The Random Walk of Understanding Diffusion

Comment on the paper “Diffusion and adsorption selectivities of hydrocarbons over FCC catalysts” by A.M. Ávila, C.M. Bidabehere and U. Sedran [Chem. Eng. J. 132 (2007) 67–75]

“…The correctness of PFG NMR data has been confirmed by numerous checks of self-consistency and cross-checks (see, e.g., Section 7.8 of [3] and [17]), including their validation by their perfect agreement with the evidence of "macroscopic" measurements when applied to macroscopic systems (where any systematic failure in the macroscopic measurements can be excluded) [18,19]. It remains to explain, therefore, why the data resulting from the fitting procedure as presented, e.g., in Refs.…”

Determination of Diffusion Coefficients in Porous Media

“…Since all techniques of diffusion measurement presented so far are based on the measurement of either mean concentrations within the catalyst particles or of the vapor pressure in the surrounding atmosphere, they are generally referred to as macroscopic techniques of measurement (or mesoscopic ones, if only one crystal is concerned) [125][126][127][128][129]. Most recently, however, two novel techniques have been introduced into heterogeneous catalysis which are likewise able to record phenomena of transport diffusion over regions which might be much smaller than the extensions of the individual zeolite crystallites and which, hence, are referred to as microscopic techniques.…”

Benefit of Microscopic Diffusion Measurement for the Characterization of Nanoporous Materials

“…As a general tendency, the diffusivities are found to be larger for "more microscopic" measuring techniques. Thus, the QENS diffusivities (which refer to displacements over nanometers [26]) tend to slightly exceed the PFG NMR diffusivities (referring to displacements over typically micrometers [45]), while the techniques following molecular uptake on the individual crystallite, i.e., mesoscopic measurement [18,19], or on the whole bed reveal the smallest diffusivities. Such findings have to be correlated with the existence of a hierarchy of transport resistances acting in addition to the genuine pore structure: If a technique is able to resolve molecular displacements much smaller than the separation between these additional resistances, the vast majority of the observed molecules and hence, the resulting diffusivities, are unaffected by these resistances.…”

“…Such investigations are referred to as "macroscopic" measurements [18,19]. Over many years, transport inhibition by the intracrystalline pore network was considered to be the rate-determining process in such experiments [20].…”

“…In fact, molecular diffusivities in zeolites are often much closer to those in the neat liquid than to the typical diffusivities in solids [24]. As a "microscopic" technique [18,19], PFG NMR is able to monitor diffusion paths over distances much smaller than the crystal sizes, so that the diffusion data are unaffected by the crystal boundary and by any bed effect.…”