Computational Optimization of Room Temperature Usable\nCapacity for Hydrogen Storage in MFU-4-Type Metal–Organic Frameworks\nvia Pairwise Metal Substitutions

The\nefficient storage of hydrogen is a critical challenge in the\nquest for sustainable energy solutions. Current adsorbent-based methods\nachieve satisfactory storage densities predominantly under cryogenic\ntemperatures and/or high pressures, which imposes problems with cost-efficient\nand safe implementation of this technology. Materials that can bind\nhydrogen gas reversibly at ambient temperatures and more moderate\npressures could play a pivotal role in enabling hydrogen-powered technologies.\nIn this study, we use reliable computational modeling to investigate\ntwo synthetically feasible paths for tuning the enthalpy of H<sub>2</sub> binding in MFU-4-type metal–organic frameworks (MOFs),\naiming to maximize usable capacity. This study examines M<sup>I</sup>M<sub>4</sub><sup>II</sup>Cl<sub>3</sub>(bta)<sub>6</sub> (bta<sup>–</sup> = benzotriazolate) Kuratowski-type clusters as a model\nfor strong binding sites in MFU-4<i>l</i> frameworks. We\nsystematically evaluate the impact of separately tuning the central\nM<sup>II</sup> metal ion (which plays a structural role) and the peripheral\nM<sup>I</sup> metal ion (which binds the substrate) on the energetics\nof H<sub>2</sub> binding. Our computational study reveals that H<sub>2</sub> binding at an M<sup>I</sup> site mostly follows the trend\nAg<sup>I</sup> < Cu<sup>I</sup> < Ni<sup>I</sup> < Co<sup>I</sup> < Au<sup>I</sup> while a larger central M<sup>II</sup> site generally weakens the H<sub>2</sub> binding at a M<sup>I</sup> site. Importantly, we have identified three new combinations of\nM<sup>I</sup> and M<sup>II</sup> to achieve high fractional usable\ncapacities of the total H<sub>2</sub> adsorbed under a pressure swing\nfrom 5 to 100 bar at room temperature. Additionally, we examine the\nnature of the binding interaction between the peripheral metal atom\nand the hydrogen molecule. While charge transfer predominantly induces\nthis interaction, for several atom combinations, a change in the polarization\n(associated with variations in the ionic radius of the M<sup>I</sup> binding atom) is another important factor for adjusting the strength\nof the interaction. We suggest that the proposed compositions of Kuratowski-type\nclusters are highly desirable synthetic targets for future laboratory\nstudy.