Layered double hydroxide precipitate

Complex precipitates, such as layered double hydroxides, are frequently prepared as precursors of oxide catalysts. Hydrotalcite is a hydrated hydroxy-carbonate mineral with the formula Mg,sAl2(0H)i6C03-4H20. It represents the better known and most popular member of a family of layered double metal hydroxide com-... 

Princito. F., Ghiotti, G., Graflin, P. and Tichiu D. (2000). Synthesis and characterization of sol gcl Mg/AI and Ni/Al layered double hydroxides and comparison with co-precipitated samples. Microporous and Mesoporous Mater. 39 229-247. 

Recently, we reported that thin films with nanocrystals such as boehmite (AlOOH) [7-10], Al-Zn layered double hydroxide (LDH) [11], and anatase [12] were prepared through hot water treatment of the gel films at temperatures less than 100 C. In this process, precursor gel films prepared using the sol-gel process were immersed only in hot distilled water under ambient atmosphere at temperatures lower than 100 C. Although studies of oxide film formation from solution are usually based on the precipitation of crystals from a saturated solution of the precursor [1-6], in the present process, precursor amorphous gel films prepared using the sol-gel process are immersed in hot distilled water to form nanocrystals on the thin film surface. 

JW Boclair, PS Braterman, F Yarberry. Morphologies of layered double hydroxides by homogeneous precipitation. Unpublished Work. Denton. TX University of North Texas, 1999. 

There are a number of techniques that have been successfully applied to synthesize modified hydrotalcites (48). The most commonly method used is the co-precipitation of two metal salts in alkaline solution at a constant pH value of about 10. Another method uses the classical ion exchange process in which the guest anions are exchanged with the anions in the interlayer spaces of preformed layered double hydroxides to produce specific anion intercalated modified hydrotalcites. StiU another method is a lattice reconstruction after heating, i.e., calcination, which is based on the structural memory effect of these materials, due to which the original structure is reproduced after rehydration. 

It should be noted that magnesium aluminum layered double hydroxides (MgAl-LDHs) have also been widely used as flame retardants for polymer materials. Similarly, MgAl-LDHs can be synthesized by reaction of aluminum and magnesium salts as precursor and sodium hydroxide as precipitator via the coprecipitation method and other preparation routes. ... 

A double hydroxide with the leading bayerite layer (d = 0.88 nm) is formed in Ni(OH)2 - AI(OH)j system obtained by co-precipitation from acidic media enriched with aluminum, while the co-precipitation from alkaline media enriched with nickel results in the formation of the hydroxides with bmcite layer in the packing (0.8 nm). 

Scheinost AC, Sparks DL (2000) Formation of layered single- and double-metal hydroxide precipitates at the mineral/water interface. A multiple-scattering XAFS analysis. J Colloid Interface Sci 223 167-178 Scheinost AC, Abend S, Pandya KI, Sparks DL (2001) Kinetic controls on Cu and Pb sorption by ferrihydrite. Environ Sci Technol 35 1090-1096... 

It is well known that hydrolyzed polyvalent metal ions are more efficient than unhydrolyzed ions in the destabilization of colloidal dispersions. Monomeric hydrolysis species undergo condensation reactions under certain conditions, which lead to the formation of multi- or polynuclear hydroxo complexes. These reactions take place especially in solutions that are oversaturated with respect to the solubility limit of the metal hydroxide. The observed multimeric hydroxo complexes or isopolycations are assumed to be soluble kinetic intermediates in the transition that oversaturated solutions undergo in the course of precipitation of hydrous metal oxides. Previous work by Matijevic, Janauer, and Kerker (7) Fuerstenau, Somasundaran, and Fuerstenau (I) and O Melia and Stumm (12) has shown that isopolycations adsorb at interfaces. Furthermore, it has been observed that species, adsorbed at the surface, destabilize colloidal suspensions at much lower concentrations than ions that are not specifically adsorbed. Ottewill and Watanabe (13) and Somasundaran, Healy, and Fuerstenau (16) have shown that the theory of the diffuse double layer explains the destabilization of dispersions by small concentrations of surfactant ions that have a charge opposite to... 

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