Doctoral Degrees (Chemistry)

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2D3V electromagnetic particle-in-cell simulations of plasmas having kappa velocity distributions.
(2018) Abdul, Reginald Francis.; Mace, Richard Lester.
It is now well established that the kappa distribution is a more appropriate kinetic model for space plasmas than the Maxwellian distribution. In particular it possesses a power-law tail, frequently observed in space plasmas. The research presented in this thesis outlines the development of a two-dimensional electromagnetic particle-in-cell (PIC) simulation code, designed to run on general purpose graphics processing units (GPGPUs), and presents results from simulations of waves and instabilities obtained using it. While PIC simulations are not new, the majority have focussed on the old paradigm of initial particle loadings with a Maxwellian velocity distribution, or one of its variants. Distinguishing this research from previous PIC simulations is the use of the kappa distribution for the initial particle loading. To achieve this, a fast and e cient algorithm for generating multi-dimensional kappa distributed deviates was developed. The code is rst applied to the study of waves in an electron-ion plasma, in a stable equilibrium con guration with a constant background magnetic eld. Both species are modelled by isotropic (a) kappa and (b) Maxwellian velocity distributions. In each case, spectral analysis of the eld uctuations is performed, allowing mode identi cation. For parallel propagation, the maximum uctuation intensities follow the dispersion relations for the L and R modes, respectively, while those at perpendicular propagation follow the dispersion relations for the X, O and electromagnetic electron and ion Bernstein waves. The variation of wave intensity for the oblique angles is also investigated. For the kappa case, this yields new and important information presently unavailable by analysis alone. The e ects of the kappa distribution on wave intensity, as well as its e ect on the dispersion relations of the modes is discussed in detail. The second application is to the simulation of the electron temperature anisotropy driven whistler instability in an electron-ion plasma, where the electron species is modelled by the (a) bi-kappa and (b) bi-Maxwellian velocity distribution. For parallel propagation, the maximum eld uctuation intensities agree well with the dispersion relation for the whistler instability in a kappa plasma. While most of the wave intensity is in the parallel whistler mode, the oblique modes also contribute signi cantly to the overall uctuation spectrum, but their intensities vary with angle of propagation relative to the magnetic eld. The dependence of the growth rate on the index e of the electron kappa distribution is discussed in detail and compared with the well known Maxwellian results. Saturation of the instability via pitch angle scattering, reducing the electron temperature anisotropy, is observed.
An ab initio molecular orbital study of some binary complexes of water.
(1996) Tshehla, Tankiso Michael.; Ford, Thomas Anthony.
Ab initio molecular orbital theory has been successful in predicting the stabilities of many weak complexes; typical of these are the complexes formed between water and various small molecules. To account for the correlation effect, Moller-Plesset perturbation theory truncated at the second order level was employed. In order to account for the hydrogen bonding, the 6-3lG** basis set was used. The geometry optimisations of the complexes were carried out using the Gaussian-92 suite of programs installed on a Hewlett-Packard 720 computer operating under UNIX. The interaction energies of the complexes were subjected to further analysis by applying the Morokuma decomposition scheme. The electrostatic interaction component accounts for over 40% of the total stabilisation energy in all the typical hydrogen bonded complexes. Gas phase enthalpies were computed and compared with the experimental values of similar systems. For the systems studied here, the prediction is that all complexes are stable at 25° C. A second program, Vibra, was used for carrying out a normal coordinate analysis. A third computer program for the graphical representation of molecular and crystallographic models, Schakal-92, was employed to illustrate the predicted equilibrium geometries and the fundamental vibrational modes. The predicted geometries, interaction energies, charge redistributions, vibrational wave numbers, infrared intensities and force constants are listed and compared with those in the literature, where applicable. Correlations between the various predicted properties show some interesting chemistry.
AB inito studies of a pentacyclo-undecane cage lactam.
(2011) Singh, Thishana; Bissety, K.; Kruger, Hendrik Gerhardus.
Abstract available in PDF.
Activation of n-hexane using vanadium-exchanged zeolites.
(2010) Naicker, Thirusha.; Friedrich, Holger Bernhard.
The influence of the form of the ZSM-5 zeolite, vanadium content and the elimination of the exterior surface acidity on the activity and selectivity of n-hexane oxidation was studied using a fixed bed reactor. Blank reactor studies (carborundum packed reactor) showed no conversion below 450°C with the highest conversion (8%) at 500°C. The dominant products were found to be carbon oxides (Sel./% = 90) with minor selectivities to the hexene isomers (7%) and the remainder being cracked products, THF and benzene. H-ZSM-5 with different SiO2/Al2O3 ratios (100 and 320) and Na-ZSM-5 (SiO2/Al2O3 ratio of 100) were tested under non-oxidative and oxidative conditions. Under oxidative conditions as the ratio of the SiO2/Al2O3 increased, the aluminium content decreased and so too did the cracking ability of the zeolite (i.e. yield of cracked products dropped from 36% to 8%). However, the use of the Na- form of ZSM-5 completely eliminated acid cracking. Under oxidative conditions H-ZSM-5 (100) was found to be more active and resulted in higher formation of cyclic and aromatic compounds. With increasing time on-stream and higher temperatures the catalyst was found to deactivate. Evidence of this was seen by a decrease in surface area and pore volumes of the spent catalyst. The Na-ZSM-5 (100) showed lower activity, but deactivation was shown to be lower. These findings led to the investigation of vanadium ion-exchanged Na-ZSM-5 catalyst for n-hexane activation. Catalysts with different vanadium loadings were prepared using the solid state ion-exchange method. Catalysts were characterised using various methods. These techniques showed that vanadium was successful loaded onto the catalyst and that the highest vanadium loading that could be achieved was 2.5%. The lower loadings were not found to alter the catalyst structure while the highest loading of 2.5% was found to show some pore blockage and to possibly alter the structural environment of the zeolite. Time on stream experiments were conducted and temperature (350, 400 and 450°C), contact time (0.5, 0.8, 1.1 and 1.5 s) and fuel-air ratios (0.7, 1.3 and 2) were varied. The optimum conditions (Conv./% = 39) for terminally activated products were found using the Na-V-ZSM-5 (0.9%) at a temperature of 400°C, a contact time of 1.1 s and fuel-air ratio of 1.3. With the lower fuel-air ratio of 0.7 (oxygen rich conditions), hexanal formation was favoured. The Na-V-ZSM-5 catalyst could be regenerated with initial activity and selectivity being regained. Silanisation was found to be possible, however, the extent and degree of silanisation was difficult to control. Pore blockage was possibly responsible for the lower activity and selectivity obtained using the silanised Na-V-ZSM-5.
Activation of n-octane and cyclohexane to oxygenates using modified zeolites.
(2014) Cele, Mduduzi N.; Friedrich, Holger Bernhard.; Bala, Muhammad Dabai.
Three types of zeolites namely ZSM-5, Faujasite Y and MOF-5 were synthesized. The synthesis of Na-Fe-silicalite-1(34), H-Fe-silicalite-1(34), Na-Fe-silicalite-1(41), Na-Fe-silicalite-1(68), Na-Fe-silicalite-1(80), Fe-silicalite-1(128), Na-Fe-ZSM-5(66) and Na-Fe-ZSM-5(114) was conducted using a solid gel method. Further to this, Na-Fe-silicalite-1(41), Na-Fe-silicalite-1(80), Fe-silicalite-1(128), Na-Fe-ZSM-5(66) and Na-Fe-ZSM-5(114) were modified by silanisation using tetraethoxysilane (TEOS) as the silanisation agent to produce Na-Fe-silicalite-1(41:Sil), Na-Fe-silicalite-1(80:Sil), Na-Fe-silicalite-1(128:Sil), Na-Fe-ZSM-5(66:Sil), Na-Fe-ZSM-5(114:Sil). The numbers in brackets represent Si/Fe molar ratio while Sil represent silanisation. Powder XRD results showed that only the ZSM-5 phase was obtained. The second type was synthesized by encapsulation of Fe-TPP (tetraphenylporphyrin) inside faujasite Y to produce Fe-TPP-NaY. Powder XRD results confirmed the faujasite structure after encapsulation. Finally, Fe-MOF-5(1), Fe-Zn-MOF-5(0.5) and Fe-Zn-MOF-5(0.2) were synthesized using conventional methods with the numbers in brackets representing the Fe wt%. Also, XRD results showed that the MOF-5 phase was obtained with a sharp peak at 2θ below 10° which is characteristic of a highly crystalline material. All synthesized catalysts were tested in the oxidation of n-octane to oxygenates with H2O2 as the oxidant in MeCN. Furthermore, Fe-TPP-NaY was also used to activate 1-octene, 4-octene and cyclohexane while Fe-MOF-5 was used to activate cyclohexane. Na-Fe-silicalite-1(34), H-Fe-silicalite-1(34) and Na-Fe-silicalite-1(68) produced selectivities of 24, 2 and 27% respectively to terminal products at 80 °C in 13 mL MeCN. Furthermore, Na-Fe-silicalite-1(41), Na-Fe-silicalite-1(80), Fe-silicalite-1(128), Na-Fe-ZSM-5(66) and Na-Fe-ZSM-5(114) achieved selectivities to terminal products of 20.2, 28.1, 17.6, 24.5 and 21.3 respectively while Na-Fe-silicalite-1(41:Sil), Na-Fe-silicalite-1(80:Sil), Na-Fe-silicalite-1(128:Sil), Na-Fe-ZSM-5(66:Sil) and Na-Fe-ZSM-5(114:Sil) showed selectivities to terminal products of 20.7, 14.3, 12.3, 25.7 and 27.3 % respectively at 80 °C in 80 mL MeCN. Fe-TPP-NaY showed 13% selectivity to terminal products in oxidation of n-octane at 80 C in 13 mL MeCN. In the oxidation of n-octane using Fe-MOF-5 catalysts, selectivity to terminal products was found to increase with a decrease in the wt% of Fe. Hence, selectivities of 9.5, 12.9 and 20.7% were recorded for Fe-MOF-5(1), Fe-Zn-MOF-5(0.5) and Fe-Zn-MOF-5(0.2) respectively.
Adsorption of selected pollutants from aqueous solutions onto modified carbon nanotubes.
(2015) Oyetade, Oluwaseun Akinwole.; Martincigh, Bice Susan.; Nyamori, Vincent Onserio.; Jonnalagadda, Sreekantha Babu.
The significance of wastewater remediation before its discharge into the aquatic environment cannot be overemphasized. Adsorption has been proven to be effective for the removal of toxic pollutants from industrial effluents and/or wastewater, due to its simplicity in operation and the possibility of regenerating sorbents for reuse. This concept was exploited to achieve the effective removal of toxic contaminants from simulated wastewater. Carbon nanotubes, a fascinating member of the carbon family, possessing unique physical and chemical properties, have been reported as superior adsorbents for wastewater remediation purposes. Their large specific surface areas and porosity, hollow and layered structures, and great mechanical and thermal stability, makes them good candidates as sorbents for wastewater treatment and contamination control. This thesis interrogates the efficacy of carbon-structured nanomaterials containing multiwalled carbon nanotubes (MWCNTs) as the backbone, for the removal of divalent metal ions and organic contaminants from aqueous solutions. In this work, a novel adsorbent was successfully synthesized by incorporating a nitrogen-donor ligand (4-phenyl-2, 2':6', 2''-terpyridine) onto MWCNTs to afford nitrogen-functionalized MWCNTs (MWCNT-ttpy). The effectiveness of this sorbent towards the removal of divalent metal ions (Pb2+, Cd2+, Zn2+, Hg2+ and Cu2+), and organic contaminants (bisphenol A and ibuprofen) from aqueous solutions was investigated. The adsorption uptake of these pollutants onto MWCNT-ttpy was compared with that of acid-functionalized MWCNTs (MWCNT-COOH) to determine the sorbent with best removal efficiencies. Further, magnetic nanocomposites containing cobalt ferrite nanoparticles and MWCNT-COOH were synthesized in varying ratios to investigate their effectiveness for the removal of rhodamine B from aqueous solutions. All nanomaterials synthesized were characterized by means of TEM, SEM, TGA, BET, FTIR and Raman spectroscopy before application. Batch adsorption experiments were conducted to determine the effects of pH, contact time, adsorbent dose, initial adsorbate concentration and temperature for each sorption process in order to evaluate the best experimental conditions necessary for pollutant removal. The experimental data were fitted into the pseudo-first order, pseudo-second order, intraparticle diffusion and Elovich models to determine the dynamics and rate-determining step of the adsorption processes. The mechanism of the process was investigated by fitting the experimental data into various two- and three-parameter isotherms. iii The application of MWCNT-ttpy for the removal of both heavy metal ions and organic pollutants demonstrated much enhanced uptakes than MWCNT-COOH. The incorporation of nitrogen onto MWCNT-COOH significantly improved the affinity towards the removal of metal ions, forming strong electrostatic and coordination interactions between the active sites on the adsorbent and metal ion cations. Increasing hydrophobicity of MWCNT-ttpy over MWCNT-COOH accounted for the enhanced removal of bisphenol A and ibuprofen, since their uptake is primarily decided on by the hydrophobic nature of sorbates. Further, the application of both MWCNT-COOH and magnetic carbon nanotube-cobalt ferrites nanocomposites showed good removal efficiencies for rhodamine B from aqueous solution, with the best uptake achieved by using MWCNT-COOH. However, the magnetic nanocomposites give an advantage of separation under magnetic influence, hence, limiting inconveniences encountered during separation. The kinetics of adsorption were mostly described by the pseudo-second order and the Elovich models, while the equilibrium data were best described by the Langmuir and the Sips isotherm models. The thermodynamic parameters of adsorption, namely, the change in Gibbs energy (ΔGº), change in enthalpy (ΔHº) and change in entropy (ΔSº) were estimated for each adsorption process. The adsorption of all adsorbates were endothermic in nature except in the case of ibuprofen and Cd2+ which exhibited an exothermic process. All adsorption processes described in this study were spontaneous, implying the feasibility of the sorbents for the removal of targeted pollutants from wastewater. Desorption studies aimed at regenerating the adsorbents for reuse were successful. High recovery efficiencies between 60-95% were achieved by using eluents such as 0.1 mol dm-3 HCl for metal ions, and ethanol and acetone/acetic acid for organic contaminants. This process averts the production of secondary pollutants, supporting the reutilization of both the adsorbents and the adsorbates. Thus, all adsorbents used in this study were efficiently regenerated by using simple conventional chemicals and can be reused for the removal of targeted pollutants from aqueous solutions. The competitive adsorption of Pb2+, Cd2+, Zn2+ and Cu2+ and the binary adsorption of bisphenol A and ibuprofen onto MWCNT-ttpy was also investigated in both single-solute and multi-component adsorption systems. The sorption of metal ions onto MWCNT-ttpy was in the sequence Cd > Pb > Cu > Zn and Pb > Cu > Cd > Zn in single-solute and multicomponent systems, respectively, while the removal of ibuprofen was higher than that of bisphenol A in a typical binary adsorption system. For the first time, the competitive sorption of organic contaminants (bisphenol A and ibuprofen) in the presence of metal ions (Cd2+ and Pb2+) onto nitrogen-functionalized MWCNT was investigated. The iv study revealed a cooperative mechanism of adsorption between metal ions and organic pollutants in a multicomponent system. Thus, the novel adsorbent proved effective for the removal of metal ions, bisphenol A and ibuprofen in both single-solute and multicomponent adsorption systems. MWCNT-ttpy also proved remarkably effective for removing three heavy metal ions, Pb2+, Cu2+ and Zn2+, in three different real-life water samples, obtained from the Umgeni River. Removal efficiencies greater than 95% were achieved for all three metal ions. The modification of MWCNTs to afford both nitrogen-functionalized MWCNTs and cobalt-ferrite/MWCNT nanocomposites was successful. These sorbents exhibited excellent pollutant removal abilities, attributed to improved textural characteristics of the nanomaterials synthesized. The application of these sorbents for wastewater and industrial effluent remediation should be further explored for prudent management of water resources.
The alkaloids of the Amaryllidaceae : the isolation and structures of two new alkaloids from Haemanthus natalensis and Nerine krigeii and contributions to the chemistry of coccinine : the absolute configuration of alkaloids based on the 5:10b-Ethanophenanthridine nucleus.
(1960) Jeffs, P. W.; Warren, Frank Louis.
No abstract available.
The alkaloids of the Amaryllidaceae : the isolation of the alkaloids of Boophone disticha Herb., the structure of buphanitine and contributions to the chemistry of buphanidrine.
(1959) Goosen, Andre.
No abstract available.
Analysis and monitoring of persistent organic pollutants in the Umgeni River, KwaZulu-Natal, South Africa.
(2016) Gakuba, Emmanuel.; Moodley, Brenda.
Abstract available in PDF file.
Analytical and mycochemical studies on selected wild growing mushrooms of the Genus Boletus, Russula, Lactarius and Termitomyces distributed in KwaZulu-Natal, South Africa.
(2021) Rasalanavho, Muvhango.; Jonnalagadda, Sreekantha Babu.; Moodley, Roshila.
Abstract available in PDF.
The application of organically modified heterogeneous catalysts in continious flow hydrogenation reactions.
(2013) Miller, Stuart Frank.; Friedrich, Holger Bernhard.
No abstract available.
The application of semiconductors as oxidants in synthetic organic chemistry.
(2012) Jeena, Vineet.; Robinson, Ross Stuart.
Oxidation of alcohols is a vital transformation in synthetic organic chemistry as evidenced by their numerous applications in natural product synthesis. However, the traditional oxidants employ hazardous, toxic and malodorous reagents. A welcome addition to the field of alcohol oxidation is the emergence of tandem coupling reactions in which the oxidized alcohol is immediately trapped by an appropriate nucleophile. In previous research within the group, the synthesis of quinoxalines using a photocatalyzed tandem coupling approach was demonstrated. However, an extension of this research to other tandem coupling reactions was unsuccessful due to the high redox potential of the active oxidizing species. Thus, the immediate goal of this project was to develop a photocatalyzed oxidative system that was selective and high yielding. The assembling of this photooxidative system began with the choice of TiO2 and ZnO as the photocatalysts, Alizarin Red S as the dye, silver (I) ions as the electron acceptor and 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) as the active oxidizing species. A test reaction was conducted using the dye sensitized TiO2/silver/TEMPOand dye sensitized ZnO/silver/TEMPO systems. While the dye sensitized ZnO system afforded a good yield the dye sensitized TiO2 system produced only a trace amount of product. Thus, a full study was conducted on a range of alcohols using the developed dye sensitized ZnO system. In addition, the oxidation of alcohols using the dye sensitized ZnO system could also be scaled-up with notable success. The developed system was applied to a one-pot tandem Wittig reaction which unfortunately was unsuccessful. The dye sensitized ZnO/silver/TEMPO system was however successful when applied to a pseudo- tandem Wittig reaction. Subsequently, an electron paramagnetic resonance (EPR) study was conducted using the dye sensitized TiO2/silver/TEMPO and the dye sensitized ZnO/silver/TEMPO system. The results of the EPR study supported the proposed mechanism for the dye sensitized ZnO system and revealed a ‘break’ in the photooxidative chain for the dye sensitized TiO2 system which accounted for the low yields obtained. As part of our interest in developing new photocatalyzed oxidative systems, attention was directed towards the application of impure diamondoid powder as a potential tandem coupling reagent. Using the diamondoid powder, the synthesis of highly conjugated quinoxalines was effected in moderate to excellent yields.
The application of “PNP” aminodiphosphine complexes in the oxidation of n-octane and styrene.
(2015) Naicker, Dunesha.; Friedrich, Holger Bernhard.
The oxidation of hydrocarbons provides a cost effective method of converting cheap starting material to bulk chemicals and more importantly in the synthetic transformation to fine chemicals. Transition metals effectively catalyze these oxidation reactions. However, the use of a good ligand system is imperative in controlling the activity of the metal complexes. Aminodiphosphine or “PNP” ligands have been used extensively in ethylene oligomerisation with chromium as the active metal. In this study six PNP ligands were synthesized and the substituent on the nitrogen atom was varied by making use of alkyl substituents such a cyclohexyl, iso-propyl and pentyl, as well as phenyl and substituted phenyl (chlorophenyl and methoxy phenyl) substituents. The ligands were complexed to the transition metals Co, Rh, Ir and Ru. These new bidentate complexes were fully characterized by NMR analysis, IR spectroscopy, HRMS and melting point determination. X-ray quality crystals were grown for eight of the metal complexes (all novel, R% < 10). These complexes were then compared in the oxidation of styrene and n-octane. This includes the comparison of two structural types of "PNP" cobalt complexes having the cyclohexyl, isopropyl and pentyl substituents on the nitrogen atom. In the oxidation of n-octane, the complex with the flexible ligand backbone showed higher activity. The ketones were the dominant product with highest selectivity to 2-octanone (34%). In the oxidation of styrene under optimum conditions, the complexes bearing the rigid ligand backbone were most active with good yields to benzaldehyde (25%). In the oxidation of styrene, of the six Ir and Rh complexes investigated, the Ir complexes were slightly more active than the Rh complexes, with the complex bearing the chlorophenyl substituent on the nitrogen atom being the most active (88% conversion). Higher yields to benzaldehyde than styrene oxide were obtained. In the oxidation of n-octane, the ketones were the dominant product formed over both the Ir and Rh catalysts. For both studies the catalysts were recovered and reused over 3 cycles. Ruthenium catalysts bearing the alkyl substituents were also applied in both oxidation studies. In the oxidation of styrene, > 80% conversion was obtained with a greater yield to benzaldehyde. In the oxidation of n-octane, the alcohols were the dominant product with good selectivity to 2 and 3-octanol (> 23%)
Approaches to the total synthesis of a novel diarylheptanoid
(2008) Vela, Nomandla Magnificent
The total synthesis of a novel diarylheptanoid isolated from a South African medicinal plant, Siphonochilus aethiopicus, was investigated. S. aethiopicus (Indungulu in Zulu) is the only South African species of the Zingiberaceae plant family and is widely used in traditional medicine. One of the compounds isolated from this plant is a novel diarylheptanoid. Diarylheptanoids constitute a distinct group of natural plant metabolites characterized by two aromatic rings linked by a linear seven-carbon aliphatic chain, with varying functional groups on the aryl and the aliphatic chain. The target molecule for our synthesis contains two highly oxygenated aryl rings linked by an aliphatic chain with two stereogenic centres and a trans-alkene. In this study we present our investigation of different strategies to a viable synthetic method that could provide material to supplement the relatively small quantity of product that can be isolated from the plant extract. The major challenges of this synthesis were to develop procedures for the preparation of the homobenzylic trans-alkene, the stereogenic centres and to attach the electron-rich aromatic rings to the aliphatic chain. In this thesis the following aspects are described: • Various types of olefination reactions (including Wittig, Julia and organometalic-mediated type of olefination reactions) • Various types of alkylation reactions (including Grignard, Friedel-Crafts and organometalic-mediated type of alkylation reactions) • Incorporation of the stereogenic centres (including asymmetric hydroxylation and use of chiral starting materials) The synthesis will not only give a viable synthetic route to the target compound but is also versatile enough to allow the preparation of analogues.
Artificial neural networks for the classification of Meliaceae extractives.
(1998) Fraser, Leigh-Anne.; Mulholland, Dulcie Aca.
The goal of this project was the development of a computer-based system using artificial intelligence to classify the limonoids, protolimonoids and triterpenoids isolated from the family Meliaceae by the Natural Products Research Group of the University of Natal, Durban. A database of samples was obtained between 1991 and 1996, part of which time the author was a member of the group and isolated compounds from Turraea obtusifolia and Turraea floribunda. Over and above the problem of complexity and similarity in structures of the above mentioned natural products, are other difficulties. These include very small amounts of sample being isolated producing very weak peak signals in the C-13 NMR spectra, extraneous peaks in the NMR spectra due to different impurities and instrument noise, non-reproducible spectra due to the pulsed Fourier transform intervals and the nuclear Overhauser effect, impure samples often isolated as stereoisomeric mixtures or as mixed esters and superposition of peak signals in the NMR spectra due to carbons in the same environment within the same compound. These factors make identification by traditional computational and expert systems impossible. As a result of these shortcomings, the author has developed a novel approach using artificial neural network techniques. The artificial neural network system developed used real data from the 300 MHz NMR spectrometer in the Department of Chemistry, Durban. The system was trained to discriminate between limonoids, triterpenoids and flavonoids/coumarins from the C-13 NMR spectra of pure, impure and unseen compounds with an accuracy of better than 90%. Further differentiation of the glabretals from the rest of the protolimonoids as well as from the rest of the triterpenoids showed similarly significant results. Finally, individual limonoid discrimination within the limonoid dataset was extremely successful. Apart from its application to the extractives from Meliaceae, the methodology and techniques developed by the author can be applied to other sets of extractives to provide a robust method for the spectral classification of pre-identified natural products.
Assessment of antiretroviral drugs uptake by vegetables from contaminated soil and their adsorption by exfoliated graphite in river and wastewater.
(2022) Kunene, Philisiwe Nganaki.; Mahlambi, Precious Nokwethemba.
This study was directed toward vegetable uptake of the commonly used antiretroviral drugs (ARVDs), abacavir, nevirapine, and efavirenz. Antiretroviral drugs are used to treat the human immune-deficiency virus (HIV). South Africa (SA) is one of the countries with a high number of infected people on ARV therapy, therefore, the ARVDs are anticipated to be existing at high concentrations in the South African environment than in other countries worldwide. In recent years, the presence of ARVDs in the environment has drawn attention; hence studies have reported their presence in aquatic environments while very few studies have been conducted on their uptake using vegetables. This work was therefore based on the optimization and application of sensitive, simple, cost-effective, and robust techniques for quantifying ARVDs in vegetables. Based on this information, ultrasonic extraction (UE) and microwave-assisted extraction (MAE) were used to isolate target compounds from vegetable samples to the aqueous phase. Dispersive liquid-liquid microextraction (DLLME) and solid-phase extraction (SPE) were utilized to preconcentration and clean up the extracts from UE and MAE, respectively. A liquid chromatography photodiode array detector (LC-PDA) was utilized to detect and quantify the extracted compounds. The UE with and without DLLME cleanup were compared with each other, also, MAE with and without SPE cleanup were compared with each other. The methods comparison was done in terms of their detection (LOD) and quantification limits (LOQ), extraction efficiencies (%Recovery), relative standard deviations (%RSD), and concentrations of ARVDs found in vegetable samples. In comparison of UE and ultrasonic-assisted dispersive liquid-liquid microextraction (UADLLME), the LOD and LOQ obtained ranged between 0.0081 - 0.015 μg/kg and 0.027 - 0.049 μg/kg for UE and 0.0028 -0.0051 μg/kg and 0.0094 - 0.017 μg/kg for UADLLME respectively. High recoveries ranging from 93 to 113% in UE and 85 to 103% in UADLLME with less than 10% RSD in both procedures were obtained. These results indicated that UADLLME is more sensitive than the UE method, although they are both accurate and precise. The UE can be recommended for routine analysis as UADLLME showed the inability to extract analytes from root vegetables. The optimized UE and UADLLME methods were applied to extract ARVDs from vegetables bought from local fruit and veggie supermarket. Vegetables were categorized as root (carrot, potato, and sweet potatoes), leaf (cabbage and lettuce), and fruit (green paper, butternut, and tomato). The target ARVDs were quantified in most samples with concentrations up to 8.18 μg/kg. The concentrations obtained were slightly high in UADLLME than in UE as a result of its high sensitivity. Efavirenz was the most dominant drug, while the potato was the most contaminated vegetable. In the comparison of MAE and MAESPE, the obtained LOD and LOQ ranged from 0.020 to 0.032 μg/kg and 0.068 to 0.109 μg/kg for MAE and 0.019 to 0.066 μg/L and 0.065 to 0.22 μg/L for MAE-SPE. The obtained recoveries ranged from 85 to 103% for MAE and from 82 to 98 % for MAE-SPE, respectively, and the RSDs were all less than 6%. These results showed that both methods have comparable sensitivity; however, the recoveries values for MAE were slightly higher than those obtained in MAE-SPE, which signals MAE’s high accuracy. The optimized MAE and MAE-SPE methods were applied to remove ARVDs in the root (potatoes, onions, and beetroot), leaf (lettuce, and spinach), and fruit (green paper, cucumber, and eggplant) vegetables bought from local fruit and veggie supermarket. The obtained ARVDs concentration range was 1.48 ± 0.5 - 27.9 ± 1.2 μg/kg. The MAE-SPE resulted in low concentration compared to MAE without cleanup. Beetroot exhibited high concentrations of the target ARVDs, while nevirapine was found to have high concentration and as a dominant compound. The results obtained revealed that the vegetables from the studied area are contaminted with ARVDs, which could indicate their possible irrigation with wastewater effluent or the use of sludge as biosolids in the agricultural areas. This is a concern as it leads to unintentional consumption by consumers which could lead to drug resistance by the human body or have human health effects. The study was then expanded by conducting the phytoremediation approach to investigate the uptake of abacavir, nevirapine, and efavirenz by beetroot, spinach, and tomato from the contaminated soil. The three selected vegetable plants were planted and irrigated with ARVDs spiked (at 2000 and 5000 μg/L) water over a period of three months. The optimized UE and LC-PDA methods were used to extract and quantify the selected ARVDs from the target vegetables and soil. The obtained results showed that the studied vegetables have the potential to take up abacavir, nevirapine, and efavirenz from contaminated soil, be absorbed by the root, and translocate to the aerial part of the plants. Abacavir was found at high concentrations to a maximum of 40.21 μg/kg in the root, 18.43 μg/kg in the stem, and 6.77 μg/kg in the soil, while efavirenz was the highest concentrations, up to 35.44 μg/kg in leaves and 8.86 μg/kg in fruits. Spinach root accumulated more ARVDs than beetroot and tomato. The bio-accumulation factor ranged from 2.0-14 μg/kg in beetroot, 3.6 - 15 μg/kg in spinach, and 6 – 10 μg/kg in tomato. The root concentration factor range was 0.047 – 17.6 μg/kg; 0.34-5.9 μg/kg, and 0.14-2.82 μg/kg in beetroot, spinach, and tomato, respectively. The translocation factor range obtained was 0.40 – 38 μg/kg, 0.08 – 19 μg/kg, and 0.14 – 49 μg/kg in beetroot, spinach, and tomato, respectively. However, the accumulation of ARVDs in all studied plants showed that they could be used in phytoremediation. The results obtained in the phytoremediation approach revealed that the utilization of the contaminated water has an influence on the presence ARVDs in vegetables; hence this work also focused on evaluating the exfoliated graphite adsorption of ARVDs in water. Natural graphite was intercalated with acids and exfoliated with thermal shock to obtain the exfoliated graphite. The scanning electron microscopy images showed that the exfoliated graphite had increased c-axis distance between the layers with accordion-like structure which were confirmed by the lower density of exfoliated graphite material (0.0068 gmL-1) compared to the natural graphite (0.54 g mL-1). Fourier Transformed Infrared Spectroscopy results showed the C=C in natural and exfoliated graphite at 1635 cm-1 stretching. The phenolic, alcoholic, and carboxylic groups were observed from 1000 to 1700 cm-1 for the intercalated and exfoliated graphite. The Energy-dispersive X-ray results further confirmed these results, which showed carbon and oxygen peaks in the intercalated and exfoliated graphite spectrum, whereas natural graphite showed only a carbon peak. Raman spectroscopy results showed that the material’s crystallinity was not affected by the intercalation and exfoliation processes as observed from the ratios of the G and D peaks and the G' and D'. Natural, intercalated and exfoliated graphite contained the D, G, D', and G' peaks at about 1350 cm-1, 1570 cm-1, 2440 cm-1, and 2720 cm-1, respectively. The exfoliated graphite material showed the characteristic of a hexagonal phase graphitic structure by (002) and (110) reflections in the X-ray diffraction results. The exfoliated graphite adsorption method was optimized based on the pH of a solution, adsorbent dosage, and adsorption time prior to application to water samples. The optimum pH solution, adsorbent dosage, and adsorption time were 7, 30 mg, 0.01 μg/L, and 30 minutes respectively. The kinetics and isotherm studies were conducted to assess the model that best fit and explain the experimental data obtained. The kinetic model and adsorption isotherm studies showed that the experimental data fit well pseudo-second-order kinetics and is well explained by Freundlich’s adsorption isotherm. The maximum adsorption capacity of the exfoliated graphite (EG) for ARVDs ranges between 1.660-197.0, 1.660-232.5, and 1.650-237.7 mg/g for abacavir, nevirapine, and efavirenz, respectively. These results showed that under proper operating conditions, the EG adsorbent could potentially be applied as a water purifying tool for the removal of ARVDs pollutants.
Asymmetric synthesis with an ephedrine based chiral auxilliary.
(1992) Malissar, Dean Graham Shane.; Drewes, Siegfried Ernst.
Abstract available in pdf file.
Boron- and nitrogen-doped carbon nanotubes : synthesis, characterization and application in solar cells.
(2015) Keru, Godfrey Kamitha.; Nyamori, Vincent Onserio.; Ndungu, Patrick Gathura.; Mola, Genene Tessema.
Abstract available in PDF file.
The Brix-Free water capacity and sorption behaviour of fibre components of sugar cane (Saccharum officinarum).
(2008) Hoi, Yin Lun Wong Sak.; Martincigh, Bice Susan.; Lionnet, Georges Raoul Edouard.; Autrey, Louis Jean Claude.
Milling data from sugar factories in Mauritius were examined from 1960 to 2004 to assess the trend in the quality of cane received at mills and the change in factory performance. A deterioration in overall quality was apparent due to the increased level of extraneous matter delivered in the cane supply. Comparison was made with available data from other countries in the world, notably those of South Africa and Australia. Controlled addition of extraneous matter to clean cane was effected under laboratory conditions to determine the relative impact of dry leaves, green leaves and cane tops on the quality of cane and the resulting juice, and to predict through derived equations, their impact on cane processing. The addition of dry leaves was found to have the most adverse effect followed by green leaves and cane tops. In the case of dry leaf addition to cane the detrimental effects were found to be masked by an increase in the concentration of solutes in the juice extracted. This phenomenon was thought to be due to the selective sorption of water (so-called Brix-free water) by dry leaves. To test this assertion, the sugar cane stalks of four different cane varieties aged 52, 44 and 36 weeks were separated into their component parts by means of a method devised in this work. There were nine component parts: stalk fibre, stalk pith, rind fibre, rind fines, top fibre, dry leaf fibre, dry leaf fines, green leaf fibre and green leaf fines which, on characterisation by Fourier transform infrared spectroscopy and scanning electron microscopy, were very similar except that stalk pith was more flaky and had a higher surface area than the others. Various analytical techniques were tested for the determination of Brix-free water. The most convenient method proved to be a refractometric method which was improved so as to be applicable to the wide range of cane components fibres studied. Statistical analysis of the Brix-free water content of the separated samples showed that when the combined effect of fibre and pith in the cane stalk of three ages was considered, the four cane varieties were not different. This was not the case for dry leaf, green leaf, top and rind. Of the nine cane components, stalk pith exhibited the highest Brix-free water value of about 20 g/100 g fibre, whereas all the other components exhibited values of about 15 g/100 g fibre, which are much lower than the traditionally accepted value of 25% for cane. The latter was found to be the fibre saturation point of bound water determined at 20 oC, which is the sum of dissolved and hydrated waters, and which is normally greater than the Brix-free water value as determined in this work. The water sorption characteristics of the various cane component parts were further investigated by making measurements to determine the equilibrium moisture contents at various water activity values. These data were used to construct adsorption isotherms. These were fitted to 17 existing isotherm models, of which two, namely, the Hailwood-Horrobin and Guggenheim-Anderson-de Boer models, gave the best fit. The sorbed water was subsequently characterised in terms of various parameters, namely, the monolayer moisture content, the number of adsorbed monolayers, the percentage of bound water, the total surface area for hydrophilic binding, the heats of sorption of the monolayer and multilayers, the net and total isosteric heats of sorption and the entropy of sorption. From the monolayer moisture content and the amount of “hydrated water” as calculated from the Hailwood-Horrobin model, it is clear that at EMC values between 0 and 5% (aw = 0 – 0.3), the non-freezable water is tightly bound to the surface of the fibre. The second region starts at EMC values from 5% to 10 – 15% (aw = 0.3 to 0.6 – 0.8) depending on the cane components, and the bound water in this region is termed the freezable water. The third type of water is essentially free water, it exists after the second region and ends at EMC values of about 25%. From this study, it is apparent that the Brix-free water as measured in this work measures the amount of water bound in the first two regions.
Carboxamide ruthenium(II) and manganese(II) complexes: structural, kinetic, and mechanistic studies in the transfer hydrogenation of ketones.
(2022) Kumah, Robert Tettey.; Ojwach, Stephen Otieno.
The carboxamide ligands N-(benzo[d]thiazol-2-yl)pyrazine-2-carboxamide (HL1), N-(1H-benzo[d]imidazol-2-yl)pyrazine-2-carboxamide (HL2), were prepared by condensation of pyrazine-carboxylic acid and appropriate heteroaromatic amines. Reactions of HL1 and HL2 with ruthenium(II) precursors, [RuH(CO)Cl(PPh3)3] and [RuH2(CO)(PPh3)3] afforded the mononuclear complexes [RuL1(PPh3)2(CO)Cl] (Ru1), [RuL1(PPh3)2(CO)H] (Ru2), [RuL2(PPh3)2(CO)Cl] (Ru3), [RuL2(PPh3)2(CO)H] (Ru4). The solid-state structures of complexes Ru1, Ru2, and Ru4 reveal bidentate modes of coordination of the ligands and distorted octahedral geometries around the Ru(II) centre. The complexes formed active catalysts in the transfer hydrogenation of ketones and achieved turnover number (TON) up to 530 in 6 h. The ruthenium(II)–hydride complexes, Ru2 and Ru4, were capable of catalysing transfer hydrogenation of ketones reactions under base free reaction conditions and demonstrated higher catalytic activities compared to the corresponding non-hydride analogues (Ru1 and Ru3). An inner sphere monohydride mechanism involving dissociation of one PPh3 group was proposed from in situ 31P{1H} NMR spectroscopy studies. Dipicolinamide ligand system, N,N'-(1,4 phenylene)dipicolinamide (H2L3), N,N'-(1,2-phenylene)dipicolinamide (H2L4), N,N'-(4,5-dimethyl-1,2-phenylene)dipicolinamide (H2L5), N,N'-(4-methoxy-1,2-phenylene)dipicolinamide (H2L6) were synthesised following a similar protocol described for HL1 and HL2. Treatment of the ligands H2L3 and H2L4 with RuH(CO)Cl(PPh3)3 afforded bimetallic complexes [Ru2(H2L3)(PPh3)4(CO)2][2Cl] (Ru5), [Ru2(H2L3)(H)2(PPh3)4(CO)2] (Ru5b), [Ru2(HL4)(PPh3)3(CO)2Cl3] (Ru6) and a mononuclear complex [RuCl2L4(PPh3)2(CO)] (Ru7). The solid-sate structure of the dinuclear ruthenium(II) complexes confirmed a bidentate coordinate mode, with PPh3, CO, and chlorido auxiliary ligands occupying the remaining coordinating sites to afford distorted trigonal bipyramidal geometries (Ru5 and Ru6) while the mononuclear complex Ru7 adopted a distorted octahedral geometry around its ruthenium(II) atom. The reaction of the ligands H2L4-H2L6 with the [RuCl2-η6-p-cymene]2 precursor produces half-sandwich diruthenium complexes [{Ru(η6-p-cymene)}2-μ-Cl(L4)][Ru(η6-p-cymene)Cl3] (Ru8), [{Ru(η6-p-cymene)}2-μ-Cl(L4)][PF6] (Ru9), [{Ru(η6-p-cymene)}2-μ-Cl(L5)][PF6] (Ru10), and [{Ru(η6-p-cymene)}2-μ-Cl (L6)][PF6] (Ru11). The molecular structure of cationic complexes, Ru8-Ru11, was confirmed by single-crystal X-ray crystallography analysis. The complexes Ru8-Ru11 display a bidentate Npyridine ^ Namidate mode of coordination to give pseudo-octahedral geometry (piano-stool-like geometry). The ruthenium(II) complexes demonstrated remarkable enhanced catalytic activity (TON values up to 1.71 x 104) in the transfer hydrogenation of ketones at a very low catalyst loading of 2.75 x10-2 mol% (275 ppm). The dinuclear ruthenium(II) complexes showed higher catalytic activity compared to the corresponding mononuclear complex Ru5. The half-sandwich diruthenium complexes Ru8-Ru11 displayed relatively higher catalytic activity than the ruthenium complexes Ru5 and Ru6 bearing the PPh3 co-ligands. Monohydride inner-sphere catalytic cycle was proposed for the transfer hydrogenation of ketones catalysed by both Ru1 and Ru9, and the formation of the reactive intermediates was supported with low-resolution mass spectrometry data. The dinuclear ruthenium complexes of pyridine and pyrazine-carboxamide bearing quinolinyl motif were synthesised by reacting, N-(quinolin-8-yl)pyrazine-2-carboxamide, (HL7), 5-methyl-N-(quinolin-8-yl)pyridine-2-carboxamide, (HL8), 5-chloro-N-(quinolin-8-yl)pyridine-2-carboxamide, (HL9), and 2-pyrazine-carboxylic acid (HL10) with equimolar [RuCl2(η6-p-cymene)]2 to afford the dinuclear complexes [{Ru(η6-p-cymene)}2Cl3(L10)] (Ru12), [{Ru(η6-p-cymene)Cl}2(L7)] [PF6] (Ru13), [{Ru(η6-p-cymene)Cl}2(L8)][Ru(L8)Cl3] (Ru14), and [{Ru(η6-p-cymene)Cl}2(L9)][PF6] (Ru15), respectively. The solid-state structures of the dinuclear complexes Ru12 and Ru13 reveal a typical piano-stool geometry around the Ru(II) ions. The dinuclear ruthenium complexes Ru12-Ru15 were used as catalysts in the transfer hydrogenation of a broad spectrum of aldehydes and ketones and demonstrated excellent catalytic activity, TON values up to 4.8 x 104, using catalyst loading of 2.0 x10-3 mol% (20 ppm). The catalytic performance of the complexes was affected by the ligand architecture and the substituents on the pyridyl ring. Complexes Ru13-15 exhibited higher catalytic activities compared to the complex Ru12 which could be ascribed to the role of quinoline in stabilising the complexes. The pyridine and pyrazine motifs have a significant impact on the reactivity and the catalytic activity of the complexes. In-situ low-resolution ESI-MS analyses of the reactive intermediates aided in proposing a monohydride inner-sphere mechanism for the transfer hydrogenation of ketones catalysed by Ru15. To develop a more sustainable, environmentally compatible and cost-efficient protocol for transfer hydrogenation of ketones, a new catalytic system based on manganese(II) metal was synthesised. New manganese(II) complexes Mn1-Mn4, ligated on dipicolinamide ligands were synthesized by treating the N,N'-(1,4-phenylene)dipicolinamide (H2L3), N,N'-(1,2-phenylene)dipicolinamide (H2L4), N,N'-(4-methoxy-1,2-phenylene)dipicolinamide (H2L5) and N,N'-(4,5-dimethyl-1,2-phenylene)dipicolinamide (H2L6) with MnCl2.4H2O salt to afford dinuclear manganese(II) complexes [Mn2(H2L3)2Cl4] (Mn1), [Mn2(H2L4)2Cl4] (Mn2), [Mn2(H2L5)2(Cl)4] (Mn3) and [Mn2(H2L6)2Cl4] (Mn4). The solid-state structure of complex Mn2 showed a six-coordinate dinuclear complex with the two Mn(II) ions adopting a distorted octahedral environment surrounded by two tetradentate ligands and chlorido co-ligands, respectively. The Mn(II) complexes formed active catalysts in transfer hydrogenation of ketones to achieve TON values up to 5.12 x 104. The presence of electron-donating substituents -OCH3 and -CH3 in complexes Mn3 and Mn4 displayed minor effects in the transfer hydrogenation of ketones. The new carboxamide-manganese(II) complexes are among the most active manganese-based catalysts capable of hydrogenating a large scope of ketones ranging from aliphatic to aromatic ketones. A dihydride catalytic cycle has been proposed and supported with in-situ low-resolution mass spectrometry data.

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