Metal-Specific Interactions of H₂ Adsorbed within Isostructural Metal–Organic Frameworks

Abstract

Diffuse reflectance infrared (IR) spectroscopy performed over a wide temperature range (35-298 K) is used to study the dynamics of H(2) adsorbed within the isostructural metal-organic frameworks M(2)L (M = Mg, Mn, Co, Ni and Zn; L = 2,5-dioxidobenzene-1,4-dicarboxylate) referred to as MOF-74 and CPO-27. Spectra collected at H(2) concentrations ranging from 0.1 to 3.0 H(2) per metal cation reveal that strongly red-shifted vibrational modes arise from isolated H(2) bound to the available metal coordination site. The red shift of the bands associated with this site correlate with reported isosteric enthalpies of adsorption (at small surface coverage), which in turn depend on the identity of M. In contrast, the bands assigned to H(2) adsorbed at positions >3 Å from the metal site exhibit only minor differences among the five materials. Our results are consistent with previous models based on neutron diffraction data and independent IR studies, but they do not support a recently proposed adsorption mechanism that invokes strong H(2)···H(2) interactions (Nijem et al. J. Am. Chem. Soc.2010, 132, 14834-14848). Room temperature IR spectra comparable to those on which the recently proposed adsorption mechanism was based were only reproduced after contaminating the adsorbent with ambient air. Our interpretation that the uncontaminated spectral features result from stepwise adsorption at discrete framework sites is reinforced by systematic red shifts of adsorbed H(2) isotopologues and consistencies among overtone bands that are well-described by the Buckingham model of molecular interactions in vibrational spectroscopy.

Orientationalortho−H2pair interactions in the microporous framework MOF-5

“…The first is the shift of the vibrationally excited J = 1 level relative to that of gas phase H 2 . This is set to agree with previous published experimental data 21 and is consistent with the empirically established relationship between the H 2 vibrational redshift and site binding energy 24,34,35 . The second parameter is the crystal field splitting of the m = ±1 levels for an isolated H 2 in MOF-5.…”

Effect of modified linkers of MOF-5 on enhancing interaction energies: A theoretical study

“…A number of methods are employed to increase the interaction energy between H 2 molecules and MOFs surface [3][4][5][6][7], including creation of open metal sites, controlling pore size, catenation, ligand functionalization, alkaline-earth metal ion or alkali metal ion inclusion, doping with metal ions, and embedding MOFs with Pd or Pt nanoparticles [8].The interaction energy between H 2 and MOFs is very low (4-6 kJ.mol -1 ) [9]. Metal-organic frameworks have shown an enhancement of H 2 adsorption.…”

Caracterización de la Adsorción de Hidrógeno en MOFs por Métodos Químico-Computacionales

“…Alternativamente, una determinación de la fortaleza de la interacción adsorbatoadsorbente puede obtenerse mediante espectroscopia infrarroja [83,116,117]. Esta técnica permite la caracterización de los diferentes centros de adsorción presentes en la superficie de un material en función de la fortaleza de la interacción.…”

“…Esta complejidad es remarcada por el estudio de Fitzgerald et al [117], quienes a partir de estudios IR similares concluyen que los desplazamientos de las bandas espectrales asignadas al H 2 adsorbido, responden principalmente a las interacciones con la superficie del material y en menor medida a la incidencia de la adsorción en centros próximos.…”

“…De la misma manera, la adsorción simultanea de varias moléculas de adsorbato sobre un mismo centro promueve la aparición de efectos adicionales que potencian (efectos cooperativos) o afectan (efectos repulsivos) la fortaleza de las intereacciones, alterando en cierta medida la imagen obtenida a partir de los cálculos con una molécula. Computacionalmente, no existe una descripción detallada de estos efectos y aunque recientemente se han reportado algunos estudios experimentales al respecto [116,164,117,100], algúnos efectos observados aún requieren estudios detallados para su completa comprensión.…”