Super Metalated Frameworks as Hydrogen Sponges

The most promising strategy towards the development of practical on-board hydrogen fuel cells for light-duty-vehicles is the application of materials that store hydrogen via physisorption. Such materials require high hydrogen storage capacities and stabilities. Metal-Organic Frameworks (MOFs) are crystalline porous materials comprised of metal clusters linked by organic struts where each component can be systematically altered or functionalized. This tunability coupled with their highly porous nature make MOFs ideal candidates for gas storage applications. The state-of-the-art MOF based absorbent (MOF-74 analogue) utilizes coordinatively unsaturated metal sites (open metal sites) which exhibit strong interactions with H<sub>2</sub> (Qst = 13 KJ/mol) producing the record figure of merit (12 g/L and 0.9 wt% at 25 ºC 100 bar) [3]. However, these values remain below the DOE 2025 target for hydrogen storage. To increase the interaction strength between the framework and H<sub>2</sub>, we have prepared a highly porous, functionalizable MOF (Mg-IRMOF-74-III). The organic linker of this MOF was functionalized with primary amines that were used to install metal-binding ligands for subsequent metalation. The newly installed open metal sites have slightly improved H<sub>2</sub> storage capacity at 77 K via weak interactions. However, the primary amines of the organic linkers have lead to a dramatic decrease in the H<sub>2</sub> capacity due to the interaction of the amines with the open metal-sites of the inorganic nodes. The conclusions of the present project pave the way for design of supermetalated crystalline materials for hydrogen storage to meet the DOE 2025 target in the future