Metal Clusters in Zeolites: Nearly Molecular Catalysts for Hydrocarbon Conversion

Abstract

Metals supported in the pores of zeolites have long been used as catalysts, but only recently have samples been prepared and characterized that incorporate metal particles smaller than about 1 nm in diameter, referred to here as clusters. Nearly uniform zeolite-supported metal clusters are the focus of this review, which addresses their synthesis, physical characterization, and catalytic properties. Supported clusters represented as Ir4, Ir6, Rh4, and Rh6 were prepared in the supercages of faujasite zeolites by ship-in-a-bottle synthesis of the metal carbonyl cluster precursors [Ir4(CO))12], [Ir6(CO)16], [Rh4(CO)12], and [Rh6(CO)16], respectively, which were decarbonylated. Alternatively Ir4 and Ir6 clusters in zeolite NaX were prepared by synthesis and then decarbonylation of [HIr4(CO)11]− and [Ir6(CO)15]2−. Metal clusters that are nearly as small and uniform as these have also been formed from metal salt precursors in, various zeolites, including zeolites NaY and KLTL. The most important of these are clusters of 5–6 Pt atoms each, on average in basic zeolite LTL. These were made from platinum ammine salts by calcination and reduction in hydrogen. Similarly, clusters of iridium nearly this same size were prepared from an iridium ammine salt. The supported platinum clusters are active and selective industrial catalysts for dehydrocyclization of naphtha to give aromatics, whereas the supported iridium clusters are unselective. The unique selectivity and activity of the zeolite-supported platinum clusters is associated with the basicity of the support and the smallness of the platinum clusters; the role of the zeolite is not fully understood. Opportunities for discovery of supported metal clusters with new catalytic properties appear to be excellent.