Abstract
Zeolites are becoming more versatile in their chemical functions through rational design of their frameworks. Therefore, direct imaging of all atoms at the atomic scale, basic units (Si, Al, and O), heteroatoms in the framework, and extra‐framework cations, is needed. TEM provides local information at the atomic level, but the serious problem of electron‐beam damage needs to be overcome. Herein, all framework atoms, including oxygen and most of the extra‐framework Na cations, are successfully observed in one of the most electron‐beam‐sensitive and lowest framework density zeolites, Na‐LTA. Zeolite performance, for instance in catalysis, is highly dependent on the location of incorporated heteroatoms. Fe single atomic sites in the MFI framework have been imaged for the first time. The approach presented here, combining image analysis, electron diffraction, and DFT calculations, can provide essential structural keys for tuning catalytically active sites at the atomic level.
Highlights
Zeolites are formed by TO4 units (T, Si or Al-atoms) tetrahedrally coordinated through O bridges, and they can generally be described as Mm+x/m[Si1ÀxA1xO2], where M is an exchangeable counter cation with valence m+ to balance the negative framework charge, and the range of x is equal to or less than 0.5 and can be as low as 0.0 for pure silica polymorphs
Electron diffraction (ED) patterns give structural information averaged over the electron-irradiated volume, which is around few hundred of nanometers, onto a unit cell in momentum space, while electron microscopy (EM) images give pinpoint structural information in real space, this information is still averaged over the crystal thickness along the electron beam (e-beam) direction
We demonstrate that two fundamental challenges in the direct atomic-level imaging of zeolites have been solved: First, observing all framework atoms, including O-atoms, in one of the most e-beam sensitive and lowest framework density zeolites, Na-LTA
Summary
Zeolites are formed by TO4 units (T, Si or Al-atoms) tetrahedrally coordinated through O bridges, and they can generally be described as Mm+x/m[Si1ÀxA1xO2], where M is an exchangeable counter cation with valence m+ to balance the negative framework charge, and the range of x is equal to or less than 0.5 and can be as low as 0.0 for pure silica polymorphs. We demonstrate that two fundamental challenges in the direct atomic-level imaging of zeolites have been solved: First, observing all framework atoms, including O-atoms, in one of the most e-beam sensitive and lowest framework density zeolites, Na-LTA (with Si/Al % 1). For this analysis, an annular bright field detector (ABF) was employed for the first time in the study of zeolites. Direct observation of single transition-metal (Fe) heteroatoms in silicalite-MFI zeolite, showing clear evidence of a non-periodic distribution of Fe within the framework. The sensitivity of ED for analysis of the local structure of Na-LTA and Fe-MFI is discussed, and comparison of the experimental data with DFT calculations is reported
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