In chemical production, some processes rely on catalysts. Catalysts accelerate reactions, increase target product yields, and reduce energy consumption, directly impacting the efficiency and economic benefits of production lines. For core processes like acrylonitrile synthesis and sulfur recovery, selecting a high-performance, highly stable catalyst is crucial. Today, we'll explore the key factors to consider when selecting these two types of catalysts.

What is Acrylonitrile Catalyst?

Acrylonitrile catalysts are specialized materials used to accelerate and selectively promote the propylene ammoxidation reaction to produce acrylonitrile. Their core functions are:

Accelerating the reaction: Significantly increasing the reaction rate at relatively mild temperatures and pressures.

Improving selectivity: Directing the reaction toward the target product, acrylonitrile, while minimizing the formation of byproducts (such as acetonitrile, hydrocyanic acid, acrolein, and carbon dioxide).

Extending the operating cycle: With excellent stability and wear resistance, they maintain high activity and selectivity over extended periods of time.

Uses of Acrylonitrile Catalysts

Acrylonitrile catalysts effectively lower the activation energy of the propylene ammoxidation reaction. In industrial production, they can be carried out at relatively mild temperatures of 400-500°C, significantly improving the efficiency of acrylonitrile. This temperature range is determined based on extensive industrial practice and thermodynamic research.

What is a sulfur catalyst?

Sulfur catalysts are a type of catalytic material used to promote the conversion of sulfur-containing compounds (such as hydrogen sulfide H₂S) into elemental sulfur (S). They are widely used in petroleum refining, natural gas processing, coal chemical industry and other fields, mainly for sulfur recovery and exhaust gas purification processes.

Uses of Sulfur Catalysts

Sulfur catalysts do not refer to elemental sulfur, but rather to a class of catalysts with metal sulfides (such as molybdenum, cobalt, and nickel sulfides) as active ingredients. Their most significant characteristic is their insensitivity to sulfur, requiring sulfur to maintain their catalytic activity.

Sulfur catalysts are primarily used in two areas:

1. Petroleum Refining

Core Task: Hydrodesulfurization. This is its most important and largest application. It is specifically designed to remove sulfur compounds from crude oil and its fractions (such as gasoline and diesel), directly producing low-sulfur and ultra-low-sulfur clean fuels that meet environmental standards.

Auxiliary Purification: Hydrodenitrogenation/Deoxygenation. While desulfurizing, it also effectively removes impurities such as nitrogen and oxygen from the oil product that are harmful to the environment and subsequent processes.

2. Chemical Production

Sulfur catalysts are used as hydrogenation catalysts in chemical processes where the raw materials themselves contain sulfur or where sulfur is unavoidable in the reaction environment, such as in the saturation of unsaturated hydrocarbons and the synthesis of fine chemicals. Its resistance to sulfur poisoning makes it an irreplaceable advantage in these applications.

Similarities between acrylonitrile catalyst and sulfur catalyst

1. Core Function: They are both catalysts. This means they both lower the activation energy of a chemical reaction by providing an alternative reaction pathway, thereby accelerating the reaction rate, while not being consumed after the reaction itself.

2. Equilibrium Impact: They both have the same effect on chemical equilibrium; that is, they cannot change the equilibrium constant of the reaction; they can only shorten the time required to reach equilibrium.

3. Selectivity: For certain complex catalytic systems (particularly acrylonitrile catalysts), selectivity is a key metric. While sulfur oxidation itself is not subject to selectivity, catalyst selectivity is equally important in the catalytic conversion of other sulfur-containing compounds (such as sulfur recovery in the Claus process).

The main differences between acrylonitrile catalyst and sulfur catalyst

Acrylonitrile Catalyst

• Working Principle

Heterogeneous catalysis follows a redox mechanism (Mars-van Krevelen mechanism):

1. Oxygen ions in the catalyst lattice oxidize propylene molecules.

2. The reduced catalyst reabsorbs oxygen from the gaseous oxygen, returning to its original state.

3. Ammonia molecules are activated at specific sites on the catalyst surface and participate in the reaction.

• Reaction Phase

Heterogeneous catalysis: solid catalyst and gaseous reactants.

• Main Applications

Almost exclusively used in the industrial production of acrylonitrile, it is a core catalyst in the petrochemical and organic chemical industries.

• Complexity and Selectivity

Extremely high. Precise control of the reaction pathway is required to ensure that propylene, ammonia, and oxygen primarily produce acrylonitrile, rather than byproducts such as acetonitrile, hydrocyanic acid, or carbon dioxide.

Sulfur Catalyst

• Working Principle

Homogeneous/Autocatalytic: In the gas phase, the SO₂ molecules produced by the reaction catalyze the reaction between SO₈ vapor molecules and O₂, forming a chain reaction and greatly accelerating the reaction rate. It acts more like a part of the reaction.

• Reaction Phase

Homogeneous Catalysis: All reactants and the catalyst (SO₂) are in the gas phase. Sulfur itself is a reactant, and its vapor also catalyzes subsequent reactions.

• Main Applications

It is primarily used in the first step of the sulfuric acid industry, where sulfur is burned to generate SO₂ gas, preparing for subsequent catalytic oxidation to SO₃.

• Complexity and Selectivity

Extremely Low. SO₂ is the only target product, and selectivity is rarely required; the reaction only needs to be complete and rapid.

Final Thoughts

You can understand the difference this way:

• An acrylonitrile catalyst is like a highly specialized intelligent robot, precisely picking up three distinct components—propylene, ammonia, and oxygen—on a complex assembly line and assembling them into the product "acrylonitrile" with extreme efficiency and accuracy, while also avoiding assembly errors and waste.

• A sulfur catalyst (its autocatalytic process) is more like a fuse or starter. The small amount of SO₂ produced by the sulfur's own combustion acts like a trigger to light a powder keg, triggering a violent, self-accelerating chain combustion reaction of the subsequent sulfur vapor.

In summary, although both are called "catalysts," acrylonitrile catalysts represent the pinnacle of complex, highly selective heterogeneous catalysis in modern industrial catalysis, while sulfur catalysts are a classic, relatively simple example of autocatalysis. The significant differences between them reflect the breadth of catalytic chemistry applications.