Morphology of Large ZSM-5 Crystals Unraveled by Fluorescence Microscopy

Abstract

Understanding the internal structure of ZSM-5 crystallites is essential for improving catalyst performance. In this work, a combination of fluorescence microscopy, AFM, SEM, and optical observations is employed to study intergrowth phenomena and pore accessibility in a set of five ZSM-5 samples with different crystal morphologies. An amine-functionalized perylene dye is used to probe acid sites on the external crystal surface, while DAMPI (4-(4-diethylaminostyryl)- N-methylpyridinium iodide) is used to map access to the straight channels in MFI from the outer surface. The use of these dyes is validated by studying the well-understood rounded-boat type ZSM-5 crystals. Next coffin-shaped ZSM-5 crystals are considered; we critically evaluate the seemingly conflicting 2-component and 3-component models that have been proposed to account for the hourglass structure in these crystals. The data prove that observation of an hourglass structure is essentially unrelated to a 90 degree rotation of the pyramidal crystal components under the (010) face. Hence, in perfectly formed coffin-shaped crystals, the straight channels can be accessed from (010). However, in other crystal batches, sections with a 90 degrees rotation can be found; they are indeed located inside the crystal sections under (010) but often only partially occupy these pyramidal components. In such a case, both straight and sinusoidal pores surface at the hexagonal face. The results largely support the 3-component model, but with the added notion that 90 degree rotated sections (as proposed in the 2-component model) are most likely to be formed inside the defect-rich, pyramidal crystal sections under the (010) faces.

Predicting crystal growth via a unified kinetic three-dimensional partition model

“…The interesting discovery is that, because the growth mechanism on different faces of the crystal is necessarily different, the silanols are confined to zones of different density within the crystal (Figures 2 and E5). This mimics almost exactly the optical microscopy images that show identical zoning and has been a source of debate for many years 23 . Similarly, in ETS-10 which displays rod growth ( Figure E6) rather than layer growth the incompleteness of the rods results in internal defects that congregate in a zone from the (001) facets to the centre of the crystal just as observed experimentally by Raman microscopy 24 .…”

Super‐Resolution Reactivity Mapping of Nanostructured Catalyst Particles

“…Electron diffraction patterns taken from two representative crystal zones prove that the line-like feature is actually an interface between two zones with different crystallographic orientation ( Figure 3 d and e). This type of 908 intergrowth has often been proposed for ZSM-5 crystals, [27,[30][31][32][33][34] but the activity of the resulting nanosteps in a catalytic reaction has never been measured.…”

“…AFM has proven that this growth process is prone to defect incorporation, and this results in irregularities in the developing crystal edge. [30,35,36] In summary, the new NASCA microscopy approach records organic reactivity maps of catalysts at small length scales similar to those of electron, scanning probe, or X-ray microscopy, which rather give inorganic or structural information. This labeling-free scheme is based on conversion of single fluorogenic molecules on the true active sites, and has a generic character as it is broadly applicable, even in biological samples.…”

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

“…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.…”

“…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. Whereas various micro‐spectroscopy methods previously introduced provided a wealth of information about inter‐ and intra‐particle heterogeneities in structure and reactivity,15, 19, 20, 21 direct structure–reactivity relationships remain difficult to establish.…”