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

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.