Дисертації з теми "Fischer-Tropsch Chemistry"

Progress in catalyst development for reactions such as Fischer-Tropsch synthesis (FTS) has been impeded by the time consuming characterisation and catalytic testing of new formulations. Hence, this thesis discusses the development of high-throughput (HT) techniques for studying the deactivation of cobalt based catalysts under simulated FTS conditions. Libraries were rapidly synthesised by incipient wetness methods utilising robotic platforms and then treated in arrays under conditions designed to cause rapid ageing. HT X-ray diffraction (XRD) was performed before and after the ageing test to monitor the deactivation of the catalysts by sintering of the active metal particles or loss of metallic cobalt. HT thermogravimetric analysis in 5 % H2 was utilised to probe the reducibility of the catalysts and this information was then combined with results from XRD to inform decisions on which formulations to scale-up for further testing. This approach led to the discovery of highly stable Co/Ru/Mg/γ-Al2O3 catalysts. Thorough characterisation of selected hits was carried out to understand the phase assemblage. In addition to the high stability of Co/Ru/Mg/γ-Al2O3 catalysts, there was also a lowering of intrinsic activity. The degree to which the intrinsic activity was decreased was dependent on the amount of Mg in the catalyst and more specifically, the amount of Mg in close proximity to Co as identified by the amount of Mg incorporated into MgxCo3-xO4 phases prior to activation. In addition to incipient wetness synthesis, a method was also developed to perform HT co-precipitation synthesis with the aid of robotic platforms. HT synthesis was coupled with HT XRD to determine synthesis conditions giving rise to high surface area, phase-pure magnesium aluminate supports.

The Fischer-Tropsch Synthesis (FTS) is an alternative route to produce liquid fuels from a variety of carbon feedstocks including coal and biomass. Typically iron and cobalt based catalysts have been used for the FTS reaction, in which a mixture of CO and H2 (syn-gas) reacts to form hydrocarbons. Enhanced performance has been reported for iron-based systems doped with alkali metals and chalcogenides. Sulfides are considered a poison for most catalytic processes, but sulfur in the form of sulfates (SVI) is found to enhance the performance of iron based catalysts towards the FTS when present at low levels. In this study a wide range of iron based catalysts was prepared under varying synthesis conditions and with different dopants. The standard methods of preparation used were co-precipitation and incipient wetness impregnation. A structural study of a wide range of iron based catalysts was carried out using characterisation methods such as X-ray Absorption Fine Structure (XAFS) spectroscopy, X-ray Photoelectron Spectroscopy (XPS), Powder X-ray Diffraction (PXRD), Scanning Electron Microscopy (SEM), Energy Dispersive X-ray (EDX) and Brunner-Emmett-Taller surface area determination (BET). The characterisation was performed before and after reduction of the catalysts (under H2) to form the catalytically active materials. Before reduction, PXRD, XPS and quantitative analysis identified a haematite iron oxide structure (α-Fe2O3) for all samples. The crystallinity of the iron oxide materials varied between samples prepared in various conditions. The highest crystallinity was observed for the samples synthesised at pH7, fast titrant addition rate, at room temperature. The same techniques revealed changes in the iron oxide structure after reduction. The catalysts activated at 400 oC were mainly composed of Fe3O4 and those activated at 450 oC were a mixture of Fe2+, Fe3+ oxides and metallic iron Fe0. Moreover, the study of the role of alkali metals showed that some of the alkali promoters (K, Rb) may decrease the effective iron oxide reduction temperature. The nitrogen adsorption experiment was used to establish that iron oxide doped with different promoters had a mesoporous structure with a narrow pore size distribution. The SEM analysis indicated two different types of surface: irregularly shaped agglomerates with smaller round edged particles attached to their surface and homogenous agglomerates surfaces with sharp edges for the samples with different promoters. The most homogenous were the samples with Rb. All samples had small particles attached to the surface of larger agglomerates. An increase of the alkali metals on the surface after the activation process and migration of the alkalis to the surface with rising reduction temperature were observed using bulk and surface techniques (XRF, EDX and XPS). The differences in K K-edge shape of the XANES spectrum indicated changes in the local structure of K corresponding to changes of coordination number around K+ during activation. It was also observed that reduction influenced the sulfur species in iron oxide catalyst. For all the samples prior to reduction sulfates (SO42-) were detected by XPS and XAFS. After the reduction at 400 oC and 450 oC, characteristic XPS S 2p peaks for both sulfate and sulfide, were noticed. The sulfate/sulfide ratio was higher for the catalyst samples reduced at the lower temperature

Includes bibliography.
A composition of thermally oxidised Fischer-Tropsch hard wax was proposed based on a study ofthe oxidation products of model compounds, n-Cl6, n-C24 and n-C32. The model compound oxidation yielded isomers of alcohols and ketones with carbon numbers ranging from 2 to the same carbon number as that of the parent hydrocarbon, lactones ranging from carbon 6 to two less than the carbon number equal to that of the highest parent hydrocarbon, acids having carbon numbers ranging from two up to two less than the parent hydrocarbon and straight chain esters in low concentration having molecular mass similar to or higher than the parent hydrocarbon. Only methyl, ethyl and propyl esters of acids with carbon number similar to the parent hydrocarbon, were identified. Oxidised Fischer-Tropsch hard wax was distilled and the distillate was found to contain similar products to those of the model compounds. No 'new' products were detected which indicated that the same mechanism is occurred in the Fischer-Tropsch oxidation with the production of similar oxidation products as for the model compound oxidation. Fractionation of oxidised Fischer-Tropsch wax and the analyses of the fractions using IR, DSC and HTGC techniques verified the proposed composition of thermally oxidised Fischer-Tropsch hard wax. The most important conclusion that can be drawn from the research done for this dissertation is that none of the analytical results refuted the proposed composition of thermally oxidised Fischer-Tropsch hard wax.

The technique of x-ray photoelectron spectroscopy (XPS) has been applied to characterize iron-manganese catalysts used in Fischer-Tropsch synthesis. The catalysts, which vary in composition from 10 FE/90 Mn, to 50 Fe/50 Mn are analyzed after being placed in a slurry reactor and having synthesis gas reacted over them. Changes, in catalyst composition are investigated further using in situ techniques. Additionally, 20 Fe/80 Mn catalysts containing potassium in the range of 0.1 wt.% to 1.3 wt.% are analyzed in the same manner. These studies have permitted the identification of some of the factors influencing activation and deactivation, product selectivity, and surface speciation.

The new series of μ(α, ω)-alkanediyl compounds of ruthenium, [CpRu(CO)₂]₂[μ-(CH₂)ₙ], where n=5-10, have been prepared from Na[CpRu(CO)₂] and the corresponding diiodoalkane. These compounds, which are stable crystalline solids at ambient temperature, have been fully characterised by microanalysis, infrared, ¹H and ¹³C NMR spectroscopy, melting point and mass spectrometry. The new heterodinuclear complex [Cp(CO)₂Fe(CH₂)₄Ru(CO)₂Cp] has been synthesised by the reaction of [CpFe(CO)₂(CH₂)₄I] with Na[CpRu(CO)₂] and characterised by all the above mentioned techniques.

Includes bibliographical references (leaves 63-65).
Ruthenium complexes of several types have been synthesized, supported on silica and their activity in CO hydrogenation was investigated in order to determine the cluster size of surface Ru atoms required for the formation of hydrocarbons. Previous studies have shown that more than one metallic site is needed for the Fischer-Tropsch synthesis.

Thesis (PhD)--University of Stellenbosch, 2011.
ENGLISH ABSTRACT: The analysis of Fischer–Tropsch–derived (FT–derived) synthetic crude and derived products is very challenging because of the highly complex nature of these products. In this study, the use of comprehensive multidimensional gas chromatography (GCxGC) with time-of-flight mass spectrometry (TOF-MS) and flame ionisation detection (FID) was investigated for the analysis of these products and the technique was found to be invaluable for the analysis of these complex mixtures. The compositions of FT synthetic crude, produced at low temperature (LT–FT) and high temperature (HT–FT) processes were compared and the effect that changes in FT reaction temperature has on product formation was investigated. Results for conventional onedimensional GC (1D-GC) and GCxGC were compared. It was found that conventional 1D–GC does not have sufficient peak capacity to separate the thousands of compounds in the HT FT products. GCxGC provides a huge peak capacity of tens-of-thousands to separate highly complex mixtures. Structured chromatograms, where groups of compounds with similar properties are grouped together, aid in peak identification. Moreover, sensitivity at low microgram per milliliter levels is obtained. These attributes enabled accurate analysis of various complex feed and product streams in the FT refinery, and also various final fuel products. The use of GCxGC alone was demonstrated, and also combined with high performance liquid chromatography (HPLC), supercritical fluid chromatography (SFC) and nuclear magnetic resonance (NMR) when even more separation power was needed. HPLC–GCxGC enabled the separation of alkene and cyclic alkane compound classes in oligomerisation products. These compound classes have similar mass spectra, elute in adjacent regions and co–elute even to some extent on the GCxGC contour plot, making differentiation difficult. SFC is a good replacement for HPLC for these applications because it does not use solvents as mobile phases. CO2 is easily evaporated after the separation and does not interfere with the GCxGC separation of the analytes. SFC is also a very good technique to separate the compound classes of alkanes, alkenes, aromatics and oxygenates, and is therefore highly complementary to GCxGC. The combination of GCxGC with NMR data was also found to be very valuable for the identification of branched alkane isomers in LT–FT diesels. GCxGC provides excellent separation of individual compounds but the identification of isomers (except for mono–methyl branching) is difficult because the mass spectra of most of these isomers are similar and not all compounds are in the mass spectral libraries. NMR, on the other hand, is able to distinguish between the individual types of branched isomers but has limited separation power for the complex mixtures. By combining the two techniques, the best of both was obtained. The study found GCxGC to be invaluable for the analysis of the highly complex FT–derived products, while its combination with other techniques such as HPLC, SFC and NMR provided even more separation power.
AFRIKAANSE OPSOMMING: Die hoogs komplekse samestelling van sintetiese ru–olie en afgeleide produkte, afkomstig van Fischer–Tropsch (FT) sintese, bied groot uitdagings aan die analis. Die studie het die gebruik van GCxGC met ’n TOF-MS en FID bestudeer vir die analise van FT produkte en het bevind dat die tegniek van onskatbare waarde is vir die analise van die hoogs komplekse mengsels. Die samestellings van produkte van lae- en hoë-temperatuur FT prossesse is vergelyk en die effek van ’n verhoging in die reaksie–temperatuur op die produk samestelling is ondersoek. Resultate vir 1D–GC and GCxGC is vergelyk en dit was duidelik dat 1D-GC nie naastenby voldoende piekkapasiteit het om al die komponente van die produkte wat tydens die hoëtemperatuur prosses gevorm word, te kan skei nie. Die GCxGC se piekkapasiteit daarteenoor is in die orde van tienduisende wat die skeiding van hoogs komplekse mensels moontlik maak terwyl die tegniek hoogs gestruktureerde kontoerplotte verskaf wat help met identfikasie van komponente. Die tegniek is verder ook baie sensitief en kan komponente op lae μg/mL vlakke waarneem. Hierdie eienskappe het akkurate analise van verskeie FT produkstrome moontlik gemaak. Die kombinasie van GCxGC met HPLC, SFC en KMR het selfs meer skeidingskrag verskaf waar nodig. HPLC–GCxGC het die skeiding van alkene en sikliese alkane moontlik gemaak. Hierdie komponent klasse se massaspektra is feitlik dieselfde en terselfdertyd elueer die twee groepe reg langs mekaar, en oorvleuel soms selfs tot ’n mate, op die GCxGC kontoerplot, sodat dit moeilik is om daartussen te onderskei. SFC is ’n goeie alternatief vir HPLC in meeste toepassings aangesien die tegniek net CO2 gebruik, wat maklik verdamp by kamertemperatuur en nie oplosmiddels gebruik wat se pieke steur met die van die laekookpunt komponente op die GCxGC kontoerplot nie. Skeidings van die komponentgroepe alkane, alkene, aromate en oksigenate is moontlik met SFC en daarom komplimenteer dit die GCxGC skeiding goed aan. Die kombinasie van GCxGC met kern–magnetiese resonansie (KMR) is van waarde gevind om die verskillende tipes vertakkings in ’n lae-temperatuur FT diesel te identifiseer. GCxGC verskaf uitstekende skeiding van individuele komponente maar die identifikasie van die verskilende isomere, behalwe vir die mono-metiel vertakkings, is moeilik aangesien die massaspektra van baie van die komponente soortgelyk is en die komponente nie in die massa spektrum–biblioteke voorkom nie. KMR, aan die ander kant, kan tussen die individuele vertakkings onderskei maar het beperkte skeidingskrag vir komplekse mensels. Deur die twee tegnieke te kombineer is die beste van albei tegnieke bekom. Die studie het bevind dat GCxGC van onskatbare waarde is vir die analise van die komplekse sintetiese FT produkte terwyl die kombinasie met ander tegnieke soos HPLC, SFC and KMR selfs meer skeidingskrag verskaf.

Includes abstract.
Includes bibliographical references (leaves 157-163).
The differential scanning calorimetry (DSC) analyses of the Fischer-Tropsch (FT) hard waxes display multiple melting peaks, the origin of which is unknown. The phenomenon is sometimes referred to in the literature, but no attempt has been made to explain its cause. There are a few known causes of melting bimodality in n-alkanes and their mixtures, petroleum waxes and polymers. These are: polymorphism, chain folding and bimodal molecular weight distributions

The objective of this study is to improve the catalytic performance of silica-supported Fischer-Tropsch cobalt based catalyst. Iron and nickel catalyst were also briefly studied. Initial work focused on synthesis of porous silica via oxidative thermal decomposition of polydimethylsiloxane (PDMS) and its characterisation. It was shown that PDMS undergoes at least two thermal degradation steps to form silica powder. It was also demonstrated that increase in isothermal time at constant temperature and increase in temperature at constant time could be used to tune the surface area and pore volume of the synthesized silica powder. Subsequently, a novel one pot technique called the swelling in method (SIM) was developed, and employed to synthesize silica-supported cobalt, iron and nickel based Fischer-Tropsch catalyst. The results of silica-supported cobalt based catalyst prepared by the swelling in method were compared with those synthesized by incipient wet impregnation method. The colloidal method was also combined with the swelling in method to prepare silica-supported cobalt nanoparticles catalyst. Characterisation of cobalt, iron and nickel based catalyst prepared by the swelling in method showed that PDMS as the initial catalyst support converted to silica powder after oxidative calcination. Physicochemical properties of silica-supported cobalt, iron and nickel catalyst prepared by the swelling in method suggest that the oxides of each metal were present inside the silica pores while cobalt based catalyst prepared by the same method had better surface area and pore volume compared to the catalyst synthesized by the incipient wetness impregnation technique. Catalytic performance of the catalyst synthesized by the swelling in and incipient wetness methods were studied in High Temperature Fischer-Tropsch synthesis reaction condition. The results showed that silica-supported cobalt based catalyst prepared by the swelling in method was overall more active, generated less methane and less susceptible to deactivation by sintering and carbon deposition when compared to the catalyst prepared by the impregnation technique. Silica-supported cobalt nanoparticles catalyst had the best catalytic activity in comparison to all the catalyst studied in this work. Silica-supported cobalt based catalyst prepared by the swelling in method using cobalt nitrate exhibited the best catalytic activity while the catalyst synthesized from cobalt acetate had the least activity. The addition of ruthenium to silica-supported cobalt catalyst contributed in minimising the formation of methane when compared to the catalyst without ruthenium. Silica-supported iron and nickel based catalyst showed reasonable catalytic activity, and as expected the amount of methane generated by nickel catalyst was relatively very high compared to all the catalyst studied in this thesis.

The primary objective of this study was to characterise standard iron based Fischer- Tropsch catalysts by using the hydrogenation of CO at elevated temperatures and ambient pressure as a test reaction. The reaction test data is supplemented by the following analytical techniques; X-ray diffraction (XRD), Raman scattering, temperature programmed oxidation (TPO), transmission electron microscopy (TEM) and also inelastic neutron scattering (INS). The aim of these studies is to characterise the hydrocarbonaceous species present on the catalyst surface after reaction and to propose what role these hydrocarbonaceous species may play in the Fe/CO/H2 surface chemistry; be it active or not. These initial characterisation studies of the reacted iron catalysts were then extended to study the temporal dependence of these species with increasing reaction time. The application of inelastic neutron scattering is shown to provide a great deal of information regarding the hydrocarbonaceous component present in these types of systems. Therefore the application of INS and the subsequent result constitute the majority of the discussion of this study. The research objectives for this study can be listed as follows; 1. To prepare an iron oxide catalyst using a reproducible and controlled method and to characterise this material using the methods outlined previously. 2. To react the iron oxide catalysts using a representative test reaction for the purpose of characterising the surface species present after reaction. 3. To use inelastic neutron scattering to probe and gain a vibrational spectroscopic insight to the hydrocarbonaceous species present. 4. To study the temporal dependence of the hydrocarbonaceous species and how they alter with increasing time-on-stream. The thesis will begin with an introduction to Fischer-Tropsch catalysis, with a focus on iron-based Fischer-Tropsch catalysis. The discussion will move to some heightened analysis of some previously reported INS measurements of iron Fischer-Tropsch catalysts. This prelude will be followed by the main discussion of results, that is the characterisation of the reacted iron oxide catalyst using inelastic neutron scattering and the temporal study with increasing time-on-stream. The thesis will then finish with a discussion on a brief study investigating the role of promoters in iron based Fischer-Tropsch catalysts, which constitutes future work.

The effects of precursor, support and calcination procedure on the physical and chemical properties of supported cobalt catalysts have been investigated. A multiple characterisation approach of thermogravimetric analysis, differential scanning calorimetry, X-ray diffraction and transmission electron microscopy was employed in order to gain understanding into the calcination and reduction processes. In addition, the catalysts were screened on a purpose built fixed bed reactor, under industrially relevant conditions, to determine effect of catalyst preparation on Fischer-Tropsch activity.

Iron Fischer-Tropsch (FT) catalysts are typically prepared as iron oxides which are reduced to FT-active iron metal and iron carbide prior to FT synthesis. The iron oxides contain a variety of different chemical and structural promoters to alter FT-activity. Silica is a common structural promoter which stabilises the formation of small crystallites and provides mechanical integrity to the catalyst. However, silica inhibits the reduction of the oxide precursor to the FT-active phases. This ultimately affects catalyst activity and product selectivity. It has been proposed that the silica interacts with the iron to form encapsulating shells of fayalite (Fe2SiO4), or fayalite rafts between the iron oxide and the silica support. In this study, six silica-promoted iron oxide samples were prepared using a simple co-precipitation technique. Samples contain varying amounts of silica, and the samples are named 100/x Fe/SiO2, where x is the weight of silica for 100 weight iron, with x taking on values of 0, 10, 25, 50, 100 and 200. The resulting iron oxides were characterised using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray powder diffraction (XRPD), M¨ossbauer spectroscopy (MS), magnetic susceptibility measurements (MM), Raman spectroscopy, thermal gravimetric analysis (TGA) and nitrogen physisorption. Their reduction in a hydrogen atmosphere was investigated using temperature programmed reduction (TPR), in situ XRPD and TEM. The reduction in hydrogen of 100/0 Fe/SiO2 and 100/10 Fe/SiO2 was also studied using in situ gas flow TEM cells. These cells allow the samples to be studied in the electron microscope at temperature and pressure conditions approaching those experienced in a real reactor environment. In the absence of a silica promoter (100/0 Fe/SiO2), hematite particles are formed with mean particle diameters of 39 ± 12 and 52.7 ± 0.2 nm determined using TEM and XRPD respectively. MM data reveals a magnetic transition (Morin transition) at≈230 K, consistent with a mean particle size of≈50 nm. In a hydrogen atmosphere, the hematite reduces to metallic iron via a two-step process viz. hematite → magnetite → iron. The final iron particles have an average crystallite size of 68.0 ± 0.2 nm. The presence of lower amounts of silica in the samples 100/10 Fe/SiO2, 100/25 Fe/SiO2 and 100/50 Fe/SiO2 results in the formation of silicasubstituted 2-line ferrihydrite particles. Bands in the Raman spectra of these samples shift on increasing silica content, which indicates an increasing number of Fe-O-Si bonds within the ferrihydrite framework. MM reveals typical superparamagnetic (SPM) behaviour above a blocking temperature in the range 39 - 68 K which gives mean particle sizes of 4.2, 3.6 and 3.5 nm for 100/10 Fe/SiO2, 100/25 Fe/SiO2 and 100/50 Fe/SiO2 respectively, in good agreement with particle sizes determined using TEM (3.1±0.4, 2.4±0.3 and 2.4±0.3 nm respectively). MS data at 300 K and 4.2 K were fitted with distributions of ∆EQ and Bhf respectively. The median values of Bhf decrease with increasing silica content, indicating greater degrees of distortion in the Fe3+ environments induced by increased silica substitution. The reduction to metallic iron occurs via a three-step process viz. hematite → magnetite → wu¨stite → iron, with the silica stabilising the wu¨stite phase. The increasing amount of Fe-O-Si bonds on increasing silica content shifts reduction to higher temperatures broadens each reduction step as a result of local Fe-O-Si concentration variations. Fractions of each sample are not completely reduced even at 1000°C, with the relative proportion increasing with increasing silica content. In situ gas flow TEM studies reveal that the mechanism of reduction involves the liberation of atomic iron atoms from the silica-substituted iron oxides which agglomerate and grow into final iron particles. This leaves a poorly crystalline Fe-O-Si bonded framework behind. STEM-EDS and STEM-EELS reveal low concentrations of silicon at the surface of the resulting iron particles, however they do not form encapsulating shells of fayalite as previously suggested. The majority of the silica remains in the Fe-O-Si material which may crystallise into separate fayalite particles at elevated temperature. The presence of silica in high proportions (100/100 Fe/SiO2 and 100/200 Fe/SiO2) results in the formation of a two-phase system consisting of silicasubstituted 2-line ferrihydrite particles which are encapsulated in an ironinfused amorphous silica network. As with the other silica-bearing samples, there is an increase in Fe-O-Si bonds and an increase in the degree of distortion at Fe3+ sites with increasing silica content. The large amount of silica suppresses the blocking temperature of the SPM crystallites. In a hydrogen atmosphere, the reduction to metallic iron follows the same three step process as the other silica-bearing samples. Reduction temperatures are further shifted to higher values and given reduction steps are considerably broader with increasing silica content. The fraction of iron not fully reduced also increases. Iron particle diameters are very small, since encapsulation by the silica matrix prevents growth of particles.

10 % Co/ZnO, 10 % Co/SiO2 and 10 % Co/TiO2 catalysts were prepared and screened using a fixed-bed reactor under industrially relevant F-T conditions. These materials have been characterised using a range of analytical techniques, including N2 porosimetry, XRD, TPR and CO pulse chemisorption analysis, in order to better understand how their physical and chemical properties are related to F-T performance. Physical and chemical properties of each catalyst varied significantly depending on the nature of the support material. A positive correlation between Co3O4 crystallite size and the average pore diameter of the support was observed. Therefore, the average Co3O4 crystallite size was thought to be determined by the nature of the support pore structure. Co/SiO2 reduced at a slightly lower temperature than Co/TiO2 and Co/ZnO. TPR profiles were also broader in nature for Co/TiO2 and Co/ZnO indicating increased metal-support interactions during catalyst activation. F-T activity of each catalyst was screened and the nature of the support material significantly influenced performance. Specific activity increased with increasing XRD crystallite size, metal dispersion and average pore diameter. However, turn over frequencies did not vary significantly between the three supported catalysts. Therefore, activity per active site was largely independent of support or dispersion effects. The difference in specific activity was thought to be a result of increased metal-support interactions influencing the extent of catalyst reduction. C7+ selectivity was seen to increase with decreasing XRD crystallite size, Co particle diameter and average pore diameter. Co/SiO2 had the highest C7+ selectivity value under steady state conditions. This trend was reflected in the shape of the liquid hydrocarbon product distributions which showed a shift towards higher molecular weight products. No correlation between C7+ selectivity and o/p ratio was observed in. Therefore, re-adsorption of α-olefins did not appear to account for differences in C7+ selectivity. Wax product distributions, ASF α-values and o/p ratios were consistent for all three catalysts. The primary aim of this thesis was to further develop the understanding of how sulphur poisons typically found as impurities in coal, natural gas and biomass derived syngas alter the performance of cobalt based F-T catalysts. The sulphur tolerance of an industrially supplied 10 % Co/ZnO catalyst was tested in a range of in-situ poisoning studies. Various concentrations of H2S, DMS and C2H5SH were added directly to a purpose built fixed-bed reactor and the effects on catalytic activity and selectivity were monitored. As the concentration of DMS added to the reactor increased, the length of time required for complete catalyst deactivation decreased. At 1 ppm the total DMS inlet required for deactivation was 13.5 μmol.g-1 giving a surface sulphur coverage of 0.4. Therefore at 1 ppm, one molecule of DMS was seen to deactivate approximately 2.5 surface cobalt sites. The total DMS inlet required for complete deactivation decreased with decreasing concentration of S fed to the reactor. Therefore, the effects of DMS on F-T activity of 10 % Co/ZnO were more marked at lower concentrations. Effects of H2S and C2H5SH were also tested and total S inlet required for complete activity loss increased with increasing molecular size. S poisoning also brought about an increase in o/p ratio without influencing chain growth probability. It was concluded that Co/ZnO has specific catalytic sites that independently catalyse the secondary hydrogenation of olefins and their chain-growth to higher molecular weight products. Sulphur was thought to poisons those sites responsible for hydrogenation and not those responsible for chain growth. The extent to which secondary hydrogenation was suppressed was higher using H2S compared to DMS and C2H5SH. H2S pulse chemisorption studies were carried out on all three supported catalysts. The nature of the support material did not significantly influence the amount of H2S required to saturate the catalyst surface. No bulk sulphides were detected to have formed during pulse chemisorption analysis. The ZnO support also offered no selective adsorption of H2S over the active Co metal.

90 leaves on CD format, preliminary i-ix pages and numbered pages 1-81. Includes bibliography, list of figures in color to pdf format (OCR).
Thesis (MSc (Chemistry and Polymer Science))--University of Stellenbosch, 2005.
ENGLISH ABSTRACT: The composition of petrochemical products obtained from Fischer Tropsch (FT) technologies is of the highest complexity possible and may contain thousands of components. Chemicals produced from FT feedstocks often contain trace level contaminants that can poison catalysts or that affect product performance in down-line processes. Single dimension GC analysis of these mixtures provides incomplete information because of lack of separation power. This study evaluates the separation power of heart-cut GC-GC, comprehensive GCxGC and sequential GC-GC for three selected challenging petrochemical applications. The fundamental theoretical aspects of the techniques are discussed. Oxygenates are removed as far as possible in C10 – C13 alkylation feedstocks, used in the production of linear alkyl benzenes, because the oxygenates may have deactivating effects on some expensive alkylation catalysts. Residual oxygenates may still be present and can consist of hundreds of components. Detection of individual components at ng/g levels is required. Heart-cut GC-GC is used to illustrate the separation and enrichment power for oxygenates in an alkylation feedstock. The stationary phase in the first dimension column was selected to provide separation of the oxygenates from the hydrocarbons in a relatively narrow window. The oxygenate fraction is then enriched by repeated injections and collection on the cryotrap. After sufficient enrichment, the trap is heated and the oxygenates are analysed on the second dimension column. Comprehensive GCxGC and Sequential GC-GC are compared for the separation and analysis of the oxygenated chemical component classes in the alkylation feedstock, before removal of oxygenates. Cyclic alcohols can occur in detergent alcohols produced from FT feedstocks. These cyclics are regarded as impurities because they affect the physical properties of the detergents. The cyclic and noncyclic alcohols in a narrow C12 – C13 detergent alcohol distillation cut have similar boiling points and polarities, and separation of individual components is thus difficult to achieve. Comprehensive GCxGC and sequential GC-GC are evaluated for the separation of the alcohol component classes. The study shows that both approaches provide component class separation but the high resolving power of the second column and the optimal chromatographic operating conditions of sequential GC-GC provide better separation of the individual components. The study illustrates the immense power of the three multidimensional GC techniques namely heart-cut GC-GC, comprehensive GCxGC and sequential GC-GC. The three multidimensional GC techniques each have their own advantages, disadvantages and unique applications and should be used as complementary rather than as competitive analytical tools.
AFRIKAANSE OPSOMMING: Fischer Tropsch (FT) petrochemiese produkte is van baie hoë kompleksiteit en kan uit duisende komponente bestaan. Chemikalië afkomstig van dié voerstrome bevat soms spoorhoeveelhede onsuiwerhede wat deaktiverend op kataliste kan inwerk of wat die werkverrrigting van finale produkte kan beïnvloed. Enkeldimensie GC analises van die komplekse mengsels is meesal onakkuraat as gevolg van geweldige piekoorvleueling. Die studie evalueer die skeidingsvermoë van drie multidimensionele tegnieke, Heart-cut GC-GC, Comprehensive GCxGC en Sequential GC-GC vir geselekteerde petrochemiese toepassings. Die fundamentele teoretiese aspekte van die tegnieke word bespreek en drie analitiese toepassings word beskryf. Oksigenate word so ver moontlik verwyder uit C10 – C13 paraffien-voerstrome, wat gebruik word in die vervaardiging van liniêre alkielbenzene, aangesien dit deaktiverend kan inwerk op alkileringskataliste. Die oorblywende oksigenate kan uit honderde komponente bestaan sodat analise van individuele komponente tot op lae ng/g vlakke nodig is. Heart-cut GC-GC word gebruik om die skeiding en verryking van die oksigenate in die alkileringsvoerstroom te illustreer. Die stationêre fase in die eerste-dimensie kolom is so gekies dat skeiding tussen oksigenate en koolwaterstowwe verkry word. Met herhaalde inspuitings verhoog die oksigenaat-konsentrasie op die cryo val en - na voldoende verryking - word die val verhit en die oksigenate geanaliseer op die tweede dimensie kolom. Die skeiding en analises verkry met Comprehensive GCxGC en Sequential GC-GC word vergelyk vir die chemiese klasse-skeiding van die alkileringsvoer (voor verwydering van oksigenate). Sikliese alkohole kan voorkom in detergent-alkohole vervaardig vanaf FT voerstrome. Dit word as onsuiwerhede beskou aangesien dit die fisiese eienskappe van die finale produkte beïnvloed. Die sikliese en nie-sikliese alkohole se kookpunte en polariteite is baie naby aanmekaar sodat skeiding van individuele komponente moeilik verkry word. Comprehensive GCxGC en Sequential GC-GC word evalueer vir die skeiding van die alkohol. Die studie toon aan dat albei die tegnieke skeiding gee van die chemiese komponent-klasse maar dat die hoë-resolusie tweede-dimensie kolom en die optimisering van die experimentele kondisies van die Sequential GC-GC sisteem beter skeiding van individuele komponente gee. Die uitsonderlike skeidingsvermoë van die drie multidimensionele tegnieke, Heart-cut GC-GC, Comprehensive GCxGC en Sequential GC-GC word geïllustreer in die studie. Elke tegniek het sy eie voordele, nadele en unieke toepassings en die drie tegnieke behoort as komplementêre eerder as kompeterende tegnieke gebruik te word.

Today, a majority of the world’s energy need is supplied through sources that are finite and, at the current usage rates, will be consumed shortly. The high energy demand and pollution problems caused by the widespread use of fossil fuels make it increasingly necessary to develop renewable energy sources of limitless duration with smaller environmental impact than the traditional energy sources.

Three fuels – rapeseed methyl ester (RME), Fischer-Tropsch (FT) diesel and FT jet fuel – derived from biomass, coal or gas were evaluated in this project. The fuel properties evaluated are in most cases listed in standards, often with recommendations, developed for biodiesel, petroleum diesel and jet fuel.

Biodiesel is monoalkyl esters, e.g. RME, produced by transesterification of triglycerides in vegetable oil and an alcohol to esters and glycerin. This produce a fuel that is suitable as a direct substitution for petroleum diesel. Biodiesel may be used in pure form or in a blend with petrodiesel. Oxidative degradation and weak low temperature performance of biodiesel are properties of concern when substituting petrodiesel with biodiesel, as was shown in this project. The experiments show that oxidative stability can be improved with a synthetic antioxidant, e.g. butylated hydroxytoluene (BHT).

The FT process converts syngas (a mixture of hydrogen and carbon monoxide) to a range of hydrocarbons. Syngas can be generated from a variety of carbon sources, e.g. coal, natural gas and biomass. The high-temperature (300-350 °C) FT process with iron-based catalysts is used for the production of gasoline and linear low molecular mass olefins (alkenes). The lowtemperature (200-240 °C) FT process with either iron or cobalt catalysts is used for the production of high molecular mass linear waxes. By applying various downstream processes, fuels suitable for substitution of petrodiesel and conventional jet fuel can be obtained. The FT fuels have lower densities than the conventional fuels. However, conclusions from this project are that most of the properties of FT fuels are better, or equal, than conventional petroleum fuels.

La chimie de surface de petites nanoparticules métalliques ( ~ 1 nm), principalement de ruthénium ou d'alliages de ruthénium, a été étudiée par une approche théorique au niveau DFT. Cela est appuyé par le développement d'outils d'analyse de propriétés structurales, électroniques et thermodynamiques de ces nanoparticules. Une première partie est consacrée à l'étude des propriétés structurales de nanoparticules métalliques. La variété de morphologie des nanoparticules ainsi que la nécessité de pouvoir générer des modèles appropriés sont mises en évidence. En particulier, l'affinement de la génération de modèles structuraux théoriques est rendu possible via l'implémentation de méthodes de modélisation de nanoparticules génériques couplées à l'utilisation de la méthode de Monte Carlo inversé permettant se rapprocher au plus près de la réalité expérimentale. L'application à ces nanoparticules de descripteurs électroniques ou morphologiques, tels que le d-band center ou le nombre de coordination généralisé, est par la suite proposée en relation avec leur capacités d'adsorption, et plus généralement dans le cadre du principe de Sabatier. Un descripteur électronique de la liaison chimique (COHP) est appliqué aux différentes nanoparticules, pour mettre en évidence les différences entre structures aussi bien que la nature des interactions au sein du cœur métallique, ainsi qu'entre ce cœur et les espèces de surface. Enfin, l'adsorption d'espèces à la surface de ces modèles est étudiée. L'adsorption d'un seul ligand à la surface d'une nanoparticule modèle est utilisée comme sonde de détermination de sites d'adsorption préférentiels, puis des taux d'adsorption plus élevés sont considérés dans le but d'étudier l'influence de celui-ci sur l'adsorption de ligands surnuméraires, ainsi que pour rendre compte de l'influence des ligands de surface sur la morphologie du cœur métallique. Pour cela, les propriétés thermodynamiques des systèmes adsorbés ont été modélisées par prise en compte de l'influence de la pression et de la température sur la stabilité relative des diverses structures via une modélisation de thermodynamique ab initio. Enfin, cette même approche à été utilisée pour étudier la co-adsorption de ligands H2 et CO à la surface de nanoparticules de ruthénium et de rhénium dans le cas particulier de la synthèse de Fischer-Tropsch, permettant notamment de proposer un intermédiaire thermodynamiquement favorable pour cette réaction. Une étude préliminaire de cette réaction, d'un fort intérêt chimique et sociétal, conclut ce manuscrit. L'utilisation combinée des approches structurale, électronique et thermodynamique permet alors d'avoir un point de vue élargi sur certains aspects de la chimie de ces nanoparticules de ruthénium
Surface chemistry of small metallic nanoparticles ( ~ 1 nm), mainly ruthenium or ruthenium alloys, has been studied at the DFT level via a theoretical approach. This study is supported by the development of analytical tools, that allow to investigate structural, electronic and thermodynamical properties of those nanoparticles. A first part is dedicated to the structural properties of metallic nanoparticles. Morphological diversity is highlighted as well as the necessity of being able to desing reliable models. The refinement of structural models is made possible via the combined use of generic nanoparticles structure design and of the reverse Monte Carlo method in order to fit experiments. Electronic or morphologic descriptors such as d-band center or generalized coordination number are applied to those nanoparticles, in relationship with their adsorption possibilities and, to a larger extent, with the Sabatier principle. An electronic descriptor of the chemical bond (COHP) is applied to the considered nanoparticles in order to show differences between structures, as well as the interactions within the metallic core and between the core and surface species. Finally, adsorption of surface species is studied. A single ligand probe is used to spot favorable adsorption sites, then higher coverages are considered so as to test its influence on the adsorption of extra ligands, and to investigate the effect of surface ligands on the metallic core morphology. To do this, thermodynamical properties of adsorbed systems have been modeled by taking into account the effect of pressure and temperature on the nanoparticles relative stabilities via ab initio thermodynamics. The same approache was eventually applied to H2/CO coadsorbed at ruthenium and rhenium nanoparticles surface, in the context of the Fischer-Tropsch synthesis, allowing to propose a thermodynamically favorable intermediate for this reaction. Preliminary study of this reaction, of high chemical and societal interest, conclude this manuscript. The combined use of structural, electronic and thermodynamical approaches widens the overview on some aspects of ruthenium nanoparticles chemistry

Preliminary studies of the Fischer-Tropsch synthesis (FTS) process were begun in 2010 at the University of Birmingham. A mini-scale F-T plant was designed and built at the School of Mechanical Engineering to study the production of long-chain hydrocarbons over a cobalt-based FTS process. For this purpose, a series of eggshell cobalt catalysts supported with silica powder with a dissimilar porous structure were investigated to examine the effect of support variables on the catalysts’ performance. The prepared catalysts were characterized with nitrogen adsorption/desorption, X-Ray Diffraction (XRD), Temperature-Programmed Reduction (TPR), Scanning-Electron Microscopy (SEM) and Energy-Dispersive X-ray Spectroscopy (EDS) experiments to ensure the qualification of the catalysts for the F-T plant. A highly metal-dispersed catalyst was achieved by controlling three key parameters: (i) cobalt content, (ii) impregnation solution and (iii) meso-porous silica of average pore diameter during catalyst preparation. The catalysts were relatively activated at high temperature because of the formation of small particles. The concentration of the active site was maximized in order to enlarge the hydrogenation activity of the cobalt-based eggshell catalyst to produce middle distillates products. The optimisation study of the F-T process at low-temperature and low/medium pressure was performed to acquire the maximum production of liquid diesel fuel in a single-pass F-T process. The orthogonal arrays’ approach was employed to design a set of experiments. The investigations were successful to maximise the conversion in reactants (up to 98%) and lower the activity of the co-reactions at the same time. The change in reactant consumption and hydrocarbons’ selectivity was monitored over the time on stream and the responsible mechanisms for short-term deactivation within the first reaction cycle were studied, to achieve the optimum reaction conditions in terms of later deactivation of the catalyst.

Ce mémoire est consacré à l'élaboration et à l'étude physico-chimique de nouveaux catalyseurs bifonctionnels pour l'électroréduction du CO2. Dans ce contexte, une approche dite « moléculaire » et une dite « inorganique » ont été développées.
Pour l'approche « moléculaire », des catalyseurs bifonctionnels, nouveaux complexes hétérobimétalliques du type [Cl(CO)3Re(L)M(Cp*)Cl]+ (L = ligand bisdiimine ; M = Ir, Rh ; Cp* = η5-pentaméthyl-cyclopentadiényle) ont été synthétisés, et les interactions intramoléculaires entre les centres métalliques ont été étudiées. Des électrocatalyses préparatives de réduction du CO2 ont été conduites avec ces complexes en solution homogène mais aussi avec des électrodes modifiées obtenues par électropolymérisation anodique des pyrrole fonctionnalisés par ces mêmes complexes en milieu hydro-organique ou aqueux.
Pour la deuxième approche « inorganique », nous avons mis au point la synthèse des précurseurs adéquats pour élaborer des films fonctionnalisés par des complexes carbonyle de ruthénium cationiques [Ru(L)(CO)2(MeCN)2]2+ et [Ru(L)(CO)2(MeCN)]22+ (L = bipyridine substituée par des pyrroles), substrats nécessaires à la préparation de matériaux composites associant des nanoparticules métalliques et un polymère rédox. Ces complexes ont été déposés à la surface d'électrodes par électropolymérisation anodique des pyrroles et ont ainsi permis d'obtenir des films cationiques précurseurs de catalyseurs bifonctionnels.
Les résultats des électrocatalyses de réduction du CO2 avec les composés issus des deux approches montrent qu'il existe des effets coopératifs au sein des catalyseurs bifonctionnels.

School of Chemical and Metallurgical Engineering, Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, South Africa 26 February 2016
Many reports in the open literature have focused on Fischer-Tropsch (FT) kinetics, yet none of them appear to be able to explain FTS completely. Few of the FT models consider the production of olefins and paraffins separately. To study whether the selectivity to olefins and paraffins follows similar trends and if kinetics alone suffices to explain FT phenomena, a series of FT experiments were conducted in a fixed bed reactor loaded with 10% Co/TiO2. FT feeds were periodically switched from syngas to syngas + N2 by adjusting the total reactor pressure so that the reactant partial pressures (PCO and PH2) remained constant. During the initial deactivation (the first 1200 hours), it was found that the formation rates of olefins remained fairly constant (in some cases they increased) while those of paraffins decreased. This indicates the deactivation is mainly caused by the decrease in the paraffin formation rate. Currently, none of the published kinetic models can explain the phenomenon that the decay of the reaction rates of olefins and paraffins were not the same during the deactivation. At steady state (1055 to 2700 hours, overall reaction rate fairly constant), adding extra N2 decreased the selectivity to the light hydrocarbons. These results suggest that by feeding the extra N2 there could be an increase in selectivity and formation rates to long chain hydrocarbons (C5+). Plotting molar ratios of paraffin to olefin (P/O) with carbon number n+1 versus the ratio with carbon number n revealed linear relationships which are independent of feed gases, catalyst activity and reaction temperature. These results imply that product distributions might be determined by some sort of equilibrium. Another plot of normalised mole fractions of CnH2n, Cn+1H2n+2, and CnH2n+2 in ternary diagrams showed that after disturbances these product distributions tended to stable points. It is suggested that this could be due to slow changes in the liquid composition after the disturbances. Although not all the results are explained, the researcher emphasises that normal kinetics alone cannot explain these results completely. There might be factors, iii including vapour-liquid equilibrium or reactive distillation, which are worthy of consideration to explain FTS.
MT2016

This thesis is concerned with studies on the conversion of syngas to liquid hydrocarbons. A major part of the effort is aimed at synthesising the catalyst and use of it in a speci cally designed high pressure-high temperature reactor to produce liquid hydrocarbons with Fischer-Tropsch synthesis. This study was motivated by two important considerations: (a) The predominant need to produce biomass derived liquid fuels such as gasoline and diesel. (b) Identify the key catalyst properties that influence the hydrocarbon yield and accordingly synthesize catalysts that compare with the data available in the literature. The specific areas of this research are: (i) Produce active, efficient Co based catalysts using different methods and characterise them for Fischer- Tropsch process, (ii) Build a reactor for operations at suitable pressure and temperatures and test the conversion process that involves various catalysts depending on the process used for producing them with the principal control parameter being the residence time and (iii) Examine the overall biomass to liquid fuel conversion and the economics of building such systems at smaller throughputs of particular reference to India (or other developing countries). Based on a review of the literature on catalysts, amongst the two FT active metals, iron and cobalt, the latter was chosen for its high activity (60 - 70% conversion in a single pass) along with high selectivity and stability in the synthesis of linear hydrocarbons. The catalysts were supported on alumina and silica-doped alumina (SDA) catalyst carriers and synthesized by incipient wetness impregnation (IWI) method and combustion synthesis (CS) method. During the course of this study, the functionality of IWI catalysts were compared to that of CS catalysts and also the effect of supports on FT reaction were investigated. Several sets of experiments were conducted with each spanning for a duration of 150 - 160 hours to address the syngas conversion and the liquid fuel generation. The fi rst part of this thesis deals with the synthesis of supported cat- alysts that contain cobalt loading of 20 wt.% (expected range of 15 - 25% from literature), the Co0 crystallite size in the range of 15 - 25 nm, and metal dispersion in the range of 9 - 16%. The selected cobalt loading level was due to the fact that loadings less than 10% results in the formation of surface cobalt aluminates (due to the diffusion of cobalt ions into the rst few layers of aluminate lattice) and at loadings greater than 25 - 30 wt.%, no additional increase in the FT activity is observed. Further, the cobalt crystallite sizes below 6 nm show reduced activity due to larger rates of deactivation and carbide formation and crystallite sizes larger than 50 nm show reduced fraction of active sites available for the FT reaction. The Al2O3 (1 mm diameter spheres, BET surface area = 158 m2/g, pore volume = 0.45 ml/g) supported cobalt catalysts were synthesized using the CS method and compared with the conventionally synthesized IWI catalysts. CS catalysts were synthesized using hexamethylenetetramine (HMT) as the fuel. While the CS catalysts have been developed and used for several industrial reactions, especially environmental catalysts, its use for FT reaction has been limited to a few laboratory scale studies. The major limitation for CS process is the high heat release and the consequential high temperature rise rates, resulting in the evolution of combustion products with uncontrolled explosion, eventually powdering the catalysts and in most cases resulting in the loss of active components. Such vigorous behaviour of CS reaction is distinctly evident for metal loading above 5%. The use of these powdered catalysts in a fixed bed reactor demands either re-pelletizing or re-moulding which is considered very disadvantageous. The two instances in the litera- ture that have reported CS catalysts for FT reactions have metal loadings limited to 15% and exist in powdered form, for use in a slurry type reactor or in micro-channel reactors [Shi et al., 2012, LeViness et al., 2014]. The performance of FT catalysis using CS is limited and unavailable in the open literature. In particular, investigations related to the increase in Co3O4 reduction temperatures and the effect of CS process on the cobalt support interaction and the metal dispersion still need further probing. The CS catalysts used in this thesis have been deposited over Al2O3 support spheres with a metal loading of 20 wt.% and without a ecting the integrity of the support structure using a novel technique. The synthesized catalysts resulted in an average cobalt oxide crystallite size of 7 - 10 nm and metal dispersion ranging from 11 - 13.5%. The X-ray photoelectron spectroscopy and the H2 chemisorption analysis of the synthesized catalysts showed that the CS catalysts display reduced metal support interaction in comparison to the IWI catalysts. Strikingly, the Al2O3 supported CS catalysts reduced at temperatures that are 350 K higher than reduction temperatures of IWI catalysts, a feature not explicit in literature. The high reduction temperatures were associated to the reduction of surface Co2+ ions in the Co2+-Al3+ spinel structure. A further effort was made to synthesize cobalt catalysts directly in a single step without the need for further reduction by employing fuel rich conditions (equivalence ratio ( ) of 1.2 and 1.5). However, the XRD analyses of these catalysts revealed the presence of Co3O4. It was observed that, even under the fuel rich conditions, the redox mixture interacts with the atmospheric oxygen, yielding Co3O4. These catalysts (CS- = 1.2, and CS- = 1.5) were characterised by higher degree of reduction (DOR = 75% and 77% respectively) and higher dispersion (D = 12.8% and 13.2% respectively) compared to the CS catalysts synthesized with unity equivalence ratio (DOR = 69% and D = 11%). The Al2O3 supported CS catalysts resulted in an increased FT activity, as the CO conversion increased from 32% for IWI catalysts to 41% for CS catalysts. Similarly, enhanced CO conversion rates were observed for CS catalysts synthesized with = 1.2 and 1.5, with a highest CO conversion of 61% for CS ( = 1.2) catalysts. Strikingly, the FT product spectrum reported a maximum weight fraction of wax hydrocarbons (C24+), allowing for higher degree of surface polymerization for CS catalysts. The formation of waxes reduced with increasing equivalence ratios. Al2O3 supported cobalt catalysts showed a strong cobalt support interac- tion. The maximum metal dispersion that could be attained was limited to 12 - 13% and the degree of metal reduction extended to a maximum of 77% (CS- = 1.5). In order to further examine the influence of the support material on catalytic activity, cobalt was impregnated into 40 wt.% silica doped alumina supports by IWI and CS method. The earlier literature study that reported investigations on the effect of 30 - 40 wt.% SiO2 doping into Al2O3 supports, revealed a silica enriched surface containing phases of only SiO2 and aluminosilicate. A lower concentration of SiO2 doping (1.5 - 30 wt.%) showed phases of silica, alumina and a minimum concentration of aluminosilicate, while higher concentration of silica doping (>40 wt.%) showed a silica enriched surface [Daniell et al.,2000]. Invariably, the least cobalt support interaction for an Al2O3 characteristic support is bound to occur for 40 wt.% silica doping, due to the maximum concentration of aluminosilicate. Therefore, the second part of this thesis investigates the e ect of 40 wt.% silica doping in alumina support on the properties of the synthesized catalysts and thereupon the FT activity and selectivity. The effect of metal support interaction and its consequent effect on the FT activity and hydro- carbon selectivity were investigated. The XRD spectra of SDA support when compared to -Al2O3 and SiO2 support indicated silica coverage of alumina support, in a way that exposed only minimum fraction of -Al2O3 phase on the support surface. Consequently, the SiO2 doping reduced the formation of cobalt aluminates, resulting in lower metal support interaction as com- pared to cobalt deposited in Al2O3 supports. A 34% increase in the degree of cobalt reduction is observed for SDA-CS catalysts, compared to Al2O3 -CS catalysts. In addition, the fraction of active cobalt sites, as measured by the H2 chemisorption experiments, increases by a margin of 48% for SDA-CS catalysts. A 16% increase in the CO conversion was recorded for SDA-CS catalysts compared to Al2O3 -CS catalysts, with a 12% increase in the C5+ hydrocarbon yield. In the available literature, the e ect of SDA (5% SiO2 in Al2O3 [Jean- Marie et al., 2009]) catalysts on the FT reactions have been only limited to the CO conversion and the C5+ hydrocarbon selectivity. The e ect of these catalysts on the nature of product spectrum was notably lacking. Hence, studies were carried to identify the major components of the FT reaction that are predominantly a function of the catalyst apart from the process conditions employed. The results of the product spectrum of SDA supported cobalt catalysts explicit the formation of middle distillate hydrocarbons (C10- C20) as the primary liquid hydrocarbon product, compared to waxes (C24+) for Al2O3 supported cobalt catalysts. The di erence in the product spectrum for alumina and SDA supported catalysts was attributed to the enhanced surface acidity of the SDA support. NH3-TPD analysis of the SDA support showed a 91% excess surface acidity compared to the Al2O3 support. The SDA surface acidity, which was observed as the summation of Lewis and Bronsted acidity, occurs due to the formation of hydroxyl group formation across aluminium and silica atoms. The aluminosilicate behaviour of SDA support was further supported by the FT-IR spectrum. Also, the SDA sup- ported catalyst displayed higher selectivity to (C2 - C5 hydrocarbons ( 2.4 times higher than Al2O3 supported catalysts) owing to its aluminosilicate structure, rendering it a zeolite like behaviour. The third part of this thesis deals with the overall energy balance and presents an economic assessment of biomass to liquid fuel for an annual con- sumption of a nominal 10000 tonne woody biomass system with an expected liquid hydrocarbon output of 1500 tonnes, a size if found economical would be of great importance to the fi eld. Cost analysis involving a 1000 kg/h steam-oxy biomass gasi cation plant paired with FT plant has been evaluated. The system con guration uses an oxy-steam gasi er, a gas cleaning system, CO2 separation system, a compressor to raise the pressure to 3 MPa and a single-pass FT reactor. Detailed capital cost estimates for each sec- tion involved in the BTL system are set out, on the basis of which future commercial large-scale facilities producing fuel and/or power are evaluated. Several elements of the total system are available as o -the-shelf items and the gasi er itself has originated from long experience of such systems for air- gasi cation at the laboratory (see http://cgpl.iisc.ernet.in). The oxy-steam gasi er operates with an equivalence ratio of 0.1 and a steam to biomass ratio in the range of 0.8 - 1.2. The exit syngas from the gasi er comprises 47% H2, 22% CO, 27% CO2 and 4% CH4 (by volume; H2/CO ratio of 2.1:1). The FT reactor consists of a single casing enclosed multi-tubular reactor maintained at 503 K and 3 MPa. Syngas conversions using the combustion synthesised SDA supported cobalt catalysts were considered for this analysis for varying space velocities (WHSV ranging from 2610 ml/h gcat - 873.3 ml/h gcat. The syncrude (C24+, C6-C12 and aqueous products) are separated and processed in a hydrocracker for conversion into high quality liquid transportation fuel. For a once-through FT reactor con guration, substantial energy exists in the gas phase, which includes C1- C5 hydrocarbons and unconverted syngas; the BTL system was therefore designed such that, the fuel-gas energy is con- verted to electricity using an internal combustion engine, either for in-house electricity consumption of for sale to the grid. The analysis elucidates that the fuel ratio, which is the ratio of mass of the liquid fuel to the mass of the gas phase hydrocarbons (including the unconverted syngas), decreases with a reducing WHSV, a ecting the net electricity cost or the gross electricity units sold to the grid. Furthermore, the analysis shows that a market competitive liquid fuel can be produced with a CO conversion greater than 60%, at a cost ranging from 35 to 40 Rs/litre (0.5 - 0.6 USD/litre). The investment of Rs 45,000/ton of liquid fuel (681 USD/tonne) will have a will have a payback of 3.5 years. These results emphasize that an economically a ordable and environmentally favourable BTL system can be produced even at the levels discussed here.

MSc., Faculty of Science, University of the Witwatersrand, 2011
A study was performed in order to investigate the effect of preparation method and the effect of microwave heating on a manganese promoted iron based Fischer-Tropsch catalyst. The effects of preparation method and microwave heating on the structure and morphology of the catalyst, its surface area and reduction behavior were investigated using various techniques such as Transmission electron microscopy (TEM), Powder x-ray diffraction (PXRD), surface area measurements (BET) and temperature programmed reduction (TPR). The FTS performance of the catalysts were also studied using a fixed bed reactor with Fischer-Tropsch Synthesis conditions (270 C, flow rate of 30 ml/min, H2/CO ratio = 2, pressure of 10 bar). Characterization of the catalysts calcined at 350 C revealed that manganese enriched the surface of impregnated Mn/Fe catalysts and suppressed the reduction of the iron catalyst. However, the Mn acted as a structural promoter in the co-precipitated catalysts and also promoted the reduction of Fe2O3 as the manganese content increased. The co-precipitated catalyst calcined at 650 C suppressed the reduction of iron. The impregnated catalysts showed similar conversion (~ 70%) for catalysts with Mn loadings 5%, 10% and 20%. This suggests Mn promotes the activity of the iron catalyst since less iron is present in the catalyst as the manganese loading is increased. The co-precipitated catalysts showed a 10 wt% Mn loading to be the optimum amount for increased activity and selectivity to C2 – C4 hydrocarbons, lower molecular weight olefins and a lower selectivity to heavier molecular weight hydrocarbons relative to Mn loadings of 5, 20 and 50 wt%. Mn loadings in excess of 10 wt% showed a slight increase in selectivity to heavier weight hydrocarbons. The impregnated catalysts showed very little difference in activity and selectivity but the co-precipitated catalyst showed a decrease in activity after the catalyst was microwave heated. A slight increase in selectivity to lower weight olefins and heavier molecular weight hydrocarbons was noted after microwave heating. The TPSR (Temperature programmed surface reaction) results revealed that this may be due to the stronger adsorption of CO on the surface of the catalyst after microwave heating. A similar trend was observed for catalysts promoted with 0.1 wt% potassium i.e. a slight increase in selectivity to heavier weight hydrocarbons after microwave heating.

The aim of this research project was to investigate the oxidation reactions of olefins to ketones. Initial studies revolved around the oxidation reactions of terminal olefins to symmetrical dialkyl ketones. The inability to isolate pure products, and the consumption of large amounts of the expensive palladium catalyst for each run as well as the extremely low yields that resulted from these oxidation reactions, made it difficult to thoroughly investigate this oxidation system. It was then decided to embark on the investigation of oxidation reactions of a-olefins to methyl ketones. For these studies, six terminal olefins were oxidised to methyl ketones employing seven different oxidation reactions. One of the most important and pioneering reactions m this field is the system employing PdCl2 / CuCl2 / O2 for the oxidation of terminal olefins to methyl ketones, namely the Wacker oxidation reaction. Experimental results, however, indicated that high product contamination from by-products resulted from these oxidation reactions despite the fairly good yields of product from the Wacker oxidation system. Some reaction systems that have been developed from the Wacker oxidation system were also investigated. The oxidation system employing PdCl2 / p-benzoquinone for the oxidation of terminal olefins to ketones was studied. The oxidation reactions resulted in incomplete oxidation with higher olefins (l-decene, l-nonene and l-octene), and complete oxidation of lower olefins (l-heptene, l-hexene and l-pentene) under the same reaction conditions. The products from lower olefins oxidised under these reaction conditions were pure and high yielding Another system that proved efficient both with feasibility and good yields of products was the oxidation system employing Pd(OAc)2 / H202 catalyst to oxidise terminal olefins to methyl ketones. Phase transfer catalysis has been employed in organic chemistry to effect different reactions. In this case two types of phase transfer agents were employed to effect the oxidation of terminal olefins to ketones. The first oxidation system involved the use of a PdCl2 / CuCl2 / O2 catalyst with a quaternary ammonium salt, cetyltrimethylammonium bromide (CTAB), to govern the course of the reaction. Reasonable yields were obtained, and moderate purity of products was also observed. The second phase transfer catalysis system employed polyethylene glycol (PEG-200) as a phase transfer agent, and PdCl2 / CuCl2 / O2 as a catalyst for oxidation of olefins to ketones. This oxidation system resulted in different isomers of a ketone from a terminal olefin. Pure methyl ketones were not isolable from the mixture of methyl and ethyl ketones. The oxidation reactions of olefins to ketones employing Pd(OAc)2 / p-benzoquinone in combination with electrolysis were also investigated. The unique feature about these reactions was the fact that cyclic olefins could also be oxidised under these conditions. Good yields were obtained, and high product purity was observed. One of the important oxidation reactions investigated during the project was the reaction that used an alternative metal to the expensive palladium catalyst for the oxidation reactions to ketones. This oxidation system employs CuCl2 / 18-C-6 / acetaldehyde as a catalyst for the oxidation of hydrocarbons to ketones and alcohols. It was discovered during the investigation that olefins can also be used as substrates and are oxidised to the corresponding ketones. The use of olefins as substrates resulted in higher yields than the hydrocarbon oxidation reactions, and less contamination in the product mixture was also observed.
Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 1997.