Hydrogen Physisorption on Metal–Organic Framework Linkers and Metalated Linkers: A Computational Study of the Factors That Control Binding Strength

Abstract

In order for hydrogen gas to be used as a fuel, it must be stored in sufficient quantity on board the vehicle. Efforts are being made to increase the hydrogen storage capabilities of metal-organic frameworks (MOFs) by introducing unsaturated metal sites into their linking element(s), as hydrogen adsorption centers. In order to devise successful hydrogen storage strategies there is a need for a fundamental understanding of the weak and elusive hydrogen physisorption interaction. Here we report our findings from the investigation of the weak intermolecular interactions of adsorbed hydrogen molecules on MOF-linkers by using cluster models. Since physical interactions such as dispersion and polarization have a major contribution to attraction energy, our approach is to analyze the adsorption interaction using energy decomposition analysis (EDA) that distinguishes the contribution of the physical interactions from the charge-transfer (CT) "chemical" interaction. Surprisingly, it is found that CT from the adsorbent to the σ*(H2) orbital is present in all studied complexes and can contribute up to approximately -2 kJ/mol to the interaction. When metal ions are present, donation from the σ(H2) → metal Rydberg-like orbital, along with the adsorbent → σ*(H2) contribution, can contribute from -2 to -10 kJ/mol, depending on the coordination mode. To reach a sufficient adsorption enthalpy for practical usage, the hydrogen molecule must be substantially polarized. Ultimately, the ability of the metalated linker to polarize the hydrogen molecule is highly dependent on the geometry of the metal ion coordination site where a strong electrostatic dipole or quadrupole moment is required.

Orientationalortho−H2pair interactions in the microporous framework MOF-5

“…Most interestingly, the splitting of the FS peaks is somewhat greater than predictions based on unmodified H 2 · · ·H 2 interactions. This is consistent with recent work that suggests that even for weakly bound H 2 a significant degree of charge-transfer and H 2 polarization occurs 15 . Future work will look to incorporate these modifications and examine the FS behavior of adsorbed H 2 in more strongly binding MOFs where the H 2 · · · H 2 interactions should deviate more dramatically from that of solid hydrogen.…”

“…S6 28 ) indicates an EQQ interaction ∼ 15% greater than the scaled value based on solid H 2 . This increased interaction could occur from H 2 modification by the MOF-5, either through polarization or chargetransfer 15 . Also, our model is very simple and does not include center-of-mass translational state averaging or effects due to triply and quadruply occupied MOF clusters.…”

“…However, to date no MOF has been achieved with the necessary properties to store H 2 under ambient conditions. In particular obtaining a material with the optimal H 2 adsorption enthalpy of -15 to -20 kJ/mol is seen as key 15 . MOF· · ·H 2 interactions arise from dispersion (van der Waals), polarization, and charge-transfer mechanisms 15 .…”

“…In particular obtaining a material with the optimal H 2 adsorption enthalpy of -15 to -20 kJ/mol is seen as key 15 . MOF· · ·H 2 interactions arise from dispersion (van der Waals), polarization, and charge-transfer mechanisms 15 . These are notoriously difficult to model due to the inherent electron correlation terms and the complexity of the MOF unit cell 16 .…”

An assessment of strategies for the development of solid-state adsorbents for vehicular hydrogen storage

“…For adsorbent materials that feature significant H 2 interactions beyond dispersion, as is generally the case for the most promising hydrogen storage materials, the results can be more varied. While permanent electrostatic effects are typically adequately described even by the simplest DFT XC functionals, treatment of more complex contributions such as Lewis acid-base interactions and backbonding, which involve charge transfer, induction, and orbital hybridization, 59,60 can differ broadly across functionals (see Fig. 6 for an example).…”