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A unique feature of metal-organic frameworks (MOFs) in contrast to rigid nanoporous materials is their structural switchabilty offering a wide range of functionality for sustainable energy storage, separation and sensing applications. This has initiated a series of experimental and theoretical studies predominantly aiming at understanding the thermodynamic conditions to transform and release gas, but the nature of sorption-induced switching transitions remains poorly understood. Here we report experimental evidence for fluid metastability and history-dependent states during sorption triggering the structural change of the framework and leading to the counterintuitive phenomenon of negative gas adsorption (NGA) in flexible MOFs. Preparation of two isoreticular MOFs differing by structural flexibility and performing direct in situ diffusion studies aided by in situ X-ray diffraction, scanning electron microscopy and computational modelling, allowed assessment of n-butane molecular dynamics, phase state, and the framework response to obtain a microscopic picture for each step of the sorption process. Switchable porous materials offer adaptive voids mutating in size and shape in response to molecular stimuli. Here authors use PFG NMR to reveal the decisive role of metastable states leading to counterintuitive negative gas adsorption transitions in responsive MOFs. [ABSTRACT FROM AUTHOR]

One-dimensional nanochannels, hundreds of microns in persistence length but with elliptical cross-sectional dimensions of only ~3.7 Å x 4.8 Å, are formed by the columnar assembly of phenylether bis-urea macrocycles. Hyperpolarized Xe-129 NMR is utilized to investigate the Xe atom packing and Xe diffusion inside the needle shaped crystals. The elliptical channel structure produces a Xe-129 powder pattern characteristic of an asymmetric chemical shift tensor extending to well over 300 ppm with respect to the gas phase, reflecting the highly anisotropic electronic environment and extreme confinement of the atom. Consistent with the simple geometrical criterion, hyperpolarized tracer exchange NMR data reveals single-file diffusion in the bis-urea nanochannels. Key words: xenon-129, nanotubes, bis-urea, single-file diffusion, hyperpolarization, SEOP, spin exchange optical pumping. Des nanocanaux unidimensionnels d'une longueur de persistance de centaines de microns, mais de dimensions de section elliptique de seulement ~3.7 Å x 4.8 Å, se forment par l'assemblage colonnaire de macrocycles de bis-uree de phenylether. Nous avons utilise la RMN du Xe-129 hyperpolarise pour etudier l'empilement des atomes de Xe et la diffusion du Xe a l'interieur des cristaux en forme d'aiguille. La structure elliptique des canaux produit un signal typique d'une de poudre de Xe-129, caracterise par un tenseur de deplacement chimique asymetrique s'etendant bien au-dela de 300 ppm, par comparaison a la phase gazeuse. Ce comportement temoigne d'un environnement electronique fortement anisotrope et d'un confinement extreme des atomes. En accord avec le critere geometrique simple, les donnees RMN du traceur hyperpolarise revelent une dynamique de diffusion en file indienne dans les nanocanaux de bis-uree. [Traduit par la Redaction] Mots-cles: xenon-129, nanotubes, bis-uree, diffusion en file indienne, hyperpolarisation, SEOP, pompage optique par echange de spin., Introduction The exploration of crystalline materials consisting of one-dimensional channel structures with sub-nm cross-sectional dimensions has proven to be one of the most intriguing uses of [sup.129]Xe NMR. (1-15) The [...]

Freezing and melting behaviors of a fluid confined to pores of mesoporous silicon with a modulated structure have been studied using NMR techniques. The molecular self-diffusivities, measured along the freezing and melting transitions, unveiled essential differences in the configuration of the frozen domains. This suggests that freezing is dominated by a pore-blocking mechanism. Freezing kinetics is found to exhibit very slow long-time dynamics, following a ln2(t) dependence. This type of time dependence may result if the front of the frozen phase is assumed to propagate in the random potential field created by the disorder of the porous silicon channels, similar to the mechanism of Sinai diffusion. The free energy barriers calculated from the kinetic measurements and estimated using a thermodynamical model yield a consistent picture of the freezing process in the presence of quenched disorder. [ABSTRACT FROM AUTHOR]

The pulsed field gradient nuclear magnetic resonance method has been used to study self-diffusion of cyclohexane in a commercial MCM-41 material at different external gas pressures from zero to saturated vapor pressure. It is found that the effective diffusivities exhibit three different regions with increasing pressure: decrease at low pressures, a sudden drop at intermediate pressures, and increase at higher pressures. In addition, in the region of irreversible adsorption (hysteresis loop) the diffusivities are also found to differ on the adsorption and the desorption branches. A simple analytical model taking account of different molecular ensembles with different transport properties due to the complex architecture of the porous structure is developed which provides a quantitative prediction of the experimental data. The analysis reveals that the effective diffusivity is predominantly controlled by the adsorption properties of the individual mesoporous MCM-41 crystallites which, in combination with high transport rates, provide a simple instrument for fine tuning of the transport properties by a subtle variation of the external conditions. [ABSTRACT FROM AUTHOR]

The unique potentials of pulsed field gradient (PFG) NMR are used for studying the molecular diffusion in nanopores by simultaneously determining the fluid densities and diffusivities during transition from subcritical to supercritical conditions. The novel option of direct diffusion measurement and SC fluids are exploited for their manifold applications, such as in extraction, separation and heterogeneous catalysis.

In the future, (electro-)chemical catalysts will have to be more tolerant towards a varying supply of energy and raw materials. This is mainly due to the fluctuating nature of renewable energies. For example, power-to-chemical processes require a shift from steady-state operation towards operation under dynamic reaction conditions. This brings along a number of demands for the design of both catalysts and reactors, because it is well-known that the structure of catalysts is very dynamic. However, in-depth studies of catalysts and catalytic reactors under such transient conditions have only started recently. This requires studies and advances in the fields of 1) operando spectroscopy including time-resolved methods, 2) theory with predictive quality, 3) kinetic modelling, 4) design of catalysts by appropriate preparation concepts, and 5) novel/modular reactor designs. An intensive exchange between these scientific disciplines will enable a substantial gain of fundamental knowledge which is urgently required. This concept article highlights recent developments, challenges, and future directions for understanding catalysts under dynamic reaction conditions. [ABSTRACT FROM AUTHOR]