Rational design of hierarchical metal-organic framework nanostructures for environmental remediation

Abstract

Metal Organic Frameworks (MOFs), and their subclass Zeolitic Imidazole Frameworks (ZIFs), are highly crystalline porous materials with a plethora of applications due to unique crystal properties. However, predictable fabrications to optimize the physical and chemical properties of these materials is challenging. This is attributed to large variety of material building blocks, potential structures and synthetic parameters. More specifically, ambient, aqueous based fabrications have proven extremely challenging, with many MOF and ZIF materials exhibiting a low hydrothermal stability. Herein, adaptations to synthetic parameters, such as temperature, precursor concentration and solvent content, are studied in order to manipulate crystals properties and underpin the potential ZIF crystal formation mechanisms. Overall, this thesis focuses on the fabrication principles of ZIFs, optimization of crystal morphology and properties and the impact on their efficiency for environmental remediations. Additionally, the use of simple fabrication methods in primarily aqueous media, with reduced fabrication temperatures and minimized waste are studied. Primarily, the application of a continuous fluid circulation system in the fabrication of ZIF-L coated hollow fibers was studied. A simple impregnation strategy was found to optimize nuclei formation, reducing the chemical consumption by 50% and significantly reducing the size of ZIF-L crystals. The use of small quantities of ethanol was found to enhance membrane performance, with 10 v/v% ethanol fabrications increasing membrane hydrophilicity, under water oleophobicity and oil-in-water emulsion separation efficiency. Considering the importance of morphological and structural properties of crystalline materials, a range of EtOH concentrations and reaction temperatures were employed to study their effect on crystal formation. The morphological evolution of the ZIF materials proved that the materials could be controlled from leaf-like morphologies to star-like, stadium like and dodecahedral. Subsequently, a phase change from ZIF-L to ZIF-8 was observed with various morphologies and surface wettability’s. The effect of phase and morphology on the antibacterial performance was explored. Further understanding the increased metal coordination, the influence of various solvents in primarily aqueous solutions were studied. The solvents, alcohols, ketone and amide, are shown to require different concentrations to fabricate ZIF-8 owing to their hydrogen bond accepting capabilities and kinetic diameters to successfully increase ligand deprotonation and act as a structural directing agent. Furthermore, the growth kinetics demonstrates that the solvents effect the rate of nucleation and growth of ZIF-8 to form various sized particles. Moreover, the stages of crystal growth were investigated for the optimal dye adsorption capabilities. The results presented in this thesis make a valuable contribution to understanding the effect of various synthetic conditions on the control of morphology and phase of MOF materials. The adaptations of synthesis parameters and synthetic procedures developed in this work reduce chemical consumption, reduced use of solvents during MOF synthesis and are typically carried out in ambient conditions. In addition, this work could contribute to the efficient fabrication of a variety of MOFs in order to enhance their physical and chemical properties. The fabrications in aqueous media with low temperatures enable the formation of polymorphs with desirable wettability, stability and morphology as well as allowing for the preferential formation of different structural phases.