Neutron Scattering and Spectroscopic Studies of Hydrogen Adsorption in Cr₃(BTC)₂—A Metal−Organic Framework with Exposed Cr²⁺ Sites

Abstract

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 (BTC3− = 1,3,5-benzenetricarboxylate) consists of dinuclear paddlewheel secondary building units connected by triangular BTC3− 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 D2 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.