Page 48 - Adsorption, Ion Exchange & Catalysis- 2007, Elsevier - Copy

Else_AIEC-INGLE_cH002.qxd 6/20/2006 11:31 AM Page 44
44 2. Adsorption, Ion Exchange, and Catalysis

often the rate-determining step in a catalytic reaction. Then, desorption of the product
occurs because the surface bond is broken, and the fdif- ulk phase, inal product enters the b
fusing through and out of the catalyst pores. This very simple picture highlights the basic
v
,
idea of heterogeneous catalysis. Consequently the deelopment of catalysis is closely
related to the eolution of adsorption. v
Most of the adsorbents used in the adsorption process are also useful to catalysis,
because they can act as solid catalysts or their supports. The basic function of catalyst sup-
ports, usually porous adsorbents, is to keep the catalytically actie phase in a highly dis- v
persed state. It is obvious that the methods of preparation and characterization of
adsorbents and catalysts are very similar or identical. The physical structure of catalysts is
investigated by means of both adsorption methods and various instrumental techniques
derived for estimating their porosity and surface area. Factors such as surface area, distri-
bution of pore v pore sizes, stability and mechanical properties of materials used
,
olumes,
are also very important in both processes—adsorption and catalysis. Activated carbons, sil-
ica, and alumina species as well as natural amorphous aluminosilicates and zeolites are
widely used as either catalyst supports or heterogeneous catalysts. From the above, the fol-
wski, lowing conclusions can be easily drawn (Dabro 2001):

• adsorption and catalysis are closely related to each other ,
• the action of solid catalysts results from their capacity to adsorb reacting substances,
• the same porous solids can be used as adsorbents, catalyst supports, and catalysts,
• their porous structure, mechanical the chemical character and size of solid surface areas,
properties, and thermal stability play an essential role in both adsorption and catalysis,
v
• the deelopment of theoretical studies on adsorption, design, and manufacture of ne w
elopment. v adsorbents affects heterogeneous catalysis de

2.3.3 Catalysis and ion exchange

Catalysis of reactions by ion exchangers can be explained in terms of the catalytic activity
of the exchanging ions and is analogous to homogeneous-phase catalysis by dissolv ed
electrolytes (Hellferich, 1995). Ion-exchange resins can act as insoluble acids and bases
for the catalysis of chemical reactions. Heterogeneous catalysis with resins can be carried
out in aqueous or nonaqueous solvent solutions (Guzzo, 1997). It is interesting that ion
exchangers can be used for catalyzing reactions in the gas phase. Ho its relation to , we v er
the ion-exchange properties of the catalyst is less distinct.
Despite the presence of two phases, solid and liquid, catalysis by ion exchangers is not
a true case of heterogeneous catalysis and may be described more adequately as homoge-
neous catalysis in the pore phase. This is because the ions of the exchangers that are
involved in the catalytic reaction are dissolved in the pores of the solid, where they act as
in a homogeneous solution (Helfferich and Hw 1988). A good example is the acid-
ang,
catalyzed hydrolysis of cane sugar which is carried out in solution using a strong acid
,
cation-exchange resin in its hydrogen form. The “in of sucrose is a commercial
ersion”
v
process in which ion-exchange catalysis has been widely used (Purolite Co.).
Despite the fact that it is not clear whether the catalytic activity is related to the ability
,
of the solid to act under different conditions as an ion exchanger there is a variety of