Preparation of Ultrafine Palladium Particles on Cationic and Anionic Clays, Mediated by Oppositely Charged Surfactants: Catalytic Probes in Hydrogenations

Finely divided Pd particles (2−3 nm in diameter) were synthesized by the reduction of Pd 2+ precursor ions in the presence of cationic (myristyltrimethylammonium bromide, MTA + Br - ) or anionic (sodium dodecyl sulfate, Na + DS - ) surfactants. The protective adsorption layer of the ionic surfactants around the particles ensured the long-term stability of the aqueous dispersions. When the palladium hydrosol stabilized with MTA + Br - was mixed with an aqueous suspension of sodium montmorillonite, Na + MM - (a cationic clay), the cation-exchange reaction between Na + and MTA + rendered the montmorillonite surface hydrophobic, in parallel with the incorporation of the released Pd particles into the MTA + MM - organoclay host. In a similar way, after addition of the palladium hydrosol stabilized with Na + DS - to an aqueous suspension of hydrotalcite nitrate, HT + NO 3 - (an anionic clay), the ion exchange between NO 3 - and DS - resulted in the formation of a hydrophobic clay, HT + DS -, with simultaneous deposition of the released Pd particles onto the clay lamellae. The low-loaded, highly dispersed Pd-organoclay materials displayed extremely high catalytic activities under mild conditions in the liquid-phase hydrogenations of styrene, hex-1-ene, and cyclohexene. Furthermore, the catalysts exhibited high selectivities for the partial hydrogenation of 1-phenyl-1-pentyne to 1-phenyl- cis -1-pentene. These high activities and selectivities were explained in terms of the high degree of dispersion of the Pd particles and the hydrophobic nature of the catalysts.