Surface and Supported Organometallic Catalysis

Organometallic complexefi can be supported in a variety of ways to give catalysts that arc more readily separable from the soluble products of the reaction than are soluble homogeneous catalysts. 

Classical heterogeneous catalysts, consisting of metal particles supptnted on a solid surface such as silica or carbon, are of great commercial importance. Tt has also proved possible to support a variety of organometallic species on silica so as to obtain mononuclear complexes covalently anchored to the silica surface. Silica has surf ace SiOH groups, often denoted sSi—O—H, which can form sSi—O—M links to the attached metal. The oxophilic early metals are particularly well suited to this approach. 

Once the organometallic species is bound to a surface, many of the usual solution characterization methods no longer apply. A combination of EXAFS (extended X-ray absorption fine structure see Chapter 16), solid-state NMR, and IR spectroscopy, however, can often give sufficient infonnation to characterize the surface-bound species. 

This remarkable C—C cleavage reaction also takes place on treatment of the hydride 9.31 with external NpH. The initial products in this case are MeH and MejCH, The reaction probably goes by a preferential )3-alkyl elimination of a neopentyl zirconium species as outlined in Eq. 9.29. The resulting isobutene is hydrogenated to MejCH and the zirconium methyl hydrogenated to methane. 

A number of commercially important catalysts consist of organometallic compounds covalently attached to surfaces. In the Phillips alkene polymerization catalyst, for example, CrCp2 is supported on silica. While there is not full agreement on the nature of the species formed, the Si—OH groups of the silica surface are believed to bind the metal via Si—O—Cr linkages in a sinular way to 9.31. 

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