Design, characterization and performance of a molecular imprinting Rh-dimer hydrogenation catalyst on a SiO2 surface

Physical Chemistry Chemical Physics Pub Date: 2002-10-28 DOI: 10.1039/B207357A

Abstract

A new active SiO2-attached Rh-dimer catalyst for the hydrogenation of alkenes was designed by combining two ways of metal-complex immobilization and molecular imprinting on an oxide surface. The preparation was conducted step-by-step in three steps: (i) the attachment of Rh dimer precursor on an Ox.50 surface; (ii) coordination of the template P(OCH3)3 to the attached Rh dimer; and (iii) subsequent molecular imprinting by hydrolysis-polymerization of Si(OCH3)4 to form SiO2-matrix overlayers on the surface. Characterizations at each step were performed by means of elemental analysis, FFIR, XPS, ICP, solid-state MAS NMR, BET, EXAFS, and DFT calculation. The attached Rh dimer was converted to a pair of Rh monomers by coordination of two P(OCH3)3 on a Rh atom. The surface-imprinting process by Si(OCH3)4 drove the rebonding of the Rh monomer pair and extraction of half of the template ligands. Removal of the template ligands provided both highly active unsaturated Rh dimer with a Rh–Rh bond distance of 0.268 nm and a space of the size of the template around the dimer inside the SiO2-matrix overlayers on the surface. The template P(OCH3)3 was regarded as an analogue to a half-hydrogenated species of 3-ethylpent-2-ene. The catalytic activity of the imprinted Rh dimer catalyst for hydrogenation of 3-ethylpent-2-ene was 35 times higher than that of the Rh monomer pair before imprinting. For hydrogenation of 4-methylhex-2-ene which possesses an ethyl group at the 4-carbon position of the pent-2-ene main chain and oct-2-ene with a long alkyl chain, promotion of the catalytic activity by the imprinting was relatively slight, indicating shape discrimination of the alkenes by the surface molecular imprinting.

Recommended Literature