Hydrotalcite double-layered metal hydroxide structure

Figure 20.2 Schematic of the double-layered metal hydroxide structure of a typical hydrotalcite. Reproduced with permission from Ref [8].

Layered metal hydroxides can be categorized into several classes according to their structure (Figure 13.1). Layered double hydroxides (LDHs), which have a hydrotalcite-like structure [27-34], can be expressed as [M2+1 xM3+x(0H)2] (Am )x/m,nH20,... 

Hydrotalcites are layered double hydroxides with the general formula Mg6Al2(0H)i6[C03].4H20. Loading these compounds with potassium carbonate strongly increases their C02 uptake [25, 35], Notably, the hydrotalcite structure already breaks down below 400 °C [26] into a mixed metal oxide. 

Layered Cobalt Hydroxides. The above results demonstrate the importance of the interlayer Co rather than the framework cobalt, in forming the required metallic Co catalyst. A structure with optimal catalytic activity would consist of one where there were only Co/Al hydroxide sheets, a structure closely approximated to by the hydrotalcite series of double layered hydroxides. 

Although there are numerous families of lamellar solids, only a handful of them exhibit the kind of versatile intercalation chemistry that forms the basis of this book. In arriving at the content of this volume, the editors have accurately identified six classes of versatile layered compounds that are at the forefront of materials intercalation chemistry, namely, smectite clays, zirconium phosphates and phos-phonates, layered double hydroxides (known informally as hydrotalcites or anionic clays ), layered manganese oxides, layered metal chalcogenides, and lamellar alkali silicates and silicic acids. Graphite and carbon nanotubes have not been included, in part because this specialty area of intercalation chemistry is limited to one or two molecular layers of comparatively small guest species that are capable of undergoing electron transfCT reactions with the host structure. 

Layered double hydroxides (LDHs), with a hydrotalcite-like structure, are a class of materials which have received considerable attention in the last decade. The structure of LDHs is based on the stacking of metal cation hydroxide (brucite-like) layers, with a positive charge on the layers resulting from the isomorphous substitution of some of the bivalent... 

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. 

Figure 6.1 Stacking of brucite-like layers forming the hydrotalcite structure. Reprinted from Coordination Chemistry Reviews, Vol. 181, Vicente Rives and Maria Angeles Ulibarri, Layered double hydroxides (LDH) intercalated with metal coordination compounds and oxometalates, pp. 61 - 120, 1999, with permission from Elsevier.

---