Simulations of H₂ Sorption in an Anthracene-Functionalized rht-Metal–Organic Framework

Abstract

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 Cu 2+ ions coordinated to 5,5′,5″-(benzene-1,3,5triyltris-(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 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-112 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 Cu 2+ ion that is part of the [Cu 2 (O 2 CR) 4 ] cluster. Second, in MFM-132, the H 2 molecules showed preference for the chemically distinct Cu 2+ ion facing toward the interior of the linker body, while in NOTT-112, open-metal sorption occurred primarily on the Cu 2+ 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.

Pristine Metal–Organic Frameworks and their Composites for Renewable Hydrogen Energy Applications

“…[123] Also, the functional groups can improve the interaction between framework and H 2 , resulting in higher storage capacity via enhanced adsorption heats of H 2 . [124][125][126] Moreover, organic linkers with functional groups in MOFs (e.g., -NH 2 ) can offer chelating sites for grafting active species to create MOFs com posites (vide infra). [127] In addition to this, the organic ligands like polyheterocyclic organic ligand, can also serve as a scaffold to anchor metal atom (i.e., by designing metallolinker, in which heteroatoms of the linker are coordinated by metal atoms), and the incorporation of metal atoms can serve as catalyti cally active sites to improve the activity of MOFs for H 2 energy applications.…”