Self-assembly in metal-organic frameworks: monitoring the established and refining the novel

Parsons, Amanda P. (2025) Self-assembly in metal-organic frameworks: monitoring the established and refining the novel. Masters thesis, Memorial University of Newfoundland.

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Abstract

Metal-organic frameworks (MOFs) are highly porous materials formed via the self-assembly of organic bridging ligands (linkers) to metallic nodes. The tunability and high surface areas of these crystalline materials provide them a wide range of possible applications, such as gas separation, catalysis, and drug delivery. Despite the popularity of MOFs, the self-assembly process through which these materials form is underexplored, especially in the field of NMR spectroscopy. This is heavily owed to the high cost of the deuterated solvent found in most MOF syntheses making ¹H-NMR experiments expensive to prepare. In Chapter 2 of this work, the synthesis of a MOF called UiO-66 is followed via time-resolved in situ solution-phase ²H-NMR spectroscopy, allowing inexpensive protonated solvents to be used. This work examines the differences in the synthesis of UiO-66 under the influence of different acidic modulators – namely, acetic acid (AA), hydrochloric acid (HCl), and benzoic acid (BA). By utilizing and following either H₂BDC-d₄, for information regarding linker concentration, or D₂O, for information regarding acidity and chemical exchange, defining features of the synthesis were observed. It was found that the choice of modulator had a significant effect on the behaviour of the reaction mixture in the form of producing different rate constants, different lengths of induction periods and different changes to solution acidity. The solvent appeared to participate in some syntheses far more than others, as the HCl and BA syntheses showed evidence of DMF hydrolysis, while the AA synthesis did not. It was found that thermally conditioning the solvent and modulator before the addition of the linker caused significant changes to the synthesis; it caused the rate constants for all three syntheses to drop, but it accelerates the formation of MOF as determined by visual check. Data regarding the porosimetry of these materials was used alongside the ²H-NMR data to create a proposed mechanism explaining this paradoxical behaviour. New MOFs are always being developed, due in part to the ongoing invention of new organic bridging ligands. MOFs with linkers based on porphyrins are plentiful, but, despite their structural similarity to porphyrins, phthalocyanines are far less present in MOFs as linkers; this is largely due to the structural challenges associated with converting these macrocycles into symmetrical linkers. Previously in the Katz group, a new phthalocyanine linker that relied on benzimidazole groups to provide it symmetry was synthesized and incorporated into a MOF. In Chapter 3 of this work, various facets of the synthesis of a MOF with this phthalocyanine linker, called BzPcMOF in this work, are experimented with in order to enhance the replicability of the synthesis, as well as probe further the role of reagents in the synthesis. The concentration of HCl and metal salt used in the synthesis was found to have significant effects on the surface areas of MOF samples. The XRD, elemental composition, and surface area of BzPcMOF were obtained, and possible crystal structures of the MOF were explored. Investigations into the temporal and hydro stability of BzPcMOF produced intriguing results: Over time, the external surface area of samples of BzPcMOF increased significantly, with no penalty to total surface area. When one of these old samples of BzPcMOF was immersed in water, its external surface area decreased, again with no penalty to the total surface area.

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