Les molybdates de bismuth présentent un grand intérêt en catalyse d’oxydation sélective du propylène. Leurs mécanismes de formation ont fait l’objet de nombreuses études mais leur compréhension reste encore incomplète.Dans un premier temps, la formation de γ-Bi2MoO6 au cours de sa synthèse par voie hydrothermale a été suivie in situ par diffraction X combinée à la spectrométrie d’absorption des rayons X (XAS) et à la diffusion Raman. Les mesures ont été réalisées au synchrotron à l’ESRF. Des informations sur la cinétique de réaction ont pu être déduites des données in-situ, mais ont dû être complétées par des analyses physico-chimiques ex-situ. L’étude montre que le catalyseur est généré selon un mécanisme en deux étapes, passant par la formation d’un molybdate de bismuth de structure proche de la fluorine à partir de laquelle se développent les cristaux lamellaires de γ-Bi2MoO6.Ce catalyseur a ensuite été utilisé en mode découplage redox dans l’oxydation du propène. Un Réacteur Catalytique à Membrane Dense (RCMD) dont le principe repose sur une membrane céramique dense, conductrice par ions oxydes O2-, qui sépare de façon étanche deux compartiments alimentés respectivement par l’hydrocarbure et l’air. La membrane de composition Bi25.75Mo10O68.625 adensifiée et mise en forme. L’étude montre que les activités catalytiques de la membrane (Bi25.75Mo10O68.625) seule ou avec le catalyseur (γ-Bi2MoO6) déposé à sa surface peuvent être modulées par l’application d’un courant électrique. L’apport en oxygène est suffisant pour permettre un travail en continu du catalyseur γ-Bi2MoO6 qui devrait être ré-oxydé par l’air co-alimenté avec le propène dans un réacteur classique. / Bismuth molybdates have drawn significant research interest because of their catalytic properties for propylene oxidation. Despite several studies dealing with the mechanism of their formation, uncertainties remain.The genesis of γ-Bi2MoO6 under hydrothermal conditions was first elucidated by an in-situ study combining High-Resolution Powder Diffraction (HRPD) / X-ray Absorption Spectroscopy (XAS) / Raman Scattering. Experiments were carried out at the European Synchrotron Radiation Facility. The informations on the two-step kinetics of reaction which were obtained by these experiments were completed by additional ex-situ characterizations. It was shown that the catalyst is formed via a bismuth molybdate of fluorite structure acting as a template for the layered γ-Bi2MoO6 crystals to be nucleated.In the second part, the catalytic properties of γ-Bi2MoO6 were studied in a the Catalytic Dense Membrane Reactor CDMR in which the reduction and reoxidation of catalyst were physically separated (redox decoupling). Bi25.75Mo10O68.625, known as a good oxide ion conductor, was used as the dense membrane, separating two compartments, one containing air and the other containing propylene to be oxidized selectively. The study has demonstrated that the catalytic activity of the γ-Bi2MoO6 and of the Bi25.75Mo10O68.625 dense membrane itself was modified by applying an electrical bias. In contrast to conventional reactors in which γ-Bi2MoO6 must be reoxidized by air cofed with propylene in a CDMR the oxygen supply is high enough for γ-Bi2MoO6 to operate continuously.

Thesis (Ph. D.)--University of Wisconsin--Madison, 1941. / Typescript. Includes abstract and vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 84-86).

Depuis 1993, un phénomène de précipitation mettant en jeu le molybdène et le zirconium (produits de fission) conduit à l’encrassement des équipements de dissolution des usines d’AREVA-La Hague. Le précipité formé, ZrMo2O7(OH)2(H2O)2, tend à inclure du plutonium(IV) ce qui impose une gestion de la criticité. La thèse a pour double objectif (i) d’étudier la composition du précipité dans des domaines plus riches en plutonium en prévision du recyclage de combustibles de type MOX ou RNR (ii) d’approfondir la connaissance des mécanismes d’inhibition de la précipitation. L’approche a consisté à caractériser les phases formées lorsque la teneur en plutonium du milieu de dissolution augmente et à déterminer l’évolution de la composition du solide en fonction du temps de séjour des solutions dans les équipements de dissolution. Les travaux ont d’abord été menés en système simulant (molybdate mixte de zirconium et de cérium) puis en actif. L’étude d’une solution visant à inhiber le phénomène d’encrassement et d’inclusion du plutonium dans le solide par ajout de TeVI en solution a montré qu’un tel ajout impacte très fortement le domaine de précipitation des composés de référence. Une nouvelle phase, amorphe aux rayons X, précipite en très faible quantité sur un large domaine de précipitation. Cette phase inclue peu, voire pas, d’élément IV et il convient de parler d’effet « retard » du tellure VI sur la précipitation des molybdates mixtes de zirconium et d’éléments IV : la présence de tellure VI retarde l’atteinte de l’équilibre thermodynamique via la formation d’une phase cinétique amorphe, métastable. / Since 1993, a precipitation phenomenon involving molybdenum and zirconium (produced by fission) leads to the fouling of the Areva La Hague factory dissolution equipments. The formed compound, ZrMo2O7(OH)2(H2O)2, tends to include plutonium (IV) which imposes a management of the criticality. In this way, the objective of these thesis is first to study the composition of the precipitate proceeding form higher plutonium contents in expectation of the recycling of MOX or RNR fuels. Secondly, these work aims at increasing the knowledge of the precipitation inhibition mechanisms. The approach consisted in characterizing the formed crystallographic phases when the plutonium amount in solution increases and determining the solutions residence time in dissolution equipment dependence of the solid composition. At first, the researches were realized in feigning system (zirconium and cerium mixed molybdates) then in radioactive environment. The study of a preventive solution to inhibit the fouling phenomenon and the plutonium inclusion in the solid by addition of TeVI in solution showed that such an addition impacts very strongly the precipitation domains of reference compounds. A new phase, amorphous to the X-rays, precipitates in very small quantity on a wide precipitation domain. This phase includes very low or no amount of tetravalent element and we must talk about a “delay” effect of the tellurium on the mixed zirconium and tetravalent elements molybdates precipitation: the presence of tellurium VI delays the thermodynamic equilibrium achievement through the precipitation of an amorphous kinetic metastable phase.

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Une série de molybdates d'ammonium quaternaire, amphiphiles et catalytiques baptisés « catasurfs », a été synthétisée et étudiée pour l'élaboration de systèmes de microémulsion triphasique utilisés comme milieux réactionnels pour la peroxydation de substrats organiques par la source chimique d'oxygène singulet 1O2 (1[delta]gH2O2/iMoO4 2-. Les propriétés physico-chimiques en solution aqueuse (T Kraftt. cmc, solubilité, diagramme binaire) de ces surfactifs catalytiques ont été déterminées et l'évolution du comportement des systèmes ternaires eau/solvant/surfactif en fonction de la nature du solvant a été étudiée. Certains de ces « catasurfs » sont qualifiés d'« équilibrés» car ils forment spontanément, à température ambiante, en présence d'eau et d'un solvant organique approprié des systèmes de microémulsion triphasique du type Winsor III. Ces milieux réactionnels triphasiques simples constitués uniquement de solvant, d'eau et de catasurf présentent deux avantages majeurs en termes d'application. Ils sont peu ou pas sensibles à la dilution et ils se séparent très rapidement (30 secondes) en 3 phases distinctes dès l'arrêt de l'agitation car la courbure spontanée du film interfacial est nulle. Ces milieux se sont révélés particulièrement efficaces pour la peroxydation par 1O2 d'une grande variété de substrats organiques riches en électrons dont la réactivité peut être relativement faible (kr = 8 x 10-3 L.morl-1.s-1). Enfin, la présence d'une phase solvant en excès dans laquelle se trouvent les produits d'oxydation facilite grandement le traitement du milieu réactionnel en comparaison des microémulsions monophasiques. / A series of amphiphilic and catalytic quaternary ammonium molybdates, named "catasurf', has been synthesized and studied for the elaboration of triphasic microemulsion systems used as reaction media for the peroxidation of organic substrates by the chemical source of singlet oxygen, 1O2 (1[delta]g),H2O2/MoO4 2-. The physico-chemical properties in aqueous solution (T krafft, cmc, solubility, binary diagram) of these catalytic surfactants have been determined, and their evolutions in water/oil/surfactant temary systems according to th nature of the solvent have been studied. Some of these "catasurfs" are said "ba/anced' because they spontaneously form Winsor III triphasic microemulsion systems in the presence of water and an appropriate solvent. These simple triphasic reaction media, only based on solvent, water and "catasurf', exhibit two main advantages for applications. They are nearly insensitive to dilution and separate very rapidly (30 seconds) in distinct phases as soon as stirring is stopped because of a zero spontaneous interfacial curvature. They are particularty efficient media for the dark singlet oxygenation of a wide variety of electron-rich organic substrates the reactivity of which can be relatively low (kr = 8 x 103 L.mol-1.s-1). Finally, the presence of an excess solvent phase, in which the oxidative products are, makes the treatment of the reaction media really easier in comparison to the monophasic microemulsions.

Thesis (Ph. D.)--University of Wisconsin--Madison, 1950. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaf [95]).

Thesis (Ph. D.)--Ohio State University, 2009. / Title from first page of PDF file. Includes vita. Includes bibliographical references (p. 167-180).

Bismuth molybdates have long been known as active catalysts for selective oxidation of olefins. There are several phases of bismuth molybdates but only three of them are known to be active for partial oxidation of propylene to acrolein, namely, α, β, and γ bismuth molybdates. A significant amount of work has been carried out and reported in the literature, aiming to understand the reaction mechanisms so as to control the reaction process. It has been revealed that the oxidation reaction follows the redox mechanisms and lattice oxygen plays a key role as the main oxygen source for the reaction and controls the catalyst performance. The properties of the lattice oxygen are influenced by the bulk crystalline structure of the catalyst. Therefore, it is possible that the crystal structure influences the performance of the catalyst in promoting the partial oxidation reaction. However, there appears to be a lack of detailed reports in the literature on the relationship between the bulk crystal structure and the activity and selectivity of the catalyst for the partial oxidation reaction. The work reported in this thesis has been designed to achieve an improved understanding of the catalyst structure in relation to the activity and selectivity of the catalyst for the partial oxidation of propylene to acrolein. / In order to fulfil the objectives of this study, several investigation steps have been taken, namely 1) acquiring and analysing the catalyst structural parameters under real reaction conditions as well as at room temperature by means of neutron diffraction and X-ray diffraction, 2) obtaining kinetics from experimentation using a packed-bed reactor operating under differential reactor mode so as to eliminate the mass diffusion effect, and 3) developing and proposing reaction mechanisms which contain events that occur on the crystalline structure of the catalysts, particularly lattice oxygen, during the reaction. Characterisation of the structure of the catalysts has been carried out by means of In-situ neutron diffraction, which has the ability to probe the crystal structure at atomic level. The structure is characterised under simulated reaction conditions to investigate the dynamics of the crystal structure, particularly lattice oxygen, during the reaction. The In-situ diffraction studies have uncovered the relationship between the crystal structure of bismuth molybdates and their selectivity and activity towards the catalytic partial oxidation of propylene to acrolein. The possible active lattice oxygen in the bismuth molybdate structures has been identified. The active lattice oxygen ions are responsible for maintaining redox balance in the crystal lattice and thus control the catalyst activity and selectivity. Mobile oxygen ions in the three bismuth molybdate crystal phases are different. The mobile oxygen ions are O(1), O(11), and O(12) in the α phase; O(3), O(11), O(16), and O(18) in the β phase; and O(1) and O(5) in the γ phase. / The mobile lattice oxygen ions are proposed to be the source of the oxidising oxygen responsible for the selective oxidation of propylene to acrolein. One common feature of all mobile oxygen ions, from a catalyst crystal structure point of view, is that they are all related to molybdenum ions rather than bismuth ions in the lattice. By modifying the physical and chemical environment of the molybdenum oxide polyhedra, it is possible to modify the catalyst selectivity and activity. The diffraction diagnoses have also shown that molybdenum oxide polyhedra in all bismuth molybdate are unsaturated. In contrast, the bismuth oxide polyhedra are over charged. The co-existence of molybdenum ions that are co-ordinately unsaturated with bismuth ions that are over valence-charged promote the formation of allyl radical such as those found in the partial oxidation of propylene to acrolein. The molybdenum ions become propylene-adsorbing sites while the bismuth ions are the active sites to attract hydrogen from the adsorbed propylene, leading to the formation of the allyl intermediate. Oxygen ions from the mobile lattice oxygen are a more moderate oxidant than molecular oxygen. With their mild activity, the partially oxidised products are the main products such as acrolein and formaldehyde when oxygen ions react with the allyl intermediate while more complete combustion products such as carbon oxides and organic acids become the side products. / Investigation into the kinetics and reaction mechanisms has revealed the aforementioned evidence to support the role of the mobile lattice oxygen ions in the partial oxidation of propylene to acrolein. The kinetic experiments have employed the power rate law to model the kinetic data. The model shows that the reaction orders in propylene and oxygen concentrations are a function of the reaction temperature. The reaction order in propylene increases with reaction temperature, from 0.6 at 300°C to 1.0 at 450°C for all the bismuth molybdate catalysts, while the reaction order in oxygen decreases from 0.6 at 300°C to 0 at 450°C. The activation energies are 99.7, 173, and 97.7 kJ.mol-1 for α-Bi2Mo3O12, β-Bi2Mo2O9, and γ-Bi2MoO6, respectively. The changes in reaction orders with respect to propylene and oxygen indicate that the reaction occurs through the redox mechanisms, using the mobile lattice oxygen. The structural dynamics identified earlier explains the decrease in the acrolein selectivity at high temperatures (ca above 390°C). At these temperatures, the mobile oxygen becomes more mobile and more active. As a result, as the mobility of the oxygen ions increase, their reactivity also increases. The increase in the oxygen reactivity leads to unselective, complete oxidation reaction, forming the complete oxidation products CO2 and H2O. The reduction-reoxidation of bismuth molybdate is controlled by the diffusion of oxygen ions in the lattice, because the reduction sites do not have to be adjacent to the oxidation sites. The oxygen diffusion rate is in turn controlled by how mobile the lattice oxygen ions are. / Hence, the mobile oxygen ions discussed earlier control the catalyst activity in catalysing the reaction of propylene partial oxidation. The examination of several reaction mechanism models has given further evidence that the propylene partial oxidation to acrolein occurs via the redox mechanism. In this mechanism, the rate of acrolein formation depends on the degree of fully oxidised sites in the bismuth molybdate. The oxidised sites affect the apparent reaction orders in propylene and oxygen and thus control the kinetics of partial oxidation of propylene to acrolein. The more easily the reduced catalysts are reoxidised, the more active the catalysts in converting propylene to acrolein. A set of reaction steps has been proposed, which adequately reassembles the reaction mechanism. Side product reactions are also identified and included in the mechanisms. The present thesis has revealed a much detailed insight into the role of lattice oxygen in the catalytic partial oxidation of propylene to acrolein over bismuth molybdates and established the relationship between structure and activity and selectivity of the catalyst. This work has laid a foundation for future catalyst design to be based on structural knowledge of the catalysts.

Thesis (Ph. D.)--Oregon State University, 2010. / Printout. Includes bibliographical references (leaves 103-119). Also available on the World Wide Web.