Synthesis and characterisation of metal-organic framework materials with carboxylate ligands

Doctoral thesis English OPEN
Munn, Alexis S.
  • Subject: QD

The adsorption of various guest molecules by the flexible 1,4-benzenedicarboxylate\ud metal-organic framework (MOF), MIL-53, has been followed using in situ diffraction\ud techniques. This crystalline MOF displays a structural expansion upon the adsorption of\ud guest molecules, evident by a change in unit cell parameters, which allows adsorption to\ud be followed using diffraction techniques.\ud Adsorption studies were performed with the Fe(III) form of MIL-53 using both liquidand\ud gas-phase guest molecules. The results from in situ X-ray diffraction (XRD) studies\ud show that small aliphatic-alcohols give rise to a two-step expansion whereas branchedalcohols\ud and larger aromatic molecules result in only a one-step expansion. The solvent\ud used for liquid phase studies was shown to affect guest adsorption; benzothiophene and\ud benzothiazole were adsorbed more quickly from a solution of heptane than from a\ud solution of isopropanol. Gas phase studies were used to investigate the effect of\ud occluded water molecules inside the pores of the framework upon the adsorption of\ud methanol. A combination of XRD, thermogravimetry and inelastic neutron scattering\ud studies were used to show that methanol does not displace water when hydrated MIL-\ud 53(Fe) is used as an adsorbent. Two equivalents of methanol can be packed inside the\ud pores of the framework, irrespective of the adsorbent being hydrated or dehydrated.\ud Eight linker-modified MIL-53(Al) materials were supplied by research partners and in\ud situ XRD studies were performed to reveal the effect of the modifications upon the\ud adsorption of gas-phase molecules. The results show that the linkers change the\ud behaviour of the material towards certain guest molecules and the most dramatic effect\ud was seen when the benzene ring of the linker was replaced by cyclohexane.\ud A series of cobalt (II) MOFs synthesised with carboxylate and pyridine-N-oxide linkers\ud is presented. The effect of functionalising the pyridine ring upon the topology of the\ud resulting framework was studied. Resonance and steric effects were found to influence\ud the structure of the final product. One of the new materials has the potential to be used\ud as an adsorbent due to a 3D porous structure.
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