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.