3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Hydrodechlorination of aromatic compounds.(1998) Karparova, Marina AlexandrovaIt is well known that chlorinated organic compounds behave as highly toxic substances in the environment. Catalytic hydrodechlorination .... one of the more convenient approaches used to treat chlorinated organic substances with the aim of obtaining compounds with lower or null toxiciy. Further, the selective cleavage of the C-Cl bond plays a key role in processes related to the manufacture of fine chemicals. These reactions can be performed over noble-metal catalysts, either in the gas or liquid phase. (Abbreviation abstract)Item Development of an optimized process for commercial production of (-) ambrafuran(2016-05-11) Mongwe, JoyAmbergris is a large lump excreted by whales, when exposed to sunlight, air and sea water, it oxidatively decomposed through inorganic reactions to form different compounds including (-) ambrafuran. (-) Ambrafuran is a highly fragrant compound and it is considered to be a stronger perfume ingredient than other constituents in ambergris. It is used as a fixative agent to stabilize perfumes by reducing the rate of evaporation of volatile substances. A two-step process for production of (-) ambrafuran starting from sclareol was developed at CSIR for an industrial partner, Teubes cc. The current commercial production of (-) ambrafuran entails a chemical process consisting of at least 8 steps and require very harsh chemicals and elevated temperatures. In the current study, relevant technologies for the optimization of a process for commercial production of (-) ambrafuran were investigated. The project objective has been to optimize fermentation conditions on laboratory scale for the conversion of sclareol to an intermediate diol using the microorganism Hyphozyma roseoniger and to subsequently test different zeolites for conversion of diol to (-) ambrafuran. Production of ambradiol was achieved in potato dextrose broth media in 13 days compared to a patented method which took 16 days. The method was also scaled-up in a 2 L fermentation bioreactor and the yield of 93% was achieved after 24 hours of reaction. Following the initial use of the zeolite CBV320, two new zeolites (CFG-1 and ZD0614) were identified which have the potential to convert ambradiol to (-) ambrafuran without undergoing an activation process. Zeolite CFG-1 have been recognized to be highly effective for converting the intermediate ambradiol to the resulting (-) ambrafuran. One of the greatest outcomes of this research project is that the amount of zeolite required per substrate has been reduced from between 1:6 and 1:9 to 1:2. The substrate concentration has been increased from 5 mg/mL to 100 mg/mL which also resulted in the reduction of the volume of solvent required for the cyclodehydration step. The study allowed for scale-up and following further optimisation on larger scale should result in a process on commercial scale.Item The suzuki-miyaura cross coupling reaction as a key step for the synthesis of oxygen and nitrogen containing hetero-aromatic compounds(2016-01-22) Pradeep, PriyamvadaThe first two chapters of this thesis deals with the synthesis of 6H-benzo[d]-naphtho[ 1,2- b]pyran-6-one motif found in gilvocarcin as well as related aromatic compounds containing the aromatic pyranone moiety. The synthesis was undertaken by employing the Suzuki- Miyaura cross coupling reaction and a novel N-bromosuccinimide induced ring cyclization reaction to afford the pyranone. It was established that the treatment of both [2-(1,4- dimethoxynaphthalen-2-yl)phenyl]methanol and (2',5'-dimethoxy-[1,1'-biphenyl]-2- yl)methanol separately with N-bromosuccinimide results in the unexpected synthesis of a naphthopyranone ring system in the form of 12-methoxy-6H-dibenzo[c,h]chromen-6-one and 2-methoxy-6H-benzo[c]chromen-6-one respectively. Application of the same methodology for the attempted synthesis of related compounds namely, 1-hydroxy-12-methoxy-6Hdibenzo[ c,h]chromen-6-one and 8-fluoro-12-methoxy-6H-dibenzo[c,h]chromen-6-one unfortunately did not generate the desired results. Attempts were made to elucidate the mechanism of this reaction. The most apparent mechanism indicates that Nbromosuccinimide, in the presence of air, oxidizes the benzylic alcohol to an aldehyde which is then converted to an acid bromide allowing for the ring closure with the adjacent aromatic ether to afford the desired pyranone. In Chapter 3 and 4 of this thesis we dealt with the synthesis of benzo[b]phenanthridine-7,12- dione motif, the backbone of biologically important secondary metabolite jadomycin B. Again, a key step involves employing the Suzuki-Miyaura cross coupling reaction. The synthetic methodology also sheds some light on the dynamics of the ring closure of benzylic amines onto naphthoquinones resulting in the synthesis of benzo[i]phenanthridine-11,12- dione, 12-methoxybenzo[i]phenanthridine and 1-hydroxybenzo[i]phenanthridine-11,12-dione. The synthesis of benzo fused phenanthridines has been undertaken in Chapter 5 and 6 by employing Suzuki-Miyaura cross coupling reaction and a potassium t-butoxide and light mediated cyclization reaction as the key steps. The synthesis of 5- phenylbenzo[i]phenanthridine was undertaken successfully but attempts to execute the same methodology to form a compound library of related benzo-fused phenanthridines was unsuccessful. The same methodology employing a Suzuki-Miyura cross coupling reaction and potassium tbutoxide and light mediated cyclization reaction was applied in Chapter 7 and 8 of the thesis directed towards the synthesis of 13H-indolo[3,2-c]acridine and 3-methoxy-13H-indolo[3,2- c]acridine. The successful synthesis of the 13H-indolo[3,2-c]acridine is reported using this methodology.Item Biocatalytic synthesis of novel oxidized aromatic compounds as potential anti-bacterial and anti-cancer agents(2016-01-19) Ogunleye, TozamaAccording to the World Health Organization (WHO), cancer is a leading cause of death worldwide and has accounted for 7.6 million deaths (13% of all deaths) in 2008. The number of effective drugs available has been reduced by chemo resistant malignant tumors. Similarly, bacterial infections are one of the world’s most pressing public health problem. The major challenge in anti-bacterial treatment is due to the development of bacteria strains that are resistant to antibiotics. Each year more than 11 million people die from major infections such as MDR tuberculosis. In 2013, 9 million people fell ill with TB and 1.5 million died from the disease (WHO). Therefore there is a need for novel therapeutic alternatives such as the discovery of new anti-cancer and anti-bacterial agents. Benzofurans have attracted much attention due to their broad spectrum of pharmacological activities such as anti-cancer and anti-bacterial activities and one classical example is usnic acid. Most of the published synthesis of the benzofuran moiety involved the formation of annellated furan ring by intramolecular cyclisation of benzene, and these procedures involved a multi-step, rigorous reaction conditions and expensive catalyst. This research investigated the novel synthesis of benzofurans through the application of biocatalysis, where the reactions involved the use of the oxidative enzyme laccase to generate carbon-carbon bonds, carbon-oxygen and carbonnitrogen bonds between aromatic compounds. The substrates used were o-diols from catechols 1, p-quinone 2 from naphthoquinones and naphthohydroquinone 3, which, when activated by the enzyme action, could be reacted with 1,3-diketones 4, 5 or coumarins 6 (Figure 1). The aim of synthesising different classes of compounds was to vary the functional groups and to increase the number of rings, so as to possibly increase the biological activities.Item Non-oxidative conversion of methane into aromatic hydrocarbons over molybdenum modified H-ZSM-5 zeolite catalysts(2014-07-02) Tshabalala, Themba EmmanuelDehydroaromatization of methane (MDA) reaction was investigated over platinum modified Mo/H-ZSM-5 catalysts which were pre-carbided at 750 oC. The influence of platinum on the catalytic performance and product selectivity of Mo/H-ZSM-5 catalysts for the MDA reaction at 700 oC was studied. The presence of platinum led to a slight decrease in methane conversion. As the platinum loading increased, the methane conversion decreased further and the catalytic stability increased with time-on-stream (TOS) during the MDA reaction. Aromatic selectivities above 90% were obtained with catalysts containing low platinum loadings (0.5 and 1.0 wt.%), with benzene being the most prominent product. A decrease in coke selectivity and coke deposits was noted with the platinum modified Mo/H-ZSM-5 zeolite catalysts. A comparative study was performed to compare platinum, palladium and ruthenium promoted Mo/H-ZSM-5 zeolite catalysts with un-promoted Mo/H-ZSM-5. The ruthenium promoted catalyst proved to be superior in catalytic performance, with a higher methane conversion obtained than found for platinum promoted and palladium promoted Mo/H-ZSM-5 catalysts. Benzene selectivity of about 60% was obtained for ruthenium and palladium promoted Mo/HZSM- 5 catalysts and the total aromatic selectivity was maintained at 90%. TGA results showed a total reduction of 50% by weight of carbon deposited on the promoted Mo/H-ZSM-5 catalyst. Dehydroaromatization of methane was studied over tin modified Pt/Mo/HZSM-5 catalysts and compared to Pt/Mo/H-ZSM-5 catalyst at 700 oC. Addition of tin decreased the activity towards methane aromatization. However, the formation of aromatic compounds was favoured. The CO FT-IR adsorption and CO chemisorption techniques showed that the catalyst preparation method had an effect on the catalytic performance of tin modified Pt/Mo/H-ZSM-5 catalysts. High aromatic selectivity and low coke selectivity were obtained with co-impregnated and sequentially impregnated Pt/Sn catalysts. While a decrease in the formation rate of carbonaceous deposits is mainly dependent on the availability of platinum sites for the hydrogenation of carbon. The order of sequentially loading platinum and tin has an effect on the electronic and structural properties of platinum as shown by XPS and FT-IR studies. CO chemisorption and the FT-IR adsorption studies showed that addition of tin decreased the adsorption capacity of the platinum surface atoms. Catalyst preparation methods and successive calcination treatments affected the location of both tin and platinum atoms in the catalyst. Catalysts prepared by the coimpregnation method showed a good platinum dispersion, better than found for the sequentially impregnated catalysts. The MDA reaction was carried out at 800 oC over manganese modified H-ZSM-5 zeolite catalysts prepared by the incipient wetness impregnation method. The effect of a number of parameters on the catalytic performance and product selectivity was investigated, such as reaction temperature, manganese precursor-type, tungsten as promoter, manganese loading and use of noble metals. The study of the effect of reaction temperature showed that the methane conversion increased linearly with increase in reaction temperature from 700 to 850 oC. The selectivity towards aromatic compounds (of about 65%) was attained for the reactions performed at 750 and 800 oC. Formation rate of carbonaceous deposits increased linearly with increase in reaction temperature. The use of different manganese precursors to prepare Mn/H-ZSM-5 catalysts had an effect on both the catalytic behaviour and the product distribution. High catalytic activities were obtained for the catalysts prepared from Mn(NO3)2 and MnCl2 salts. However, the product distribution was significantly different, with the Mn(NO3)2 catalyst being more selective towards aromatic compounds while the MnCl2 catalyst was more selective toward coke. The effect of manganese loading was studied at 800 oC and an optimum catalyst activity was obtained at 2 and 4 wt.% manganese loadings. The aromatic selectivity above 70% and coke selectivity of 20% were obtained for a 2 wt.% loaded catalyst. Addition of tungsten as a promoter onto the 2 wt.% loaded catalyst (2Mn/H-ZSM-5) lowered the catalytic activity but the catalyst remained fairly stable with increase in TOS. Tungsten modified catalysts favoured the formation of carbonaceous deposits over aromatic compounds. TGA results showed a coke deposit of 164 mg/g.cat, an 88% increase in coke deposit when tungsten was used a promoter. Noble metals were added to reduce the total amount of coke on the tungsten modified Mn/H-ZSM-5 catalysts. The presence of a noble metal favoured the formation of aromatic compounds and suppressed the formation of coke. Platinum and ruthenium promoted catalysts were the active catalysts and aromatic selectivity increased from 12% to 55% and 46% respectively. A reduction in the total amount of coke deposit on the platinum promoted catalyst (42%) and the ruthenium promoted catalyst (31%) was noted.