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Materials 2016, 9(7), 578; doi:10.3390/ma9070578

H2 Adsorbed Site-to-Site Electronic Delocalization within IRMOF-1: Understanding Non-Negligible Interactions at High Pressure

Department of Chemistry, Washington State University, Pullman, WA 99164, USA
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Academic Editor: Sammy Chan
Received: 12 April 2016 / Revised: 18 June 2016 / Accepted: 27 June 2016 / Published: 15 July 2016
(This article belongs to the Section Energy Materials)
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Abstract

Isoreticular metal organic frameworks (IRMOFs) have shown high uptake capabilities for storage of H2 (11.5 wt % at 77 K and 170 bar). A significant literature has employed fragment models and a single adsorbed H2 to identify adsorption sites within IRMOFs, as well as the necessary adsorbate–adsorbent interactions needed to reach sufficient adsorption enthalpy for practical usage, however at high pressures it remains to be seen if H2···H2 intermolecular interactions may influence the energetics. This study focuses upon IRMOF-1 (also known as MOF-5), and examines the individual H2 stabilization energies at different sites using Möller–Plesset perturbation theory and density functional theory alongside chemical models that consist of isolated fragment models and a cubic super cell cluster consisting of both the face- and edge-cube’s of IRMOF-1. Optimization of twenty stable configurations of singly adsorbed H2 in the super-cell cluster is observed to be essential to obtain energy ordering of the five primary sites consistent with experiment and prior benchmark calculations (α >> β > γ > δ ≈ ε). To examine site-to-site interactions that may occur in the high-pressure regime, 64 co-adsorbed H2 within a super-cell cluster have been studied (a theoretical maximum of all adsorption sites, 14 wt %). There, delocalization and/or charge transfer of electrons is observed from the σ orbitals of the H2 bound at the γ positions into the σ* orbitals of H2 bound at the α sites leads to stabilization of the interaction of H2 at the γ, by 1.4 kJ/mol, respectively (using M06-2X/LANL2DZ). This effect has been confirmed to be charge transfer, and not a manifestation of enhanced dispersion at high loading, through natural bond order (NBO) analysis and by comparisons of the square of off-diagonal NBO Fock matrix elements for both density functionals that account for dispersion interactions and Hartree–Fock calculations that ignore dispersion. View Full-Text
Keywords: IRMOF-1; H2 adsorption; MOF-5; high-pressure interactions IRMOF-1; H2 adsorption; MOF-5; high-pressure interactions
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

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MDPI and ACS Style

Wu, J.; Kucukkal, M.U.; Clark, A.E. H2 Adsorbed Site-to-Site Electronic Delocalization within IRMOF-1: Understanding Non-Negligible Interactions at High Pressure. Materials 2016, 9, 578.

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