82
5.1 Zeolite
Zeolites are synthetic microporous aluminosilicates. It is the key ingredient of the
FCC catalyst, which contributes to motor gasoline selectivity. Zeolites make up
about 10–50% of the catalyst’s volume. To understand the cracking that occurs
within the zeolite, it is essential to be familiar with the morphology of the zeolite
itself (Fig. 12).
Zeolites have a fixed lattice structure made up of repeating tetrahedrons. Each
tetrahedron has either silica or an aluminum atom at its center and oxygen atoms at
the corners. These lattices form very small pores of approximately 8 Å diameter.
Such small pores contribute to the high selectivity toward gasoline and LPG olefins
since large hydrocarbon molecules cannot diffuse into these pores. Hence, zeolites
are also called molecular sieves. Zeolites tend to get de-aluminated in the extreme
hydrothermal conditions in the regenerator. This causes the collapse of the lattice
structure and formation of extra framework alumina. This reduces the acid site density of the zeolite. To counter this effect, rare earths are added to zeolites to modify
selectivity and activity and increase hydrothermal stability. Its concentrations vary
up to 16 wt% (Figs. 13 and 14).
5.2 Matrix
Matrix refers to the active mesoporous, amorphous alumina in the FCC catalyst.
Sometimes, the alumina in the matrix can be of crystalline nature as well. It makes
up about 5–25% of the catalyst volume. It provides the primary cracking sites for
heavier hydrocarbons. It has a pore size suitable for the diffusion of larger hydrocarbons. These hydrocarbons are pre-cracked on the acid sites of the matrix. The
Fig. 11 FCC catalyst manufacture [72]
A. R. Khande et al.
5.1 Zeolite
Zeolites are synthetic microporous aluminosilicates. It is the key ingredient of the
FCC catalyst, which contributes to motor gasoline selectivity. Zeolites make up
about 10–50% of the catalyst’s volume. To understand the cracking that occurs
within the zeolite, it is essential to be familiar with the morphology of the zeolite
itself (Fig. 12).
Zeolites have a fixed lattice structure made up of repeating tetrahedrons. Each
tetrahedron has either silica or an aluminum atom at its center and oxygen atoms at
the corners. These lattices form very small pores of approximately 8 Å diameter.
Such small pores contribute to the high selectivity toward gasoline and LPG olefins
since large hydrocarbon molecules cannot diffuse into these pores. Hence, zeolites
are also called molecular sieves. Zeolites tend to get de-aluminated in the extreme
hydrothermal conditions in the regenerator. This causes the collapse of the lattice
structure and formation of extra framework alumina. This reduces the acid site density of the zeolite. To counter this effect, rare earths are added to zeolites to modify
selectivity and activity and increase hydrothermal stability. Its concentrations vary
up to 16 wt% (Figs. 13 and 14).
5.2 Matrix
Matrix refers to the active mesoporous, amorphous alumina in the FCC catalyst.
Sometimes, the alumina in the matrix can be of crystalline nature as well. It makes
up about 5–25% of the catalyst volume. It provides the primary cracking sites for
heavier hydrocarbons. It has a pore size suitable for the diffusion of larger hydrocarbons. These hydrocarbons are pre-cracked on the acid sites of the matrix. The
Fig. 11 FCC catalyst manufacture [72]
A. R. Khande et al.