Zeolite Chemistry Studied at the Level of Single Particles, Molecules and Atoms

Abstract

Zeolites are microporous aluminosilicates that find a wide-spread application as catalysts in the oil refining and petrochemical industries. Zeolite acidity and related chemistry play a major role in numerous catalytic processes and it is of significant practical interest to understand their reactivity and the location of acid sites. The latter is related to the exact location of aluminium atoms in zeolite framework. Optimizing the catalytic activity of zeolites demands the fundamental understanding of the single particle structure and reactivity. The structure of individual catalyst particles is often very complex and spatiotemporal changes in reactivity remain challenging to be measured by bulk methods. This PhD thesis makes a step forward towards using novel characterization methods to study zeolite chemistry at the level of single particles, molecules and atoms. The applied methodologies are capable of resolving the structure and reactivity of individual catalyst particles, ultimately reaching the sensitivity to derive the 3-D atomic distribution of aluminium and detect the formation of individual catalytic events taking place at Brønsted acid sites. Throughout the thesis, a set of advanced characterization tools is used to study three distinct types of differently steamed zeolite ZSM-5 model crystals and real-life fluid catalytic cracking particles based on zeolite ZSM-5. The crystallographic architecture of zeolite ZSM-5 crystals was studied by micro-diffraction imaging. This method resolved the crystallographic intergrowth structure of the zeolite ZSM-5 crystals and Al zoning taking place within the near-surface regions of the crystals. The changes in the 3-D distribution of Al were further studied by time-of-flight secondary ion mass spectrometry and atom probe tomography. The latter technique provided unprecedented insights into the 3-D distribution of Al atoms in parent and steamed zeolite crystals. It was confirmed that clustering of Al atoms takes place upon steaming resulting in a severe deactivation of the catalyst. The structural analysis was further validated by applying single molecule fluorescence microscopy to study the reactivity of individual zeolite ZSM-5 crystals. Two different labeling reactions based on furfuryl alcohol and styrene derivates were employed to quantify in 3-D the reactivity and surface diffusion barriers of large zeolite ZSM-5 crystals. The results indicate large spatial heterogeneities in reactivity of steamed zeolite ZSM-5 crystals. Furthermore, by extending single molecule fluorescence methodology to real-life fluid catalytic cracking particles it was possible to localize 100-500 nm large zeolite domains within the particles and provide more information about the reactivity of individual zeolite domains. The last part of the PhD thesis combines micro-X-ray-diffraction imaging and styrene oligomerization reaction to simultaneously study the structure and reactivity of a single steamed zeolite crystal. One X-ray beam was used to simultaneously record the structural information via X-ray diffraction and Brønsted reactivity via X-ray induced optical fluorescence. The method correlated spatial differences in dealumination with the formation of different types of fluorescent species. Diffraction imaging, atom probe tomography and single molecule fluorescence microscopy emerge as novel tools to characterize the structure and reactivity of individual catalyst particles, providing unprecedented insights into zeolite chemistry at the single particle, molecule and atom level.