Metal-Specific Interactions of H_2 Adsorbed within Isostructural Metal-Organic Frameworks

摘要

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_2L (M = Mg, Mn, Co, Ni and Zn; L = 2,5-dioxidobenzene-l,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 en- thalpies 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 ? 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.