Browsing by Author "Leckie, Laura"
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- ItemImmobilized catalysts for the oxidation of hydrocarbons based on triazole complexes of ruthenium(Stellenbosch : Stellenbosch University, 2017-03) Leckie, Laura; Mapolie, S. F.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.ENGLISH ABSTRACT: The synthesis of model and siloxane functionalized ruthenium complexes and the subsequent immobilization of the latter onto mesoporous silica, as well as the application of the prepared substances in the oxidation of hydrocarbons, is described in this thesis. Model and siloxane functionalized pyridine triazole (ML1 and SL1), pyridine N-oxide triazole (ML2 and SL2) and quinoline triazole (ML3 and SL3) ligands were successfully synthesized utilising a copper mediated click-type cycloaddition reactions. These ligands were reacted with the ruthenium arene dimer, [RuCl2(p-cymene)]2, to produce mononuclear cationic complexes (MC1-MC3) which were stabilized with tetraphenylborate as counterion. The ligands together with the model and siloxane functionalized complexes, MC1-MC3 and SC1-SC3, were fully characterized using FT-IR spectroscopy, NMR (1H and 13C) spectroscopy, ESI-MS analysis and microanalysis. The siloxane functionalized complexes, SC1-SC3, were immobilized onto mesoporous silica supports, MCM-41 and SBA-15, to afford the immobilized catalysts IC1-IC6. The immobilization is effected by a condensation reaction between the siloxane functionality of the complexes and the surface silanols on the MCM-41 and SBA-15 supports, thereby affording a ruthenium complex that is covalently immobilized onto the support. Furthermore the model complex, MC1, was physically adsorbed onto MCM-41 and SBA-15 to afford the adsorbed catalysts AC1 and AC2. The native silicas as well as the supported catalysts were fully characterized using a variety of solid state characterization techniques including infrared spectroscopy, nitrogen adsorption/desorption (Brunauer–Emmett–Teller, BET) surface analysis, low-angle powder X-ray diffraction, transmission electron microscopy (TEM), scanning electron microscopy (SEM), thermal gravimetric analysis (TGA) and ICP-OES. ICP-OES was used to determine the ruthenium loading on the silica supports, thereby facilitating an accurate and direct comparison between the model, adsorbed and immobilized systems during catalysis. The model (MC1-MC3) and immobilized (IC1-IC6) ruthenium catalysts were employed in the oxidative cleavage of 1-octene. The catalysts were successful in transforming 1-octene to heptaldehyde and subsequently to heptanoic acid at extended reaction times. The immobilized N,N (pyridine triazole and quinoline triazole) catalysts were significantly more active than their model counterparts at a low catalyst loading of 0.1 mol%. At this same catalyst loading the N,O (pyridine N-oxide triazole) complexes gave similar results when comparing the model and immobilized systems. The N,O catalyst systems performed better in the oxidative cleavage of 1-octene than the N,N systems. The role of the mesoporous silica supports in the oxidative cleavage reaction was investigated using various catalytic systems including: model complex MC1, model complex MC1 in the presence of either MCM-41 or SBA-15 and model complex MC1 adsorbed on MCM-41 and SBA-15 (AC1 and AC2, respectively). From the results of this investigation it was noted that the immobilized catalysts employed appear to be hydrophilic in nature. This was deduced from the fact that in these biphasic systems, the silica material was found closely associated with the water layer. Thus the immobilization of the ruthenium complex on the silica support facilitates the transfer of the ruthenium pre-catalyst from the organic phase into the aqueous phase which contains the oxidant. This promotes the oxidation of the precursor to RuO4 which is the actual active species. This phase transfer allows thus the proposed active species, RuO4, to form at a faster rate and leads to enhanced reaction rates for the immobilized catalysts compared to their model analogues in the oxidative cleavage of 1-octene. The presence of RuO4 as an important intermediate in the oxidative cleavage reaction was confirmed using UV-Vis studies. The ruthenium model complexes (MC1-MC3) and their immobilized counterparts (IC1-IC6) were also applied in the oxidation of n-octane. The catalysts were successful in transforming n-octane into octanones and octanols in the presence of tert-Butyl hydroperoxide (TBHP), with a catalyst loading of only 0.01 mol% being required for the catalytic oxidation. Reaction mixtures were reduced with an excess of triphenylphosphine after the oxidation reaction, from this an estimate of the selectivity for octyl hydroperoxides could be obtained. Hydrogen peroxide was found to be ineffective as terminal oxidant for these reactions. The immobilized catalysts IC4 and IC5 could be recycled once, while IC3 could be recycled twice. Very little drop in product yield was observed in the recycling of the immobilized catalysts.