Probing Adsorbed Water in Al(HCOO)3 Metal–Organic Framework by Multinuclear and Multidimensional Solid-State NMR Spectroscopy

Water affects the local structure, dynamics, and adsorption properties of metal–organic frameworks (MOFs), yet the molecular-level characterization of confined water remains difficult due to the lack of long-range ordering. Here, we study the water adsorption in microporous MOF Al(HCOO)3 using multinuclear solid-state NMR spectroscopy combined with ab initio molecular dynamics (AIMD) simulations. 1H NMR resolves two distinct confined-water environments and reveals water–framework and water–water contacts. 13C and 27Al NMR further show that water adsorption perturbs the local environments of the formate linkers and AlO6 coordination units. Powder X-ray diffraction shows that the long-range crystalline framework is preserved after water loading, whereas 13C NMR detects a minor mobile formate-containing species that disappears upon re-dehydration, consistent with the partial local hydrolysis of the framework. 2H NMR distinguishes both highly mobile and confined water undergoing rapid anisotropic reorientation. The effective quadrupolar line shape is consistent with rapid anisotropic reorientation, for which C2-axis flips provide one possible motional model. 1H–2H CP-HETCOR NMR further suggests dipolar contacts between restricted water molecules, the formate framework, and neighboring water molecules. AIMD simulations support water–framework interactions associated with the two pore environments and the formation of an interconnected water network. These results provide molecular-level insights into the water adsorption sites and dynamics in Al(HCOO)3 and the previously unreported partial hydrolysis of this MOF.