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.