Neutron scattering and spectroscopic studies of hydrogen adsorption in Cr₃(BTC)₂-A metal-organic framework with exposed Cr ²⁺ sites

Microporous metal-organic frameworks possessing exposed metal cation sites on the pore surface are of particular interest for high-density H₂ storage at ambient temperatures, owing to the potential for H₂ binding at the appropriate isosteric heat of adsorption for reversible storage at room temperature (ca. -20 kJ/mol). The structure of Cr₃(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 m²/g, respectively, with open axial Cr²⁺ coordination sites on the paddlewheel units. Its relatively high surface area facilitates H₂ 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 Cr₃(BTC)₂ have been probed using both neutron powder diffraction and inelastic neutron scattering experiments, revealing that the Cr²⁺ 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 Cr²⁺ site. Below this loading, the D₂ is adsorbed primarily at the apertures of the octahedral cages. The H-H stretching frequency corresponding to H₂ 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 H₂ of 4155 and 4161 cm^-1. The infrared data have been used to compute a site-specific binding enthalpy for H₂ of -6.7(5) kJ/mol, which is in agreement with the zero-coverage isosteric heat of adsorption derived from gas sorption isotherm data.