Defect engineering of UiO-66 metal-organic framework (MOF) for improved hydrogen storage applications
Date
2021-03
Authors
Journal Title
Journal ISSN
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Publisher
Stellenbosch : Stellenbosch University
Abstract
ENGLISH ABSTRACT: Metal–organic frameworks (MOFs) as a relatively new class of porous crystalline materia ls
have attracted much interest in many applications due to their high porosity, diverse structures,
and controllable chemical structures. However, the specific geometrical morphologies, limited
functions and unsatisfactory performances of pure MOFs hinder their further applicatio ns .
Several modifying strategies for engineering MOF crystals have been developed based on their
desired features and applications. In nature there are no "ideal crystals" with an infinite periodic
repetition or ordering of the same groups of atoms in space. The structure of "real crystals"
often deviates from the ideal arrangement and includes a significant density of structural
irregularities or deficiencies. Crystal irregularities may arise from compositio na l
inhomogeneities, and this term is often used interchangeably. By using defective technology
strategies during their synthesis, crystal imperfections may be introduced into the MOF
framework, thereby enhancing their performance in the envisioned applications.
Defect
engineering is one of the most effective approaches that one can use to change their physical
and chemical features such as thermal stability, textural properties, mechanical properties and
gas adsorption abilities. In order to achieve the desired changes, it is essential to control the
defects, otherwise these defects may have an adverse effect on the MOFs. Therefore, it is vitally
important to apply synthetic control over defects; the exact nature and concentration of the
defects may be controlled by modifying the synthetic conditions and post-synthet ic
modifications. Structurally characterising inherent or engineered defects is very challenging
and this challenge has not been addressed substantially.
This thesis explores the experimental creation of structural defects via post-synthetic
modification, the role of structural defects and their relationship to gas adsorption, with emphasis on hydrogen adsorption. Through a combination of techniques, including powder X-
ray diffraction (PXRD), thermogravimetric analysis (TGA), acid-base titration and Brunauer–
Emmett–Teller (BET) surface area and pore size measurements, missing linkers and missing
cluster defects have been identified and analysed. In Chapter 4, we seek to understand the
relationship between some of the major synthetic parameters and the physicochem ic a l
characteristics of UiO-66 (Universitetet i Oslo) MOF and discover a "non-defective" sampling
technique for this material. The technique produces non-defective UiO-66 MOFs at a specific
temperature (493 K), with the linker ratio being greater than that of the salt previously reported
by Shearer et al. As described in Chapter 5, by varying the concentrations of modulator and the
linker, we demonstrate that the linker vacancies can be systematically tuned, leading to
significantly increased surface areas. The defects are caused by partial terephthalic acid eplacement with smaller formate groups from the formic acid modulator. The BET surface areas of the obtained samples range from 1200 to 1600 m2.g-1, and the best sample has a surface area that is about 30% higher than the theoretical value of the surface area of defect-free UiO-
66. Additionally, linker vacancies are proven to have profound effects on the gas adsorption
behaviour of UiO-66 by improving the hydrogen uptake from 1.51 wt. % to 2.0 wt. % at 77 K
and 1 bar. Chapters 5 and 6 include detailed studies of two conventional methods for generating
defects (i.e., de novo defect technology and post-synthetic modification (PSM)) discussed in
each chapter respectively. Still in chapter 5, experimental investigations are discussed that
show the impact of modulator and linker concentration on H2 adsorption and thermal stability.
Chapter 6 provides insight into the impact on thermal stability and adsorption properties
brought about by the post-synthetic modification methods. The resultant materials typically
have high surface areas, large pore volumes and structures with hierarchical pores, which
makes them more practical for hydrogen storage applications.
AFRIKAANSE OPSOMMING: Raadpleeg teks vir opsomming
AFRIKAANSE OPSOMMING: Raadpleeg teks vir opsomming
Description
Thesis (PhD)--Stellebosch University, 2021.
Keywords
Metal Organic Frameworks, Hydrogen -- Adsorption, UiO-66, Defects in manufactures, UCTD