Differential calorimetric enthalpies of adsorption of p-xylene and m-xylene on Y faujasites at 25 degrees C

The differential enthalpies of adsorption of p-xylene and m-xylene on NaY, KY and BaY zeolites were measured by isothermal calorimetry coupled with isothermal volumetry at 25 degrees C. Whatever the zeolite, the enthalpies of adsorption of p-xylene and m-xylene at low filling were of the same order of magnitude. They did not show significantly the effect of the dipolar moment of m-xylene. Their absolute values varied in the sequence Qo(pX/NaY)<Qo(pX/KY)=Qo(mX/KY)<Qo(pX/BaY)<Qo(mX/NaY) = eo(mX/BaY) During the adsorption of the third molecule of xylene per alpha-cage, BaY zeolite exhibited specific behaviour: the differential enthalpies of adsorption decreased with the filling of the alpha-cages in such a way that they became lower than those of the other two zeolites. Some arguments concerning the structures of the zeolite and xylene molecules can explain such behaviour. Whatever the zeolite, the adsorption capacity of the alpha-cages was 3.5 molec.alpha(-1). For relative pressures ranging from 0.2 to 0.5, an additional adsorption of about 0.1 molec.alpha(-1) occurred on the external surface.