Low-field 1H NMR relaxometry of water molecules confined in microporous UiO-66

Water molecules confined in a microporous metal-organic framework (MOF) UiO-66 are characterized by a low-field H-1 nuclear magnetic resonance (NMR) spectroscopy. Measurements are performed of the longitudinal (T-1) and transverse (T-2) relaxation times as a function of water content from fully saturated to incomplete coverage of the first-adsorbed monolayer. The results obtained indicate that the relaxation of water molecules confined in UiO-66 is within the fast-exchange regime. When the amount of water exceeds filling ratio f = 0.4, the averaged relaxation time is approximately linearly dependent on water filling ratio in pore. When the water amount cannot support a full coverage of surface monolayer, the relaxation rate increases with less filling ratio, illustrating that the mobility of water molecules is more restricted. Analysis of the measured values and the simulated ones leads to the conclusion that the surface-affected zone in UiO-66 is not confined to the surface monolayer.