Synthesis and Structural Characterization of ... - Jacobs University

Chapter 1
Introduction
dioxygen is the most effective oxidant, other oxidants can be used for the same purpose, e.g.
H + and M n+ . This reaction produces O − 2 (O 2·), H 2 , or M 0 and decolorizes the solution.
However, the ability of photoreduced POM to be oxidized by metal ions depends on the
reducing ability of POM, the oxidizing ability of the metal ion and a lesser extent on the
nature of the organic substrate;
POM (e − ) + M n+ POM + M (n−1)+
By using better oxidizing reagents for roxidation of the reduced POM leads to produce more
catalyst in the oxidized form which is available to reabsorb light and oxidize organic
compounds. Therefore, a faster photocatalytic degradation process can be achieved.
1.6.2 Acid catalysis
Acid catalysis by heteropoly acids (HPAs) can be related to the known data on their acidity.
Keggin compounds of the type H n [XM 12 O 40 ] are very strong Brønsted acids. Therefore, HPAs
are often completely dissociated in aqueous solution. Upon reduction, the anions basicity
increase and, consequently, the anions have a tendency to become protonated (the protontransfer/storage
ability) and may themselves be reoxidized with oxygen.
HPAs as catalysts have the advantages of being nonvolatile, odorless, and thermally stable
which make them more attractive rather than mineral acids like sulfuric and hydrochloric acid.
Their acid-base and redox properties can be varied by changing the chemical composition
where the acid strength of the HPAs in solution decreases in the series PW > SiW ≥ PMo >
SiMo. They can be used in homogeneous acid catalysis as well as in heterogeneous catalysis
using supports like silica gel or activated charcoal. An additional advantage of POMs is the
fact that heteropoly acids can be separated and enriched by extraction into organic
solvents. 1a,b,f, 23
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