Andrei Nossov

Breathing Transitions in MIL‐53(Al) Metal–Organic Framework Upon Xenon Adsorption

MOFs come alive: The combination of gas‐adsorption experiments at various temperatures with an osmotic thermodynamic model produced a generic temperature–loading phase diagram that displays unexpected re‐entrant behavior. The breathing effect in the metal–organic framework MIL‐53(Al) is predicted to be general and should be observed over a limited temperature range regardless of the guest adsorbate.

129Xe NMR study of the framework flexibility of the porous hybrid MIL-53(Al).

The metal-organic framework MIL-53 exhibits a structural transition between two possible porous structures, so-called large-pore (lp) and narrow-pore (np) forms, depending on the temperature or when guest molecules are adsorbed. (129)Xe NMR has been used to study the lp --> 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 x 10(4) 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.

Continuous flow hyperpolarized 129Xe-MAS NMR studies of microporous materials

A magic angle spinning (MAS) NMR probe has been developed to allow in situ measurements of NMR spectra. Two applications are targeted with this device: i) in situ and operando MAS NMR spectroscopy of working catalysts and ii) the hyperpolarized (HP) 129Xe spectroscopy of porous materials under MAS and continuous flow conditions. The construction of the MAS probe is described and the usefulness of this system is demonstrated by studying the adsorption of hyperpolarized xenon on AlPO-41 and ITQ-6 zeolites. The high stability of the HP xenon flow allowed us to perform two-dimensional exchange experiments under MAS conditions, in a short time and with very good resolution.

Application of continuously circulating flow of hyperpolarized (HP)129Xe-NMR on mesoporous materials

The continuously circulating flow of hyperpolarized (HP) 129Xe NMR technique has been used to study the porous structure of ordered purely siliceous and Al-containing MCM-41 and SBA-15 mesoporous materials. In this paper, we report 129Xe chemical shift measurements of xenon adsorbed on mesoporous materials having different porosities and surface composition. The use of hyperpolarized xenon allows us to measure spectra at very low xenon concentration where xenon reflects mainly the interaction between the adsorbed xenon atoms and the surface. The effect of compression on the mesopore structure has also been studied by HP 129Xe NMR. The obtained NMR spectra are interpreted in terms of an exchange between adsorbed xenon in the pores and gazeous Xe atoms in the interparticle spaces. Variable temperature measurments allowed us to obtain information on the heat of adsorption of xenon on the surface and to evaluate the chemical shift of xenon in interaction with the surface.

The first direct probing of porosity on supported mesoporous silica thin films through hyperpolarised 129Xe NMRElectronic supplementary information (ESI) available: materials, NMR measurements, modification of silica surface. See http://www.rsc.org/suppdata/cc/b2/b207127d/

The continuous flow hyperpolarised (HP) 129Xe NMR technique has been used for the first time on mesoporous silica thin films. This study demonstrates the possibility of recording information about the porosity of a very small quantity of materials and the high sensitivity of the NMR response to pore size and pore functionality.