(529f) High-Throughput Screening of Alkane and Organophosphate Diffusion in Metal-Organic Frameworks

Metal-Organic Frameworks (MOFs) that contain zirconium (Zr₆) nodes catalyze the decomposition of organophosphate chemical warfare agents (CWAs), yet the ambient conditions required for practical CWA decomposition may introduce mass transfer limitations for bulky, polar adsorbates. These limitations may prevent reactants from reaching active sites or products from leaving the crystal. For a catalytic Zr₆ node with varying connectivity and an organic linker of interest, there are numerous unique topologies that can be formed, each with distinct pore structure. We developed a database of Zr₆ MOFs with 20 different topologies to systematically evaluate the role of MOF topology on diffusion. Molecular dynamics simulations were used to simulate diffusion of alkanes and organophosphates to identify MOFs with favorable diffusion properties. We used high-throughput and automated spatial analysis tools to map pore regions with similar framework distances or adsorption energies to quantitatively describe diffusion at finite loadings.

Simulated diffusion coefficients of alkanes and organophosphates vary by orders of magnitude depending on MOF topology, even for the similar linker sizes used here. This is due in part to the Zr₆ node connectivity, which can be 6-, 8-, or 12-connected to linkers. For topologies that are 12-connected, average diffusion coefficients of alkanes are lower at all adsorbate loadings relative to less-connected topologies, due to smaller average pore apertures. Average diffusion coefficients for both 6- and 8- connected topologies, however, do not differ from each other. The connectivity of the linkers does not significantly impact diffusivity. At high adsorbate fractional loading, diffusion coefficients correlate with pore size, but at low loadings, pore connectivity and pore shape control diffusion. For polar organophosphates, stronger adsorption to Zr₆ nodes reduces diffusion at low adsorbate loadings, which are most relevant for CWA decomposition. We found that the choice of topology and linker were crucial to control organophosphate diffusion at low loadings, as diffusion coefficients vary by 3 orders of magnitude among topologies for structurally similar nodes and linkers. Coulombic interactions between nodes and adsorbates were shown to be dominant by removing node partial charges and comparing the distribution of adsorbates between frameworks with and without partial charges. We then extended these results to develop design rules for the diffusion of bulky polar adsorbates in other crystalline microporous materials.