Graph theoretic techniques provide an invaluable computational toolkit for studying periodic crystalline materials. They are the drivers, for example, of rapid advances in diverse sub-applications such as the design of novel metal-organic frameworks (MOFs),1 the prediction of layered zeolites,2 and even feature construction for the machine learning of inorganic materials’ properties3 (just to name a few). In this work, we add another use case: identifying specific realizations of flexibility in MOFs without requiring simulations. This is an important topic since most computational studies that predict promising adsorbents assume rigid structures due to computational efficiency concerns, but even small degrees of flexibility may significantly impact gas storage or separation performance. Requiring no atomistic simulations, our graph theoretic identification of flexible MOF subcomponents can efficiently screen thousands of materials. Among the potentially flexible materials, we seek those whose ground state structure are nominally considered non-porous by standard geometric arguments, hypothesizing that the energy penalty to deform the framework to accommodate guest adsorption will be lower (or even negligible) in comparison to the energy stabilization from guest adsorption. We confirm this hypothesis for a number of screened materials via density functional theory calculations that demonstrate a net energy decrease for the combined deformation and adsorption process, even in the case of very weakly adsorbing H2 gas. These materials are overlooked by computational screening studies due to the rigid structure approximation, and our most promising materials are further synthesized and characterized experimentally to test our approach for identifying overlooked H2 physisorbents. Finally, we envisage that the same approach can be used to identify similar MOFs for the storage of other small molecules as well.

Identification of previously untested, high-potential H2 physisorbents via graph theory-based descriptors of MOF flexibility

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.