Comparative study of size-quantized Pd-montmorillonite catalysts in liquid-phase semihydrogenations of alkynes Original Research Article

Organophilic Pd-montmorillonite (Pd-M) samples with controlled particle size were synthetized in a micellar system, by using a cationic surfactant stabilizer. The three samples examined had the same Pd loading of 0.15% and differed only in the mean particle diameter and the size distribution, as confirmed by structural investigation. The mean crystallite diameter was 1.5, 3.0 and 6.2 nm for Pd-M1, Pd-M2 and Pd-M3, respectively. The smaller the particle size, the narrower the distribution observed. All the samples proved to be active and selective catalysts for the liquid-phase semihydrogenation of phenylacetylene under mild conditions. The catalytic activity was found to increase with increasing Pd dispersion, but the selectivity of styrene formation remained unaffected. In correlation with the values of the apparent activation energies determined from the Arrhenius plots, a crystallite size effect was established. When the particle diameter surpassed 4 nm, the catalytic activity of Pd-M dropped considerably. A similar observation was made as concerns the semihydrogenation of 4-octyne, for which the Pd-Ms proved to be less active, but highly stereoselective catalysts: a cis-alkene stereoselectivity of ∼90% was obtained for each sample, irrespective of the Pd dispersion. H2 sorption measurements afforded evidence that β-hydride formation occurred in the bulk phase of the Pd particles. Nevertheless, the β-hydride was found to be catalytically inactive as it did not decrease the alkene selectivity for either reaction.