Quantitation of diffusion in zeolite catalysts

Recent access to measuring techniques covering the whole scale of diffusion phenomena relevant for heterogeneous catalysis demands reconsideration of the transport phenomena inherent to heterogeneous catalysis with respect to the options of their quantitation. With respect to the relevant diffusion paths the measuring techniques may be classified as microscopic and macroscopic and with respect to the thermodynamic regime as equilibrium techniques and non-equilibrium techniques. The PFG NMR method is presented as a particularly versatile technique, being able to reflect aspects of both microscopic and macroscopic observation. In addition to the direct measurement of the coefficients of intracrystalline and intraparticle diffusion, it is as well able to provide information about deviations from normal diffusion such as, e.g., in the case of single-file diffusion, and about the existence of transport barriers. With diffusion studies of FCC catalysts an example of the practical application of PFG NMR, including its inherent limitations, is given.