Molecular Engineering in Catalysis: Immobilization of Shvo's Ruthenium Catalyst to Silica Coated Magnetic Nanoparticles

Abstract

The functionalized Shvo’s catalyst precursor {3,4-[p-(EtO)₃Si(CH₂)₃OPh]₂-2,5-Ph₂(η4-C₄CO)}Ru(CO)₃ (1) was covalently immobilized to the surface of magnetic nanoparticles, MNPs, including magnetite (Fe₃O₄) and magnetite covered by one, two and three independently added silica (SiO₂) coatings (Fe₃O₄@SiO₂, Fe₃O₄@SiO₂@SiO₂, Fe₃O₄@SiO₂@SiO₂@SiO₂) resulting in the corresponding ruthenium catalysts Fe₃O₄@Ru (2a), Fe₃O₄@SiO₂@Ru (2b), Fe₃O₄@SiO₂@SiO₂@Ru (2c), and Fe₃O₄@SiO₂@SiO₂@SiO₂@Ru (2d). These catalysts were characterized by FT-IR, TEM, EDX, powder XRD, BET surface area analysis and BJH pore size and volume analysis. The catalytic performances of 2a–2d were tested for the conversion of levulinic acid (LA) to gamma-valerolactone (GVL) using formic acid (FA) as the hydrogen source. The catalysts were separated from the reaction mixture by using an external magnet. Catalysts on the silica coated MNPs showed higher activity than that of immobilized directly to Fe₃O₄. There were no significant differences in TONs, TOFs and yields of GVL using catalysts 2b–2d. Leaching test of the four catalysts showed that by increasing the number of independent silica coatings on the surface of magnetite significantly decreased iron leaching. The recyclability of 2b was investigated and it was reused several times without significant loss of the catalytic activity. Hot filtration test of 2c and 2d has established that the catalytic activity was due to the supported ruthenium catalyst and not from some active ruthenium species leached from the solid support to the solution under the reaction conditions.