Browsing by Author "Storm, Ené"
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- ItemOxidative organic transformations using catalyst systems based on metal nanoparticles supported on organic/inorganic composite materials.(Stellenbosch : Stellenbosch University, 2020-10) Storm, Ené; Mapolie, Selwyn Frank; Malgas-Enus, Rehana; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.ENGLISH ABSTRACT: The use of metal nanoparticles as catalysts in a variety of transformations has been widely reported. Their prevalent use in catalysis is based on their increased surface area and thus an increase in active sites which could improve their catalytic ability compared to the respective bulk metals. This is specifically true for Au nanoparticles. Au and Pd nanoparticles have previously been successfully used as catalysts in oxidation reactions. Based on this, the dissertation describes the use of supported Au and Pd nanoparticles applied as catalysts in the oxidation of alcohols and the oxidative transformation of amines. A variety of support materials, including inorganic, organic and hybrid materials, have been reported in the literature to be able to stabilize metal nanoparticles. Organic support materials containing a high nitrogen content is envisaged to enhance the stability of metal nanoparticles, specifically in the case of Au and Pd nanoparticles. Introducing inorganic functional groups into an organic support material could also improve the thermal stability without significantly impacting on the high nitrogen content. With this in mind, two organic support materials and one hybrid silica-organic support material containing triazine units were prepared. The organic support materials, polymeric melamine formaldehyde microspheres (PMF) and polymeric melamine resorcinol formaldehyde microspheres (MRF), were synthesized through hydrothermal condensation. The hybrid support material, a triazine siloxane hybrid material (TSH), was prepared in a one-pot process with in-situ formation of the monomer followed by subsequent hydrolysis and condensation of the siloxane units to produce a highly cross-linked polymeric material. All three support materials were fully characterized employing solid-state characterization techniques and found to be thermally stable and insoluble in common organic solvents and water. Au and Pd monometallic nanoparticles were stabilized on all three support materials through an in situ chemical reduction method. Small and well-dispersed nanoparticles were prepared in all cases. The interaction between the TSH material and the Au nanoparticles supported on it was also investigated and revealed that the Au nanoparticles were stabilized by the Si atoms in the TSH material and not the N atoms as expected. The supported metal nanoparticles were then applied as catalysts in the oxidation of 1-phenylethanol and 2-octanol as well as the oxidative self- and cross-coupling of amines. 1-Phenylethanol could be fully converted to acetophenone under relatively mild conditions in the presence of TBHP as oxidant and Au/PMF as catalyst. Improved catalyst activity was observed using water as solvent compared to acetonitrile. This was attributed to the formation of a 3-phase emulsion droplet catalytic system. An investigation into the mechanistic pathway was also conducted, including a study into the factors that influence the decomposition of the oxidant, TBHP. The Au/PMF catalyst was also tested in the oxidative self-coupling of benzylamine and the oxidative cross-coupling of benzylamine with aniline to produce its corresponding imine products. Here, it was found that the use of excess aniline and longer reaction times increased the selectivity to the target imine product. The influence of the catalyst and oxidant on the reaction pathway was also explored.