Specific surface area and I-point evidence for microporosity in nanostructured MSU-S aluminosilicates assembled from zeolite seeds

Abstract

In this work we provide evidence of microporosity in the nanostructured mesoporous aluminosilicates MSU-S/W_(BEA) synthesized from zeolite BEA seeds, based on the so called I-point plots (refs. 18–20) of the BET equation. Seventeen solids were characterized for their porous characteristics as follows: (i) one mesoporous aluminosilicate 2%Al-MSU-S/W_(BEA), (ii) two typical mesoporous MCM-41 materials, one pure silica and one aluminosilicate 2%Al-MCM-41, (iii) three typical microporous zeolites, i.e., zeolites Y, ZSM-5 and beta, (iv) two typical amorphous silica–alumina (ASA) samples, (v) five mechanical mixtures of mesoporous Al-MCM-41 with zeolite beta at different addition percentages, and (vi) four amorphous or partially crystalline ZSM-5 based aluminosilicates. The P,V data from the nitrogen-adsorption isotherms of those porous solids were plotted according to the I-point modification of the BET equation in the form V[1 − (P/P₀)]/(P/P₀) = CV_m − (C − 1)V[1 − (P/P₀)]. Plots of V[1 − (P/P₀)]/(P/P₀) vs. V[1 − (P/P₀)] provided lines which have the shape of an inclined V, i.e. >, and variable slopes corresponding to (C − 1) from which the values of the C parameter were determined for the whole range of partial pressures 0 < P/P₀ < 1. The inversion point of the above plots, called the I-point, projected onto the V[1 − (P/P₀)] axis corresponds exactly to the volume of the monolayer V_m = V_I[1 − (P_I/P₀)], from which the value of the specific surface area S_I (m² g⁻¹) was determined. The shape of the I-plots depended strongly on the type of porous solids, i.e. microporous/zeolitic and partially crystalline zeolitic materials with combined micro- and meso/macroporosity; ordered mesoporous MCM-type materials; ASA materials with random mesoporosity; and mechanical mixtures of zeolite beta with MCM-41. The I-plot for the MSU-S/W_(BEA) solid exhibited the combined characteristics of the plots of ordered mesoporous MCM-41 samples and of partially crystalline zeolitic materials, thus proving the presence of microporous domains within the homogeneous mesoporous structure of the MSU-S material. The I-plot result supports the proposed presence of protozeolitic subunits in the framework walls, of MSU-S, even though XRD and electron diffraction TEM studies have shown the material to be free of any zeolitic crystalline phase. The surface areas estimated by the I-method were similar to those provided by the BET model for the zeolitic and the ASA samples, while in the case of the ordered MCM-41 and MSU-S/W_(BEA) mesostructures, two surface area values can be estimated based on different interpretations of the I-method, one similar to the value from the BET model and one supported by the modified CPSM model (refs. 31–34).