Systematic Analysis of the Nitrogen Adsorption–Desorption Isotherms Recorded for a Series of Materials Based on Microporous–Mesoporous Amorphous Aluminosilicates Using Classical Methods

Amorphous aluminosilicates are applied in adsorption and catalysis because they can be designed to possess micro-, meso-, and macropores. In this work, we analyze the nitrogen physisorption isotherms recorded for a series of materials based on amorphous aluminosilicates synthesized by sol–gel via hydrolysis in acidic medium (ACSG), sol–gel with a poly(ethylene glycol) template (P), and sol–gel with gel skeletal reinforcement (GRS). We found that the porosity of the ACGS and GRS types of materials was dominated by mesopores, while that of the P-type materials was dominated by micropores but with a non-negligible fraction of mesopores. We catalogued the isotherms produced by the latter type of materials as type IV(c) as a supplement to the current IUPAC classification. In addition, we found that the hysteresis loops shown by the amorphous aluminosilicates of the ACGS-type materials present inflection points not found in the current IUPAC classes; therefore, we propose classifying them as H3(b) types. We also assessed the surface area and porosity of the materials by classical methods, namely, the Brunauer–Emmett–Teller (BET) surface area, t-plot microporosity, Barrett–Joyner–Halenda (BJH) mesopore size distribution, and fractal dimension. First, we found a semiexponential correlation between the CBET constant of the materials and their relative fraction of the microporous surface area. Second, we found that under the conditions used herein, the impregnation of a NiMo phase over the synthesized amorphous aluminosilicates increased the CBET constant and reduced the fractal dimension of the materials. These changes were thus correlated to the changes observed in the relative microporosity and mesoporous size distributions of the materials.

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