Investigations of the High Pressure Effects on Structural Properties and CO2 Adsorption Performance of MOFs using Vibrational Spectroscopy

Shan Jiang, The University of Western Ontario

Abstract

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.