Diffusion in microporous materials

Frequency Response Measurements of Diffusion in Microporous Materials... 

The current PEP setup allows two types of experiments to measure diffusion in microporous materials, hi the first type, labeled molecules are injected as a small pulse into a steady-state feed stream of either an inert carrier gas or of unlabeled molecules of the same kind. The propagation of the pulse through the reactor is followed using the PEP detector. Information about the diffusive processes can be obtained from the delay and broadening of the pulse, and quantitative information can be obtained by analysis of the measurements using an appropriate model, as will be discussed in more detail in the next section. This type of experiments is especially suited for diffusion measurements under zero loading conditions. A drawback of this method is that it is limited to the determination of single-component diffusion coeffi-... 

Thus, Volume 7 of the series Molecular Sieves - Science and Technology presents descriptions, critical analyses, and illustrative examples of applications of the most important methods for investigations of sorption and sorption kinetics in zeolite systems and related materials. The editors hope that the volume will be helpful for researchers as well as technologists who are confronted with the important phenomena of adsorption and diffusion in microporous materials as they occur, for instance, in separation processes and catalysis. 

Furthermore, it should be mentioned that NMR spectroscopic techniques have found numerous applications for the characterization of the acidity of zeolites and related materials (see Vol. 6 of this series) and for the investigation of the molecular diffusion in microporous materials (see Vol. 7 of this series). 

The mechanisms of diffusion in these two systems (gas and liquid) are different and unrelated diffusion in gases is the result of the collision process, whereas that in liquids is an activated process (Bird et al., 1960). Diffusion in microp-orous materials is neither gaseous nor liquid diffusion. The closest case for such diffusion is surface diffusion, where molecules hop within the surface force field (see review by Kapoor et al., 1989b). Pick s law is used for both application (in modeling of adsorption processes) and experimental measurement of diffusion. Extensive reviews are available on diffusion in microporous materials and zeolites (Karger and Ruthven, 1992 Do, 1998). A lucid discussion on the nonlinear, and in some cases peculiar, phenomena in zeolite diffusion was given... 

A molecular dynamics approach can also be used to predict mixed gas diffusivities in microporous materials, at the expense of computation cost (e.g., Qureshi and Wei, 1990 Chitra and Yashonath, 1995 Trout et al., 1997 Snurr and Karger, 1997). The empirical correlation of Vignes (1966) for binary diffusivities in liquid solutions and also metallic alloys has been used extensively for calculating binary diffusivities, using the Maxwell-Stefan formalism for flux equations (e.g., Krishna, 1990). 

---