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The use of hierarchically organized nanoporous materials has proven to be well suited for counteracting transport hindrance in micropores and the resulting reduction in their technological performance. As a typical example, we consider materials with the structure of mesoporous zeolites, where a microporous continuum is traversed by a network of mesopores. Mass transfer in such material has recently been shown to be effectively quantitated by application of the two-region approach of diffusion (the “Kärger model”). It operates with the diffusivities in the two pore spaces, their relative occupations and the mutual exchange rates as the only free parameters. In the present paper, the validity of this approach for the combined consideration of mass transfer and catalytic conversion in such materials will be confirmed by comparison with the outcome of kinetic Monte Carlo simulations.

Diffusion is an omnipresent, most fundamental phenomenon in nature and thus critical for the performance of numerous technologies. This is in particular true for nanoporous materials with manifold applications for matter upgrading by separation, purification and conversion. The path lengths of molecular transportation within the industrial plants range from the elementary steps of diffusion within the micropores of the individual particles up to the matter flow over macroscopic distances. Each of them might be decisive in determining overall performance so that detailed knowledge of all modes of mass transfer is crucial for a knowledge-based optimization of the devices with reference to their transport properties. The rate of mass transfer is particularly complicated to be assessed within the individual (adsorbent) particles/crystallites with pore sizes of the order of molecular dimensions. We are going to present two powerful techniques exactly for this application, operating under both equilibrium (Pulsed Field Gradient (PFG) NMR) and non-equilibrium (Microimaging by interference microscopy and IR microscopy) conditions. The potentials of these techniques are demonstrated in a few showcases, notably including the options of transport enhancement in pore hierarchies. The contribution concludes with a survey on present activities within an IUPAC initiative aiming at the elaboration of “guidelines for measurements and reporting of diffusion properties of chemical compounds in nanoporous materials”.

Adsorption and desorption of hydrocarbons in a realistic model (2496 atoms) of ZSM-5 zeolite (MFI), including an external surface and a reservoir for molecules, have been studied using classical molecular dynamics, with special focus on surface barriers. Different degrees of surface blocking have been modeled according to experimental observations. Using previ-ous molecular dynamics results, from the analysis of adsorption and desorption path lengths, we demonstrate that surface barriers are symmetric, i.e. with equal paths for desorption and adsorption, in agreement with the principle of microscopic reversibility and in contradiction with a model proposed recently. A new thermodynamic analysis confirms the symmetry of adsorption/desorption paths.

An editorial and a selection of papers presented at the 14th International Bologna Conference Magnetic Resonance in Porous Media (MRPM 14), February 2018, Gainesville, FL, USA

s of the conference 'Diffusion Fundamentals VIII - Deffusion Phenomena Moving People', September 1-5, 2019, Erlangen, Germany

Through IR microimaging the spatially and temporally resolved development of the CO2 concentration in a ZIF‐8@6FDA‐DAM mixed matrix membrane was visualized during transient adsorption. By recording the evolution of the CO2 concentration, it is observed that the CO2 molecules propagate from the ZIF‐8 filler, which acts as a transport "highway", towards the surrounding polymer. A high‐CO2‐concentration layer is formed at the MOF/polymer interface, which becomes more pronounced at higher CO2 gas pressures. A microscopic explanation of the origins of this phenomenon is suggested by means of molecular modeling. By applying a computational methodology combining quantum and force‐field based calculations, the formation of microvoids at the MOF/polymer interface is predicted. Grand Canonical Monte Carlo simulations further demonstrate that CO2 tends to preferentially reside in these microvoids, which is expected to facilitate CO2 accumulation at the interface.

Pulsed field gradient (PFG) nuclear magnetic resonance (NMR) allows recording of molecular diffusion paths (notably, the probability distribution of molecular displacements over typically micrometers, covered during an observation time of typically milliseconds) and has thus proven to serve as a most versatile means for the in-depth study of mass transfer in complex materials. This is particularly true with nanoporous host materials, where PFG NMR enabled the first direct measurement of intracrystalline diffusivities of guest molecules. Spatial resolution, i.e., the minimum diffusion path length experimentally observable, is limited by the time interval over which the pulsed field gradients may be applied. In “conventional” PFG NMR measurements, this time interval is determined by a characteristic quantity of the host-guest system under study, the so-called transverse nuclear magnetic relaxation time. This leads, notably when considering systems with low molecular mobilities, to severe restrictions in the applicability of PFG NMR. These restrictions may partially be released by performing PFG NMR measurements in combination with “magic-angle spinning” (MAS) of the NMR sample tube. The present review introduces the fundamentals of this technique and illustrates, via a number of recent cases, the gain in information thus attainable. Examples include diffusion measurements with nanoporous host-guest systems of low intrinsic mobility and selective diffusion measurement in multicomponent systems.

Die Diffusion in nanoporösen Wirt-Gast-Systemen wird oft als viel zu kompliziert angesehen, als dass sie mit so “einfachen” Beziehungen wie dem ersten und zweiten Fickschen Gesetz vereinbar sein könnte. Wir zeigen nun, dass die mikroskopischen Diffusionsmesstechniken, vor allem die NMR-Spektroskopie mit gepulsten Feldgradienten (PFG) und die Mikrobildgebung mittels Interferenzmikroskopie (IFM) und IR-Mikroskopie (IRM), den direkten experimentellen Nachweis für die Anwendbarkeit der Fickschen Gesetze auf solche Systeme liefern. Dies gilt oft auch dann noch, wenn die zugrundeliegenden Transportmechanismen komplex sind. Die Grenzen des Diffusionsmodells werden unter Bezugnahme auf die umfangreiche Literatur diskutiert., Diffusion in nanoporous host-guest systems is often considered to be too complicated to comply with such “simple” relations as Fick’s first and second laws of diffusion. However, we show here that the microscopic techniques of diffusion measurement, notably the pulsed field gradient (PFG) technique of NMR and micro-imaging by interference microscopy (IFM) and IR microscopy (IRM), provide direct experimental evidence of the applicability of Fick’s laws to such systems. This remains true in many situations, even when the detailed mechanism is complex. The limitations of the diffusion model are also discussed with reference to the extensive literature of this subject.