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

Abstract

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 Å 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 H₂, in agreement with theory. The onset of hydrogen desorption for both Ti-doped and undoped nano-NaAlH₄@MOF-74(Mg) is ∼50 °C, nearly 100 °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 H₂ desorption is reduced from 79.5 kJ mol^–1 in bulk Ti-doped NaAlH₄ to 57.4 kJ mol^–1 for nanoconfined NaAlH₄. The structural properties of nano-NaAlH₄@MOF-74(Mg) were probed using ²³Na and ²⁷Al 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.