  Monolithic metal-organic frameworks (MOFs) represent a promising solution for
the industrial implementation of this emerging class of multifunctional
materials, due to their structural stability. When compared to MOF powders,
monoliths exhibit other intriguing properties like hierarchical porosity, that
significantly improves volumetric adsorption capacity. The mechanical
characterization of MOF monoliths plays a pivotal role in their industrial
expansion, but so far, several key aspects remain unclear. In particular, the
fracture behavior of MOF monoliths has not been explored. In this work, we
studied the initiation and propagation of cracks in four prototypical MOF
monoliths, namely ZIF-8, HKUST-1, MIL-68 and MOF-808. We observed that shear
faults inside the contact area represent the main failure mechanism of MOF
monoliths and are the source of radial cracks. MIL-68 and MOF-808 showed a
remarkably high resistance to cracking, which can be ascribed to their
consolidated nanostructure.
