Oliver Görke

Catalyst coatings for microstructure reactors

Catalytic active coatings for microchannels have been developed for metallic microstructure reactors in heterogeneous catalyzed gas phase reactions. To enhance the inner surface of microchannel reactors, porous metal oxide coatings have been made by various deposition techniques such as sol-gel processes, anodic oxidation, and deposition of nanoparticles. The ceramic thin films have been developed as supports for catalytically active components like precious metals or as catalyst themselves. In an aluminum microstructure reactor the walls of the micro channels of one passage was successfully coated with a porous alumina layer by a modified anodic oxidation method. Moreover, microchannels of microstructure reactors were successfully coated with SiO 2 , Al 2 O 3 and TiO 2 by the sol-gel process. First promising results have been obtained from depositing oxide nanoparticles (ZnO, CuO and Al 2 O 3 ) by washcoat or electrophoretic deposition methods.

Process of Applying Al2O3 Coatings in Microchannels of Completely Manufactured Microstructured Reactors

The methods presented allow vacuum-tight microstructured components to be produced, in which the microchannel walls are provided with aluminum oxide coatings. These coatings have a large surface area and may be applied as catalyst carriers for heteregeneously catalyzed reactions, as was demonstrated by the catalytic conversion of hydrogen with oxygen.

Feasibility study of the Mn–Na2WO4/SiO2 catalytic system for the oxidative coupling of methane in a fluidized-bed reactor

The catalytic system Mn–Na2WO4/SiO2, known for its relatively stable performance for oxidative coupling of methane (OCM), has been thoroughly investigated in the past. In order to evaluate its catalytic performance, micro-fixed-bed reactors were used almost exclusively. This study aims to answer the question of whether this catalytic system would be applicable on a larger scale using a miniplant fluidized-bed quartz glass reactor. Special consideration was given for finding the optimal operating conditions and investigating whether catalyst abrasion and agglomeration could be limiting factors. In this study different compositions of the Mn–Na2WO4/SiO2 catalyst were tested. High sodium content catalysts were difficult to fluidize at the optimal reaction temperature due to severe agglomeration by melting. Low sodium content catalysts showed low selectivity to C2+ hydrocarbons. Catalysts containing intermediate levels of sodium were used for detailed testing as they showed promising performance as well as good fluidizability. The influence of the different reaction parameters on performance was tested, resulting in 19.4% C2 yield at 40% C2 selectivity. Catalysts before and after reaction were characterized regarding composition, crystalline phases, surface morphology and thermal stability. After time on stream, all catalysts exhibited a reduction in specific surface area, changes in Mn valence state (Mnδ+ (2 ≤ δ ≤ 3)) and changes in morphology due to grain growth.

Li/MgO Catalysts Doped with Alio-valent Ions. Part II. Local Topology Unraveled by EPR/NMR and DFT Modeling

The role of Li in Li/MgO as a catalyst for oxidative coupling of methane (OCM) is to promote MgO surface morphology change rather than serve as a constituent of catalytically active sites. While Li/MgO is unstable at realistic conditions with respect to loss of Li, the resulting samples show enhanced selectivity towards C2 hydrocarbons versus CO2, although activity is low and close to pristine MgO. The way (co‐)doping with alio‐valent metal ions affects the catalytic performance of Li/MgO has now been explored. To analyze the structure and the stability of the samples, catalysts with well‐defined stoichiometry were prepared using a co‐precipitation method with freeze‐drying and subsequent annealing. Gd and Fe were used as dopants. Apart from their potential direct role in catalysis, these dopants are anticipated to stabilize Li in the catalyst under the reaction conditions, allowing further clarification of the role of Li. In the case of Gd/Li co‐doping, changes observed in EPR and 7Li‐NMR spectra indicate the formation of correlated, next‐neighbor Li−Mg⋅⋅⋅Gd+Mg pairs co‐existing with “isolated” Gd3+ ions at octahedral Mg lattice sites. For Li/Fe co‐doping, no significant change in the EPR pattern is observed in the presence of Li+ ions, indicating a larger distance between the Li+ and Fe3+ cations in the MgO lattice. Hybrid DFT calculations explain the difference between Fe and Gd co‐doping by a less efficient screening of the Coulomb repulsion between Gd3+ and neighboring cations in Gd doped samples, leading to the stabilization of LiMg near GdMg.

Contributions of phase composition and defect structure to the long term stability of Li/MgO catalysts

Abstract The phase composition and defect structure of the system Li2O–MgO was investigated in terms of the long term stability of Li/MgO catalysts. The Li content was varied from 0 to 7 mol.%. Pure Li · MgO solid solutions were prepared via a special washing procedure. Li contents below 0.04 wt.% were stabilized within the MgO host lattice, whereas higher Li contents were found to segregate as Li2O and Li2CO3 phases. The catalytic activity in the oxidative coupling of methane was found to decay for all catalysts over a period of 19 h on stream, accompanied by a loss of Li as LiOH. Li in the Li · MgO solid solution was found to be more stable in the lattice than in the surface region of the solid. However, impedance measurements on transition metal stabilized Li/MgO catalysts indicated that even the Li ions within the Li · MgO solid solution are not sufficiently stabilized. Thus, neither the Li compounds nor the dissolved Li ions within the Li/MgO solution seem to be truly stable at 750°C under catalytic conditions.

Li/MgO Catalysts Doped with Alio-valent Ions. Part I: Structure, Composition, and Catalytic Properties

Doping of Li/MgO with additional metal ions is suggested leading to an improved system with respect to the catalytic performance and stability when used for oxidative coupling of methane. We used Gd and Fe as dopants and characterized the resulting materials, showing that Fe seems to be completely and Gd partly incorporated into the MgO lattice. The catalytic performance is improved in most cases, but all materials still suffer from severe deactivation. A loss of Li is observed when being used under reaction conditions, but this Li loss is retarded for Fe‐Li/MgO as compared to undoped Li/MgO.

Thermally activated redox-processes in V2O5-x under high oxygen partial pressures investigated by means of impedance spectroscopy and Rutherford backscattering

Electrochemical methods have been applied in the catalytic system V2O5 in order to investigate the redox properties and their correlation with catalytic properties. Temperature programmed conductivity measurements using electrochemical impedance spectroscopy enabled us to determine the onset of a thermally induced reduction at about 380°C. Rutherford backscattering analysis provides evidence for a reduction from V+5 to V+4. Experiments under different oxygen partial pressures showed that the vanadyl oxygen is involved in the reduction process and it was possible to determine the energy of formation for an oxygen vacancy as 1.23 ± 0.03 eV. The removability of the vanadyl oxygen is assumed to be a key factor for the catalytic activity so that it can be characterized by macroscopic transport properties.

CHAPTER 3:Oxidative Coupling of Methane in Membrane Reactors

Oxidative Coupling of Methane (OCM) processes have been investigated as an alternative promising approach for ethylene production for the last three decades. Having considered the performance of the state-of-the-art OCM catalysts and the OCM reaction mechanism, improving the performance of the OCM membrane reactor could be considered as an important contribution to address such a complicated reactor engineering task. In this context, a systematic methodology implementing inorganic membranes, properly modified via silica-based materials, and the thereby achieved outstanding OCM membrane reactor performances are reported here. Moreover, the most important aspects of the performance analysis of OCM membrane reactors, especially in the context of the thermal-engineering characteristics of these systems, are discussed. Such analysis, for the most part, can be applied similarly to analyze other highly exothermic reaction systems in membrane reactors. Interactions between the membrane and the benchmark Mn–Na2WO4/SiO2 catalyst are also discussed. Furthermore, along with reviewing the general aspects of the model-based analysis of OCM membrane reactors, the potential of integrated OCM membrane reactors, such as dual-membrane reactors, is also highlighted. The special characteristics of modeling such non-isothermal reaction systems with significant mass and heat integration in both radial and axial dimensions are also reviewed.