Abstract
This study made use of a recently developed combination of advanced methods to reveal the atomic structure of a disordered nanocrystalline zeolite using exit wave reconstruction, automated diffraction tomography, disorder modelling and diffraction pattern simulation. By applying these methods, it was possible to determine the so far unknown structures of the hydrous layer silicate RUB-6 and the related zeolite-like material RUB-5. The structures of RUB-5 and RUB-6 contain the same dense layer-like building units (LLBUs). In the case of RUB-5, these building units are interconnected via additional SiO4/2 tetrahedra, giving rise to a framework structure with a 2D pore system consisting of intersecting 8-ring channels. In contrast, RUB-6 contains these LLBUs as separate silicate layers terminated by silanol/sil-oxy groups. Both RUB-6 and RUB-5 show stacking disorder with intergrowths of different polymorphs. The unique structure of RUB-6, together with the possibility for an interlayer expansion reaction to form RUB-5, make it a promising candidate for interlayer expansion with various metal sources to include catalytically active reaction centres.
Highlights
Zeolites are tetrahedrally connected aluminosilicate frameworks with nanometre-sized pores and channels
After determining the average structure of RUB-5, the disordered real structure was studied: in order to simulate the diffuse scattering from RUB-5, complex stacking disordered structures based on three different layers were modelled and the corresponding electron diffraction patterns were calculated with the DISCUS software package (Proffen & Neder, 1997)
The majority of the particles from RUB-5 and RUB6 samples observed by scanning electron microscopy (SEM) consist of thin plate-like crystals that are highly agglomerated
Summary
Zeolites are tetrahedrally connected aluminosilicate frameworks with nanometre-sized pores and channels. 3D electron diffraction (3D ED) has been established as an alternative method for ab initio structure determination of nanometre-sized crystals (Gemmi et al, 2019), including complex zeolites and layered silicates (Mugnaioli & Kolb, 2015; Yun et al, 2015). We presented an alternative approach in which we first determine the average crystal structure of a material based on ADT data using direct methods for ab initio structure analysis and subsequently compare the resulting model with structural images produced by exit wave reconstruction (Krysiak et al, 2018). Possible deviations from the periodic structure, such as layer shifts, can be determined and modelled, for example, with the DISCUS software package (Proffen & Neder, 1997) These simulated electron diffraction patterns can in turn be compared qualitatively with the reconstructed experimental reciprocal space. The structure of RUB-5 including disorder and intergrowth was solved and analysed in detail by applying the combination of 3D ED, XRPD, HRTEM, structural modelling and diffraction pattern simulations
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