Diffusion in Porous Materials: Insights, Surprises and Challenges
After, with the first measurement of guest diffusion in nanoporous materials over microscopic dimensions, pulsed field gradient NMR gave rise to a paradigm shift in our understanding of zeolitic diffusion, with the potentials of microimaging to observe the evolution of molecular ensembles, finally also microscopic diffusion measurement under non-equilibrium conditions became possible1. Examples of the novel insight thus accessible include
- the measurement of transport resistances of the external surface of the adsorbent particles (“surface barriers”) as a crucial, but so far inaccessible parameter deciding about the technological feasibility of these materials where, depending on the host-guest system under study, the surface barriers are found to be caused by essentially impermeable layers with dispersed “holes” or by quasi-continuous layers of dramatically reduced permeability2,
- the potentials for immediately deciding about the applicability of Fick’s diffusion laws for describing mass transfer in a given nanoporous host-guest system 3,
- simultaneous recording of adsorption and conversion within catalytically active nanoporous crystals/particles, with direct (“one-shot”!) measurement of effectiveness factors as the key number describing the efficiency of the industrial application of such materials4,
- measurement of two-component adsorption and diffusion, with the option of the direct recording of “uphill” fluxes and “overshooting”, i.e. of profiles with concentrations temporarily exceeding the equilibrium ones5, and
- recording of pore filling and emptying upon gas pressure variation as a function of space and time as a novel route towards exploring the fundamentals of sorption hysteresis 6.
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