Supported molecular catalysts: metal complexes and clusters on oxides and zeolites

Abstract

When metal complexes and clusters are bonded to oxide or zeolite supports, they may combine the technological advantages of solid catalysts (robustness for high-temperature operation, lack of corrosiveness, ease of separation from products) with the selectivity of soluble molecular catalysts. Supported mononuclear metal complexes are typically synthesized by the reaction of a mononuclear organometallic compound with oxygen atoms or OH groups of the support, giving structures shown by infrared, X-ray absorption, and NMR spectroscopies and density functional theory to be analogous to those of molecular species, but with the support playing the role of a multidentate ligand, bonding strongly to the metal and holding the groups apart from each other on the support surface. Some supported metal complexes have new and unexpected catalytic activities. Supported metal clusters are formed by adsorption or surface-mediated synthesis of metal carbonyl clusters, which under some conditions may be decarbonylated on the support with the metal frame remaining essentially intact. The decarbonylated clusters are bonded to the supports by metal–oxygen bonds similar to those characterizing supported metal complexes; even noble metals in clusters on supports at the metal–support interface are cationic, and the metal–oxygen distances are about 2.1–2.2 A, matching the distances in mononuclear metal complexes with metal–oxygen bonds. Metal clusters are preferentially bonded at defect sites on oxide surfaces. The catalytic activities of supported metal clusters of only a few atoms are distinct from those of bulk metals; the supports act as ligands affecting the catalysis.