Developing design criteria and scale up methods for water-stable metal-organic frameworks for adsorption applications
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Metal-organic frameworks (MOFs) are a relatively new class of porous materials, assembled from inorganic metal nodes and organic ligands. MOFs have garnered significant attention in the porous materials and adsorption fields in recent years due to their various attractive features such as high surface areas and pore volumes, tunable and uniform pore sizes, chemically functionalized adsorption sites, and potential for post-synthetic modification. These features give MOFs enormous potential for use in applications such as air purification, methane and hydrogen storage, separations, catalysis, sensing, and drug delivery. Therefore, synthesis and adsorption studies of MOFs have increased tremendously in recent years. Among the aforesaid applications, air purification and air quality control are important topics because existing porous media are ineffective at the adsorptive removal of toxic industrial chemicals (TICs) and chemical warfare agents. Thus, there is a critical need for radical improvements in these purification systems. MOFs have shown great potential to become next-generation filter media as they outperform the traditional porous materials such as activated carbons and zeolites in the air purification of TICs such as ammonia and sulfur dioxide. In spite of the numerous desirable attributes of MOFs, the practical use of these new materials in most applications hinges on their stability in humid or aqueous environments. The sensitivity of certain MOFs under humid conditions is well known, but systematic studies of the water stability properties of MOFs are lacking. This information is critical for identifying structural factors that are important for development of next-generation, water stable MOFs. In addition to the water stability issue, difficulty in the scale up of MOF synthesis has also plagued MOFs. Hence, the goal of this Ph.D. dissertation research is to design ammonia-selective, water stable MOFs that can be synthesized on a large scale. This work will have a direct impact on moving the MOF field forward to the commercial level. To achieve the aforementioned goal, this Ph.D. dissertation research has been divided into following three objectives: (1) Advance our understanding of the water stability of MOFs and develop design criteria for the construction of water stable MOFs. (2) Design water stable, ammonia-selective MOFs for next-generation chemical, biological, radiological, and nuclear (CBRN) filter media. (3) Investigate the scale-up of the UiO-66 MOF scaffold. Through the research efforts over the past four years, it is discovered that it is possible to adjust the water stability of pillared MOFs in both positive and negative directions by proper shielding of the ligand via functional groups. This study is the first of its kind and is of high value for the MOF community. This shielding concept is further extended by synthesizing 4 novel isostructural MOFs with methyl functional groups at different positions on the ligand. For the first time, light is shed on the important distinction between kinetic and thermodynamic water stability and experimental evidence for a kinetically governed water stability mechanism in these MOFs is provided. It is also demonstrated that, using catenation in combination with a pillaring strategy, it is possible to obtain water stable MOFs even when the pillar ligand has lower basicity (pKa value). Ammonia breakthrough measurements have shown that a hydroxyl functionalized Zr-based UiO-66 material is promising as it could offer a method for targeting the removal of specific chemical threats in a chemically stable framework that does not degrade in the presence of water. Large scale synthesis of a water stable MOF, UiO-66, is studied using glass vials and Teflon lined autoclaves. UiO-66 synthesis methods have been refined such that it is now possible to produce more than 70 times the yield obtained from the original synthesis report using the same reaction volume. This would result in a significant reduction of the MOF production cost at the industrial scale. Methane and hydrogen are ‘clean fuels’ (less CO2 emissions than petroleum) and MOFs are being tested for their on-board storage in cars due to the extremely high storage capacities of MOFs being promising enough to meet the requirements. Hence, more broadly, this Ph.D. dissertation work will lead to commercial applications of MOFs, which can revolutionize a variety of gas separation and storage problems such as CO2 capture, natural gas upgrading, and methane and hydrogen storage for clean fuel technologies. This would greatly reduce the environmental concerns faced by our society today.