Two different clay minerals (namely BBn and VLn), mined in Boa Vista District of Paralba (Brazil), were acid-activated using a 10 wt% suspension of clay in 4mol/l sulfuric acid at 90 °C for 2 h to produce the BBa and VLa solids, respectively. The solids were filtered under reduced pressure and washed with distilled water until the washing water had the same pH of the original material. The material was dried in
an oven at 110 °C for 24 h and finally ground until pas­sage through a 60 mesh sieve. The surface area of the acid-activated clays was improved from 71 m2/g to 238 m2/g for the VLn sample and from 123 m2/g to 170 m2/g for the BBn sample. The catalytic activity of these compounds was evaluated in the esterification of different organic acids, using different acylating agents and reflux conditions. In the case of methyl and hexyl esterification of lauric acid (Figure 16.6), the sulfuric acid activation of both clays greatly improved their cat­alytic activity and this was also valid for other acids and acylating agents. As reported in Case 2, acid activation led to catalytic activities higher than the standard Lewis acid catalyst (K10).

In conclusion, clay minerals are cheap inorganic ma­terials that are readily available worldwide, environ­mentally friendly and suitable for the development of reusable acid catalysts for the esterification of fatty acids and the transesterification of oils and fats. The natural acidity can be improved by thermal treatment and selec­tive acid activation. Depending on the clay minerals’ origin and genesis, different chemical compositions are possible and different acid treatments are needed to optimize the acidic properties. Normally, the catalysts can be used in several consecutive reaction cycles and, after deactivation, the residual solids can be easily disposed of or even incorporated in native clays for the production of ceramic materials, bricks and roofs, as well as in the production of porcelains.