Layer Double Hydroxides LDH

Natural layer double hydroxides (or hydrotalcite-like compounds as they are sometimes called) are, unlike clay minerals, relatively rare. Where they occur they are associated with metamorphic rock formations or saline deposits. The structure of LDHs is very similar to that of brucite, Mg(OH)2, in which magnesium is octahedrally surrounded by six oxygen atoms in the form of hydroxide with the octahedral units extending to form infinite sheets through edge sharing. If some of the magnesium in the sheets is isomorphously substituted by a higher charge cation such as Al3#, the resulting Mg2 + -Al3 + -OH layer gains a positive charge. Sorption of an equivalent amount of hydrated anions occurs so as to maintain electrical neutrality; in nature the charge-balancing hydrated anion is usually carbonate. The OH groups of the positively charged brucite-like sheet are linked to the CO2" groups either directly (via OH-CO3-HO linkages) or via intermediate water (i.e. OH-H2O-CO3-HO). The interlayer carbonate anions adopt an orientation parallel to the layers, i.e. they lie fiat surrounded by loosely bound water (Figure 7). The resulting natural LDH may exist in either of two dimorphic forms, i.e. as a rhombohed-ral hydrotalcite or a hexagonal manasseite.

LDHs may be described by the general formula

where M represents a metal cation and X represents an anion. M2 + may be Mg2 +, Fe2 +, Co2 +, Ni2 +, Zn2+ and M3+ may be Al3 + , Cr3 + or Fe3 + . M2 + /M3 + ratios between 1 and 5 are possible but are typically 0.25 < x < 0.33 and 0 < n < 6. Synthetically there is a wide range of variables such as: (i) different combinations of M2 + and M3 + ; (ii) different charge balancing anions; (iii) different amounts of interlayer water; and (iv) crystal morphology and size. To form LDHs, the M22 and M3 + cations must be of a size that can be contained in

Figure 7 Illustration of the top view of LDH (Mg6Al (OH)16CO3• 4H2O) lattice.

the holes (octahedral sites) between the close-packed OH groups in the brucite-like layers. This limits the possibilities to cations of ionic size between 0.5 and 0.8 A and, in the main, excludes cations such as Be2 + (0.35 A), Ca2+ (0.99 A) and Cd2+ (0.97 A). The formation of LDHs is not, however, limited to M2 + /M3+ cations; it is, for example, possible to incorporate monovalent cations (M # ) such as Li + in a Li/Al material, or to have divalent/tetravalent materials such as Co/Ti.

The number of exchangeable anions in LDHs depends on the charge density on the host layers. However there are no particular restrictions on the nature of the anion. Inorganic charge-balancing anions include Cl", OH", NO2", ClO4~ and SO2". Organic acids such as adipic, succinic, oxalic, malonic, sebacic and terephthalic may also serve as charge-balancing species. However, as mentioned above, nature favours the carbonate ion which is tenaciously held in the interlayer region due to its relatively high polariz-ability and synthesis of pure LDHs with other anions requires special preparation procedures (see below). LDHs may undergo swelling in a manner not unlike that of silicate clays. For example sulfate-containing LDH may be solvated with glycol or glycerol. In general swelling of LDHs depends on the nature of the interlayer anion (charge, mass, structure), nature of the solvent (polarity, molecular dimensions) and of course the layer charge.

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