Moffat J.B. Metal-Oxygen Clusters. The Surface and Catalytic Properties of Heteropoly Oxometalates

Kluwer Academic Publishers, NY, Boston, Dordrecht, London, Moscow, 2000. — 322 p. — ISBN: 0-306-47378-X.Heterogeneous catalysts, as their name implies, are employed in multiphase systems, the ¯uid phase most frequently being gaseous. Although heteropoly oxometalates or metal-oxygen cluster compounds appear in a variety of forms, the present book is largely restricted to those with Keggin structure. In their acidic form, these present relatively small surface areas to the reactant molecules in the fluid phase.
Equally interesting and important, the structures and largely the morphology of the heteropoly oxometalates, which are monophasic, one-component systems, can be retained while the catalytic functionality is altered by variations of the elemental compositions of the anions. Because this allows the study of compositional effects on the catalysis without the intrusion of other variable effects, it is particularly useful for fundamental studies in catalysis.
Although, with the simplest acidic forms, the syntheses are reasonably straightforward, in contrast the preparation of the salts and those solids containing atoms of more than three elements in the anion must be performed carefully with thorough characterization of the products, particularly elemental analyses. The relative quantities of the preparative elements are not necessarily reflected in the resulting solids.
Not surprisingly, the stabilities of these materials are often less than desired, although again these vary with both the nature of the cation and the anionic composition. The use of high-surface-area supports has been investigated for the purpose of both augmenting the interphase interactions as well as increasing the stabilities of the supported heteropoly oxometalates.
In spite of the low suce areas and the absence of a pore structure in the acidic forms of the heteropoly oxometalates, polar molecules have been shown to be capable of penetrating into the structures, that is, between the cations and anions. Although this does not increase the real surface area available to reactant molecules in the fluid phase, nevertheless the portion of the solid available to these molecules is effectively increased. Although the crystal structure of the solids is retained during such diffusional phenomena, the lattice constants, not surprisingly, are found to vary. Thus, the heteropoly oxometalates can be considered as three-dimensional intercalates.A Brief Look at the Early History of Heteropoly Oxometalates
Synthesis
Characterization
Structure and Bulk Properties
Stability
Supported Heteropoly Acids and Their Derivatives.
Microporosity
The Two Functions: Acidity and Oxidation±Reduction
Acid-Catalyzed Processes
Oxidation Processes
Environmentally Related Processes