EFFECT OF WATER-CONTENT ON THE SELECTIVE COADSORPTION OF GASEOUS P-XYLENE AND M-XYLENE ON THE BAY ZEOLITE

The selective coadsorption of gaseous p-xylene and m-xylene is studied at 150 degrees C on BaY with a water content which varies up to 3.5% in weight. The preadsorbed water seems to be located only in the alpha -cages, probably around the compensation cations in sites II. The wate r, which is the more strongly adsorbed component, is not replaced duri ng the coadsorption of xylene's, and this leads to a weakening of the interactions between the aromatic ring and the compensation cation. As for the anhydrous zeolite, two selective adsorption processes are dis cerned according to the filling. At low filling, BaY adsorbs preferent ially the more abundant isomer in the adsorptive mixture, and the pres ence of water has no significant effect on the selectivity except for mixtures rich in p-xylene with which the selectivity is up to three ti me as great as that with the anhydrous zeolite. At high filling, BaY i s selective for p-xylene whatever the composition of the adsorptive mi xture, and the higher the water content, the higher is the selectivity for p-xylene. Obviously the enhancement of the selectivity caused by water preadsorption is to the detriment of the adsorption capacity of alpha-cages, which is decreased up to 20%.