Intracrystalline diffusivities in silicalite were measured by using the gravimetric sorption up-take method for several branched and cyclic C6 paraffins (2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, 2,3-dimethylbutane, methylcyclopentane, cyclohexane). The diffusivity of 2-methylpentane in silicalite at 423K was 2 x 10-12m2/s, with an activation energy of 46kJ/mol. The zero-length column method was also used to check and validate some of the results. Branching and cyclization of saturated hydrocarbon structures led to significant changes in the critical molecular diameter and hence to differences in kinetic behavior. The general trend observed for the diffusivities was: linear > single-branched > double (ternary C)-branched > cyclic paraffins > double (quaternary C)-branched. Steric hindrance was most pronounced for compounds having a quaternary carbon atom; such as 2,2-dimethylbutane. The kinetic data conformed well to a one-dimensional (slab) diffusion model, reflecting the anisotropic diffusion behavior characteristic of the silicalite/ZSM-5 structure.

Adsorption of Branched and Cyclic Paraffins in Silicalite. 2. Kinetics. C.L.Cavalcante, D.M.Ruthven: Industrial and Engineering Chemistry Research, 1995, 34[1], 185-91