A NEW APPROACH TO THE DETERMINATION OF THE EXTERNAL SURFACE AND MICROPORE VOLUME OF ZEOLITES FROM THE NITROGEN ADSORPTION-ISOTHERM AT 77-K

Nitrogen adsorption isotherms have been carried out on Y zeolites and dealuminated Y zeolites, mordenites and dealuminated mordenites, and Z SM-5 zeolites. The treatment of these isotherms by the Dubinin-Radushk evich method has led to a first estimation of the microporous volume o f these zeolites. The t method has been used to determine their extern al surface and to obtain estimation of the microporous volumes. It is shown that the Dubinin-Radushkevich method overestimates the microporo us volume of solids with important external surface. It is also shown that the choice of the standard t function and the part of the V-t cur ve considered for the Linear fitting may drastically influence the mic roporous volume and the external surface obtained by the t method. A n ew way of treating the N-2 adsorption isotherm to establish the microp orous volume and the external surface of zeolites is proposed. This me thod assumes that the experimental adsorption isotherm may be decompos ed into two parts corresponding, respectively, to the adsorption on th e external surface and to the adsorption in the micropores. It is cons idered that the BET equation governs the adsorption on the external su rface, while the adsorption in the micropores can be described by the Dubinin-Radushkevich equation. The correlation coefficient of the BET curve obtained from the experimental isotherm corrected for the adsorp tion in the micropores is optimized by adjusting the limiting adsorpti on volume and the adsorption energy of the Dubinin-Radushkevich equati on. The C-BET, the microporous volumes, and the energy of adsorption i n the micropores obtained in this way are consistent with the data of the literature.