Modeling of Diffusion in the Metal-Organic Framework Uio-66

Highly porous metal-organic frameworks (MOFs) have been studied for the capture and destruction of chemical warfare agents (CWAs), with the UiO-6x family showing great promise. This work explores UiO-66 specifically using molecular dynamics. Transport phenomena are especially important in understanding the behavior of CWAs in UiO-66. To explore diffusion in UiO-66, isopropanol (IPA) has been chosen as the primary analog molecule due to its ability to hydrogen bond to hydroxide groups within the MOF, like many CWAs are expected to, and its relatively rapid diffusion in pristine UiO-66, unlike most CWAs, which are only expected to diffuse appreciably in defective UiO-66. Self and corrected diffusivities were examined, with the latter used to calculate transport diffusivity by application of a thermodynamic correction factor. Diffusion free energy profiles of several additional molecules of relevance, including notable CWA simulants dimethylphosphite (DMP) and dimethyl methylphosphonate (DMMP), were calculated. Hydrogen bonding was found to play a substantial role in the diffusion of polar species, with an order of magnitude decrease in diffusivity and a twofold increase in the activation energy to diffusion of IPA when hydrogen bonding was properly accounted for in the parameterization of the system. An understanding of base interactions in the pristine MOF serves as a starting point for exploring more complex defective structures and a wider range of guest species in future studies.