Interactions of Multiple Water Molecules with MIL-53(AI) and Understanding the Mechanism of Breathing: The DFT Study

Geometries and energies of large clusters of the benchmark metal-organic framework MIL-53(Al) with and without water molecules were studied by density functional theory (DFT) using the M06-2X functional. First, a discrete model of the MIL-53(Al) cluster with a cavity is investigated, where the large-pore (lp) and narrow-pore (np) forms and the transition between the two have been modeled as a function of temperature. An additional investigated model is the same cluster but with three water molecules in the cavity. The model with three water molecules reproduces that the np form of MIL-53(Al) is more stable than the lp form at low temperatures, while at the higher temperatures the lp form becomes more stable; the computed frequencies allow the calculation of free energies as a function of temperature. The computations suggest that structural transition (breathing) in the complex of MIL-53(Al) with water could occur without desorbing water. The computations also suggest that the three water molecules are not identical but belong to the two substantially different kinds: (a) the central water strongly bonded to the mu(2)-OH groups of MIL-53(Al) and to other water molecules and (b) the peripheral waters which are bonded more weakly. The calculation of transient structures along the reaction coordinate for opening of the np structure allows an investigation of the changing hydrogen bonding environment as well as an approximation of the activation energy.