Specific Surface Area Determination for Microporous/Mesoporous Materials: The Case of Mesoporous FAU-Y Zeolites

A methodology for determining the micropore, mesopore, and external surface areas of hierarchical microporous/mesoporous materials from N₂ adsorption isotherms at 77 K is described. For FAU-Y zeolites, the microporous surface area calculated using the Rouquerol criterion and the Brunauer-Emmett-Teller (BET) equation is in accord with the geometrical surface determined by the chord length distribution method. Therefore, BET surface area ( S_BET) is the well representative of micropore surface areas of microporous materials and of total surface area of microporous/mesoporous materials. Mechanical mixtures of mesoporous MCM-41 and microporous FAU-Y powders of known surface areas were used to calculate the respective surface areas by weighted linear combination and the results were compared to the values obtained by the t-plot method. The first slope of the t-plot determined the mesopore and external surface areas ( S_mes+ext). The linear fit of the first slope is in general in the range 0.01 < p/ p₀ < 0.17 and contains the volumes and relative pressures at which all micropores are filled ( p/ p₀ > 0.10). Overestimation of S_mes+ext values was evident and appropriate corrections were provided. External surface areas ( S_ext) were obtained from the second slope of the t-plot, without noting an overestimation of S_ext, thus allowing the determination of mesopore surface areas ( S_mes) by difference. Micropore surface areas were calculated by subtracting S_mes+ext from the total surface area, S_BET. As an example, this methodology was applied to characterize a family of hierarchical microporous/mesoporous FAU-Y (FAUmes) synthesized from H-FAU-Y (H-Y, Si/Al = 15) using C18TAB as the surfactant and different NaOH/Si ratios (0.05 < NaOH/Si < 0.25). By increasing the NaOH/Si ratio in the synthesis of FAUmes, it was shown that as the micropore surface area decreases, the mesopore surface area increases, whereas the micropore and mesopore surface area remains constant. This methodology allows accurate characterization of the surface areas of microporous/mesoporous materials.