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46 2. Adsorption, Ion Exchange, and Catalysis

Catalysis with ion-exchange resins provides effective and efficient answers to a number
of catalytic problems:

• the resins do not introduce counterions (which would have to be removed from the final
product made under homogeneous catalysis conditions),
• they may be regenerated and reused oely long periods, er relati v v
• corrosion arising from the presence of strong acids in the bulk phase is eliminated,
• pore vsurf ents, olume, ace area, swelling in solv etc. resins may be tailored in particle size,

However, it should be noted that the maximum operating temperature recommended for
ion-exchange resin catalysts is in the range 137–145°C. Thus, the use of resins as catalysts
v
is limited to systems that operate at relatiely low temperatures. In the case of ele v ated
temperatures, zeolites could be used instead, because the xhibit higher stability for tem- y e
peratures as high as 800°C (e.g. clinoptilolite). In many applications, acid-treated clays
could be used as an alternative. Clays, being naturally occurring aluminosilicates, are read-
ily available and inexpensive compared to other types of heterogeneous acid catalysts, e.g.
ion-exchange resins (Chitnis and Sharma, 1997). Clays, in general, are thermally stable up
to 200 °C and they can be greatly improved by the pillaring process. Furthermore, in many
cases, they exhibit higher selectivity than resins. The disadvantage of clays is that their
activity is lower than resins, their use is restricted to nonaqueous reactions systems only ,
w . and their mechanical strength is lo

2.4 ENVIRONMENTAL APPLICATIONS OF ADSORPTION,
ION EXCHANGE, AND CATALYSIS

2.4.1 Adsorption

There are many environmental applications of adsorption in practice and many others are
Acti being developed (Noble and Terry, 2004). vated carbons and clays are frequently used
v
for the remoal of organic contaminants, such as phenol and aniline, both of which are
aste w prevalent in industry waters and are known to hae a significant nee impact on v v gati
marine life and human health (IRIS, 1998; Dabro wski et al ., 2005). Moreo the adsorp- er , v
xpensi
v
tion on inee and eficient solid supports has been considered a simple and eco-
f
nomical viable method for the removal of dyes from water and wastewater (Forgacsa et al .,
2004). Activated carbon, clays, coal, v and other adsorbents ha ermiculite, e been used for v
this purpose. Specif adsorption can be employed in (Noble and T 2004;
,
erry
ically
,
Dabrowski, 2001):
• the remoal of water from organic solv v ents
• v the remoal of organics from w ater
• taste and odor regulation in w aste ater treatment w
• v the remoal of radon, hydrogen sulf and other sulfur compounds from gas streams ide,
• fluent v mercury remoal from chlor-alkali-cell gas ef