Understanding Hydrogen Sorption in In- soc -MOF: A Charged Metal-Organic Framework with Open-Metal Sites, Narrow Channels, and Counterions
Forrest, Katherine A.
Belof, Jonathan L.
KAUST Grant NumberFIC/2010/06
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Abstract© 2015 American Chemical Society. Grand canonical Monte Carlo (GCMC) simulations of hydrogen sorption were performed in In-soc-MOF, a charged metal-organic framework (MOF) that contains In3O trimers coordinated to 5,5′-azobis(1,3-benzenedicarboxylate) linkers. The MOF contains nitrate counterions that are located in carcerand-like capsules of the framework. This MOF was shown to have a high hydrogen uptake at 77 K and 1.0 atm. The simulations were performed with a potential that includes explicit many-body polarization interactions, which were important for modeling gas sorption in a charged/polar MOF such as In-soc-MOF. The simulated hydrogen sorption isotherms were in good agreement with experiment in this challenging platform for modeling. The simulations predict a high initial isosteric heat of adsorption, Qst, value of about 8.5 kJ mol<sup>-1</sup>, which is in contrast to the experimental value of 6.5 kJ mol<sup>-1</sup> for all loadings. The difference in the Qst behavior between experiment and simulation is attributed to the fact that, in experimental measurements, the sorbate molecules cannot access the isolated cages containing the nitrate ions, the most energetically favorable site in the MOF, at low pressures due to an observed diffusion barrier. In contrast, the simulations were able to capture the sorption of hydrogen onto the nitrate ions at low loading due to the equilibrium nature of GCMC simulations. The experimental Qst values were reproduced in simulation by blocking access to all of the nitrate ions in the MOF. Furthermore, at 77 K, the sorbed hydrogen molecules were reminiscent of a dense fluid in In-soc-MOF starting at approximately 5.0 atm, and this was verified by monitoring the isothermal compressibility, βT, values. The favorable sites for hydrogen sorption were identified from the polarization distribution as the nitrate ions, the In3O trimers, and the azobenzene nitrogen atoms. Lastly, the two-dimensional quantum rotational levels for a hydrogen molecule sorbed about the aforementioned sites were calculated and the transitions were in good agreement with those that were observed in the experimental inelastic neutron scattering spectra.
CitationPham T, Forrest KA, Hogan A, Tudor B, McLaughlin K, et al. (2015) Understanding Hydrogen Sorption in In- soc -MOF: A Charged Metal-Organic Framework with Open-Metal Sites, Narrow Channels, and Counterions . Crystal Growth & Design 15: 1460–1471. Available: http://dx.doi.org/10.1021/cg5018104.
SponsorsWe thank Youssef Belmabkhout and Jens Moellmer for presenting us with experimental data for high-pressure hydrogen sorption in In-soc-MOP at 77 and 298 K, respectively. This work was supported by the National Science Foundation (Award No. CHE-1152362). Computations were performed under a XSEDE Grant (No. TG-DMR090028) to B.S. This publication is also based on work supported by Award No. FIC/2010/06, made by King Abdullah University of Science and Technology (KAUST). We also thank the Space Foundation (Basic and Applied Research) for partial support. We acknowledge the use of the services provided by Research Computing at the University of South Florida.
PublisherAmerican Chemical Society (ACS)
JournalCrystal Growth & Design