Structural Characterization of Crystalline Microporous Materials Using Transmission Electron Microscopy

Abstract

In this thesis, the structural characterization of functional, especially crystalline microporous materials, was performed by electron diffraction tomography (EDT) technique using transmission electron microscope (TEM) and supported in combination with complementary methods. The structural elucidation of materials at the atomic level is a key step to understand their chemical and physical properties and is therefore of great importance for the development of specific applications and for the targeted design of novel materials with desired properties. Microporous materials show unique structural features - a periodic arrangement of cavities and channels with high internal surface areas. This type of material is suitable for numerous applications in industry as well as in daily life. Since microporous materials often emerge with the factors of nano crystal size, disorder, multiple phase and low crystallinity, the structural characterization of these materials is challenging with traditional and conventional diffraction methods like single-crystal X-ray diffraction (XRD) or X-ray powder diffraction (XRPD). The used method in this dissertation, namely automated diffraction tomography (ADT), enables structure analysis directly from single nanosized crystals and can overcome the mentioned challenges. In this work, the structural characterization started with an electron beam stable ceramic (Al4B2O9) with a disordered structure and then focused on several beam sensitive microporous materials. The first phase of structural analysis of microporous materials comprises of crystal structure determinations of two metal-organic frameworks (Zr-MOFs) with large lattice parameters and a novel zeolite (THK-2) in a multiphase mixture. Subsequently, zeolites with industrial interests were structurally investigated after targeted modifications. On basis of the known crystal structure, the crystal disorder could be described for the metal interlayer expanded zeolites (M-IEZ-RUB-36) by structural modelling and simulation of electron diffraction patterns in the program package DISCUS. In addition, the positions of organic structure directing agents (OSDAs) in the pore of the porous materials: SSZ-51 and SOD; the Cu atom position in a dehydrogenated selective catalytic reduction (SCR) catalyst (Cu-chabazite) were determined from three-dimensional ADT data. This work provides an important contribution to the overall structure characterization of microporous nanomaterials including ab initio structure determination, disorder analysis, determining the position of OSDAs in zeolites and detecting the metal atom position in dehydrated zeolite, which would not be accessible to elucidate the structural features with a reliable accuracy as shown in this work using the conventional methods.