Catalytic hydrogenation activity of zerovalent palladium dispersed in functional porous polystyrene matrices

Zerovalent palladium catalysts, dispersed within functional, porous, crosslinked styrene-divinylbenzene copolymers were prepared by impregnation of lipophylic (C₈H₁₇)₂NH+]PdCl₄ = ] complexes followed by reduction with hydrazine or formaldehyde. A qualitative survey of the catalytic reactivity in hydrogenation of various alkenes differing in substitution degrees (styrene, +-methylstyrene, etc.), and competitive reduction of exo/endo bonds (4-vinylcyclohexene) was performed. The functional groups in the polymer, 2,4-dinitrophenyl, aminomethyl, methoxybenzyl, dialkylaminomethyl, and pseudocrown were shown to have a major effect on catalyst activity. In the two series of polymeric supports, the polymeric adsorbents (Amberlites XAD-2, XAD-4, and XAD-7) and functionalized Amberlite XE-305, the general trend indicated preference of Pd₀ catalysts dispersed on hydrophobic π-acceptor type supports over hydrophobic supports, over polar basic or hydrophilic supports. This is generally true for both non-polar (e.g., styrene) and polar (e.g., allylacrylate) olefins. The most active catalysts, carrying 2,4-dinitro phenyl groups, also showed higher selectivity in reduction of the exo over endo double bond in 4-vinylcyclohexene, in comparison to commercial Pd/C catalyst. Electron microscopy (SEM) showed very little change in the inner porous polymer structure and almost homogeneous metal distribution profiles. TEM provided particle sizes. The activity of the catalyst was 8-fold higher with the smallest crushed particles (0.05–0.12 μm) than with the large (600 μm) noncrushed beads. The catalyst showed exceptional stability on storage (98% activity after 1 year) and marginal loss of activity after 21,000 catalytic cycles per Pd atom.