Hybride von Metall-Nanopartikeln mit hochverzweigten Polyethylenimin als lösliche, abtrennbare Katalysatoren

Nanoscale metal colloids have generated significant interest because of their unique chemical and physical surface properties. These metal particles consisting of a few hundred atoms, most of them located at the surface, are not easily stabilized in solution, and controlling their size remain a key challenge. Thermodynamically favored aggregation is usually suppressed by adsorption of low-molecular weight surfactants or linear polymers on the nanoparticle surface. Recently, motivated by the challenge of efficient nanoparticle stabilization and the concept of particle size control by means of synthesis in a confined environment, hybrids of dendrimers with metal nanoparticles have been investigated. Whereas the ideally perfectly branched dendrimers are tedious to prepare, which is a major drawback of using them as stabilizers, hyperbranched polymers, which possess a randomly branched topology, can be prepared in a controlled fashion in one step syntheses. <br>The aim of this work was the synthesis of hybrids of metal particles with hyperbranched amphiphilic polyethylenimine. Amphiphilic polyethylenimine-amides with a hyperbranched polyamine core and an apolar lypophilic alkyl chain periphery were obtained by amidation of PEI with carboxylic acids. With carbonyldiimidazole activated acids, the primary amine end groups can be reacted completely selectively. <br>PEI-amide stabilized silver-, palladium -, and rhodium dispersions can be prepared by reduction of metal salts or decomposition of metal precursors in presence of the hydrophobically modified PEI. Ag nanoparticles were obtained with H2, CO, Li[BH(C2H5)3] and hydrazine as reducing agents. The structure of Ag nanoparticles templated by polymers based on different core molecular weights (PEI Mw 5,000 g mol-1 and 25,000 g mol-1) and having the same degree of derivatization was studied by SANS. Measurements at different concentrations revealed that the neat polymer is present as non-aggregated unimolecular micelles for a polymer based on a Mw 25,000 g mol-1 core, and that silver particles are stabilized by a single polymer molecule. For a polymer based on a Mw 5,000 g mol-1 core, some aggregation occurs upon loading with the silver salt (AgNO3) and particle formation. The Ag dispersions showed antimicrobial properties. Pd nanoparticles were prepared and studied as catalysts for the hydrogenation of cyclohexene as a model reaction. The dispersions displayed low activities compared to Pd on charcoal, and to amphiphilic polyester-polyol stabilized nanoparticles of similar size. Coordination of the N-containing functional groups may block surface sites. Rhodium dispersions stabilized by PEI-amide were studied as catalysts for hydroformylation of 1-hexene. By comparison to rhodium(I) complexes as catalyst precursors (in the presence of polymer), a similar reaction profile, in terms of the gas consumption over time, was observed. The selectivity of the colloid is somewhat higher for hydrogenation and hydroformylation by-products.The effect of the reaction conditions was studied. Catalytic activities increased with pressure (in the range 10-50 bar, 120°C) and temperature (80 to 120°C, 50 bar). The catalyst could be recycled by dialysis. Upon repeated use, a ca. 25% reduction in activity was observed, which can be due to rhodium losses during recovery.

Author dc:creator

• Tuchbreiter, Lydie

Contributors dc:contributor

• Mecking, Stefan

• polyethylenimine
• catalysis
• hydroformylation
• nanoparticles

Repository record source_url

https://freidok.uni-freiburg.de/data/2506

OAI identifier oai:identifier

oai:freidok.uni-freiburg.de:2506