Abstract
In metal-organic frameworks (MOFs), mixed-metal clusters have the opportunity to adsorb hydrogen molecules due to a greater charge density of the metal. Such interactions may subsequently enhance the gravimetric uptake of hydrogen. However, only a few papers have explored the ability of mixed-metal MOFs to increase hydrogen uptake. The present work reveals the preparation of mixed metal metal-organic frameworks M-MOF-5 (where M = Ni2+, Co2+, and Fe2+) (where MOF-5 designates MOFs such as Zn2+ and 1,4-benzenedicarboxylic acid ligand) using the post-synthetic exchange (PSE) technique. Powder X-ray diffraction patterns and scanning electron microscopy images indicate the presence of crystalline phases after metal exchange, and the inductively coupled plasma–mass spectroscopy analysis confirmed the exchange of metals by means of the PSE technique. The nitrogen adsorption isotherms established the production of microporous M-MOF-5. Although the additional metal ions decreased the surface area, the exchanged materials displayed unique features in the gravimetric uptake of hydrogen. The parent MOF-5 and the metal exchanged materials (Ni-MOF-5, Co-MOF-5, and Fe-MOF-5) demonstrated hydrogen capacities of 1.46, 1.53, 1.53, and 0.99 wt.%, respectively. The metal-exchanged Ni-MOF-5 and Co-MOF-5 revealed slightly higher H2 uptake in comparison with MOF-5; however, the Fe-MOF-5 showed a decrease in uptake due to partial discrete complex formation (discrete complexes with one or more metal ions) with less crystalline nature. The Sips model was found to be excellent in describing the H2 adsorption isotherms with a correlation coefficient ≅ 1. The unique hydrogen uptakes of Ni− and Co-MOF-5 shown in this study pave the way for further improvement in hydrogen uptake.
Highlights
As a clean alternative fuel with zero-CO2 releases and high energy density, hydrogen characterizes a possible source of energy
The PXRD pattern of the as-synthesized metal-organic frameworks (MOFs)-5 is comparable with the simulated pattern of MOF-5 crystal
The metal-exchanged Ni-MOF-5 and Co-MOF-5 showed slightly higher hydrogen uptake in comparison with the parent MOF-5; the Fe-MOF-5 showed a decrease in the hydrogen uptake due to partial complex formation with less crystalline nature
Summary
As a clean alternative fuel with zero-CO2 releases and high energy density, hydrogen characterizes a possible source of energy. There is no storage material that has fulfilled the recent U.S Department of Energy capability objectives of 4.5 wt.% or 30 g/L at pressures under 100 atm and a temperature of −40 to 60 ◦C [3]. In this regard, metal-organic frameworks (MOFs) are good candidates as physical adsorbents for hydrogen storage [4,5,6,7,8].
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