Synthesis and catalytic properties of large ligand stabilized palladium clusters.
Ligand stabilized palladium clusters in the size range of 3-4 nm on various supports are used as catalysts in gas phase and liq. phase hydrogenation reactions. 1,10-Phenanthroline and O2 form the ligand shell for a mixt. of seven- and eight-shell clusters, (Pd7/8(phen)), on TiO2, Al2O3, and CaCO3. The gas phase hydrogenation of 1,3-butadiene under various conditions gives results like established metal catalysts, i.e. low selectivities. In contrast with these observations liq. phase reactions show high selectivities which can addnl. be influenced by variation of the ligand shell. Semihydrogenation of 2-hexyne with Pd7/8(phen) on TiO2 in n-octane results in 93% of cis-2-hexene. If 3-n-Bu or 3-n-heptyl-phenanthroline is used instead of the unsubstituted ligand the selectivity is about the same, however, the formation of daughter products such as isomers or n-hexane is almost completely suppressed. The hydrogenation of a series of unsatd. carbonic acids again shows the influence of the nature of the ligands on the products. Here, phenanthroline and (-)-cinchonidine are compared. Finally, the hydrogenation of acetophenone is studied using di-2,9-(2-methyl-butyl)-1,10-phenanthroline and (-)-cinchonidine for comparison. The iso-pentyl substituted phenanthroline enforces high activities, while cinchonidine prevents the same reaction almost quant. TEM and STM studies of the clusters have been performed and will be described.
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