Combining Electron Microscopy and Elemental Mapping for the Investigation of Zeolite Crystallization

An elemental mapping method using electron microscopy with energy dispersive X-ray spectroscopy has proved to be a versatile tool to track the crystallization of zeolites. We have observed that disparities in local concentration of inorganic structure-directing agent (e.g., alkali metal) is an effective indicator of the degree of crystallization in zeolites. In this study, we demonstrate this approach for zeolite ZSM-22 (TON) with very small crystal sizes (<1 μm), where the high spatial resolution of elemental mapping in combination with scanning transmission electron microscopy allows one to obtain a high sensitivity for the detection of early-stage crystals. The crystallization of TON proceeds through a primary alkali rich amorphous phase that evolves to a secondary poorly crystalline phase which already possesses the composition of the final zeolite crystals. This fact gives us the possibility to determine the onset of disorder-to-order transitions in individual crystals of materials, that are amorphous by X-ray diffraction.