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

1990; Wiley; Volume: 28; Issue: 6 Linguagem: Inglês

10.1002/pola.1990.080280601

1099-0518

Gad Bar-Sela, Abraham Warshawsky,

Asymmetric Hydrogenation and Catalysis

Abstract Zerovalent palladium catalysts, dispersed within functional, porous, crosslinked styrene‐divinylbenzene copolymers were prepared by impregnation of lipophylic [(C 8 H 17 ) 2 NH+][PdCl 4 = ] 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 Pd 0 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.