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Metal-Organic Framework

Abstract:

Metal-Organic Frameworks (MOFs) are a class of hybrid microporous materials built up from metal ions or clusters linked together with organic ligands into an ordered structure. MOFs are a very interesting class of materials with properties that resemble those of a crossbreed between zeolites and polymers. They have a highly ordered framework which is often crystalline and possess a well-defined porosity similar to that of zeolites. Additionally the organic linker and metal that make up the MOF can often be customized as is done with polymers. MOFs are currently being investigated for a wide field of applications from catalysis and adsorption to separations and sensors. To perform this variety of functions, different MOF structures are used but often they are also specially modified for the desired task. In this thesis we will focus on the latter, shaping existing MOFs to create a predetermined functionality that is desired for applications. The first and second part of this work focus on smart materials that can respond to external triggers to perform a specific action. The first is a smart catalyst that can be activated by UV-irradiation. Catalytically active MOF particles are encapsulated in polyurea microcapsules with photocleavable walls. These capsules keep reactants and MOFs separated effectively preventing catalytic reaction from occurring. UV-irradiation can initiate the catalytic reaction because it degrades the capsule walls bringing the MOFs into contact with the reactants. The MOFs are encapsulated via interfacial polymerization on oil droplets in an oil-in-water emulsion. A critical condition for MOF encapsulation is that the MOFs remain in the oil phase of the emulsion during interfacial polymerization. This is done by modifying the framework with long chain fatty acids, resulting in a hydrophobic layer on the MOF surface. Incorporating photolabile oligomers in the polymer shell results in a photolabile shell that breaks under UV-light irradiation. The second smart material is designed to inhibit adsorption/desorption if the surrounding temperature is increased above a certain threshold. Some MOFs are great candidates for adsorption and desorption of certain molecules, but uptake and release of adsorbates from a MOF is governed by an equilibrium. Temperature has an effect on this equilibrium and this is used for temperature swing adsorption/desorption processes; however this is a gradual effect over large temperature intervals. The goal is to create a binary (ON/OFF) system that operates on a small temperature difference (< Δ10°C). For this we attach temperature-sensitive poly(N-isopropylacrylamide) polymers (PNIPAM) on the surface of MOF particles. This polymer has an Lower Critical Solution Temperature (LCST) of around 32°C in water, which means it becomes insoluble above this temperature. When coated on a MOF particle, the polymers contract above 32°C and should block pore openings preventing adsorption and desorption. PNIPAM chains can coordinate with their terminal carboxylic acid or thiol group to the metal clusters on the MOF surface. The amount of PNIPAM bound to the MOF surface this way however is insufficient to block the pores at higher temperatures. Another research group showed that by covalently binding PNIPAM to linkers of the MOF framework, the PNIPAM density can be increased and this results in a temperature-sensitive PNIPAM-MOF adsorption/desorption system. The third and fourth part of this work revolve around materials combining polyethylene terephthalate (PET) with terephthalic acid based MOFs. Both materials consist for a large part out of terephthalic acid and this creates interesting opportunities. In the third part of this work we convert (waste) PET into high quality MOFs in a one pot reaction. Certain MOFs can be made under the same conditions that hydrolyze PET, allowing terephthalic acid to be generated from PET, and utilized in MOF synthesis at the same time. MIL-53(Al) and MIL-47(V) can be made from PET and the corresponding metal chlorides with water as solvent and reactant. HCl is generated in situ and increases PET hydrolysis, but excess HCl protonates terephthalic acid and reduces MOF yield. An optimal HCl concentration is obtained by using a mixture of metal chlorides and metal oxides to obtain complete depolymerisation and high MOF yield. Activation of the MOF is done by calcination in air. MOFs that are not stable under the harsh hydrolytic conditions can be made by first depolymerizing PET and adding the metal salt to the same reactor in a second step for MOF synthesis. The fourth part originates from observations made in the third part. PET proved to be an excellent support material and coatings of MOF can be grown on PET bottles. Using PET textiles instead of bottles increases the available surface of PET and the polymer thus can support more MOF. Improved attachment of the MOFs is realized by creating carboxylic acid groups on the textiles by surface hydrolysis. By selecting the right pre-treatment this can be done without deteriorating the mechanical properties of the treated textiles. Short synthesis times give MOFs with a large number of internal defects and a high internal surface area. As synthesis time increases, the defects are removed and the MOF layer becomes thicker. The loss of internal surface with longer synthesis time is more than compensated by the larger amount of MOF on the PET textile.