Abstract
This paper provides a general overview of the phenomenon of guest diffusion in nanoporous materials. It introduces the different types of diffusion measurement that can be performed under both equilibrium and non-equilibrium conditions in either single- or multicomponent systems. In the technological application of nanoporous materials for mass separation and catalytic conversion diffusion often has a significant impact on the overall rate of the process and is quite commonly rate controlling. Diffusion enhancement is therefore often a major goal in the manufacture of catalysts and adsorbents.
Highlights
Diffusion, i.e. the random movement of objects, is among the omnipresent and most fundamental phenomena in our world
With seemingly identical crystals of commercially produced zeolites of type SAPO-34 from the same batch, surface permeabilities were found to vary over almost two orders of magnitude (Remi et al 2015), suggesting that very minor differences in synthesis conditions may have a major effect
With (32), the time constants of uptake and release under single-file conditions are seen to scale with even the third power of the file length and, of the crystal size for crystalline material. Such a situation would be given during tracer exchange but, more importantly, for chemical reactions if the diffusion properties of the species under consideration do not appreciably differ so that, under stationary conditions, their diffusion properties are reasonably well approximated by the self-diffusivities (Carvill et al 1993; Liu et al 1996; Rödenbeck et al 1999; Gauw et al 2001) [see chapter 5 of (Kärger et al 2012)]
Summary
I.e. the random movement of objects, is among the omnipresent and most fundamental phenomena in our world. It is no surprise that our knowledge about the diffusion of atoms in a solid (Philibert 1991; Mehrer 2007) or of molecules in a liquid (Cussler 2009), with trillions of identical objects under study is far more advanced than our options to predict, for example, the spreading or extinction of a given species in a certain habitat (Vogl 2019) Within this spectrum of possibilities, molecular diffusion within nanoporous host systems (Kärger et al 2012) occupies a medium position. The contents are rounded off with a discussion of diffusion in the individual nanoporous particles, superimposed on mass transfer in a macroscopic column, with examples illustrating the application of diffusion theory and modelling In addition to their relevance for fundamental research such an approach can enhance the accuracy and reliability of diffusion measurement. In particular, to the benefits that can be derived from parallel simulations and theoretical analysis
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