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_NH2), 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, pH2 = 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.