Catalyst/catalytic activity/center/properties stability

A recent elegant example of the tailoring the chemical properties of encapsulated metal complexes is the work of Balkus etal. who prepared and studied perfluorinated phthalocyanine complexes of Fe, Co, Cu and Ru (Scheme 25)[230] in NaX. Perfluorinating the complexes enhances the stability and catalytic activity of the catalysts in the oxyfiinctionalisation of light alkanes. The rapid deactivation of the catalysts based on Fe, Co and Cu Fj Pc complexes was overcome by using Ru as the metal center. Similar catalysts, i.e.,Co-phthalocyanine (Co-Pc) encapsulated in zeolite Y, are active catalysts for cyclohexene and 1-hexene epoxidation (Scheme 27)[231]. Comparison of the activity of free and encapsulated Co-Pc has shown that the interaction with the zeolite stabilizes the complex. Co-Pc is still active after 24 hrs reaction whereas the free complex in solution is virtually inactive after 15 minutes. 

Accidental contamination of AlEt3 by water greatly increased Z-N catalytic activity and led to the development of diverse metallocene catalysts. By contrast, with earlier catalysts, the metallocene copolymers have well-defined structure, a single active polymerization center leading to high polymerization efficiency (1 kg polymer per 1 g catalyst), narrow MWD, and uniform properties. Metallocenes show better thermal stability, facile incorporation, high a-olefins even at high concentration (plastomers), and control of stereostructure. Most frequently, the active part of Z-N catalyst is a zirconocene cation... 

Another benefit of the structural rigidity of many intermetallic compounds is that it prevents the dissolution of reactants underneath the surface. This prevents the formation of subsurface compounds, such as hydrides and carbides, which have been identified as being the catalytically active phase in elemental Pd hydrogenation catalysts as dealt within more detail below. Embedding the centers of reactivity in a dense atomic matrix like in the intermetallic compounds provides mechanical and structural stability and excellent thermal properties preventing the subsurface chemistry. 

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