Dendrimer encapsulated gold nanoparticles as catalyst precursors for oxidative transformations of unsaturated hydrocarbons

Slazus, Ene (2015-04)

Thesis (MSc)--Stellenbosch University, 2015.

Thesis

ENGLISH ABSTRACT: In an attempt to produce active catalysts for the oxidation of alkanes, hydrophobic dendritic micelle encapsulated gold nanoparticles were prepared. Dendrimers are well suited as templates for the encapsulation of metal nanoparticles as they can control the size and distribution of the particles. Using hydrophobic dendritic micelles it was found that the mode of encapsulation is driven by the solubility of the metal ions and not complexation of these ions, as is the case with conventional dendrimers. The dendritic micelles also provide the possibility of producing the dendrimer encapsulated nanoparticles in organic solvents, simplifying the encapsulation process as well as their subsequent application in catalysis. With this in mind, two types of dendritic micelles were synthesized. The first type, based on commercially available DAB PPI dendrimers, contained a diaminobutane core while the second type, containing a PAMAM interior architecture, has a cyclam core. Three generations of DAB PPI dendrimers were modified on their periphery with palmitoyl chloride to give the alkyl chain terminated hydrophobic DAB PPI dendritic micelles. The PAMAM-type cyclam-cored dendrimers were synthesized from the core outwards to produce two generations of cyclam-cored amine-terminated dendrimers. Their periphery could then be modified with palmitoyl chloride to produce two generations of alkyl chain terminated hydrophobic cyclam-cored dendritic micelles. The dendritic micelles were used as templates for the encapsulation of gold nanoparticles and these were fully characterized by UV/Vis spectroscopy and HR-TEM. Au13, Au31 and Au55 nanoparticles were encapsulated in each dendrimer template by varying the dendrimer to gold ratio. HR-TEM results indicate relatively uniform particles with an average particle size falling in the range of 4-6 nm. Finally, the dendrimer encapsulated nanoparticles (DENs) were applied as catalysts in the oxidation of n-octane. To the best of our knowledge DENs have not been applied as catalysts in the oxidation of linear alkanes. High substrate conversions, falling in the range of 70-90%, were achieved with all of the catalysts. Longer reaction times and lower catalyst loadings resulted in higher conversions with the optimum condition determined to be 0.1 mol% catalyst and 72 hours reaction time. It was also concluded that the nanoparticle size has a bigger influence on the conversion than the nature and generation of the dendrimer template. Overall the gold DENs show great potential as oxidation catalysts.

AFRIKAANSE OPSOMMING: In die poging om aktiewe katalisators vir die oksidasie van alkane te produseer is goud nanopartikels in die binne ruimtes van hidrofobiese dendritiese miselle ge-enkapsuleer. Dendrimere is geskikte template vir die enkapsulering van metaal nanopartikels a.g.v die feit dat dit die grootte en distribusie van die partikels kan beheer. Deur gebruik te maak van hidrofobiese dendritiese miselle verander die wyse van enkapsulering van kompleksering van metaal ione (die geval in konvensionele dendrimere) na oplossing gedrewe enkapsulering. Dendritiese miselle bied ook die moontlikheid om die dendrimer-ge-enkapsuleerde nanopartikels in organiese oplosmiddels voor te berei wat die enkapsulerings proses sowel as die toepassing in katalise vergemaklik. Met hierdie in gedagte is twee verskillende tipe dendritiese miselle gesintetiseer. Die eerste tipe, gebasseer op kommersieel beskikbare DAB PPI dendrimere, bevat ‘n diaminobutaan kern, terwyl die tweede tipe, bestaande uit ‘n PAMAM binne-struktuur, ‘n siklaam kern bevat. Drie generasies van DAB PPI dendrimere was gemodifieer op die periferie met palmitoïelchloried om alkiel ketting getermineerde hidrofobiese DAB PPI dendritiese miselle te produseer. Die PAMAM siklaam kern bevattende dendrimere was gesintetiseer van die kern uitwaarts om twee generasies amien getermineerde dendrimere te produseer. Dit was toe moontlik om die periferie met palmitoïelchloried te modifieer om twee generasies van alkiel getermineerde siklaam kern bevattende hidrofobiese dendritiese miselle op te lewer. Die dendritiese miselle was gebruik as template vir die enkapsulasie van goud nanopartikels en volledig gekarakteriseer deur UV/Vis spektroskopie en HR-TEM. Au13, Au31 and Au55 nanopartikels was ge-enkapsuleer in elk van die dendrimeer template deur die verhouding van dendrimeer tot goud te wissel. HR-TEM resultate dui aan dat die partikels goed versprei is met ‘n gemiddelde partikel grootte tussen 4-6 nm. Die dendrimeer ge-enkapsuleerde goud nanopartikels (DENs) was as katalisators in die oksidasie van n-oktaan toegepas. Volgens ons kennis is DENs nog nie toegepas as katalisators in die oksidasie van lineêre alkane nie. Hoë substraat omskakelings, tussen 70 en 90%, was deur al die katalisators bereik. ‘n Langer reaksie tyd en laer katalisator konsentrasies het hoër omsettings tot gevolg gehad. Die optimale kondisies sluit ‘n 0.1 mol% katalisator konsentrasie en 72 uur reaksie tyd in. Die gevolgtrekking was gemaak dat die nanopartikel grootte ‘n groter invloed op die substraat omsetting het as die aard en generasie van die dendrimeer templaat. Alles in ag geneem, wys die goud DENs groot potensiaal as oksidasie katalisators.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/96790
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