Synthesis, Structure and Catalytic Activity of ... - Jacobs University

Ch.1 Introduction
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other hand, when reduced restricts the delocalization of the added electron to
the belt tungstens. 1,2,5
1.6 Applications
The unique properties of POMs including mass, size and electron/proton
storage capacities have granted them the application in different areas such
as materials science, medicine, and catalysis which was by far the most
active area.
Their fairly high thermal stability in the solid state, their solubility in polar
solvents and non-polar solvents as well upon choosing the appropriate cation,
high H + mobility and reversible redox properties gave POMs the advantage to
be employed as acid as well as oxidation catalysts. 10
Heteropolyacids and their salts have even found their way to industrial large
scale acid and oxidation catalyzed processes. These include homogeneous
systems (liquid phase hydration of butene) and heterogeneous systems (gas
phase oxidation of methacrolein to methacrylic acid and of ethylene to acetic
acid). 2
1.6.1 Acid catalysis
The general mechanism of reactions catalyzed by heteropolyacids can be
represented by the conventional single proton transfer reaction:
S1 +H + ↔ S1H + + S2 → P + H +
Where S1 and S2 are the substrate and P is the product. Consequently, the
higher the Brønsted acidity of the heteropolyacid, the more efficient catalyst
it becomes. Thus the order of catalytic activity of the Keggin POMs, being the
ones that were extensively studied, parallels that of acid strength in
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