Patent Application
20180170764
The present invention relates to a process for preparing very small size particles of layered double hydroxides (LDHs).
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 119, 2005 Layered Double Hydroxides ed. X Duan and D. G. Evans. The hydrotalcites, perhaps the most well-known examples of LDHs, have been studied for many years. LDHs can intercalate anions between the layers of the structure. WO 99/24139 discloses use of LDHs to separate anions including aromatic and aliphatic anions.
Owing to the relatively high surface charge and hydrophilic properties of LDHs, the particles or crystallites of conventionally synthesised LDHs are generally highly aggregated. The result of this is that, when produced, LDHs aggregate to form “stone-like”, non-porous bodies with large particle sizes of up to several hundred microns and low specific surface area of generally 5 to 15 m2/g (as disclosed for example in Wang et al Catal. Today 2011, 164, 198). Reports by e.g. Adachi-Pagano et al (Chem. Commun. 2000, 91) of relatively high surface area LDHs have specific surface areas no higher than 5 to 120 m2/g.
For use in certain applications (for example, adsorbents, coatings and catalyst supports), it is advantageous to provide LDHs of very small size.
It has previously been suggested that small particle LDHs can be obtained using, as solvent, a mixture of water and one or more organic solvent. Generally, such processes require ageing the mother liquor for a few hours at an elevated temperature, e.g. 50-200° C. to provide the required LDH particles. The use of organic solvents increases costs and introduces the need for solvent recovery procedures. Ageing at elevated temperatures not only increases production costs but also lengthens the production time required for obtaining the LDH particles.
It is an object of the present invention to provide a process for preparing small size LDH particles which overcomes disadvantages mentioned above, particularly one which does not require the use of organic solvents, elevated temperatures or essential ageing and/or drying steps.
When the LDH being produced is a Ca—Al LDH, it is often the case in prior art processes that the obtained product contains CaCO3 as an impurity. It is, therefore, a further object of the present invention to provide a process which can produce Ca—Al LDH which is not contaminated by CaCO3.
In particular, it is an object of the present invention to provide a process for preparing very small size LDHs, especially Ca—Al LDHs, which is quick, efficient and can be conducted with low costs.
According to a first aspect of the present invention, there is provided a Ca—Al—NO3 layered double hydroxide, in a substantially pure form, and having a particle size of not greater than 2000 nm.
According to a further aspect of the present invention, there is provided a process for preparing particles of a layered double hydroxide of the general formula
[Mpz+M′qy+(OH)2]a+(Xn−)a/n.bH2O (I)
wherein Mz+ and M′q+ are metal cations or mixtures of metal cations, z=1 or 2; y=3 or 4; p+q=1; b=0 to 10, Xn− is an anion, n is 1 to 5 and a is determined by p, q, y and z such that a=zp+yq−2
which method comprises
According to a further aspect of the present invention, there is provided a layered double hydroxide of the general formula
[Mpz+M′qy+(OH)2]a+(Xn−)a/n.bH2O (I)
wherein Mz+ and M′y+ are metal cations or mixtures of metal cations, z=1 or 2; y=3 or 4; p+q=1; b=0 to 10, Xn− is an anion, n is 1 to 5 and a is determined by p, q, y and z such that a=zp+yq−2, the layered double hydroxide being obtainable, obtained or directly obtained by a process defined herein.
When compared with prior art techniques, the present process provides numerous advantages. Chiefly, the present process provides a rapid method for producing small particle size LDH, the rapid nature of which being such that the method can be conducted under an atmosphere of air without detriment to the purity of the product. Accordingly, the present process obviates the need for an inert (e.g N2) blanket, which has until now been necessary to avoid generating unwanted side products, such as calcium carbonate.
According to the process of the invention, step (a) comprises rapidly mixing Mz+ cations, M′y+ cations and Xn− anions, with a base. The aqueous solution may be prepared by mixing together, in any order, an aqueous solution containing at least one salt of metal M, an aqueous solution containing at least one salt of metal M′, an aqueous solution containing Xn− anions and a solution containing a base, for instance, NaOH. Alternatively, the anion Xn− may be present in the solution containing Mz+ cations or in the solution containing M′y+ cations, or in both of these solutions, or in the basic solution. According to a preferred embodiment, a solution will comprise a salt of metal M with the anion X and a salt of metal M′ with the anion X. A solution containing the base, such as NaOH, may then be added to this. When all of the cations, anion X and base are in solution together, the solution is preferably mixed rapidly.
It is preferred that M is Li, Mg, Zn, Fe, Ni, Co, Cu, or Ca, or a mixture of two or more thereof.
Further preferred is that y is 3 and M′ is Al, Ga, In, or Fe or a mixture of two or more thereof. When M′ comprises a mixture, it is preferably a mixture of Al and Fe.
According to a preferred embodiment, M′ is Al.
According to another preferred embodiment, in the layered double hydroxide, M/M′ is selected from Zn/Al, Ni/Al, Mg/Al, and/or Ca/Al, preferably Ca/Al.
In a further embodiment, it is preferred that Xn− is an anion selected from halide, inorganic oxyanion, anionic surfactants, anionic chromophores, and/or anionic UV absorbers.
Most preferred, the inorganic oxyanion is a carbonate, bicarbonate, hydrogenphosphate, dihydrogenphosphate, nitrite, borate, nitrate, sulphate, sulphite or phosphate anion or a mixture of two or more thereof, preferably a nitrate anion.
According to one preferred embodiment, step (a) in the process of the invention is carried out in a high speed mixer and mixing is preferably carried out at a mixing speed not lower than 5000 rpm, more preferably not lower than 8000 rpm.
In another embodiment, step (a) in the process of the invention is carried out at a mixing speed not lower than 12,000 rpm. Suitably, step (a) in the process of the invention is carried out at a mixing speed not lower than 15,000 rpm. More suitably, step (a) in the process of the invention is carried out at a mixing speed not lower than 17,000 rpm. Such mixing speeds may, for example, be achievable by using a disperser or a homogeniser.
In a particular embodiment, step (a) in the process of the invention is carried out at a mixing speed of 18,500 rpm to 25,000 rpm (using, for example, a disperser or a homogeniser).
Mixing step (a) may be performed using a disperser or homogeniser having a rotor and a stator.
In another embodiment, the total volume of material mixed during step a) does not exceed 2 litres.
According to a different preferred embodiment, step (a) in the process of the invention is carried out in a colloid mill and mixing is preferably carried out at a mixing speed not lower than 300 rpm.
Typically, mixing in step (a) of the process of the invention is carried out for a period of from 1 to 15 minutes.
In an embodiment, step (a) is performed for a period not longer than 10 minutes. Suitably, step (a) is performed for a period not longer than 5 minutes.
In an embodiment, step (a) is performed for a period of between 0.5 and 5 minutes at a mixing speed not slower than 1500 rpm. Suitably, step (a) is performed for a period of between 0.5 and 3 minutes at a mixing speed not slower than 1500 rpm. More suitably, step (a) is performed for a period of between 0.5 and 5 minutes at a mixing speed not slower than 10,000 rpm. In a particular embodiment, step (a) is performed for a period of between 0.5 and 3 minutes at a mixing speed not slower than 17,500 rpm.
In an embodiment, 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 1000 nm, preferably not greater than 800 nm, more preferably not greater than 500 nm, more preferably not greater than 300 nm, and most preferably not greater than 100 nm.
According to an embodiment, the base comprises OH− anions. Preferably, the base is NaOH.
Furthermore, it is preferred that the pH value of the aqueous solution is adjusted by the addition of NaOH or a mixture of NaOH and NaX (X=anion) to the solution.
Typically, in step (a) above, whether a high speed mixer or a colloid mill is used as the mixing apparatus for performing the rapid mixing, in aqueous solution of the Mz+ cations, M′y+ cations and Xn− anions with a base, solutions containing the ions and base are preferably added to the mixing apparatus simultaneously. Mixing, in step (a), is preferably commenced within 30 min after the addition of all of the cations, anion Xn− and base, in aqueous solution, to the mixing apparatus and, most preferably, immediately.
A further object is achieved by a Ca—Al—NO3 layered double hydroxide, in a substantially pure form, and having a particle size of not greater than 2000 nm, preferably not greater than 300 nm and most preferably not greater than 100 nm.
It has been found that a rapid mixing of the solution promotes rapid nucleation of the LDH. The rapid nucleation under rapid mixing conditions causes quick precipitation of the LDH but halts the growth of LDH crystals such that an LDH colloid is formed having very small particle size, typically not greater than 2000 nm, preferably not greater than 800 nm, more preferably not greater than 500 nm, even more preferably not greater than 300 nm, yet even more preferably not greater than 200 nm and, most preferably, not greater than 100 nm.
The particle size was determined as the mean platelet diameter from a study of 100 particles by Transmission Electron Microscopy (TEM).
Rapid precipitation also improves the purity of the LDHs, particularly in the case of the preparation of Ca-containing LDHs in air where CaCO3 precipitation is a highly favoured side reaction.
According to a preferred embodiment of the process of the invention, an aqueous solution of a salt of metal(s) M with the anion X and an aqueous solution of a salt of metal(s) M′ with the anion X are added to a mixer. These may be added separately or a solution containing all of the ions may be prepared first and then added to the mixer together with a base.
The aqueous solutions of the metal salts prepared and added to the mixer in the process are substantially pure. By the term “substantially pure” it is meant that the aqueous solutions do not contain any deliberately or intentionally added substances or compounds, such as organic solvents or aqueous anions other than X. In addition, purity of the product may be enhanced by using de-ionised water in the preparation of the solution or degased de-ionised water.
In the context of Ca—Al—NO3 LDHs, the term “substantially pure” also means that the LDH contains no calcium carbonate. This can be determined, for example, by XRD analysis, since the Ca—Al—NO3 LDHs of the invention contain no other observable Bragg reflections from other crystalline contaminants, such as metal carbonates. In this sense, the Ca—Al—NO3 LDHs of the invention are considered to be phase pure.
The metal salt aqueous precursor solutions typically have a high concentration of the metal salt. More typically, the concentration of the salt of metal M with the anion X in the aqueous solution will be in the range of 0.1 to 3 M, preferably 0.1 to 1.5 M. Alternatively, the concentration of the salt of metal M with the anion X in the aqueous solution will be in the range of 0.1 to 1 M, preferably 0.1 to 0.8 M, more preferably 0.1 to 0.7 M, yet more preferably 0.3 to 0.7 M. The concentration of the salt of metal M′ with the anion X in the aqueous solution will be chosen according to the requirement for M′ in the LDH and based on the concentration of the salt of metal M used. For instance, in the case where z in the formula I is 2 and y in the formula I is 3, the concentration of the M′ salt in its aqueous precursor solution will typically be about one half of the concentration of the M salt in its aqueous precursor solution so as to avoid the use of excess metal cations. Highly concentrated metal salt solutions promote rapid LDH precipitation under alkali conditions (pH>7) which further improves the phase purity of the LDHs, particularly in the case of the preparation of Ca-containing LDHs in air, where CaCO3 precipitation is a highly favoured side reaction.
Preferably, a base such as NaOH is added to the metal ion solution, during mixing, in order to raise the pH of the solution to a pH value greater than 7, preferably greater than 9, more preferably greater than 10.
It has been found by the inventors that, surprisingly, under rapid mixing conditions, there is a relationship between the concentration of the M and M′ metal salt precursor solution used and the size of the LDH crystals obtained by the process of the invention. For instance, using a solution of 0.64 M Ca(NO3)2.4H2O and 0.32 M Al(NO3)3.9H2O and mixing this with 2.2 M NaOH for 90 s at a mixing speed of 20,000 rpm, LDH crystals of 100 nm are obtained whereas LDH crystals of 200-300 nm are obtained by mixing a solution of 0.32 M Ca(NO3)2.4H2O and 0.16 M Al(NO3)3.9H2O under the same mixing conditions. If, under the same conditions, a solution which is 0.16 M in Ca(NO3)2.4H2O and 0.08 M in Al(NO3)3.9H2O is subjected to rapid mixing then the LDH crystals obtained are in the size range of 400-500 nm.
Mixing apparatus which can be used to carry out the rapid mixing of the aqueous solution containing the metal cations, the anion Xn− and the base according to the present invention, may be any apparatus known to provide the required mixing speed. Examples of such apparatus known to the person skilled in the art of rapid mixing technology include high speed mixers, blenders and colloid mills.
After subjecting, in aqueous solution, Mz+ cations, M′z+ cations, Xn− anions and base, to rapid mixing, in accordance with the process of the invention, the mixed solution may, if desired, be subjected to ageing. Ageing the mixture, if desired, may typically be carried out by maintaining the mixture in the mixer, reducing the mixing speed of the mixer and maintaining mixing at the lower speed for a period of time. Typically, the mixing speed of the mixer during an ageing step, if used, will be about 8000 rpm or, preferably, lower, e.g. 5000 rpm or lower. The ageing step, at a reduced mixer speed, may typically be carried out for at least 1 hour and preferably at least 2 hours. It is also possible, after this ageing step, to continue ageing at an even lower mixing speed, e.g. 600 rpm, for up to 24 hours. Since ageing the mother liquor has been found to increase the particle size of the LDH product and/or increase the size distribution of the LDH product, the use of ageing in the process of the invention is not preferred.
According to step (b) of the process of the invention, the layered double hydroxide is allowed to precipitate from the solution mixed in step (a). The precipitated material is, thus, obtained as an aqueous slurry or paste. In such a slurry or paste, the LDH particles obtained tend not to form aggregates. Removal of water in order to concentrate an aqueous slurry or paste may be achieved by centrifugation of the liquor containing the precipitated material. Typically, the liquor containing the LDH particles may be subjected to centrifuge at 9000 rpm for a few to several minutes, for example 10 minutes. The treatment in the centrifuge may be repeated one or more times washing with de-ionised water between each centrifugation. In general, recovery may also, or instead, be facilitated by filtration, in particular when rather big particles are prepared, such as by the use of a filter candle.
Following step b), the precipitated LDH may be washed one or more times with water. Such washing steps may be necessary to remove excess salts.
Following isolation of the LDH, the LDH may be contacted with acetone or ethanol. Suitably the LDH is contacted with acetone at a weight ratio of LDH to acetone of 1:5 to 1:15 (e.g. 1:10) for 1 minute to 5 hours (e.g. 1 hour).
The isolated LDH may, in some embodiments, be dispersed in a solvent (e.g. ethyl acetate). Such a step may be necessary when it is desirable to form an organic solvent dispersion of the LDH for use in, for example, coating applications.
Typically, the aqueous slurry/paste obtained, for instance from the centrifugation step, will have a dry solids content in the range of from 12 to 45% by weight. If desired, LDH particles may be recovered from the slurry/paste containing the LDH particles by subjecting the slurry or paste to a drying procedure, so as to produce a dry, particulate product. Preferably, if a dry LDH product is desired, the drying procedure used should be selected from those procedures that minimise the possibility that the LDH particles will form aggregates during drying. If drying the product is desired, preferably a drying procedure such as vacuum drying at low temperature (e.g. 20° C.) or spray drying should be used to minimise any aggregation of the particles.
The process of the invention may be used to prepare particles of an LDH of the formula I above. According to a preferred embodiment, the LDH has the formula I in which z is 2 and M is Mg, Zn, Fe, Ni, Co, Cu or Ca or a mixture of two or more of these, when z is 1, M is preferably Li. Preferably, M is Ca. According to another embodiment, the LDH has the formula I in which y is 3 and M′ is Al, Ga, In, or Fe or a mixture of Al and Fe. Preferably, M′ is Al. According to an embodiment, the LDH is selected from Zn/Al, Ni/Al, Mg/Al and Ca/Al LDHs. It is an especially preferred embodiment of the invention that the LDH is a Ca/Al LDH.
The anion X is an anion preferably selected from halide (for example, chloride), inorganic oxyanion, anionic surfactants, anionic chromophores, and/or anionic UV absorbers. Examples of inorganic oxyanions include carbonate, bicarbonate, hydrogenphosphate, dihydrogenphosphate, nitrite, borate, nitrate, sulphate, sulphite and phosphate and mixtures of two or more of these. According to a preferred embodiment, the anion X is nitrate.
According to a particularly preferred embodiment of the process of the invention, the LDH prepared is a CaAl—NO3 LDH.
The present invention provides, according to a particularly preferred embodiment, a process for preparing particles of a Ca—Al—NO3 LDH, which particles have a size of not greater than 2000 nm, preferably not greater than 300 nm and most preferably of not greater than 100 nm which process comprises
The precipitated LDH produced according to this embodiment may, if desired, be recovered. Typically, recovery of the precipitated LDH will be achieved according to any of the various procedures described above.
According to an alternative embodiment, the present invention provides a process for preparing particles of a Ca—Al—NO3 LDH, which particles have a size of not greater than 2000 nm, preferably not greater than 300 nm and most preferably of not greater than 100 nm which process comprises
The precipitated LDH produced according to this embodiment may, if desired, be recovered. Typically, recovery of the precipitated LDH will be achieved according to any of the various procedures described above.
Preferably, the precursor aqueous metal solution consists essentially of Ca(NO3)2.4H2O and Al(NO3)2.9H2O in degased deionised water. Preferably, the precursor aqueous metal solution is greater than 0.1 M in Ca(NO3)2.4H2O, more preferably greater than 0.3 M, even more preferably greater than 0.6 M and most preferably greater than 1.0 M. Preferably, the Al(NO3)2.9H2O in the precursor aqueous metal solution has a concentration which is approximately half the concentration of Ca(NO3)2.4H2O in the solution, more preferably half the concentration of Ca(NO3)2.4H2O in the solution.
Preferably, the precursor aqueous metal solution is adjusted to a pH value greater than 7, more preferably greater than 9, and even more preferably greater than 10, during the rapid mixing operation. Typically, an addition of NaOH will be used to adjust the pH value of the solution.
The fine, particulate Ca—Al—NO3 LDH obtained according to this embodiment has great purity and is typically in the form of an aqueous slurry or paste. The LDH may be recovered according to any of the procedures described above. The particle size of the Ca—Al—NO3 LDH is sufficiently small that it finds use as an adsorbent, in coating compositions or as a catalyst support.
According to a further preferred embodiment, the invention provides a Ca—Al—NO3 layered double hydroxide, in a substantially pure form, which has a particle size of not greater than 1000 nm, preferably not greater than 800 nm, more preferably not greater than 500 nm, even more preferably not greater than 300 nm, and most preferably not greater than 100 nm.
If necessary, the aqueous slurry or paste recovered can be subjected to a drying operation that minimises the formation of aggregates of the LDH. Examples of drying operations include drying in a vacuum oven at low temperature under vacuum and spray drying using a conventional spray drying apparatus.
The following numbered paragraphs describe particular embodiments of the invention:
[Mpz+M′qy+(OH)2]a+(Xn−)a/n.bH2O (I)
[Mpz+M′qy+(OH)2]a+(Xn−)a/n.bH2O (I)
wherein Mz+ and M′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 is an anion, n is 1 to 5 and a is determined by p, q, y and z such that a=zp+yq−2, obtainable according to the process of any one of paragraphs 1 to 18.
Powder X-Ray Diffraction (XRD).
Powder X-ray diffraction patterns were recorded on a PANalytical X'Pert Pro instrument using a Cu anode and K-alpha 1 λ=1.540598 and K-alpha 2 λ=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−1 from 3 to 70° with a slit size of 1°. Samples were ground in powder form and loaded onto stainless steel sample holders.
Thermogravimetric Analysis (TGA).
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.
Dynamic Light Scattering (DLS).
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.
Transmission Electron Microscopy (TEM).
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.
Fourier Transform Infrared (FTIR) Spectroscopy.
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−1 with 50 scans at 4 cm−1 resolution.
Solid State Nuclear Magnetic Resonance (NMR) Spectroscopy.
27Al DPMAS and 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 27Al NMR spectroscopy, in order to obtain quantitative MAS spectra, a single pulse excitation was applied using a short pulse length (0.15 μs). 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 27Al NMR spectroscopy chemical shift is referenced to an aqueous solution of Al(NO3)3. 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 pulse1 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 13C NMR spectra were referenced to adamantane (the upfield methine resonance was taken to be at δ=29.5 ppm2 on a scale where δ(TMS)=0) as a secondary reference.
Scanning Electron Microscopy (SEM).
SEM images were obtained using a JEOL JSM 6610 scanning microscope.
Brunauer-Emmett-Teller Surface Area Analysis (BET).
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.
The particle sizes of the Ca—Al—NO3 LDH's obtained were measured. The results are shown in Table 1. T.E.M. images of the Ca—Al—NO3 LDH crystals obtained in (B), (C) and (D) are shown in
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.
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 Ca—Al—NO3 LDH crystals are shown in
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.
Carbonate intercalated Mg3Al-LDH (Mg3Al(OH)8(CO3)0.5.4H2O, Mg3Al—CO3 LDH) has been synthesised using rapid mixing method. 59.97 g of Mg(NO3)2.6H2O and 29.25 g of Al(NO3)3.9H2O are mixed in 100 ml of degassed DI water called solution A. 24.96 g of NaOH and 4.134 g of Na2CO3 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.
Nitrate intercalated Ca2Al-LDH (Ca2Al(OH)6(NO3).2H2O, Ca2Al—NO3 LDH) has been synthesised using rapid mixing method. 44.42 g of Ca(NO3)2.4H2O and 35.36 g of Al(NO3)3.9H2O 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.
Nitrate intercalated Ca2Al-LDH (Ca2Al(OH)6(NO3).2H2O, Ca2Al—NO3 LDH) has been synthesised using rapid mixing method. 266.52 g of Ca(NO3)2.4H2O and 212.16 g of Al(NO3)3.9H2O 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.
Synthesis of Ca2AlNO3-LDH 7.56 g of Ca(NO3)2 and 6.00 g of Al(NO3)3 (to give a 2:1 Ca:Al ratio of cations) were dissolved in 50 mL of deionised and degassed water (purged with N2 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% HNO3 and then twice again with deionised water. The Ca2AlNO3-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.
Ca2AlNO3-LDH was synthesised using the rapid mixing method in the colloid mill as detailed below. The powder X-ray (XRD) pattern of Ca2AlNO3-LDH shown in
The Ca2AlNO3-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.
Paste Ca2AlNO3-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 (
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 Å in 4 weeks, Table 3. This is an important discovery for the future storage of Ca2AlNO3-LDHs as wet pastes for their use as additives in cement technology where particle size is important.
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 (
Scanning electron microscopy (SEM) was also be used to study the particles morphology. The morphology directly after synthesis had well defined hexagonal plate-like Ca2AlNO3-LDH particles,
The DLS data in
It is possible to see a strong effect due to the speed of colloid mill,
Three different Ca2AlNO3-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.
To investigate how temperature affects the rate of growth and change in morphology of the LDH particles paste samples of Ca2AlNO3-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 (
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 Ca2AlNO3-LDH paste after 1 week ageing at 23° C. appears to be impure as extra diffraction peaks are seen. This indicates that the Ca2AlNO3-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 Ca2AlNO3-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.
Particle size was also studied using the TEM images. These data suggest an increase in particle size with ageing temperature,
TEM images were also used to study the particle morphology (
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 (
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).
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1502832.7 | Feb 2015 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/GB2016/050420 | 2/19/2016 | WO | 00 |
