Understanding the Carbon Dioxide Binding Potential Landscape near Open Metal Sites in M-MOF-74 (M = Mg, Mn, Fe, Ni,...

Metal-organic frameworks (MOFs), especially those with undercoordinated open metal sites (OMS), are attractive candidates for applications involving adsorption of small molecules, including CO₂. Among MOFs with OMS, MOF-74 variants have drawn considerable interest due to a high gravimetric density of such adsorption sites. The electronic nature of adsorbate-adsorbent interactions at OMS typically results in the accuracy of conventional force fields being insufficient to describe adsorption energetics, motivating the need for more accurate predictive methods. Toward this goal, we have utilized density functional theory (DFT) to explore the binding characteristics of CO₂ in M-MOF-74s. We focus specifically on examining how various parameters, such as binding distance, variation in adsorption angles, and dihedral (precession) angles influence adsorption energetics. We consider M-MOF-74 variants across a range of d-band occupations, including Mg (3d⁰), Mn (3d⁵), Fe(3d⁶), Ni (3d⁸), and Zn (3d¹⁰) in this work, illustrating the impact of electronic structure of the OMS metal on the energetics of CO₂ adsorption both at and in the vicinity of the primary adsorption sites at the OMS. Our findings, particularly at the minimum energy binding configurations, closely align with experimental structures and isosteric heats of adsorption. Our exploration of potential energy surfaces (PES) at OMS in off-minimum binding configurations uncovers more complicated interaction mechanisms, highlighting local minima and maxima within the PES. Developing these comprehensive energy profiles provides vital insights, potentially aiding in the creation of force fields that can accurately capture adsorbate-adsorbent interactions in MOFs with OMS.