Antoine Gédéon

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.

Direct synthesis of AlSBA mesoporous molecular sieves: characterization and catalytic activities

Aluminium-incorporated SBA mesoporous materials have been obtained for the first time by direct synthesis; the resulting materials retain the hexagonal order and physical properties of purely siliceous SBA-15 and present higher catalytic activities in the cumene cracking reaction than AlMCM-41 materials.

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.

Evaluation of the characteristic dimensions for porous sound‐absorbing materials

The concept of characteristic thermal dimension was recently introduced to predict the behavior at high frequencies of the bulk modulus of fluids saturating a porous frame. This thermal dimension is closely related to the specific surface of the solid porous frame. It is shown that the standard Brunauer, Emmett, and Teller method (BET) of measuring the specific surface and acoustical measurement of the bulk modulus are in good agreement, and that the viscous dimension can be evaluated unambiguously from the BET method plus an acoustical measurement.

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.

Monitoring the crystallization process of a zeolite structure on SBA-15 mesopore walls

Hierarchical materials based on crystallization of the MFI structure on mesoporous Al-SBA-15 have been synthesized by a hydrothermal procedure in water. The effect of the crystallization temperature and time as well as the proportion of zeolite template has been investigated. In addition to XRD, TEM and N2 sorption measurements, the structural and textural properties of the materials have also been studied by hyperpolarized (HP) 129Xe NMR and FTIR. These techniques emphasized that the main issue during the synthesis procedure is to find the optimal parameters (time, temperature and concentrations) in order to maintain the structure of the Al-SBA-15 mesoporous solids under the conditions required for zeolite crystallization. Embryonal MFI crystals have been identified on SBA-15 walls which remained ordered for materials obtained with TPABr : Al molar ratio of 0.7 : 1 after 8 h of hydrothermal treatment at 180 °C. Evidence for these zeolite nanocrystals has been obtained for the first time by hyperpolarized 129Xe NMR spectroscopy. Moreover, this was corroborated by the appearance of an IR absorption band at 542 cm−1 due to the asymmetric stretching vibration of the double five-membered rings present in ZSM-5.

Application of 129Xe nuclear magnetic resonance to the study of CuY zeolites: dehydration and redox effects

Copper-exchanged sodium Y zeolites have been studied by using 129Xe nuclear magnetic resonance (NMR) spectroscopy. The oxidation state as well as the location of copper in the zeolite have been determined for different samples at various levels of exchange (lambda = 15, 35, 58%) and hydration, and at different stages of redox treatment. Information about the nature of Xe-Cu+ or Xe-Cu2+ interactions has been obtained by a combination of electron spin resonance (ESR) and 129Xe NMR measurements on Cu-Y and Cu-Rho zeolites.

X-ray diffraction analysis of mesostructured materials by continuous density function technique

Abstract A continuous density function technique has been developed for X-ray diffraction (XRD) structural investigations of mesostructured materials. The technique is designed for the analysis of the density distribution in materials exhibiting nanoscale (2-50 nm) ordering of structural elements without atomic long-range order. The results of structure investigations of a series of silicate, metallosilicate and carbon mesostructured materials with hexagonal and cubic symmetry are presented.

Transport parameters and sound propagation in an air-saturated sand

Measurements at ultrasonic frequencies of the transmission coefficient and the sound speed in layers of quarry sand saturated by air and helium are performed at different pressures. The measurement of surface impedance at audible frequencies is also performed for air-saturated layers. Evaluation of transport parameters can be obtained from these measurements. Close sets of parameters can be obtained at high and low frequencies with the model by Pride et al. [Phys. Rev. B 47, 4964 (1993)].