Accurate Prediction of Hydrogen Adsorption in Metal-Organic Frameworks with Unsaturated Metal Sites via a Combined Density-Functional Theory and Molecular Mechanics Approach

The incorporation of eoordinatively unsaturated metal sites in microporous metal-organic frameworks (MOFs) has emerged as an important synthetic strategy for the development of potential room-temperature hydrogen storage materials, because the relatively strong, localized interaction of hydrogen with the metal centers induces an increase of the isosteric heat of hydrogen adsorption. Previous modeling studies have shown that these interactions are not adequately modeled when literature force-field parameters are used. Typical results of grand-canonical Monte Carlo (GCMC) simulations exhibit a pronounced underestimation of the hydrogen uptake at low pressures and low temperatures. In this study, it is shown that this shortcoming can be resolved by deriving a new set of potential parameters to represent the metal dihydrogen interaction from ab initio calculations for molecular model systems. The approach is computationally efficient and could be applied for any coordination environment of the metal center. The present work focuses on three MOFs with unsaturated copper centers. The newly derived Cu-H-2 potential model is combined with literature force-field parameters to model the dispersive interactions with other framework atoms. At cryogenic temperatures and pressures up to I bar, GCMC simulations using these parameters provide for a massively improved prediction of the hydrogen storage characteristics when compared to parameters from a literature force field. On the other hand, the unmodified literature parameters perform best in predicting the saturation uptake. At room temperature, the effect of the potential modification is much smaller, and the best agreement with experiment is obtained when the localized metal dihydrogen interaction is not accounted for in the simulations. This indicates that the metal dihydrogen interaction is too weak to permit a significant adsorption at the metal sites under these conditions. Calculations using an artificially enhanced potential model show that a drastic increase of the interaction strength could boost. the hydrogen storage capacity at room temperature, although the attainable uptake remains limited by the number of available metal sites. The implications of these results for the synthesis of new MOFs are critically discussed.