Pd nanoparticles in hydrotalcite: mild and highly selective catalysts for alkyne semihydrogenation

Pd nanoparticles incorporated in a hydrotalcite (HT) host were prepared via anion exchange between a dilute suspension of HT–nitrate and a Pd hydrosol stabilized by the anionic surfactant sodium dodecyl sulfate. Samples of two of the resulting Pd organoclays, Pd–HT1 and Pd–HT2, with metal contents of 0.1 and 0.42%, respectively, were subjected to further investigations. Characterization via ICP-AES, XRD, and TEM measurements indicated the deposition of fairly monodispersed Pd particles, predominantly on the external surface of the HT layers. The Pd–HT samples proved to be efficient catalysts for the liquid-phase semihydrogenations of both terminal and internal alkynes under mild conditions. For the transformation of phenylacetylene to styrene, a bond selectivity of 100% was obtained, and the cis stereoselectivities for the hydrogenations of the internal alkynes 4-octyne and 1-phenyl-1-pentyne reasonably approached 100%. The catalytic activity of Pd–HT increased with the Pd dispersion, whereas the alkene selectivity remained essentially unaffected.