Chapter 15 - Microporous Metal—Oxygen Cluster Compounds (Heteropoly Oxometalates): Synthetic Variables, Nature and Source of the Porosity, Catalytic Applications, and Shape Selectivity

Abstract

Heteropoly oxometalates (also known as metal-oxygen cluster compounds, MOCC) have been known for more than 150 years. Although another form of oxygen polyanion exists, namely the isopolyanions the heteropoly ions are those that contain two different elements in addition to oxygen. While many possible structures are known for the heteropoly anions, that elucidated by Keggin has been more commonly studied for its catalytic and surface properties. A number of isomers of the Keggin structure are known; of these the α form appears to be the most stable and as a consequence catalytic investigations have generally focused on this isomer. The Keggin structure has a central atom (X) bonded to four oxygen atoms arranged tetrahedrally. Most of the surface and catalytic studies have employed P or Si as the central atom although, as noted earlier, many possibilities exist. The selection of P or Si as the central atom undoubtedly relates to factors of thermal stability, ease of preparation, and cost. The central XO4 tetrahedron is surrounded by 12 octahedra with a peripheral metal atom (M) at each of their approximate centers and oxygen atoms at their vertices. For catalytic purposes the more common peripheral metal atoms are W, Mo, and V. The octahedra share oxygen atoms with each other and with the central atom. The 12 octahedra are arranged in four groups of three edge-shared octahedra, M3013, which are linked by shared corners to each other and to the central XO4 tetrahedron. There are two types of bridging oxygen atoms, those connecting the central atom to the peripheral metal atoms and those joining each of the peripheral metal atoms.