ADSORPTION OF BRANCHED AND CYCLIC PARAFFINS IN SILICALITE .2. KINETICS

Intracrystalline diffusivities in silicalite have been measured by the gravimetric sorption uptake method for several branched and cyclic Cg paraffins (2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, 2,3-dimethylbutane, methylcyclopentane, and cyclohexane). The zero-length column (ZLC) method was also used to check and validate some of the results. Branching and cyclization of saturated hydrocarbon structures lead to significant changes in critical molecular diameter and hence to differences in kinetic behavior. The following general trend was observed for the diffusivities: linear > single-branched > double (ternary C)-branched > cyclic paraffins > double (quaternary C)-branched. Steric hindrance was most pronounced for compounds with a quaternary carbon atom, such as 2,2-dimethylbutane. The kinetic data conform well to a one-dimensional (slab) diffusion model, reflecting the anisotropic diffusion behavior characteristic of the silicalite/ZSM-5 structure.