Process Section – Daily Hydrogenation Process Knowledge Question No. 009

For most industrial catalysts, their physical and chemical properties undergo slight changes as the catalytic reaction proceeds; these changes are difficult to detect in the short term. However, over time these changes accumulate, leading to a significant decline in the catalyst’s activity and selectivity – this is the process of catalyst deactivation. In addition, external factors such as toxins and impurities present in the reactants, dust from upstream processes, and carbon deposition on the reactants during the reaction also cause a decrease in the catalyst’s activity and selectivity.

1 Coking and carbon deposition on the catalyst surface； 2 Metal and ash deposition on the catalyst ； 3 Metal aggregation and changes in crystal size and morphology.

Poisoning, sintering and thermal deactivation, coking and blockage

1. Coking and carbon deposition on the catalyst surface； 2. Metal and ash deposition on the catalyst ； 3. Metal aggregation and changes in crystal size and shape.

The reasons for catalyst deactivation are generally classified into three categories: poisoning, sintering and thermal deactivation, and coking and blockage.

The main issues include: coking and carbon deposition on the catalyst surface； Metal and ash deposition on the catalyst ； Metal aggregation and changes in crystal size and morphology. ①During the hydrogenation process, the cracking of hydrocarbons in the feed oil and the condensation of unstable compounds both lead to coking on the surface of the catalyst. This results in the coverage of its metal active centers and the blockage of its micropores, which are major causes of catalyst deactivation. ②The metals in the crude oil, particularly Fe, Ni, V, Ca, etc., exist in the form of soluble organometallic compounds; upon decomposition during the hydrogenation process, they deposit on the surface of the catalyst and clog its micropores ； As, Pb, Na, etc. react with the active centers of the catalyst, leading to the destruction of the zeolite structure. In addition, ash-forming substances such as graphite, alumina, aluminum sulfate, and silica gel block the pores of the catalyst, cover the active sites, and undergo solid-phase reactions with the carrier at excessively high regeneration temperatures; this constitutes permanent deactivation. ③Hydrogenation catalysts that are not made of precious metals suffer from issues such as metal aggregation, crystal growth, morphological changes, and destruction of the zeolite structure during long-term operation. Among the above three deactivation mechanisms, only catalyst deactivation caused by coking can have its activity restored through a coking treatment using oxygen-containing gases.

Toxicity, sintering and thermal deactivation, coking and blockage

The reasons for catalyst deactivation are generally classified into three categories: poisoning, sintering and thermal deactivation, and coking and blockage. 1. Inactivation caused by poisoning (1) Temporary poisoning (reversible poisoning): When a toxin adsorbs onto or reacts with the active site, the bonds formed are relatively weak; it is possible to use appropriate methods to remove the toxin, thereby restoring the catalyst’s activity without affecting its properties. This type of poisoning is known as reversible poisoning or temporary poisoning. (2) Permanent poisoning (irreversible poisoning): When a toxin interacts with the active components of a catalyst, strong chemical bonds are formed, making it difficult to remove the toxin using conventional methods in order to restore the catalyst’s activity. This type of poisoning is known as irreversible or permanent poisoning. (3) Selective poisoning: After catalyst poisoning, it may lose its catalytic ability for a certain reaction, but remain catalytically active for other reactions; this phenomenon is known as selective poisoning. In a chain reaction, if the poison only poisons the active sites that initiate subsequent reactions, it can halt the reaction at an intermediate stage, yielding intermediates in high yield. 2. Deactivation caused by coking and blockage: Carbonaceous deposits on the surface of the catalyst are known as coking. Coking almost always occurs in heterogeneous catalytic reaction processes that use organic substances as raw materials and solids as catalysts. The phenomenon in which the pore size decreases (or the pore openings shrink) due to the deposition of carbonaceous substances and/or other materials within the catalyst pores, preventing reactant molecules from diffusing into those pores, is known as clogging. Therefore, clogging is often classified under coking, and the overall decline in activity is referred to as coking deactivation; it is the most widespread and common form of catalyst deactivation. Typically, carbonaceous deposits can be removed by gasification in reaction with water vapor or hydrogen, so coking and deactivation are reversible processes. Compared to catalyst poisoning, there are far more substances that cause catalyst coking and blockage than catalyst poisons. In actual coking studies, it has been found that catalyst coking involves a rapid initial deactivation, followed by a quasi-steady state in terms of activity. It has been reported that coking deposition occurs mainly during the initial stage (within 0.15 seconds), and it has also been observed that approximately 50% of the carbon formed gets deposited within the first 20 seconds. Coking deactivation is also reversible; by controlling coking during the early stages of the reaction, the activity of the catalyst can be significantly improved. This is precisely why research on coking deactivation is becoming increasingly active. 3. Sintering and thermal deactivation (solid-state transformation): The sintering and thermal deactivation of catalysts refer to the changes in the structure and properties of the catalysts caused by high temperatures. In addition to causing sintering of the catalyst, high temperatures can also lead to other changes, including alterations in chemical and phase composition, semi-melting, grain growth, the encapsulation of active components by the carrier, and the loss of active components due to the formation of volatile or sublimable substances. In fact, at high temperatures, all catalysts will gradually undergo irreversible structural changes; the rate of these changes varies depending on the catalyst. Sintering and thermal deactivation are related to various factors, such as the pretreatment, reduction, and regeneration processes of the catalyst, as well as the promoters and carriers added. Of course, the reasons for catalyst deactivation are complex; the deactivation of each catalyst does not occur based on just one of the categories mentioned above, but is rather caused by two or more factors.

1. Carbon deposits clog the channels. 2. Collapse of the carrier molecular sieve framework. 3. Deposition of basic nitrides at the acidic centers of the catalyst. 4. Aggregation of active metals.

⑴For complex catalysts, the main issue is excessive temperature – most complexes decompose and become inactive at temperatures above 250°C. ⑵ For biological catalysts, factors such as overheating, contamination by chemical substances and microorganisms, and imbalances in pH can lead to inactivation. ⑶ For solid catalysts, the reasons for inactivation include: ① Excessive temperature, which causes sintering of the catalyst surface, as well as changes in crystal structure or phase; ② Presence of toxic impurities in the feed gas, which poisons the catalyst; ③ Accumulation of dirt or carbon deposits on the catalyst surface.