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
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Abstract
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 CO<sub>2</sub> 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 <i>closed-pore</i> phase to the <i>very-narrow-pore</i> phase (symmetry change
from <i>P</i>2<sub>1</sub>/<i>c</i> to <i>C</i>2/<i>c</i>) 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 CO<sub>2</sub> adsorption of MIL-53(Sc) in silico by allowing the MIL-53(Sc) framework
to evolve freely in response to CO<sub>2</sub> loadings corresponding
to the two steps in the experimental adsorption isotherm. The resulting
structures enabled the structure determination of the two CO<sub>2</sub>-containing <i>intermediate</i> and <i>large-pore</i> phases observed by experimental synchrotron X-ray diffraction studies
with increasing CO<sub>2</sub> pressure; this would not have been
possible for the <i>intermediate</i> structure via conventional
methods because of diffraction peak broadening. Furthermore, the strong
and anisotropic peak broadening observed for the <i>intermediate</i> 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 CO<sub>2</sub> adsorption. These simulations
illustrate the power of the AIMD method for the prediction and understanding
of the behavior of flexible microporous solids