Page 108 - Fluid Catalytic Cracking Handbook

FCC Catalysts 85

* Binder
• Filler

Zeolite

Zeolite, or more properly, faujasite, is the key ingredient of the FCC
catalyst. It provides product selectivity and much of the catalytic
activity. The catalyst's performance largely depends on the nature and
quality of the zeolite. Understanding the zeolite structure, types,
cracking mechanism, and properties is essential in choosing the "right"
catalyst to produce the desired yields.

Zeolite Structure
Zeolite is sometimes called molecular sieve. It has a well defined
lattice structure. Its basic building blocks are silica and alumina
tetrahedra (pyramids). Each tetrahedron (Figure 3-1) consists of a
silicon or aluminum atom at the center of the tetrahedron, with oxygen
atoms at the four corners.
Zeolite lattices have a network of very small pores. The pore diameter
of nearly all of today's FCC zeolite is approximately 8.0 angstroms (°A).
These small openings, with an internal surface area of roughly 600 square

Figure 3-1. Silicon/aluminum-oxygen tetrahedron [15].