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 &gt; In-soc-MOF-1a &gt; 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.

Investigating H2 adsorption among soc-MOF analogues using grand canonical Monte Carlo simulations

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