News Flash  

Sep. 2016: Second Workshop on Hierarchical Materials in Erlangen: RV with invited lecture on transport properties of hierarchical porous solids.

Sep. 2015: IUPAC has established the task group "Diffusion in nanoporous solids" involving us as the active members.

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Sep. 2015Our work on diffusion in hollow core-shell nanoparticles just appeared in Langmuir.

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Seminars  

R. Valiullin: Mesoporous materials -- Perspectives revealed by fluids confined in their pore spaces
Mo, 27.06.2016, 17:15–18:00
Universität Hamburg, Fachbereich Chemie, Martin-Luther-King-Platz 6, Hörsaal C

October 21-23, 2015
R. Valiullin: NMR Cryoporometry: From fundamentals to application
Workshop: Methods of Porosimetry and Applications (HZDR Dresden-Rossendorf)

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Sep. 2015: IUPAC has established the task group "Diffusion in nanoporous solids" involving us as the active members.

Diffusion is an ubiquitous phenomenon in nature. Its random patterns may be found in an extreme variety of very different processes, like in haphazard trajectories of animals seeking a better environment or food, in distribution pathways of ideas in societies, or in chaotic journeys of chemical species searching for active reaction pairs in porous catalysts. Therefore, it is not surprising that it was a botanist and a medical doctor who most ingeniously recognized hidden traces of randomness in macroscopic observations of epidemic spread of diseases or elegant dances of pollen in water observed in a microscope and exerted thus the initial thrust to understand physics and mathematics of diffusion. Since that, substantial developments in theoretical descriptions of diffusion-related phenomena and their experimental measurements have been done. Despite long history of diffusion studies, some areas still remain hot topic of current research. This concerns, in particular, diffusion processes occurring in nanoporous materials.

Being an important constituent of chemical technology, porous solids have continuously been attracting an elevated interest. Among their different properties, mass transfer within their bodies is recognized to be one of the key elements. As the most illustrative example, during their exploitation in mass separation and catalytic conversion the gain in value-added products can never be faster than allowed by the rate of mass transfer. Hence, experimental assessment of transport properties in these materials turns to be of vital importance for process engineering. From the early days of diffusion studies in porous and, especially, in microporous solids, experimental scientists faced, however, various challenging problems. Most exceptionally, the second part of 20th century has puzzled scientists with the observation of dramatic differences in the diffusion rates in microporous solids probed by different experimental techniques, approaching, in some cases, several orders of magnitude. This situation is exemplified in the accompanying cartoon showing that, from the perspective of different observers which are using different experimental tools, the rate of a chaotic underwater excursion of the Loch Ness Monster may appear to approach the rates of antelope run, snake creep, bird fly, or turtle crawl.

In the last decades, the advent of “microscopic” techniques of diffusion measurement, together with a concurrent refinement of the “macrosocopic” techniques, has revolutionized our knowledge. It has been rationalized that, rather than being controlled by the diffusional resistance of the genuine micropore network as so far generally assumed, diffusive mass transfer in nanoporous materials is now in many cases known to be subject to a hierarchy of resistances. Though nowadays techniques and methodologies to experimentally determine all these resistances are available, our knowledge of mass transfer in the nanoporous materials in technological use is generally rather limited, if not even totally wrong. This situation is mainly caused by the fact that transport resistances occur over essentially all length scales. The sensitivity of the measuring techniques must, correspondingly, be adjusted to these very length scales. Inappropriate use of these techniques will therefore lead to wrong conclusions, which (as a common case given the complexity of the given situations) the unexperienced user is scarcely able to avoid.

Exactly this problem, which has increasingly been recognized by scientific community, shall be in the focus of a recently established task group “Diffusion in nanoporous solids” under the IUPAC auspices. It brings together worldwide leading specialists in the different fields of diffusion measurement, representing both academia and industry. This initiative follows previous local activities aiming at a clarification of the discrepancy so far often encountered in diffusion measurement. Providing the broadest coverage of the experimental techniques, including also experts from theory and quickly-growing computer modeling, the group intends now to develop the practical procedures for diffusion measurements in nanoporous solids with emphasis on the conditions that must be fulfilled and any experimental checks that should be undertaken to ensure reliable results. A special effort shall be made to formulating the criteria under which the results obtained within different approaches and for different classes of porous materials may be compared to each other. Recalling the figure, the Loch Ness Monster indeed may move with quite different speeds of either an antelope or a turtle just depending on the surrounding environment. Thus, only by collecting the information from different hunters acquired on different time- and length scales, the mystery of Loch Ness may finally be solved. 

Chemistry International, Vol. 38, Issue 1, Page 24

   
© Rustem Valiullin, 2006-2013