Geoscience Reference
In-Depth Information
Figure 6.40 Columnar AlPO
4
-5
crystals aligned (a) and embedded (b)
in nickel
[146]
.
The application of zeolites as catalysts is basically dependent upon the accessibility
to reactants of intra-crystallization, catalytically active centers, and upon the shape
and size of the penetrant molecules. Specific catalytic actions can depend upon zeolite
basicity, the generation of acid sites within the crystals, or upon the introduction of
particular cations (by ion exchange), or of elements such as S, Te, and Se, or metals
like Pt into the crystals.
The catalytic oxidation of H
2
SbyO
2
exemplifies a process in which the best cata-
lysts are the basic Na- and K-forms of zeolites X and Y
[150]
. The catalytic activity is
directly proportional to the number of AlO
4
tetrahedra and inversely related to the elec-
trostatic potential of the exchange ions introduced (K
1
,Na
1
,L
1
,andCa
2
1
).
However, it is the hydrocarbon chemistry which has placed zeolites in a unique place
in science and technology, especially with H-zeolites providing Br¨nsted acid sites and
reactions proceeding via carbonium ions. The acid centers form the following revers-
ible reaction:
Al
Si
H
1
1
"
Al Si
Various shape-selective catalyses have been described in several zeolites. In Ca-
A zeolite, chemical dehydration of n-butanol (which penetrates the crystals), but
not of isobutanol (which does not penetrate), was observed. Whereas Ca-X zeolite
does not show this kind of selectivity, but it sorbs both alcohols readily. Similarly,
