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2.3 Adsorption, Ion Exchange, and Catalysis: Three Related Phenomena 45

materials used in both catalysis and ion exchange, such as zeolites and clays. Some
catalytic applications of zeolites and clays, materials used as ion exchangers, are pre-
sented in T able 2.6.
Furthermore, in the manufacture of zeolite catalysts, ion e xchange plays an outstanding
role. Brönsted acid sites can be readily generated by introducing ammonium ions followed
v
by a heat treatment or by introducing multialent metal cations, again followed by heat
,
eitkamp,
we
er
v
treatment (W 2000). Ho not all these applications incorporate the ion
exchange and catalysis phenomena at the same time, i.e. simultaneous action of these tw o
mechanisms.
Catalysis by ion exchangers exhibits some advantages oer homogeneous catalysis by v
dissolved electrolytes, such as
• the catalyst can be easily separated from the liquid phase by filtration or other suitable
means,
• continuous operation in fed beds is possible, ix
• in some cases, it is possible to isolate reaction intermediates that cannot be obtained by
homogeneous catalysis,
• the ion exchanger is generally more selecti i.e. it distinguishes more sharply
e,
v
between the various reactant molecules than the dissolved catalyst.

Table 2.6
Catalytic reactions for which zeolites and clays are used (Tsitsishvili et al ., 1992; Mumpton, 1999;
Armbruster, 2001; Chitnis and Sharma, 1997)
Material Application
Natural erionite-clinoptilolite Selective-forming catalyst
Hydrogen-exchanged natural mordenite Hydrocarbon conversion catalyst for
the deprotonization of toluene to benzene and xylene
Cation-exchanged clinoptilolite Hydromethylation of toluene
Clinoptilolite Isomerization of n -b the dehydration of methanol utene,
to dimethyl ether and the hydration of acetylene to ,
acetaldehyde
Acid-treated clays Alcylation reactions
(e.g. of benzene with benzyl chloride)
Dimerization reactions (e.g. of a -methylstyrene)
Etherification reactions
(e.g. of tert -butanol with methanol)
Condensation reactions (e.g. of c xanone) yclohe
Separation of close boiling aromatic amines
Separation of isomers of xylene
Thermally pillared clays Dimerization of unsaturated fatty acids to dimer acids
Removal of olefins from “BTX”, ethylbenzene,
cumene, etc. steams
Decolorization of industrial oil
derivatives
Purification of kerosene and other mineral oil
derivatives