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

Abstract

We demonstrate that NaAlH(4) 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 NaAlH(4) within these pores alters the decomposition pathway by eliminating the stable intermediate Na(3)AlH(6) 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-NaAlH(4)@MOF-74(Mg) is ∼50 °C, nearly 100 °C lower than bulk NaAlH(4). 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 79.5 kJ mol(-1) in bulk Ti-doped NaAlH(4) to 57.4 kJ mol(-1) for nanoconfined NaAlH(4). The structural properties of nano-NaAlH(4)@MOF-74(Mg) were probed using (23)Na and (27)Al solid-state MAS NMR, which indicates that the hydride is not decomposed during infiltration and that Al is present as tetrahedral AlH(4)(-) 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.

A mechanical-force-driven physical vapour deposition approach to fabricating complex hydride nanostructures

“…Moreover, the desorption of hydrogen from nano-NaAlH 4 confined within the metal-organic framework material MOF-74(Mg) begins at 50°C, nearly 100°C lower than the temperature for bulk NaAlH 4 (ref. 22). Nano-LiBH 4 embedded in Ni-doped porous carbon absorbs approximately 10 wt% H 2 (versus LiBH 4 ) at 320°C under 40 bar of H 2 , exhibiting a significantly improved hydrogen storage reversibility 23 .…”

“…Nanoconfinement is a more recently developed strategy that creates smaller, more uniform nanoscale hydrides, typically through the infiltration of scaffolds with hydrides. This bottomup approach has been successfully utilised to generate nanoconfined ammonia borane, alanates, borohydrides and magnesium hydrides within mesoporous silicas, nanostructured carbons and metal-organic frameworks [16][17][18][19][20][21][22][23][24] . Magnesium nanocrystals incorporated into a polymer (poly(methyl methacrylate)) display high hydrogen density (up to 6 wt% of Mg) and rapid kinetics (loading times o30 min at 200°C) 18 .…”

Organometallics and Related Molecules for Energy Conversion

“…The decomposition reaction equations of bulk NaAlH 4 are usually described by the three steps as follows: (15.5) to be accessible and reversible under ambient conditions. In this regard, the studies focused on metal-organic frameworks proposed it as a template for formation of nanoscale NaAlH 4 on dehydrogenation and reversible hydrogen storage [226][227][228]. Other transition metals containing additives have also proven to be effective catalysts for improving the hydrogenation and dehydrogenation kinetics of NaAlH 4 [211,212].…”

Metal‐Organic Frameworks: Metal‐Uptake

“…58,59 For example, approximately 1.2 molecules per pore of [RuPcF16] (PcF16 = perfluorophthalocyanine) can be incorporated into MIL-101(Cr) and this material is an order of magnitude more active as an oxidation catalyst for tetralin than is free [RuPcF16]. 58 Small hydrogen-rich metal-containing species such as LiBH 4 60 and NaAlH 4 61, 62 can be bound in the pores of [Cu 3 (btc) 2 ] (dehydrated HKUST-1, btc = benzene-1,3,5-tricarboxylate). An alternative approach to the problem of hydrogen storage is chemisorption where such hydrides release H 2 on heating, and the H 2 release characteristics of the hydrides embedded in MOFs were distinct from that of bulk hydrides.…”