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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Chemistry and characterization of vanadyl phosphate catalysts

Nguyen, Phong T. 21 June 1995 (has links)
Graduation date: 1996
2

N-butane activation over ruthenium and iron promoted VPO catalysts.

January 2009 (has links)
The Fe- and Ru-promoted vanadium phosphorus oxide (VPO) catalysts were synthesized via the organic route in iso-butanol to form the VPO precursor, VOHPO4·0.5H2O. The resulting precursor was then activated in a stream of nitrogen to form an amorphous (VO)2P2O7, which crystallized after conditioning in the reactor in the presence of n-butane. The promoted catalysts were synthesized at 0.1%, 0.3% and 1% loading, pelletized and sieved to give a 300-600 μm pellet size. The catalysts were tested in a fixed-bed continuous flow micro-reactor and the products were analyzed by GC’s equipped with a flame ionization detector (FID) to monitor maleic anhydride and n-butane and a thermal conductivity detector (TCD) to monitor the carbon oxides. A range of characterization techniques were employed to determine the influence of the promoting elements on a VPO catalyst and to associate the composition of the catalysts obtained from such techniques with their performance. The characterization techniques used include X-ray diffraction (XRD), BET-surface area, ICP-OES, EDS, 31PNMR, TPR, redox titrations, ATR and SEM to determine the phase composition of the catalysts, the surface area of the promoted catalysts relative to the un-promoted VPO, elemental mole ratios, the reducibility of the catalysts, average vanadium oxidation state, determination of the anions present in the surface of the catalysts and the variations in the morphology of the catalysts, respectively. Optimization of the system involved variation of the GHSV, the reactor temperature and the promoter loading. (Activation of a 0.75% n-butane in air mixture was performed at an optimum temperature of 400oC while varying the gas hourly space velocity to establish a range of feed conversions and subsequently determine the activity of each catalyst with respect to n-butane conversion). The promoted catalysts modified the morphology of the catalysts as evidenced by the scanning electron microscopy and the X-ray diffraction patterns. Furthermore an improved conversion was obtained with these catalysts. However, only the 0.1% iron-promoted catalyst improved maleic anhydride yield leading to ca. 10% maleic anhydride yield increment. Yields of 46% and 55% were obtained at GHSVs of 2573 and 1450 per hour respectively and a temperature of 400oC. Electronic and structural modifications were encountered leading to an improved catalytic performance. The performance of this catalyst is associated with a vanadyl pyrophosphate phase (XRD), and a limited and controlled amount of V5+ species as illustrated in the TPR, and solid state 31P NMR data. Moreover, this modification can be considered both structural and electronic in nature as observed in the SEM images and FTIR spectra of this catalyst. Furthermore, this improved performance is possible at higher conversions 80 to 90% conversion. / Thesis (M.Sc.)-University of KwaZulu-Natal, Westville, 2009.
3

The oxidative dehydrogenation of n-Hexane and n-Octane over vanadium magnesium oxide catalysts.

Chetty, Jonathan. January 2006 (has links)
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.
4

The selective oxidation of n-butane to maleic anhydride.

January 2003 (has links)
Industrial catalysts used in commercial processes for the production of maleic anhydride are mainly Vanadium Phosphorous Oxide (VPO) catalysts. The VPO catalyst used is Vanadyl Pyrophosphate (VO)2P207 made from its precursor Vanadium Phosphorous Hemi-Hydrate VOHP04.O.5H20 in an non-aqueous medium. In order for the VPO catalyst to perform optimally, a metal promoter, Ru, was selected as the doping agent in this study. Four catalysts of different metal doping concentrations (undoped, 0.2%, 0.6% and 1%) were subjected to the oxidation of n-butane. Promoters are added to facilitate the oxidation of n-butane to maleic anhydride. n-Butane gas is now being used in many industrial processes, in fixed bed reactors to convert the gas to maleic anhydride. Catalysts were calcined under high temperatures under a nitrogen atmosphere. It was found that with an increase in reaction temperature, there was an increase in conversion of n-butane to maleic anhydride. Selectivity of the product also showed an increase with an increase in temperature at a Gas Hourly Space Velocity (GHSV) of 1960-2170hr-1. Catalysts were characterized using different techniques such as Electron Dispersive X-Ray Spectroscopy, Inductively Coupled Plasma-Atomic Emission Spectroscopy, Fourier Transform - Infra Red, Average Oxidation State, Brunauer Emmett and Teller (surface area), X-Ray Diffraction and Scanning Electron Microscopy. The 0.6% Ru promoted VPO catalyst showed to be most effective in terms of conversion, selectivity and yield, at a temperature of 450°C as compared to the other catalysts studied. The catalysts degenerated after being subjected to higher temperatures. The selectivity obtained by this catalyst was at 70.2% and the yield obtained was 37%. This study showed that with an increase in Ru up to a certain concentration (0.6%), an increase in selectivity and yield was observed, thereafter, with additional Ru doping, a decrease in selectivity and yield was obtained. / Thesis (M.Sc.)-University of Natal, 2003.
5

A comparative study of VPO catalysts in the oxidation of butane to maleic anhydride.

Govender, Nishlan. January 2002 (has links)
Co promoted and unpromoted vanadium-phosphorous-oxide (VPO) catalysts were synthesized via an organic route. The catalyst precursor was calcined and then conditioned in a reactor, forming the active vanadyl pyrophosphate, (VO)2P2O7, phase. Different promoter loaded catalysts were synthesized and their effect on the yield of maleic anhydride (MA) from n-butane oxidation was examined at different temperatures and gas hourly space velocities (GHSV). The catalysts were tested as a powder. The catalysts were examined in the oxidation of n-butane gas, over air as an oxidant, in a specially designed and constructed continuous flow, fixed-bed catalytic micro-reactor equipped with an on-line gas chromatography (GC) monitoring system. A thermal conductivity detector (TCD) was employed for carbon oxide monitoring and a flame ionization detector (FID) for all other products. The catalysts were characterised by X-ray diffraction (XRD) to determine the phases present in the precursor, calcined and used catalysts. The Brunauer-Emmet-Teller (BET) surface area was calculated for the different promoter loaded catalysts. Fourier transform infrared (FT-IR) spectra, via the KBr pellet method, and attenuated total reflectance (ATR) spectra were recorded to determine the anions present in the bulk and surface of the catalyst respectively. Energy dispersive X-ray (EDX) and inductively couple plasma-atomic emission spectroscopic (ICP-AES) techniques were employed to determine the elemental composition on the surface and in the bulk of the catalyst respectively. Scanning electron microscopic (SEM) images of the catalysts during different stages of their investigation were recorded. The average vanadium oxidation state (AV) in the bulk of the catalyst was determined via a titrimetric method. The catalysts were optimized to a high yield and selectivity of MA. The operating temperature, GHSV and promoter loading on the catalyst were the parameters that were changed during the testing of the catalyst. Different stages of the catalyst's life were characterised via the techniques mentioned above. The catalysts were monitored over a 200-hour period on average, usually taking approximately 24 hours to equilibrate. One such Co promoted catalyst yielded 45 % MA at 275°C and GHSV of 2878 hr-1 on equilibration, with an n-butane conversion of 73 %, whilst all previously reported VPO catalysts produce far lower MA yields at this temperature. / Thesis (M.Sc.)-University of Natal, Durban, 2002.
6

An investigation into the antidiabetic and catalytic properties of oxovanadium(IV) complexes

Walmsley, Ryan Steven January 2012 (has links)
In part 1 of this thesis, the antidiabetic activity of a series of novel oxovanadium(IV) complexes was investigated. A range of bidentate N,O-donor ligands, which partially mimic naturally occurring bioligands, were prepared and reacted with the vanadyl ion to form the corresponding bis-coordinated complexes. Initially, 2-(2ˊ-hydroxyphenyl)-1R-imidazoline (where R = H, ethyl and ethanol) ligands were prepared. The aqueous pH-metric chemical speciation was investigated using glass electrode potentiometry which allowed for the determination of protonation and stability constants of the ligands and complexes, respectively. The species distribution diagrams generated from this information gave an indication of how the complexes might behave across the broad pH range experienced in the digestive and circulatory systems. This information was used to create an improved 2nd generation of ligands that were constructed by combining the imidazole and carboxylic acid functionalities. These corresponding bis[(imidazolyl)carboxylato]-oxovanadium(IV) complexes displayed a broader pH-metric stability. Both sets of complexes improved glucose uptake and reduced coagulation in vitro. In part 2 of this thesis, a range of homogeneous and heterogeneous oxovanadium(IV) catalysts were prepared. Firstly, Merrifield beads were functionalized with ligands from Part 1 and then reacted with vanadyl sulfate to afford the corresponding heterogeneous catalysts. These displayed promising catalytic activity for the peroxide facilitated oxidation of thioanisole, styrene and ethylbenzene as well as the oxidative bromination of phenol red. Smaller imidazole-containing beads with higher surface areas than the Merrifield beads were prepared by suspension polymerization. These beads similarly demonstrated excellent catalytic activity for the oxidation of thioanisole and were highly recyclable. In attempt to increase the exposed catalytic surface area, while retaining the ease of separation achieved in the before mentioned systems, micron to nano sized electrospun fibers containing coordinating ligands were fabricated. The corresponding oxovanadium(IV) functionalized fibers were applied to the oxidation of thioanisole using a continuous flow system. The flexible and porous nature of the fiber mats was well suited to this approach. After optimization of the reactant flow rate and catalyst amount, near quantitative (> 99%) oxidation was achieved for an extended period. In addition, leaching of vanadium was mitigated by modification of the attached ligand or polymer material.

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