Department of Chemistry and Polymer Science

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    Synthesis and characterization of fluorous-stabilized metal nanoparticles for evaluation in fluorous biphasic catalysis
    (Stellenbosch : Stellenbosch University, 2022-04) Hensberg, Joshua; Malas-Enus, Rehana; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
    ENGLISH ABSTRACT: A fluorous biphasic approach as a green strategy for the facile recycling and re-use of expensive catalysts, has been probed. A series of fluorous-stabilized Au NPs were successfully synthesized using a micelle-template strategy. The strategy entailed modifying a hydrophilic G3-DAB PPI-NH2 dendrimer to include peripheral palmitoyl groups yielding an amphiphilic unimolecular micelle (referred to as the modified dendrimer in this work). The modified dendrimer was characterized by FT-IR spectroscopy and 1H NMR spectroscopy; and displayed complete solubility in CHCl3. Using the modified dendrimer as a template, organic- soluble Au DENs were prepared by the encapsulation of Au ions into the interior of the dendrimer, followed by reduction. These Au DENs were extracted from the organic phase into a fluorous phase (S1 or S2) with the use of fluorous ligands (L1 and L2). This extraction step was found to be the most challenging and much effort was placed on optimizing the extent of extraction into the fluorous phase. In instances incorporating high Au quantities, little or no extraction was observed and was ascribed to the larger size of the Au DENs making phase transfer more unlikely. It was identified that for the formation of small, uniform Au DENs, it was necessary that we identify the maximum quantity of Au ions which could be encapsulated by the dendrimer. Failure to determine this value could lead to overloading the dendrimer and subsequent reduction would form DSNs. For the purposes of this research, it was critical to prepare organic DENs and prevent the formation of DSNs, therefore an additional study was executed to identify the endpoints in a series of UV-Vis spectrophotometric titrations involving the dendrimer and metal salt being investigated. In the study, two dendrimers were investigated and included an unmodified, hydrophilic G3-DAB-PPI-NH2 dendrimer and the aforementioned modified dendrimer. The metal loading capacities of these dendrimers were determined for a range of metal ions in triplicate, which include; Cu(II), Ni(II), Co(II), Zn(II), Cd(II), Pb(II), Ru(III), Rh(III), Pd(II), Pt(II) and Au(III). The results showed that the unmodified dendrimer, in most cases, housed fewer metal ions in comparison to the analogous modified dendrimer. This was attributed to the improved solubility of the modified dendrimer in organic solvents. This, in effect, causes the loading interaction to be driven by solubility differences between the hydrophilic interior of the modified dendrimer and the hydrophobic solvent as opposed to fixed stoichiometric ratios. Subsequently, eight unique, spherical, monodisperse and small fluorous-stabilized Au NPs were generated and characterized by UV-Vis spectroscopy, TEM and ICP-OES analysis. Systems incorporating both L1 and L2 as the fluorous stabilizer were produced. Moreover, the use of perfluoro-1,3- dimethylperfluorocyclohexane (S2) provides smaller fluorous-stabilized Au NPs in comparison to the use of perfluoromethylcyclohexane (S1) in the extraction step. It was found that an increase in temperature during the extraction did not aid it but instead promoted the oxidation of the NPs or accelerated agglomeration in the organic phase. Thus it was discovered that the extraction of the organic soluble DENs into the fluorous phase was highly dependent on their size which in turn was dependent on the outcome of the reduction step. An alternative novel synthetic method (called the direct method) was designed and optimized for the preparation of fluorous-stabilized Au NPs stabilized by L1. Not only did this method offer a significantly reduced preparation time, but it also entailed a fluorous-aqueous biphasic reduction to yield the fluourous-stabilized Au NPs. Furthermore, it was shown by way of this method that it was possible to tailor different sizes of NPs by varying the Au: L1 ratio. It was found that increasing the quantity of ligand to gold resulted in smaller fluorous-stabilized Au NPs. The ratios of Au: L1; with Au = 1 eq. and L1 = 0.28 eq.; 0.56 eq.; 1.12 eq. and 2.24 eq. yielded Au NPs of sizes; 43.4 ± 22.2 nm, 17.8 ± 13.7 nm, 11.8 ± 15.8 nm and 2.0 ± 0.3 nm, respectively. From this work, it has been shown that a simple strategy exists to produce fluorous-stabilized Au NPs within 3 h at ambient temperatures using L1. Other attempts were made with the use of other fluorous ligands such as L2, L3 and L4. In these cases, no fluorous-stabilized Au NPs were attained. Ten fluorous-stabilized Au NP catalyst systems were prepared using both methods, using varying ratios of Au: L1 or L2, in S1 or S3, respectively. These systems were assessed as catalysts in the biphasic catalytic oxidation of 1-octene under the optimized catalytic reaction conditions. It was found that all the fluorous-stabilized Au NPs which were examined in these aforementioned experiments, were active in the fluorous biphasic catalytic oxidation of 1-octene. Not only this, but even after recycling up to five times, the catalyst continued to show steady activity and always performed better than the blank reaction (without catalyst) in terms of % conversion of substrate. There appeared to be a distinct relationship between the average particle diameter (nm) of the Au NPs in the system and the conversion of 1-octene. This was demonstrated for JHD12 and JHD16 which comprised the largest NPs and afforded the lowest conversions of 1-octene in relation to the other catalyst systems tested at this constant metal loading (Cf (Au)) of 5 × 10-7 mol/mL and optimized experimental conditions. Although not tested at the same metal loading as the other catalyst systems, JHD15, was found to be the most active catalyst. This is because the catalyst was tested at a concentration ten times less than all the other catalysts and still provided a higher conversion of 1-octene. The high activity was attributed to the size of the Au NPs of JHD15 which were 2.0 ± 0.3 nm being much smaller than those associated with the other catalyst systems. GC-FID was employed to quantify the relevant chemical species after the catalysis runs. The recyclability and re-use of the catalysts was also investigated. In each case the epoxy product was the major product.