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

Boqing Li, The University of Western Ontario

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.