Permeation of aromatic hydrocarbon vapors through silicalite-zeolite membranes

Vapor permeances of pure aromatics through silicalite zeolite membranes follow the relative order m-xylene > p-xylene > benzene approximate to toluene > ethylbenzene approximate to o-xylene near 380 K with permeance ratios as high as 20. This trend does not follow the size order of the kinetic diameters, and therefore the kinetic diameter is not the determining factor for the permeation. Activation energies for the aromatics varied from 13 to 57 kJ/mol. In binary and ternary mixtures, the faster permeating compounds were slowed to rates similar to slower permeating molecules, and thus no separation was obtained for any mixture. These findings are consistent with single file transport, where all molecules have equal chances of entering the pores, but they cannot pass each other in the narrow channels and the slowest species determines the permeation rates.