Understanding Porosity Loss in MOF Pelletization: Intrinsic vs Extrinsic Mechanical Stability in Zn-MOF-74

Area of Science:

• Materials Science
• Chemical Engineering
• Crystallography

Background:

• Metal-organic frameworks (MOFs) show promise for industrial gas storage and separation.
• Industrial applications often require MOFs to be pelletized, a process that can degrade their performance.
• The mechanical stability of MOFs during shaping is critical for their practical implementation.

Purpose of the Study:

• To investigate the mechanical origins of performance degradation in MOFs during pelletization.
• To evaluate the mechanical response of Zinc-Metal-Organic Framework-74 (Zn-MOF-74) under different stress conditions.
• To provide insights into improving MOF processability for industrial applications.

Main Methods:

• High-pressure single-crystal X-ray diffraction (HP-SCXRD) and periodic density functional theory (DFT) for mechanical response analysis.
• Powder X-ray diffraction (PXRD) and nitrogen sorption measurements to assess structural integrity and porosity.
• Evaluation of MOF-74 under both hydrostatic (ideal) and uniaxial (practical) stress conditions.

Main Results:

• Zn-MOF-74 exhibits significant intrinsic stability under hydrostatic pressure, with a bulk modulus of ~15 GPa.
• Uniaxial pelletization at low pressures (0.08-0.77 GPa) causes significant loss in BET surface area and increased microstrain.
• The shear yield strength of Zn-MOF-74 is low (0.16-0.50 GPa), indicating susceptibility to shear-induced damage during pressing.

Conclusions:

• Nonhydrostatic stress, particularly shear, plays a crucial role in MOF degradation during pelletization.
• Bulk modulus alone is insufficient to predict MOF resilience during industrial shaping processes.
• Strategies like hydrostatic compaction or binder-assisted shaping can mitigate porosity loss and improve MOF processability.