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 = in diameter) were synthesized by the red uction of Pd2+ precursor ions in the presence of cationic (myristyltrimethy lammonium bromide, MTA(+)Br(-)) or anionic (sodium dodecyl sulfate, Na+DS-) surfactants. The protective adsorption layer of the ionic surfactants arou nd 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), th e cation-exchange reaction between Na+ and MTA(+) rendered the montmorillon ite 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 su spension of hydrotalcite nitrate, HT+NO3- (an anionic clay), the ion exchan ge between NO3- and DS- resulted in the formation of a hydrophobic clay, HT +DS-, with simultaneous deposition of the released Pd particles onto the cl ay lamellae. The low-loaded, highly dispersed Pd-organoclay materials displ ayed extremely high catalytic activities under mild conditions in the liqui d-phase hydrogenations of styrene, hex-1-ene, and cyclohexene. Furthermore, the catalysts exhibited high selectivities for the partial hydrogenation o f 1-phenyl-1-pentyne to 1-phenyl-cis-1-pentene. These high activities and s electivities were explained in terms of the high degree of dispersion of th e I'd particles and the hydrophobic nature of the catalysts.