Working Principles and Classification of Hydrogenation Catalysts

Overview

Hydrotreating catalysts are generally made up of active components, promoters, and a support. They are used to remove sulfur, nitrogen, oxygen, and heavy metals from petroleum fractions through hydrogenation. They can also hydrogenate and saturate polycyclic aromatic hydrocarbons.

During hydrotreating, the molecular structure of the feedstock does not change significantly. Depending on the process requirements, some hydrocracking may also occur, but the conversion depth is limited. The conversion rate is generally around 10%.

Hydrotreating catalysts need to provide two main functions:

• Hydrogenation
• Hydrogenolysis, which means breaking chemical bonds with the help of hydrogen

However, hydrogenolysis does not require a highly acidic catalyst.

Working Principle

Mechanism of Catalytic Hydrogenation Catalytic hydrogenation changes the reaction pathway and lowers the activation energy.

Hydrogen molecules are first adsorbed onto the catalyst surface. They then split into highly active hydrogen atoms. These hydrogen atoms add to the double or triple bonds of alkenes and alkynes, whose bonds have been weakened by adsorption on the catalyst.

Stability of Alkenes and Alkynes

The stability of an alkene is affected by the number of alkyl groups attached to the carbon atoms of the double bond.

In general, the more alkyl groups attached to the double-bonded carbon atoms, the lower the heat of hydrogenation and the more stable the alkene:

R2C=CR2 > R2C=CHR > R2C=CH2 > RCH=CH2 > CH2=CH2

A trans isomer is generally more stable than the corresponding cis isomer.

The heat of hydrogenation of acetylene is approximately: -313.8kJ·mol－1，This value is greater than twice the heat of hydrogenation of ethylene, which is approximately: -274.4kJ·mol－1

Therefore, acetylene is less stable than ethylene.

Applications

In the presence of catalysts such as platinum, palladium, or nickel, alkenes and alkynes react with hydrogen through addition reactions to form the corresponding alkanes. Heat is released during the reaction. This released heat is called the heat of hydrogenation.

The heat of hydrogenation is the amount of heat released when 1 mole of an unsaturated hydrocarbon is hydrogenated.

The basic mechanism of catalytic hydrogenation is as follows:

1. Hydrogen molecules are adsorbed onto the catalyst surface.
2. The hydrogen molecules split into active hydrogen atoms.
3. The catalyst weakens the double or triple bonds of the unsaturated hydrocarbons.
4. The active hydrogen atoms add to these bonds, forming saturated hydrocarbons.

Classification

1. Hydrocracking Catalysts

A hydrocracking catalyst is used in the hydrocracking of heavy oil during petroleum refining. Under high temperatures of approximately 360–450°C and high pressures of about 15–18 MPa, heavy oil is converted into products such as:

• Gases

• Gasoline

• Jet fuel

• Diesel fuel

Hydrocracking is a secondary processing step in petroleum refining. Its feedstocks are usually heavy distillates, but atmospheric residue and vacuum residue can also be processed.

The main advantages of hydrocracking include:

• High flexibility in product selection
• Adjustable product distribution through changes in operating conditions
• The ability to produce gasoline, low-freezing-point jet fuel, and diesel
• The ability to produce large quantities of unconverted oil for use as a steam-cracking feedstock or for producing lubricating oil

The products have good stability. However, the octane number of hydrocracked gasoline is generally not very high.

Because hydrocracking requires severe operating conditions, its equipment investment and operating costs are high. As a result, it is less widely used than catalytic cracking.

One important advantage of hydrocracking is that it can process feedstocks containing high levels of sulfur and other impurities, as well as feedstocks with high aromatic content. Before entering the hydrocracking reactor, the feedstock is usually sent through a hydrotreating reactor for thorough purification.

Depending on the desired products, the feed to the cracking reactor may or may not include recycle oil. The recycle oil range can be broad. It may include all fractions lighter than gasoline, or it may consist of heavy fractions below the diesel range, generally with a boiling point above 350°C.

Most feedstocks can be processed in fixed-bed reactors. However, residue hydrocracking must generally use an ebullated-bed reactor. Hydrocracking catalysts must be selected to match the feedstock, operating conditions, and reactor type.

2. Hydrotreating Catalysts

Hydrotreating catalysts are used to remove sulfur- and nitrogen-containing compounds from petroleum products. They can also hydrogenate aromatic hydrocarbons and convert them into cycloalkanes.

The active components are usually composite oxides or sulfides of metals such as:

• Molybdenum

• Cobalt

• Tungsten

• Phosphorus

Common catalyst systems include combinations of cobalt–molybdenum or nickel–molybdenum supported on alumina.

Catalyst Selection Guidelines

The following factors should be considered when selecting a hydrogenation catalyst:

1. ActivityThe catalyst should promote the desired reactions efficiently.
2. SelectivityThe catalyst should favor the desired products and limit unwanted side reactions.
3. StabilityThe catalyst should maintain its performance for a long operating period.
4. Mechanical strengthThe catalyst should resist crushing, abrasion, and damage during loading and operation.
5. Regeneration performanceThe catalyst should be suitable for regeneration when its activity decreases.
6. SafetyThe catalyst and its operating conditions should not create unnecessary safety risks.
7. Cost-effectivenessThe overall performance and service life should justify the purchase and operating costs.

In addition, the following factors must also be considered:

• The type and composition of the feedstock
• The physical and chemical properties of the feedstock
• The required product quality and product distribution
• The hydrotreating or hydrogenation process
• The operating pressure
• The hydrogen-to-oil volume ratio
• The liquid hourly space velocity, or LHSV

The correct catalyst and operating conditions should be selected so that product quality and product distribution meet the required standards. At the same time, unwanted side reactions and hydrogen consumption should be minimized to improve the overall economic performance of the process.

Safety and Handling Precautions

• Do not roll catalyst containers on the ground during transportation. Rolling can crush and damage the catalyst.
• When opening the containers, take approximately 10 mL of sample from each drum or container. Mix the samples together and use the combined sample for catalyst analysis.
• After opening the containers, decide whether the catalyst needs to be screened according to its condition.
• Load the catalyst to the designed bed height.
• Transfer the catalyst into the loading hopper and then load it into the reactor through a canvas loading bag.
• Handle the catalyst carefully. Do not drop it, throw it, or roll the metal drums.
• Personnel involved in catalyst loading must ensure that no foreign objects are taken into the reactor. Objects such as keys, knives, iron wire, and lead wire must not be brought inside.
• Anyone entering the reactor must obtain the required entry permit and wear a dust mask or suitable respiratory protection.
• Before anyone enters the reactor, the atmosphere inside the reactor must be sampled and analyzed. Entry is permitted only after the test results meet the safety requirements.
• The reactor must be completely isolated from the process system by installing blind flanges or spectacle blinds before personnel enter it.

Conclusion: Choosing the Right Hydrogenation Catalyst for Long-Term Process Performance

Hydrogenation catalysts play a critical role in modern refining and petrochemical processes by improving product quality, reducing impurities, and enhancing overall process efficiency. However, selecting the right catalyst is not simply about choosing a high-activity material—it requires a comprehensive understanding of feedstock characteristics, operating conditions, reactor design, and target product requirements.

At CHEMPACK, we understand that every refining and petrochemical process has unique challenges. With more than 30 years of experience in catalyst and adsorbent technologies, we provide advanced catalyst solutions designed to meet different application requirements, including hydrotreating, hydrocracking, sulfur recovery, and other hydrogen-related processes.

Our technical team works closely with customers to evaluate process conditions, optimize catalyst selection, and improve long-term operational performance. By combining catalyst technology, engineering expertise, and reliable manufacturing capabilities, CHEMPACK helps customers achieve higher efficiency, longer catalyst cycles, and more sustainable production.

Choosing the right hydrogenation catalyst is not only a technical decision—it is a strategic investment in process reliability, product quality, and long-term operational value. CHEMPACK is committed to delivering customized catalyst solutions that support the evolving needs of the global refining and petrochemical industry.