COKE ON CATALYSTS - HARMFUL, HARMLESS, INVISIBLE AND BENEFICIAL TYPES

The deactivating influence of coke on catalysts depends very much on the nature of the coke, its structure and morphology, and the exact location of its deposition on the catalyst surface. When coke deposition occurs mainly on the carrier surface and not directly on the active sites, the catalyst can tolerate even 6-10 wt.% coke without much loss in activity. A relatively low coking rate (sometimes achieved by a proper feedstock blending) can prevent pore-mouth blockage and ensure efficient use of the available surface and activity of a catalyst. Continuous gasification of the coke formed in oxidation reactions can maintain steady activity and selectivity over long periods. The importance of a carbonaceous overlayer or intermediate in many hydrocarbon reactions and even the beneficial role of coke in some cases have also been recognized in several catalytic reactions on metals and oxides. Considerations of such less-negative aspects of coke on catalysts suggest that catalytic reactions may be broadly classified into (relatively) coke-sensitive and coke-insensitive types. The latter type very often results when the catalyst contains a strong hydrogenating function and there is an abundance of hydrogen under pressure in the system. © 1990.