Page 118 - Fluid Catalytic Cracking Handbook

FCC Catalysts 95

Alumina is the source for an active matrix. Most active matrices used
in FCC catalysts are amorphous. However, some of the catalyst suppliers
incorporate a form of alumina that also has a crystalline structure.
Active matrix contributes significantly to the overall performance
of the FCC catalyst. The zeolite pores are not suitable for cracking
of large hydrocarbon molecules generally having an end point > 900°F
(482°C); they are too small to allow diffusion of the large molecules
to the cracking sites. An effective matrix must have a porous structure
to allow diffusion of hydrocarbons into and out of the catalyst,
An active matrix provides the primary cracking sites. The acid sites
located in the catalyst matrix are not as selective as the zeolite sites,
but are able to crack larger molecules that are hindered from entering
the small zeolite pores. The active matrix precracks heavy feed mole-
cules for further cracking at the internal zeolite sites. The result is a
synergistic interaction between matrix and zeolite, in which the activity
attained by their combined effects can be greater than the sum of their
individual effects [2].
An active matrix can also serve as a trap to catch some of the
vanadium and basic nitrogen. The high boiling fraction of the FCC
feed usually contains metals and basic nitrogen that poison the zeolite.
One of the advantages of an active matrix is that it guards the zeolite
from becoming deactivated prematurely by these impurities.

Filler and Binder

The filler is a clay incorporated into the catalyst to dilute its activity.
Kaoline [Al 2(OH) 2, Si 2O 5] is the most common clay used in the
FCC catalyst. One FCC catalyst manufacturer uses kaoline clay as a
skeleton to grow the zeolite in situ.
The binder serves as a glue to hold the zeolite, matrix, and filler
together. Binder may or may not have catalytic activity. The importance
of the binder becomes more prominent with catalysts that contain high
concentrations of zeolite.
The functions of the filler and the binder are to provide physical
integrity (density, attrition resistance, particle size distribution, etc.),
a heat transfer medium, and a fluidizing medium in which the more
important and expensive zeolite component is incorporated.
In summary, zeolite will effect activity, selectivity, and product
quality. An active matrix can improve bottoms cracking and resist