Synthesis and catalytic activity in selective hydrogenation of palladium complexes anchored in montmorillonite

Abstract

Functional groups such as diphenylphosphine, triphenylphosphine, and 4-picoline have been intercalated in montmorillonite, a swelling smectite clay; subsequent complexation with palladium has led to syntheses of various interlamellar montmorillionite-anchored palladium(II) complexes. These montmorillonite ligands and their complexes are characterised by elemental analyses, IR, XRD, and ESCA. Diphenylphosphinomontmorillonite-palladium(II) complex (4B(ii), the best catalyst of the series for hydrogenation reactions, hydrogenated alkynes to alkenes and to alkanes selectively, yields are quantitative. The internal double bonds are also hydrogenated, but with decreased rate of activity. Catalyst 4B(ii) has shown consistent activity for a number of recycles in the hydrogenation of styrene, and hex-1-ene. Used catalyst was also tested for hydrogenation activity with various organic reactants. The rate of hydrogen absorption is almost the same as that with fresh catalyst under identical conditions. These data indicate that transient homogeneous complex species formed in the pretreatment remain unchanged during the recycles. Catalyst 4B(ii) hydrogenated mono and disubstitued alkynes and alkenes in ethanol. On the other hand, diphenylphosphinite-montmorillonite-palladium(II) complex (2B) selectively hydrogenated monosubstituted alkynes and alkenes only under identical conditions. Catalyst 2B, with its lower initial interlamellar spacings, has induced size selectivity as evidenced in the selective hydrogenation of monosubstituted reactants only, because the preferred perpendicular orientation of the transient reactant-palladium complex formed with monosubstituted olefins in the interlayer spacings of montmorillonite requires a space smaller than that expected for disubstituted ones. Easy adjustment of interlayer spacings by the incorporation of complex moieties of different sizes in the interlayers makes montmorillonite a practical alternative to zeolites for inducing size selectivity.