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

Ch.1 Introduction
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incapable of further polymerization and thus account for the existence of
POMs as discrete species. The strong polarizability and the nonbasicity of the
surface oxygen atoms make it unlikely for them to get protonated or even
form hydrogen bonds of average strength. The distribution of the negative
charge over the large size of heteropoly complexes places the cations at large
distances thus minimizing the electrostatic interaction and lowering the
lattice energies and solvation energies 3 . In consequence, the hydrodynamic
radii and crystallographic radii of POMs are frequently comparable.
Lipscomb rule states that it is very unlikely for a POM to have MO6
octahedra with more than two unshared oxygen atoms. This can be reasoned
based on the strong trans influence of the M=O bonds that facilitates the
dissociation of MO3 from the polyanion. 1,2,7 The 1983 monograph by Pope 1
again classifies polyanions as type I “mono-oxo” and type II “cis-dioxo”. The
number of terminal oxygen atoms M=O per addenda atom is one for type I
and two in cis positions for type II.
In the next two sections, two of the most common POM structures will be
introduced and discussed in the form of tungstates only. These are the
Keggin and the Wells-Dawson ions.
1.3 The Keggin ion
First discovered by Berzelius in 1826 upon reacting ammonium molybdate
with phosphoric acid and later structurally elucidated by J.F. Keggin in 1933
in the form of 12-tungstophosphoric acid, 8 the Keggin structure and its
derivatives are by far the most widely investigated.
The Keggin ion is a highly symmetric heteropolyanions generally represented
by the formula [XM12O40] x- , it is composed of a central XO4 tetrahedron
surrounded by a neutral shell of 12 WO6 octahedra grouped as four M3O13
units, called triads (Fig. 1.1). Each triad is formed by three edge shared WO6
3