Formation and catalytic activity of Pd nanoparticles on silica in supercritical CO2

Metal complexes of polydimethylsiloxane-derived ligands can be adsorbed onto silica and subsequently reduced in-situ in supercritical CO2 (scCO(2)) to generate metal nanoparticles. Pd nanoparticles on silica, generated during C - C coupling reactions in scCO(2), can be recycled several times without any loss in activity. Focusing on Heck and Suzuki coupling reactions, the products showed no contamination of the organic products with Pd using quantitative ICP emission spectroscopy. The use of scCO(2) prevents the desorption of the Pd nanoparticles from their support. Build-up of ammonium salts as by-products in these coupling reactions leads to reduced activity for these heterogeneous catalysts after four runs.