No description available.

No description available.

Most biodiesel plants operate batch-wise using homogeneous alkali catalysts. Recently, several heterogeneous catalysts have been suggested in literature, as they have shown potential for overcoming most of the challenges associated with the application of homogeneous catalysts. Previous published techno-economic comparisons of the two technologies on large-scale processes located in the developed world, have revealed the economic superiority of heterogeneously catalysed processes. Hence, prospect exists for current homogeneously catalysed process plants to be converted to heterogeneously catalysed ones. The objective of this research was to investigate the actual cost benefit of converting a small-scale batch biodiesel plant from homogeneous to heterogeneous catalysed process. For this purpose, a small-scale batch biodiesel plant located in South Africa was taken as the base case homogeneous process. Aspen Batch Process Developer® software was used to perform the process simulations. The homogeneous process was converted to the heterogeneous one and results from process simulation were used to evaluate the economics of both processes, which were compared in terms of fixed capital cost, total manufacturing cost and profitability indicators. During economic evaluation, two types of cost factors were used: one prevailing in developed world and the other one relevant to South Africa. The sensitivity analysis of both processes was further performed in order to investigate the impact of some uncertain parameters on their profitability. Finally, a debottlenecking study was carried out. Results obtained from this study showed an increase in the annual throughput of biodiesel as well as significant savings in the total capital cost for the heterogeneous catalysed process relative to the homogeneous one. As regards the estimation of the total unit manufacturing cost of biodiesel, significant differences arose when using the two types of cost factors. Results of economic analyses estimated using cost factors relative to South Africa suggest an increase in the unit manufacturing cost of biodiesel while using the developed world’s cost factors suggests the opposite. This is due to the higher raw material and energy requirement for the CaO process, while knowing that the direct costs are a bigger proportion of the manufacturing costs estimated using the South African cost factors. Profitability and sensitivity analyses only provided positive results when estimated using the South African cost factors. In all cases, the heterogeneous catalysed process was found to be more promising than the homogeneous one over the prescribed project life. The study showed the importance of using cost factors relevant to a particular economic environment during techno-economic assessment of a process. It was also shown that there are economic benefits when replacing settling with centrifugation in biodiesel production processes. In summary, this thesis makes some important contributions. It presents the first process simulation for biodiesel production using Aspen Batch Process Developer® software and thereby proposes a methodology that is currently scarce in the literature. It also reports the first techno-economic analysis applied to the biodiesel field in South Africa and provides a preliminary insight to owners of biodiesel plants as regards the decision to convert or not their homogeneous catalysed plant to heterogeneous one.

No description available.

Fire presents a great risk to humans and their possessions. Polymeric compounds are highly flammable and compounds are added to increase their fire resistance. These additives are referred to as flame retardants. Intumescent flame retardants are systems that form a carbon rich foam on the surface of the polymer during thermal degradation. A new field of metal catalysed intumescence is discussed in this document. The influence of both the metal ion and organic backbone of polyhydroxylcarboxylic acid metal salts was studied. The intumescence and foaming properties of selected salts were studied in more detail. Gluconic acid proved to be the best intumescent and showed the best combination of carbon char yield, foam structure and volume, stability and open flame characteristics. As a general rule of thumb, the char volume and structure improved with an increased number of hydroxyl groups in the compound. Carbon char yield increased with an increase in the number of hydroxyl groups for a constant number of carbons in the complex. The number of acid groups in the compound played a lesser role in intumescence. Most metal complexes catalyse intumescent decomposition of polyhydric compounds such as polyhydroxylcarboxylic acids and pentaerythritol. The reduction in the volatilisation losses implies that the metal cations catalyse carbonisation reactions in the polyhydroxyl compound. Unfortunately, the resultant carbon-foams are unstable: the metal residues also catalyse the further oxidative degradation of the char. The low thermal stability of the compositions tested, as well as the afterglow effect makes them unsuitable for use as flame retardant additives in plastics. From the studies of the different metal complexes with acetylacetone and gluconic acid, it was concluded that the calcium ion exhibited the most promise. When pyrolysed in air with an open gas flame or in a furnace, calcium gluconate monohydrate produces voluminous foam. The foam is of a closed cell structure, densely packed and has no mechanical strength. The cell walls are very thin (5 to 50 nm) and the cells are between 50 µm and 200 µm in size. If the sample is compressed before heating, the resultant foam produced during pyrolysis is less voluminous than that of the loose powder. The BET surface area of the calcium gluconate monohydrate foamed at 300°C for 5 min. is 16.0 m2/g. The foam produced after 5 min heating at 300°C has a thermal conductivity similar to standard polystyrene foam. Adding leached silica to the calcium gluconate monohydrate increases mechanical strength of the foam, but influences the foam volume negatively. The optimum silica level was calculated to be 1:1 gluconate to silica on a mole ratio (11.8% SiO2 by mass). The mechanical strength of the foam can also be increased with the addition of expandable graphite. The graphite has a lesser influence on the foam volume than the silica, but also reduces the foam volume. However, the addition of the expandable graphite gives the sample more .opening. force, as the foam volume of a compressed sample pyrolysed, is similar to that of the heated loose powder. Calcium gluconate monohydrate starts to degrade at 120°C, losing its crystal water and shows a mass loss of 4% at 180°C. The exothermic peak (shown in the DSC/TGA data) associated with the metal catalysed carbon oxidation (afterglow) is observed at 570°C in air. For the calcium gluconate monohydrate the transition from CaCO3 to CaO occurs above 650°C in air. It has been shown that the foaming of polyhydroxylcarboxylic acids is due to the formation of water vapour during degradation. For calcium gluconate monohydrate, foaming starts due to the loss of crystal water and is continued by the loss of hydroxyl groups as water. The bulk of the foaming is due to the second reaction. It has been shown that compounds with crystal waters produce a more voluminous and lower density foam. The foam is an amorphous carbon rich residue. The molecular mass of the carbon residue increases up to a heating temperature of 300°C. This implies that the carbon residue crosslinks during formation, forming a stretchable cell wall for the foam. Compounds with a .free. hydroxyl group at the end of the carbon chain produce a foam of larger volume and lower density. This supports the crosslinking theory. The foam produced when calcium gluconate monohydrate is heated for 5 min at 300°C in air is of very low density . 2.5 kg/m3 based on residual mass. The density of the calcium gluconate monohydrate pyrolysed at 1000°C for 5 min in air yields a CaO with a density of 20 kg/m3 . This implies that the high temperature pyrolysis of calcium gluconate can produce an inorganic oxide of low bulk density and possible high specific surface area. The BET surface area of CaCO3 from the gluconate pyrolysed at 600°C is ~ 12 m2 /g. An intumescent coating containing calcium gluconate monohydrate, leached silica and expandable graphite as a system was prepared and compared to commercial formulations. This gluconate based system was at least as efficient as the commercial formulations when painted on balsa wood planks or aluminium plates. On cardboard sheets it did not perform as well as the commercial systems. More work should be done to overcome the afterglow effect observed with metal- based intumescent systems. The crystal structure of calcium gluconate monohydrate should be determined as to understand the decomposition better. It is unclear whether the decomposition is catalysed inter- or intramolecularly. / Thesis (PhD(Chemical Engineering))--University of Pretoria, 2005. / Chemical Engineering / unrestricted

The cobalt-catalysed alkylative aldol cyclisations of α,β-unsaturated amides with an appendant ketone were studied using a range of trialkylaluminium reagents. Investigations revealed that Co(acac)2·2H2O acts as an effective precatalyst for this transformation and the reaction provides β-hydroxylactam products containing three contiguous stereocentres with high levels of diastereoselection. The substrate scope of the reaction was explored and attempts were made to develop an asymmetric variant of this process. Two plausible reaction mechanisms have been proposed; the first invokes a π-allylcobalt species as a key intermediate in the reaction mechanism and the second involves a radical-mediated pathway. A stereochemical rationale for the observed relative stereochemistry of the β-hydroxylactam products has been discussed. II. Rhodium-Catalysed Carbometallations of Ynamides in the Preparation of Multisubstituted Enamides A highly stereo- and regioselective rhodium-catalysed carbozincation of ynamides using organozinc reagents has been disclosed. A careful examination of ligand effects on the rhodium catalyst yielded a complementary set of hydrozincation conditions. The alkenylzinc intermediates produced during the course of these reactions have been harnessed in further transformations with electrophilic species and in cross-couplings, thus providing access to multisubstituted enamides in a stereo- and regioselective fashion. Additionally, a rhodium-catalysed tandem carbometallation−conjugate addition with ortho-boronate substituted cinnamic acid derivatives has been described. The enamide−indene products were obtained in good yields and regioselectivities. Preliminary work has been undertaken on an asymmetric variant of this transformation and the initial results have been reported.

The construction of bonds in a controlled and selective manner and the development of operationally simple, general and reliable methods to achieve these aims remains a key goal of chemical synthesis and the countless industries it impacts upon. With this in mind, the chemo-, regio- and stereoselective introduction of a number of functionalities into small molecules was investigated. Traditionally the majority of functionalisations have used precious metals; the scope of transformations that can be achieved using these catalysts is remarkable. Palladium in particular has found widespread application in new bond-forming processes and, in addition to cross-coupling reactions, palladium catalysis has been used to effect a wide variety of asymmetric reactions. This work describes investigations into the palladium-catalysed enantioselective electrophilic fluorination of azaarylacetates and amides A2 and the oxidative annulation of ferrocene derivatives A4 (Scheme A1). Both products have structural significance; heterocycles and stereogenic fluorinated centres, present in A3, are important motifs in the pharmaceutical industry, and ferrocenes are important rigid scaffolds in chiral ligands such as those in the Josiphos family e.g. A7. Scheme A1. Palladium catalysis for a) enantioselective fluorination and b) oxidative annulation Whilst a general catalyst remained elusive for the asymmetric fluorination of azaarylacetates and amides, benzoxazole-containing substrates were consistently fluorinated with excellent enantioselectivity (up to 96% ee) using palladium catalyst A1 (Scheme 1a). The oxidative annulation of ferrocene derivatives proved challenging and although the reaction was successful, the product could only be isolated in up to 24% yield (Scheme 1b). In order to determine the yield-limiting step of the reaction, mechanistic studies were conducted and palladacycle A6 was synthesised as a possible reaction intermediate. Recently there has been a shift towards the development of more sustainable, environmentally benign and economic catalyst systems and iron is quickly becoming recognised as a viable alternative owing to its high natural abundance and low toxicity. A general iron-catalysed hydrofunctionalisation procedure is described that was used to form a wide variety of carbon-carbon and carbon-heteroatom bonds (Scheme A2). Scheme A2. Iron catalysis for carbon-carbon and carbon-heteroatom bond formation With just 0.5 mol% iron catalyst, the broad scope formal hydrofunctionalisation of styrene derivatives was achieved using commercially available and bench-stable catalysts and reagents. An iron-catalysed highly regioselective hydromagnesiation gave a common benzylic Grignard reagent, which was reacted with an array of electrophiles in a highly chemo- and regioselective manner. Significantly, the products of formal hydroboration, hydrosilylation and cross-coupling reactions were obtained.

Organometallic compounds derived from tin, boron, and zinc, have been used extensively in transition metal-catalysed cross-coupling, and continue to hold status as the go-to reagents to form new carbon-carbon bonds. Recently, organosilicon compounds have emerged as an attractive alternative to these established reagents, benefitting from low toxicity, low cost, and general ease of handling. While the fundamental reactivity of arylsilane reagents (Ar-SiR3) is well known, their role in transition metal-catalysed reactions is generally less well studied. This thesis comprises an investigation into the effect of ortho-substitution of these arylsilane reagents, and specifically, their application in gold-catalysed direct arylation. In Chapter 2, the transmetalation of these reagents to gold(III) is assessed using a combination of in situ reaction monitoring coupled with kinetic simulations. This allowed a scale of reactivity to be constructed for a range of structurally diverse arylsilanes, and uncovered that more sterically hindered arylsilanes actually exhibit accelerated rates of transmetalation. In Chapter 3, the reactivity of ortho-substituted arylsilanes in gold-catalysed arylation is addressed. The majority of arylsilanes tested in the previous chapter were found to be unable to undergo coupling, despite the viability of transmetalation having been demonstrated. Slight modification of the ortho-substituent, to incorporate a tethered ligand, was found to have a dramatic effect on reactivity, and allowed the coupling of a variety of substrates. The nature of the ligand, as well as the substitution of the tether was found to have a significant impact on the rate of coupling. Chapter 4 describes the way in which the reactivity of ortho-substituted arylsilanes might be exploited in a 'Catch and Release' protocol for catalyst recovery. This aims to combine the established benefits of homogeneous and heterogeneous catalysis to offer an alternative to current methods of catalyst recycling in industrial chemistry. A number of different 'Catch and Release' mechanisms were considered, and the validity of the concept was demonstrated in a monophasic system.

Since its discovery in 1979, the Suzuki-Miyaura (SM) reaction has become one of the most widely utilised tools for carbon-carbon bond formation. The palladium catalysed coupling of an organoboron and organohalide compounds proceeds through a three-stage mechanism of oxidative addition, transmetalation and reductive elimination. The transmetalation of boronic acids to a palladium(II) complex has been widely studied. However, very little is known about the transmetalation of boronic esters, which are commonly used as an alternative to unstable boronic acids. Whether these species undergo direct transmetalation or prior hydrolysis to the boronic acid under SM conditions remains unknown. This research aimed to elucidate the mechanism of this cross-coupling process. Initial results under typical SM conditions created a biphasic reaction, promoted by the inorganic base and solvent composition, and showed that the boronic esters and corresponding boronic acid couple at the same absolute rate. This is thought to be a consequence of the formation of a biphasic mixture, rendering phase transfer the turnover-limiting step. The conditions were thus adapted to maintain a monophasic system using an organic soluble base, 2-tert-butyl-1,1,3,3-tetramethylguanidine, enabling the focus to be transmetalation as the turnover-limiting step. These new conditions show a significant difference in both reaction rate and induction period when using a boronic ester compared to the corresponding boronic acid. The use of guanidine was also shown to have an interesting effect on the boronic acid/ester species by 19F and 11B NMR. Further studies found the use of guanidine to create a boronate species, with this species being an aryl trihydroxyboronate or the hydroxyl"ate"-complex of the boronic ester, depending on the presence of diol in the system. Formation of a boronate species was found to be crucial for efficient cross-coupling. When testing weaker bases, unable to form a boronate species, poor SM cross-coupling conversion was found using the newly developed phosphine-free guanidine conditions, showing the importance of the boronate species under these conditions. The results suggest that depending on the strength of base used, the pathway of transmetalation pathway can be switched, between the boronate pathway and the oxo-palladium pathway, under the specific conditions developed.

The iron-catalysed hydromagnesiation of styrene derivatives has been developed further from previous publications, expanding the electrophile scope to enable the regioselective formation of new carbon-carbon and carbon-heteroatom bonds (Scheme A1). A commercially available pre-catalyst and ligand were used to give an operationally simple procedure that did not require prior synthesis of a catalyst. This work also investigated the hydromagnesiation of dienes, using a screen of ligands commonly used in transition metal catalysis. An investigation into the magnesium-catalysed hydroboration of olefins was also carried out. Although mostly unsuccessful, it was demonstrated that in the presence of a magnesium catalyst, a small amount of vinyl boronic ester could be formed from an alkyne (Scheme A2). Simple magnesium salts were also investigated for the reduction of carbonyls. Lastly, this work explored the titanium-catalysed hydrosilylation of olefins, using a novel activation method developed within the group (Scheme A3). The results were compared to those published previously using traditional organometallic activation methods and attempts at identifying conditions to improve chemoselectivity were carried out.