Performance and kinetic behavior of a new SiO2-attached molecular-imprinting Rh-dimer catalyst in size- and shape-selective hydrogenation of alkenes

A new size- and shape-selective Rh-dimer catalyst was designed by combination of a metal-complex-attaching method and a molecular-imprinting method on an Ox.50 surface. Coordinatively unsaturated, air-stable Rh dimers with a Rh-Rh bond of 0.268 nm were prepared in 0.74-nm micropores of SiO2-matrix overlayers on the Ox.50 silica surface by attaching a Rh2Cl2(CO)(4) precursor on the surface (Rh2Cl2(CO)(4)/SiO2), followed by coordination of template ligands P(OCH3)(3) to the attached Rh species (Rh-sup catalyst), and then by surface imprinting of the template with SiO2-matrix overlayers formed by hydrolysis-polymerization of Si(OCH3)(4) (Rh-imp catalyst). The Rh dimers, micropores, and SiO2-matrix overlayers in the molecular-imprinting Rh-imp catalyst were characterized by EXAFS, BET analysis, and Si-29 solid-state MAS NMR, respectively. It was found that activity of the Rh-sup catalyst for hydrogenation of alkenes was promoted remarkably (35-51 times) after the imprinting. The alkene hydrogenation proceeded on the imprinted Rh dimers with a monohydride without any breaking of the Rh-Rh bond. Size and shape selectivities of the molecular-imprinting Rh-imp catalyst were examined by measuring the hydrogenation rates of eight alkenes of different sizes and shapes. It was also found that the Rh-imp catalyst exhibited not only high activity and stability but also size and shape selectivities for the alkene molecules, probably due to a template-size cavity, created behind the removed template ligand, being used as a reaction site. Activation energies and activation entropies for the hydrogenation of large and branched alkenes were much smaller than those for small alkenes, which implies a shift in the rate-determining step in the reaction sequence for alkene hydrogenation. The performance of the molecular-imprinting Rh-dimer catalyst is discussed from structural and kinetic viewpoints. (C) 2002 Elsevier Science (USA).