| 1. |
- Kapaca, Elina, 1987-
(författare)
-
A journey towards complete structure determination of zeolites by electron crystallography methods
- 2020
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Electron crystallography has recently become very successful for structural studies of materials with sub-micrometer sized crystals. In this thesis two major techniques have been applied for structure elucidation – 3-dimensional electron diffraction (3D ED) and high-resolution transmission electron microscopy (HRTEM) imaging. Both can provide information about the structure at the atomic level and have been used for structure determination. During the last decade, two 3D ED methods have been used in our group; the stepwise rotation electron diffraction (RED) method developed in our lab and continuous rotation electron diffraction (cRED) where improvements on the already existing RED method were implemented. Both 3D ED methods can be used for fast structure determination of ordered crystalline materials. HRTEM imaging is very useful for structure determination of more complex and severely disordered materials. For complex structures it is often necessary to combine several methods including powder X-ray diffraction (PXRD). Zeolites are microporous crystalline materials. They have complex structures and often synthesized as polycrystalline powders. The aforementioned electron crystallography methods have unique advantages in elucidation of atomic structures of such zeolites. In this thesis, the development of 3D ED methods, especially from RED to cRED, is described through the journey of structure determination of four zeolites; a known pure silicate silicalite-1 for testing the RED method, and three new zeolites. The new zeolites include two extra-large pore germanosilicates ITQ-56 and SYSU-3 and one small-pore aluminosilicate EMM-37. The thesis shows the limitations and advantages of the RED and cRED methods and how different challenges in the structure determination of zeolites are tackled by the advances of 3D ED methods. Finally the thesis presents a detailed structural study of disorders in an aluminosilicate zeolite ITQ-39 by combining HRTEM, RED with sample preparation by ultramicrotomy. The structure of ITQ-39 was determined in 2012 by our group. Here three new zeolite polytypes of ITQ-39 were identified from the HRTEM images and their structure models are proposed. A complete structure determination of zeolites includes elucidation of the framework structure, guest species such as structure directing agent (SDA) molecules and ions in the pores, and any structural disorder in the crystal. This thesis reflects to all of these structural characteristics of zeolites, presenting the power of electron crystallography.
|
|
| 2. |
|
|
| 3. |
- Korde, Akshay, et al.
(författare)
-
Single-walled zeolitic nanotubes
- 2022
-
Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 375:6576, s. 62-66
-
Tidskriftsartikel (refereegranskat)abstract
- We report the synthesis and structure of single-walled aluminosilicate nanotubes with microporous zeolitic walls. This quasi-one-dimensional zeolite is assembled by a bolaform structure-directing agent (SDA) containing a central biphenyl group connected by C10 alkyl chains to quinuclidinium end groups. High-resolution electron microscopy and diffraction, along with other supporting methods, revealed a unique wall structure that is a hybrid of characteristic building layers from two zeolite structure types, beta and MFI. This hybrid structure arises from minimization of strain energy during the formation of a curved nanotube wall. Nanotube formation involves the early appearance of a mesostructure due to self-assembly of the SDA molecules. The biphenyl core groups of the SDA molecules show evidence of π stacking, whereas the peripheral quinuclidinium groups direct the microporous wall structure.
|
|
