Thesis (MScEng (Process Engineering))--University of Stellenbosch, 2007. / The catalytic selective oxidation of hydrocarbon molecules is the process where a selectively oxidized intermediate molecule is formed instead of the thermodynamically favoured total oxidation products, in the presence of a suitable catalyst. Examples are the selective oxidation of methane to synthesis gas at moderate temperatures, for which a catalyst is still needed and the selective oxidation of propene to acrolein over α-Bi2Mo3O12. The selective oxidation of propene over α-Bi2Mo3O12 occurs via a Mars-van Krevelen mechanism where the bulk oxygen in the catalyst is inserted into the propene molecule and leaves as part of the product, while being replaced with gaseous oxygen. From an economic perspective there is a need to produce synthesis gas from methane at low temperatures. It was seen in the literature that α-Bi2Mo3O12 is a mixed metal oxide that might be capable of achieving this. The feasibility of the selective oxidation of methane to synthesis gas with α-Bi2Mo3O12 was therefore investigated. However, it was found that the selective oxidation of methane over α-Bi2Mo3O12 is not feasible at moderate temperatures. To circumvent the problem of producing synthesis gas at low temperatures a membrane reactor was suggested that might be able to produce synthesis gas at moderate temperatures with conventional selective methane oxidation catalysts that thermodynamically favours carbon dioxide formation at low temperatures. No time on-stream experiments had been done previously for the selective oxidation of ...

Thesis (MScEng (Process Engineering))--University of Stellenbosch, 2005. / Although selective oxidation catalysts are widely used and extensively studied for their industrial and academic value, their complex mechanisms are, to a large extent, still unclear. The field of so-called allylic (amm)oxidations reactions was chosen for further investigation, in particular the simplistic selective oxidation of propene to acrolein over an α-Bi2Mo3O12 catalyst. One of the most important approaches in selective oxidation is to try to correlate the physicochemical properties of catalysts with their catalytic performance (activity and selectivity). The most interesting, and seemingly most widely invoked parameter, is lattice oxygen mobility. The problem, however, is the difficulty encountered in measuring oxygen mobility. It is hypothesised that the depth of oxygen utilisation and lattice oxygen mobility of bismuth molybdate during the partial oxidation of propene to acrolein may be determined by measuring the rate of acrolein formation and lattice oxygen usage over a range of discrete particle sizes that could be synthesised using reverse micelle technology. Catalyst Preparation A preliminary investigation into the reverse micelle technique showed that discrete nanosized particles could be synthesised, but that there was no size control over the outcome and that, in most cases there were some degree of particle agglomeration. It was also found that nanorod formation occurred due to adsorbtion of surfactant. More in-depth investigation had to be done in order to achieve particle size control and the liberation of the calcined α-Bi2Mo3O12 catalyst particles required for kinetic experiments. Simple precipitation methods, the catalyst calcination step, and the formation and stability of reverse micelles were investigated. A simple precipitation method to prepare α-Bi2Mo3O12, suitable to be integrated into the reverse micelle technique was found by buffering the mixture of bismuth nitrate and ammonium molybdate solutions with an excess of molybdate. This prevented the pH from decreasing below a critical value of 1.3 (at which β-Bi2Mo2O9 forms as an impurity). The excess molybdenum caused the formation of MoO3 in the calcined product, which was selectively and successfully removed using a warm ammonium wash followed by a water rinse and a recalcination step. XRD of a temperature range calcination shows that the calcination starts at temperatures as low as 200°C and almost complete calcination of the catalyst at 280°C. DSC analyses show a 47.15 J/g crystal formation peak only at 351°C. The Mo18O56(H2O)8 4- anion or its double, Mo36O112(H2O)16 8-, is responsible for the formation of α-Bi2Mo3O12 in the precipitation calcination reaction. Reverse micelles were investigated using a Malvern Zetasizer and showed a complex dynamic system in which the reverse micelle sizes and size distributions change over time as a function of surfactant and aqueous concentrations, the salt used and aqueous phase salinity. Although much was accomplished in this study, more investigations into the constituent steps of the reverse micelle technique are needed to develop a method to synthesise the range of discrete catalyst particle sizes required for kinetic studies. Kinetic Studies For the purpose of kinetic experiments a metal reactor was found to be superior to that of a glass reactor. The reactor rig was adequate for these kinetic studies but do not meet the requirements for detailed reaction order experiments. The analysing apparatus could not measure CO2 formation accurately and it had to be calculated using a carbon balance. Only the model proposed by Keulks and Krenzke [1980a] was able to describe the kinetic result, but the model parameter describing the oxidative state of the catalyst surface could not be calculated due to the lack compatibility between published data. Values were awarded to this parameter so to give an Arrhenius plot which corresponded to published data. The parameter describing the oxidative state vs. temperature took on a function that was consistant with the reasoning of Keulks and Krenzke [1980a]. Comprehensive preliminary kinetic studies are needed, both in catalyst reduction and reoxidation, in order to determine the reaction conditions, explore more advanced kinetic models and investigate model parameters that are theoretically and/or empirically obtainable and quantifiable.

Ce projet fait suite à une thèse effectuée en amont décrivant le portrait-robot du catalyseur idéal pour cette réaction. Une première partie de la thèse a donc porté sur la synthèse d’un nouveau catalyseur plus productif que le catalyseur commercial utilisé actuellement. Une nouvelle voie de synthèse a pour cela été explorée et a menée à la synthèse de deux nouveaux catalyseurs, ayant le design adéquat et collant avec le portrait-robot, qui sont respectivement deux et cinq fois plus productifs lorsqu’ils sont testés dans les mêmes conditions expérimentales. L’obtention de propriétés catalytiques élevées, nécessite dans le catalyseur la présence de plusieurs phases à base de molybdate de bismuth, et d’autres métaux Obtenir ces phases en équilibre et dans des proportions relatives adéquates a été un challenge difficile à relever. Les catalyseurs préparés et la voie de synthèse mise au point font l’objet d’un brevet en cours de dépôt. Des essais d’enduction de ce catalyseur produit sous forme de poudre est en cours dans un laboratoire de l’université de Limoges.Si l’enduction est la méthode la plus simple pour introduire un catalyseur si complexe dans un réacteur millimétrique, elle présente néanmoins beaucoup de difficultés qui ne sont pas toutes résolues. L’un des problèmes majeurs est l’ajout de produits divers permettant la dispersion de la poudre dans une solution visqueuse qui conduit à la contamination légère des catalyseurs. Nous avons donc travaillé en parallèle sur une nouvelle méthode de synthèse du catalyseur directement à la surface des tubes d’un milli-réacteur. Cette méthode est basée sur un dépôt des éléments les plus importants du catalyseur classique. Les paramètres de dépôt et les caractéristiques du catalyseur ont été optimisés pour former un mélange de phases actives et sélectives / A first part of the thesis focused on the synthesis of a new catalyst more productive than the commercial catalyst currently used. A new way of synthesis has been explored and led to the synthesis of two new catalysts, having the appropriate design and sticky with the portrait-robot, which are respectively two and five times more productive when they are tested in the same experimental conditions. Obtaining high catalytic properties, requires in the catalyst the presence of several phases based on bismuth molybdate, and other metalsObtaining these phases in equilibrium and in adequate relative proportions has been a difficult challenge. The catalysts prepared and the synthetic route developed are the subject of a patent being filed. Coating tests of this catalyst produced in powder form is underway in a laboratory at the University of Limoges.While coating is the simplest method of introducing such a complex catalyst into a millimeter reactor, it nevertheless presents many difficulties that are not all solved. One of the major problems is the addition of various products allowing the dispersion of the powder in a viscous solution which leads to the slight contamination of the catalysts. So we worked in parallel on a new method of catalyst synthesis directly on the surface of the tubes of a milli-reactor. This method is based on deposition of the most important elements of the conventional catalyst. Deposition parameters and catalyst characteristics were optimized to form a mixture of active and selective phases

The goal of this project was to develop a mixed matrix membrane with enhanced properties for propylene/propane separations. To start with the project, one of the high performance 6FDA based polyimides was identified as the polymer matrix for the rest of the project. The chosen polymer (6FDA-6FpDA) was successfully synthesized in the laboratory. During the synthesis process the key objectives for high molecular weight and low polydispersity index polymer were identified. High molecular weight 6FDA-6FpDA was achieved via laboratory synthesis and was tested successfully. After successful synthesis of the high performance polymer, pure polymer dense films were tested for transport properties. One problem identified with 6FDA-6FpDA polymer films for propylene/propane separations was plasticization. A major objective of this research was to develop a method for plasticization suppression. A carefully controlled annealing procedure with high temperature permeation experiments was used in this research to suppress plasticization in a mixed gas environment. To the best of our knowledge, this is for the first time plasticization suppression was achieved with pure polymeric membrane material for propylene/propane separations with pure and mixed gases. The observed mixed gas experimental selectivity was lower than the pure gas selectivity which was explained by the combination effect of dual mode and bulk flow effect. The last objective of this project was to successfully incorporate molecular sieve materials to form a mixed matrix membrane hybrid material with enhanced transport properties First, an ideal molecular sieve for propylene/propane separation was identified and characterized. AlPO-14 was chosen for this research following its success with propylene/propane pressure swing adsorption. Mixed matrix membranes were successfully produced and tested for enhanced transport properties. Both pure and mixed gas results showed promising results with enhanced propylene permeability and propylene/propane selectivity. The experimental results were modeled with the Cussler and Maxwell models. A modified Cussler model was presented in this work. This is the first time an enhancement in the transport properties with mixed matrix membrane for propylene/propane separations has been observed. This fundamental dense film work holds a bright future for the scale up of propylene/propane separations.

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Cílem této práce bylo prostudovat vliv předpolymerace na vybraný komerční MgCl2–nosičový TiCl4 katalyzátor (tzv. Zieglerův-Nattův). Výsledkem práce bylo nalezení optimálních podmínek, které umožní ideálně využít této techniky k modifikaci struktury a morfologie zvoleného katalyzátoru a dosažení stabilnějšího průběhu polymerace a lepších vlastností produkovaného polypropenu. U předpolymerací byl studován vliv teploty, koncentrace triethylhliníkového kokatalyzátoru a externího donoru, množství polyolefinu syntetizovaného během předpolymerace (tzv. stupeň předpolymerace) na následné chování předpolymerovaného katalyzátoru při plynofázní polymeraci propenu.

The aim of the study will be the determination of the kinetic profiles of the polymerization of propene, ethene and their mixtures with commercial Ziegler-Natta catalyst by application of two different experimental procedures. The prepared polymer material will be utilized for the SEC analysis and subsequent evaluation of the concentration of active sites. The determined molecular mass distribution curves will be further investigated by the application of the procedure, which deconvolute the overall profile on the basic Flory's distribution functions. Then the comprehensive study about character and behavior of various types of active centers in dependence on time and type of monomer will result from this investigation.

<p>This work studies propene aromatization reaction on H-MFI (Si/Al = 40) and physical mixtures of H-MFI (Si/Al = 40) and PtZn/SiO2 (2 wt% Pt, 3 wt% Zn) at 723 K - 823 K and 3 kPa C3H6. The influence of PtZn alloy (dehydrogenation function) is investigated on the product distribution and selectivity of metal-acid catalyzed propene aromatization. Typical product distribution consists of methane, ethane, ethene, propane, C4-C6 alkanes and alkenes, and benzene, toluene, xylene (BTX). On comparing the BTX carbon selectivity over the two catalysts at first equivalent space velocity and then equivalent propene conversion, higher BTX selectivities are observed on PtZn+H-MFI than H-MFI in both the cases. The higher BTX selectivities were previously attributed in the literature to the dehydrogenation pathway on the metal function. However, space velocity is an inadequate descriptor of reaction progress because the conversion of reactants can be different at same space velocity. Similarly, propene conversion is an incomplete descriptor for reaction progress because intermediates such as ethene and C4-C6 hydrocarbons react to form higher molecular weight hydrocarbons and subsequent aromatics as the reaction progresses. Such reactive hydrocarbons were lumped together as reactive intermediates and the remaining hydrocarbons were classified as non-reactive species or products. When BTX selectivities over PtZn-H-MFI and H-MFI are compared at equivalent temperature and equivalent conversion of all the reactive intermediates, both the catalysts exhibit similar BTX selectivities, suggesting that the presence of the dehydrogenation metal function doesn’t influence the selectivity towards BTX products. Further, we hypothesize cyclohexene as an intermediate in aromatic formation and use cyclohexene conversion as a probe reaction to understand how aromatics are formed over Brønsted acid sites and PtZn alloy. Cyclohexene conversion results at 723 K and 823 K shows the presence of an alternate route of aromatic formation via dehydrogenation of cycloalkenes, and this dehydrogenation pathway has an order of magnitude higher rates than the hydride transfer route on Brønsted acid sites. Further, we propose dominant reaction pathways of C1 – BTX hydrocarbon formation on H-MFI and bifunctional PtZn+H-MFI. Finally, we discuss the implications of using PtZn+H-MFI on developing a commercial propylene aromatization process and provide our recommendations for chemical and fuel production. In summary, these findings reveal previously unknown mechanistic details of metal bifunctionality for propene aromatization catalysis.  </p>

Developing a fundamental understanding of the interactions between catalytic surfaces and adsorbed molecules is imperative to the rational design of new materials for catalytic, sorption and gas separation applications. Experiments that probed the chemistry at the gas-surface interface were employed through the utilization of in situ infrared spectroscopic measurements in high vacuum conditions to allow for detailed and systematic investigations into adsorption and reactive processes. Specifically, the mechanistic details of propene epoxidation on the surface of nanoparticulate Au supported on TiO2 and dimethyl chlorophosphate (DMCP) decomposition on the surface of TiO2 aerogel-supported Cu nanoparticles were investigated. In situ infrared spectroscopy illustrates that TiO2-supported Au nanoparticles exhibit the unprecedented ability to produce the industrially relevant commodity chemical, propene oxide, through the unique adsorption configuration of propene on the surface of Au and a hydroperoxide intermediate (-OOH) in the presence of gaseous hydrogen and oxygen. Whereas, TiO2-supported Cu aerogels oxidize the organophosphate-based simulant, DMCP, into adsorbed CO at ambient environments. Through a variety of spectroscopic methods, each step in these oxidative pathways was investigated, including: adsorption, oxidation and reactivation of the supported-nanoparticle systems to develop full mechanistic pictures. Additionally, the perturbation of vibrational character of the probe molecule, CO, was employed to characterize the intrinsic µ3-hydroxyls and molecular-level defects associated with the metal-organic framework (MOF), UiO-66. The adsorption of CO onto heterogeneous surfaces effectively characterizes surfaces because the C-O bond vibrates differently depending on the nature of the surface site. Therefore, CO adsorption was used within the high vacuum environment to identify atomic-level characteristics that traditional methods of analysis cannot distinguish. / Doctor of Philosophy / The interaction between small gas molecules and solid surfaces is important for environmental, industrial and military applications. In order to chemically change molecules, surfaces act to lower activation barriers and provide a low energy plane to create new chemical bonds. To study the fundamental interactions that occur between gas molecules and surfaces, we employ infrared spectroscopy in order to probe the vibrations of bonds at the gas–surface interface. By tracking the chemical bonds that break and form on the surface of different materials, we can develop surface reaction pathways for a variety of different chemical reactions. We focus our efforts on two different applications: the conversion of propene to propene oxide for industrial applications and the decomposition of chemical warfare agents. Using the techniques described above, we were able to develop reaction pathways for both propene oxidation and chemical warfare agent simulant degradation. Our work is critical to the further development of catalysts that harness the specific structural and chemical properties we identify as important and exploit them for further use.

Im Rahmen der vorliegenden Arbeit wurde eine neuartige Methode und die entsprechenden Katalysatoren für die heterogen katalysierte Gasphasen-Epoxidation von Propen entwickelt und optimiert. Das Propen wurde an FeOx/SiO2-Katalysatoren mit N2O als Oxidationsmittel epoxidiert. Die Katalysatoren wurden mittels XRD, TEM, XPS, Physi- und Chemisorption, TPR/TPO, TPD und IR untersucht und charakterisiert. Der Einfluß der Reaktionsbedingungen auf die Oxidationsergebnisse wurde bestimmt und Untersuchungen zum Reaktionsablauf durchgeführt. Es wurden Selektivitäten zu Propenoxid von 40-70%, bei Propenumsätzen von 3-12%, erreicht. Die maximalen erzielten PO-Ausbeuten betrugen über 5%, und sind damit den berichteten Ergebnisse aus der Literatur überlegen.