Apart from clay materials, different types of layered compounds have been tested as heterogeneous catalysts in processes traditionally mediated by homogeneous catalysts, which are in some cases expensive and highly toxic. Reactions such as Michael addition, cyanoethyl — ations of alcohols, aldolic condensations and condensa­tion of nitro compounds with aldehydes and ketones, and ring openings can be used as examples (Centi and Perathoner, 2008).

Layered materials have also been used as solid cata­lysts for biodiesel production through esterification and transesterification. Most applications involving this class of compounds refer to the use of layered dou­ble oxides (LDOs), which are derived from layered dou­ble hydroxides (LDHs) by controlled calcination. LDHs, layered hydroxide salts (LHSs) and layered carboxylates are less commonly used for this purpose. This section presents a brief review of the structure of these layered materials in addition to the description of their use and performance as catalysts.

Layered Double Hydroxides

LDHs are compounds whose individual layers are of brucite-like (Mg(OH)2) structure. In brucite, the layers are electrically neutral with magnesium cations located in the center of an octahedron with six hydroxyl groups in the vertices. The isomorphic substitution of magnesium by trivalent cations forms positively charged layers, which are stabilized by the presence of anions in the interlayer space (Bravo-Suarez et al., 2004). LDHs are represented by the formula [M^M^+tpH^JA!^ nH20, where M2+ and M3+ are divalent and trivalent cations and Am~ represents an anion with an m-charge and x usually has a value between 0.25 and 0.33 (Crepaldi and Valim, 1998).

In this work, the chemical composition of a specific LDH will be condensed as M2+/M3+-A. Thus, an LDH whose layers contain Mg and Al and the counterion between the layers is nitrate will be written as Mg/ Al-N03.

0ne peculiar characteristic of LDHs is the memory ef­fect. Calcination of Mg/Al or Zn/Al LDHs forms mixed and nanostructured mixed metal oxides described as LD0, which are able to reassemble the LDH structure if added back to an aqueous solution containing a salt whose anions will be intercalated between the layers in order to stabilize the LDH structure (Carlino, 1997). These anions can be different from the anions found in the original LDH. This kind of materials can substitute basic homogeneous catalysts like ammonia, ammonium salts or amines and offer an option as nonpollutant solid catalysts that can be easily separated from the reaction system and recovered. Their catalytic activity is related to the large surface area of LDHs, its solid base character and layered morphology. For instance, Zn/Al LDH con­taining nitrate, sulfate or orthophosphate anions have catalytic activity in esterification reactions (Hu and Li, 2004), while Mg/Al LDH intercalated with t-butoxide is active in transesterification (Choudary et al., 2000).