Detailed structure features from 3D electron diffraction : Heteroatoms, framework disorders, and guest species in zeolites

Abstract: 3D electron diffraction (3D ED) has evolved as a powerful method for ab initio structure determination from sub micrometer-sized crystals. It can be used to elucidate the arrangement of atoms in crystalline materials and to provide insights into the laws of nature that govern the properties of matter. This thesis explores the advantages, challenges, and applications of 3D ED in structure determination of zeolites. It demonstrates that 3D ED can be used to reveal not only the framework structures but also structure details, which facilitates the study of zeolite chemistry.Zeolites are porous silicate materials used in a wide range of applications as shape-selective ion-exchangers, catalysts, and adsorbents. They feature regularly arranged pores of molecular dimensions that can discriminate between molecules with sub-Ångström precision. However, zeolites often crystallize as polycrystalline powders, and their structures are complex and difficult to determine.In this thesis, eight zeolites have been investigated by 3D ED. The structures of three novel materials, PST-24, EMM-59, and EMM-25 are determined. The silicate PST-24 exhibits columnal disorder that yields varying intracrystalline channel dimensionality, which is unprecedented in zeolites. The borosilicate EMM-59 consists of intersecting 3D 12 × 10 × 10-ring channels and is one of the most complex zeolites. The boron sites in the framework can be located by both 3D ED and integrated differential phase contrast scanning transmission electron microscopy (iDPC-STEM). Structure refinement reveals structural changes upon calcination associated to the change of boron coordination. EMM-25 is also a borosilicate with 2D 11 × 10-ring channels. 3D ED reveals that the EMM-25 structure contains zigzag chains that are disordered with two configurations. Further investigations show that similar disorders also exist in other zeolites containing zigzag chains, i.e., EU-1, ITQ-27, and nonasil. We show that disordered atomic sites that are beyond the data resolution can also be identified and refined using 3D ED data.Furthermore, factors that impact the location of organic guest species in zeolites using 3D ED are investigated. Because of the disorder and flexibility of the organic species in EMM-25 and EMM-59, only their average locations can be found using 3D ED. Therefore, we selected a STW-type zeolite HPM-1 with chiral channels for further study. HPM-1 was synthesized using 2-ethyl-1,3,4-trimethylimidazolium cations, and the guest species are intact and ordered in the channels of HPM-1, as previously determined by single crystal X-ray diffraction. We demonstrate that is possible to locate guest species using continuous rotation 3D ED data. Their atomic positions are refined against 3D ED data through both kinematical and dynamical refinements. Finally, the effect of electron fluence on the location of the organic guest species in the zeolite is investigated.