Abstract
Magnesium–aluminium–bismuth-layered double hydroxides (LDH) intercalated with carbonate were studied in respect of maximal rate of substitution of Al3+ by Bi3+ for the first time. LDH with the nominal compositions of Mg3Al1 - x Bi x –CO3 (x = 0 to 0.5) were prepared using both the conventional super saturation co-precipitation method and sol–gel processing via hydration of the mixed oxide powders in carbonate-containing solutions. The mixed oxides were obtained either by calcination of the LDH (prepared by co-precipitation) or by using a novel alkoxide-free sol–gel method. All the LDH products were characterised using the methods of X-ray diffraction, scanning electron microscopy and thermogravimetry. The observed values of the lattice parameters of LDH phases were compared with the calculated values. It has been found that, regardless of the preparation method used and the conditions (pH, temperature, time) applied, the maximum rate of substitution of aluminium by bismuth in LDH is about 20 mol.%. A schematic representation of LDH structure of a 3R polytype [4, 5] where the lattice parameter c and the basal spacing d relate to each other as c = 3d.
Highlights
Layered double hydroxides (LDH), known as hydrotalcite-type compounds, belong to a family of anionic clays whose crystal structure is derived from brucite, Mg (OH)2
The powder XRD patterns of LDH synthesised by coprecipitation and sol–gel methods are shown in Figs. 4, 5, respectively
These results confirm that alkoxide-free sol–gel method is not suitable for the direct synthesis of LDH
Summary
Layered double hydroxides (LDH), known as hydrotalcite-type compounds, belong to a family of anionic clays whose crystal structure is derived from brucite, Mg (OH). In LDH, the positively charged layers of double metal hydroxides alternate with charge-compensating interlayer of anions coordinated by water molecules [1]. MI–MIII LDH are known [2, 3], the great majority of layered hydroxides are of the MII−MIII type. The general chemical formula of such LDH can be represented as [MII1 − nMIIIn(OH)2]n+(Am−)n/m]·zH2O. The metal cations in the hydroxide layers are coordinated by six oxygen atoms forming 2-D structures of the facelinked oxygen octahedra (Fig. 1). The octahedra are compressed in the direction perpendicular to the layer planes [1]
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