Performance and Kinetic Behavior of a New SiO₂-Attached Molecular-Imprinting Rh-Dimer Catalyst in Size- and Shape-Selective Hydrogenation of Alkenes

Author： Tada M. Sasaki T. Iwasawa Y.

Publisher： Academic Press

ISSN： 0021-9517

Source： Journal of Catalysis, Vol.211, Iss.2, 2002-10, pp. : 496-510

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Abstract

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 ²⁹Si 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. © 2002 Elsevier Science (USA).