Hydrogen-bonding receptors for anion recovery in a capacitive deionisation system

Abstract

Receptors are ubiquitous throughout nature and are found heavily within biological systems. This has led to synthetic supramolecular chemists to modify or develop analogous mimics of these receptors with high affinity and specificity for a range of target compounds, for potential commercial use. One group of particular interest are receptors that function through the formation of hydrogen bonds to the guest species. This class of receptor has been shown to have a range of different structural geometries and binding motifs, that allow for the sequestration of a number of different species. In the context of this work, anionic hydrogen-bonding receptors, specifically for ‘phosphate’- in most cases dihydrogenphosphate- and bicarbonate are of interest. Phosphate is an integral part of the DNA backbone, however a organophosphorus containing compounds also comprise a large group of chemical weapons which can have a devasting impact on the bodies ability to function. Chemical weapon compounds, such as sarin and Novichok, are based on the functionalisation of a central phosphate core which can be biotransformed into a highly potent active species within the body. Phosphate is also an essential component of plant fertilizers and is used on a huge scale in order to maintain global food security. However, phosphate loss as a consequence of agricultural run-off leads to reduced availability of essential minerals as well as large scale eutrophication. One such method that could be utilised for the recovery of phosphate is electrochemical capacitive deionisation. The principle and idea of capacitive deionisation has been around since the late 1960’s to early 1970’s and has been shown to be a suitable method for the desalination of low-to-medium salinity input streams. The purpose of the work within this thesis was to modify and synthesise receptors that could be covalently attached to porous carbon electrodes and impart selectivity to a capacitive deionisation system. In Chapter 1, the importance of ‘phosphate’, biologically and commercially is addressed before an in depth look at some of the phosphate specific hydrogen bonding receptors that have been reported in the literature. The design of a successful hydrogen bonding receptor relies on the correct orientation of the binding motifs and the range of structural scaffolds have been shown to be useable. Following this, the electrochemical principles of capacitive deionisation and its suitability for the recovery of phosphate are detailed, including some examples of capacitive deionisation set-ups and the overall processes involved. Chapter 2 details the theory of the techniques used throughout this thesis, which include, but not limited to, 1H and 13C NMR for the structural elucidation of the synthesised receptors and cyclic voltammetry which was used for the attachment of organic groups to an electrode. The historical and theoretical background established in Chapters 1 and 2 will lead into the work undertaken in Chapters 3-5. Chapter 3 focusses on the first of three hydrogen bonding receptors synthesised. Building upon previous work within the field, two neutral indole-based receptors were modified to include two different potential attachment points for the electrode- a carboxylic acid and an alkyne. Following the successful synthesis of the alkyne-based receptor, 1H NMR titrations were used to confirm the affinity of the new receptor for dihydrogenphosphate. Chapter 4 introduces the second anion of interest, bicarbonate. The underlying principles for hydrogen bonding are the same for bicarbonate, as in phosphate, however a different receptor was synthesised. The carbazole receptor synthesised contained free amine groups that were proposed to act as points of attachment to an already surface bound organic spacer group. 1H NMR titrations are once again used to determine the affinity of the receptor for the bicarbonate anion. Finally, Chapter 5 introduces the second of the dihydrogenphosphate-specific receptors, this time based on the amino acid leucine. UVVis titrations with a number of different anions were used to determine the affinity of the receptor. Within this chapter, methods for the attachment of organic groups are detailed including the electroreduction of 4-nitrobenzene diazonium and the direct oxidation of the alkyne

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