APPLICATION OF LARGE-PORE MCM-41 MOLECULAR-SIEVES TO IMPROVE PORE-SIZE ANALYSIS USING NITROGEN ADSORPTION MEASUREMENTS

MCM-41 siliceous molecular sieves were used to test the applicability of the Kelvin equation for nitrogen adsorption in cylindrical pores of the size from 2 to 6.5 nm. It was shown that the Kelvin equation for the hemispherical meniscus, corrected for the statistical film thickne ss, is in quite good agreement with an experimental relation between t he pore size and the capillary condensation pressure. The agreement ca n be made quantitative in the pore size range from ca. 2 to 6.5 nm, if a simple correction to the Kelvin equation is introduced. The require d statistical film thickness curve (t-curve) was calculated using nitr ogen adsorption data for large pore MCM-41 samples and the obtained re sults were extrapolated using an adsorption isotherm for a macroporous silica gel. Moreover, an accurate analytical representation of the t- curve was found. Since both the corrected Kelvin equation for cylindri cal pores and the t-curve have simple analytical forms, they can conve niently be used in a variety of methods to evaluate porosity. It was s hown that the BJH method with the corrected Kelvin equation accurately reproduces pore sizes of MCM-41 materials. A comparison was made betw een the specific surface areas for the MCM-41 samples calculated on th e basis of the BET equation and those obtained using other independent methods. The results strongly suggest that when nitrogen adsorption d ata are used, the BET method overestimates the specific surface area o f siliceous materials. The latter conclusion was supported by the exam ination of the obtained statistical film thickness curve.