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As levels of carbon dioxide continue to increase in the atmosphere, it is appealing to consider the prospect of utilizing CO2 as a C1 building block for the synthesis of value-added organic chemicals. Such transformations offer potential to directly counteract environmental concerns, and could also enhance the recyclability of current materials. To meet this challenge, the development of metal catalysts capable of promoting the functionalization of carbon dioxide is necessary. Furthermore, there is great interest in employing main group metals for these transformations, particularly those metals that are earth-abundant, non-toxic and affordable. To address these needs and others, the research herein has been driven by the synthesis and characterization of main group metal hydride, alkyl and fluoride complexes with the ultimate aim of developing catalysts for CO2 functionalization. Chapter 1 investigates the synthesis of magnesium, zinc and calcium complexes supported by the tris[(1-isopropylbenzimidazol-2-yl)dimethylsilyl)]methyl ligand, [TismPriBenz]. Most significantly, the magnesium carbatrane compound, [TismPriBenz]MgH, which possesses a terminal hydride ligand, has been synthesized and structurally characterized. The corresponding magnesium methyl derivative, [TismPriBenz]MgMe, was also prepared, and the reactivity of these compounds with respect to both metathesis and insertion is explored in great detail. The synthesis and characterization of the corresponding zinc hydride complex, [κ3 TismPriBenz]ZnH, is also described, as well as the preparation of a rare example of a monomeric calcium benzyl compound, [TismPriBenz]CaCH2Ph. Some reactivity of the zinc and calcium derivatives is also described. In Chapter 2, the aforementioned magnesium and zinc compounds and their reactivity towards CO2 is described in detail. Systems comprised of [TismPriBenz]MH (M = Mg, Zn) and tris(pentafluorophenyl)borane are highly effective for the room temperature reduction of CO2 with R3SiH to afford sequentially the bis(silyl)acetal, H2C(OSiR3)2, and CH4 (R3SiH = PhSiH3, Et3SiH and Ph3SiH). Notably, the selectivity of the catalytic system may be controlled by the nature of the silane. Catalytic intermediates were isolated and structurally characterized, including an interesting magnesium formatoborate complex, which has helped elucidate an understanding of the mechanism of the catalysis. Most significantly, it was found that H2C(OSiPh3)2 can be prepared on a multi-gram scale as a crystalline solid and can be converted directly into formaldehyde (CH2O), which is an important industrial chemical. Thus, H2C(OSiPh3)2 can serve as a formaldehyde surrogate and its ability to provide a means to incorporate CH and CH2 moieties into organic molecules is described. Isotopologues of H2C(OSiPh3)2, namely D2C(OSiPh3)2, H213C(OSiPh3)2, and D213C(OSiPh3)2, may be synthesized from the appropriate combinations of (12C/13C)O2 and Ph3Si(H/D), thereby providing a direct and convenient means to use carbon dioxide as a source of isotopic labels in complex organic molecules. In Chapter 3, details pertaining to other transformations catalyzed by [TismPriBenz]MgR (R = H, Me) are provided and their mechanisms are discussed. Notably, [TismPriBenz]MgR is a catalyst for hydrosilylation and hydroboration of styrene to afford exclusively the Markovnikov products, Ph(Me)C(H)SiH2Ph and Ph(Me)C(H)Bpin; the magnesium alkyl intermediate in the catalytic process, [TismPriBenz]MgCH(Me)Ph, has been isolated and structurally characterized, providing the first structural evidence for the insertion of an olefin into a magnesium hydride bond. Other catalytic transformations are described, including hydroboration of carbodiimides to form N-boryl formamidines and hydroboration of pyridine to provide N-boryl 1,4- and 1,2-dihydropyridines. Additionally, the ability for the magnesium hydride and methyl complexes to catalyze dehydrocoupling reactions is discussed. Finally, the ability for [TismPriBenz]MgMe to catalyze the isomerization of a terminal alkyne is reported. Chapter 4 outlines the chemistry of magnesium and zinc compounds supported by a different scaffold, namely, the tris(3-tert-butyl-5-methylpyrazolyl)hydroborato, [TpBut,Me], ligand. The magnesium methyl compound, [TpBut,Me]MgMe, was used as a precursor to prepare [TpBut,Me]MgF via metathesis with Me3SnF, and is the first example of a structurally characterized monomeric magnesium fluoride complex. The reactivity of [TpBut,Me]MgF is described, including its ability to serve as a hydrogen bond and halogen bond acceptor, such that it forms adducts with indole and C6F5I. Corresponding zinc chemistry was studied, including interesting reactivity of [TpBut,Me]ZnH and [TpBut,Me]ZnF. Finally, new heterobimetallic compounds containing magnesium or zinc supported by the [TpBut,Me] ligand and tungsten were synthesized and structurally characterized.

A dissertation submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science in Engineering. 10 February, 2017. / Climate change due to the ever-increasing emission of anthropogenic greenhouse gases arising from the use of fossil fuels for power generation and most industrial processes is now a global challenge. It is therefore imperative to develop strategies or modern technologies that could mitigate the effect of global warming due to the emission of CO2. Carbon capture and storage (CCS) is a viable option that could ensure the sustainable use of cheap fossil fuels for energy generation with less CO2 emission. Amongst existing CCS technologies, absorption technology using monoethanolamine (MEA) is very mature and widely embraced globally. However, the absorption technology has a lot of challenges such as, low CO2 loading, high energy requirement for solvent regeneration, corrosive nature etc. On this note, the adsorption technology using solid sorbents is being considered for CO2 capture due to its competitive advantages such as flexibility, low energy requirement for sorbent regeneration, non-corrosive nature etc. On the other hand, adsorbents have a very vital role to play in adsorption technology and there is need to understand the behaviour of adsorbents for CO2 capture under different operating conditions in order to adapt them for wider applications. On this note, the study contained in this dissertation investigated the adsorption behaviour of a novel polymer-based adsorbent (polyaspartamide) during post-combustion CO2 capture using experimental study and mathematical modelling approach. Polyaspartamide is an amine-rich polymer widely used in drug delivery. In addition, its rich amine content increases its affinity for CO2. Its porosity, thermal stability and large surface area make it a promising material for CO2 capture. In view of this, polyaspartamide was used as the adsorbent for post-combustion CO2 capture in this study. This dissertation investigated the kinetic behaviour, the diffusion mechanism and rate limiting steps (mass transfer limitation) controlling the CO2 adsorption behaviour of this adsorbent. Furthermore, effect of impurities such as moisture and other operating variables such as temperature, pressure, inlet gas flow rate etc. on the CO2 adsorption behaviour of polyaspartamide was also investigated. Existing mathematical models were used to understand the kinetics and diffusion limitation of this adsorbent during CO2 capture. Popularly used gas-solid adsorption models namely; Bohart- Adams and Thomas model were applied in describing the breakthrough curves in order to ascertain the equilibrium concentration and breakthrough time for CO2 to be adsorbed onto polyaspartamide. Lagergren’s pseudo 1st and 2nd order models as well as the Avrami kinetic models were used to describe the kinetic behaviour of polyaspartamide during post-combustion CO2 capture. Parameter estimations needed for the design and optimization of a CO2 adsorption system using polyaspartamide were obtained and presented in this study. The Boyd’s film diffusion model comprising of the interparticle and intra-particle diffusion models were used to investigate the effect of mass transfer limitations during the adsorption of CO2 onto polyaspartamide. Data obtained from continuous CO2 adsorption experiments were used to validate the models in this study. The experiments were conducted using a laboratory-sized packed-bed adsorption column at isothermal conditions. The packed bed was attached to an ABB CO2 analyser (model: ABB-AO2020) where concentrations of CO2 at various operating conditions were obtained. The results obtained in this study show that temperature, pressure and gas flow rate had an effect on the adsorption behaviour of polyaspartamide (PAA) during CO2 capture. Polyaspartamide exhibited a CO2 capture efficiency of 97.62 % at the lowest temperature of 303 K and pressure of 2 bar. The amount of CO2 adsorbed on polyaspartamide increased as the operating pressure increased and a decrease in the adsorption temperature resulted in increased amount of CO2 adsorbed by polyaspartamide. The amounts of CO2 adsorbed on polyaspartamide were 5.9, 4.8 and 4.1 mol CO2/kg adsorbent for adsorption temperatures of 303, 318 and 333 K, respectively. The maximum amount of CO2 adsorbed by polyaspartamide at different flow rates of 1.0, 1.5 and 2.5 ml/s of the feed gas were 7.84, 6.5 and 5.9 mmol CO2/g of adsorbent. This shows that higher flow rates resulted in decreased amount of CO2 adsorbed by polyaspartamide because of low residence time which eventually resulted in poor mass transfer between the adsorbent and adsorbate. Under dry conditions, the adsorption capacity of polyaspartamide was 365.4 mg CO2/g adsorbent and 354.1 mgCO2/g adsorbent under wet conditions. Therefore, the presence of moisture had a negligible effect on the adsorption behaviour of polyaspartamide. This is very common with most amine-rich polymer-based adsorbents. This could be attributed to the fact that CO2 reacts with moisture to form carbonic acid, thereby enhancing the CO2 adsorption capacity of the material. In conclusion, this study confirmed that the adsorption of CO2 onto polyaspartamide is favoured at low temperatures and high operating pressures. The adsorption of CO2 onto polyaspartamide was governed by film diffusion according to the outcome of the Boyd’s film diffusion model. It was also confirmed that intra-particle diffusion was the rate-limiting step controlling the adsorption of CO2 onto polyaspartamide. According to the results from the kinetic study, it can be inferred that lower temperatures had an incremental effect on the kinetic behaviour of polyaspartamide, external mass transfer governed the CO2 adsorption process and the adsorption of CO2 onto polyaspartamide was confirmed to be a physicochemical process (both physisorption and chemisorption). / MT2017

<p>This thesis considers the effect of a large scale wind power introduction into an electricity system and the focus has been on the carbon dioxide emissions. Two different systems were studied, the Swedish and the Danish electricity system. When studying the Swedish electricity system different scenarios were created to see what might happen with the CO₂ emissions with an introduction of a large amount of wind power. The model that was used is based on parameters such as regulating power, transmission capacity, export possibility, and the electricity generation mixes in the Nordic countries. Given that the transmission capacity is good enough, the conclusion is that the carbon dioxide emissions will be reduced with a large scale introduction of wind power. In the Danish electricity system wind power is already introduced to a large extent. The main purpose here was to investigate the development of the CO₂ emissions and if it is possible to decide the actual change in carbon dioxide emissions due to the large scale introduction of wind power. The conclusions to this part are that the CO₂ emissions per kWh produced electricity have decreased since the electricity generation mix has changed but the total amount of CO₂ emissions fluctuates depending on weather, in a dry year less hydro power from Norway and Sweden can be used and more electricity from the fossil fuelled CHPs are generated. It has not been possible to determine the influence of the wind power on the CO₂ emissions.</p>