Determination of micropore volume and external surface of zeolites

Original and thermally degraded EAU type zeolites were characterized by nitrogen adsorption at 77 K and compared with isotherms for amorphous silica from literature. The isotherms were evaluated by the method of Remy and Poncelet which is an additive composition of the Dubinin-Radushkevich and the BET equation. Nonlinear fits typically generate two optimal values. One is attributed to very low, the other to very high values of the C-constants of the BET part. The conversion of the DR-isotherm to a set of Langmuir isotherms with different affinities shows that the low value is correct. For comparison all data were compared with those obtained by the t-plot method. The t-plot generally yields a less good fit with the experimental data, lower micropore volumes and deviating external surfaces, especially in the case of thermally degraded zeolites with an enhanced degree of mesopores. The entire isotherms from p/p(0) = 0.5 to 0.98 are covered by an additive combination of the Dubinin-Radushkevich equation and the general Aranovich equation. That equation, however, fails for the determination of the external surface. (C) 2016 Elsevier Inc. All rights reserved.