Deactivation modes coke content effect

The first mode of deactivation is clearly shown with HZSM5, At low coke content, Vr/Va is close to 1 4 coke molecules are needed to deactivate one acid site, This weak deactivating effect can be explained by a competition for adsorption on the acid sites between the reactant and the coke molecules which are too weakly basic to be "irreversibly adsorbed at the reaction temperature. However limitations in the rate of diffusion of the reactant can also be responsible for deactivation. The size and the basicity of the coke molecules increase with the coke content, which causes an increase in the deactivating effect of the coke molecules. Beyond a certain size of the coke molecules the channel intersection is completely inaccessible to the reactant and to the adsorbates and Vr/Va can be lower than T This first mode of deactivation occurs also with USHY. However the deactivating effect of coke molecules is initially very high because coke molecules are formed on the strongest (hence the most active) acid sites. 

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