Comparison between homogeneous and sol-gel-encapsulated rhodium-quaternary ammonium ion pair catalysts

The ion pairs RhCl₃/Aliquat 336® and RhCl₃/[Me₃N(CH₂)₃Si(OMe) ₃]Cl were entrapped in SiO₂ sol-gel matrices, and used as catalysts for isomerization of allylbenzene, disproportionation of 1,3-cyclohexadiene, hydrogenation of alkenes, alkynes, arenes, nitriles and nitro-compounds, and for hydroformylation of C-C double bonds. The immobilized ion pairs proved to be stable, leach-proof and recyclable. Occasionally the catalytic efficiency dropped upon recycling, owing to pore blockage but the activity could be restored by treatment with boiling water. The performances of the sol-gel-entrapped ion pairs were compared with those of the homogeneous RhCl₃/Aliquat 336 catalyst. In most cases the immobilized catalysts proved superior to their homogeneous version.