Thesis Format

Integrated Article

High-Pressure Studies of Flexible Metal-Organic Frameworks and Their Performance for CO2 Adsorption using Infrared Spectroscopy

Author

Boqing Li, The University of Western OntarioFollow

Degree

Master of Science

Program

Chemistry

Supervisor

Yining Huang

2nd Supervisor

Yang Song

Joint Supervisor

Abstract

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.

Recommended Citation

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.
https://ir.lib.uwo.ca/etd/7665