Palladium nanoparticles supported on hyperbranched aramids: Synthesis, characterization, and some applications in the hydrogenation of unsaturated substrates

Hyperbranched (HB) aromatic polyamides (aramids), synthesized from A(2) + B-3 reagents (A = p-phenylenediamine; B = trimesic acid), have been used as polymeric supports for palladium(0) nanoparticles. The chemical-physical properties of these aramids, denoted as pPDT, have been compared to those of the polymer pABZAIA obtained from the AB(2) monomer 5-(4-aminobenzamido)isophthalic acid. The two HB polymers show structural differences in the shape of the macromolecule as well as the nature of the end groups. The pPDT materials exhibit a less regular structure with both amino and carboxy terminal groups, while the pABZAIA polymer contains a single amine focal unit per molecule and carboxy end groups. HB aramid-supported Pd-0 nanoparticles were obtained by reduction of polymer-supported PdCl2 with an aqueous solution of NaBH4. Some model compounds, mimicking the polymeric hyperbranched structure, were investigated to gain insight into the nature of the interactions occurring between PdCl2 and the support. The nature of the interactions between the polymeric matrix and PdCl2 was investigated by means of FT IR and XPS techniques. XPS spectroscopy, performed on both polymers and model molecules, indicated that the interaction between the support and the metallic precursor involves the terminal amino groups. The particle dimensions and the metallic dispersion were determined by TEM analysis. This study showed that the dimensions and dispersion of the metal clusters on pABZAIA are smaller than those on pPDT. A preliminary study of the catalytic performance of a specific Pd-0/ pPDT catalyst in the hydrogenation of relevant unsaturated substrates (benzene, benzylideneacetone, phenylacetylene, diphenylacetylene, and quinoline) has been carried out in either nonpolar or polar solvents. In most cases, the catalyst proved to be efficient, selective, and recyclable, yet the nature of the solvent affected remarkably the catalysis outcome in terms of both activity and selectivity.