Deactivation modes site coverage

The mode of deactivation depends on the zeolite pore structure. With monodimensional zeolites such as MOR and with zeolites with trap cavities (large cavities with narrow apertures) such as ERI deactivation occurs mainly through pore blockage while with tridimensional zeolites such as Y and ZSM5 it occurs mainly through site coverage. 

The wide-ranging effects of pore structure on coke toxicity lead us to define four modes of deactivation (Figure 8) instead of the two (site coverage and pore blockage) which are generally proposed. Thus, deactivation could be due to ... 

In Modes 1 and 2 the limitation or the blockage is due to chemical reasons i.e., the coke molecules are (1) reversibly or (2) quasi-irreversibly adsorbed on the acid sites (site poisoning or site coverage) and/or to steric reasons the diffusion of reactant molecules through the cavity or the channel intersection is (1) limited or (2) blocked. With these modes the toxicity of the coke molecules is low as only the sites located in the cavity or at the chaimel intersection, often only one site, are partially (Mode 1) or totally (Mode 2) deactivated. 

With mode b the deactivating effect is similar to that of a site coverage (provided however that only one acid site is located in the cavity or at channel intersection). Mode c can be considered as pore blockage. The deactivating effect of the coke molecules will increase progressively from mode a to mode c, provided however that all the acid sites are identical. 

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