Internal architecture of coffin-shaped ZSM-5 zeolite crystals with hourglass contrast unravelled by focused ion beam-assisted transmission electron microscopy

Studying Reaction Mechanisms in Solution Using a Distributed Electron Microscopy Method

Electron microscopy (EM) of materials undergoing chemical reactions provides knowledge of the underlying mechanisms. However, the mechanisms are often complex and cannot be fully resolved using a single method. Here, we present a distributed electron microscopy method for studying complex reactions. The method combines information from multiple stages of the reaction and from multiple EM methods, including liquid phase EM (LP-EM), cryogenic EM (cryo-EM), and cryo-electron tomography (cryo-ET). We demonstrate this method by studying the desilication mechanism of zeolite crystals. Collectively, our data reveal that the reaction proceeds via a two-step anisotropic etching process and that the defects in curved surfaces and between the subunits in the crystal control the desilication kinetics by directing mass transport.

Local anisotropy in single crystals of zeotypes with the MFI framework structure evidenced by polarised Raman spectroscopy

Polarised Raman spectroscopy is used to characterise the local structure in single crystals of zeotypes, namely silicalite-1 and ZSM-5, which share the MFI framework structure. Attributes favourable for applying polarised Raman spectroscopy are the orthogonal axes of these single crystals and their size, i.e. 10 to 30 micrometers in all three directions. We show that the intensity of certain vibrational modes in silicalite-1 depends on the polarisation of the incident light, reflecting the anisotropic character of the molecular bonds contributing to these vibrations. Using these observations, and by estimating the depolarisation ratio (ρ) and the pseudo-order factor (f), we propose a more accurate assignment of the Raman active modes. More precisely, Raman intensities peaked at 294, 360, 383 and 472 cm^-1 are attributed to bending modes in 10-, 6-, 5- and 4-membered rings, respectively. In the region of stretching modes, the vibration at 832 cm^-1 is assigned to Si-O-Si bonds shared between 5-membered rings, which have an orientation parallel to the a-axis of the crystal. By virtue of having a strongly polarised character, the modes at 472 and 832 cm^-1 can be used as orientational indicators. The proposed assignment is supported by the good agreement between experimental and simulated polar plots, where Raman intensities are plotted as a function of the polarisation angle of the incident light. Finally, upon partial substitution of Si atoms by Al, the crystalline structure is maintained and almost no spectroscopic changes are observed. The only significant difference is the increased width of most vibrational modes, which is consistent with the local lower symmetry. This is also seen in the angular dependence of selected vibrational modes that compared to the case of pure silicalite-1 appear less polarised. In the Raman spectrum of ZSM-5 a new feature at 974 cm^-1 is observed, which we attribute to Al-OH stretching. In the high frequency range, the O-H stretching modes are observed which arise from the Si-O(H)-Al Brønsted acid sites. The intensity of the characteristic mode at 3611 cm^-1 reveals an anisotropic character as well, which is in line with previous findings from solid state NMR that Al atoms distribute nonrandomly within the MFI framework structure.

Characterization at the Level of Individual Crystals: Single-Crystal MFI Type Zeolite Grains

Electron-diffraction data on the zeolites Silicalite-1 and ZSM-5 (both MFI framework type) were collected from individual grains of about 150×100×50 nm³ . Crystals were synthesized with tetrapropylammonium as structure-directing agent. The resolution extended to about 0.8 Å for Silicalite-1 and about 0.9-1.0 Å for ZSM-5 crystals. Analysis of several data sets showed that at the nanometre-scale, these zeolite crystals are single crystals and not intergrown.