Catalytic significance of organometallic compounds immobilized on mesoporous silica: economically and environmentally important examples

Thomas, J.M. and Raja, R. (2004) Catalytic significance of organometallic compounds immobilized on mesoporous silica: economically and environmentally important examples. Journal of Organometallic Chemistry, 689 (24), 4110-4124.

Abstract

Because of the availability of well-defined nanoporous silicas, possessing (controllable) pore diameters of 30 Å to ca. 500 Å, bulky organometallic species may be readily introduced on to the inner walls of these thermally stable, solid supports. Four kinds of highly active catalysts that may be derived from the combination of these two materials are illustrated. First, starting from titanocene dichloride, single-site TiIV-centered epoxidation catalysts of exceptional activity may be formed. These are now used in the preparation of epoxy polymers and other products from naturally occurring methyl esters of fatty acids, a novel example of sustainable development. Second, asymmetric organometallic species that exhibit good enantioselectivity as homogeneous catalysts (in the hydrogenation of keto-esters, for example) may be significantly improved in their catalytic performance when they are anchored to a concave silica surface, their ee values are boosted by the spatial restrictions imposed by the mesopores within which they are anchored. Examples of three distinct diamino-complexes of Rh(I) are illustrated for the selective hydrogenations of methylbenzoylformate and E-?-phenylcinnamic acid. In the third kind of catalyst immobilized on nanoporous silica, several mixed-metal carbonylates are used as precursors. They are firmly anchored to the inner walls and may, by gentle calcining, be converted to bimetallic catalysts [e.g., Ru6Pd6, Ru5Pt and Ru10Pt2] that exhibit very high activity, under mild conditions, in the single-step, solvent-free hydrogenation of a range of economically important unsaturated compounds. The fourth category of immobilized, organometallics illustrated here is of the "ship-in-bottle" kind, where, typically, a chlorinated copper-phthalocyanine incarcerated within the cages of zeolite-X efficiently converts methane to methanol and also smoothly oxyhalogenates a range of aromatic compounds (e.g., benzene, phenol, toluene, aniline, anisole and resorcinol) in air, again under mild conditions. The unconventional techniques, both in situ and ex situ, required to characterize all these immobilized catalysts are outlined.

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Contributors

Author: R. Raja

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