Most of the clay minerals have low Bronsted and Lewis acidity. The Bransted acidity and surface area can be slightly improved during drying when interstitial and intercalated cation hydration water molecules are removed. Another way to improve the acidity is to exchange the intercalated species with highly polarizing species such M3+ cations, where the hydrolysis of the solvated water molecules release protons.

The Lewis acidity is normally associated with exposed Al3+ or Fe3+ at the broken crystalline edges, which can be increased by heating the clay material to temperatures above 300 °C or by acid treatment (Moronta, 2004). As the thermal treatment can lead to the collapse of the clay mineral structure, the acid treat­ment is the most effective way to improve both Lewis and Bransted acidity. This procedure was already
described in the 1960s (Ryland et al., 1960), when acid-modified smectites provided high gasoline yields when used as a petroleum-cracking catalyst.

Acid activation of clay minerals using mineral acids is not new and this procedure causes disaggregation of clay particles, elimination of impurities and improve­ment of their surface area, porosity and catalytic proper­ties. Acid-activated clays are broadly spread in different industrial processes, being especially used as bleaching agent and catalysts. Although this process was found to be dependent on several factors such as the type of mineral, its crystallinity, morphology and particle size, the effects of the selective acid leaching of a 2:1 clay min­eral, which is a first-order process, can be schematically seen as shown in Figure 16.2.

The treatment of clay minerals with mineral acids at room temperature tends to replace the intercalated cat­ions by hydrated protons and/or by the leached cations, improving Bransted acidity while the structure is mostly preserved (Figure 16.2(b)). By contrast, the thermal acid activation has several effects on the structure of the clay minerals, which depend on the temperature, time of treatment and concentration and strength of the acid used. Under mild temperatures, times and acid con­centration, the first effect is usually the removal of acid-soluble impurities and partial leaching of the octa­hedral coordinated metals from the octahedral sheet

(Figure 16.2(c)). During this process, not only the Lewis acidity is generated but also the anions of the acid can be incorporated in the structure.

As clays having Mg or Fe in their structure are more easily leached than octahedra occupied by Al, the acid activation must be optimized to extract the best charac­teristic of each clay mineral. Under extreme conditions, regardless of the type of clay mineral, all the octahedral metals are removed to produce inactive hydrated fibrous silica as reported previously (Figure 16.2(d)) (Wypych et al., 2005). The effect of an effective acid acti­vation is the broadening of the basal X-ray diffraction peak due to the damage of the layers and, finally, to the total collapse of the structure. The physical effects of this activation process are the improvement of the surface area and pore volumes up to a specific point from which these properties are decreased. The pore radii are also constantly reduced during the treatment. For each acid and activation conditions, a new optimiza­tion needs to be reported since each clay mineral has a characteristic that depends not only on the mineralog — ical classification but also on the mine from which it was extracted.

As an example for montmorillonites (Wilson and Clark, 2001; Zatta et al., 2012), Figure 16.3 shows a possible mechanism for the formation of Bronsted and Lewis acid sites after mineral acid treatment of a 2:1 clay mineral constituted basically of Al in the octahedral sites and Si in the tetrahedral sites.