to alter the reaction pathway and reduce the reaction activation energy. It is generally believed that in the hydrogenation reaction, H2 and olefins are simultaneously adsorbed on the catalyst surface; the catalyst facilitates the breaking of the σ bond in H2, resulting in the formation of two M-H σ bonds, which then react with the olefins coordinated to the metal surface.

A hydrocracking catalyst is a catalyst used in the petroleum refining process, where heavy oil undergoes hydrocracking reactions at high temperatures of 360–450°C and high pressures of 15–18 MPa, to be converted into products such as gases, gasoline, jet fuel, and diesel. The hydrocracking process is a secondary processing step in petroleum refining. Its feedstock consists of heavy distillates, as well as atmospheric residue and vacuum residue. The main advantage of hydrocracking is its high production flexibility; the composition of the products can be controlled by adjusting the operating conditions. It is possible to produce gasoline, jet fuel with a low freezing point, and diesel, while large quantities of residual oil can also be produced for use as feedstock in cracking processes or for manufacturing lubricants. The resulting product has good stability, but the octane rating of the gasoline is not high. Due to harsh operating conditions, high equipment investment and operating costs, its application is less widespread than that of catalytic cracking. However, the hydrocracking process can handle feedstocks containing impurities such as sulfur and those with a high aromatic content; these feedstocks are first subjected to hydrorefining in a hydrorefining reactor before entering the cracking reactor for further refinement. Depending on the desired product, the feed to the cracking reactor may or may not include recycled oil. The range of circulating oil is wide; it can include all fractions below gasoline, as well as heavy fractions (>350°C) below diesel. Most feedstocks can be processed using fixed-bed reactors, but residue hydrocracking requires a fluidized-bed reactor; the hydrocracking catalyst must be suitable for the respective feedstocks, reaction conditions, and equipment.

(1) The catalyst reacts with the reactants to form intermediate compounds, thereby altering the reaction pathway. (2) The role of a catalyst is to shorten the time required to reach equilibrium, but it cannot change the equilibrium state. (3) Catalytic reactions are selective

Fixed-bed reaction； It does not participate in the reaction, and its mass remains unchanged before and after the reaction ； Increasing the reaction temperature speeds up the reaction rate ；

A substance that can change the rate of a chemical reaction while its own composition and weight remain unchanged before and after the reaction is called a catalyst. A catalyst that accelerates the reaction rate is called a positive catalyst ； A retardant negative catalyst. The catalysts referred to in general are positive catalysts. Catalysis alters the pathway of chemical reactions. At the end of the reaction, although the catalyst remains unchanged compared to its initial state, it participates in the reaction – for example by forming activated adsorbed states and intermediate products – thereby reducing the activation energy required for the reaction. The action of a catalyst cannot change the state of chemical equilibrium, but it reduces the time required to reach equilibrium. In reversible reactions, it can increase the rates of both the forward and reverse reactions by the same factor. Catalysts can only accelerate reactions that are thermodynamically possible, not those that are thermodynamically impossible. Selectivity of catalyst action. A catalyst can cause the same reactants to react in different directions to produce different products, but under certain conditions a single catalyst can only accelerate one reaction. For example, CO and H2 use copper and nickel as catalysts respectively, to produce CH3OH and CH4+H2O under corresponding conditions. The emergence of a new catalytic process or a new catalyst often fundamentally changes the conditions of a certain chemical processing method, thereby driving the development of industrial production processes and generating substantial wealth. There are countless examples of this in modern inorganic chemistry, organic chemistry, petroleum processing, and the emerging petrochemical industry. Catalysis plays an increasingly important role in many aspects closely related to human survival, such as the efficient use of resources, improving the efficiency of chemical processing processes, synthesizing products with specific properties, making effective use of energy, reducing and managing environmental pollution, as well as in life sciences.

Answer: The catalyst reacts with the reactants to form intermediate compounds, thereby altering the pathway of the reaction. (2) The role of a catalyst is to shorten the time required to reach equilibrium, but it cannot change the equilibrium state. (3) Catalytic reactions are selective.