Oxidative coupling of methane (OCM) represents a potentially viable method to convert methane directly into more desirable products such as ethane, and ethylene. In this dissertation, a comprehensive kinetic study of oxidative coupling of methane was performed over La2O3-based catalysts. An accurate and reliable gas-phase model is critical for the entire mechanism. The gas-phase kinetics was first studied using a jet-stirred reactor without catalyst. Both experiments and simulations were conducted under various operating conditions using different gas-phase models. Quantities of interest and rate of production analyses on hydrocarbon products were also performed to evaluate the models. NUIGMech1.1 was selected as the most comprehensive model to describe the OCM gas-phase kinetics and used for the next study. Next, microkinetic analysis on La2O3-based catalysts with different dopants was performed. The Ce addition has the greatest boost over the performance. The kinetics at low conversion regimes were analyzed and correlated to the catalysts’ properties. The activation energy for methane hydrogen abstraction was estimated, with the formation rate of primary products, which suggested that the initiation reaction steps were similar for La2O3-based catalyst. A homogeneous-heterogeneous kinetic model for La2O3/CeO2 catalyst was then constructed. By applying in situ XRD, the doping of CeO2 not only enhanced catalytic performance but also improved catalyst stability from CO2 and H2O. A wide range of operating conditions was investigated experimentally and numerically, where a packed bed reactor model was constructed based on the dimensions of experimental setup and catalyst characterization. The rate of production (ROP) was also performed to identify the important reactions and prove the necessity of surface reactions for the OCM process. Laser-induced fluorescence was implemented to directly observe the presence of formaldehyde. The last section includes the implementation of in situ laser diagnosis techniques at the near-surface region to solve the existing challenges. Raman scattering was implemented to quantitate the concentration profiles of major stable species near the surface and measure the in situ local temperatures at different heights above the catalyst surface, to study the kinetics transiting from the surface edge to the near-surface gas phase and provide a new perspective in OCM kinetic studies.

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Le couplage oxydant du méthane (OCM) permet la transformation directe du méthane en éthylène (C2). A ce jour, le procédé catalytique n'atteint pas les critères requis de sélectivité et de rendement. La présence d'oxygène gazeux à haute température (T>700°C) favorise l'oxydation totale. L'utilisation d'un réacteur à membrane (RM) dense, composé de matériaux conducteurs ioniques et électroniques limite la présence de O2(g) dans le compartiment de réaction améliorant ainsi la sélectivité en C2. Cette thèse a pour cadre le développement de membranes catalytiques ayant un flux d'oxygène assez élevé pour atteindre une conversion supérieure à 25% et dont le revêtement catalytique entraine une sélectivité en C2 à 80%. Une méthode innovante, basée sur une approche microcinétique, a été développée pour déterminer le flux d'oxygène à travers les membranes à partir de mesures sur les poudres correspondantes (échange isotopique et ATG). Les constantes d'adsorption et de diffusion obtenues sont ensuite intégrées dans une équation de flux simulant la semi-perméabilité. Cette méthodologie, validée sur trois matériaux : Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF), La0.6Sr0.4Co0.2Fe0.δ et Ba0.95La0.05FeO3-δ, permettra d'accélérer la découverte de nouveaux matériaux conducteurs d'oxygène. Les tests d'OCM ont été réalisés sur une membrane BSCF modifiée en surface par une couche mince catalytique. Deux catalyseurs ont été sélectionnés : Mn/NaWO4 très sélectif et LaSr/CaO très actif. Le rendement en réacteur membranaire est limité à 6%. Une analyse critique a été réalisée afin de concevoir une géométrie de réacteur membranaire optimale pour cette réaction / The oxidative coupling of methane (OCM) allowed the direct transformation of methane into ethylene (C2). Until now, the catalytic process does not reach the required criteria of selectivity and yield. The presence of gaseous oxygen at high temperature (T>700°C) favors the total oxidation. The use of a dense membrane reaction (MR), made of mixed ionic and electronic materials, limits the gaseous oxygen in the reaction compartment and thus improve the C2 selectivity. The goal of this PhD is to develop catalytic membranes exhibiting a flux high enough to reach a conversion higher than 25% and of which the catalytic coating leads to a C2 selectivity of 80%. An innovative method, based on a microkinetic approach, has been developed to determine the oxygen flux across a membrane from measurements on corresponding powders (isotopic exchange and TGA). The adsorption and diffusion constants obtained are then introduced in the flux equation simulating the semi-permeability. The methodology, validated on three materials: Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF), La0.6Sr0.4Co0.2Fe0.δ et Ba0.95La0.05FeO3-δ, will allow to accelerate the discovery of new oxygen conducting. The OCM tests have been achieved on BSCF membrane modified by a thin layer of catalyst. Two catalysts have been selected: Mn/NaWO4 highly selective and LaSr/CaO highly active. The yield in membrane reactor cannot overstep 6%. A critical analysis has been achieved in order to design an optimal membrane reactor geometry for this reaction

No / Oxidative coupling of methane (OCM) is suggested to be a promising process for the conversion of the abundant natural gas into useful chemicals. However, this reaction faces many drawbacks such as low yields for higher hydrocarbons, fast catalyst deactivation, and huge heat effects of the reaction. Only a well-designed fluidized bed reactor is able to overcome effectively those disadvantages and to provide a satisfactory continuous operation. However, design approaches for fluidized bed reactors are still based on models developed during 70s and 80s, which cannot take into account various hydrodynamic effects on the reactor performance. Thus, a reactor designer has usually to rely on extensive experiments in order to improve the classical fluidized bed reactor design. In this work, the relevance of hydrodynamics, reactor geometry, and feeding policy on the performance of a fluidized bed reactor for the OCM is shown. For this purpose, several case studies of fluidized bed reactors are simulated in full 3D geometry under the same reaction conditions, but with different reactor geometries and feeding policy. These studies show the significance of hydrodynamic parameters for the reactor performance, and moreover, how fluidized bed reactor performance can be improved by a careful study of coupled momentum-mass transport-reaction phenomena. Furthermore, it can be demonstrated that a suitable distributed feeding policy of oxygen provides an improved yield while a traditional fluidized bed reactor design results in an inferior performance among all investigated cases.

Die vorliegende Dissertation präsentiert eine detaillierte quantenchemische Untersuchung der H-Abstraktion von Methan durch MgO und Li dotiertes MgO. Motiviert wurde die durch das UniCat-Excellenz-Cluster, welches sich zum Ziel gesetzt hat, die oxidative Kupplung von Methan (OCM) im Detail zu verstehen. Basierend auf der Hypothese, dass Li+O•– Spezies für den H-Abstraktion Schritt verantwortlich sind, wurden kleine kationische MgO- und Li dotierte MgO-Cluster (die O•– Sites modellieren) untersucht. Zur Erstellung von möglichst realen Gasphasen-Modell-Systemen, wurde die globalen Minima der Gasphasencluster mittels eines genetischen Algorithmus bestimmt. Zum Vergleich wurden auch die Strukturen von neutralen MgO-Cluster bestimmt, um eventuelle strukturelle Unterschiede zu erkennen. Anschließend wurden die optimierten Cluster in Bezug auf ihre Eignung zur C-H-Bindungsaktivierung von Methan untersucht. Die Verwendung von kleinen Cluster-Größen ermöglicht es, die Reaktion im Detail zu studieren und verschiedene Methoden der Berechnung zu vergleichen. Die Ergebnisse für die kleinen Cluster wurden anschließend mit realistischeren Modellen verglichen, die eine genauere Beschreibung der Li dotierten Oberflächen ermöglichen, wie zum Beispiel non-embedded und embedded-Cluster und slab Modelle. Unerwartete Ergebnisse für die Betrachtung der Li+O•– Spezies haben zur Untersuchung von zusätzliche Arten von Defekten in MgO (wie niedrig koordinierten O2-Seiten, O-Leerstellen mit unterschiedlicher Ladung und Verunreinigungen) geführt, die als aktive Zentren in OCM fungieren können. Insbesondere wurden morphologische Defekte und verschiedene Arten von F Zentren untersucht. Die Aktivierung von Methan an defekten MgO-Oberflächen wurde innerhalb eines Cluster-Ansatzes untersucht und durch periodische Berechnungen mittels periodic slab models verifiziert. Die Ergebnisse wurden mit vorhandenen experimentellen Daten verglichen. / This work presents a detailed quantum chemical (mostly DFT) study of H abstraction from methane by MgO and Li doped MgO. It is motivated by the UniCat effort to understand the oxidative coupling of methane (OCM). Based on the hypothesis that an Li+O•– species is responsible for the H abstraction step in OCM small cationic MgO and Li doped MgO clusters (which model O•– sites) were investigated. Because we were interested in real gas phase model systems the global minimum structures of (MgO)n+ and LiO(MgO)n-1 clusters were first determined (by means of genetic algorithm) and then used in subsequent reactivity studies. To check if there are any structural differences between neutral and cationic MgO clusters the neutral species were studied as well. After structure determination, the activation of methane by the O•– radical sites was investigated. The small cluster sizes enabled to study the reaction in detail and to compare different methods of calculations. The results were verified by comparison with more realistic models that mimic Li doped MgO surface, like non-embedded and embedded clusters and slab models. However, unexpected results for the Li+O•– sites led to the consideration of additional types of sites in MgO that may be active for OCM – such as low-coordinated O2- sites, O vacancies with different charge and impurity defects. In particular morphological defects and different types of F centers were investigated. Methane activation by defective MgO surface was studied by a cluster approach and then followed by periodic calculations on periodic slab models. The results were compared to existing experimental data.