Molecular Hydrogen “Pairing” Interaction in a Metal Organic Framework System with Unsaturated Metal Centers (MOF-74)

Abstract

Infrared (IR) absorption spectroscopy measurements of molecular hydrogen in MOF-74-M (M = metal center) are performed as a function of temperature and pressure [to 45 kTorr (60 bar) at 300 K, and at lower pressures in the 20-200 K range] to investigate the nature of H(2) interactions with the unsaturated metal centers. A small shift (∼ -30 cm(-1) with respect to the unperturbed H(2) molecule) is observed for the internal stretch frequency of H(2) molecules adsorbed on the metal site at low loading. This finding is in contrast to much larger shifts (∼ -70 cm(-1)) observed in previous studies of MOFs with unsaturated metal centers (including MOF-74) and the general assumption that H(2) stretch shifts depend on adsorption energies (FitzGerald et al., Phys. Rev. B 2010, 81, 104305). We show that larger shifts (∼ -70 cm(-1)) do occur, but only when the next available site ("oxygen" site) is occupied. This larger shift originates from H(2)-H(2) interactions on neighboring sites of the same pore, consistent with the short distance between H(2) in these two sites ∼2.6 Å derived from an analysis of neutron diffraction experiments of D(2)-D(2) at 4 K (Liu et al., Langmuir 2008, 24, 4772-4777). Our results at 77 K and low loading can be explained by a diffusion barrier against pair disruption, which should be enhanced by this interaction. Calculations indicate that the vibrational shifts do not correlate with binding energies and are instead very sensitive to the environment (interaction potential and H(2)-H(2) interactions), which complicates the use of variable temperature IR methods to calculate adsorption energies of specific adsorption sites.

Ammonia Adsorption and Co-adsorption with Water in HKUST-1: Spectroscopic Evidence for Cooperative Interactions

“…Probing adsorptive interactions in-situ with chemical specificity can reveal mechanisms not observed by adsorption isotherms alone. [18][19][20][21][22][23][24] Therefore, an in-situ study of the co-adsorption is critical for revealing adsorbate-metal center and adsorbate-adsorbate interactions. Here, competitive interactions of water and ammonia are interrogated by the APXPS technique at low relative humidity and partial pressure of ammonia to investigate interactions leading to degradation.…”

Interaction of Small Molecules within Metal Organic Frameworks Studied by In Situ Vibrational Spectroscopy

“…Copyright (2010) American Chemical Society. with respect to the unperturbed molecules) is observed in the low loading regime when H 2 is dominantly adsorbed on the metal site [2]. Additional ~−32 cm −1 IR shift and a large variation in dipole moment are observed once the neighboring oxygen site was occupied with H 2 molecule to form a "pair" with H 2 molecules on the metal site.…”

Understanding Small‐Molecule Interactions in Metal–Organic Frameworks: Coupling Experiment with Theory

“…[ 104,106,115 ] In addition, integrated peak intensities are often assumed to be associated with the loading level; [ 114 ] however, in systems where increased loading results in additional intermolecular interactions, this correlation does not always hold true. This was highlighted by a combined experimental and theoretical study of Nijem et al, [ 116 ] who studied H 2 adsorption in the M 2 (dbodc) series. They found that high H 2 loadings in Mg 2 (dobdc) resulted in a counterintuitive decrease in IR intensity due to a decrease in the effective charge of H 2 at the open metal site.…”