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 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.