Vanadium magnesium oxide (VMgO) catalysts with different vanadium loadings were synthesized and tested for catalytic activity using pure «-hexane and «-octane as feeds. High surface area catalysts were obtained by the wet impregnation of magnesium oxide with an aqueous ammonium metavanadate solution. The optimum loading of vanadium was shown to be 19 % (calculated as weight % of V205). Catalysts were characterized by x-ray diffraction (XRD), inductively coupled plasma - atomic emission spectroscopy (ICP-AES), Brunauer-Emmet-Teller (BET) surface area, differential scanning calorimetry - thermogravimetric analysis (DSC-TGA), Fourier transform infrared spectroscopy (FTIR), laser Raman spectroscopy (LRS), x-ray induced photoelectron spectroscopy (XPS), energy dispersive x-ray spectroscopy (EDS) and scanning electron microscopy (SEM). Magnesium oxide (MgO) and magnesium orthovanadate (Mg3(V04)2 were the only phases observed in each catalyst. VMgO catalysts were tested under both oxygen-rich and oxygen-lean conditions. «-Hexane as feed yielded benzene, 1-hexene, 2-hexene, propane, propene, carbon oxides and water as products, n- Octane as feed yielded styrene, ethylbenzene, xylene, benzene, octenes, carbon oxides and water. 19VMgO was promoted with different loadings of molybdenum oxide (M0O3), cesium oxide (Cs20), antimony oxide (Sb20s), niobium oxide (Nb205), bismuth oxide (Bi203) and tellurium oxide (Te02). The promoted catalysts were tested in specially designed and constructed parallel fixed bed continuous flow reactors. / Thesis (Ph.D.)-University of KwaZulu-Natal, 2006.

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. / Thesis (Ph.D.)-University of KwaZulu-Natal, Westville, 2010.

Bibliography: leaves 230-242 / 242 leaves : ill ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, 1986

Proefschrift Universiteit van Amsterdam. / Met lit. opg. - Met samenvatting in het Nederlands.

Frankfurt (Main), Univ., Diss., 2004.

Th.--Sci. phys.--Toulouse--I.N.P., 1983. N°: 69.

Th. doct.-ing.--Sci. des mater.--Nancy--I.N.P.L., 1982.

No description available.

An investigation of the low temperature tensile properties of vanadium single crystals was carried out, using zone-refined metal. Single crystals of predetermined axial [110] orientation were grown by a melt solidification technique, using an electron beam, floating-zone refiner. Tensile specimens were prepared from these single crystals. A plot of yield stress versus test temperature was found to be discontinuous and consisted of two curves which intersected at -125° C. The plot of log yield stress versus reciprocal temperature yielded two straight lines which also intersected at -125° C. The slip system was identified as <111>{112}, which is different from that found for iron single crystals. X-ray, metallographic and electrical resistance data indicate that the phenomenon is primarily a yield point effect. The results of tensile tests performed on single crystals at a very low strain rate, and on polycrystalline specimens indicated that the temperature dependence of yield stress is itself orientation dependent. Three possible explanations of the unusual temperature dependence of yield stress are given: (1) A change in deformation mechanism occurs, for example, from slip to twinning. (2) A minor ordering reaction occurs. (3) A change in the mechanism by which dislocations are unlocked from their atmospheres occurs; for example, two impurity atmospheres surrounding different dislocations, each impurity showing a separate temperature dependence of yield stress. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

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.