Synthesis and characterization of fluorous-stabilized metal nanoparticles for evaluation in fluorous biphasic catalysis

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Stellenbosch : Stellenbosch University
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.
AFRIKAANSE OPSOMMING: n Fluoor-bifasiese benadering as 'n groen strategie vir die maklike herwinning en hergebruik van duur katalisators, is ondersoek. 'n Reeks fluoor-gestabiliseerde Au NP's is suksesvol gesintetiseer met behulp van 'n misel-templaat strategie. Die strategie het behels dat 'n hidrofiele G3-DAB PPI-NH2 dendrimeer gewysig moet word om perifere palmitoielgroepe in te sluit wat 'n amfifiliese unimolekulêre misel oplewer (in hierdie werk verder verwys as die gemodifiseerde dendrimeer). Die gemodifiseerde dendrimeer is gekenmerk deur FT-IR spektroskopie en 1H KMR spektroskopie; en het volledige oplosbaarheid in CHCl3 vertoon. Deur die gemodifiseerde dendrimeer as 'n templaat te gebruik, is organies-oplosbare Au DEN'e voorberei deur die inkapseling van Au-ione in die binnekant van die dendrimeer, gevolg deur reduksie. Hierdie Au DEN'e is uit die organiese fase in 'n fluoorfase (S1 of S2) onttrek met die gebruik van fluooragtige ligande (L1 en L2). Daar is gevind dat hierdie ekstraksiestap die mees uitdagendste was en baie moeite is gedoen om die mate van ekstraksie in die fluoorfase te optimaliseer. In gevalle waar hoë Au-hoeveelhede ingesluit is, is min of geen ekstraksie waargeneem en dit is toegeskryf aan die groter grootte van die Au DEN'e wat fase- oordrag meer onwaarskynlik maak. Dit is geïdentifiseer dat vir die vorming van klein, eenvormige Au DEN'e dit nodig was dat ons die maksimum hoeveelheid Au-ione identifiseer wat deur die dendrimeer inkapsuleer kan word. Versuiming om hierdie waarde te bepaal kan lei tot oorlading van die dendrimeer en sal dan DSN'e vorm. Vir die doeleindes van hierdie navorsing was dit van kritieke belang om organiese DEN'e voor te berei en die vorming van DSN'e te voorkom, daarom is 'n addisionele studie uitgevoer om die eindpunte te identifiseer in 'n reeks UV-Vis spektrofotometriese titrasies wat die dendrimeer en metaalsout behels wat ondersoek word. In die studie is twee dendrimere ondersoek en het 'n ongemodifiseerde, hidrofiele G3-DAB-PPI-NH2 dendrimeer en die voorgenoemde gemodifiseerde dendrimeer ingesluit. Die metaallaaivermoë van hierdie dendrimere is bepaal vir 'n reeks metaalione in drievoud, wat insluit; Cu(II), Ni(II), Co(II), Zn(II), Cd(II), Pb(II), Ru(III), Rh(III), Pd(II), Pt(II) en Au(III). Die resultate het getoon dat die ongemodifiseerde dendrimeer in die meeste gevalle minder metaalione gehuisves het in vergelyking met die gemodifiseerde dendrimeer analoog. Dit is toegeskryf aan die verbeterde oplosbaarheid van die gemodifiseerde dendrimeer in organiese oplosmiddels. Dit veroorsaak in werklikheid dat die laai-interaksie gedryf word deur oplosbaarheidsverskille tussen die hidrofiliese binnekant van die gemodifiseerde dendrimeer en die hidrofobiese oplosmiddel in teenstelling met vaste stoïgiometriese verhoudings. Vervolgens is agt unieke, sferiese, monodisperse en klein fluoor-gestabiliseerde Au NP's gegenereer en gekenmerk deur UV-Vis spektroskopie, TEM en ICP-OES analiese. Stelsels wat beide L1 en L2 insluit as die fluoor stabilisator is vervaardig. Boonop verskaf die gebruik van perfluoro-1,3-dimetielperfluorosikloheksaan (S2) kleiner fluoor-gestabiliseerde Au NP's in vergelyking met die gebruik van perfluorometielsikloheksaan (S1) in die ekstraksiestap. Daar is gevind dat 'n toename in temperatuur tydens die ekstraksie dit nie aangehelp het nie, maar eerder die oksidasie van die NP's of versnelde agglomerasie in die organiese fase bevorder het. So is dit ontdek dat die ekstraksie van die organies oplosbare DEN'e in die fluoorfase hoogs afhanklik was van hul grootte, wat weer afhanklik was van die uitkoms van die reduksiestap. 'n Alternatiewe nuwe sintetiese metode (genoem die direkte metode) is ontwerp en geoptimaliseer vir die voorbereiding van fluoor-gestabiliseerde Au NP's wat deur L1 gestabiliseer is. Hierdie metode het nie net 'n aansienlik verminderde voorbereidingstyd gebied nie, maar dit het ook 'n fluoor-waterige bifasiese reduksie behels om die fluoor- gestabiliseerde Au NPs te lewer. Verder is daar deur middel van hierdie metode getoon dat dit moontlik was om verskillende groottes NP's te verkry deur die Au:L1-verhouding te verander. Daar is gevind dat die verhoging van die hoeveelheid ligand na goud tot kleiner fluoor-gestabiliseerde Au NP's gelei het. Die verhoudings van Au: L1; met Au = 1 vgl. en L1 = 0,28 ekw.; 0,56 ekw.; 1,12 vgl. en 2,24 ekw. Au NP's van groottes opgelewer; 43,4 ± 22,2 nm, 17,8 ± 13,7 nm, 11,8 ± 15,8 nm en 2,0 ± 0,3 nm, onderskeidelik. Uit hierdie werk is dit getoon dat 'n eenvoudige strategie bestaan om fluoor-gestabiliseerde Au NP's binne drie uur te produseer by omgewingstemperature met behulp van L1. Ander pogings is aangewend met die gebruik van ander fluooragtige ligande soos L2, L3 en L4. In hierdie gevalle is geen fluoor- gestabiliseerde Au NP's bereik nie. Tien fluoor-gestabiliseerde Au NP katalisatorstelsels is voorberei deur beide metodes te gebruik, met behulp van verskillende verhoudings van Au: L1 of L2, in S1 of S3, onderskeidelik. Hierdie sisteme is geassesseer as katalisators in die bifasiese katalitiese oksidasie van 1-okteen onder die geoptimaliseerde katalitiese reaksie toestande. Daar is gevind dat al die fluoor-gestabiliseerde Au NP's wat in hierdie voorgenoemde eksperimente ondersoek is, aktief was in die fluoor-bifasiese katalitiese oksidasie van 1-okteen. Nie net dit nie, maar selfs na herwinning tot vyf keer, het die katalisator aangehou om bestendige aktiwiteit te toon en het altyd beter presteer as die blanko reaksie (sonder katalisator) in terme van % omsetting van substraat. Daar was 'n duidelike verwantskap tussen die gemiddelde deeltjie deursnee (nm) van die Au NP's in die sisteem en die omskakeling van 1-okteen. Dit is gedemonstreer vir JHD12 en JHD16 wat die grootste NP's uitgemaak het en die laagste omsettings van 1-okteen verskaf het in verhouding tot die ander katalisatorstelsels wat getoets is by hierdie konstante metaallading (Cf (Au)) van 5 × 10-7 mol/mL en geoptimaliseerde eksperimentele toestande. Alhoewel dit nie teen dieselfde metaallading as die ander katalisatorstelsels getoets is nie, is gevind dat JHD15 die mees aktiewe katalisator is. Dit is omdat die katalisator teen 'n konsentrasie tien keer minder as al die ander katalisators getoets is en steeds 'n hoër omsetting van 1-okteen verskaf het. Die hoë aktiwiteit is toegeskryf aan die grootte van die Au NPs van JHD15 wat 2.0 ± 0.3 nm was wat baie kleiner was as dié wat met die ander katalisatorstelsels geassosieer word. GC-FID is aangewend om die relevante chemiese spesies na die kataliselopies te kwantifiseer. Die herwinbaarheid en hergebruik van die katalisators is ook ondersoek. In elke geval was die epoksieproduk, die hoofproduk.
Thesis (PhD)--Stellenbosch University, 2022.
Nanoparticles, Biphasic catalysis, UCTD, Fluoropolymers, Catalysis, Activation (Chemistry)