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Therefore anions with high charge density are needed

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Unformatted text preview: can be exchanged. Next to the fundamental necessities, mentioned above, there are several other factors concerning the applicability of this method. The anion present in the HT must be easily exchangeable. Therefore carbonated HTs are only of very limited use for the classical ion exchange as carbonate is not easy exchangeable and its destruction by diluted mineral acids can lead simultaneously to the destruction of the HT [27, 40]. Hence carbonate free HTs must be used as precursors. The crystal size has also a great influence on the rate of the ion exchange, too big crystals may even inhibit the exchange [27]. For the intercalation of very large anions, like many polyoxoanions, the interlayer space of the HT must be increased before the ion exchange. This can be achieved by keeping the HT in an aqueous suspension, the use of swelling agents such as glycerol, glycol or long chain alcohols or by using HT precursors that contain large organic anions such as terephtalate, p-toluensulphonate or 4-hydroxybenzoate [2, 25, 35, 45]. In the method of structure reconstruction the first step is transformation of the HTs into mixed metal oxides by calcination at temperatures between 250 and 850°C. Such mixed metal oxides derived from HT precursors have a ‘memory’ of the HT. Due to this 1. Introduction 15 ytrtrghrhrhhrhrhhffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff ‘memory effect’ the mixed metal oxides prepared from HT precursors can rebuild the HT structure. The memory can be activated in contact with water, water vapour and especially aqueous solutions containing anions [1, 2, 7, 25, 38, 41, 47, 49, 53, 54, 55, 56]. In contrast to the other two methods usually carbonate containing HTs are used as precursors, because of the high volatility of carbonate. It must be pointed out that the reconstruction of the mixed metal oxide to the HT is never complete, because during the formation of the mixed metal oxide a certain amount of a spinel phase is formed which does not reconstruct. The amount of the spinel phase formed generally increases with increasing calc...
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