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

Metal-organic frameworks (MOFs) are highly studied materials for gas storage and separation, with increasing interest in their application in industrial settings. Industrial processes typically require MOFs to be shaped into robust pellets. Unfortunately, pelletization processing can often result in significant loss in porosity and/or performance. This research examines the mechanical origins of such degradation using Zn-MOF-74 as a model system. A combination of high-pressure single-crystal X-ray diffraction (HP-SCXRD), periodic density functional theory (DFT), powder X-ray diffraction (PXRD), and nitrogen sorption measurements was undertaken to evaluate the mechanical response of Zn-MOF-74 under both hydrostatic (ideal) and uniaxial (practical) stress. SCXRD and DFT show that Zn-MOF-74 remains crystalline under isotropic compression beyond 2.8 GPa, yielding a bulk modulus of ∼15 GPa, confirming significant intrinsic stability under hydrostatic pressure. In contrast, uniaxial pelletization at pressures as low as 0.08-0.77 GPa results in an increasing reduction of BET surface area and microstrain. This difference can be attributed to extrinsic effects, including shear-induced microstructural damage and particle fragmentation. The shear yield strength, estimated empirically from the shear modulus, is only 0.16-0.50 GPa, consistent with the onset of degradation during pressing. These findings reveal the crucial role of nonhydrostatic stress in MOF shaping and demonstrate that bulk modulus alone is insufficient to predict mechanical resilience during pelletization. Strategies such as hydrostatic compaction, binder-assisted shaping, or solvation during pressing have the potential to mitigate porosity loss. This work provides a mechanistic understanding of how to improve MOF processability through both material and engineering solutions.