OF LAYERED DOUBLE HYDROXIDES

The present invention relates to methods for the preparation of layered double hydroxides (LDHs), in particular LDHs having anisotropic morphology. The present invention also relates to LDHs having such morphology.

Layered double hydroxides (LDHs) are a class of compounds which comprise two metal cations and have a layered structure. A review of LDHs is provided in Structure and Bonding; Vol 1 19, 2005 Layered Double Hydroxides ed. X Duan and D.G. Evans. The hydrota!cites, perhaps the most well-known examples of LDHs, have been studied for many years.

LDHs can be represented by the general formula

x/z.yH ₂ 0 or [ >o _. ^M, ¾ ⁰ H)d ⁿ⁺ A -yH20 _t where M\ M ^l! and !v are mono, di- and trivalent metai cations respectively, that occupy octahedral positions in hydroxide layers, A ^2" is an interiayer charge-compensating anion where z is an integer, such as C0 ₃ ^2" , NOs ^" or CI ^" , n~2x-1 , x is a number less than 1 and y is 0 or a number greater than 0. A large number of LDHs with a wide variety of Μ"- '" cation pairs (e.g., Ca-AI) as well as the M ^! - ^!il cation pair (e.g. Li-AI) with different anions in the interiayer space have been reported and studied.

LDHs having plate-like morphology are disclosed in Chem Mater

(1977) 9 pp2 1-247: the X-ray diffraction patterns of the materials disctosed therein clearly show the plate-like morphology of the particles.

Other LDHs with piate-like morpholog are disclosed in US-B- 4,727,167 and US-B-4,461 ,714.

The structure of the layered materials [LtAi?.{OH}6jX _! where X is CI, Br or NO ₃ , and their hydrates has been described by Besserguenev et a!., in Chem. Mater, 1997, no. 9, p.241-247. The materials can be formed by direct precipitation (see, for example, Serna et al., Clays & Ciay Minerals, (1997), 25,384). The materials can also be produced by the reaction of forms of Ai(OH)3, such as bayerite, nordstrandite, gibbsite or doy!eite, with lithium salts. The structure of the LiAl2(OH)6 ⁺ layers in the compounds is unusual amongst LDHs since ίί is based on an ordered arrangement of metai cations within the layers.

The synthesis of LiAb(OH)8 ⁺ compounds is described in US 4,348,295 and US 4,348,297.

5 intercalates into Lj-AI LDHs (and other LDHs) are described in US

4,727,187, US 4,812,245 and WG-A-02/47729.

LDHs exhibit a wide range of anion-exchange reactions with guests such as organic carboxylates, sulfonates and a range of anionic metal complexes. These materials are of significant technological importance in io diverse areas such as catalysis, optics, medical science, materials science and separation science.

The control and tailoring of the morphology and particie size of LDH materials is, in general, much more difficult than for many other materials. There is, however, a need for LDH materials of controlled morphology, in i s particular, there is need for LDH materials with a relatively high aspect ratio.

Although plate like particles of LDH have been prepared in the past (e.g. by ex-foliation methods), there is a need for materials having high one- dimensional aspect ratios (i.e. with a rod-like morphology). It would be particularly useful to have such materials with particular morphology on the0 rtano scale.

it is an aim of the present invention to address these problems.

The present invention accordingly provides in a first aspect, a method for the preparation of a layered double hydroxide comprising particies having a rod-like morphology (i.e. morphologically one-dimensionaily anisotropic5 particles), the method comprising preparing a precursor comprising particles of aluminium hydroxide having a rod-like morphology (i.e. morphologically one-dimensionaily anisotropic particies), and contacting the precursor with an aqueous lithium sail

Surprisingly, use of such a method enables nano-rod particles of LDH0 of high or extremely high one-dimensional aspect ratio to be prepared.

Preferably, the method further comprises a step of hydrothermal treatment after contacting the precursor with the aqueous lithium salt. The hydrothermai treatment will usually be performed at a temperature of 100°C or lower for 1 to 20 hours, preferably 95°C or lower for up to 14 hours.

The precursor aluminium hydroxide is preferably AI(OH)3, more preferably gibbsi e (γ AI(OH} ₃ ).

The lithium salt will usually comprise a lithium hali.de but may, alternatively, comprise lithium nitrate, lithium carbonate or other suitable, soluble, lithium salts. The preferred lithium halide is lithium chloride although lithium bromide may also be suitable,

The aqueous lithium salt will usually be in molar excess, preferably at least 3-fold molar excess and more preferably at least 5-fold molar excess.

Generally, the precursor comprising rod-like particles (I.e. having morphologically one-d!mensiona!iy anisotropic particles) is prepared by a templated hydrothermai synthesis. The templated hydrothermai synthesis will usually comprise hydrothermai treatment of a reaction mixture comprising aluminate and a templating component. The preferred aluminate is sodium aluminate prepared by e.g. mixture of aluminium chloride with an alkaline sodium compound, in particular sodium hydroxide.

Generally, the templating component will comprise a surfactant, preferably a cationic surfactant The most preferred templating component is a quaternary ammonium salt. The most preferred quaternary ammonium salt is a ammonium salt having at least one long alkyl chain. The long chain alky! will usually be Cs-Cia and is most preferably cety! (Cie). The most preferred templating component is cetyi trimethyl ammonium haiide, in particular bromide (GTAB).

The great benefit of the present invention is that the morphologically one-dimensionaily anisotropic particles are substantially rod-shaped (i.e. are elongate), in particular on the nano-scale (i.e. having dimensions <1mm), in other words are nano-rods.

Generally, the particles prepared according to the method of the present invention will have substantially hexagonal prismatic morphology (e.g. i a cross-section through the rod). Generally, the layers of the LDH will be stacked along the anisotropic dimension of the particles (i.e. along the elongate axis of the hexagonal prisms).

Generally, the LDH prepared according to the present invention, as prepared, will, have an interSayer spacing of approximately 0.75 nm, however the inter!ayer spacing may generaliy be in a range of 0,5 to 1 nm. As is well known to the skilled person, the inferiayer spacing of the LDH may be modified by intercalation of e.g. relatively long chai organic anions.

The greatly advantageous feature of the present invention is that it, for the first time, enables a lithium aluminium layered double hydroxide to be prepared having nan -rod morphology. Thus, in a second aspect, the invention provides a lithium aluminium layered double hydroxide comprising particles having rod-like morphology (i.e. morphologically one-d mensionaliy anisotropic particles).

Materials produced according to the first aspect of the present invention find use in many areas of technology including in the usual fields in which LDHs find use {catalysis, optics, medical science, separation science and material science) but particularly as additives in composites e.g. cement, polymers and other materials and also in specific medica! uses in view of the nano-rod shape of the particles. Further use of LDHs according to the invention is in nanocomposites. Examples of such composites include polymer/inorganic nanocomposites (polymers may be synthetic or bio {or bio- inspired) with polar and/or non-po!ar monomers.

The invention is illustrated by the figures in which:

Figure 1 illustrates the X Ray diffraction (XRD) patterns of {a) the gibbsite precursor and (b) U-AI LDH as synthesised, and

Figure 2 illustrates (a) scanning electron microscopy fSEM) image and {b) transmission electron microscopy (TEM) image of gibbsite nanorods; (c) TEM image of Li-Ai LDH nanorods (inset; a corresponding selected area electron diffraction {SAED) pattern); (d) Higher magnification TEM image of Li-AI LDH nanorods; and (e) Line profile perpendicular to the c~axis along the white line in (d). The invention is further illustrated by the following Example in which Li- A! LDHs of nano-rod morphology were prepared and characterised according to the method of the invention.

The Example describes an efficient method for controlling the particle size and morphology of lithium-aluminium layered doubie hydroxides (LDHs). The use of nanodispersed LDH or LDH with various crystalline morphologies has potential applications in the synthesis of organic/inorganic nanocomposi.tes and as additives in e.g. cements.

The Example discloses the synthesis of a layered double hydroxide (Li~ Al LDH) with a unique rod-like morphology. LDH nanorods were synthesised using gibbsite {A (OH} _3! often designated as y~AI{OH)3) nanorods as a precursor. Gibbsite nanorods are disclosed in Lin et ai J. Rhys, Chem. C 2008 1 12, pp 4124-4128. Characterisations with XRD, TE and SEM verified the product is single -crystalline LDHs with rod-like one-dimension nanostructure.

Example

1. Synthesis

To prepare gibbsite nanorod precursor AlC 6H ₂ O 1,207g and NaOH 0.8g were dissolved in 18 ml deionized water to form a transparent NaAID^ solution (solution A). 1.585 g of cetyl trimethyiammonium bromide (CTAB) was dissolved in 8 ml ethanol under 3G°G to form solution 8. Then solution B was added dropwise into solution under magnetic stirring. After further stirring for 1 hour, the mixture was transferred into a 40 mi Teflon lined stainless autoclave and kept under 120°C for 12 hours. The solid product was collected by centnfugation, washed three times with water and ethanol and dried in air.

The as-prepared gibbsite nanorods sample were dispersed in water with 5-foid molar excess of LiCI. After stirring for haif hour, the mixture was iransferred into a 30 ml Teflon lined stainless autoclave and kept under 90°C for 12 hours. The product was collected by centnfugation, washed three times with water and ethanol and dried in air. 2. Characterisation

X-Ray Diffraciion (XRD) - Figure 1 shows the diffraction patterns of gibbsite nanorods precursor and Li-AI LDHs nanorods. The XRD pattern of the as-synthesized gibbsite sample can be indexed to the monociinic gibbsite phase (JCPDS PDF 33-0018). The XRD pattern of the Li-AI LDHs can be indexed to lithium aluminium hydroxide■chloride hydrate (JCPDS PDF 01-087- 1788) and gives a hexagonal unit cell with a = 5.09 Λ and c ~ 15.29 A. The distance between layers is half of the c parameter, which is about. G.75nm.

Electron Microscopy - The as-prepared gibbsite sample and Li-AI LDH sample were studied by transmission electron microscopy (TEM), high resolutio TEM (HR-TEM) and scanning electron microscopy (SEM) and illustrated in Figure 2, The Sow-magnification TEW and SEM images of the gibbsite sample (Figure 2(a) and (b)) and Li-AI LDH (Figure 2(c)) clearly demonstrate the hexagonal prismatic morphology of both of the samples. The selected area electron diffraction (SAED) patterns obtained during TEM show that the samples are single-crystallized.

Figure 2(d) is the high resolution image of the Li-AI LDHs. Lattice fringes can be clearly seen, which further confirmed the single-crystalline structure of the sample and proved that the brucit layers in LDH stack along c-axis to form one-dimension nanostructure. The line profile along the lamellar layers is shown in Figure 2(e), which gives an interlame!Sar separation, d, of about 0.75 nm. This value agrees well with the interlayer separation calculated from XRD data. 3. Conclusion

Li-AI LDH nanorods have been synthesized from gibbsite nanorods precursor. X-ray crystallography verified that the precursor could be indexed as gibbsite AI(OH)3 and the product as Ll-Ai LDH with an inferiamellar distance of 0.75nm. TEM and SE imaging reveal that both the precursor and the product adopt a rod-like one-dimension nanostructure.