Mono- and Bimetallic Au-Cu dendrimer micelle encapsulated nanoparticles as catalysts in the oxidation of styrene.
Thesis (MSc)--Stellenbosch University, 2018.
ENGLISH ABSTRACT: In this thesis, we report the synthesis and characterization of mono- and bimetallic gold and copper nanoparticles, stabilized by a dendrimer micelle. This was achieved by modifying commercially available DAB G3-PPI dendrimers by introducing alkyl chains to the dendrimers peripheries. This enabled the preparation of dendrimer micelles with the unique property of possessing a hydrophilic interior and hydrophobic exterior. This dual nature of the dendrimer micelles’ solubility allowed us to exploit the advantage of solubility-driven encapsulation/stabilization of metallic NPs. Dendrimer micelles with varying alkyl chain lengths (M1-M3) were successfully synthesized, characterized and employed as both templates and stabilizers for the synthesis of monometallic Au, Cu and bimetallic AumCun NPs. HR-TEM analysis showed that the alkyl chain length on the periphery of the dendrimer micelle influences NP size, viz. the longer the chain, the higher the degree of stabilization and hence, the smaller and monodispersed the metal NPs. Having identified M1, with the C15 alkyl chain, as the most effective dendrimer micelle for stabilizing the metal NPs, the long-term stability of the prepared DSNs was evaluated. It was found that the bimetallic DSNs are more stable over extended periods compared to the monometallic DSNs, which showed agglomeration. This was likely due to a stabilizing effect of the Cu NPs on the Au NPs and the nature of the bimetallic NPs. After optimizing the synthetic method to give reproducible results, we set out to evaluate and compare the stabilities of these various DSNs. After keeping these mono- and bimetallic DSNs in solution for nine months, it was found that bimetallic DSNs were very stable and generally showed little change in size and dispersity over such an extended period of time. This showed that the composition of metallic NPs affects the stability of the NP clusters. These mono and bimetallic DSN’s were then evaluated as catalysts in the solvent-free oxidation of styrene. A general trend was observed throughout the course of testing five different mono- and bimetallic DSNs with varying metal composition. It was found that the DSNs with a greater proportion of Au exhibited slightly greater conversions. These conversions, however, was found to decrease slightly as the amount of Au in the NPs was gradually replaced with Cu. Hence, the Au DSNs generally converted the most of the substrate, followed by the Au3Cu1, Au1Cu1, Au1Cu3, and Cu DSNs, respectively. Since the Au NPs were the largest and the Cu NPs were the smallest, it was concluded that conversion of styrene was not dependent on the NP size, but rather the metal content. The rationale behind this was that NP size-dependent catalysis would show the highest conversion for the smallest NPs, however, the results obtained showed the highest conversion for the larger Au NPs, whilst the small Cu NPs showed the lowest conversion. Additionally, the selectivity of the DSNs was found to vary depending on the oxidant employed. In the case of H2O2, benzaldehyde formation was favoured over the formation of styrene oxide, whilst the converse was true when TBHP was employed as the oxidant. Of the two oxidants, TBHP produced the highest activity and favoured formation of the desired product – styrene oxide.
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