Hydrotalcites and their role in coordination of anions in Bayer liquors: Anion binding in layered double hydroxides

Adsorption of sodium dodecylsulfate on a hydrotalcite-like compound. Effect of temperature, pH and ionic strength

In the present study we investigated the adsorption of sodium dodecyl- and octylsulfate, and sodium dodecyl- and octylbenzenesulfonate, on a layered double hydroxide (LDH) under controlled conditions. The results were compared to those obtained for the adsorption of surfactants on mineral oxides and on LDHs, showing that the behavior of surfactant adsorption on LDHs can be approximately explained by the models proposed for surfactant adsorption on mineral oxides. The electrokinetic potential of the surfactant-adsorbed LDH particles in suspension was measured in order to monitor the variation of this potential as a function of the adsorption. The results showed a characteristic behavior of the surfactant-adsorbed LDH that can be correlated to a rearrangement of the adsorbed layer at concentrations above the CMC. Surfactant sorption was also studied, using SDS and the calcined LDH, showing the regeneration of the calcined material by intercalating the DS anions. Two processes can occur during the removal of surfactants from aqueous solutions by LDHs, depending on the starting material: (i) adsorption itself, when a noncalcined LDH intercalated with a nonexchangeable anion, such as carbonate, is used; and (ii) adsorption and intercalation, which occurs when a calcined LDH, like Mg–Al–CO3, is used. The potential application of this material for surfactant removal is one of the reasons for the need for a better understanding of the mechanisms involved. As we demonstrate here, LDHs are very efficient in removing surfactants from aqueous solutions.

Sorption of Anionic Surfactants on Layered Double Hydroxides

Hydrotalcites, which are anionic clays, have been synthesized by the sol−gel technique from magnesium ethoxide, aluminum tri-sec-butoxide, and aluminum acetylacetonate. Initially a colloidal solution is obtained which gels and is thermally stabilized. The local and average structure are studied comparing the electron microscopy results with the XRD patterns. One of the main features of these materials is the high specific area (around 150 m2/g), if compared with the natural hydrotalcites areas.

Synthesis and Characterization of Sol−Gel Hydrotalcites. Structure and Texture†

Vapour phase alkylation of phenol with methanol was carried out in a fixed-bed flow reactor over thermally calcined magnesium aluminium hydrotalcites, with Mg/Al atomic ratios from 3 to 10. The main products were anisole, o-cresol, and 2,6-xylenol. The influence of phenol-to-methanol molar ratio, reaction temperature and contact time on the product yields were examined. The activity of these catalysts was compared with MgO, Al2O3 and physically mixed MgO-AL2O3. Of the catalysts studied, Mg-Al 4.0:1 calcined hydrotalcite (CHT) was the most active. These catalysts are more active than magnesia and γ-alumina. On the basis of the results obtained, a suitable reaction pathway was proposed. The participation of combined acidic and basic sites in the calcined hydrotalcites for the alkylation of phenol with methanol over magnesium-aluminium calcined hydrotalcites has been suggested. Prior to the reaction, the catalyst precursors were characterised by X-ray powder diffraction and surface area measurements.

Alkylation of phenol with methanol over magnesium-aluminium calcined hydrotalcites

The effect of by-products obtained during the low-temperature synthesis of Mg, Al- hydrotalcite on further thermal decomposition of hydrotalcite material was studied by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy and infrared spectroscopy. By-products, which thermally decomposed below 450°C, led to the formation of α-Al2O3 and MgO; by-products which completely decomposed above 500°C resulted in the formation of MgAl2O4 and a sharp increase in MgO dispersity.

Thermal decomposition of Mg, Al-hydrotalcite material

The transformation of methane into C2 hydrocarbons over thermally degraded Mg, Pb, Al-hydrotalcite material proceeds with the active participation of β-PbO. The conditions of pre-treatment leading β-PbO formation in the catalyst have been examined on the Pb, Al-hydrotalcite material as a model system. By use of X-ray diffraction, scanning electron microscopy, transmission electron microscopy, infrared spectroscopy and diffusereflectance spectroscopy techniques, it was found that the temperature interval 600–750 °C is optimal for β-PbO crystallization. On the basis of the present and previous results, the formation of all components of the catalytically active thermally degraded Mg, Pb, Al-hydrotalcite material, has been explained. During calcination of the Mg, Pb, Al-hydrotalcite material, the lead-containing components form β-PbO and γ-Al2O3 and the magnesium-containing components transform to a finely dispersed MgO-Al2O3 matrix.

M. L. Valcheva-Traykova

Papers

Thermal decomposition of Mg, Al-hydrotalcite material

Thermal decomposition of the Pb, Al-hydrotalcite material