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
2016-02-18T13:58:55Z (GMT) by
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.
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