(465g) Understanding Small Molecule Binding Potential Landscapes Near Open-Metal Sites in M-MOF-74 (M=Mg, Mn, Ni, Zn)

Metal–organic frameworks (MOFs), a class of materials of interest for their highly customizable and tunable properties, often exhibit high adsorption selectivity and capacity, motivating substantial research interest in energy-related applications such as adsorptive separations. Among MOFs, those with undercoordinated metals, or “open metal sites” (OMS), have drawn particular interest for the ability to leverage exposed metal cations for highly selective and tunable adsorptive separations. Because MOFs with OMS are often of interest for leveraging the unique properties at these exposed metals, classical force field-based modeling approaches have struggled to rigorously account for the electronic details relevant for fully describing adsorption. Additionally, adsorption may affect structural changes in the adsorbent that are often neglected but may play a role in adsorption behavior. In this work, we use density functional theory (DFT) to study adsorption of small molecules H₂, N₂, CO₂, and NO in M-MOF-74s, a class of OMS MOFs with a range of d-occupations from among 3d transition metals. We studied binding potentials of these small molecules as a function of distance from the open metal sites in Mg (3d⁰)-, Mn (3d⁵)-, Ni (3d⁸)-, and Zn (3d¹⁰)-MOF-74s, investigating different structural constraints ranging from complete structural optimization of adsorbate and adsorbent to partial geometrical optimization to single point (static) calculations. We characterize path-dependent energetics and structural changes to facilitate model development for adsorption of small molecules in OMS MOFs. This work presents progress in both rational design of materials for gas separations and development of next-generation force-fields to capture the structural and energetic changes relevant to adsorptive performance in OMS MOFs.