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Unformatted text preview: he gallery height in order to obtain larger porous volume and a larger surface area in the HT as well as providing space in the interlayer region for other guest species [1, 2, 25]. 13 1. Introduction Tab. 3: Gallery heights for HTs with different anions [25,35] Anion Gallery height [pm] CO32- 290 SO42- 390 CrO42- 390 HVO42- 460 V10O286- 690 HV4W 8PO406- 970 NaP5W 30O11016- 1590 Most important for their synthesis is that the anions are stable at the same pH value as the HT and that they are stable with respect to hydrolysis. Only anionic complexes can be incorporated into the interlayer structure. Cationic complexes are only adsorbed on the outer surface of the HT crystallites [25]. Three methods are generally used for the preparation of HTs with unusual anions. Coprecipitation of the HTs in the presence of the desired anion is the most straightforward method. Without special care taken for the exclusion of carbon dioxide sources (degassed water, inert gas atmosphere, etc.) it is impossible to prepare carbonate free HTs, due to the high affinity of CO2 to the positively charged brucite-like layers. The only exception is when the preparation of the HT is possible at pH-value ≤ 5, because at this pH-value virtually no carbonate is present in solution which can be explained by Equation 2 and 3 [4, 33]. Despite all precautions coprecipitation is not always suitable due to the possible formation of side phases during the synthesis. 14 1. Introduction H2CO3 H+ + HCO3- pK1 = 6.4 (Eq. 2) HCO3- H+ + CO32- pK2 = 10.3 (Eq. 3) Another method is to exchange the anion present in the interlayer. This method is comparable to the widely studied cation exchange of cationic clays, but in HTs anions are exchanged. However, the ion exchange in HTs is more difficult than in smectitetype clays, their cationic counterpart, due to the higher charge density of HTs in comparison to smectites, 4e/nm2 to 1e/nm2, respectively [2, 25, 35]. Therefore anions with high charge density are needed. Inorganic as well as organic ions...
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This document was uploaded on 10/07/2013.

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