LDH Interlayers

The structure of the interlayer galleries is discussed in detail in Sect. 3.2. 

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Figure 9.9. [2 + 2] photodimerization pathways within the LDH interlayer. Simplified drawing of anti-parallel packing of (a) />-phenylcinnamate and (b) stilbenecarboxylate anions (SBC), (c) Effect of coadsorbate, />-phenylbenzoate (PEB), on product selectiv-ities of intercalated SBC in water. A formation of head-to-head dimers. B formation of head-to-tail dimers. C isomerization to cis-SBC. After Sasai et al. [110]. 

Figure 3.9. Hypothetical reaction process illustrating transfonnation of an initially precipitated Ni-Al LDH into a phyllosilicate-like phase during aging. The initial step involves the exchange of dissolved silica for nitrate within the LDH interlayer followed by polymerization and condensation of silica onto the octahedral Ni-Al layer. The resulting solid possesses structural features common to 1 1 and 2 1 phyllosilicates. (From Ford et al., 2001.)...

In situ polymerization. The monomer anion, such as acrylate, is first intercalated into the LDH interlayer and then polymerized in situ within the interlayer spacing (223,224). 

Unsaturated organic anions can undCTgo photochemical cycloaddition within the LDH interlayer, provided that the distance between them is suitable. Tagaki et al. examined the cycloaddition of two kinds of unsaturated carboxylates intercalated in the MgAl-LDH (701-703). They found that (a) the conversion is higher than that of reaction without LDH (b) the addition is very stereoselective for head-to-head isomers. In conttast, photoaddition without LDH is nonselective, giving both head-to-head and head-to-tail isomers in approximate amounts. 

Ion exchange synthesis method is suitable when the coprecipitation synthesis method cannot be applied, in cases in which the divalent and trivalent metal cations or anions, which participate in the synthesis, are unstable in alkaline environment or have greater affinity to guest ions. Therefore, this synthesis method is used for preparing LDH materials with interlayer anions that are different from CO " due to the higher affinity of these anions to be incorporated into the LDH interlayer [20]. Earlier studies made an observation that the treatment of LDH with diluted HCl did not obstruct its structure, but lead to the formation of new LDHs with different crystal lattice parameters and to the substitution of anions with CL ions [2]. Recent studies claim that for the COj anion substitution in the interlayer the most suitable anions are chlorides, nitrates, sulfates, and bromides [20]. 

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. 

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