Accessibility, Structure and Reactivity of Individual Catalyst Particles Studied by Fluorescence Microscopy

Abstract

This PhD thesis is aimed at using fluorescence microscopy to study accessibility, structure and reactivity of two types of systems. The first part of this thesis is focused on model zeolite crystals. Fundamental insights into the accessibility and internal structure of zeolite powders and crystals is gained. Four fluorescent organic probe molecules were synthesized, varied in size by adding bulky substituents. The ability of these probe molecules to enter zeolite powders with different pore sizes was studied; the results show a clear correlation between the size of the molecule versus the total amount of molecules are taken up. Furthermore, UV-Vis absorption spectroscopy showed that the electronic properties of these molecules are influenced by the highly polar interior of the zeolite. Next, this series of molecules of different size was used to probe the large zeolite crystals MFI and BEA, giving insight into the accessibility of these important zeolite model systems. Because the probe molecules fluoresce without the need for external activation, their distribution inside a zeolite crystal was studied using confocal fluorescence microscopy and we showed that certain subunits in the crystals are prone to defects. Furthermore, for BEA, the pore orientation proposed in earlier studies was confirmed. Finally, crystals of zeolite ferrierite (FER) were studied using these probes and a range of alternative techniques. Crystals of this zeolite are platelet-shaped with a 2-dimensional pore system with different pore sizes in each direction. Several techniques, based on optical and confocal fluorescence microscopy, are discussed that can be used to provide the pore orientation of this system in a facile way. In the second part, real industrial FCC catalysts are studied, which add a layer of complexity because the catalyst particles consist of zeolites embedded within a matrix of multiple types of materials. Using single-molecule fluorescence microscopy, the diffusion of individual feedstock-like organic probe molecules in the pore system of a single FCC particle was studied. Two states were detected for the probe molecules: immobile (i.e. trapped or adsorbed to the pore wall) and mobile, moving through the catalyst pore network. The obtained information is highly localized and can be used to evaluate and map heterogeneities in diffusion properties within hierarchically structured catalysts. At the same time, the results shine light on the macroscopic properties of the catalyst particles such as the diffusion coefficient and the catalyst efficiency under reaction conditions. To study structure activity relationships, a multimodal approach to study thin sections of industrially used fresh and aged FCC particles was developed. A novel sample preparation method allowed a combination of transmission electron microscopy (TEM) to determine structure of the catalyst materials, correlated with single-molecule fluorescence (SMF) microscopy, to map the reactivity. Ultrastructure components of zeolitic and non-zeolitic material could be distinguished and showed differences in reactivity in the thiophene oligomerization reaction. It was found that seemingly identical zeolite crystals can exhibit large differences in reactivity. The approached allowed structure-activity relationships to be visualized, for the first time with nanometer precision.