Marie-Anne Springuel-Huet

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.

Flexibility of ZIF-8 materials studied using 129Xe NMR

Structural changes in a porous hybrid inorganic-organic ZIF-8 compound have been explored using hyperpolarized (129)Xe NMR of adsorbed xenon at various temperatures. Upon xenon adsorption at low temperature, the organic linkers undergo a reorientation leading to a stepwise increase in xenon adsorption and in chemical shift.

Probing the mobility of ibuprofen confined in MCM-41 materials using MAS-PFG NMR and hyperpolarised-129Xe NMR spectroscopy

The continuous-flow hyperpolarised (HP)-(129)Xe NMR and magic angle spinning-pulsed field gradient (MAS-PFG) NMR techniques have been used for the first time to study the distribution and the dynamics of ibuprofen encapsulated in MCM-41 with two different pore diameters.

Evidence for oxygen vacancy formation in HZSM-5 at high temperature

The properties of HZSM-5 zeolite (Si/Al=25) were investigated, at high temperature (623–973 K), by ac conductivity and 129Xe-NMR. The 129Xe-NMR and conductivity results suggest, for the first time, that HZSM-5 dehydroxylation at temperatures higher than 673 K leads to the formation of oxygen vacancies. It was found that the concentration of oxygen vacancies is proportional to the degree of dehydroxylation (temperature of treatment). Gas-phase O2 as well as H2O are incorporated into oxygen vacancies. Different conduction mechanisms were established for HZSM-5 in the temperature range investigated. Defect chemistry equations are used to describe the formation and evolution of lattice defects of HZSM-5 with temperature. Furthermore, the role of the HZSM-5 lattice defects in molecule activation at high temperatures is discussed. It is suggested that the active sites are either oxygen vacancies (in the absence of gas phase oxygen) or paramagnetic O- species formed by gas-phase oxygen incorporation into oxygen vacancies.

3D Study of the Morphology and Dynamics of Zeolite Nucleation.

The principle aspects and constraints of the dynamics and kinetics of zeolite nucleation in hydrogel systems are analyzed on the basis of a model Na-rich aluminosilicate system. A detailed time-series EMT-type zeolite crystallization study in the model hydrogel system was performed to elucidate the topological and temporal aspects of zeolite nucleation. A comprehensive set of analytical tools and methods was employed to analyze the gel evolution and complement the primary methods of transmission electron microscopy (TEM) and nuclear magnetic resonance (NMR) spectroscopy. TEM tomography reveals that the initial gel particles exhibit a core-shell structure. Zeolite nucleation is topologically limited to this shell structure and the kinetics of nucleation is controlled by the shell integrity. The induction period extends to the moment when the shell is consumed and the bulk solution can react with the core of the gel particles. These new findings, in particular the importance of the gel particle shell in zeolite nucleation, can be used to control the growth process and properties of zeolites formed in hydrogels.

An Analytical Study of Molecular Transport in a Zeolite Crystallite Bed

Analytical solutions of the diffusion equations to obtain the diffusant concentrations in the macro- and micropores which constitute the pore system of a zeolite bed are presented. The parameter which determines the influence of each pore type on the evolution of the adsorbate/adsorbant system towards the equilibrium state is described. Examples are given to illustrate a qualitative and quantitative study based on the curves obtained from these equations.