DIFFUSION OF C-8 AROMATIC-HYDROCARBONS IN SILICALITE

Diffusion of p-xylene, ethylbenzene, and o-xylene in large (150 x 50 x 40-mu-m) crystals of silicalite has been studied by the zero-length column (ZLC) method and by gravimetric uptake rate meaurements. The form of the uptake curves suggests that diffusion occurs primarily through the straight channels of the silicalite pore structure so both ZLC and uptake rate curves were interpreted on the basis of diffusion in a parallel-sided slab. The experimental data obtained by both techniques are consistent. The diffusivity of p-xylene is almost the same as that of benzene and about an order of magnitude larger than that of o-xylene, but there is very little difference in activation energy (approximately 30 kJ/mol). The results, therefore, do not support the commonly accepted view that the difference in diffusivity between the ortho and para isomers results from the difference in their critical molecular diameters.