Morphology-dependent zeolite intergrowth structures leading to distinct internal and outer-surface molecular diffusion barriers

Abstract

Zeolites play a crucial part in acid-base heterogeneous catalysis. Fundamental insight into their internal architecture is of great importance for understanding their structure-function relationships. Here, we report on a new approach correlating confocal fluorescence microscopy with focused ion beam-electron backscatter diffraction, transmission electron microscopy lamelling and diffraction, atomic force microscopy and X-ray photoelectron spectroscopy to study a wide range of coffin-shaped MFI-type zeolite crystals differing in their morphology and chemical composition. This powerful combination demonstrates a unified view on the morphology-dependent MFI-type intergrowth structures and provides evidence for the presence and nature of internal and outer-surface barriers for molecular diffusion. It has been found that internal-surface barriers originate not only from a 90 degrees mismatch in structure and pore alignment but also from small angle differences of 0.5 degrees-2 degrees for particular crystal morphologies. Furthermore, outer-surface barriers seem to be composed of a silicalite outer crust with a thickness varying from 10 to 200 nm.

X‐ray Excited Optical Fluorescence and Diffraction Imaging of Reactivity and Crystallinity in a Zeolite Crystal: Crystallography and Molecular Spectroscopy in One

“…The 90° intergrowth of the selected ZSM‐5 crystal seems to be interconnected in an anomalous manner, when compared to previous reports 14, 46. The spatial distribution of the crystallographic phases was resolved by integrating the contributions of the higher‐order (16 0 0) and (0 16 0) Bragg reflections for a given range of X,Y positions (Figure 2 b).…”

“…A remarkable example of the compositional and structural complexity of zeolites is ZSM‐5 with the MFI topology, often found with pronounced Al zoning10, 11, 12, 13 and complex internal intergrowth structures 14, 15. Both Al zoning and architecture of the crystals may strongly affect the outcome of post‐synthesis modifications and lead to remarkable differences in mesoporosity8, 16 and reactivity 17, 18.…”

Infrared and Raman imaging of heterogeneous catalysts

“…5b). The intergrowth architecture of the MFI-type crystals has unfortunately not been elucidated at that time; with today's knowledge on the orientation of the channel system in different regions of the crystal 48 the results can be straightforwardly rationalized. In the centre of the silicalite crystal the straight channels accommodating p-xylene molecule are directed perpendicularly to the crystal long axis.…”

Review. Fluorescence Microscopy Studies of Porous Silica Materials

“…In another study, also by Karawacki et al, the authors related the intergrowth structure of zeolites to internal and outer-surface molecular diffusion barriers [76]. In their study, they used a combination of confocal fluorescence microscopy, electron backscatter diffraction from samples prepared by focused ion beam sectioning, transmission electron microscopy imaging and diffraction, atomic force microscopy, and X-ray photo-electron spectroscopy to study a wide range of coffinshaped MFI-type zeolite crystals, and presented an overview of the morphology-dependent MFI-type intergrowth structures.…”