Polytungstate clusters

The structural features of large polytungstate clusters can be visualized in terms of subunits based on lacunary fragments of the Keggin ion. In such representations many of the fragments may not actually exist independently from the cluster containing these subunits. This also applies to the building blocks identified as part of the polyoxometallate clusters discussed in Sections 7.1.4 and 7.1.5. 

Mixed WOj/Al Oj/HY catalysts prepared by calcination of physically mixed WO3, Al Oj and HY zeolite showed unique behavior in the metathesis between ethene and 2-butene to produce propene [147]. Monomeric tetrahedrally coordinated surface tungstate species responsible for the metathesis activity were formed via the interaction with Bronsted acid sites of HY zeolite. Polytungstate clusters are supposed to be less active in the metathesis reaction. The best catalyst demonstrates the 2-butene conversion close to the thermodynamic equilibrium value ( 64%) at 453 K. The catalysts are bifunctional [148] they catalyze first isomerization of 1-butene to 2-butene and then cross-metathesis between 1-butene and 2-butene to produce propene and 2-pentene. 10%W03/Al203-70%HY exhibits the highest propene yield. 

Fig. 8.10 Representation of the evolution of supported WOj[ domains from isolated mono tungstates to two-dimensional polytungstates and three-dimensional WO3 clusters. The increasing W-O-W connectivity with increasing domain size is related to the extent of oligomerization (from Ref. [87])...

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