Phase Transitions in Metal–Organic Frameworks Directly Monitored through In Situ Variable Temperature Liquid-Cell Transmission Electron Microscopy and In Situ X‑ray Diffraction

Zr6-based
metal–organic frameworks (MOFs) with
tetratopic organic linkers have been extensively investigated owing
to their versatile structural tunability. While diverse topologies
and polymorphism in the resulting MOFs are often encountered with
tetratopic linkers and Zr6 nodes, reports on phase transitions
within these systems are rare. Thus, we have a limited understanding
of polymorph transformations, hindering the rational development of
pure phase materials. In this study, a phase transition from a microporous
MOF, scu-NU-906, to a mesoporous MOF, csq-NU-1008, was discovered
and monitored through in situ variable temperature
liquid-cell transmission electron microscopy (VT-LCTEM), high-resolution
transmission electron microscopy (HRTEM), and in situ variable temperature powder X-ray diffraction (VT-PXRD). It was
found that the microporous- to-mesoporous transformation in the presence
of formic acid occurs via a concomitant dissolution–reprecipitation
process.