(720c) Pd-Impregnated Hyperbranched Aminosilica (HAS) Materials As Hybrid Catalysts for the Selective Production of Olefins From Alkynes

The partial hydrogenation of alkynes to alkenes is an important class of chemical transformations with wide application in industry, for example in the elimination of alkynes from alkene feedstocks or the preparation of cis-olefins of high value. The Lindlar catalyst (Pd/CaCO₃ poisoned with Pb(OAc)₂) along with base additives (eg. quinoline) is commonly used in the semihydrogenation of alkynes or dienes. However undesirable lead compounds and other additives are required to achieve good selectivity. Thus it is desirable to develop alternative, green catalysts for these catalytic transformations.

In liquid phase hydrogenations, N or S donor ligands are often used as Pd modifiers to achieve improved selectivity.^1-3 To this end, we sought to use hybrid silica-aminopolymer composite materials as supports for Pd centers in these reactions and a new aminopolymer-silica composite supported Pd catalyst was developed for the selective hydrogenation of alkynes to alkenes. Hyperbranched aminosilica (HAS) materials, which are prepared via the in-situ ring-opening polymerization of aziridine on porous silica supports (originally developed as carbon dioxide adsorbents,^4-5) were used as ligand/modifier-containing supports for the Pd species. The HAS solids were then impregnated with Pd(OAc)₂, followed by various reduction treatments before use in catalytic tests. The resulting Pd-HAS materials were applied as catalysts using four different substrates, yielding good selectivity in the conversion of alkynes to alkenes in all cases, as well as excellent cis-alkene selectivity for disubstituted substrates.⁶The catalysts were characterized in detail with multiple techniques to shed a light on the naure of the Pd species in the materials. Recycle studies of the catalysts showed a negligible amount of palladium leaching during reaction, and the catalysis appeared to be occurring on the solid material.

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6. Long, W.; Ping, E. W.; Didas, S. A.; Brunelli, N. A.; Jones, C. W.; manuscript in preparation, 2011.