A study into the crystallisation and synthesis of zirconium and yttrium metal-organic frameworks

Since the discovery of MOFs in the 1990s, research into these materials has grown at an exceptional rate, with over 70,000 MOFs currently reported in the Cambridge Structural Database. Their high stabilities and permanently porous nature gives them a wide range of applications, with materials often being designed with specific application in mind, for example gas storage and separation, biomedicine, or catalysis. This thesis focuses on the task of obtaining a better understanding of MOF crystallisation, specifically Zr and Y MOFs, through a variety of methods, aiming to ultimately aid in the synthesis of new materials tailored to specific application. This knowledge was then implemented in the synthesis of a series of functionalised UiO-68 derivatives for methane storage application. An introductory first chapter covers the common synthetic procedures for a range of prototypical MOFs, along with outlining the various applications and postsynthetic modifications available to MOFs. Following, chapter 2 begins to probe the effect of concentration, temperature and additives/modulators on the rate of crystallisation of UiO-66 through the use of the relatively unexplored technique of turbidity measurement. Correlations were found between crystallisation time and various parameters such as temperature and the addition of additives. Overall, the study underlines the delicate nature and complexity of MOF crystallisation, and provides invaluable information which was integrated into future synthetic protocols. Chapter 3 details work carried out in collaboration with the University of Cambridge in the synthesis of zirconium MOFs for methane storage application. A series of structures were identified computationally which are expected to show exceedingly high surface areas and methane uptake. The six MOFs were synthesised both as powders and single crystal via solvothermal synthesis, allowing for crystal structure determination via single crystal X-ray diffraction characterisation. The materials show good thermal stability, a characteristic of zirconium MOFs, and particularly high surface areas, a vital property for gas storage materials. The materials are currently undergoing extensive stability tests along with methane adsorption studies in order to determine their suitability as energy storage materials. Following this, a comprehensive study into the crystallisation of yttrium frameworks has been conducted, examining the effect of coordination modulation on crystallisation. Coordination modulation – the addition of monotopic linkers as capping agents or crystallisation promotors – is a technique routinely implemented in the synthesis of MOFs not only to enhance crystallinity, but also often to control particle size and physical properties whilst maintaining the same overall structure. Two series of Y-MOFs connected either by 2,6-naphthalenedicarboxylate or 1,4- benzenedicarboxylate linkers have been synthesised by coordination modulation, each showing different topologies and connectivities through single crystal X-ray diffraction characterisation. The modulators chosen varied in their size, connectivity and pKa. Contrary to many MOF series in which a change in modulator merely alters crystallinity or particle properties while maintaining the same crystal structure, the resultant crystal structures of the yttrium materials varied depending on the modulator employed, ranging from coordination polymers to porous 3D structures, showing coordination modulation can also be used as a method to direct the crystallisation of a diverse range of polymorphs. Under the same synthetic conditions, alteration of the linker in some cases also showed variance in the framework produced, indicating linker properties could also influence the resultant structure. Finally, exchange between conventional and microwave assisted heating methods also led to the formation of different materials when implementing the same modulation procedure, suggesting a control between the formation of thermodynamic and kinetic products. This study shows the potential of modulation as a structure-directing synthetic protocol for the discovery of new materials with varied applications. The development of turbidity measurement as a simple yet effective tool in the probing of crystallisation kinetics, and the deeper exploration into the role of coordination modulation in MOF synthesis could enable the future elucidation of a range of MOF self-assembly processes, allowing the rational design and synthesis of highly functionalised materials targeted for specific application