Vergleichende Untersuchungen zur Synthese, Charakterisierung und katalytischen Testung schichtartiger Zeolithe vom Typ Faujasit (FAU)
Year: 2020
Type: dissertation
Abstract: Zeolites are porous, crystalline materials with a three-dimensional pore network and consist mainly of silicon and aluminium atoms linked by oxygen atoms, so-called aluminosilicates. Their pore system consist of uniformly ordered micropores (< 2 nm) and can be composed of cages or channels. The dimensions of the pores, which are similar to molecule diameters, give these materials outstanding properties for applications in adsorption, separation, ion exchange or catalysis. However, the small pore diameters of these materials lead to a slower diffusion of molecules in the pore system causing these materials to suffer from a limitation of the intracrystalline mass transport. In the worst case, molecules can be completely excluded from the pore system due to their size and the resulting lack of accessibility. This occasionally leads to a reduction in the activity, selectivity or lifetime of zeolite catalysts. To compensate for this disadvantage, it is possible to shorten the diffusion path length of a molecule in the pore system of the zeolite by reducing the crystal dimensions (layered (-1D), needle-shaped (-2D) or nano-zeolites (-3D)) or by introducing an additional pore system (mesopores and/or macropores). If the different pore systems are interconnected, these materials are referred to as hierarchical zeolites. The occurrence of intergrowth of isostructural crystal phases can lead to the formation of layered, branched zeolite crystals. This can observed for the zeolite type Faujasite (FAU) if an intergrowth with the crystal phases of the zeolite type Elf Mulhouse Two (EMT) is present, which can be achieved by suitable additives or synthesis parameters. Bottom-up synthesis routes for layer-like zeolites of the faujasite type are already known from the working group and the literature. These routes contain organosilanes or inorganic salts as additives, but can also be present as additive-free routes with a reduced crystallization temperature. Therefore, the focus of this thesis was on the investigation of the additive-free synthesis route of layer-like FAU zeolites and the comparison of the products with the layer-like faujasites from other synthesis routes. For the first time, layer-like faujasites produced by different synthesis routes were compared with respect to their textural, morphological, chemical, physical and catalytic properties. Electron microscopic images showed that zeolite samples synthesized with the additive inorganic salt or additive-free with a reduced crystallization temperature, in contrast to samples with the additive organosilane, have microporous areas at the edges of the plates and show intergrown parts with assumed mesoporosity just towards the center of the plate. The variation of the molar H2O/Al2O3 ratio in the synthesis gel of all synthesis routes showed that a reduction of this ratio leads to a directly proportional decrease of the particle size. More detailed investigations of the additive-free synthesized samples showed an additional directly proportional decrease of the plate thickness, whereby the ratio of plate thickness to particle size remained constant. With an ion exchange of potassium and caesium ions, respectively, with different layer-like faujasites it could be shown for the first time that these zeolite samples behave almost identically to a conventional faujasite and achieve similar degrees of exchange. The catalytic testing of layer-like faujasites in a Knoevenagel condensation demonstrated that the catalytic activity correlates directly with both the mesopore volume and the thickness of the plates. In addition, the ion-exchanged faujasites showed a significant increase in catalytic activity with no significant difference between potassium and caesium forms of a zeolite. The additive-free synthesis of layer-like zeolite X samples with reduced crystallization temperature was also successfully applied to zeolite Y. The molar Si/Al ratio could be varied from 1.38 to 1.79 with constant particle size and plate thickness. Furthermore, the already described effect of the molar H2O/Al2O3 ratio in the synthesis gel could also be shown here. The used range of the compositions of the synthesis gels for layer-like zeolite X and Y samples (additive-free and reduced crystallization temperature) could be successfully described mathematically as a function of the important properties molar Si/Al ratio and particle size as variables, in order to calculate the composition of the synthesis gel for the synthesis of layer-like faujasites with desired properties.
Author: Reiprich, Bastian
Language: German
Subfield: Inorganic Chemistry
Field: Chemistry
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7053248984