Functionalised Metal-Organic Frameworks

Student thesis: Doctoral ThesisPhD

Abstract

Chapter 1 introduces the research space of MOFs, outlining definitions and terminology associated with this class of crystalline coordination polymer. Framework building blocks are discussed along with the concept of reticular synthesis. The key properties of MOFs are emphasised and some of the characteristic structural features highlighted. Examples pertaining to the most commonly studied applications for MOFs including gas adsorption, catalysis, sensing and drug delivery are also detailed.
Chapter 2 presents the design and synthesis of the new organic molecule 5-((carboxymethyl)amino)isophthalic acid (H3cmai). This species was targeted for study due to its semi-rigid nature, variety of coordination sites on anions derived from this and unexplored research space in the MOF area. H3cmai was reacted with cadmium(II) salts, leading to the formation of six different frameworks. Zinc(II), copper(II) and cobalt(II) salts were also reacted with H3cmai, resulting in the formation of a further four new MOFs. The cadmium MOFs showed extensive variety in dimensionality, ligand coordination modes, thermal behaviour and internal void sizes. The frameworks based on the first-row d-block metals showed less structural variation, with the cobalt, copper and one of the zinc MOFs being isostructural. The copper-based material was found to be photocatalytically active for the decomposition of the environmentally persistent dye Rhodamine B. Common themes identified within the 10 MOFs were the chelating bidentate binding mode of the flexible arm intrinsic to the cmai-based linker and the presence of water ligands. Notably, only the cadmium-based MOFs formed 3-D frameworks.
Chapter 3 focuses on the synthesis of cmai-containing frameworks that also contained neutral N-donor ligands. These were targeted as a way of increasing framework dimensionality in an effort to produce networks with larger cavities. Five new compounds were obtained with cadmium(II) centres and four new compounds were synthesised containing zinc(II) centres. As with the compounds in Chapter 2, the MOFs formed were diverse in structure and properties. A higher proportion of the synthesised materials were three-dimensional networks, suggesting the addition of the N-donor ligands had the intended effect. Two charged frameworks were observed, one containing a balancing nitrate ion whilst the other contained a second charge balancing framework, a very rare structural feature.
Chapter 4 introduces another new linker, 2,2'-((1,4-phenylenebis(methylene))bis(azanediyl))-diterephthalate (tpat). This linker contains a semi-rigid tethering p-xylene group, connecting two terephthalate units together. The combination of this linker with zinc(II) led to the synthesis of two novel cross-linked frameworks. The MOFs share their base topology with previously reported structures, the difference being the presence of the tethering group spanning the pores. The location of the tethering group was postulated using geometric considerations as it could not be viewed in the single-crystal data. The presence of the cross-linking tether was shown to increase guest retention within the framework. The synthesis of mixed-linker MOFs, varying the amount of cross-linking within the framework, was shown to provide a route to control the extent of the guest entrapment.
Chapter 5 describes the use of MOFs as hosts for guest semiochemicals for application in lure-based traps for pest species. Three different semiochemicals were targeted for study, 3-octanone, isoamyl acetate and 1-hexanol. Out of a range of investigated MOF systems IRMOFs, [Zn4O(bdc-X)3] (bdc = 2-X-1,4-benzenedicarboxylate, X = H, NH2, NHPr and OPr), were shown to be most promising hosts due to high semiochemical uptake in combination with their propensity to allow functionalisation of the dicarboxylate linker. Careful selection of the linker functionalisation was shown to enhance uptake of the semiochemicals. The release of the semiochemicals was also tracked and the linker functionalisation was again highlighted as a method of controlling the system’s behaviour. The three different semiochemicals studied all interacted with the frameworks differently, giving unique release profiles, leading to the conclusion that optimisation of a semiochemical-MOF system must be carried out on a case by case basis.
Date of Award16 Sep 2020
Original languageEnglish
Awarding Institution
  • University of Bath
SupervisorMary Mahon (Supervisor) & Andrew Burrows (Supervisor)

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