Rational Design and Activation of Microporous Coordination Polymers Towards Targeted Structures and Porosity

Abstract

Microporous coordination polymers (MCPs), a class of materials composed of metal clusters connected by organic linkers, generally provide superior porosity, tunable pore size, and pore shape when compared to traditional sorbents such as activated carbon, zeolites and silica. In spite of the discovery of a vast number of MCPs, the design of MCPs with targeted structure and porosity is still largely an empirical exercise. In this thesis, two key aspects towards achieving targeted MCP structure and porosity are discussed to address this problem. The first aspect focuses on the design and control of MCP structures using additives to alter structure formation and geometric analysis of linkers with an emphasis on shape and flexibility. The second aspect involves exploratory research into critical steps in the MCP activation process involving solvent exchange and solvent evacuation to reveal inherent porosity. Among efforts towards targeted MCP structures, strategies such as applying additives and geometric analysis of linkers are found to be effective in precise control over MCP structures and porosity. In the first strategy, additives, such as monocarboxylic acids, were found to play a crucial structural role in a newly discovered 2D Zr MCP, UMCM-309a. The additives can be incorporated and then subsequently removed to affect the interlayer spacing of the resulting structure. The findings suggest the proper use of additives is an emerging synthetic approach leading to novel MCP structures. In the second strategy, an analysis of linker geometry and flexibility was performed in the context of achieving predicted topologies for tetratopic-linker based Zr MCPs. Tetratopic linkers were categorized into tetrahedral, planar square or planar rectangular groups with an emphasis on linker flexibility. A combination of these three linker shapes and known Zr6 cluster generates a prediction framework. Using this strategy, two new Zr MCPs (UMCM-312 and UMCM-313) were produced, both of which confirmed topology predictions, demonstrating the robustness of this predictive method for targeted MCP structures. Moreover, this strategy highlights the importance of understanding linker flexibility, an overlooked yet vital element of MCP design. Activation failures in MCPs are common, leading to lower than predicted surface area and inconsistent/irreproducible gas-storage properties. To better understand activation and factors impacting this process, a detailed study of the critical steps in MCP activation was performed. It was discovered that solvent exchange kinetics are extremely fast, and minutes rather than days are appropriate for solvent exchange in many MCPs. Additionally, it has also been determined that very low surface tension solvents are critical in activating challenging MCPs. MCPs that failed to be activated previously can achieve predicted surface areas upon application of lower surface tension solvents, such as n-hexane and perfluoropentane The insights aid in the efficient activation of MOFs in both laboratory and industrial settings towards targeted porosity.