Doctor of Philosophy
The pre- and post-combustion carbon dioxide capture has drawn much attention in the past few decades owing to the increasing concentration of CO2 in the atmosphere. Among all the potential solid adsorbents for CO2 capture, metal-organic frameworks (MOFs) are a promising class of materials due to their large surface areas, high tunability and their high selectivity for gas adsorption applications. It has been widely demonstrated that the application of high external pressure in gigapascal level can substantially tune the structure, pore size and opening of porous material. Consequently, the structural, as well as gas adsorption properties of these materials, can be modified and optimized. In this thesis, we focused on the high-pressure studies of 6 types of MOFs with various topologies, structures and properties. In situ vibrational spectroscopies and are synchrotron X-ray diffraction used as preliminary methods for characterizing the structural and gas adsorption properties of MOFs under high pressure. In PbSDB and CdSDB, strongly contrasting guest-host interactions in terms of the pressure-regulated CO2 adsorption sites have been observed. SIFSIX-3-Zn features ultra-micro pore and the one-dimensional channel was then studied. The framework fluorine and hydrogen atoms were found to play important roles in interacting with CO2 under high pressure, and ultimately formed a new CO2 binding site. ZIF-8 and UiO-66 are two noble MOFs that possess enormous cages and are anticipated to have great potential in accommodating CO2 under high pressure. However, it was severely limited by the phase change of the free CO2 at 0.6 GPa under room temperature. With the aid of temperature (i.e. 30-100 °C), the CO2 storage capacity in ZIF-8 and UiO-66 was significantly improved. ZnAtzOx(H2O) is constructed by zinc-3-amino-1,2,4-triazolate sheets that are linked into the third dimension by oxalate pillars, creating ultra-micro pores with different sizes along three directions. The high pressure studies on the framework itself, CO2 loaded, D2O loaded, as well as CO2-D2O co-loaded ZnAtzOx(H2O) have shown excellent structural stability of the framework and better CO2-framework affinity under high pressure even in the presence of water (i.e. D2O). Overall, it is hoped that the information our study provides is insightful for designing and modifying MOFs and porous material for CO2 adsorption, and provides guidance for optimizing the CO2 capture and storage conditions.
Summary for Lay Audience
Metal-organic frameworks (MOFs), are a class of crystalline materials constructed by bridging metal-containing units with organic linkers to create open rigid frameworks with permanent porosity. Due to their high thermal stability, enormous surface area, finely tunable chemical functionality, MOFs are believed to have great potential in CO2 capture and storage. Compared to the conventional CO2 absorption methods, using MOFs as solid adsorbent has the advantages of being less corrosiveness and more energy efficient. It is well known that temperature, pressure and volume are three basic macroscopic parameters to describe a thermodynamic system. Among these parameters, pressure spans over 60 orders of magnitude in the universe, from 10-32 in intergalactic space to 1032 Pascal in the center of neutron star. Under such a broad range of pressure, materials could exhibit various structures as well as novel properties, especially under high pressure. When applying pressure to materials, the general effect is to reduce the volume and thus shorten then inter-/intra-molecular distances. As a result, lots of interesting phenomena and novel materials can be generated under high pressure, including phase transitions, chemical reaction, novel bondings and new properties. Previous studies have demonstrated a wide variety of pressure behavior of MOFs, including unusual elastic responses, phase transitions, chemical reactions and high-pressure guest insertion. In terms of CO2 storage, it has been reported that high pressure can efficiently tune the channel size and shape, pore volume and surface area in MOFs. Consequently, these pressure-induced modifications on the framework will affect the adsorption capacity, selectivity, and thus better adsorption performance. In this thesis, our studies on six MOFs featuring distinctive structures and topologies are performed under high pressure and other conditions (i.e. the presence of water, high temperature). All these MOFs have shown extraordinary stability and a well-enhanced CO2 adsorption performance under high pressure. The structural changes, enhanced guest-host interactions, as well as improved CO2 adsorption capacity are characterized by vibration spectroscopy, which allows the understanding of local structures, chemical bonding, and thus the nature of guest-host interactions. It is anticipated that our findings would inspire the design of new MOF structures to address practical applications in the future.
Jiang, Shan, "Investigations of the High Pressure Effects on Structural Properties and CO2 Adsorption Performance of MOFs using Vibrational Spectroscopy" (2019). Electronic Thesis and Dissertation Repository. 6721.