Upgrading the Hydrogen Storage of MOF-5 by Post-Synthetic Exchange with Divalent Metal Ions

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 H-2 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 H-2 adsorption isotherms with a correlation coefficient approximately equal to 1. The unique hydrogen uptakes of Ni- and Co-MOF-5 shown in this study pave the way for further improvement in hydrogen uptake.

Automated summary

This study prepared mixed metal metal-organic frameworks M-MOF-5 using the post-synthetic exchange technique and found that Ni-MOF-5 and Co-MOF-5 demonstrated higher hydrogen uptake capacities compared to the parent MOF-5. The Fe-MOF-5 exhibited decreased uptake due to the formation of discrete complexes with less crystalline nature. The Sips model was effective in describing the hydrogen adsorption isotherms in this study, with Ni- and Co-MOF-5 showing unique hydrogen uptakes, paving the way for further improvement in hydrogen uptake.