Electron Microscopy Studies of Local Structural Modulations in Zeolite Crystals

Abstract

Zeolites are widely used in catalysis, gas separation, ion exchange, etc. due to their superior physicochemical properties, which are closely related to specific features of their framework structures. Although more than two hundred different framework types have been recognized, it is of great interest to explore from a crystallographic perspective, the atomic positions, surface terminations, pore connectivity and structural defects that deviate from the ideal framework structures, namely local structural modulation. In this article, we review different types of local modulations in zeolite frameworks using various techniques, especially electron microscopy (EM). The most recent advances in resolving structural information at the atomic level with aberration corrected EM are also presented, commencing a new era of gaining atomic structural information, not only for all tetrahedral atoms including point vacancies in framework but also for extra‐framework cations and surface terminations.

Three-dimensional electron diffraction for porous crystalline materials: structural determination and beyond

“…In combination with imaging techniques, electron diffraction (ED) taken along well-dened zone axes had been applied for structure determination of nano-and submicron-sized crystals for more than 50 years. [14][15][16][17][18][19][20][21][22] However, collection and processing of such in-zone ED data for 3D structure determination is both demanding and time-consuming, requiring strong electron microscopy and crystallography expertise. More importantly, ED intensities suffer from dynamical effects generated by multiple scattering events of electrons in the crystal.…”

“…The readers are referred to recent review articles on HRTEM imaging of zeolites. 17,22 Compared to HRTEM imaging, 3DED is more feasible and less demanding in terms of both instrumentation and expertise. During the past decade, 3DED has played a dominating role in the discovery of novel zeolites and determination of their structures.…”

Direct Imaging and Location of Pb2+ and K+ in EMT Framework-Type Zeolite

“…It has been previously mentioned that due to the structural similarities (the use of the same building units differing only on how the layers stack), FAU and EMT can coexist in the same crystals. ,, Along the [110] orientation, both hexagonal EMT and cubic FAU exhibit different ways of pillaring the faujasite layers, where for EMT , there is an AA′AA′... stacking (Figure a) and for FAU (Figure b), it follows ABC... pillaring. These layers are formed by sodalite cages ( t-toc ), in pink, linked by d6Rs ( t-hpr ), in purple, which are subsequently also connected by d6Rs ( t-hpr ) shown in green between different layers.…”

“…Therefore, long-range techniques such as X-ray diffraction and Rietveld analysis do not always give the short-range information needed. In this sense, transmission electron microscopy is the most powerful methodology as it can provide direct visualization of the framework and the guest species at an atomic level together with crystallographic and chemical information. − All of these results have been achieved with thorough control of the electron dose, as zeolites are strongly beam sensitive. Working in the probe mode offers a quick scan, reducing the sample damage.…”

Metal Sites in Zeolites: Synthesis, Characterization, and Catalysis

“…Right now, scanning transmission electron microscopy (STEM) has become a powerful tool for atomic-resolution imaging of materials that can operate under low-dose conditions with short acquisition time to preserve the zeolite structure. Furthermore, with the successful implementation of spherical aberration (C s ) correctors in STEM, subangstrom resolution has been achieved for investigating the atomic crystallographic positions, structural defects, pore connectivities, and surface terminations of zeolite frameworks . Furthermore, high-angle annular dark field (HAADF) and annular bright field (ABF) detectors can be synergistically used to observe elements with different atomic numbers ( Z number), in which ADF is suitable for analyzing elements with higher Z numbers and ABF for analyzing elements with a lower Z numbers .…”