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 t he determination of the components of the diffusion tenser of reactant or adsorbate molecules in microporous crystals with anisotropic chann el system. By first orienting molecular sieve crystals vertically or h orizontally and then embedding. them in sputtered copper, one finds th at an abrasion of the resultant composites opens selective pathways al ong 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 t he 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 mea n net diffusion flux densities through crystal faces of different labe ls. However, the simplicity of the approach is complicated by crystal twinning effects.