Developing chemically stable metal-organic frameworks for clean energy technologies

Abstract

As global energy demand increases, breakthroughs in clean energy technologies will be needed to meet these demands in a sustainable manner. Metal-organic frameworks (MOFs) are nanoporous materials with extreme promise for addressing these challenges. Formed by the assembly of inorganic metal nodes and organic ligands, the exceptionally high internal surface areas and chemically tunable structures of MOFs give them extreme promise for a wide range of clean energy technologies such as methane and hydrogen storage, catalysis, and carbon dioxide capture. Chemical instability in the presence of water is one of the last remaining challenges to the widespread use of MOFs in a number of clean energy applications. This dissertation elucidates the underlying structure-property relationships that govern the chemical stability and adsorption properties that are critical to their success in such technologies. To enable the development of chemically stable MOFs for clean energy technologies, three key scientific questions are addressed: (1) What are the structure-property relationships governing MOF chemical stability? (2) How can ligand functionalization be used to adjust MOF chemical stability properties? (3) Can we simultaneously improve both chemical stability and adsorption properties in MOFs in order to engineer next-generation adsorbents for post-combustion CO2 capture? Question one is addressed in chapter 3 through a comprehensive review on the topic of water stability and adsorption in MOFs. This work provided the scientific community with the first insight into the overarching structural features that govern the chemical stability properties of these materials. From this understanding, strategies that can be exploited to impart greater stability in next-generation MOFs were also identified. Overall, this chapter lays critical groundwork for both understanding and overcoming the water stability challenge in MOFs. Question two is addressed in chapters 4 to 6 through studies where modifications to ligand functional groups are used to tune the stability properties of isostructural MOFs in the presence of moisture. In chapter 4, experimental studies identifying key trends related to the effect of ligand functionality on water stability properties are presented. Chapters 5 and 6 then provide molecular-level insight into the origin of these stability differences via complimentary experimental and molecular modeling techniques. In these instances, molecular modeling was used to provide information that is not only consistent with experiment but also allows critical insight into phenomena occurring over length and time scales that are difficult to observe experimentally. Question three is addressed in chapter 7, where the incorporation of methyl functional groups on the organic ligand is identified as a promising approach for simultaneously improving both chemical stability and CO2 selectivity properties in MOFs. The experimental CO2 capture findings presented in this chapter were rationalized through a detailed analysis involving multi-scale molecular simulations, and through this analysis a novel approach for quantitatively understanding the intermolecular interactions governing these trends was developed. These findings are relevant to the development of next-generation materials for post-combustion CO2 capture from coal power plants. As the single largest point-source of CO2 emissions into the atmosphere, CO2 capture from such sources can play a significant role in reducing our greenhouse gas emissions in order to sustainably meet our generation's growing energy demands.