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.
