Hydrotalcite clays

Hydrotalcite can be produced by a complete synthetic procedure (2). It is intended to replace heavy metal containing stabilizers. 

The stabilization activity results from the capacity of the layered double hydroxides to react with the HCI formed during the degradation of PVC. The reaction between the layered double hydroxides and the HCI occurs in a two-step process (3)  

The counterions between the layers tend to react with the HCI gas, and 

The layers themselves react with the HCl imder complete destruction of the layered structure and the formation of metal chlorides 

The special property when used as nanofiller materials is their thermal decomposition behavior, which makes them also interesting as a potential flame retardant for poljnners (4). 

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Hydrotalcite clays, for example, are built up of positively charged brucite layers, for reviews see Cavani et al. (1991). Upon calcination they become active as solid bases in e.g. aldol and Knoevenagel condensations (see Fig. 2.26) (Fgueras et al., 1998 Corma and Martin-Aranda, 1993 Climent e/a/., 1995). 

The hydrotalcite clays and their synthetic analogues are a large class of anion exchangers [3,87-90], The structure of hydrotalcite (see Section 2.6.2) can be envisaged as being made of brucite-type octahedral layers, where the M(III) cations partially substitute for M(II) cations [88] this replacement... 

Brindley, G. W. and Kikkawa, S. (1980). Thermal behavior of hydrotalcite and of anion-exchanged forms of hydrotalcite. Clays Clay Miner. 28, 87. 

Takagi, K., Shichi, T., Usami, H. and Sawaki, Y. (1993). Controlled photocycloaddition of unsaturated carboxylates intercalated in hydrotalcite clay interlayers. J. Am. Chem. Soc. 115, 4339. 

Sasai, R., Shinya, N., Shichi, T., Takagi, T. and Gekko, K. (1999). Molecular alignment and photodimerization of 4 -chloro-4-stilbenecarboxylic acid in hydrotalcite clays bilayer formation in the interlayers. Langmuir 15, 413. 

Olguin, M. T., Bosch, P., Acosta, D. and Bulbulian, S. (1998). 131I sorption by thermally treated hydrotalcites. Clays Clay Miner. 46, 567. 

The replacement of conventional bases, such as NaOH, KOH and NaOMe, by recyclable solid bases, in a variety of organic reactions, is also a focus of recent attention [27, 40]. For example, synthetic hydrotalcite clays, otherwise known as layered double hydroxides (LDHs) and having the general formula Mg8 xAlx (0H)16(C03)x/2 nH20, are hydrated aluminum-magnesium hydroxides possess-... 

Table I. Exchange Reactions of Terephthalate-Pillared Hydrotalcite Clays with Decavanadate...

The condensation of 4-phenyl-2-butanone (45) with ethyl cyanoacetate took place over Cs-X to give the product, 46, in only a 39% yield but when a magnesium oxide containing hydrotalcite clay was used a 75% yield of 46 was obtained (Eqn. 22.43). 8 Formaldehyde was condensed with acetaldehyde at 425°C over a ZSM-5 zeolite containing MgO. Acrolein was obtained in almost 70% yield. 9... 

Dimerization - As reported in Part II, Chapter I the control of photochemical reactions in the constrained environment of a hydrotalcite clay as the supporting medium has been examined. This particular study examined the irradiation (X > 280 nm) of a mixture of 4-benzoylbenzoic acid and 2-phenylethe-nylbenzoic acid in this environment. While the regioselective formation of oxetanes was observed, dimerization of the phenylethenylbenzoic acid also takes... 

Hydrotalcite clays as solid bases can catalyze aldol-condensations or JCnoevena-gel-condensations... 

To a 16.6 g portion of preceding adipate-hydrotalcite-clay slurry, a solution consisting of 10.72 g of ammonium tetraborate tetrahydiate in lOOmL of deionized water was added. The suspensioi (pH=9.5) was stined and heated at 363 K (or given temperature) for 6 h, followed by filtering, washing with deionized wat , and drying at 323 K. 

Figure 6.1 shows the lamellar structures of montmorillonite and hydrotalcite clays. Because the ions in the interlayer space can easily be replaced by foreign ions (cation/anion), these clays can be modified under appropriate conditions by intercalation with suitable ions to satisfy various practical applications. 

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