Interfacial Synthesis of Metal-organic Frameworks

Abstract

<p>Metal-organic frameworks (MOFs) are considered as a type of very useful materials for the gas separation/purification industries. However, control over the growing position and growing shape of the crystals remains a challenge and must be overcome in order to realize the commercial potentials of MOFs.</p> <p>In this thesis, a method based on interfacial coordination is developed to address this issue. Zinc-benzenedicarboxyl (Zn-BDC) is chosen as a model system for the proof of concept. In a typical liquid-liquid interface protocol, the MOF precursors, zinc nitrate [Zn(NO3)2] and terephthalic acid (TPA or H2BDC), and the catalyst, triethylamine (TEA), were dissolved into two immiscible solvents, dimethylformamide (DMF) and hexane, respectively. The reaction site, i.e. the MOF growing position could thereby be confined at the interface of the two solvents. It was found that a free-standing membrane could be formed with the combinations of high Zn-H2BDC and low TEA concentrations. The combinations of low Zn-H2BDC and high TEA concentrations yielded MOF particles precipitated out from DMF. Similar results were obtained by changing the liquid-liquid interface to liquid-gas interface, with the TEA-hexane solution replaced by saturated TEA vapor. The dependence of product shape on precursor and catalyst concentrations can be explained by the competition between MOF formation and TEA diffusion into the precursor phase.</p> <p>The morphology, constitution and surface area of the MOF products were characterized by SEM, XRD and nitrogen adsorption testing, respectively. The particles were found to be exclusively MOF-5. The membranes were characterized as asymmetric. The top layer was particulate while the bottom layer had a sheet-like morphology. This was further revealed by XRD data as MOF-5 and MOF-2 (ZnBDC·DMF), respectively. This asymmetry was caused by a change of TEA diffusion rate during the synthesis process, which might result in a change in pH value for the membrane growth. Decent surface areas of the particles and membranes were measured.</p> <p>Apart from the free-standing membranes, MOF membranes on Anodisc support were also synthesized employing the same interfacial techniques. The MOF formation site, i.e. the interface, was confined to the upper end Anodisc pores and sealing the pores after the reaction. The difference in wetting force between DMF and hexane with Anodisc membrane material resulted in the difference of MOF layer morphology from liquid-liquid protocol and liquid-gas protocol. The later gave a continuous MOF membrane due to the absence of air bubble interference.</p>