We present a series of isostructural M(II) MOFs based on a biosourced gallate linker, with a focus on their structural evolution and H2 sorption behavior. Among them, MgGallate shows the highest H₂ uptake (1.4 wt.% at 77 K and 1 bar) and a constant isosteric heat of adsorption (Qₛₜ = 8.6 kJ/mol), despite lacking open metal sites. To elucidate the activation mechanism, in situ synchrotron PXRD reveals a reversible two-step dehydration, transitioning from a dihydrate to a stable, fully activated phase under vacuum. This phase retains crystallinity and exhibits well-defined ultramicropores that enable efficient H2 adsorption. Complementary in situ neutron powder diffraction with D₂ identifies two distinct adsorption sites located within the pore volume, with no interaction observed at the metal centers. These sites are symmetrically independent and become fully occupied at saturation, as revealed by sequential Rietveld refinement across multiple pressures. This confirms a purely confinement-driven sorption mechanism, further validated by quantitative agreement with macroscopic isotherms and site occupancies determined from Rietveld refinement at different pressures. Importantly, the heat of adsorption remains constant throughout the loading range, indicating uniform adsorption energy across the pore space, distinct from typical MOFs with open metal sites where Qₛₜ decreases upon saturation. Additionally, a mechanochemical synthesis route further enhances the material’s sustainability, offering rapid, scalable access to this material. The reaction proceeds under solvent-free conditions with high atom economy, achieving a space-time yield of over 600 kg/m³/day.
Esser et al. (Wed,) studied this question.