Just a small number of papers have been devoted so far to the use of acid-activated clay minerals in the cata­lytic esterification of fatty acids (Vijayakumar et al., 2005;

Konwar et al., 2008; Nascimento et al., 2011; Neji et al., 2011; Zatta et al., 2012,2013; Rezende et al., 2012; Olowo — kere et al., 2012), as well as in the transesterification of oils and fats (Bokade and Yadav, 2009). Some specific cases will be evaluated in sequence.

Case 1 (Zatta et at., 2012)

Standard Texas Montmorillonite STx-1 with the chemical formula ((Ca0.27Na0.04K0.01)[Al2.41Fe(III)0.09

Mg0.71Ti0.03]Si8.00O20(OH)4), supplied by the Clay Min­eral Society repository, was activated using phosphoric, nitric and sulfuric acids under different conditions of temperature, time and acid concentrations and the resulting materials were characterized by X-ray diffrac­tion (XRD), nitrogen adsorption isotherms and Fourier transform infrared spectroscopy. Also, the presence of Lewis and Bronsted acid sites in the structure of the cata­lyst was characterized by pyridine adsorption. After­ward, the materials were evaluated as catalysts in the methyl esterification of lauric acid. Blank reactions car­ried out in the absence of any added catalyst presented conversions of 32.64, 69.79 and 79.23% for alcohol:lauric acid molar ratios of 60:1,12:1 and 6:1, respectively. In the presence of the untreated clay and using molar ratios of 12:1 and 6:1 with 12 wt% of catalyst, conversions of 70.92 and 82.30% were obtained, respectively. For some key samples obtained by the acid activation, conversions up to 93.08% of lauric acid to methyl laurate were ob­tained, much higher than those observed for the thermal conversion (TC) or using raw montmorillonite. Relative good correlations were observed between the catalytic activity and the development of acid sites and structural and textural properties of the acid-leached materials.

Case 2 (Zatta et at., 2013)

The same sample of montmorillonite STx-1 described above was submitted to acid activation using aqueous solutions of phosphoric acid. The acid treatment was carried out under vigorous stirring at 100 °C in a flat- bottomed flask connected to a reflux condenser and a heating mantle. The mineral clay and the acid solution were mixed in a 1:4 ratio (mass per volume) using acid concentrations of 0.5, 1, 2 and 4 mol/l.

After the acid activation process, the samples were repeatedly washed with distilled water until pH close to 7, dried at 110 °C for 24 h and then heated in an oven at 250 °C for 2 h. To check the influence of other acids in the activation of montmorillonite STx-1, this clay mineral was subjected to activation with hydrochlo­ric (37% proof), nitric (65% proof) and sulfuric (98% proof) acids. The resulting acid-activated clay materials were characterized and subsequently used in the cata­lytic conversion of lauric, oleic and stearic acids, as well as of a complex mixture of fatty acids (tall oil) to their corresponding fatty acid methyl esters (FAMEs).

The results obtained for the best phosphoric acid — activated sample (PA) were compared to those of the TC and from a standard commercial Lewis acid catalyst (K10) and raw montmorillonite (STX). In all experi­ments, conversion of all samples were carried out for 2 h at a methanol:fatty acid molar ratio of 12:1 and 160 °C with 12 wt% of the catalyst in relation to the oil mass.

In general, the PAs and the standard Lewis acid cata­lyst (K10, Sigma—Aldrich), which is produced by HCl activation of mineral clays at boiling temperatures, had similar catalytic activities. However, in some cases, the catalytic performance of PA was even better than that of K10 (Figure 16.4). These data were a strong indication that PA and K10 have similar chemical and physical characteristics, even though part of the layered structure is still retained in PA after acid activation.

Tests of reuse of the best PA were performed (Figure 16.5) and no significant losses of activity were observed during the first four consecutive reaction cy­cles (see dotted horizontal line in Figure 16.5). This is a very important observation since, from any practical use in industrial processes, the catalysts must last for long time before deactivation.