Reversible Hydrogen Storage by NaAlH4 Confined within a Titanium-Functionalized MOF-74(Mg) Nanoreactor

We demonstrate that NaAlH4 confined within the nanopores of a titanium-functionalized metal-organic framework (MOF) template MOF-74(Mg) can reversibly store hydrogen with minimal loss of capacity. Hydride-infiltrated samples were synthesized by melt infiltration, achieving loadings up to 21 wt %. MOF-74(Mg) possesses one-dimensional, 12 angstrom channels lined with Mg atoms having open coordination sites, which can serve as sites for Ti catalyst stabilization. MOF-74(Mg) is stable under repeated hydrogen desorption and hydride regeneration cycles, allowing it to serve as a nanoreactor. Confining NaAlH4 within these pores alters the decomposition pathway by eliminating the stable intermediate Na3AlH6 phase observed during bulk decomposition and proceeding directly to NaH, Al, and H-2, in agreement with theory. The onset of hydrogen desorption for both Ti-doped and undoped nano-NaAlN4@MOF-74(Mg) is similar to 50 degrees C, nearly 100 degrees C lower than bulk NaAlH4. However, the presence of titanium is not necessary for this increase In desorption kinetics but enables rehydriding to be almost fully reversible. Isothermal kinetic studies Indicate that the activation energy for H-2 desorption Is reduced from 793 kJ mol(-1) In bulk Ti-doped NaAlH4 to 574 kJ mol(-1) for nanoconfined NaAlH4. The structural properties of nano-NaAlH4@MOF-74(Mg) were probed using Na-23 and Al-27 solid-state MAS NMR, which indicates that the hydride Is not decomposed during Infiltration and that Al is present as tetrahedral AlH4 anions prior to desorption and as Al metal after desorption. Because of the highly ordered MOF structure and monodisperse pore; dimensions, our results allow key template features to be identified to ensure reversible, low-temperature hydrogen Storage.