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The random motion (the diffusion) of guest molecules in nanoporous host materials is key to their manifold technological applications and, simultaneously, a ubiquitous phenomenon in nature quite in general. Based on a specification of the different conditions under which molecular diffusion in nanoporous materials may occur and of the thus resulting relevant parameters, a survey of the various ways of the measurement of the determining parameters is given. Starting with a condensed introduction to the respective measuring principles, the survey notably includes a summary of the various parameters accessible by each individual technique, jointly with an overview of their strengths and weaknesses as well as of the respective ranges of observation. The presentation is complemented by basic relations of diffusion theory and molecular modeling in nanoporous materials, illustrating their significance for enhancing the informative value of each measuring technique and the added value attainable by their combination. By providing guidelines for the measurement and reporting of diffusion properties of chemical compounds in nanopores, the document aims to contribute to the clarification and standardization of the presentation, nomenclature, and methodology associated with the documentation of diffusion phenomena in nanoporous materials serving for catalytic, mass separation, and other relevant purposes. [ABSTRACT FROM AUTHOR]

The separation of xylene isomers still remains an industrially challenging task. Here, porous purine-based metal–organic frameworks (MOFs) have been synthesized and studied for their potential in xylene separations. In particular, Zn(purine)I showed excellent para-xylene/ortho-xylene separation capability with a diffusion selectivity of 6 and high equilibrium adsorption selectivity as indicated by coadsorption experiments. This high selectivity is attributed to the shape and size of the channel aperture within the rigid framework of Zn(purine)I. [ABSTRACT FROM AUTHOR]

The tracer exchange of molecules between the phase adsorbed in one-dimensional channels and the surrounding gas phase by molecular dynamics simulations is studied. It is seen that equilibrium concentration is reached faster at the boundaries and slower in the middle part of the channel in the case of single-file diffusion, which can be explained by the mechanisms of the normal diffusion laws and that which admits additional displacements causing an accelerated particle exchange.

Research was conducted to examine the visible light-induced reaction between 2,3-dimethyl-2-butene and O2 in zeolite NaY at -100 degrees C using rapid scan time-resolved FT-infrared spectroscopy. The growth of the sole product, 2,3-dimethyl-3-hydroperoxy-1-butene during a 150 ms photolysis pulse was observed. Results confirm that the sole physical property of the zeolite cage is responsible for the visible light-induced olefin oxidation.

Incrystalline concentration profiles of water, adsorbed in large crystals of CrAPO-5 and SAPO-5 under equilibrium with water vapor at one and 20 mbar, are determined with the use of inference microscopy. The results indicate high inhomogeneous distribution of incrystalline water in both crystal types at lower pressure.

The report stated that the equilibrium intracrystalline concentration profile of methanol was recorded in CrAPO-5 crystals by interference and FTIR microscopy. The study stated that two-dimensional concentration profiles of an unprecedented quality were reported, which showed a nonhomogeneous distribution of adsorbate in zeotite cyrstal under equilibrium with the adsorbate vapor.

Pulsed field gradient (PFG) NMR was used to investigate the self-diffusion of carbon dioxide in alumina stabilized samaria aerogel catalyst, a promising porous catalyst for gas-phase reactions featuring high porosity and high surface area. For diffusion studies, the catalyst was prepared in two sample packing types, macroscopic monoliths (i.e., macroscopic cylindrical particles) and powder beds with particle sizes around 200 μm that are considered for catalytic applications. Studies of diffusion in these samples revealed how macroscopic packing influences the catalyst transport properties. Application of a high magnetic field of 17.6 T in the reported PFG NMR studies enabled diffusion measurements for relatively low carbon dioxide densities in the catalyst samples corresponding to a gas loading pressure of around 0.1 atm. As a result, it was possible to perform diffusion measurements for a large range of carbon dioxide loading pressures between 0.1 and 10 atm. The measured carbon dioxide diffusivities in the beds of catalyst particles are interpreted in the context of a simple diffusion-mediated exchange model previously used for zeolites and other porous materials. [ABSTRACT FROM AUTHOR]

Mesoscopic kinetic Monte Carlo simulations and pulsed field gradient nuclear magnetic resonance (PFG NMR) measurements are compared in order to investigate the transport of ethane in a bed of NaX crystals. A novel molecular mechanics particle-based reconstruction method is employed for the digital representation of the bed, enabling for the first time a parallel study of the real system and of a computer model tailored to reproduce the void fraction, particle shape and average size of the real system. Simulation of the long-range diffusion of ethane in the bed over the Knudsen, transient, and molecular diffusion regimes is consistent with the PFG NMR measurements in yielding tortuosity factors which depend upon the regime of diffusion; more specifically, tortuosity factors defined in the conventional way are higher in the Knudsen than in the molecular diffusion regime. Detailed statistical analysis of the computed molecular trajectories reveals that this difference arises in a nonexponential distribution of the lengths and in a correlation between the directions of path segments traversed between collisions with the solid in the Knudsen regime. When the Knudsen tortuosity is corrected to account for these features, a single, regime-independent value is obtained within the error of the calculations. [ABSTRACT FROM AUTHOR]

Intracrystalline concentration profiles of methanol during its adsorption into the one-dimensional channels of CrAPO-5 crystals are reported. The profiles were recorded by the interference microscopy technique recently introduced in our laboratory. The exceptionally high spatial resolution, which can be achieved by this technique allowed us to obtain detailed information on the interplay of intracrystalline diffusion, the permeability of the crystal surface and the role of the intergrowth effects on molecule uptake. Comparison of the measured concentration profiles with those simulated by the dynamic Monte Carlo method provides quantitative information on the intracrystalline diffusivity of methanol and the permeability of the transport barriers on the crystal surface. [ABSTRACT FROM AUTHOR]