Industrial aspects of cluster chemistry

Abstract

The reasons for industrial interest in metal clusters and cluster catalysis are described. These may be summarized as attempting to bridge the gap between homogeneous and heterogeneous catalysis, specifically by combining the high selectivities typical of the former with the high activities associated with the latter. Progress towards the realization of this objective is illustrated by using examples of our work in both areas. Thus, some results from an investigation of homogeneous ruthenium and rhodium catalysts (as both separate components and mixtures) for the synthesis of oxygenated products from CO-H 2 are summarized and correlated with high-pressure infrared spectroscopic measurements. The cluster anion [Rh 5 (CO) 15 ] - is shown from spectroscopic evidence to be very closely related to the catalytically active species in the rhodium-catalysed reactions. Also, the distinction between the behaviour of supported catalysts derived from Group VIII metal cluster compounds and those obtained by more conventional methods of heterogeneous catalyst preparation is discussed. For example, in the case of ruthenium, cluster-derived catalysts are shown to display greatly enhanced activity for the complete hydrogenolysis of straight-chain aliphatic hydrocarbons to methane and provide a temperature advantage of 150°C relative to conventionally prepared ruthenium catalysts, where only moderate hydrocarbon conversions are noted. The increased activity superficially correlates with the smaller metal crystallite sizes (15-20 A; 1.5-2.0 nm) reproducibly obtainable with metal cluster compounds as catalyst precursors.