Ulla Simon

Eco-fabrication of hierarchical porous silica monoliths by ice-templating of rice husk ash

In this study, within a sustainable chemistry approach, a clean and eco-friendly synthesis process of silica monoliths compatible with environmental limitations is developed. Rice husk (RH), a surplus agricultural byproduct, was used to engineer monoliths with a hierarchical pore structure. Amorphous and crystalline silica powders were extracted from rice husk and ice-templated using water as a liquid vehicle. The effect of silica crystallinity and silica content, as well as processing parameters such as the freezing rate and sintering temperature on the microstructural characteristics and mechanical properties of the obtained monoliths were investigated. Depending on the freezing regime, microstructural analysis confirmed the formation of macropores with lamellar or honeycomb-like structures. By varying the processing parameters, the pore size, wall thickness and porosity can be tailored. While monoliths from crystalline rice husk ash (CRHA) show higher mechanical strength, monoliths from amorphous rice husk ash (ARHA) possess a high specific surface area, demonstrating aligned nano-channels and non-uniform mesoporosity.

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.