MOLECULAR MOBILITY MEASUREMENT OF HYDROCARBONS IN ZEOLITES BY NMR TECHNIQUES

This chapter reviews the most relevant experimental methods to follow molecular translations and/or reorientations of guest molecules in zeolite pores. The benefit of combining these techniques is illustrated by a series of diffusion studies of hydrocarbons adsorbed in zeolite catalysts. Because of the key role of structure-related diffusion processes in shape-selective catalysis and the unique possibilities of the nuclear magnetic resonance (NMR) self-diffusion techniques to investigate such processes, the fundamentals of the NMR self-diffusion techniques are presented in the chapter and examples are given of their application to characterize zeolite catalysts. The NMR results (self-diffusion coefficients, anisotropic diffusivities, jump lengths, and residence times) can be correlated with corresponding neutron scattering data and sorption kinetics as well as molecular dynamics calculations, thus giving a comprehensive picture of molecular motions in porous solids. Analyzing the self-diffusion behavior of guest molecules in a microporous catalyst by the combined application of pulsed-field gradient NMR self-diffusion techniques reveals the spatial distribution of transport resistances over the catalyst particles. © 1993, Academic Press Inc.