Elucidating the Breathing of the Metal–Organic Framework MIL-53(Sc) with ab Initio Molecular Dynamics Simulations and in Situ X‑ray Powder Diffraction Experiments

Ab initio molecular dynamics (AIMD)
simulations have been used
to predict structural transitions of the breathing metal–organic
framework (MOF) MIL-53­(Sc) in response to changes in temperature over
the range 100–623 K and adsorption of CO2 at 0–0.9
bar at 196 K. The method has for the first time been shown to predict
successfully both temperature-dependent structural changes and the
structural response to variable sorbate uptake of a flexible MOF.
AIMD employing dispersion-corrected density functional theory accurately
simulated the experimentally observed closure of MIL-53­(Sc) upon solvent
removal and the transition of the empty MOF from the closed-pore phase to the very-narrow-pore phase (symmetry change
from P21/c to C2/c) with increasing temperature, indicating
that it can directly take into account entropic as well as enthalpic
effects. We also used AIMD simulations to mimic the CO2 adsorption of MIL-53­(Sc) in silico by allowing the MIL-53­(Sc) framework
to evolve freely in response to CO2 loadings corresponding
to the two steps in the experimental adsorption isotherm. The resulting
structures enabled the structure determination of the two CO2-containing intermediate and large-pore phases observed by experimental synchrotron X-ray diffraction studies
with increasing CO2 pressure; this would not have been
possible for the intermediate structure via conventional
methods because of diffraction peak broadening. Furthermore, the strong
and anisotropic peak broadening observed for the intermediate structure could be explained in terms of fluctuations of the framework
predicted by the AIMD simulations. Fundamental insights from the molecular-level
interactions further revealed the origin of the breathing of MIL-53­(Sc)
upon temperature variation and CO2 adsorption. These simulations
illustrate the power of the AIMD method for the prediction and understanding
of the behavior of flexible microporous solids.

基于从头计算分子动力学（Ab initio molecular dynamics，AIMD）的模拟被用于预测呼吸型金属有机框架（Metal-Organic Framework，MOF）MIL-53(Sc)在100-623 K温度变化范围及196 K时0-0.9 bar的CO2吸附作用下的结构转变。该方法首次成功预测了温度依赖性的结构变化以及柔性MOF对可变吸附剂吸收量的结构响应。采用校正了分散能的密度泛函理论进行的AIMD模拟精确地再现了MIL-53(Sc)在溶剂去除过程中的闭合现象，以及随着温度升高，空MOF从闭合孔相转变为极窄孔相（对称性从P21/c变为C2/c）的过渡，这表明其能够直接考虑熵变以及焓变效应。我们还将AIMD模拟用于在计算机上模拟MIL-53(Sc)框架对CO2吸附，允许框架自由演化以响应与实验吸附等温线两个步骤相对应的CO2负荷。所得到的结构使得通过实验同步辐射X射线衍射研究随着CO2压力的增加确定了含有CO2的中间结构和大型孔相，这在传统方法中由于衍射峰展宽而不可能实现。此外，观察到的中间结构的强烈各向异性峰展宽可以通过AIMD模拟预测的框架波动来解释。从分子水平相互作用中获得的根本见解进一步揭示了MIL-53(Sc)在温度变化和CO2吸附时的呼吸机制。这些模拟展示了AIMD方法在预测和理解柔性微孔固体行为方面的强大功能。