title: Nanostructured metal-organic frameworks with exceptional thermal and hydrothermal stability: synthesis, characterisation and applications in environmental remediation
Creators: name: Butt, Fraz Saeed
Publisher: name: The University of Edinburgh
Year: 2024
Subjects: subject: Nanostructured metal-organic frameworks
subject: exceptional thermal and hydrothermal stability
subject: Nanostructured metal-organic frameworks with exceptional thermal and hydrothermal stability
subject: environmental remediation
subject: Metal-organic frameworks (MOFs)
subject: porous-coordination polymers
subject: zeolitic imidazolate frameworks (ZIFs)

description: Metal-organic frameworks (MOFs), or porous-coordination polymers, have emerged as an exciting new class of highly structured porous materials composed of both organic (ligand) and inorganic (metal) entities. MOFs benefit from the flexibility of structural tunability, where an enormous number of MOF structures have been prepared simply by varying the types of ligands, metal ions and/or reaction conditions. Owing to this structural versatility, MOFs find their applications in catalysis, energy production, separations and storage. Recent advancements in MOF chemistry have led to the discovery of zeolitic imidazolate frameworks (ZIFs), a sub-class of MOFs manifesting the highly desirable structural attributes of both MOFs and zeolites. ZIFs, with extraordinary structural properties, have been mainly synthesised in organic solvent mediums, resulting in a high synthesis cost and difficult disposal. Although aqueous-based ZIF synthesis routes are available, however, they require a high ligand(L)/metal(M) ratio. Moreover, the as-reported (aqueous-based) ZIF structures suffer due to high-temperature thermal degradation and low aqueous-phase stability (due to the breakage of the M–L bond), resulting in lower material performance. Therefore, a highly optimised, cost-efficient and environmentally friendly formation of ZIF crystals, having excellent thermal and aqueous-phase stability, is urgently needed. For this reason, a thorough understanding of the influence of different synthesis parameters on ZIF structural properties, especially in cosolvent systems, is necessary. Benefiting from the structural tunability of ZIFs, the effect of the addition of an organic cosolvent, in a predominantly aqueous-based environment, on improving ZIFs’ structural properties was studied systematically. Different organic cosolvents, including methanol, ethanol, isopropanol, n-propanol, DMF and acetone, were added to the ZIF synthesis mixtures and their desired concentrations for ZIF-L→ZIF-8 phase direction were evaluated. Additionally, the ZIF formation mechanism was drawn for the used organic cosolvents in terms of solvent properties, metal site interactions, and deprotonation capabilities. This newly proposed mechanism of ZIF growth provides vital information regarding the cosolvents’ roles in aqueous-based fabrications and provides a thorough insight into the synthesis and structure control/modifications of ZIFs and their composite materials. The study on the role of organic cosolvent, especially in ZIF-8’s thermal stability improvement, was further extended with the use of long and straight-chain organic cosolvents such as n-hexane, c-hexane, n-heptane and n-dodecane. It was found that the straight-chain organic cosolvents (n-hexane and n-heptane) resulted in the formation of highly crystalline ZIF-8 structures with improved thermal stability and large surface, later, the mechanism for the thermal stability improvement was proposed. The best-obtained cosolvent-stabilised ZIF-8 samples showed only ~1% of material loss in 2 h of tube furnace treatment (under N2) at 500 °C in comparison to conventional aqueous-phase synthesised ZIF-8 (~15%). The cosolvent-directed ZIF structural tunability and property enhancement were studied with other types of ZIFs, such as ZIF-L-Co and ZIF-67. The presence of an organic cosolvent (n-hexane) resulted in phase direction and improvement in ZIF-67’s structural hydrophobicity. These as-prepared ZIF-67 samples showed excellent hydrostabilty with no loss/change in structural properties such as crystallinity, morphology and molecular structure over long-term aqueous exposure (up to 7 days). Additionally, the mechanism for the cosolvent-directed ZIF-67 hydrostability improvement was proposed. Furthermore, owing to their long-term aqueous endurance, the catalytic samples obtained from hydrotreated ZIF-67s showed no significant loss in CO2 conversion, CH4 and CO selectivity in the ‘Sabatier reaction’ for CO2 to CH4 conversion, in comparison to conventional (aqueous-based) ZIF-67. Owing to the luxury of application-based ZIF synthesis, the soft-templating formation of hollow ZIF-8 nanospheres was studied on cosolvent(oil) in water (O/W) mixtures. The effect of pre-synthesis ultrasonication, cosolvent (n-hexane) concentration, surfactant amount, synthesis time, ligand concentration and the order of the reactant addition on hollow ZIF-8 structural properties were studied. Furthermore, the best-prepared samples showed excellent CO2 adsorption/desorption, large CO2/N2 selectivity and remarkable CO2 storage and material regeneration. In a further study, owing to ZIF’s structural versatility, surfactant-directed two-dimensional ZIF-8 nanosheet assemblies (having a large surface-to-volume ratio) were prepared through careful recipe manipulation. The as-formed two-dimensionalised ZIF-8 structures showed highly efficient and large-capacity removal of, especially, the large-sized dye molecules. Furthermore, the dye adsorption models and isotherms were studied, and a suitable mechanism for dye adsorption was proposed accordingly. Adsorbent-adsorbate electrostatic interaction, nanosheet structures, molecular size and structural aromaticity of the dye, and the presence of SDS functionalities were found as the essential components for the high-capacity dye adsorption as compared with previous studies. In summary, the results included in this thesis present a remarkable contribution towards ZIF structural property control and related improvements through systematic tuning of ZIF nanostructures and variations in the synthesis recipes. This study provides well-optimised ZIF synthesis routes that assure a significant reduction in the excessive use of toxic chemicals by mainly adopting aqueous-based fabrications, therefore resulting in a more economical and environmentally friendly ZIF synthesis. Furthermore, this thesis includes the applications of the as-designed ZIF structures in catalysis, carbon capture and dye rejection.