A sustainable ultramicroporous MOF for hydrogen storage

Hydrogen is a promising candidate for renewable energy storage, yet its efficient storage remains a challenge. Here, we investigate ultramicroporous metal-organic frameworks (MOFs) derived from biosourced gallic acid. Among a series of M(II) gallate MOFs (M = Mg, Co and Ni), magnesium gallate (MgGal) exhibited the highest gravimetric hydrogen uptake, reaching 1.4 wt.% at 77 K and ambient pressure. In situ synchrotron X-ray diffraction under vacuum reveals a transition from the phase I (dihydrate to monohydrate) to the phase II (monohydrate to fully dehydrated state). The fully activated phase II displayed a high affinity for hydrogen, with the isosteric heat of adsorption Qst of 8.6 kJ/mol, determined from physisorption experiments, constant up to full coverage. Kinetic measurements showed that adsorption equilibrium was reached within minutes, with perfect reversibility. In situ neutron diffraction, consistent with physisorption data, revealed that hydrogen adsorption occurs solely through confinement within the ultramicropores, leading to the high Qst. Finally, the synthesis of MgGal was optimized for sustainability, achieving 100% yield and atom economy in under an hour via mechanochemistry, with a space-time yield of 605 kg/m³/day. These findings highlight the potential of green ultramicroporous MOFs for hydrogen storage via confinement mechanisms.