Page 90 - Principles of Catalyst Development
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CATALYTIC MATERIALS 77
are combined, as in Fig. 4.22, a cavity with 0.7-nm openings is produced.
This is the zeolite type A structure, designed mostly for separation processes
in which molecules less than 0.4-0.5 nm in size are preferentially adsorbed.
On the other hand, if the sodalite units are bridged across six-membered
faces, the resulting structure (Fig. 4.23) has a 0.73-nm opening to a 12-nm
cavity, forming faujasite. Still another arrangement gives the parallel pores
of mordenite (Fig. 4.23).
There are over 70 known different kinds of building arrangements,
each resulting in a distinct structure. Some of these occur as natural minerals;
some are synthetic. The search for new combinations goes on. Catalytically
important zeolites are listed in Table 4.11. More details on exact structures
will be found in the references. (137.138.139) In recent years, much attention
has been directed toward a new zeolite, ZSM-5, whose structure is shown
in Fig. 4.24. This is a zeolite with openings of 0.54-0.56 nm, intermediate
between the zeolite A (0.4 nm) and faujasite X and Y (0.74 nm). It has
been successfully applied to the production of gasoline from methanol and
for other specialized purposes.(140)
0.6 nm
(a)
Figure 4.23. Structure of (a) faujasite and (b) mordenite.