The modes of deactivation of the zeolite catalysts
were established during n-heptane cracking at 450°C
on protonic zeolites with different structures: monodimensional
or tridimensional - small, average or
large pore apertures - tubular or with cavities [13].
Deactivation could be due to: (a) a limitation of the
access of the reactant to the active sites of a cavity or a
pore intersection in which is located a coke molecule,
or (b) a blockage of the access; (c) and (d) a limitation
or a blockage of the access of the reactant to the active
sites of cavities, of pore intersections or parts of
channels in which coke molecules are not located.
In modes a and b the limitation or the blockage are
either due to steric reasons, namely the diffusion of the
reactants in the cavity or at the pore intersections is
limited or blocked, or chemical reasons - the coke
molecules are reversibly or quasi-irreversibly
adsorbed on the acid sites (site coverage) [25]. With
these modes the deactivation of the zeolite is generally
limited since only the sites located in the cavity or at
the channel intersection (often only one site) are
partially or totally deactivated.
Modes c and d correspond to what is generally
called pore blockage [25]. With these modes the
deactivating effect of the coke molecules is very
pronounced, because a large number of active sites
are generally located in the inner pores. The access of
the reactant to these pores is limited or blocked.
With tridimensional zeolites having no trap cavities
such as Y and ZSM5 only site coverage (modes a and