is observed at low coke content while pore blockage
appears at high coke content caused by coke molecules
located on the outer surface of the crystallites
(Fig. 2). Therefore, with HZSM5 whose acid sites are
of identical strength the greater the coke content the
greater the deactivating effect of coke molecules. The
situation is more complex when the acid sites of the
zeolites are of different strengths as is the case with
HY zeolites. Indeed, the strongest acid sites, hence the
most active, are the first to be deactivated, resulting in
a large deactivating effect of the coke molecules at low
coke content [13].
With monodimensional zeolites such as HMOR or
with zeolites presenting trap cavities (large cavities
with small apertures) such as HERI, the deactivation
occurs only through pore blockage. With HMOR, one
coke molecule located in the large channel (most
likely at the intersection of this channel with the
narrow pores inaccessible to the organic molecules)
is able to block the access of the reactant to all the
active sites located in the channel resulting in a large
deactivating effect of the coke molecules. This large
deactivating effect is also observed with HERI
because the coke molecules initially formed, located
in the cages near the outer surface, limit and then
block the access of the reactant to the inner cages [ 13].