Page 89 - Principles of Catalyst Development
P. 89
76 CHAPTER 4
;--
@ ,
-
, "
SOLID TRUNCATED
TETRAHEDRON OCTAHEDRON
FACE OF CUBIC ARRAY
OF TRUNCATED OCTAHEDRA
Figure 4.21. Zeolite building blocks.
Third, ions introduced into cavities through ion exchange have separate
activities of their own. Opportunities for dual functional catalysis involving
acidity with other activities are greatly facilitated.(57) These three factors
have made zeolites a powerful addition to the arsenal of catalytic
materials.( 135,136)
Zeolites are built from basic units consisting of four oxygen ions packed
like spheres (Fig. 4.21). Interstices between the oxygen ions are filled with
3
A1 + and Si + ions in the ratio x: y. By joining these tetrahedra together
4
through apical oxygen atoms, a truncated octahedron "sodalite" unit is
produced. With this as a building block, a vast number of structures can
be produced. For example, by joining them through the four-membered
faces in a cubic array, the structure in Fig. 4.21 results. When two of these
0.42 nm
Figure 4.22. Structure of type A zeolite.