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 coupl ed with isothermal volumetry at 25 degrees C. Whatever the zeolite, th e enthalpies of adsorption of p-xylene and m-xylene at low filling wer e 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 (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 a lpha-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 zeol ites. Some arguments concerning the structures of the zeolite and xyle ne molecules can explain such behaviour. Whatever the zeolite, the ads orption capacity of the alpha-cages was 3.5 molec.alpha(-1). For relat ive pressures ranging from 0.2 to 0.5, an additional adsorption of abo ut 0.1 molec.alpha(-1) occurred on the external surface.