Zeolite-supported metal complexes of rhodium and of ruthenium: a general synthesis method influenced by molecular sieving effects

Abstract

A general method for synthesis of supported metal complexes having a high degree of uniformity is presented, whereby organometallic precursors incorporating acetylacetonate (C₅H₇O₂⁻, acac) ligands react with zeolites incorporating OH groups near Al sites. The method is illustrated by the reactions of Rh(acac)(CO)₂ and of cis-Ru(acac)₂(η²-C₂H₄)₂ with zeolites slurried in n-pentane at room temperature. The zeolites were H-Beta, H-SSZ-42, H-Mordenite, and HZSM-5. Infrared (IR) and extended X-ray absorption fine structure spectra of the zeolites incorporating rhodium complexes indicate the formation of Rh(CO)₂⁺ bonded near Al sites; similar results have been reported for the formation of zeolite-supported Rh(η²-C₂H₄)₂⁺ from Rh(acac)(η²-C₂H₄)₂. IR spectra of the supported rhodium gem-dicarbonyls include sharp, well-resolved ν_CO bands, demonstrating that the sites surrounding each metal complex are nearly equivalent. The frequencies of the ν_CO bands show how the composition of the zeolite influences the bonding of the supported species, demonstrating subtle differences in the roles of the zeolite as ligands. When the zeolite has pore openings larger than the critical diameter of the precursor organometallic compound, the latter undergoes facile transport into the interior of the zeolite, so that a uniform distribution of the supported species results, but when the precursors barely fit through the zeolite apertures, the mass transport resistance is significant and the supported metal complexes are concentrated near the pore mouths.