Neutron Scattering and Spectroscopic Studies of Hydrogen Adsorption in Cr-3(BTC)(2)-A Metal-Organic Framework with Exposed Cr2+ Sites

作者：Sumida Kenji^*; Her Jae Hyuk; Dinca Mircea; Murray Leslie J; Schloss Jennifer M; Pierce Christopher J; Thompson Benjamin A; FitzGerald Stephen A; Brown Craig M; Long Jeffrey R

来源：Journal of Physical Chemistry C, 2011, 115(16): 8414-8421.

DOI：10.1021/jp200638n

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) (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 m(2)/g respectively, with open axial Cr2+ 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 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 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.

• 出版日期2011-4-28