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.

晶体材料可发生极具科研价值的相变过程，支撑此类相变的能量机制，可通过热力学与动力学研究的联合手段得到完整表征。本研究首次报道了金属-有机框架（metal–organic framework, MOF）互穿现象的完整反应能量景观，具体针对NU-1200相变为其双互穿对应相STA-26的相变过程。我们通过实验与密度泛函理论（density functional theory, DFT）计算相结合的方法，对该相变的热力学特性进行了表征。分析结果表明，晶体密度提升等因素会驱动Zr基与Hf基NU-1200向STA-26发生互穿相变；而空间位阻等其他化学相互作用，则会阻碍Th基NU-1200发生互穿。借助时间分辨原位X射线衍射技术，我们监测了相变反应的进程曲线，并通过Avrami-Erofe’ev模型提取了定量动力学信息。据此，我们分别得到了Zr基、Hf基NU-1200转化为对应Zr基、Hf基STA-26的活化能，结果揭示出化学键更强的MOF其相变动力学更为迟缓。最后，我们将动力学数据与实验观测结果相结合，将该相变的机理模型归类为部分溶解机制。我们预计，本研究获得的热力学、动力学与机理认知，将为后续结晶过程能量学相关研究提供广泛的参考与启示。