Date of Degree
PhD (Doctor of Philosophy)
Forbes, Tori Z.
First Committee Member
Forbes, Tori Z.
Second Committee Member
Third Committee Member
Fourth Committee Member
Fifth Committee Member
Metal-organic nanotubular (MON) materials have garnered significant attention in the recent years not only due to the aesthetic architecture but also due to the interesting chemical and physical properties that have been reported for these compounds. The number of MONs reported in the literature are limited compared to metal organic frameworks due to synthetic challenges and difficulties in crystal engineering. These types of materials are of interest given their one-dimensional channels that lead to their potential application in advanced membrane technologies.
In Forbes group, a uranium-based metal-organic nanotube (UMON) was synthesized using zwitterionic like iminodiacetic acid (IDA) as the ligand. IDA ligand chelates to the U(VI) metal center in a tridentate fashion and doubly protonated IDA linker connects the neighboring uranyl moieties until it forms hexameric macrocycles. These macrocycles stack into a nanotubular array due to supramolecular interactions. Single crystal X-ray diffraction studies displayed there are two crystallographically unique water molecules that can be removed reversibly at 37 °C. UMON indicated selectivity to water, the selectivity of this material was analyzed using solvents with different polarities, sizes, and shapes. In the current body of work, dehydrated UMON crystallites were exposed to these solvents (in liquid and vapor phase) and studied using TGA coupled FTIR set up, confirming the highly selective nature of UMON. Kinetic studies were also conducted using an in-house built vapor adsorption setup confirmed the water uptake rate of the nanotube depends on the humidity of the environment. Uptake rates were estimated using a simple kinetic model and indicated enhanced hydration compared to other porous materials. One of the hypotheses regarding the interesting properties of UMON is that the uranium metal center might play a central role in the selectivity of this material. To test this hypothesis, a similar uranium based metal-organic nanotube containing 2,6-pyridine dicarboxylic acid (UPDC) as the ligand was synthesized and its properties were compared to that of the UMON material. UPDC did display some selectivity based upon size exclusion but did not exhibit the same selectivity to water that is observed for UMON. Different transition metals were also incorporated into the nanotubular structures to determine the influence of dopants on the observable properties. Only small amounts of transition metal dopants were incorporated into the structure, but it increased the stability under high humid environment. Attempts to incorporate transition metal dopants in the UPDC led to the formation of novel chain structures.
Water is essential for the survival of living organisms and 75% of the earth surface is covered with water, leading to the name “blue planet”. Although water is abundant on the earth’s surface, access to fresh water can be challenging in some parts of the world as most of the water exists in oceans or inaccessible ice. Given the difficulties in obtaining cleaning drinking water, understanding the basics of water chemistry and developing new methods of water purification are important areas of research.
In our group, we developed a novel material that can trap pure water inside one- dimensional channels. This material is referred to as a metal organic nanotube (MON) and is formed by connecting metals and organic linkers into a tubular shape. The metal and the organic linker that was used in our material is uranium (so will be referred to as UMON) and iminodiacetic acid, respectively. This material is unique because only water can enter the nanotube. In this body of work, the selectivity of this material was extensively analyzed by exposing the UMON to different solvents and vapors then determining the weight loss upon heating and identifying the molecules removed by the heating process. These studies confirmed that only water can enter the nanotubular material and could be a potential candidate for advanced water purification.
In some industrial processes, the presence of water can be harmful. As an example, water vapor can mix with vehicle fuel, especially in aircraft, can cause serious problems in the fuel line and engine. The drawbacks of the current desiccants are they lack selectivity to water and reactivation requires high temperatures heat cycles that consume more energy. For our UMON material, the water in the material can be removed by heating up to 37 ˚C, which is dramatically lower than typical temperature (100-250˚C). Also once the temperature is decreased it gets rehydrated. As UMON inherits selectivity as well as low- temperature water removal this is a prospective desiccant. My work focused on the rates of water uptake in the UMON material and confirmed that the material maintains selectivity even when exposed to a wide range of vapors.
The origin for all the interesting properties of the UMON material is a mystery, so I also investigated the root of these characteristics by comparing to other uranium based metal-organic nanotubes and doped materials. The unique properties observed for the UMON was not found for other uranium based porous materials. Transition metals, such as copper, nickel and cobalt were also incorporated into the UMON material and the selectivity to water and low-temperature rehydration was maintained.
Doping, Metal-organic, Nanotubes, Rates, Uranium, Water
xxii, 166 pages
Includes bibliographical references (pages 156-166).
Copyright © 2017 Ashini Shamindra Jayasinghe
Jayasinghe, Ashini Shamindra. "Exploring the unique water properties of metal-organic nanotubes." PhD (Doctor of Philosophy) thesis, University of Iowa, 2017.