Coking, also known as carbon buildup, is a common problem in catalysis, particularly in processes involving hydrocarbons or carbon oxides. It describes the formation of carbonaceous residues that can accumulate on the surface of catalysts and block their active sites. These deposits—often referred to as coke—can account for up to 20% of the catalyst weight and lead to significant deactivation, either by physically blocking the active sites or by clogging the pores.

Mechanisms of coke formation

The mechanisms of coke formation vary depending on the catalyst type (metal, oxide, or sulfide catalysts) and the specific reaction conditions. On metallic catalysts, such as nickel (Ni), various carbon species are formed by the disproportionation of carbon monoxide (CO). These include atomic carbon, amorphous carbon, graphitic carbon, and carbides. Generally, two main types of coke formation are distinguished:

Coke on metallic centers: This type comprises carbon-rich deposits with graphitic structures down to atomic carbon. Its formation occurs through disproportionation and cleavage reactions catalyzed on metallic centers.

Coke on acidic centers or supports: These are usually aromatic deposits formed by catalytic cracking reactions. These lead to the formation of so-called coke precursors, such as alkenes. Subsequent dehydrogenation and cyclization reactions result in highly aromatic compounds that condense as coke on the catalyst surface.

The chemical composition of the coke formed depends strongly on the reaction conditions, the composition of the feed gas stream, and the age of the catalyst. High proportions of olefins or aromatic compounds in the feed promote coke formation because these act as hydrogen acceptors and facilitate the formation of carbon-containing precursors. Additionally, coke may be unevenly distributed within the catalyst pores, creating diffusion barriers. Deposits near the pore entrances act as diffusion resistance, blocking the access of reactants to the active sites.

Prevention and control of coke formation

Avoiding coke formation is a key objective in catalyst development and process optimization. Catalyst composition plays a crucial role in this. Promoters such as potassium can neutralize acidic centers in support materials that would otherwise promote coke formation due to unsaturated hydrocarbons. One example is the doping of nickel-based steam reforming catalysts with potassium to minimize coke formation.

If coke formation cannot be completely avoided, the catalyst is regenerated by gasification. Gases such as oxygen (O₂), air, or hydrogen (H₂) are used to convert the coke into CO₂ or CH₄. In processes like catalytic cracking on acidic zeolites, the lifetime of a catalyst is often only a few seconds, which is why continuous regeneration by burning off the coke is necessary.

Conclusion

Coke formation is a complex phenomenon influenced by chemical, physical, and operational factors. A detailed understanding of the mechanisms and causes is crucial for designing industrial processes to maximize catalyst lifetime and maintain efficiency.