Accurate H2 Sorption Modeling in the rht-MOF NOTT-112 Using Explicit Polarization

Hydrogen sorption was characterized theoretically, in NOTT-112, an rht-metal-organic framework (MOP) that consists of Cu2+ ions coordinated to 1,3,5-tris(3',5'-dicarboxy[1,1'-biphenyl]-4-yl)-benzeoe ligands. Grand-canonical Monte Carlo (GCMC) simulations of H-2 sorption were performed using three different sorbate potentials of increasing complexity to elucidate the mechanism of interaction in NOTT-112. Reasonable agreement with the experimental H-2 sorption isotherms, isosteric heats of adsorption (Q(st)), and H-2-Cu2+ distances was obtained with the model that includes explicit many-body polarization. This highlights the physical importance of induction in H-2 sorption in MOFs with open-metal sites. The two Cu2+ ions within the paddlewheels in NOTT-112 have distinct electrostatic profiles and sorption characteristics. The Cu2+ ions projecting into the cuboctahedral (cub-O-h) cage of the MOF are more electropositive than the Cu2+ ions projecting toward the organic linker, and therefore serve as the initial loading sites. Such a sorption mechanism has been shown to be sensitive and tunable-via the magnitude of the metal charge. Simulated sorption onto the open-metal sites was compared with extant experimental neutron powder diffraction (NPD) data in NOTT-112. Simulated annealing calculations on sorbates and analysis of the radial distribution function for H-2 sorbed about the Cu2+ ions revealed metal-sorbate interaction distances that were in reasonable agreement with NPD data in NOTT-112 for the most realistic model. This study demonstrates how the inclusion of explicit polarization in classical GCMC simulation is able to reproduce experimental observables and elucidate the gas sorption behavior in a MOF possessing open-metal sites. Additionally, this study shows how careful modeling of the MOF-sorbate interactions can provide complementary atomistic resolution to data obtained from NPD measurements.