Mapping the Complete Reaction Energy Landscape of a Metal–Organic Framework Phase Transformation

Crystalline materials undergo valuable
phase transformations, and
the energetic processes that underlie these transformations can be
fully characterized through a combination of thermodynamic and kinetic
studies. Here, we report the first complete reaction energy landscape
of metal–organic framework (MOF) interpenetration, specifically
in the phase transformation of NU-1200 to its doubly interpenetrated
counterpart, STA-26. We characterized the thermodynamics of this phase
transformation by pairing experiments with density functional theory
(DFT) calculations. This analysis revealed that factors such as the
increase in crystal density likely drive Zr- and Hf-NU-1200 to STA-26
interpenetration, while other chemical interactions such as steric
repulsions prevent Th-NU-1200 from interpenetrating. Using time-resolved in situ X-ray diffraction, we monitored phase transformation
reaction profiles and extracted quantitative kinetic information using
the Avrami-Erofe’ev model. As a result, we obtained activation
energies for the Zr- and Hf-NU-1200 transformations to Zr- and Hf-STA-26,
respectively, revealing slower phase change kinetics for MOFs with
stronger bonds. Finally, we paired the kinetic data with experimental
observations to classify the mechanistic model of this phase transformation
as partial dissolution. We anticipate that this thermodynamic, kinetic,
and mechanistic understanding will broadly inform further studies
on the energetics of crystallization.