Authors: (P. Schneider, P. Hudec and O. Solcova1 - Institute of Chemical Process Fundamentals, Czech Academy of Sciences, Czech Republic and others)
Abstract: Textural analysis of zeolites is usually based on the analysis of physical adsorption of inert gases (e.g. nitrogen or krypton at 77K or argon at 87K). The simultaneous presence of micropores and mesopores in zeolites makes the textural analysis based on transformation of physical adsorption isotherms of inert gases much less straightforward than in case of porous solids with mesopores only. The simple BET analysis  is, nevertheless, usually (and incorrectly) performed. The reason is that the simple BET isotherm was developed explicitly for non-microporous (mesoporous) solids and in the relative pressure range, xBET, which guarantees no capillary condensation in pores (e.g. 0.05 < xBET < 0.25; x = p/po with p the adsorbate pressure and po the saturation adsorbate pressure at the measurement temperature)). Basic information required for zeolites is the volume of micropores, Vμ, and the specific surface area of mesopores, Sm. The correct way is to use the comparative plots (t-plots, α- plots) which can supply the micropore volume as well as the mesopore surface area. The complication with comparative plots arises from the requirement to know the adsorption isotherm for a non-porous solid with same chemical character as the analyzed porous sample (standard/master isotherm). The generalized standard isotherms of Lecloux and Pirard , a = a(x, C), is also of less use because the required C parameter obtained by application of the BET equation is invalid when micropores are present.