Abstract

With the introduction of interference microscopy to diffusion studies in zeolites, the direct, space-resolved observation of transport diffusion in nanoporous materials has become possible for the first time. The method is based on recording transient concentration profiles during molecular uptake or release so that the local intracrystalline diffusivities directly result by a microscopic (“differential”) application of Fick's second law.

We show that under certain conditions the thus accessible wealth of information on the transport characteristics of nanoporous materials may even be surpassed by the application of an integration method that Ludwig Boltzmann invented more than a century ago and that thus encounters a large new field of application. The procedure is illustrated by discussing its application to the adsorption process of methanol on FER-type zeolite crystallites. The advantages and limitations of the method in comparison with the differential application of Fick's second law are discussed.