Design of a novel molecular-imprinted Rh-amine complex on SiO2 and its shape-selective catalysis for α-methylstyrene hydrogenation

A new molecular-imprinted Rh-amine complex with a template-shaped cavity acting as a shape-selective reaction space was prepared by metal-complex attaching and molecular imprinting techniques on a SiO2 surface. Rh(eta(3)-C3H5)(3) reacted with SiO2, and the precursor was attached on the surface to form a bidentate structure Rh(C3H5)(OSi)(2). alpha-Methylbenzylamine with a shape similar to that of a half-hydrogenated alkyl intermediate of alpha-methyl styrene coordinated to the attached Rh as a template ligand. Then SiO2-matrix overlayers were stacked on the surface by the hydrolysis-polymerization of Si(OCH3)(4) surrounding the attached Rh complexes with the amine template. Final evacuation at 373 K promoted the removal of the template ligand from the Rh complex embedded in the surface pore of the SiO2-matrix overlayers. All the structures of the attached Rh complexes were characterized by the means of ICP, XPS, ESR, FT-IR, solid-state MAS NMR, EXAFS, DFT, GC, and H-2 adsorption. The imprinted Rh complex has an unsaturated structure Rh-(amine)(OSi)(HOSi) and exhibits high catalytic activity for alpha-methylstyrene hydrogenation. On the contrary, the other supported Rh complexes before imprinting are inactive for the hydrogenation. Furthermore, the active Rh complex is durable under the reaction condition despite the unsaturated conformation which usually tends to gather each other. The imprinted cavity locating on the active Rh center acts as a shape-selective reaction space for alpha-methylstyrene. The inhibition of the hydrogenation by the use of seven amines with different shapes demonstrates the shape-selective behavior of the imprinted Rh complex discriminating one methyl group and the position of the phenyl ring.