Neutron Scattering and Spectroscopic Studies of Hydrogen Adsorption in Cr3(BTC)2鈥擜 Metal鈭扥rganic Framework with Exposed Cr2+ Sites

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• 作者：Kenji Sumida ; Jae-Hyuk Her ; Mircea Dinca虇 ; Leslie J. Murray ; Jennifer M. Schloss ; Christopher J. Pierce ; Benjamin A. Thompson ; Stephen A. FitzGerald ; Craig M. Brown ; Jeffrey R. Long
• 刊名：Journal of Physical Chemistry C
• 出版年：2011
• 出版时间：April 28, 2011
• 年：2011
• 卷：115
• 期：16
• 页码：8414-8421
• 全文大小：1016K
• 年卷期：v.115,no.16(April 28, 2011)
• ISSN：1932-7455

文摘

Microporous metal鈭抩rganic 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. 鈭?0 kJ/mol). The structure of Cr₃(BTC)₂ (BTC^{3鈭?/sup> = 1,3,5-benzenetricarboxylate) consists of dinuclear paddlewheel secondary building units connected by triangular BTC3鈭?/sup> 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 鈭?.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鈭扖r 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鈭扝 stretching frequency corresponding to H₂ molecules bound to the primary site is observed in the form of an ortho鈭抪ara pair at 4110 and 4116 cm鈭?, respectively, which is significantly shifted compared to the frequencies for free H₂ of 4155 and 4161 cm鈭?. The infrared data have been used to compute a site-specific binding enthalpy for H₂ of 鈭?.7(5) kJ/mol, which is in agreement with the zero-coverage isosteric heat of adsorption derived from gas sorption isotherm data.}