Green Synthesis of Zeolitic Imidazolate Frameworks (ZIFs) for Sustainable Development

Abstract

Metal organic frameworks (MOFs) are a broad family of micro-/mesoporous materials with a significant potential for many applications due to their tunable pore size and very high surface area. However, it remains a significant challenge to produce well-dispersed MOF nanopowders and hierarchically-structured MOF films using scalable and environmentally friendly methods. In order to utilize MOFs for sustainable development, it is of great importance to not only minimize the waste generation and environmental impacts in their syntheses, but also develop suitable applications to mitigate environmental damages, energy problems and health risks. This thesis focuses on zeolitic imidazolate frameworks (ZIFs), a subclass of MOF family, in particular ZIF-8 (made of Zn2+ and 2-methyilimidazolate) and cobalt-doped-ZIF-8. In this thesis, new approaches to synthesizing ZIF-8 in powder and film forms were investigated using environmentally friendly methods, and their new applications in the energy and environmental sectors were explored. In the first study, a green and scalable synthesis method, mechanochemical processing of a stoichiometric ratio of ZnO nanoparticles and 2-methylimidazole, was investigated to produce well-dispersed ZIF-8 nanopowders with a high yield and minimal waste. It was found for the first time that mechanochemically-produced nanopowders have higher efficacy in adsorbing organic pollutants than ZIF-8 nanopowders synthesized via common solution-based methods. In the second study, the conversion of highly porous films of ZnO nanoparticles into ZIF-8 films was studied. It was demonstrated for the first time that ZIF-8 films with a uniform intergrown grains and controlled structural porosity can be obtained via vapour-phase conversion: the hierarchically porous structure of ZIF-8 films can be engineered in a wide range of porosity, by controlling the reaction temperature. The use of dense and crack-free ZIF-8 monoliths as a separator film in a lithium-sulfur battery system showed significantly improved long-term stability and performance, compared to benchmark separators. In the third study, the stability of ZIFs in water was studied systematically using a new combination of analytical tools. It has been widely believed that ZIFs show excellent structural stability in water and thus ideal for aqueous applications. However, this study revealed otherwise. The study generated new knowledge on the effects of ZIF concentrations in water, cobalt doping levels, and amounts of ligands in water on the water stability of ZIF powders. It also determined the minimum amount of additional ligands in water required to stabilize ZIF structures. The use of mechanochemical processing in the synthesis of 0-100 at% cobalt-doped ZIF-8 was critical in producing ZIF-8 nanoparticles with similar sizes, independent of cobalt-doping levels, to enable the study. In the last study, the instability of ZIF-8 in aqueous system was explored in a biomedical application. Antibacterial properties of ZIF-8 are studied in comparison with ZnO, a widely used antimicrobial agent, against E. coli. The study demonstrated superior antibacterial properties of ZIF-8 to ZnO and the reason was elucidated by conducting detailed investigation of the stability of ZIF-8 in common bacterial culture media. This feature of ZIF-8 was further used to improve the antibacterial properties of superhydrophobic surfaces. The present study has made significant contributions to the progress of MOF research in the following areas: developing new green techniques to synthesise MOF nanoparticles and hierarchical MOF films from metal oxide precursors; demonstrating the tunability of their physical properties for sustainability-related applications such as energy storage and health security; revealing the instability of ZIFs in difference aqueous media using quantitative techniques and elucidating the implication of the instability in developing new applications of ZIFs. Show more