A Lightweight Beryllium Metal-Organic Framework for Combined Physical and Chemical Hydrogen Storage.

In this work, we report the synthesis and structural characterization of the beryllium-based metal-organic framework of general formula [Be4O-(BDC-NH2)2.5(OAc)] (Be_BDC_NH 2 ), designed for combined physical and chemical hydrogen storage applications. The material was extensively characterized through a plethora of solid-state techniques (including less conventional 9Be NMR-MAS spectroscopy). Structural analysis by X-ray powder diffraction confirmed the formation of a crystalline porous framework of fcu topology isostructural to MOF-5 and to MOF-5-(Be), while thermogravimetric studies revealed remarkable thermal stability up to 830 K. Nitrogen adsorption measurements demonstrated a high specific surface area (2264 m2/g after removal of residual acetic acid), confirming the accessible porosity of the material. Hydrogen adsorption experiments (physical hydrogen storage) performed at cryogenic temperatures showed fast, fully reversible physisorption with a gravimetric H2 density of 8.0 wt % H2 (T = 77 K, p H2 = 80 bar) and a H2 isosteric heat of adsorption of 2.7 kJ/mol (at 0.1 wt % H2 coverage), consistent with weak, noncovalent interactions between the hydrogen molecules and the framework. To enable chemical hydrogen storage, ammonia borane (NH3·BH3, AB, 19.6 wt % H) was successfully impregnated into the MOF pores by suspending it on concentrated methanol solutions of AB. Solid-state multinuclear (11B, 15N) NMR spectroscopy revealed the presence of several boron-containing species, indicating partial chemical transformations of ammonia borane within the framework triggered by the formation of an initial B-H···H-N dihydrogen bonding interaction with the amino dangling group on the MOF linker. 11B NMR quantification determined a maximum hydride loading of 2.1 AB molecules per formula unit. To our knowledge, this is the first example of a beryllium MOF able to host either physisorbed molecular hydrogen or chemically bound hydrogen in the form of BN-based lightweight inorganic hydrides, highlighting its potential as a multifunctional material for advanced hydrogen storage strategies.