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Infrared (IR) microimaging is employed for selectively recording two‐dimensional plots of the individual distributions of the components n‐hexane (nC6) and 3‐methylpentane (3MP) over a single crystal of zeolite ZSM‐5 during mixture adsorption. At a pressure change from 0 to 0.02 mbar, the scenario of "overshooting" is observed, with the concentration of nC6 temporarily exceeding its equilibrium value. After the next pressure step from 0.02 to 2 mbar, 3MP is progressively driven out of the pores, in parallel with an increase in nC6 concentration. The IR microimaging results are corroborated by grand canonical Monte Carlo simulations, which help explain the peculiarities of mass transfer by providing visual representations of the distribution and packing densities of the guest molecules in the pores. [ABSTRACT FROM AUTHOR]

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]

Diffusion in nanoporous host-guest systems is often considered to be too complicated to comply with such "simple" relationships as Fick's first and second law of diffusion. However, it is shown herein that the microscopic techniques of diffusion measurement, notably the pulsed field gradient (PFG) technique of NMR spectroscopy and microimaging 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 on this subject.