Layered double hydroxides
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Layered double hydroxides (LDHs) were discovered in the mid-19th century. At this time, the naturally-occurring hydrotalcite mineral was first reported. The formula of this material was determined to be [Mg6Al2(OH)16]CO3∙4H2O in 1915. However, the structure was not fully understood until the 1960s. Both naturally occurring and synthetic LDHs are known. An LDH has the generalised formula [Mz+1-xM3+x(OH)2]q+(Xn-)q/n•yH2O. Usually, z = 2, and M2+ = Ca2+, Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+ or Zn2+; hence q = x. M3+ can be Al3+, Cr3+, Mn3+, Fe3+, Co3+, Ni3+, or Ga3+. Also known are materials with z = 1, where M+ = Li+ and M3+ = Al3+; in this case q = 2x - 1. Pure phases exist for 0.2 ≤ x ≤ 0.33, but values of x have been reported in the range 0.1 ≤ x ≤ 0.5.
The structure of LDHs is best understood by considering the brucite (Mg(OH)2) structure. This features layers of MgO6 octahedra, which share four edges and three vertices with adjacent octahedra to form infinite sheets. Adjacent sheets are linked by hydrogen bonding. If some of the Mg2+ cations are replaced by M3+ cations, the layers will have a net positive charge. To preserve electroneutrality, charge-balancing anions are located between the layers. Generally, there are also some water molecules found between the layers. The amount of water in the interlayer region is governed by a number of factors, including the temperature, water vapour pressure and the nature of the anions present. The water molecules and anions are usually in a state of flux. The water molecules are bound both to the layers and to the interlayer anions by hydrogen bonding. The anions in the interlayer region may be exchanged for other anions by reaction with a large excess of the latter.
From this structure, it is possible to rationalise why pure compounds exist only over a certain range of x values. Considering a system with Mg2+ and Al3+, at intermediate values of x the Al3+ cations in the layers will be distant from one another owing to electrostatic repulsions. Therefore, if x < 0.33, the AlO6 octahedra are not adjacent. However, at high x values, the AlO6 octahedra will be forced into neighbouring positions. This causes the formation of an Al(OH)3 phase. In an analogous fashion, very low values of x will lead to a high density of MgO6 octahedra, and hence Mg(OH)2 is formed.
LDHs exist as two polymorphic forms: a hexagonal form, with an aba layer stacking sequence, and a rhombohedral form, with an abca stacking sequence. In the hexagonal form, there are two layers in the unit cell, and the layers are related by a 180 ° rotation. In contrast, in the rhombohedral form the layers are related by a 120 ° rotation, and there are three layers in the unit cell. These stacking sequences are illustrated in Figure 1.
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Figure 1: The generalised structure of an LDH, showing (a) hexagonal and (b) rhombohedral stacking sequences. MO6 octahedra are shown in blue, and interlayer anions are depicted as green spheres. H atoms have been omitted for clarity.
A wide variety of LDHs are known; recent reviews have discussed these in detail. LDHs may be formed with all dipositive metal ions from Mg2+ to Zn2+, and all transition metal trivalent ions (except Ti3+, owing to the relative instability of this oxidation state for Ti). The work in this thesis is focussed on Al-containing layered hydroxides. Alcontaining LDHs may be synthesised with a variety of M2+ cations, such as Mg2+, Ca2+, Mn2+, Co2+, Ni2+, Zn2+ and Cd2+, in addition to the highly unusual family of LDHs [LiAl2(OH)6]X∙yH2O. A range of interlayer anions, including CO32-, NO3-, OH- and Cl- have been reported. Materials with three metal cations in the layers have also been reported, and some authors have described materials with +4 cations such as Zr4+ doped for trivalent metal ions.
When z = 2, the cations in the layers tend not to exhibit long-range order. However, short-range order has been established through extended X-ray absorption fine structure (EXAFS) studies, for instance for [Zn2Al(OH)6]Cl•yH2O and [Zn2Cr(OH)6]Cl•yH2O. There are two systems which are exceptions to this: the LDHs [Ca2Al(OH)6]X∙yH2O (X = Cl, Br, I, NO3 and SO4: denoted Ca2Al-X) and [LiAl2(OH)6]X∙yH2O (X = Cl, Br, NO3: LiAl2X). In these cases, there is complete ordering of the cations in the layers (Figure 2). Recent results have also shown cation ordering to occur in the related [Ca2M(OH)6]Cl•yH2O systems, where M = Ga, Fe and Sc.45
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Figure 2: Views of the layers in (a) [LiAl2(OH)6]X∙yH2O and (b) [Ca2Al(OH)6]X∙yH2O, showing the ordering of the cations. AlO6, LiO6 and CaO6(OH2) polyhedra are shown in blue, red and green respectively.
A consideration of the bonding in these structures can give some insight into why ordering is seen here but not elsewhere. In the LiAl2-X systems, the layers are identical to the Al(OH)3 layers, except with Li cations located in all the octahedral vacancies of the parent structure. Li+ is a small, hard cation, and therefore ionic bonding to the oxides will be the dominant bonding interaction. The valences of the oxide ligands are fulfilled by two Al3+ cations in the ordered structure. If there were no ordering present, then there would be instances in which the oxide was bound to three Al cations, which would result in it being overbonded. This is not the case for LDHs such as [Mg2Al(OH)6]X∙yH2O, since Mg and Al bond in a similar fashion.
Ordering is also seen in the Ca2Al-X case. This can be rationalised using a different argument. Close inspection of Figure 2(b) reveals the Ca2+ cations to have an expanded coordination number. A 3 + 3 + 1 coordination sphere is seen. The Ca2+ ion binds to 6 oxide ligands in the layer, three at a distance of 2.357(1) Å and three at 2.455(1) Å. Additionally an interlayer water molecule is bonded to the cation, occupying the seventh coordination site at a distance of 2.497(3) Å. Presumably, having two expanded coordination polyhedra adjacent to one another would result in excessive strain in the layers, and hence the Ca and Al cations are ordered to minimise strain.
LiAl2-X is usually synthesised via direct reaction of Al(OH)3 with concentrated solutions of LiX. However, other synthetic methods exist, including a hydrothermal preparation of LiAl2-OH from hydrated alumina gel and LiOH. Other LDH materials may be synthesised in a variety of ways, but by far the most common route is a coprecipitation method using nitrate or chloride salts. This is the route usually employed to access the Ca2Al-X materials.
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