PROCESS FOR PREPARING SMALL SIZE LAYERED DOUBLE HYDROXIDE PARTICLES

Abstract

A process for preparing particles of a layered double hydroxide of the general formula

[M.sub.p.sup.z+M.sub.q.sup.y+(OH).sub.2].sup.a+(X.sup.n).sub.a/n.bH.sub.2O(I)

wherein M.sup.z+ and M.sup.y+ are metal cations or mixtures of metal cations, z=1 or 2; y=3 or 4; p+q=1; b=0 to 10, X.sup.n is an anion, n is 1 to 5 and a is determined by p, q, y and z such that a=zp+yq2, comprises (a) mixing, in aqueous solution, M.sup.z+ cations, M.sup.y+ cations and X.sup.n anions, with a base; and (b) allowing the layered double hydroxide of formula (I) to precipitate from the solution mixed in step (a).

Preferably, M is Li, Mg, Zn, Fe, Ni, Co, Cu, Ca, or a mixture of two or more. Preferably, y is 3, and M is Al, Ga, In, Fe or a mixture of two or more thereof. Also provided are particles obtainable by the process, especially wherein M is Ca, M is Al, and X.sup.n is NO.sub.3.sup.. Particles of a layered double hydroxide wherein the particles have a particle size of not greater than 2000 nm, preferably not greater than 300 nm and especially not greater than 100 nm, are also provided. The layered double hydroxides according to the invention are useful in certain applications, for example, as adsorbents, coatings and catalyst supports.

Claims

1. A process for preparing particles of a layered double hydroxide of the general formula
[M.sub.p.sup.z+M.sub.q.sup.y+(OH).sub.2].sup.a+(X.sup.n).sub.a/n.bH.sub.2O(I) wherein M.sup.z+ and M.sup.y+ are metal cations or mixtures of metal cations; z=1 or 2; y=3 or 4; p+q=1; b=0 to 10; X.sup.n is an anion, wherein n is 1 to 5; and a is determined by p, q, y and z such that a=zp+yq2; which process comprises (a) mixing M.sup.z+ cations, M.sup.y+ cations and X.sup.n anions, with a base; and (b) allowing the layered double hydroxide of formula to precipitate from the solution mixed in step (a); wherein step (a) is performed in aqueous solution, and under an atmosphere of air, for a period not longer than 15 minutes; and wherein the mixing speed and duration of step (a) are such that the layered double hydroxide that precipitates in step (b) has a particle size of not greater than 2000 nm.

2. The process according to claim 1, wherein the mixing speed and duration of step (a) are such that the layered double hydroxide that precipitates in step (b) has a particle size of not greater than 500 nm.

3. The process according to claim 1, wherein the mixing speed and duration of step (a) are such that the layered double hydroxide that precipitates in step (b) has a particle size of not greater than 300 nm.

4. The process according to claim 1, wherein, in the layered double hydroxide, M/M is selected from Mg/Al, and Ca/Al.

5. The process according to claim 1, wherein, in the layered double hydroxide, M/M is Ca/Al.

6. The process according to claim 1, wherein X.sup.n is an anion selected from halide, inorganic oxyanion, anionic surfactants, anionic chromophores, and anionic UV absorbers.

7. The process according to claim 6, wherein the inorganic oxyanion is selected from carbonate, bicarbonate, hydrogenphosphate, dihydrogenphosphate, nitrite, borate, nitrate, sulphate, sulphite, phosphate, and a mixture of two or more thereof.

8. The process according to claim 6, wherein the inorganic oxyanion is nitrate.

9. The process according to claim 1, wherein step (a) is conducted at a speed not slower than 5000 rpm.

10. The process according to claim 1, wherein step (a) is conducted at a speed not slower than 8000 rpm.

11. The process according to claim 1, wherein step (a) is conducted at a speed not slower than 12,000 rpm.

12. The process according to claim 1, wherein step (a) is conducted at a speed not slower than 17,000 rpm.

13. The process according to claim 1, wherein step (a) is carried out for a period of from 1 to 15 minutes.

14. The process according to claim 1, wherein the base is a compound comprising OH.sup. anions.

15. The process according to claim 14, wherein the base is NaOH.

16. The process according to claim 1, wherein the pH of the aqueous solution obtained by the mixing step (a) is greater than 7.

17. The process according to claim 16, wherein the pH value of the aqueous solution of step (a) is adjusted by the addition of NaOH or a mixture of NaOH and NaX, where X.sup. is an anion, to the solution.

18. The process according to claim 1, wherein the process further comprises a step c) of recovering the precipitated layered double hydroxide.

19. Particles of a layered double hydroxide of the general formula
[M.sub.p.sup.z+M.sub.q.sup.y+(OH).sub.2].sup.a+(X.sup.n).sub.a/n.bH.sub.2O(I) wherein M.sup.z+ and M.sup.y+ are metal cations or mixtures of metal cations z=1 or 2; y=3 or 4; p+q=1; b=0 to 10; X.sup.n is an anion, wherein n is 1 to 5; and a is determined by p, q, y and z such that a=zp+yq2; wherein the particles are obtainable, obtained or directly obtained by a process comprising (a) mixing M.sup.z+ cations, M.sup.y+ cations and X.sup.n anions, with a base; and (b) allowing the layered double hydroxide of formula (I) to precipitate from the solution mixed in step (a); wherein step (a) is performed in aqueous solution, and under an atmosphere of air, for a period not longer than 15 minutes; and wherein the mixing speed and duration of step (a) are such that the layered double hydroxide that precipitates in step (b) has a particle size of not greater than 2000 nm.

20. A CaAlNO.sub.3 layered double hydroxide, in a substantially pure form, and having a particle size of not greater than 2000 nm.

21. The CaAlNO.sub.3 layered double hydroxide according to claim 20, wherein the layered double hydroxide contains no calcium carbonate.

22. The CaAlNO.sub.3 layered double hydroxide according to claim 21, wherein the layered double hydroxide contains no metal carbonates, or calcium aluminium oxide carbonate hydrate.

23. The CaAlNO.sub.3 layered double hydroxide according to claim 21, having a particle size of not greater than 300 nm.

24. The CaAlNO.sub.3 layered double hydroxide according to claim 20, having a particle size of not greater than 100 nm.

25. The CaAlNO.sub.3 layered double hydroxide according to claim 20, wherein individual crystals of the layered double hydroxide have substantially hexagonal morphology.

Description

EXAMPLES

Materials and Methods

[0107] Powder X-Ray Diffraction (XRD).

[0108] Powder X-ray diffraction patterns were recorded on a PANalytical X'Pert Pro instrument using a Cu anode and K-alpha 1 =1.540598 and K-alpha 2 =1.544426 with a K-alpha 2/K-alpha 1 ratio of 0.5. The generator voltage was set to 40 kV and the tube current to 40 mA at 0.01 s.sup.1 from 3 to 70 with a slit size of 1. Samples were ground in powder form and loaded onto stainless steel sample holders.

[0109] Thermogravimetric Analysis (TGA).

[0110] Thermogravimetric analysis was carried out using a Mettler Toledo TGA/DSC 1 System. Around 20 mg of the sample was heated in a crucible from 25 to 700 C. at a rate of 5 C. per minute, and then left to cool.

[0111] Dynamic Light Scattering (DLS).

[0112] A Malvern Zetasizer Nano ZS in the Begbroke Science Park was used to carry out the dynamic light scattering analysis. A small amount of the sample in paste form was fully dispersed in about 10 mL of dionised water using a sonicator for 5 minutes, this dispersion was then pipetted into a plastic cuvette to the suggested level and inserted into the instrument.

[0113] Transmission Electron Microscopy (TEM).

[0114] Transmission electron microscopy images were obtained using a JEOL 2100 microscope with an accelerating voltage of 200 kV to view the samples. A small amount of the LDH sample in paste form was dispersed in ethanol in a sonicator for about 3 minutes, and then cast onto copper grids coated with Formvar film.

[0115] Fourier Transform Infrared (FTIR) Spectroscopy.

[0116] FTIR spectra were recorded on a Nicolet iS5 Spectrometer equipped with the iD3 ATR (attenuated total reflection) accessory, measuring in the range of 400-4000 cm.sup.1 with 50 scans at 4 cm.sup.1 resolution.

[0117] Solid State Nuclear Magnetic Resonance (NMR) Spectroscopy.

[0118] .sup.27Al DPMAS and .sup.13C CPMAS Solid state NMR spectra were obtained at 104.2 and 100.5 MHz respectively (9.4 T) on a Bruker Avance IIIHD spectrometer. For .sup.27Al NMR spectroscopy, in order to obtain quantitative MAS spectra, a single pulse excitation was applied using a short pulse length (0.15 s). 7000 scans were acquired with a 0.1 s delay and a MAS rate of 40 kHz using 1.9 mm O.D zirconia rotors. The .sup.27Al NMR spectroscopy chemical shift is referenced to an aqueous solution of Al(NO.sub.3).sub.3. .sup.13C CPMAS NMR spectra were measured using 4 mm O.D zirconia rotors and a MAS rate of 10 kHz using a cross-polarization sequence with a variable X-amplitude spin-lock pulse.sup.1 and spinal64 proton decoupling. 1500 transients were acquired using a contact time of 1.0 ms, an acquisition time of 12.5 ms (1024 data points zero filled to 16 K) and a recycle delay of 5 s. All .sup.13C NMR spectra were referenced to adamantane (the upfield methine resonance was taken to be at =29.5 ppm.sup.2 on a scale where (TMS)=0) as a secondary reference.

[0119] Scanning Electron Microscopy (SEM).

[0120] SEM images were obtained using a JEOL JSM 6610 scanning microscope.

[0121] Brunauer-Emmett-Teller Surface Area Analysis (BET).

[0122] The gas adsorption isotherm for nitrogen adsorption onto the LDH surface was measured using a Tristar II plus 3030. The samples were degassed at 110 C. overnight using a VacPrep degas machine. The Brunauer-Emmett-Teller (BET) method was then used to calculate the surface area.

1. Relationship Between Metal Salt Precursor Concentration and Product LDH Particle Size

[0123] (A) 50 ml of a metal salt precursor solution, wherein the solution was 1.28 M Ca(NO.sub.3).sub.2.4H.sub.2O and 0.64 M Al(NO.sub.3).sub.3.9H.sub.2O, was put into a laboratory blender together with 50 ml of 4.4 M NaOH and mixed at 20,000 rpm for 90 s. CaAlNO.sub.3 LDH precipitated during this rapid mixing stage. The liquor containing the precipitated material was centrifuged at 9,000 rpm for 10 minutes followed by washing with deionised water. The centrifugation and washing steps were repeated three times to give a product in the form of a wet paste. [0124] The preparation procedure was repeated using: [0125] (B) a metal precursor solution of 0.64 M Ca(NO.sub.3).sub.2.4H.sub.2O and 0.32 M Al(NO.sub.3).sub.3.4H.sub.2O with 2.2 M NaOH; and [0126] (C) a metal precursor solution of 0.32 M Ca(NO.sub.3).sub.2.4H.sub.2O and 0.16 M Al(NO.sub.3).sub.3.9H.sub.2O with 1.1 M NaOH; and [0127] (D) a metal precursor solution of 0.16 M Ca(NO.sub.3).sub.2.4H.sub.2O and 0.08 M Al(NO.sub.3).sub.3.9H.sub.2O with 0.55 M NaOH.

[0128] The particle sizes of the CaAlNO.sub.3 LDH's obtained were measured. The results are shown in Table 1. T.E.M. images of the CaAlNO.sub.3 LDH crystals obtained in (B), (C) and (D) are shown in FIGS. 1, 2 and 3, respectively. The CaAlNO.sub.3 LDH obtained in Example B was subjected to X-ray powder diffraction analysis. The plot of intensity (a.u.) against 2 Theta (degree) for the material is shown in FIG. 8.

TABLE-US-00001 TABLE 1 Stirring time Particle Example Ca(NO.sub.3).sub.24H.sub.2O/M Al(NO.sub.3).sub.39H.sub.2O/M NaOH/M and speed size/nm A 1.28 0.64 4.4 90 s 20000 rpm 80 B 0.64 0.32 2.2 90 s 20000 rpm 100 C 0.32 0.16 1.1 90 s 20000 rpm 200-300 D 0.16 0.08 0.55 90 s 20000 rpm 300-500

[0129] It is clear from the results shown in Table 1 that the particle size of the LDH particles obtained depends on the concentration of the metal salts in the metal precursor solution used. The highest metal salt concentration used gave the smallest LDH particles (80 nm) and the lowest metal salt concentration used gave the largest sized LDH particles with a distribution of 300-500 nm.

2. Relationship Between Ageing Time and Product LDH Particle Size

[0130] Using solutions according to (D) in Experiment 1 above, the relationship between ageing time and product LDH particle size was investigated. As in Experiment 1 above, the basified solutions were subjected to mixing at 20,000 rpm for 90 s in a laboratory blender. After this 90 s mixing, the solutions were each aged while maintaining the stirring speed of the laboratory mixer at 600 rpm. The ageing times, at this stirring speed, were 2 h (Example E), 4 h (Example F), 6 h (Example G) and 8 h (Example H). The particle size of the LDH products obtained in each case was determined. The results are shown below in Table 2. T.E.M. images of the CaAlNO.sub.3 LDH crystals are shown in FIG. 4 (Example E), FIG. 5 (Example F), FIG. 6 (Example G) and FIG. 7 (Example H).

TABLE-US-00002 TABLE 2 Ageing time dependence on particle size control Ageing time/h & Stirring time/s stirring speed/rpm + Particle Example Ca(NO.sub.3).sub.24H.sub.2O/M Al(NO.sub.3).sub.39H.sub.2O/M NaOH/M and speed/rpm N.sub.2 size/nm E 0.16 0.08 0.55 90 s 20000 rpm 2 h 600 rpm 300-500 F 0.16 0.08 0.55 90 s 20000 rpm 4 h 600 rpm 500-700 G 0.16 0.08 0.55 90 s 20000 rpm 6 h 600 rpm 600-800 H 0.16 0.08 0.55 90 s 20000 rpm 8 h 600 rpm 800-1000

[0131] It can be seen from the results reported in Table 2 that ageing for 8 hr at 600 rpm considerably increases the particle size distribution compared to the particle size range obtained in Example D of Experiment 1 reported above. It can also be seen that particle size of the LDH obtained in this experiment increases with increasing ageing time.

3. Preparation of Mg.SUB.3.Al-LDH30 g Scale

[0132] Carbonate intercalated Mg.sub.3Al-LDH (Mg.sub.3Al(OH).sub.8(CO.sub.3).sub.0.5.4H.sub.2O, Mg.sub.3AlCO.sub.3 LDH) has been synthesised using rapid mixing method. 59.97 g of Mg(NO.sub.3).sub.2.6H.sub.2O and 29.25 g of Al(NO.sub.3).sub.3.9H.sub.2O are mixed in 100 ml of degassed DI water called solution A. 24.96 g of NaOH and 4.134 g of Na.sub.2CO.sub.3 are dissolved in 150 ml of degassed DI water called solution B. These precursor solutions are mixed rapidly via homogeniser at 20,000 rpm. The LDH has been made at room temperature for 30 minutes. Vacuum filtration and washing with DI water are used to remove excess salts. The LDH is then treated with acetone with the ratio of weight of LDH powder and acetone to 1:10 for 1 hr. The LDH is separated from acetone and left to dry under vacuum oven at 65 C. for 8 hours.

[0133] FIG. 9 shows TEM of Mg.sub.3Al(OH).sub.8(CO.sub.3).sub.0.5.4H.sub.2O powder after acetone treatment and aging for 30 minutes.

4. Preparation of Ca.SUB.2.Al-LDH30 g Scale

[0134] Nitrate intercalated Ca.sub.2Al-LDH (Ca.sub.2Al(OH).sub.6(NO.sub.3).2H.sub.2O, Ca.sub.2AlNO.sub.3 LDH) has been synthesised using rapid mixing method. 44.42 g of Ca(NO.sub.3).sub.2.4H.sub.2O and 35.36 g of Al(NO.sub.3).sub.3.9H.sub.2O are mixed in 150 ml of degassed DI water called solution A. 22.57 g of NaOH is dissolved in 100 ml of degassed DI water called solution B. These precursor solutions are mixed rapidly via homogeniser at 20,000 rpm. These series of the LDHs has been made at room temperature for aging time of 2, 5, 10, 20, and 30 minutes. Vacuum filtration and washing with DI water are used to remove excess salts. The LDHs are then treated with acetone with the ratio of weight of LDH powder and acetone to 1:10 for 1 hr. The LDHs are separated from acetone and left to dry under vacuum oven at 65 C. for 8 hours.

[0135] FIGS. 10-14 show TEM of Ca.sub.2Al(OH).sub.6(NO.sub.3).2H.sub.2O powder after acetone treatment and aging at 2, 5, 10, 20, and 30 minutes respectively.

[0136] FIGS. 15-19 show XRD patterns of Ca.sub.2Al(OH).sub.6(NO.sub.3).2H.sub.2O after aging at 2, 5, 10, 20, and 30 minutes respectively.

5. Preparation of Ca.SUB.2.Al-LDH Dispersed in Ethyl Acetate150 g Scale

[0137] Nitrate intercalated Ca.sub.2Al-LDH (Ca.sub.2Al(OH).sub.6(NO.sub.3).2H.sub.2O, Ca.sub.2AlNO.sub.3 LDH) has been synthesised using rapid mixing method. 266.52 g of Ca(NO.sub.3).sub.2.4H.sub.2O and 212.16 g of Al(NO.sub.3).sub.3.9H.sub.2O are mixed in 900 ml of degassed DI water called solution A. 135.42 g of NaOH is dissolved in 1,100 ml of degassed DI water called solution B. These precursor solutions are mixed rapidly via homogeniser at 20,000 rpm. The LDH has been made at room temperature for aging time of 10, 20, and 30 minutes. Vacuum filtration and washing with 3,600 ml of DI water are used to remove excess salts. The LDH is then treated with acetone with the ratio of weight of LDH powder and acetone to 1:10 for 1 hr. The LDH is separated and dispersed in 1,800 ml of ethyl acetate for 1 hr. And then the LDH is separated and suspended in 1,800 ml of ethyl acetate.

[0138] FIGS. 20-22 show TEM of Ca.sub.2Al(OH).sub.6(NO.sub.3).2H.sub.2O dispersed in ethyl acetate after aging at 10, 20 and 30 minutes respectively.

6. Further Aging Studies

[0139] Synthesis of Ca.sub.2AlNO.sub.3-LDH 7.56 g of Ca(NO.sub.3).sub.2 and 6.00 g of Al(NO.sub.3).sub.3 (to give a 2:1 Ca:Al ratio of cations) were dissolved in 50 mL of deionised and degassed water (purged with N.sub.2 for two hours to remove any carbonate ions), to give a 0.64 M solution of calcium ions and a 0.32 M solution of aluminium ions. 4.40 g of NaOH pellets were dissolved in another 50 mL of deionised and degassed water to give a 2.2 M solution of NaOH. The colloid mill was first washed with water, and then deionised water three times. The previous two solutions were then poured into the mill for a mixing time of 90 s, the rotor speed was set to 2000 rpm and the gap to G5. After mixing the product was collected, and the colloid mill was washed once with water, once with 10% HNO.sub.3 and then twice again with deionised water. The Ca.sub.2AlNO.sub.3-LDH sample was washed using deionised and degassed water 4 times using a centrifuge at 9000 rpm for 5 minutes. The sample was then collected, 0.5 g was dried in a vacuum oven for characterisation; the rest of the paste was stored in a fridge at 8 C.

[0140] Ca.sub.2AlNO.sub.3-LDH was synthesised using the rapid mixing method in the colloid mill as detailed below. The powder X-ray (XRD) pattern of Ca.sub.2AlNO.sub.3-LDH shown in FIG. 23 is consistent with the expected pattern. The infra-red (IR) spectroscopy is shown in FIG. 24 and highlights absorptions at 3600 cm.sup.1 (OH and intercalated water), 1630 cm.sup.1 (bending mode of water), 1400 and 1350 cm.sup.1 (NO stretching mode of the intercalated NO.sub.3.sup.). The transmission electronic microscopy (TEM) and scanning electronic microscopy (SEM) images show that the LDH particles synthesised using the rapid mixing method in the colloid mill have a hexagonal plate like morphology as expected from the literature, FIGS. 25 and 26. The darker areas on the TEM image indicate stacking of the LDH sheets, or a thicker sheet, The average particle size was found to be 250 nm, with a large standard deviation of 106 nm. Thermogravimetric analysis (TGA) was used to analyse the thermal decomposition of the Ca.sub.2AlNO.sub.3-LDHs, FIG. 27. The first weight loss between room temperature and 200 C. (T1) is due the loss of the physisorbed water (or other solvent) either on the surface or in the interlayer. The second weight loss which occurs between 200 and 450 C. (T2) is due to the loss of water from dihydroxylation of the inorganic layers. The third beyond 450 C. (T3) is due to the decomposition of the intercalated nitrate group (or other guest anions). The Brunauer-Emmett-Teller (BET) demonstrates a curved shape of the adsorption isotherm suggesting a microporous structure, FIG. 28. The LDH sample has a surface area of 17.95 m.sup.2.g.sup.1, similar to publish data. The .sup.27Al solid state NMR spectrum shows one resonance peak at 10.05 ppm, consistent with a single aluminium environment in the sample, FIG. 29.

Effect of Aging Time

[0141] The Ca.sub.2AlNO.sub.3-LDH paste sample was left in a fridge at 8 C. Small amounts of the sample were extracted and tested after 1 week, and after 4 weeks.

[0142] Paste Ca.sub.2AlNO.sub.3-LDH samples synthesised by rapid mixing method were stored at 8 C. and tested after different time periods to observe the effect on the particles. The sharpness of the diffraction peaks increases as the ageing time is increased, the 002 peak increased in intensity from 931 to 11871 a.u. in 4 weeks, showing there is a significant change in the particles over time despite the low temperatures (FIG. 30 and Table 3).

TABLE-US-00003 TABLE 3 Peak intensities and CDLs of Ca.sub.2AlNO.sub.3- LDHs after different lengths of time at 8 C. CDL along c-axis CDL along a- and b- Intensity of 002 using 002 axes, using 300 reflections reflections reflections Time at 8 C. (a.u.) () () 1 day 931.0 146.6 106.7 1 week 4245.3 448.0 177.8 4 weeks 11871.0 733.2 251.9

[0143] An average crystallite size (or the mean crystallite domain length (CDL) along the a-, b- and c-axes) can be calculated using the Scherrer equation. The CDL along the c-axis increased from 143.4 to 717.0 in 4 weeks, Table 3. This is an important discovery for the future storage of Ca.sub.2AlNO.sub.3-LDHs as wet pastes for their use as additives in cement technology where particle size is important.

[0144] The average particle sizes calculated from the TEM images reveals a large increase in average particle size when the LDHs are left at 8 C., from 250 to 705 nm in 4 weeks, confirming previously analysed data. However, the standard deviation for the data is extremely large (FIG. 31). However, more importantly, the morphology begins to change (FIG. 32). Immediately after synthesis most of the LDH particles have a hexagonal plate-like morphology (circled in FIG. 32a). As ageing time is increased the particles begin to stretch in one plane forming parallelogram shaped plate-like LDH particles (circled in FIG. 32b). This may be because the surface energy of one face is lower than another, so the particles grow preferentially in one direction. The TEM images of the LDHs that have been left in the fridge for 4 weeks show 3D diamond like LDH particles (circled in FIG. 32c).

[0145] Scanning electron microscopy (SEM) was also be used to study the particles morphology. The morphology directly after synthesis had well defined hexagonal plate-like Ca.sub.2AlNO.sub.3-LDH particles, FIG. 26. The morphology of the particles after ageing appears significantly less defined, FIG. 33.

[0146] The DLS data in FIG. 34 shows a very significant increase in particle size with ageing time. The average particle size increased from 430 to 1865 and 2461 nm in 1 and 4 weeks respectively, the standard deviation of the average particle size also increased from 8 to 188 nm. TGA data also reveal an increase in particle size with ageing time. Table 4 shows that the temperatures of weight loss are much higher after ageing in the fridge. This is due to the increased particle size.

TABLE-US-00004 TABLE 4 TGA data after different ageing times at 8 C. Temperature ( C.) Ageing Time T1 T2 T3 1 day fridge 90.3 258.0 539.5 1 week fridge 105.5 273.3 565.4 4 weeks fridge 100.2 275.4 576.7

[0147] It is possible to see a strong effect due to the speed of colloid mill, FIG. 35. There is a strong effect on the particle size when the samples were aged at 8 C. from 2315 to 753 and 530 nm for 2000, 5000 and 8000 rpm speed of the rotor respectively, FIG. 35. The effect is not as noticeable on fresh samples.

Effect of Aging Temperature

[0148] Three different Ca.sub.2AlNO.sub.3-LDH paste samples were left for 1 week at different temperatures. One was left at room temperature, 23 C., one in a fridge, 8 C., and one at 20 C. in a freezer. The samples were tested after this week.

[0149] To investigate how temperature affects the rate of growth and change in morphology of the LDH particles paste samples of Ca.sub.2AlNO.sub.3-LDH synthesised by rapid mixing method were left for 1 week at different temperatures and then tested. The XRD patterns show a sharpening of the peaks after ageing at 8 and 23 C., consistent with an increase in crystallinity (FIG. 36). Use of the Scherrer equation suggests that the particles have grown after 1 week at all temperatures, even at 20 C. (Table 5). The CDL along the c-axis has increased from 143.4, to 183.4, 483.2 and 303.4 for 20, 8 and 23 C. respectively.

TABLE-US-00005 TABLE 5 Peak intensities and CDLs of Ca.sub.2AlNO.sub.3-LDHs after 1 week ageing at different temperatures. CDL along c-axis CDL along a- and b- 1 Week Ageing Intensity of 002 using 002 axes, using 300 Temperature reflections reflections reflections ( C.) (a.u.) () () Before Ageing 931.0 146.6 106.7 20 221.2 187.6 177.9 8 4245.3 448.0 177.9 23 1725.5 310.2 177.9

[0150] The particles appear to grow less at room temperature (23 C.) than in the fridge (8 C.), which were a surprising result. However, the XRD pattern for Ca.sub.2AlNO.sub.3-LDH paste after 1 week ageing at 23 C. appears to be impure as extra diffraction peaks are seen. This indicates that the Ca.sub.2AlNO.sub.3-LDHs paste synthesised by rapid mixing method are not stable at room temperature and therefore have started to decompose during the week, having a direct effect on particle growth. This explains why the Ca.sub.2AlNO.sub.3-LDH aged at 8 C. is more crystalline than at 23 C. (002 peaks have intensities of 4245.3 and 1725.5 a.u.). The impurities appear to be calcium aluminium oxide carbonate hydrate, and calcium aluminium oxide nitrate hydroxide carbonate.

[0151] Particle size was also studied using the TEM images. These data suggest an increase in particle size with ageing temperature, FIG. 37. Particles stored at 20 C. for 1 week had an average particle size of 199 nm, and particles stored at 23 C. had an average size of 415 nm. The standard deviation in particle size increased as storage temperature increased demonstrating a loss of control on the average particle size.

[0152] TEM images were also used to study the particle morphology (FIG. 38a/b stored at 20 C. for 1 week; FIG. 38c stored at 23 C. for 1 week). The particles stored at 20 C. seemed uniform. The particles stored at 8 C. are well defined but have a larger variation of particle sizes (FIG. 32), the particles stored at 23 C. appear less well defined and exhibit impurities (FIG. 38), as the XRD data also suggested.

[0153] DLS was also used to study the particles size. When the sample was left at 20 C. for a week the average particle size increased to 559 nm (FIG. 39). This shows that ageing is significantly slowed, or perhaps stopped, when Ca.sub.2AlNO.sub.3-LDH is stored at very low temperatures. The Ca.sub.2AlNO.sub.3-LDH particles left at room temperature (23 C.) appear to be smaller (1449 nm) than the sample left at 8 C. (1865 nm, see FIG. 39), certainly due to the presence of the decompositions products.

[0154] The TGA data further suggest that the particles have increased in size after ageing at all temperatures as the weight loss temperatures have increased (Table 6).

TABLE-US-00006 TABLE S4 TGA data for Ca.sub.2AlNO.sub.3-LDHs after one week ageing at different temperatures. Ageing Temperature Temperature ( C.) ( C.) T1 T2 T3 Before Ageing 90.3 258.0 539.5 20 111.8 271.7 551.0 8 102.2 272.7 568.3 23 115.4, 149.3 275.2 565.8

[0155] The features disclosed in the foregoing description, in the claims and in the accompanying drawings may, both separately and in any combination thereof, be material for realizing the invention in diverse forms thereof.