Abstract: After a catalyst has been used for a period of time, the phenomenon that the reaction activity decreases with the operating time is called catalyst deactivation. What are the causes of catalyst deactivation? The main reasons for catalyst deactivation include catalyst poisoning, catalyst carbonization, and catalyst sintering. How to regenerate the catalyst if it is deactivated? Let’s introduce the method of catalyst deactivation regeneration, let’s understand it. 1. What are the causes of catalyst deactivation

The causes of catalyst deactivation are generally divided into three categories: poisoning, sintering and thermal deactivation, coking and clogging.

1. Inactivation caused by poisoning

(1) Temporary poisoning (reversible poisoning)

The bonds generated when poisons are adsorbed or combined on the active center If the intensity is relatively weak, appropriate methods can be used to remove the poison and restore the catalyst activity without affecting the properties of the catalyst. This kind of poisoning is called reversible poisoning or temporary poisoning.

(2) Permanent poisoning (irreversible poisoning)

The poison interacts with the active components of the catalyst to form a strong chemical bond. It is difficult to remove the poison by ordinary methods to make the catalyst Activity is restored, this kind of poisoning is called irreversible poisoning or permanent poisoning.

(3) Selective poisoning

After the catalyst is poisoned, it may lose its catalytic ability for a certain reaction, but it still has catalytic activity for other reactions. This phenomenon is called selective poisoning. . In a series reaction, if the poison only poisons the active site that leads to the subsequent reaction, the reaction can be stopped in the intermediate stage and an intermediate product with high yield can be obtained.

2. Deactivation caused by coking and clogging

The carbonaceous deposits on the catalyst surface are called coking. Coking may occur in almost all heterogeneous catalytic reaction processes using organic matter as raw material and solid as catalyst. Due to the deposition of carbonaceous substances and/or other substances in the catalyst pores, the pore diameter is reduced (or the pore opening is narrowed), so that the reactant molecules cannot diffuse into the pores. This phenomenon is called clogging. Therefore, clogging is often classified as coking, and the overall activity decline is called coking deactivation. It is the most common and common form of catalyst deactivation. Usually, carbonaceous deposits can be removed by gasification by reacting with water vapor or hydrogen, so coking deactivation is a reversible process.

Compared with catalyst poisoning, there are many more substances that cause catalyst coking and clogging than catalyst poisons. In actual coking studies, it was found that there is a rapid initial deactivation of catalyst coking, followed by a quasi-stationary state in terms of activity. There are reports that coking deposition mainly occurs in the initial stage (within 0.15s), and some It was found that approximately 50% of the carbon formed was deposited within the first 20 seconds. Coking deactivation is reversible. By controlling coking in the early stage of the reaction, the activity of the catalyst can be greatly improved. This is also an important factor in the increasingly active research on coking deactivation.

3. Sintering and thermal deactivation (solid-state transformation)

Sintering and thermal deactivation of catalysts refer to changes in catalyst structure and performance caused by high temperatures. In addition to causing the sintering of the catalyst, high temperatures can also cause other changes, including: changes in chemical composition and phase composition, semi-melting, grain growth, active components being embedded in the carrier, and active components generating volatile substances or Sublimable substances can be lost, etc.

In fact, all catalysts will gradually undergo irreversible structural changes at high temperatures, but the speed of this change varies with different catalysts. Sintering and thermal deactivation are related to many factors, such as catalyst pretreatment, reduction and regeneration processes, as well as the added promoters and carriers.

Of course, the reasons for catalyst deactivation are complicated. Each type of catalyst deactivation does not only follow one of the above categories, but is often caused by two or more reasons.

2. How to regenerate a deactivated catalyst

The general rule of regeneration of industrial catalysts is that the activity of the catalyst will decrease every time it is regenerated. The operating temperature of the regenerated catalyst It is significantly higher than before regeneration. In addition, the deactivated catalyst cannot be regenerated frequently and endlessly, and will eventually have to be replaced.

1. Regeneration after coking (coke) deactivation

During the use of the catalyst, carbon deposits are gradually formed on the surface, causing the catalyst activity to decrease.

Coke regeneration (air + water vapor) is commonly used in industrial catalysts after the coke is deactivated. Catalytic activity can be restored by oxidizing carbonaceous deposits in the catalyst pores into carbon monoxide and carbon dioxide.

Purge method: Organic by-products, mechanical dust and impurities that are not very serious in carbon deposition and clog the pores of the catalyst or cover the active centers on the surface of the catalyst can be removed in situ using the purge method.

Notes during regeneration: The regeneration temperature and time should be adjusted to prevent catalyst sintering; the regeneration cycle varies with the coke accumulation rate.

2. Metal pollution deactivation regeneration

The source of metal pollution is metal compounds, metal porphyrin complexes or non-porphyrin compounds in liquids directly liquefied from crude oil or coal. Mainly V, Ni, Fe, Cu, Ca, Mg, Na, K, etc.

Prevention and control methods: Remove porphyrins from raw materials by chemical or adsorption methods, add additives (antimony compounds), and form alloys with metal impurities to passivate them.

3. Regeneration of poisoning and deactivation

A small amount of impurities in the fluid in contact with the catalyst are adsorbed on the active sites of the catalyst, causing the activity of the catalyst to significantly decrease or even disappear.

Poisoning is divided into: reversible poisoning, regenerable, and temporary poisoning.

Prevention and control measures: Remove poisons before entering the reaction section.

4. Regeneration after sintering deactivation

During the sintering of the catalyst, the size of the uncrystalline gradually increases or the primary particles grow.

Prevention and control measures: Selection of usage conditions: The working temperature is lower than the Tammann temperature, usually 0.5Tm. Carrier selection: Ni/Cr2O3 catalyst Ni/Cr2O3-Al2O3 structure, adding additives (isolating agent).

Regeneration method: After the metal with large grains is oxidized by oxygen, it is reduced by H2.