SELECTIVE SORPTION UPTAKE KINETICS OF N-HEXANE ON ZSM-5 - A NEW METHOD FOR MEASURING ANISOTROPIC DIFFUSIVITIES

Selective sorption uptake kinetics is a new experimental approach to the determination of the components of the diffusion tenser of reactant or adsorbate molecules in microporous crystals with anisotropic channel system. By first orienting molecular sieve crystals vertically or horizontally and then embedding. them in sputtered copper, one finds that an abrasion of the resultant composites opens selective pathways along the z axis or the x,y axes. The new method is demonstrated for the study of the diffusion anisotropy of n-hexane in ZSM-5. Analysis of the sorption uptake curve showed that the diffusion along the length of the crystals (z direction) is ca. 3 times slower than that occurring perpendicular to this direction. This finding corresponds with the mean net diffusion flux densities through crystal faces of different labels. However, the simplicity of the approach is complicated by crystal twinning effects.