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Crystal engineering of coordination networks for the development of regeneration optimised water vapour sorbent
Date
2025
Abstract
Climate change continues to exacerbate water scarcity by altering precipitation patterns, reducing the reliability of traditional water sources, and increasing the frequency of droughts. Therefore, addressing water scarcity requires innovative, low energy footprint approaches that are effective everywhere on our planet, especially where there is low humidity. Unfortunately, state-of-the-art water capture technologies have several drawbacks as they rely on energy intensive chemisorption mechanisms, making them less viable in regions with limited energy resources. Scalability is another issue, as many current technologies are only effective on a small scale and need substantial modifications to serve larger populations or agricultural needs.
High maintenance costs further impede their implementation, especially in resource-constrained economies. Moreover, productivity varies with environmental conditions, rendering some systems less effective in arid regions where they are most needed. In this context, desiccants that operate by physisorption offer the potential to enable energy-efficient adsorptive capture of water and it is unsurprising that there is a growing interest in this area of research. The study of a new generation of regeneration optimised sorbents (ROSs) for atmospheric water harvesting (AWH) and indoor humidity control (IHC) is the main focus of this thesis.
The approach taken is based upon crystal engineering, the field of chemistry that studies the design, properties, and applications of crystals, which has emerged as a tool for the design and construction of a class of material known as coordination networks. Coordination networks are defined by IUPAC as compounds comprised of metal or metal clusters (nodes) connected in two or more directions by organic, inorganic or combination of both organic and inorganic linker ligands. Coordination networks that are amenable to crystal engineering are particularly important in this context as they offer a means of precise control over pore size/chemistry and they have recently emerged as benchmark physisorbents for addressing energy-efficient gas/vapour/liquid separations such as C2H2/CO2, C2H2/C2H4, C3H4/C3H6, C8 aromatic isomers, AWH and IHC. An interesting feature of some coordination networks is their “stepped” or S-shaped isotherms that result from pore filling (in rigid coordination networks) or structural flexibility (in flexible coordination networks). This feature endows coordination networks with the advantages of providing new opportunities in water harvesting applications enabled by fast kinetics, hydrolytic stability (recyclability) and low regeneration temperatures when compared to conventional desiccants. Less than 1% of coordination networks archived in the Cambridge structural database are known to exhibit structural transformation triggered by external stimuli, such as guest molecules, vacuum, or temperature. This feature of flexible coordination networks can be characterised by a reversible transformation between a large-pore “open” phase and a guest-free, “closed” phase. Less commonly, flexible coordination networks can also mimic biochemistry through pore contraction, a structural transformation that mimics induced fit enzyme-substrate binding, thereby offering a new approach in the application of coordination networks such as metal-organic frameworks (MOFs) for water scarcity and indoor air quality regulation. MOFs are the most commonly studied class of coordination networks and are defined by IUPAC as a coordination network with organic ligands containing potential voids.
The objective of this thesis is to explore the crystal engineering of coordination networks as ROSs for AWH and IHC through the study of pore engineering, hydrolytic stability and water harvesting performance of selected coordination networks and their derivatives.
Supervisor
Zaworotko, Michael J.
Description
Publisher
University of Limerick
Citation
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Shabangu-2025-Crystal.pdf
Adobe PDF, 11.3 MB