The flexibility of modified-linker MIL-53 materials

Area of Science:

• Materials Science
• Crystallography
• Chemistry

Background:

• Aluminum hydroxo terephthalates with the MIL-53 type structure are known for their flexibility.
• Functionalization of the terephthalate linker can modify the properties of these metal-organic frameworks.

Purpose of the Study:

• To investigate the structural flexibility of eight functionalized aluminum hydroxo terephthalates (Al-MIL-53) under various conditions.
• To understand the influence of functional groups on the breathing behavior and structural evolution of Al-MIL-53 materials.
• To explore the energetic factors governing the distribution of functional groups on the organic linkers.

Main Methods:

• In situ powder X-ray diffraction (PXRD) was employed to monitor structural changes during thermal dehydration, superhydration, and methanol adsorption/desorption.
• Profile fitting analysis of PXRD data was used to determine lattice parameters and track structural evolution.
• Density functional theory (DFT) calculations were performed to investigate the energetic stability of different functional group arrangements.

Main Results:

• Methanol adsorption induced framework opening in most Al-MIL-53 materials, except for the -NH2 substituted variant.
• Superhydration resulted in open structures only for Al-MIL-53-NO2, -Br, and -(OH)2.
• Upon thermal dehydration, all MIL-53 solids, except Al-MIL-53-(OH)2, adopted open structures.
• DFT calculations indicated that the most energetically stable configuration of functional groups on the linkers best matched the experimental PXRD data, suggesting energetic control over substituent distribution.

Conclusions:

• The Al-MIL-53 series displays significant structural flexibility, with distinct responses to methanol adsorption and superhydration depending on the functional group.
• Energetic principles dictate the ordering of functional groups on the organic linkers, influencing the overall structural behavior of these metal-organic frameworks.
• The study highlights the tunable nature of Al-MIL-53 materials and provides insights into structure-property relationships for tailored applications.