Mannich base metal complexes and their thiocyanate analogues as catalysts in the oxidation of Catechol
- Authors: Ayeni, Ayowole Olaolu
- Date: 2018
- Subjects: Mannich bases , Catechol , Catechol -- Oxidation , Thiocyanates , Catalysts
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/62339 , vital:28156
- Description: The study focused on the design of new Cu(II) and Fe(III) complexes, with or without thiocyanate (NCS-), as possible candidates of catechol oxidation using 3,5-di-tert-butyl catechol (3,5-DTBC) as substrate. Two classes of Mannich bases were studied depending on the active methylene group from which they were formed, being either p-cresol or acetaminophen. The ligands were characterised by 1H and 13C NMR spectroscopy. Crystal structures of three of the ligands are newly reported, along with detailed discussion of polymorphism observed in one of the ligands, and the nature of the hydrogen within the ligands in the solid state as well as in solution. The Mannich bases behaved as bidentate (NO), tridentate (NNO) and tetradentate (NNOO) ligands on coordination to Cu(II) and Fe(III) ions in which the hydroxyl group may be protonated or deprotonated. Coordination was determined by IR spectroscopy, investigating shifts in vOH, vC-O and in vCNC of the Mannich bases. The vCNC stretching frequencies v1 and v2 of asymmetrical piperazine Mannich bases were observed to shift upward in few cases upon complexation and this is attributed to (chair-boat) conformational change. The mode of coordination of the thiocyanate was determined by IR spectroscopy. Of the forty metal complexes investigated, six groups of metal complexes were identified as follows: (i) Ma(Ln)aClb-cH2O; (ii) Ma(HLn)a(NCS)aClb; (iii) Ma(Ln)a(NCS)aClb; (iv) Ma(HLn)aClb-cH2O; (v) Ma(Ln)a(NCS)a-cH2O; (vi) Ma(HLn)a(NCS)a-cH2O where a = 1 - 2 ; b = 1 - 4, c = 1 - 8. Molar conductivity values of 4.38 - 161.77 Q-1.cm2.mol-1 for the Cu(II) and Fe(III) complexes in DMSO showed that they range from non-electrolytes to 1:1 and 1:2 electrolytes. Electronic spectra for the ligands and the complexes were conducted in DMF and DMSO. The ligands are characterised by and n→n* and n→n* transitions. Intraligand charge transfer transitions peculiar to the nitro group were observed at about 430 nm for the nitro containing ligands. On coordination, these bands overshadowed the d-d transitions particularly for the nitro-Mannich bases. On complexation, ligand to metal charge transfer transitions associated with the hydroxyl were observed between 320 - 420 nm. Charge transfer transitions associated with the thiocyanates were also observed and discussed. The d-d transitions for high spin Fe(III) complexes are spin forbidden and generally uninformative. Those of Cu(II) are spin allowed and allow tentative structural proposals. Square planar and octahedral geometry are generally prevalent in the Cu(II) complexes with trigonal bipyramidal observed in few instances. The Fe(III) complexes are generally octahedral. Thirty-nine of the forty synthesised Cu(II) and Fe(III) complexes were catalytically active on the substrate (3,5-DTBC) in DMF with turnover rates (kcat) reported in the range of 1.86 ± 0.09 to 112.32 ± 3.72 h-1. From this pool of complexes, sixteen isostructural pairs were identified in terms of geometry, molecular formula and the source of the Mannich base and the following conclusions were made: The presence of thiocyanate in the metal complexes reduce catecholase activity; the Cu(II) complexes generally have better activity but the Fe(III) complexes become more relatively active with highly electron donating groups while the Cu(II) complexes become less; dinuclear complexes have greater activity than the mononuclear.
- Full Text:
- Date Issued: 2018
The effect of silica on the reduction of precipitated iron-based fischer-tropsch catalysts
- Authors: Coombes, Matthew
- Date: 2016
- Subjects: Fischer-Tropsch process Reduction (Chemistry) , Catalysts
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10948/14873 , vital:27888
- Description: Iron Fischer-Tropsch (FT) catalysts are typically prepared as iron oxides which are reduced to FT-active iron metal and iron carbide prior to FT synthesis. The iron oxides contain a variety of different chemical and structural promoters to alter FT-activity. Silica is a common structural promoter which stabilises the formation of small crystallites and provides mechanical integrity to the catalyst. However, silica inhibits the reduction of the oxide precursor to the FT-active phases. This ultimately affects catalyst activity and product selectivity. It has been proposed that the silica interacts with the iron to form encapsulating shells of fayalite (Fe2SiO4), or fayalite rafts between the iron oxide and the silica support. In this study, six silica-promoted iron oxide samples were prepared using a simple co-precipitation technique. Samples contain varying amounts of silica, and the samples are named 100/x Fe/SiO2, where x is the weight of silica for 100 weight iron, with x taking on values of 0, 10, 25, 50, 100 and 200. The resulting iron oxides were characterised using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray powder diffraction (XRPD), M¨ossbauer spectroscopy (MS), magnetic susceptibility measurements (MM), Raman spectroscopy, thermal gravimetric analysis (TGA) and nitrogen physisorption. Their reduction in a hydrogen atmosphere was investigated using temperature programmed reduction (TPR), in situ XRPD and TEM. The reduction in hydrogen of 100/0 Fe/SiO2 and 100/10 Fe/SiO2 was also studied using in situ gas flow TEM cells. These cells allow the samples to be studied in the electron microscope at temperature and pressure conditions approaching those experienced in a real reactor environment. In the absence of a silica promoter (100/0 Fe/SiO2), hematite particles are formed with mean particle diameters of 39 ± 12 and 52.7 ± 0.2 nm determined using TEM and XRPD respectively. MM data reveals a magnetic transition (Morin transition) at≈230 K, consistent with a mean particle size of≈50 nm. In a hydrogen atmosphere, the hematite reduces to metallic iron via a two-step process viz. hematite → magnetite → iron. The final iron particles have an average crystallite size of 68.0 ± 0.2 nm. The presence of lower amounts of silica in the samples 100/10 Fe/SiO2, 100/25 Fe/SiO2 and 100/50 Fe/SiO2 results in the formation of silicasubstituted 2-line ferrihydrite particles. Bands in the Raman spectra of these samples shift on increasing silica content, which indicates an increasing number of Fe-O-Si bonds within the ferrihydrite framework. MM reveals typical superparamagnetic (SPM) behaviour above a blocking temperature in the range 39 - 68 K which gives mean particle sizes of 4.2, 3.6 and 3.5 nm for 100/10 Fe/SiO2, 100/25 Fe/SiO2 and 100/50 Fe/SiO2 respectively, in good agreement with particle sizes determined using TEM (3.1±0.4, 2.4±0.3 and 2.4±0.3 nm respectively). MS data at 300 K and 4.2 K were fitted with distributions of ∆EQ and Bhf respectively. The median values of Bhf decrease with increasing silica content, indicating greater degrees of distortion in the Fe3+ environments induced by increased silica substitution. The reduction to metallic iron occurs via a three-step process viz. hematite → magnetite → wu¨stite → iron, with the silica stabilising the wu¨stite phase. The increasing amount of Fe-O-Si bonds on increasing silica content shifts reduction to higher temperatures broadens each reduction step as a result of local Fe-O-Si concentration variations. Fractions of each sample are not completely reduced even at 1000°C, with the relative proportion increasing with increasing silica content. In situ gas flow TEM studies reveal that the mechanism of reduction involves the liberation of atomic iron atoms from the silica-substituted iron oxides which agglomerate and grow into final iron particles. This leaves a poorly crystalline Fe-O-Si bonded framework behind. STEM-EDS and STEM-EELS reveal low concentrations of silicon at the surface of the resulting iron particles, however they do not form encapsulating shells of fayalite as previously suggested. The majority of the silica remains in the Fe-O-Si material which may crystallise into separate fayalite particles at elevated temperature. The presence of silica in high proportions (100/100 Fe/SiO2 and 100/200 Fe/SiO2) results in the formation of a two-phase system consisting of silicasubstituted 2-line ferrihydrite particles which are encapsulated in an ironinfused amorphous silica network. As with the other silica-bearing samples, there is an increase in Fe-O-Si bonds and an increase in the degree of distortion at Fe3+ sites with increasing silica content. The large amount of silica suppresses the blocking temperature of the SPM crystallites. In a hydrogen atmosphere, the reduction to metallic iron follows the same three step process as the other silica-bearing samples. Reduction temperatures are further shifted to higher values and given reduction steps are considerably broader with increasing silica content. The fraction of iron not fully reduced also increases. Iron particle diameters are very small, since encapsulation by the silica matrix prevents growth of particles.
- Full Text:
- Date Issued: 2016
Enantioselective transformations using tetrol as a chiral mediator
- Authors: Dorfling, Sasha-Lee
- Date: 2015
- Subjects: Enantioselective catalysis , Trichothecenes , Catalysts , Titanium
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:10445 , http://hdl.handle.net/10948/d1021195
- Description: (+)-(2R,3R)-1,1,4,4-Tetraphenylbutane-1,2,3,4-tetraol (TETROL) and its derivatives were reacted with varying molar ratios of titanium isopropoxide (2:1, 1:1 and 1:2 tetraol:titanium isopropoxide) in an attempt to prepare potential titanium-based tetraol catalysts for enantioselective transformations. In each case, infrared and HNMR spectra suggested that the product was formed. We tentatively proposed that the structure of the catalyst was a spiro-type, but we could not determine conclusively what its exact structure was, despite using numerous techniques at our disposal (molecular modelling calculations, H NMR and IR spectroscopy, thermal analyses, powder diffraction, and single crystal X-ray diffraction). The catalyst and derivatives thereof were able to act catalytically for the enantioselective additions of diethylzinc compounds to aldehydes. The effects of temperature and solvent were investigated, and toluene and -78 °C were selected as optimal from the results obtained. (The reaction could, however, not be maintained at this low temperature for extended periods due to the fact that we did not have, at our disposal, the correct equipment. Each 16 h reaction was thus allowed to reach room temperature in each case.) The selectivity for the product 1-phenylpropan-1-ol (when benzaldehyde was the starting aldehyde) varied depending on the nature of the aryl substituents of the titanium-based catalyst. Using 0.2 molar equivalents of the chiral titanates, the highest selectivity was 42 percent (e.e.), but only when excess Ti(O-i-Pr)4 had been added to the reaction mixture. This was achieved with the tetra(ortho-methoxyphenyl)-TETROLate derivative. TETROL and its derivatives were also successful in metal-free catalysis where higher conversions and selectivities were observed, compared to when these were complexed to titanium. The highest selectivity was 70 percent (e.e.), achieved with the tetra(ortho-methylphenyl)TETROL derivative.
- Full Text:
- Date Issued: 2015
Application of catalysts and nanomaterials in the design of an electrochemical sensor for ochratoxin A
- Authors: Flanagan, Shane Patrick
- Date: 2011 , 2010-12-06
- Subjects: Ochratoxins , Filamentous fungi , Electrochemical sensors , Nanostructured materials , Catalysts , Food contamination
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4121 , http://hdl.handle.net/10962/d1013328
- Description: Ochratoxin A is the most potent chlorinated derivative of the ochratoxin group, consisting of a 5'-chlorinated dihydroisocoumarin moiety linked by an amide bond to l-phenylalanine. Produced as a secondary fungal metabolite by several species of Aspergillus and Penicillium, ochratoxin A has been shown to readily contaminate a large variety of commodities including cereals, groundnuts, dried fruit, spices and coffee. This has led to widespread contamination of ochratoxin in wine, beer, milk and meat products. As ochratoxin A is a potent nephrotoxin exhibiting teratogenic and carcinogenic properties, the development of a rapid screening platform for the cost effective control of ochratoxin A content in foodstuffs is therefore required. The evaluation of metallophthalocyanine and carbon nanotube electrode modification toward the development of a nanostructured biosensor capable of enhancing the electrochemical detection of ochratoxin A in complex media is presented. Cyclic voltammetry at a glassy carbon electrode allowed for the optimization of detection parameters including pH and type of supporting electrolyte. Britton-Robinson buffer was found to be the most suitable supporting electrolyte in terms of sensitivity and reproducibility obtaining a LOD of 0.28 μM as determined by differential pulse voltammetry. Subsequent analysis determined the dependence of OTA oxidation on pH in acidic media which proceeds with the transfer of two electrons to form a quinone/hydroquinone couple shown to adsorb to the electrode surface. Passivation of the electrode through adsorption of oxidation products was shown to severely limit the detection of OTA upon successive detection cycles. Comparison of various metallophthalocyanine modifiers showed an increase in sensitivity toward the detection of OTA at phthalocyanine complexes with metal based redox processes. However with the exception of NiPc and CoTCPc complexes, phthalocyanine modification was limited by the increase in deviation of current response and extent of fouling. NiPc modification showed an increase in sensitivity by two fold with fouling characteristics comparable to an unmodified electrode while low improvements in fouling was observed at CoTCPc modified electrodes with sensitivity in detection comparable to an unmodified electrode.Modification of the electrode with multi- and single walled carbon nanotubes produced a significant increase in sensitivity toward the detection of ochratoxin A. The electrocatalytic activity of nanotube modifiers was attributed to the increase in surface area and to the addition of oxygenated functional groups upon acid treatment as confirmed by Raman spectroscopy. Acid functionalization of the carbon nanotubes for a period of two hours produced the greatest increase in sensitivity obtaining a respective LOD of 0.09 μM and 0.03 μM for analysis of ochratoxin A at multi- and single walled carbon nanotube modified electrodes. Centrifugal purification of carbon nanotubes was deemed necessary to improve the electrocatalytic activity of the nanotube modifiers through the removal of carbonaceous impurities as visualized by atomic force microscopy. Furthermore, a crude lipase preparation, lipase A, was investigated as a potential biological recognition element for selective detection of ochratoxin A in complex media. Lipase A enabled the hydrolysis of ochratoxin A to the electroactive species ochratoxin α as confirmed by thin layer chromatography and voltammetric analysis. Additional isolation of a pure hydrolase from the lipase A preparation is required prior to utilization within a nanostructured biosensor platform capable of detecting ochratoxin A in complex media.
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- Date Issued: 2011