Systematic analysis of the nitrogen adsorption-desorption isotherms recorded for a series of microporous – mesoporous amorphous aluminosilicates using classical methods

Understanding the porous structure of microporous – mesoporous materials is very important for developing useful scientific concepts and principles in materials science, surface science, heterogeneous catalysis, adsorption and related technological applications. Amorphous aluminosilicates are of particular interest in this sense because it is relatively simple to design them with diverse porous structures comprising micro, meso, and even macropores. In this work, we took advantage of the latter and developed a systematic and in-depth analysis of the results of nitrogen physisorption tests at 77 K performed over a series of microporous – mesoporous amorphous aluminosilicates and of the characterization of their texture by classical models to estimate surface area and porosity. The strategy for the analysis consisted of making a thorough description of the features showed by the recorded nitrogen isotherms, first. As a result, a proposal for considering two new types of isotherms, types I(c) and IV(c), and five new types of hysteresis loops, H1(b), H2(c), H3(b), H3(c), and H4(b), in addition to the standard IUPAC classification. These new categories stemmed from the microporous – mesoporous nature of the materials and from the presence of strong network effects. The previous analysis helped interpreting and judging the results of the calculations made with classical methods to assess the texture of the materials; namely, their BET surface area, t-plot microporosity, BJH mesopore size distribution, and fractal dimension. The performed analyses allowed establishing that the relative percentage of microporosity of the materials can be correlated to the physisorption energy as described qualitatively by the CBET constant. Concerning mesopore size distributions, it was found that the BJH method remains to be very valuable for describing the porous structure of the materials particularly if the results obtained with both branches of the isotherms are considered. Finally, it was shown that the fractal dimension can complement the analysis of the porous structure of microporous – mesoporous materials if the latter are compared considering the features of their isotherms and of their hysteresis loops. Overall, the present study can thus be said to make two contributions: (i) it proposes a systematic methodology for analyzing both the raw data and the textural properties calculated by classical methods both derived from nitrogen physisorption experiments. (ii) It presents useful new insights on the texture of microporous – mesoporous materials.