Grand Canonical Monte Carlo (GCMC) simulations of H₂ adsorption were carried out on three isostructural metal–organic frameworks (MOFs) with square octahedral (soc) topology: In-soc-MOF-1a, In-soc-MOF-1b, and Fe-soc-MOF-1a. These three MOFs consist of the organic ligand, 5,5′-azobis(1,3-benzenedicarboxylate), coordinated to M₃O (M = In, Fe) clusters. Furthermore, the MOFs are balanced by uncoordinated counterions (1a = NO₃^–, 1b = Cl^–) that are localized in carcerand-like capsules of the framework. The classical force field for the MOF and the adsorbate consists of Lennard–Jones parameters, point partial charges, and static point polarizabilities, to model van der Waals, permanent electrostatic, and many-body polarization interactions, respectively. The simulated H₂ adsorption isotherms and isosteric heats of adsorption (Qst) for each MOF are in reasonable agreement with the corresponding experimental results. It was determined that the trend in the H₂ uptake and affinity is as follows: In-soc-MOF-1b > In-soc-MOF-1a > Fe-soc-MOF-1a. The In³⁺ ions in both In-soc-MOF-1a and In-soc-MOF-1b exhibit a greater partial positive charge than the Fe³⁺ ions in Fe-soc-MOF-1a on the basis of electronic structure calculations, thus potentially explaining why the simulated H₂ uptake was higher in the In analogues. When comparing the two In variants, In-soc-MOF-1b displays a higher simulated H₂ uptake likely due to the smaller size of the Cl^– counterion compared to the bulkier NO₃^– ion in In-soc-MOF-1a. Specifically, the smaller size of the Cl^– ion contributes to a higher accessible volume within the carcerand-like capsules in the MOF, which ultimately contributes to higher H₂ uptake. The most favorable H₂ binding sites for all three MOFs were identified to be the counterions followed by the exposed metal centers of the M₃O clusters. Overall, the results from this theoretical study showed how making small modifications to the soc-MOF structure (i.e., substituting the metal ion and counterion) resulted in varying H₂ adsorption properties. This could provide valuable insights into the rational design of new MOFs for improved H₂ uptake in the quest for meeting commercial H₂ storage targets.

Chloe Starkey

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