Synthesis of LDHs

LDHs can be synthesized by various techniques, such as coprecipiation, 

Structure reconstruction, sol-gel method, anion-exchange method, and so on. Detailed descriptions of the processes are available in the literature and reviews. However, this does not mean that it is easy to prepare pure LDHs rather, it means that different techniques may be adopted to obtain the desired LDH compositions. Among these different preparation methods, coprecipitation is the most common and useful method to prepared large amounts of LDHs. 

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Theoretical calculations [43] based on first principles molecular dynamics discussed in Sect. 3.2.6 have suggested that Mg Al LDHs are most stable for n = 3 (i.e. x = 0.25) and indeed many minerals, including hydrotalcite itself, have this stoichiometry [4]. It has been reported that the synthesis of LDHs (with benzoate or terephthalate anions in the interlayers) from solutions containing Mg/Al = 2, leads to LDHs having the same composition when the synthesis is carried out at moderate temperatures but LDHs with Mg/Al = 3 (plus AlOOH) when the reaction is carried out under hydrothermal conditions [44]. It was proposed that the latter ratio represents the thermodynamically most favorable product. A similar observation has been reported [45] for solutions with Ni VPe = 2, where hydrothermal preparation led to segregation of an LDH with Ni VPe = 3 and Ni Fe 204. An attempt to synthesize a Co sAl LDH resulted in partial oxidation of the Co and formation of a Co o.yCo o.s LDH with complete migration of Al " from the layers to generate interlayer aluminum oxy-species [46]. 

Abstract Layered double hydroxides (LDHs) comprise au extensive class of materials that are very easy to synthesize in the laboratory, albeit not always as pure phases. In this chapter, we review the wide variety of methods that are available for the synthesis of LDHs and focus on the way in which the physicochemical properties of the materials (such as phase piuity, crystallinity and surface area) vary with synthesis method. The flexibility of the different methods is also discussed some methods can be used to synthesize LDHs containing a wide range of constituent cations and anions, whilst others are more limited in scope. In some cases, the potential for scale-up of a method to produce larger quantities of material is also noted. 

Although simple to prepare in the laboratory in principle, LDHs are not always easy to synthesize as pure phases. In the second chapter of this volume He et al. review methods of synthesis of LDHs, with an emphasis on the way in which the physicochemical properties of the materials vary with the synthesis method. 

The PXRD patterns for both the LDH and MH are shown in Figure 1. The LDH showed a PXRD pattern with very intense and sharp peaks, characteristic of a very well ordered hydrotalcite-like compound. The basal spacing obtained was 7.68 A, very close to the reported data [17,20,21] for carbonate containing LDH. From the chemical analysis we obtain the following molecular formula Zn2 9Cr(OH)7 8(C03)o5-2,3H20 (normalized to Cr = 1). The Zn Cr ratio obtained, 2.9, was slightly lower than the one expected, 3, indicating a preferential solubilisation of Zn(II) cations, a feature widely reported in the synthesis of LDHs [11]. 

Hydrothermal synthesis involves the hydrothermal treatment of an aqueous suspension of two metal oxides in a pressurized vessel at a high temperature for a few days [7]. With this method, the synthesis of LDHs is obtained leading to the conversion of small LDH crystallites to larger and well-defined crystals. Hydrothermal synthesis method, also known as hydrothermal crystallization, is used when precise LDH structural properties are required because it enables the transformation of the amorphous precipitates to the crystalline LDH form. According to hterature, with this synthesis method the crystallization of an amorphous trivalent metal oxide (M2 "Oj) precursor in the presence of a suitable divalent metal oxide (M"0) is achieved [7]. 

Specific property is called the memory effect and is often used for the synthesis of LDHs that have anions in the interlayer different to... 

Application of LDHs is mostly based on their use after thermal treatment and mixed oxide formation. If the calcination is performed at temperatures below 550 °C, mixed oxides also have, besides the aforementioned properties, the memory effect. This very specific property of mixed oxides derived from thermal degradation of LDHs allows the reconstruction of the layered structure in mild conditions when mixed oxides are in contact with aqueous solution or air. If calcination is carried out at temperatures above 827 °C, irreversible mixed spinels are formed and the memory effect is disabled [48]. The main application of the memory effect is for the synthesis of LDHs with different interlayer anions than CO ". Taking to consideration that carbonate anions have the highest affinity toward the incorporation in the LDH interlayer, during the classical synthesis methods the contamination with carbon dioxide from the air always occurs. If, for example, the synthesis LDH with OH" ions in the interlayer is required, the reconstruction of mixed oxides can be performed by steam or contact with decarbonized water. Similarly, if synthesis of LDHs with other anions in the interlayer is anticipated, reconstruction is carried out in an aqueous solution containing the desired anions. Catalytic properties of mixed oxides obtained by reconstruc-tion/recrystallization procedure depend mainly on the conditions of each activation step. 

From the above discussion, it can be seen that the method adopted to prepare nanocomposites is highly dependent on the nature of the polymer. When the polymers or monomers are water soluble, they can be incorporated into the pristine LDH without any organo-modification due to their good affinity with the LDH. Additionally, the aqueous environment is compatible with the condition for the synthesis of LDH materials. Therefore, water-soluble polymer-LDH nanocomposites can be prepared using some special methods such as in situ synthesis, ion exchange and reconstruction. In the case of water-insoluble polymers and monomers, their nanocomposites are usually prepared in orga-nosolvent (solution intercalation method, exfoliation-absorption method and in situ polymerization method) or molten polymer (melt intercalation method). However, emulsion polymerization and suspension polymerization are methods that allow the incorporation of a water insoluble polymer into an LDH in water. The following sections are devoted to polymer-LDH nanocomposites obtained via emulsion polymerization and suspension polymerization. 

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