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Mixed-linker Approach toward the Structural Design of Metal-Organic Frameworks


Metal-organic frameworks (MOFs) are porous, crystalline, extended structures made by linking inorganic and organic molecular building blocks together through strong coordination bonds. Due to their high surface areas, pore volumes, and the possibility of decorating them with a great variety of chemical functionalities through the judicious choice and design of their inorganic joints and organic struts, MOFs have been studied extensively and found useful in widespread industrial applications, such as gas storage, separation, and catalysis.

The design and synthesis of new MOFs with desirable properties and functions is a central theme in MOF chemistry. Generally, the structural design of MOFs is realized by a reticular synthesis approach, whereby the metal-based nodes and rigid organic linkers have predetermined geometries. However, when it comes to MOFs with flexible coordination geometries, it can become challenging to successfully guide the synthesis.

The work herein describes my Ph.D. research, dealing with the structural design of two important subclasses of MOFs, zeolitic imidazolate frameworks (ZIFs) and calcium-based environmentally friendly MOFs. Due to the structural unpredictability arising from the flexible coordinative behavior of the metal centers and organic linkers, the development of both classes of materials has so far relied heavily on synthetic “trial and error”, which has greatly limited their structural diversities. The work presented here shows that such challenges can be addressed by the synergistic assembly of linkers with different functionalities (mixed-linker approach). Following this strategy, fifteen new porous ZIF structures (amongst which structures presenting the largest cage and aperture reported for ZIFs), and two new porous calcium lactate frameworks were created. Furthermore, this work allowed the identification and establishment of three general principles for the guided structural design of ZIFs.

Chapter 1 introduces the concept, synthesis, characterization, and classification of MOFs. Then the importance and existing approaches for structural design are summarized. Finaly, the structure-function relationship is discussed to address the potentials of MOF materials in practical industrial applications.

Chapter 2 reports the synthesis and characterization of a series of ZIFs with ultra-large pores and aperture sizes via the mixed-linker approach. The synthesis of fifteen new ZIFs (ZIF-303, -360, -365, -376, -386, -408, -410, -486, -412, -413, -414, -516, -586, -615, and -725) is presented. Members of this list represent ZIFs with the largest pore sizes and aperture sizes (ZIF-412 and ZIF-516, -586, -615, -725) reported so far. Morevoer, a tertiary combination of linkers with different functionalities is reported for the first time in these new structures.

Chapter 3 builds upon the work of Chapter 3 and addresses the desire to uncover general synthetic principles to guide the synthesis of ZIFs. It begins with the analysis of relationship between the imidazolate starting material and framework structure based on the fifteen new ZIFs presented in Chapter 2. The ring in the ZIF structure is identified as the bridge, and the mixed imidazolate approach is the key to tune the ring size and composition. Based on these analyses, three general principles are identified which are applicable to the whole class of ZIF materials.

Chapter 4 switches focus from the previous chapters, and explores the use of a mixed-linker approach in another subfield of MOFs: calcium-based MOFs. The successful implementation of the mixed-linker approach in the ZIF synthesis encouraged us to explore similar synthetic challenges in other fields. In this chapter, the environmentally-friendly calcium-based MOFs are selected as topic of investigation. Using a combination of naturally existing lactate and acetate linkers, the first two examples of porous of calcium lactate frameworks are synthesized and presented. I demonstrate that both of these linkers are essential to the successful formation of the final structure.

Chapter 5 presents potential applications of the newly-made MOFs described in the previous chapters. A particular attention is paid to environmental applications benefiting from the structural and componential characters of the MOFs. The hydrophobic large-pore ZIF is demonstrated to be highly efficient for air purification through the removal of volatile organic compounds, and the calcium-based MOF, exhibiting an environmentally-friendly composition, is shown to be an effective pesticide carrier for agriculture purposes.

Chapter 6 is the final chapter of my thesis, in which I provide my humble thoughts and perspectives of the future development of the structural design of MOFs. MOFs are highly potential in real world applications, and the rational design of their structures and compositions, in order to meet the requirements for targeted applications, is the key to success.

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