Tannin-coated polyurethane foams as a non-innocent structured support for Pd-catalysed alkyne semi-hydrogenation, nitroarene reduction and Suzuki coupling under air

Polyurethane foams (PUF) were coated with hydrolyzable (HT) or condensed (CT) tannins, derived from chesnut and quebracho woods, and Pd was anchored onto the HT or CT layer via simple immersion of HT@PUF and CT@PUF in a water-ethanol (1:5) solution of Pd(NH3)4Cl2⋅H2O at room temperature under air to afford Pd@HT@PUF and Pd@CT@PUF composites. XPS showed a partial reduction of the Pd(II) salt to Pd(0) in agreement with the reducing properties of the HT and CT coatings, and SEM and SEM-EDX suggested a coordination of evenly dispersed Pd(II) ions by the phenol moieties of the HT or CT layer together with the formation of Pd (nano)particles. The palladium loading, as determined by ICP-AES measurements, varied with the concentration of the aqueous tannin solution (0.1 or 0.5 g/L) used for the HT or CT coating procedure; the higher the concentration, the higher the Pd loading. A high loading of Pd, hence of Pd(0), on the as-synthesized macroscopic catalyst guaranteed a stable catalytic activity and selectivity from the beginning without prior activation procedure in alkyne-to-alkene hydrogenation. A lower Pd loading resulted in a gradual improvement of the catalytic activity overtime that conincidated with a reduction of the chelated Pd(II) ions to Pd(0) under the reaction conditions. Notably, both Pd@HT@PUF and Pd@CT@PUF proved reusable for at least 6 cycles without significant catalyst activity and selectivity decrease, in a convenient “dip-and-play” mode, in selective alkyne semihydrogenation and/or nitroarene reduction, as well as in fast Suzuki coupling under air