Investigating the effect of acid concentration on the surface and structural properties of HCl-modified natural clay

Abstract

This study examined the changes in the structural and surface properties of Omialafara natural clay upon acid modification, using different concentrations of HCl solution. The changes in the natural clay before and after the acid treatments were studied by different instrumental techniques such as powder XRD, FTIR, BET, and SEM-EDX. The surface acidity of the clays was measured via pyridine-FTIR spectroscopy. The SEM images of the clays revealed different morphological features due to changes in the clay structures upon acid modifications. The specific surface area (SSA) of the clay increased at high concentrations of acid treatment, with the highest SSA value of
528 m²/g obtained at 35% HCl. The high SSA value, which is an important parameter in adsorption studies, was caused by structural changes within the clay matrix upon acid activation. The Si/Al ratios were directly proportional to the SSA values for the modified clays. Measurement of the surface acidity revealed intense adsorption bands at 1580 cm-1 for Brønsted acid sites (BAS) and 1438 and 1483 cm^-1
for Lewis acid sites (LAS) for the acid-modified clays at modest concentrations (15% and 25% HCl). At low (5%) and high (35%) acid  concentrations, weak and low-intensity bands were observed for BAS and LAS. The surface acidities of the unmodified and modified clays showed an inverse proportionality to Si/Al ratios, revealing that high Al content may be required to maintain high acidity in the modified clays for catalysis. While the 35% HCl-modified clay with the highest SSA value (528 m² /g), Si/Al ratio of 5.35, and low surface acidity may be efficient in adsorption studies, the 25% HCl-modified clay, which has a high SSA value of 450 m² /g, Si/Al ratio of 3.81, and a very high surface acidity, may be a suitable catalyst for various acid-catalyzed reactions.