Simulations of H2 Sorption in an Anthracene-Functionalized Rht-Metal–organic Framework

Grand canonical Monte Carlo (GCMC) simulations were performed to investigate H-2 sorption in the rht-metal-organic framework (MOF) MFM-132, which is composed of two chemically distinct Cu2+ ions coordinated to 5,5',5 ''-(benzene-1,3,5-triyltris-(anthracene-10,9-diyl))triisophthalate ligands. This MOF was shown experimentally to have a Brunauer-Emmett-Teller surface area of 2466 m(2) g(-1), a high H-2 uptake of 2.83 wt % at 77 K/1 bar, and an initial isosteric heat of adsorption (Q(st)) value of 6.7 kJ mol(-1). It possesses a structure that is similar to that of NOTT-112, which was studied previously by our group [Franz, D.; et al. Cryst. Growth Des. 2016, 116, 6024-6032], with an experimental H-2 uptake of 2.3 wt % under the same conditions and an initial Q(st) value of 5.6 kJ mol(-1). In MFM-132, there are anthracenyl rings in the region between the isophthalate groups and the central aromatic ring of the linker, whereas NOTT(-1)12 possesses a single phenyl ring in the same region. The purpose of this study was to investigate the effects of the bulky anthracenyl rings on the MOF-H-2 interactions using NOTT-112 as a basis for comparison. Using GCMC simulations, three major differences in the sorption mechanisms between MFM-132 and NOTT-112 were revealed. First, the primary binding site for H-2 in MFM-132 was found to be a small binding pocket in the corner of the truncated tetrahedral cage, which is formed by the partial overlap of the anthracenyl rings from three linkers. In NOTT-112, however, the initial binding site was a Cu2+ ion that is part of the [Cu-2(O2CR)(4)] cluster. Second, in MFM-132, the H-2 molecules showed preference for the chemically distinct Cu2+ ion facing toward the interior of the linker body, while in NOTT-112, open-metal sorption occurred primarily on the Cu2+ ion facing away from the center of the linker. In addition, significant sorption within the interior of the truncated tetrahedral cages was observed for MFM-132, but not for NOTT-112. The difference in the initial binding sites between these two isostructural MOFs can be explained by the presence of strong van der Waals interaction created by the partial overlap of the anthracenyl rings. Overall, this study showed that making minor alterations to the ligand in rht-MOFs can result in dramatic differences in the H-2 sorption characteristics. Moreover, significant insights into the H-2 sorption behavior in MFM-132 was obtained through molecular simulations and these results could be important for the rational design of new MOF structures.