As the world moves forward to the development of biorefineries, the interest to replace chemicals and materials derived from petroleum is increasing exponentially. Lignin is a macromolecular by-product derived from the wood pulping industry, mainly used for heating purposes. The development of new processes to produce high value-added lignin products such as multifunctional aromatic chemicals and high-tech carbon materials are required to fulfill the needs for biorenewable feedstocks. Such processes are likely to include selective oxidation catalysis. The aim of this Thesis is to advance the state-of-the-art for the oxidation of lignin models and lignin extracts using homogeneous catalysts based on vanadium, an inexpensive and abundant transition metal, using air as the only oxidant. Lignin models containing the most important features of lignin (e.g., β-O-4 and β-1 linkages) were initially used to assess the catalytic potential. Previously reported (HQ)2V(O)(OiPr) and (dipic)V(O)(OiPr) catalysts (dipic = dipicolinate and HQ = 8-oxyquinoline) displayed different selectivity for C-H, C-O and C-C bond cleavage upon varying of the solvent, the lignin model or the catalyst. Moreover, these catalysts cleave the C-H bond of secondary alcohols through a two-electron oxidation process and the C-C bond cleavage of the oxidation product ketone in the presence of exogenous base. Several amine bis(phenolate) oxovanadium(V) catalysts were synthesized and fully characterized, and demonstrated very good activity for the oxidation of lignin models and the depolymerization of organosolv lignin. These new catalysts overcome the need for added base, display higher reaction rates of oxidation, and improve the selectivity for the disassembly of lignin models. The different selectivities involving C-H vs. C-O vs. C-C bond cleavage are discussed together with a novel redox-neutral C-C bond cleavage of lignin model 1,2-diphenyl-2-methoxyethanol. The oxovanadium(V) catalysts, along with a metal-free variant and other transition metal catalysts, were employed to assess their performance for the oxidation and depolymerization of organosolv lignin. Although most catalysts oxidized the lignin extracts, the oxovanadium(V) complexes demonstrated the highest degree of lignin depolymerization as shown by Gel Permeation Chromatography (GPC), quantitative-Heteronuclear Single Quantum Correlation (q-HSQC) and quantitative 31P NMR spectroscopy of derived phosphite esters. In a complementary study, oxovanadium(V) catalysts also established their utility for the valorization of cellulose-derived substrates (e.g., diols). Two trialkoxyamine oxovanadium(V) complexes bearing a triethoxyamine and tris[2-(3,5-di-tert-butyl-phenoxy)methyl]amine ligand respectively, selectively cleaved the C-C bond in 1,2-diols with excellent rates and using air as the only oxidant. In a stoichiometric investigation of this reaction, it was determined that the transformation proceeds through an unusual direct oxidative two-electron cleavage of diols, affording a non-oxo monometallic V(III) intermediate. DFT calculations support a single-site proton-coupled electron-transfer of the hydroxyl hydrogen to the V oxo orbital. In summary, this Thesis describes new developments in vanadium catalysis such as mechanistic implications and catalyst optimization for the valorization of lignocellulosic biomass utilizing air as an oxidant.

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A research report submitted to the Faculty of Mining Engineering. University of Witwatersrand, Johannesburg, in partial fulfillment of the requirements for the degree of Master of Science in Engineering / Hlstorically South Africa has enjoyed a privileged position with its vast natural resource base in that it has.not needed to generate wealth but has merely explored this base, The only true way to create wealth, is to add value to a commodity by means of beneficiation, South Africa is the world's largest producer of vanadium accounting for approximately 70% of the western world's requirements. Unfortunately the South African producers of vanadium mainly export vanadium in the form of slag and pentoxide, instead or ferrovanadium and other prime alloys. ( Abbreviation abstract) / AC 2018

Some bicyclam compounds are highly active as HIV inhibitor agents with high selectivity for the CXCR4 coreceptor, an important protein receptor for the human immunodeficiency virus. One particular bicyclam, AMD3100 (1,1’-(1,4-phenylenebismethylene)-bis-1,4,8,11-tetraazacyclotetra- decane, known as xylyl-bicyclam) reached Phase II clinical trials in 2001. More recently, it has been relaunched commercially as Mozobil, a stem cell mobiliser. The activity of this macrocycle and configurationally restricted analogues as antivirals against HIV, have been shown to increase on complexation with zinc. A range of metals has been studied with varying degrees of activity seen, but none show activity comparable with the zinc complexes. In order to investigate the chemistry of macrocycles in their role as anti-HIV agents, the syntheses of vanadium-cyclam, vanadium-bicyclam and zinc constrained cyclam complexes have been carried out. Of the many oxovanadium(IV) cyclam complexes synthesised, those with a sulfate or chloride axial ligand trans- to the V=O group were chosen for comparison purposes with oxovanadium(IV) bicyclam analogues for both solid state and solution analyses. X-ray crystallography showed that the oxovanadium(IV) cyclam complexes crystallised in the thermodynamically stable trans-III configuration. Aqueous solution studies using EPR spectroscopy revealed a different behaviour for the oxovanadium(IV) cyclam and bicyclam complexes studied. Results suggested that the 6th axial ligand trans- to the V=O group could undergo facile exchange in solution for the cyclam complexes but may be retained in the bicyclam complexes. Antiviral test results for oxovanadium(IV) cyclam complexes revealed these to be completely inactive against HIV. Xylyl-bicyclam was synthesised and complexed with vanadium, as oxovanadium(IV) sulfate and chloride structures. EPR spectroscopy results suggested that for the bicyclam chloride complex some displacement of the 6th axial ligand may occur, but the bicyclam sulfate appeared to retain both axial ligands. More importantly, these ligands also appear to have a significant effect on the hyperfine coupling constant, A. Antiviral test results for oxovanadium(IV) bicyclam sulfate and chloride complexes revealed these to be highly active against HIV, with the chloride structure showing similar activity to AMD3100. Possible interactions between the V=O group and the protein backbone of CXCR4 were studied using molecular modelling techniques, incorporating the X-ray crystal structure obtained for the oxovanadium(IV) cyclam sulfate complex. This revealed the possibility of a weak interaction between the oxygen of the V=O group and the proton of the alpha-carbon of a tryptophan residue, Trp195. No metal-carboxylate interaction was seen between the vanadium centre and aspartate residue Asp171 in the binding site investigated, possibly due to the distance between them being unfavourable for formation of a coordination bond (3.4 Å). Possible hydrogen bonding was seen between the carboxylate oxygens of Asp171 and the NH protons of the trans-III configured ring. These studies may demonstrate how oxo-metals interfere with the protein backbone in such a way as to distort the molecule from an optimum binding geometry. It may also indicate the importance of a second binding site known to support a cis-V configuration of the macrocycle, bearing in mind that vanadium bicyclam complexes do show activity as antivirals against HIV. It is possible that one ring of the vanadium bicyclam complexes interacts with the binding site for the trans-III configuration in a similar fashion to the cyclam complexes and the antiviral activity may result, in this case, from interactions formed through binding of the second ring. The aromatic linker is also thought to be important. Cyclams can be constrained by adding bulky substituents and/or additional rings to the structure and this has the effect of stabilising one configuration. NMR studies show how a zinc constrained cyclam retains the trans-III configuration in solution and also shows high activity against HIV, comparable with the vanadium bicyclam sulfate complex. This emphasises how the oxo-vanadium cyclams must be in an unfavourable position and unable to form crucial interactions known to be important for antiviral activity. A 4-coordinate zinc constrained macrocycle showed unusual binding to three ring nitrogens and may reveal an intermediate species in the uptake and release of metals by cyclams. Further complexation by exchange of the axial ligand revealed a 5-coordinate zinc bound to all four nitrogens on the ring. This thesis provides the first investigation into the role of oxo-vanadium complexes as antiviral agents against HIV looking at possible interactions of the oxo-metal group with the protein backbone of the CXCR4 receptor. An investigation into configurational change in solution for zinc constrained cyclams is also described.

Student Number : 0420699F - MSc Dissertation - School of Physics - Faculty of Science / This dissertation presents for the first time the conditions for the synthesis of VO2 by ultrasonic nebula-spray pyrolysis (UNSP) from a precursor solution of NH4VO3+VCl3 optimized as follows: a carrier gas of argon at a flow rate of 11 liters per minute, a furnace temperature of 400 to 700oC. This work also incorporates thermodynamic variables of Tpr-P-V into the equations that relate the mean diameter of droplets, D, to frequency of the exciting ultrasound waves, f, the density of the precursor solution, #26; and the surface tension, #27;, previously worked on independently by Lang and Jokanovic. The incorporation results in the diameters of the droplets (and consequently the collected grains) being smaller as p and Tpr are increased in a non- linear form. The variable V, however, increases the diameter of the droplets as it is allowed to increase. This study shows the departures many authors find of the theory from experiment but it also shows that the departure does not lie in the equations but rather on post- synthesis and annealing effects. From X-ray diffraction, scanning electron microscopy (SEM) and Raman spectroscopy, this study shows that as furnace temperature is increased the morphology of the sample surfaces for both VO2 and WO3 transforms from amorphous to crystalline, from spherical grains to plate-like structures, with grain mean diameter increasing non-linearly in some cases and decreasing non-linearly in other cases confirming previous findings, the latter enjoying the majority vote. In Raman spectra of the as- obtained WO3, asymmetric broadening of the Raman peaks was observed in some samples and a phonon confinement model was employed in the size distribution prediction. These findings prompted the re- workout of the phonon confinement model. In this dissertation an equation has been derived based on the Faucet-Campbell equation of the PC model. The new equation relates the ratio of neighboring peaks in a material’s Raman spectrum to the mean diameter of the grains. The present modification allows the PCM model to predict the grain size beyond the current limiting range of 0 to 100 nm. Analysis of the experimental data using this equation unveils two different equations- one for particles of size below 100 nm and the other equation for particles with larger that 100 nm. Also this analysis has enabled the present study to evaluate the phonon dispersion relations for WO3.

Inicialmente, é feita uma revisão bibliográfica dos principais métodos de obtenção de vanádio metálico, comparando-se os resultados obtidos com os diversos redutores empregados. Também os métodos de purificação usualmente empregados são revistos, e mostra-se a eficácia de alguns deles. A seguir, analisa-se o processo aluminotérmico de obtenção de vanádio metálico a partir de pentóxido de vanádio, compreendendo estudos termodinâmicos, cinéticos e de transporte de calor. É mostrada a influência de diversas variáveis sobre a recuperação do metal. Desenvolve-se na parte experimental o estudo sobre a aluminotermia de pentóxido de vanádio, utilizando-se para isto reatores abertos revestidos com argamassa refratária. Estuda-se a influência das variáveis de processo sobre a recuperação de vanádio, incluindo a adição de fluxantes, a granulometria dos reagentes, o tamanho da carga, a pureza dos reagentes, o uso de excesso de redutor e o modo de ignição da carga. Os resultados experimentais obtidos são apresentados e discutidos. O trabalho apresenta conclusões e sugestões para pesquisas futuras. / A literature survey on the processes of vanadium reduction was carried out having in mind the behavior of the reducing agents used. The refining methods for crude metallic vanadium were also covered in the review. The aluminothermic process for the reduction of vanadium pentoxide was particularly considered. Its thermochemistry features were studied, as well as the heat transfer and the rate phenomena concerning such a reaction system discussed. It was pointed out the effect of the processes parameters on the recovery of metallic vanadium. The experiments were designed to investigate the effects of the harge composition, the purity level of the reactants, the size of the solid particles of the reacting mixture and the ignition method on the recovery of vanadium. The aluminothermic reaction was carried out in an open reaction vessel. The experimental results are presented and discussed. The research work done has reached conclusive trends which allows the proposal for further interesting research.

Predicting the temperature-strain phase diagram of VO$_2$, including the various structural allotropes, from first principles is a grand challenge of materials physics, and even the phase diagram remains unclear at T = 0K. The coexistence of Peierls and Mott physics suggests that a theory which can capture strong electronic correlations will be necessary to compute the total energies. In order to understand the complex nature of the first-order transition of VO$_2$, we build a minimal model of the structural energetics using the Peirels-Hubbard model and solve it exactly using the Density Matrix Renormalization Group (DMRG) methods demonstrating that the on-site interaction $U$ has a minimal effect on the structural energetics for physical parameters. These results explain the qualitative failures of Density Functional Theory (DFT) and DFT+$U$ for the structural energetics, in addition to the partial success of the unorthodox DFT+$U$ results (i.e. non-spin-polarized and small $U$). It also guides the creation of empirical corrections to the DFT+$U$ functional which allow us to semi-quantitatively capture the phase stability of the rutile and monoclinic phases as a function of temperature and strain. Our work demonstrates that VO$_2$ is better described as a Mott assisted Peierls transition.

The aim of this work was to investigate the precipitation and stability of nm-sized Ti oxides in vanadium-based alloys, a prime candidate material for future nuclear fusion reactors based on the magnetic confinement of the plasma. Fusion energy reproduces the nuclear reactions occurring in stars. It can potentially produce more energy than current nuclear fission power plants, and it is meant to be a solution to the clash of today's increasing energy demand with the continuous decrease of fossil-based energy sources, whose use is harmful for the environment. The operating conditions in a fusion reactor will be unprecedented in terms of ultra-high temperatures, stresses, radiation fields and very corrosive media. Only a limited number of materials may be able to withstand such combination of harsh environmental conditions, and vanadium-based alloys are among them. Recent research efforts have identified V-4Cr-4Ti as the most promising vanadium-based alloy for application in the first wall of future fusion nuclear reactors such as DEMO and beyond. The presence of TiO-type precipitates, containing relatively small amounts of C and N, strongly influences the final mechanical properties and radiation resistance of the alloy. Therefore, a thorough understanding of the precipitate structure and evolution at both relatively high temperatures and radiation dose levels is primordial to predict and optimise the final performance of the structural component in the fusion reactor. This thesis is written in alternative format and collects one article already published in Scripta Materialia, and two additional articles to be submitted to peer-review scientific journals. Atomic resolution imaging of the precipitates, coupled with chemical analysis, constitutes the main body of the first article: a novel intergrowth of the fcc Ti oxide in the bcc V matrix is revealed at the precipitate/matrix interface. The evolution of the vacancies present in the TiO precipitates above 400°C, together with the recovery of dislocations in the matrix and the formation of extra precipitates, is studied in the second article by positron annihilation spectroscopy and micro-hardness measurements. The formation of additional precipitates below 400°C induced by radiation is assessed in the third article using proton irradiation as a surrogate of neutron damage. The structure of those additional precipitates and of the dislocation loops induced by the proton bombardment is characterized by advanced analytical electron microscopy.

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