Page 91 - Principles of Catalyst Development
P. 91
78 CHAPTER 4
TABLE 4.11. Catalytic Zeolites
Pore opening
Zeolite No. of 0 2 ions Dimensions (nm)
Faujasite, X, Y 12 0.74
Mordenite 12 0.74
Otfretite 12 0.64
ZSM-S 10 0.54 x 0.56
Zeolite A 8 0.41
Erionite 8 0.36 x 0.52
These materials have the general formula
Me~;,,[(AI02USi02)v]mH20 (4_10)
where Me"+ is the cation that satisfies electric neutrality in the structure_
During catalysis, the m H 20 molecules are removed, leaving a large cavity
in which the reaction takes place. Cationic positions vary from one structure
to another, but some of them are found at locations within the internal
surface of the cavities. As such, they playa vital role in establishing catalytic
properties. Zeolites are most easily prepared from sodium silicates and
aluminates, so that sodium is the cation in freshly made material. This is
readily ion-exchanged with a large number of ions and the effect is very
dramatic, as demonstrated in Table 4.12. There are various models to explain
these results. In addition to different valences, the Lennard-lones ionic
radius decreases from Na+ to HT. This suggests that a good correlation
exists with the factor fle/ r, the polarizing power. This polarizing effect could
be exerted directly across the cavity or indirectly through the zeolite lattice
to neighboring OH groups.! 1411
Shape and size selectivity are important when molecules approach the
critical dimensions of the pore opening_ In Table 4.13, the diameters of
common hydrocarbons are listed in order of size, and we expect to find the
largest effect in zeolites with pores and cages close to these dimensions.
Figure 4.24. Structure of ZSM-5.