Novel SiO2-attached molecular-imprinting Rh-monomer catalysts for shape-selective hydrogenation of alkenes; preparation, characterization and performance

Abstract

A combination of two methods for catalyst design, namely metal-complex attaching and molecular imprinting, at oxide surfaces was employed to prepare new SiO₂-attached shape-selective Rh-monomer catalysts for hydrogenation of alkenes. Attachment of Rh monomer on SiO₂ surface was performed by reaction of RhCl(P(OCH₃)₃)₃ precursor with an Ox.50 surface. The attached Rh monomers were characterized to be coordinated with two P(OCH₃)₃ ligands (Rh–P = 0.225 nm) and two surface oxygens (Rh–O = 0.204 nm) by EXAFS, XPS, and FT-IR, independent of Rh loading in the range 0.24–0.60 wt.%. Subsequent molecular imprinting of a P(OCH₃)₃ ligand in the attached Rh monomers as template ligand by hydrolysis–polymerization of Si(OCH₃)₄ depended on the Rh loading; the lower loading Rh-monomers were successfully imprinted in uniform micropores formed in SiO₂-matrix overlayers on the Ox.50 surface. Characterizations of the molecular imprinting catalysts with different heights of the SiO₂-matrix overlayers were performed by EXAFS, XPS, ²⁹Si solid-state MAS NMR, BET analysis, and H₂ adsorption. The hydrolysis–polymerization of Si(OCH₃)₄ to form the SiO₂-matrix overlayers on the surface led to removal of a template ligand P(OCH₃)₃, and as the result provided a cavity behind the template. Thus, a coordinatively unsaturated active Rh-monomer structure with a remaining P(OCH₃)₃ ligand per Rh and the template cavity as catalytic reaction space was created in the micropores of the SiO₂-matrix overlayers. The catalytic properties of the imprinted Rh monomers were examined by measuring the hydrogenation rates of nine simple alkenes (C₅–C₈ alkenes). The catalytic activity of the surface Rh monomers was promoted 2.4–13 times by the imprinting, and shape selectivity for the alkenes also appeared by the effect of template cavity, resulting in a shift of the rate-determining step and a large decrease in the activation energy and the activation entropy for alkenes with the larger shapes.