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.