Characterization of MEL defects in 2 - Dimensional MFI nanosheets

Abstract

MFI-type zeolite is a microporous silicon-oxygen framework that has been important for several decades due to its o-/p-xylene separation properties in petroleum industry. In 1980s, significant TEM characterization work was done on large-sized MFI-type zeolite crystals, leading to imaging and electrondiffraction studies of its crystal structure, identification of defects and detection of MEL intergrowth within the MFI framework [1][2]. Since then, development of novel synthesis techniques has enabled the creation of 2-dimensional MFI nanosheets [3][4], while development of aberration corrected TEM has pushed the resolution limit of electron microscopes. Capitalizing on these scientific advancements, we revisit the problem of MEL intergrowth within MFI framework by characterizing this defect at the atomic scale in 2-dimensional nanosheets using aberration corrected TEM.Electron-diffraction and BF-TEM data was collected on [010] oriented MFI zeolite nanosheets using FEI Tecnai G2 F30 (S)TEM equipped with TWIN pole piece (Cs=2 mm) and a Schottky field emission electron gun operating at 300 kV. HAADF-STEM imaging was performed on aberration corrected FEI Titan 60-300 (S)TEM, equipped with FEI SuperX EDX detector, operating at 200 kV.Analysis of experimental data was performed by developing new image processing algorithm constituting radial weiner filtering and cross-correlation to identify the MFI and MEL unit cell in BF-TEM and HAADF-STEM mode (Figure 1a-c). These unit cell images were used as templates in a matching algorithm developed to obtain the spatial distribution of MEL within MFI framework (Figure 1d). Analysis of electron diffraction data was performed by 2D-gaussian fitting of (102) diffraction spot from experiment, which showed streaking in a*-direction (Figure 2). Comparison of fitting data with Multislice simulations [5] of electron diffraction patterns and structure factor calculations allowed us to compute the average MFI domain size and MEL content within the 2D zeolite nanosheets. In this study, we observed the prevalence of MEL-type defect in a targeted synthetic route for MFI-type framework suggesting a dynamic growth process, which requires precise control and further understanding of chemical synthesis [6].

Dissolution–Recrystallization Formation of Huge Thin 2D Silicalite Lamella for Promoted Sorption

“…As shown in Figure a, the parent silicalite‐2 obtained is aggregates of rice‐like or elliptic nanoparticles (<200 nm × 760 nm in size) showing clear lattice fringes representative of MEL‐type framework topology. A direct scanning transmission electron microscopic (STEM) observation on its lattice structure detects the sequence of inversion‐center‐related layers interrupted by mirror related layers (Figure S1, Supporting Information), indicative of existence of partial MFI/MEL intergrowth stacking faults in accordance with previous reports . Upon treatment at 353 K for 16 h by Díaz and Corma's method, the silicalite‐2 nanoparticles are rather transformed to either elliptical mesoporous nanoparticles or larger mesoporous platelets than only delaminated as monitored by the high‐resolution TEM (HRTEM) images (Figure 1b).…”

“…A direct scanning transmission electron microscopic (STEM) observation on its lattice structure detects the sequence of inversion-center-related layers interrupted by mirror related layers ( Figure S1, Supporting Information), indicative of existence of partial MFI/MEL intergrowth stacking faults in accordance with previous reports. [41][42][43] Upon treatment at 353 K for 16 h by Díaz and Corma's method, [16] the silicalite-2 nanoparticles are rather transformed to either elliptical mesoporous nanoparticles or larger mesoporous platelets than only delaminated as monitored by the high-resolution TEM (HRTEM) images (Figure 1b). However, morphology observations do confirm the formation of larger 2D platelets and in particular the lateral aggregation or somehow merging of mesoporous nanoparticles into 2D platelets in the samples before and after pyrolysis ( Figure S2, Supporting Information).…”