The vapor permeances of pure aromatics through silicalite zeolite membranes followed the relative order, m-xylene > p-xylene > benzene ≈ toluene > ethylbenzene ≈ o-xylene, near to 380K; with permeance ratios as high as 20. This trend did not mirror the size-order of the kinetic diameters; therefore the kinetic diameter was not the determining factor for permeation. The activation energies for the aromatics varied from 13 to 57kJ/mol (table 30). In binary and ternary mixtures, the faster-permeating compounds were slowed to rates similar to those of slower permeating molecules; thus, no separation was obtained for any mixture. The findings were consistent with single-file transport, where all molecules had an equal chance of entering the pores, but could not pass each other in the narrow channels. The slowest species therefore determined the permeation rate.

Permeation of Aromatic Hydrocarbon Vapors through Silicalite−Zeolite Membranes. C.D.Baertsch, H.H.Funke, J.L.Falconer, R.D.Noble: Journal of Physical Chemistry, 1996, 100[18], 7676–9