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Several NMR studies are conducted to explain the large-pore to narrow-pore transitions taking place in the porous hybrid, MIL-53(AI) via the adsorption of xenon. The mechanisms involved in the framework flexibility, as exhibited by these hybrids are also analyzed.

The metal-organic framework MIL-53 exhibits a structural transition between two possible porous structures, so-called large-pore (Ip) and narrow-pore (np) forms, depending on the temperature or when guest molecules are adsorbed. 129Xe NMR has been used to study the Ip → np transition induced by the adsorption of xenon as revealed by the adsorption isotherms. The NMR spectra show that the two structures, characterized by two distinct lines, coexist for xenon pressures above 5 × 104 Pa at room temperature, but a complete transformation is achieved when the temperature is decreased. An original interpretation of the NMR results allowed us to quantify the rate of the structural transformation. In particular, at room temperature, we have shown that 28% of the channels remain open. Two possible interpretations of the hysteresis observed in the chemical shift variation versus xenon pressure are proposed. [ABSTRACT FROM AUTHOR]

Pore-space homogeneity of zeolite NaX was probed by pulsed field gradient (PFG) NMR diffusion studies with n-butane as a guest molecule. At a loading of 0.75 molecules per supercage, a wide spectrum of diffusivities was observed. Guest molecules in the (well-shaped) zeolite crystallites were thus found to experience pore spaces of quite different properties. After loading enhancement to 3 molecules per supercage, however, molecular propagation ideally followed the laws of normal diffusion in homogeneous media. At sufficiently high guest concentrations, sample heterogeneity was thus found to be of no perceptible influence on the guest mobilities anymore.