(293b) The Interaction of Water with Metal Organic Framework-5 Simulated by Molecular Dynamics Using a New Non-Bonded Forcefield

Metal organic frameworks (MOFs) are a promising new class of materials composed of a metal oxide clusters joined by organic linkers. A particular focus of the computational community is MOF-5, which consists of Zn4O tetrahedral with 1,4-benzenedicarboxylate (BDC) linkers. MOF-5 has an extremely large surface area and therefore represents a promising medium for gas storage. Molecular simulation studies of MOF-5 have treated the framework as a rigid body and general force fields were used for MOF-adsorbate interactions. We have included framework and adsorbate flexibility by combining a general force field for organic molecules with parameters that accurately model zincite (ZnO). Additionally, we use only nonbonded (electrostatic and van der Waals) parameters between Zn atoms and coordinating O atoms, so that the stability of MOF-5 in different chemical environments can be investigated. Our molecular dynamics simulations of pure MOF-5 reproduce the experimental lattice parameter to within 0.2 Å. Average bond lengths and angles are also in good agreement with experiment. This methodology can be applied to other Zn-based MOFs as well. MOF-5 is stable with water contents as high as 2.3 % by mass, although the lattice parameter decreases with added water. The mechanism for instability with adsorbed water involves 1) hydrogen bonding between water and MOF O atoms, and 2) the formation of Zn-water complexes through water O atoms. Above 3.9 % water, the framework is not stable due to replacement of MOF O atoms by water O atoms in the Zn4O tetrahedra. Results will also be presented for other Zn-based MOFs, demonstrating the potential of this method to serve as a screening tool to identify new MOFs that are less susceptible to decomposition in water. Framework flexibility also provides a chemically accurate model of MOF-5 structure when other gas-phase molecules such as benzene occupy key adsorption sites within the pore.