Fast adsorption-desorption kinetics with dead-times of 25 to 35ms were monitored. At sorption equilibrium, a zeolite sample was subjected to small (less than ±1%) pressure changes produced by very rapid, square-wave volume perturbations. By recording the gas pressure response upon compression/ expansion of the system, sorption up-take/desorption processes, which were governed by intracrystalline diffusion, could be followed. By fitting appropriate solutions of Fick's diffusion law to the adsorption/desorption curves, and correcting the as-measured diffusivities using the Darken factor, intracrystalline diffusion coefficients of short-chain hydrocarbons, n-hexane and benzene, were determined. The corrected diffusion coefficients for ethane, propane and hexane were 1.3 x 10-5cm2/s at 298K, 5.0 x 10-6cm2/s at 333K and 2.0 x 10-9cm2/s at 325K, respectively. These were smaller than the self-diffusivities measured directly using NMR pulsed-field-gradient techniques. The corresponding activation energies were 10.5, 7.4 and 17kJ/mol, respectively. The corresponding coefficients for n-hexane and benzene agreed well with those determined using piezometric measurements.

Fast Adsorption-Desorption Kinetics of Hydrocarbons in Silicalite-1 by the Single-Step Frequency Response Method. N.Van-Den-Begin, L.V.C.Rees: Zeolites, 1989, 9[4], 287-92