Preparação, caracterização, reações de intercalação e decomposição termica de hidroxidos duplos e amelares contendo Cd, Mg e Zn

AUTOR(ES)
DATA DE PUBLICAÇÃO

1992

RESUMO

In this work we have prepared Layered Double Hydroxides (LDH) using different divalent metals, aluminium as the trivalent metal, and nitrate and carbonate as the counter anions. These LDHs are bidimensional compounds which alternate positively charged layers formed by the metal atoms and hydroxyl groups and negatively charged layers formed by the counter anions and water molecules. We have then studied the intercalation reactions between these LDHs and phenylphosphonic acid, as well as 2-carboxyethylphosphonic acid, by varying some reaction parameters, such as temperature, reaction time, and the pH of the reaction media. We have also investigated the thermal decomposition of the host compounds through calcination at different temperatures. The obtained compounds were exhaustively characterized using the following techniques: Chemical analysis, X-ray diffraction, Fourier- transform infrared spectroscopy, thermogravimetric analysis, differential scanning calorimetry, real density of solids by gas dislocation, and solid state nuclear magnetic resonance with magic angle spinning. The application of these techniques has enabled us to deepen our knowledge about the intercalation reactions of LDHs, as well as the understanding of the changes that occur after the end of the reactions. A detailed study on the the thermal decomposition of these compounds was also possible, and we were able to determine some features of the solid solutions formed as result of these decompositions. These solutions may, in principle, present interesting, catalytic properties. All LDHs prepared, except one, formed intercalation compounds with both the acids used. The obtention of intercalation compounds from the LDHs with carbonate as the counter anion shows that these compounds are more reactive than it has been assumed, for a number of authors descibe the difficulties in obtaining intercalation compounds from carbonate-LDHs. From this, it can be concluded that the phosphonic acids are a good class of compounds for the intercalation in LDHs. As for the thermal decomposition, the use of nuclear magnetic resonance has shown that the aluminium atoms which innitially occupy octahedral sites, shift to tetrahedral sites during a certain temperature interval, and then move back to octahedral sites at higher temperatures, at which spinnel-like phases are formed for the Mg and Zn LDHs. We have also demonstrated that the presence of these spinnel-like phases can be detected by the combined use of infrared spctroscopy and nuclear magnetic resonance at lower temperatures than those observed by X-ray diffraction, wich is the main technique used in this kind of investigation. Finally, we could demonstrate that the techiques used are complementary, forming a powerful group of tools for the investigation of various solid state phenomena.

ASSUNTO(S)

hidroxidos quimica inorganica

ACESSO AO ARTIGO

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