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Hydrogen Adsorption Properties and Structural Stability of Metal-Organic Framework (MOF)-177

Dipendu Saha and Shuguang Deng. Chemical Engineering Department, New Mexico State University, P.O. Box 30001, MSC 3805, Las Cruces, NM 88003

Metal organic framework MOF-177 is probably one of the most promising adsorbents with the highest hydrogen adsorption capacity among all physical adsorbents. MOF-177 was synthesized with a modified approach and characterized with XRD, SEM and nitrogen adsorption for its physical properties. The hydrogen adsorption equilibrium and kinetic data were measured volumetrically at ambient pressure and gravimetrically in a magnetic suspension balance at hydrogen pressure up to 100 bar. The MOF-177 synthesized in this work was found to have a higher specific surface area (Langmuir: 6008 m2/g; BET: 3814 m2/g) and a higher hydrogen adsorption capacity (11.0 wt.% excess adsorption, 19.67 wt.% absolute adsorption) than previously reported values on MOF-177. Freundlich equation fits well the hydrogen adsorption isotherms at low and high pressures. Diffusive time constant and isosteric heat of adsorption were calculated from the hydrogen adsorption kinetics and equilibrium data.

The effects of moisture and oxygen on structural stability of MOF-177 are investigated by X-ray diffraction and thermogravimetric analysis. The MOF-177 sample is exposed to ambient air of 16% RH for five weeks and monitored for its structural changes by XRD. The crystal structure of MOF-177 gradually changes from hexagonal to orthogonal, and then to monoclinic in the five weeks period. The crystal structure of MOF-177 is completely destroyed after it is immersed in water. Weigh loss of MOF-177 is negligible at temperature below 330C in the presence of oxygen. However, MOF-177 is completely converted to zinc oxide when it is exposed to 420C. An extension of possible explanation is made for structural shift of MOF-177 compared to previous researchers.