Modes of coking and deactivation of acid zeolite catalysts

The formation of coke during hydrocarbon conversion on zeolite catalysts can be considered as a nucleation-growth process. Nucleation is due to the retention in the zeolite micropores of coke precursors (heavy secondary reaction products). At high temperature the retention is due to trapping in the cavities or at channel intersections condensation and hydrogen transfer reactions are involved both in the formation of coke precursors and in their growth. As with other catalysts, deactivation of zeolite catalysts occurs either through site coverage or through pore blockage. Pore blockage is the only mode of deactivation of monodimensional zeolites and of zeolites with trap cavities (large cavities with small apertures), both types deactivating very rapidly. With the other types of zeolites at low coke contents deactivation is due to site coverage, while at high coke contents it is due to a blockage of the pores by coke deposits on the outer surface of the crystallites.