Doctoral Degrees (Chemistry)

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Browsing Doctoral Degrees (Chemistry) by Advisor "Erasmus, E."

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    Computational and characterization studies of homogeneous and heterogeneous tris(beta-diketonato) complexes with catalytic applications
    (University of the Free State, 2017-06) Gostynski, Roxanne; Conradie, J.; Erasmus, E.
    English: Nine [Mn(β-diketonato)3] [β-diketonato = dipivaloymethanato (dpm), 1; acetylacetonato (acac), 2; benzoylacetonato (ba), 3; dibenzoylmethanato (dbm), 4; trifluoroacetylacetonato (tfaa), 5; thenoyltrifluoroacetonato (tfth), 6; trifluorofuroylacetonato (tffu), 7; trifluorobenzoylacetonato (tfba), 8 and hexaflouroacetylacetonato, 9] complexes were synthesized by adapted methods from literature. An attempt was made to graft [Mn(β-diketonato)3] complexes 2-9 onto two dimensional (2-D) Si-wafer supports (S5-S12) via a silane linker. The X-ray photoelectron spectroscopy (XPS) Mn:F atomic ratio results of fluorine containing [Mn(β-diketonato)3] complexes grafted onto amino-functionalized Si-wafer (S9-S12) gave an indicaton that the [Mn(β-diketonato)3] complexes decomposed during the grafting process. Studying Mn2O3 on the hydroxylated Si-wafer surface by the use of XPS, it was confirmed that the Mn 2p photoelectron lines observed for S5-S12 consist mostly of Mn2O3 that has no interaction with the silane linker. Eight [Mn(β-diketonato)3] complexes 1-8 was wet impregnated onto various three dimensional (3-D) solid supports (S12- S28) to form [Mn(β-diketonato)3] model catalysts. Additionally a selection of [M(acac)3] complexes ([Co(acac)3], 12; [Rh(acac)3], 13, and [Ir(acac)3], 14) were grafted onto two dimensional (2-D) Si-wafer supports (S29-S31). While [M(acac)3] complexes ([Cr(acac)3], 10; [Fe(acac)3], 11; [Co(acac)3], 12; [Rh(acac)3], 13, and [Ir(acac)3], 14) were wet impregnated onto various three dimensional (3-D) solid supports (forming S32-S36). The model 2-D and 3-D catalysts were studied and characterized by the means of XPS, TGA and computational chemistry calculations. Selected model 2-D (S5, S7, S8, S12 and S29-S31) and Mn(acac)3 immobilized onto SiO2 (3-D catalysts, S14 as is and heat treated at 100, 130, 250 and 350 °C) were tested for the catalytic self-solvating reaction between ethanol and hexamethylenediisocyanate (HDI) to form hexamethylenediurethane (HDU) to mimic the industrial production of polyurethane. The model 2-D [Mn(β-diketonato)3] (S5, S7, S8 and S12) catalysts showed that as the total group electronegativity, 3(R + R'), increase a general decrease in turnover frequency (TOF) was observed. The model 2-D [M(acac)3] catalysts (S29-S31) showed that with an increase in the metal centre’s Pauling electronegativity a general increase in TOF was observed. The catalytic test on the Mn(acac)3 immobilized onto SiO2 showed that the sample heat treated at 100°C before hand has the highest TOF, which is most probably due to the loss of only one -diketonato ligand making it more active. The chromium(0) Fischer carbene complexes (Cr-FCCs) ([Cr(CO)4(PPh3)=C(OEt)(Fu)], C1; [Cr(CO)5=C(NHCy)(Fu)], C2; [Cr(CO)5=C(NHCy)(Th)], C3 and [Cr(CO)5=C(OEt)(ThTh)], C4) study was in collaboration with the research group of Dr. M Landman at the University of Pretoria. The four Cr(0) FCCs, were obtained and characterized by electrochemistry, XPS and computational chemistry calculations in this study. Penta-carbonyl Cr-FCCs [Cr(CO)5=C(OEt)(Fu)], C5, and [Cr(CO)5=C(OEt)(Th)], C6, were also supplied and anchored onto an amino-functionalized Si-wafer to create an immobilized Cr-FCCs C7 and C8. XPS results showed that the Cr(CO)5=C(OEt)(Fu)], C5, and [Cr(CO)5=C(OEt)(Th)], C6, was successfully anchored onto an amino-functionalized Si-wafer (C7 and C8). The electrochemical study of C1-4 showed that the oxidation potential (Epa) is influenced by the varied substituents on different sites of the Cr-FCCs. The oxidation order of the Cr-FCCs is: [Cr(CO)5=C(OEt)(ThTh)], C4 > [Cr(CO)5=C(NHCy)(Th)], C3 > [Cr(CO)5=C(NHCy)(Fu)], C2 > [Cr(CO)4(PPh3)=C(OEt)(Fu)], C1. The reduction followed the same trend except for the [Cr(CO)4(PPh3)=C(OEt)(Fu)], C1, complex that is reduced at a higher potential than the aminocarbene complexes, [Cr(CO)5=C(OEt)(ThTh)], C4 > [Cr(CO)4(PPh3)=C(OEt)(Fu)], C1 > [Cr(CO)5=C(NHCy)(Th)], C3 > [Cr(CO)5=C(NHCy)(Fu)], C2. The electronic energies of the different conformations obtained by the computational study showed that certain conformations are preferred over others. The computational results were in good agreement with experimental characterization method results. Computational study of HOMO and LUMO orbitals of the Cr- FCCs confirmed that the oxidation process is metal based and the reduction is based across the carbene ligand. The correlation of the oxidation potential (Epa) of the Cr0/Cr+1 redox couple and reduction potential (Epc) of the ligand based reduction with the HOMO energy (EHOMO) and LUMO energy (ELUMO) respectively, showed that with decreasing molecular orbital energy (HOMO and LUMO) an increase to more positive Epa and Epc potentials are obtained.
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    ItemOpen Access
    Preparation, characterisation and selective hydrocarbon transformation applications of flatmodel and zeolite-supported platinum catalysts
    (University of the Free State, 2014-10) Magqi, Nceba; Erasmus, E.; Swarts, J. C.
    English: Allyl, amino and silanol (Si-OH) functionalised silicon wafers with epitaxial layer of SiO2 (SiO2/Si-wafers) were used as two-dimentional supports to prepare flat model Pt/SiO2/Si-wafer catalysts by spin coating and grafting of suitable platinum precursors followed by calcination and reduction. X-ray photoelectron spectroscopy (XPS) revealed that H2PtCl6 reacted with surface allyl groups to form π-olefin Pt2+ supported complexes. Residual Si-OH groups also reacted with H2PtCl6 to form Pt4+ species. Together, the different surface Pt species formed a heterogeneous coating on the surface of Si-wafer. The platinisation of aminated silicon surfaces with K2PtCl4 dissolved in aqueous ethanol resulted in the reduction of the Pt(II) precursor and immobilisation of the resultant metallic Pt particles on the model Si-wafer. Calcination and reduction of the surface Pt species resulted in the formation of relatively small Pt particles, 1-5 nm average particle diameter, which interacted with the silicon surface via boundary Pt2+-O-Si bonds or Pt2+-N-Si bonds. The prepared flat model Pt/SiO2/Si-wafer catalysts had catalytic activity in solvent-free aerobic oxidation of 1-octadecanol to carbonyl compounds. A selection of the prepared flat model Pt/SiO2/Si-wafer catalysts were also shown to have activity in hydrogenation of alkene and carbonyl functional groups. The preparation and characterisation of Pt catalysts supported on alkaline KL-zeolite and acidic HY-zeolites are also described. Two types of Pt/KL-zeolite catalysts were investigated. Those Pt/KL-zeolite catalysts prepared by vapour phase impregnation had a high particle dispersion (91.5 % vs. 75.8 %) and smaller particles sizes (average diameter = 1.2 nm vs.1.5 nm) compared to the equivalent Pt/KL-zeolite catalysts that were obtained by incipient wetness impregnation. In aromatisation of n-alkanes, the Pt/KLzeolite catalysts obtained by vapour phase impregnation were more active and more tolerant to oxygenates present in the feed than the Pt/KL-zeolite catalysts obtained by incipient wetness impregnation. A correlation between the concentrations of acetic acid in the feed, the amount of coke deposit on spent catalysts and catalyst deactivation was established. Pt/KL-zeolite and Pt/HY-zeolite catalysts were also applied in liquid phase oxidation of primary alcohols under mild conditions (ca.100 °C and ambient pressure). Moderate substrate conversion (10-40 %) was achieved.