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 re sults. Branching and cyclization of saturated hydrocarbon structures l ead to significant changes in critical molecular diameter and hence to differences in kinetic behavior. The following general trend was obse rved for the diffusivities: linear > single-branched > double (ternary C)-branched > cyclic paraffins > double (quaternary C)-branched. Ster ic hindrance was most pronounced for compounds with a quaternary carbo n atom, such as 2,2-dimethylbutane. The kinetic data conform well to a one-dimensional (slab) diffusion model, reflecting the anisotropic di ffusion behavior characteristic of the silicalite/ZSM-5 structure.