Uptake rates for o, m and p-xylene and ethylbenzene in 100μm crystals of NaX and 250μm crystals of natural faujasite were measured at 130 to 200C by using the gravimetric method. The uptake curves conformed quantitatively to the diffusion equation and the time constants, measured under comparable conditions for both adsorbents, were in the ratio of the square of the crystal radii; thus confirming intracrystalline diffusion control, with no significant difference in diffusivity between the two materials. This suggested that the diffusivity was controlled by the zeolite framework rather than by any specific interaction with the cations. The diffusivities increased strongly with sorbate concentration, but this effect appeared to be due entirely to the non-linearly of the isotherms. The corrected diffusivities were essentially independent of concentration. There was little difference in diffusivity between the four isomers (all were of the order of 6 x 10−9cm2/s at 443K) and the activation energies were all about 26kJ/mol. Although the activation energy was consistent with NMR data, the actual diffusivities were much lower.

Sorption and Diffusion of C8 Aromatic Hydrocarbons in Faujasite Type Zeolites. II. Sorption Kinetics and Intracrystalline Diffusivities. M.Goddard, D.M.Ruthven: Zeolites, 1986, 6[4], 283–9. See also: Zeolites, 1988, 8[4], 258–9