Synthesis and characterization of montmorillonite clays with modulable porosity induced with acids and superacids

Abstract

The structural transformation of natural montmorillonite clay with different acids and superacids (hydrochloric, HCl; sulfuric, H2SO4; hydrofluoric, HF; perchloric, HClO4; chlorosulfonic, HSO3Cl; and trifluoromethanesulfonic, HSO3CF3 acids) is evaluated by various techniques, allowing its full characterization. The process involves the modulated digestion of the aluminum and silicon atoms from the original clay without the loss of the original mass in the material from a careful drying methodology. The x-ray diffraction technique suggests that the process is disruptive, causing different magnitudes of modification, depending on the acidic source, taking the material with natural lamellar structure as reference. By 27Al and 29Si magnetic resonance, it is also shown that all the silicon and aluminum atoms were removed from the chemical structure to be physisorbed or dispersed in the interstices of the clay lattice when HF, HSO3Cl, and H2SO4 acid treatments were carried out. However, with treatment of HClO4, the results suggest that only was achieved partial digestion and physisorption of the aluminosilicate core. Treatment with HCl led to a mild digestion that increased the laminar distance of the montmorillonite. Finally, HSO3CF3 acid achieved a total disorder of the lamellar structure of the clay. In addition, by infrared spectrometry, the disappearance of the absorption peaks assigned to Al2OH, AlMgOH, and Al-O-Si are other evidence of the complete digestion of silicon and aluminum atoms caused by the acidic medium. On the other hand, the Brunauer–Emmett–Teller (BET) surface areas of the acidified clays showed a great enlargement in comparison with the unmodified clay. The porosimetry measurements demonstrate the presence of mesopores and hysteresis as observed in the physisorption isotherms, which indeed exhibited a capillary condensation into the porous structures. The present results are the first full characterization research and structural comparison between the acidic and the not-extensively-studied superacidic modified clays of this type.