Browsing by Author "Maggott, Emile Darron"
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- ItemCatalytic oxidation of lignin model compounds(Stellenbosch : Stellenbosch University, 2019-03) Maggott, Emile Darron; Mapolie, Selwyn Frank; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.ENGLISH ABSTRACT: This thesis pertains to the synthesis of a range of model and siloxane functionalized complexes containing salicylaldimine ligands. The functionalized complexes were immobilized onto two types of mesoporous silica viz. MCM-41 and SBA-15 which were employed in the oxidation of veratryl alcohol. Both model (L1) and functionalized (L2) were fully characterized by a range of analytical techniques such as FT-IR (ATR) and 1H NMR spectroscopy. The model and siloxane functionalized salicylaldimine ligands were complexed to metal precursors based on copper, cobalt and palladium salts in a bis-bidentate fashion. Full characterization of the model (MC1- MC3) and functionalized (FC1-FC3) complexes was achieved by mass spectrometry, FT-IR (ATR) spectroscopy, 1H NMR spectroscopy (MC3 and FC3), UV-Vis spectroscopy, EPR spectroscopy (MC1, MC2, FC1 and FC2), single crystal X-ray diffraction (MC3), elemental analysis and melting point determinations. The native silica supports, MCM-41 and SBA-15 and the functionalized complexes were reacted with each other to afford the immobilized complexes (IC1-IC6). The immobilization process involves a condensation reaction between the surface silanol groups of the support and the siloxane functionality of the complexes. These complexes are covalently bound to their respective supports, which ultimately led to “heterogenized” catalysts. The immobilized complexes (IC1-IC6) were fully characterized by a range of solid-state analytical techniques which include FT-IR (ATR) spectroscopy, low angle powder X-ray diffraction, thermal gravimetric analysis (TGA), solid state 29Si NMR spectroscopy, nitrogen adsorption/ desorption (BET) surface analysis, scanning and transmission electron microscopy (SEM and TEM). Inductive coupled plasma optical emission spectroscopy (ICP-OES) was used to quantitatively determine the metal loading of the immobilized complexes. Both the model (MC1-MC3) and immobilized (IC1-IC6) complexes were evaluated in the oxidation of veratryl alcohol. A comparison of the effect of different oxidants on the activity was investigated using either molecular oxygen, hydrogen peroxide or tert-butyl hydroperoxide (TBHP). Model complex, MC1, exhibited better activity than the other model catalysts (MC2 and MC3) in the oxidation of veratryl alcohol at 25 °C under 1 atm oxygen pressure. This catalyst managed to selectively produce veratrylaldehyde. Amongst the three oxidants, hydrogen peroxide exhibited the least activity when used in conjunction with MC1, which was used to optimize reaction conditions. Experiments with catalyst, MC1, were also conducted in the absence of oxygen, which resulted in only moderate activity of about 20% with 100% aldehyde formation. This most likely suggests that the reaction undergoes a β-hydride elimination reaction with subsequent generation of molecular hydrogen. The use of tert-butyl hydroperoxide as oxidant in the oxidation of veratryl alcohol, exhibited relatively high activity compared when molecular oxygen is used. The high activity can be attributed to the higher stability of TBHP as well as its solubility in the reaction mixture. This suggests that the active species forms faster with TBHP as oxidant. The best conversion was obtained for the cobalt catalyst, MC2, which was 85% followed by MC1 and MC3 respectively. The higher activity of TBHP resulted in higher veratrylaldehyde yield but unfortunately it also led to over-oxidation to veratric acid. The immobilized catalysts, IC1-IC6, were also evaluated in the oxidation of veratryl alcohol using TBHP as oxidant. These catalysts generally exhibited moderate activity although the cobalt analogues (IC3 and IC4) showed moderate conversion of up to 65%. Molecular oxygen when used as oxidant with the immobilized catalysts seemed to be ineffective in the oxidation of veratryl alcohol. All the immobilized catalysts except IC5 were recycled and reused for up to five catalytic runs. IC3 and IC4 exhibited moderate activity over the five cycles with only a slight decrease in activity. The nature of the support had an effect on the activity as well as the selectivity. The immobilized catalysts based on MCM-41 supports, showed enhanced activity. The selectivity was slightly altered when using SBA-15 as a support. Overall, the immobilized catalysts exhibited reasonable to moderate activity in the presence of TBHP while no activity is observed when using molecular oxygen as oxidant. However, the immobilized catalysts were outclassed by their model counterparts when employing both oxidants.