Master of Science
Metal-organic frameworks (MOFs) are crystalline porous materials comprising metal ions/clusters and organic linkers. MOFs feature very large surface area and broad tunability, which distinguish them from traditional CO2 adsorbents. High external pressure can significantly modify the framework structures and CO2 adsorption properties of MOFs. MIL-53(Al) and NH2-MIL-53(Al) exhibit excellent CO2 affinity by forming hydrogen bonds between bridging OH groups and adsorbed CO2. We used in situ infrared spectroscopy to investigate the high-pressure performance of their framework structures and CO2 adsorption capacities. Diamond anvil cell was employed to apply high pressures in gigapascal level.
For as-made MIL-53(Al), pressures-induced inter-framework hydrogen bonds between OH groups and free H2BDC molecules were observed. The IR spectra of activated MIL-53(Al) upon compression provided direct evidence of its extraordinary stability compared to as-made and CO2-loaded MIL-53(Al). Pressure-induced intra-framework hydrogen bonding interactions between OH groups and octahedral [AlO6] were observed in activated MIL-53(Al). Moreover, structural modifications of as-made and activated MIL-53(Al) were irreversible upon complete decompression. Pressure-enhanced CO2 adsorption in MIL-53(Al) was demonstrated. Upon complete decompression, considerable CO2 molecules remained in the framework. Activated NH2-MIL-53(Al) exhibited reversible pressure-enhanced intra-framework interactions via two types of hydrogen bonding: one was between -NH2 groups and octahedral [AlO6], the other was between OH groups and octahedral [AlO6]. For CO2-loaded NH2-MIL-53(Al), there were four different high-pressure adsorption sites co-existing upon compression: dimeric adsorption, large-pore adsorption, narrow-pore adsorption, and amino adsorption. We demonstrated that high pressures made the narrow-pore adsorption highly favored over the other three.
Summary for Lay Audience
As a rising class of porous crystalline materials, metal-organic frameworks (MOFs) are composed of metal centers connected through organic linkers to create open frameworks with channels or cages. MOFs have been well-studied as one of the most promising adsorbents for CO2 capture, owing to their porous structure and high surface area. The framework structures and gas adsorption properties of MOFs could be altered vastly upon compression. Pressure-induced phase transitions, intra-framework and guest-host interactions are the most common phenomena of MOFs under high pressure. Previous studies have demonstrated that high external pressure can enhance CO2 adsorption in many MOFs. This thesis focuses on two well-studied MOFs with excellent CO2 affinity. Diamond anvil cell was employed to apply high pressures in gigapascal level on MIL-53(Al) and NH2-MIL-53(Al) (MIL = Matériaux de l'Institut Lavoisier), coupled with infrared spectroscopy, we studied their framework structural modifications and CO2-framework interactions in the pressure range of 0 - 10 GPa.
In this thesis, we found that activated frameworks of MIL-53(Al) and NH2-MIL-53(Al) with empty channels exhibited remarkable structural stability and compressibility. Also, pressure-induced intra-framework interactions between OH groups of their frameworks and octahedral [AlO6] were observed. As for the CO2 adsorption performance, their adsorption sites are located around the OH groups of the frameworks forming hydrogen bonds with CO2. In our case, pressure-enhanced guest-host interactions were demonstrated for both MOF systems. Upon compression, a well-enhanced CO2 adsorption in MIL-53(Al) was demonstrated. Upon complete decompression, although some of the CO2 molecules were released to the air, considerable CO2 molecules remained in the framework. For CO2-loaded NH2-MIL-53(Al), there were four different CO2-framework interactions co-existing upon compression: between one dimeric CO2 species and two bridging OH groups (dimeric adsorption); between one CO2 molecule and one bridging OH group (large-pore adsorption); between one CO2 molecule and two bridging OH groups (narrow-pore adsorption); between CO2 molecules and -NH2 groups (amino adsorption). High pressures made the narrow-pore adsorption more favorable over the other three. When all the applied pressures released, it was found that the pressure-regulated CO2 adsroption behaviors of NH2‑MIL-53(Al) were fully reversible.
Li, Boqing, "High-Pressure Studies of Flexible Metal-Organic Frameworks and Their Performance for CO2 Adsorption using Infrared Spectroscopy" (2021). Electronic Thesis and Dissertation Repository. 7665.