Polymeric monolith supported Pt-nanoparticles as ligand-free catalysts for olefinhydrosilylation under batch and continuous conditions

Abstract

Pt-nanoparticles were generated within the mesopores of polymeric monolithic supports. These monoliths were prepared viaring-opening metathesis polymerization (ROMP) from (Z)-9-oxabicyclo[6.1.0]non-4-ene (OBN) and tris(cyclooct-4-en-1-yloxy)methylsilane (CL) using the 3^rd-generation Grubbs-initiator RuCl₂(pyridine)₂(IMesH₂)(CHPh) (1) in the presence of a macro- and microporogen. To generate the Pt nanoparticles inside the porous system, the epoxy-groups of the monolithic supports were hydrolyzed into the corresponding vic-diol with the aid of 0.1 N sulfuric acid. Loading with Pt⁴⁺ and reduction with NaBH₄ resulted in the formation of Pt nanoparticles <7 nm in diameter that were exclusively located within the mesopores of the support as demonstrated by transmission electron microscopy/EDX spectrometry. The nanoparticles were stabilized by both the vic-diols and the polymeric double bonds as evidenced by control experiments with poly(glycidylmethacrylate)-based monoliths that lack the double bonds. Typical metal loadings of around 1.7 mg g⁻¹ (8.7 μmol g⁻¹) were obtained. Pt(0)-loaded monoliths were used in the hydrosilylation of terminal alkenes, norborn-2-ene and norbornadiene applying both batch and continuous flow conditions. Hydrosilylation of olefins under continuous flow conditions resulted in constant product formation in 98% yield at T = 45 °C applying a linear flow of 12.0 mm min⁻¹. Metal leaching was very low, resulting in Pt-contamination in the addition products of less than 4 ppm throughout.