The design and preparation of metal-organic nanosheets for water purification applications

Metal-organic framework nanosheets (MONs) are a unique class of two-dimensional (2D) material. MONs can be defined as free-standing, 2D materials, approaching monolayer thickness, formed by the coordination of organic ligands with metal ions or clusters. The unique combination of properties such as large external surface areas, nanoscopic dimensions, high aspect ratio, periodic structures and tuneability has led to the use of MONs in a vast range of applications. In recent years, there has been a growing concern over the demand for freshwater. Factors such as industrialisation, growing populations and climate change are all putting pressure on this resource. Using separation techniques such as membrane technology can enable alternative water sources such as industrial wastewater and seawater to be utilised. However, new materials are needed to overcome fundamental limitations between the permeability and selectivity of these membranes. MONs are promising candidates thanks to their tunable porosity, surface chemistry and their 2D surface area. Early publications have shown examples of high performing membranes formed using MONs can lead to improved water purification performances. In this thesis, the use of MON-membranes for water purification was explored. Extensive screening and optimisation studies were undertaken to further understand and improve the MON-membranes prepared. The synthesis of novel MONs using defect mediated synthesis routes was also explored. In Chapter 2, the synthesis of novel MONs is investigated. Using a modulating agent, analogues of UiO66 were synthesised with defects that allowed the formation of a hexagonal close packed (hcp) crystal phase. It is the formation of this layered phase that allows the normally three-dimensional interpenetrated UiO-66 structure to form free-standing nanosheets. Synthesis of 2D UiO-66 analogues using tetrafluoro terephthalic acid yielded ultrathin nanosheets that undergo self-exfoliation upon suspension. The use of mixed linkers, tetrafluoro- and 2-amino- terephthalic acids, allowed the synthesis of a 2D UiO-66 analogue material containing amino functionality. The mixed linker 2D UiO66 analogues were subjected to ultrasonic liquid exfoliation, forming monolayer nanosheets with high aspect ratios. Chapter 3 takes the tetrafluoro terephthalate UiO-66 nanosheets and explores their use in water purification membranes. Prior to the work carried out in this thesis, the preparation and testing of water purification membranes was unknown in the Foster group. Therefore, a large focus of this chapter was development of the processes of membrane preparation, optimisation, and water purification testing. Important lessons about additive solubility and the importance of homogeneity in MON active layers were learnt. Using the optimised MON loadings from the previous chapter, an extensive screening study was conducted in Chapter 4. Five different MONs were deposited onto membranes and tested for water purification efficacy. From this screening study, it was identified that Zr-4”(pyridine-2,4,6- triyl)tribenzoate (Zr-PTB) MONs were the most promising candidate and were taken forward for optimisation in Chapter 5. Optimisation of loadings, inclusion of polydopamine as an additive and the introduction of a compression step to the membrane preparation yielded a Zr-PTB membrane with stable rejection of 84 %. Overall, this thesis demonstrates the how MONs can be utilized for the improvement of water purification membranes. It also offers some insight into modulated synthesis routes to allow the formation of MONs from non-layered structures. The growing field of MONs and their unique set of properties demonstrates why MONs have significant potential in water purification applications.