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Thermodynamics of hydrogen adsorption in MOF-177 at low temperatures: measurements and modelling.

Mon, 12/21/2009 - 2:37pm
Nanotechnology
Hydrogen adsorption measurements and modelling for the Zn-based microporous metal-organic framework (MOF) Zn4O(1,3,5-benzenetribenzoate)2, MOF-177, were performed over the 50-77 K and 0-40 bar ranges. The maximum excess adsorption measured under these conditions varies over about 105-70 mg g(-1). An analysis of the isotherms near saturation shows that hydrogen is ultimately adsorbed in an incompressible phase whose density is comparable to that of the bulk liquid. These liquid state properties observed under supercritical conditions reveal a remarkable effect of nanoscale confinement. The entire set of adsorption isotherms can be well described using a micropore filling model. The latter is used, in particular, to determine the absolute amounts adsorbed and the adsorption enthalpy. When expressed in terms of absolute adsorption, the isotherms show considerable hydrogen storage capacities, reaching up to 125 mg g(-1) at 50 K and 25 bar. The adsorption enthalpies are calculated as a function of fractional filling and range from 3 to 5 kJ mol(-1) in magnitude, in accordance with physisorption. These results are discussed with respect to a similar analysis performed on another Zn-based MOF, Zn4O(1,4-benzenedicarboxylate)3, IRMOF-1, presented recently. It is found that both materials adsorb hydrogen by similar mechanisms.

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