Neutron Scattering And Spectroscopic Studies Of Hydrogen Adsorption In Cr3(BTC)2-a Metal-organic Framework With Exposed Cr2+ Sites


Microporous metal-organic frameworks possessing exposed metal cation sites on the pore surface are of particular interest for high-density H(2) storage at ambient temperatures, owing to the potential for H(2) binding at the appropriate isosteric heat of adsorption for reversible storage at room temperature (ca. -20 kJ/mol). The structure of Cr(3)(BTC)(2) (BTC(3-) = 1,3,5-benzenetricarboxylate) consists of dinuclear paddlewheel secondary building units connected by triangular BTC(3-) bridging ligands to form a three-dimensional, cubic framework. The fully desolvated form of the compound exhibits BET and Langmuir surface areas of 1810 and 2040 m(2)/g respectively, with open axial Cr(2+) coordination sites on the paddlewheel units. Its relatively high surface area facilitates H(2) uptakes (1 bar) of 1.9 wt % at 77 K and 1.3 wt % at 87 K, and a virial-type fitting to the data yields a zero-coverage isosteric heat of adsorption of -7.4(1) kJ/mol. The detailed hydrogen loading characteristics of Cr(3)(BTC)(2) have been probed using both neutron powder diffraction and inelastic neutron scattering experiments, revealing that the Cr(2+) site is only partially populated until a marked elongation of the Cr-Cr internuclear distance occurs at a loading of greater than 1.0 D(2) per Cr(2+) site. Below this loading, the D(2) is adsorbed primarily at the apertures of the octahedral cages. The H-H stretching frequency corresponding to H(2) molecules bound to the primary site is observed in the form of an ortho-para pair at 4110 and 4116 cm(-1), respectively, which is significantly shifted compared to the frequencies for free H(2) of 4155 and 4161 cm(-1). The infrared data have been used to compute a site-specific binding enthalpy for H(2) of -6.7(5) kJ/mol, which is in agreement with the zero-coverage isosteric heat of adsorption derived from gas sorption isotherm data.


American Chemical Society

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Journal of Physical Chemistry C


Physics and Astronomy

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