GB2483801A

GB2483801A - Synthetic hydrotalcite

Info

Publication number: GB2483801A
Application number: GB1116128.8A
Authority: GB
Inventors: Emiliana Mariana Dvininov; Hazel Stephenson
Original assignee: Magnesium Elektron Ltd
Current assignee: Magnesium Elektron Ltd
Priority date: 2010-09-17
Filing date: 2011-09-19
Publication date: 2012-03-21
Anticipated expiration: 2031-09-19
Also published as: GB2483801B; GB201116128D0; GB201015603D0

Abstract

A synthetic hydrotalcite comprises the general formula M(II)xM(III)yM'(III)z(OH)aAâ cH20 or M(II)xM(III)yM(IV)z(OH)aAâ cH20, wherein; 0â ¤zâ ¤yâ ¤xâ m6, a>x>c>b; M(II) is a divalent cation selected from; Mg, Zn, Cu, Co, Fe and Ni; M(III) is at least one trivalent cation selected from Al, Mn, Co, Ga, La, Ce and Ti; M(IV) can be Zr or Ti; A defines an anion which compensates the positive charge of the brucite type layers and can be at least one of the following: carbonate, nitrate, sulphate, chloride, hydroxide and phosphate. The synthetic hydrotalcite may have a partial CO2 capacity greater than 5 wt%. A method of forming the synthetic hydrotalcite is comprises the following steps: a. preparation of a solution by mixing the metallic precursors other than carbonates to generate concentrations up to 3M; b. addition of a solution of an hydroxide and carbonate of an alkali element, said II having concentrations up to 10M, until the pH less than 12, at temperatures up to 70Â°C; and one or more of the following steps: c. filtration; d. washing; e. hydrothermal treatment; f. milling; g. drying; h. calcination.

Description

Inorganic oxides for CO2 capture

[001] Background of the invention

[002] 1. Field of the invention

[003] The invention relates to tuned compositions and properties of the mixed oxides generated through a thermal treatment applied to the corresponding layered double hydroxides and to the synthetic method involved into their synthesis. The methods minimise the time and temperature of ageing offering materials with different structural characteristics which make them suitable for CO2 sorption in various temperature and pressure conditions.

[004] 2. Description of the related art

[005] Over the last decades hydrotalcite-type compounds or layered double hydroxides have attracted much attention due to their exceptional properties, which make them suitable for an wide variety of applications such as, catalysis, wastewater treatment, acidic gaseous streams cleaning, polymer stabilizers, flame retardants, etc. [006] Layered double hydroxides are crystalline structures resulted by partially replacement of Mg2 cations in the brucite type layers by cations having the same valence but different sizes or superior valent cations.

These will generate an excess of positive charge, which will be accompanied by the presence of various anions, together with water molecules, at the level of interlayer space. When heated to elevated temperatures, both the water and anions are released, the corresponding mixed oxides being generated. If the thermal treatment is conducted at temperatures below 600°C these mixed oxides have the capacity to reform the initial structure when in contact with an anionic source. This phenomenon is known as "memory effect" and it is the milestone of the application directions of these types of materials. When heated above 750°C a mixture of spinel and MgO can be obtained, this process being irreversible. TG/DTA curves recorded for this type of materials indicated that the dehydration step usually appears between 100-300°C and is associated with the removal of both physi-and chemisorbed water. This first mass loss is followed by a second process, which corresponds to the dehydroxylation of the octahedral layers. This second mass loss is not very well defined being overlapped with the removal of the interlayer anions and occurs at temperatures ranging between 350-500°C.

[007] In terms of the synthetic procedures used for their manufacture, the literature is rich. The main approaches to obtain such type of materials includes the direct synthesis or the titration method where two solutions containing the cations source and a base are mixed together in the presence of the desired anion and the co-precipitation of the base and cations at a constant pH. Other techniques include co-precipitation in non-aqueous solutions, sol-gel processes, reaction of metal oxides/hydroxides with salts, urea method, etc. Known methods for producing layered double hydrotalcites are disclosed by the following patents: W02006/1 23284 (A2), US 2006/0276328 (Al), WO 2008/050927 (Al), US 2007/01 85251 (Al), US 7,740,818 (B2), US 2009/01 62658 (Al), US 201 0/0075847 (Al), PCT/US98/01605, US 2005/0080178, US 2004/0141907 (Al), US6514473 (B2), WO 2008/069610 (Al), RU2361814, EP20114358 (Al).

[008] By tuning the cationic content a large variety of compositions can be designed, compositions which have tailored properties such as basicity and anionic and water capacities.

[009] Despite the fact that these sorts of materials have predominant affinity for carbonate anion, by choosing the right synthetic procedure and specific cationic ratios, structures having predominantly other anions in the interlayer space can be generated. Also, by varying the reaction precursors, both the compensation anionic content and the level of impurities can be easily controlled.

[0010] 3. Summary of the invention

[0011] Accordingly, it is an object of the present invention to provide synthetic procedures for obtaining binary and tertiary hydrotalcites with controllable compositions and adequate particle sizes for various applications. The methods described here propose organic free simplified routes for the preparation of various hydrotalcite-type compounds.

[0012] In accordance with one aspect of the present invention, the methods described here aim to prepare M(ll)xM(lll)yM'(lll)z(OH)aAbcH2O (I) and M(ll)xM(lll)yM(lV)z(OH)aAbcH2O (II) type materials, wherein, M(ll) is a divalent cation selected from a series of Mg, Zn, Cu, Co, Fe and Ni; M(lll) is a trivalent cation selected from a series of Al, Mn, Co, Ga, La, Ce and Ti which posses a deviation of ionic radius of 50% of the ionic radius of Mg2, shares similar octahedral positions as Mg2 and are responsible for the generation of the positive charge along hydroxide layers of hydrotalcites; M(IV) can be Zr or Ti and also can occupy octahedral positions in the hydroxide layer of the hydrotyalcites being responsible for the enhancement of the excess of positive charge along these layers.

As anions can be present at the level of interlayer space any of the following: C032, N03, Cr, POt, Ha.

Also, in the case of the former composition 0«=zcycx«=6, while a>x>c>b. In the second case, 0«=z«=ycx«=6 and a>x>c>b.

[0013] In accordance with another aspect of the present invention, the above mentioned layered double hydroxides have been used as precursors to generate the corresponding mixed oxides. Basically, under specific thermal conditions the divalent and/or tetravalent cations migrate at the level of interlayer space contributing to the appearance of atomic level mixed oxides responsible for the enhanced catalytic and sorptive activities of these materials. Moreover, by incorporation of tetravalent elements, not only the anion exchange capacity was improved by increasing the positive charge density along the layers, but also the basicity of the generated mixed oxides.

[0014] In accordance with another aspect of the present invention, the synthetic procedures involved for the preparation of these materials are variations of the constant pH coprecipitation technique and titration method and, some of them, allow the obtaining of various compositions whose water content (both physi-and chem i-sorbed water) is released at far lower temperatures than similar materials. This is of a tremendous importance when talking about CO2 physi-sorption at low temperatures.

Also, the anionic content can be tailored so its releasing can be achieved at higher or lower temperatures depending upon the applications.

[0015] The enhancement of particles surface area can be achieved using two methods: increasing the tetravalent cation content which generates a higher disturbance along the octahedral layers and PT and/or bead milling of the reslurried material in water.

[0016] The bead milling procedures allows also reduction of the particle sizes of the hydrotalcite-type materials.

[0017] Brief description of the drawings

[0018] Figure 1. Evolution of a and c cell parameters with the zirconium content.

[0019] Figure 2. XRD patterns of the samples having a general formula of Mg3Al1Zr, where x varies between 0 and 0.3 and prepared by titration method (i).

[0020] Figure 3. XRD patterns of the samples having a general formula of Mg2Al1Zr, where x varies between 0.1 and 0.3 and prepared by titration method (i).

[0021] Figure 4. XRD patterns of the samples having a general formula of Mg3Al1Zr, where x varies between 0 and 0.5 and prepared by co-precipitation method (iii) in the absence of Na2CO3.

[0022] Figure 5. XRD patterns of various samples prepared by titration - (a) and (c) and coprecipitation -(b) and (d) methods.

[0023] Figure 6. XRD patterns showing the variation of cell parameters with the decreasing of magnesium content.

[0024] Figure 7. XRD patterns of the hydrotalcites with and without zinc in their composition prepared by route (iii).

[0025] Figure 8. XRD patterns of Mg1.5Zn1.5A1-HTs prepared using route (ii) and (iii).

[0026] Figure 9. The evolution of XRD patterns during a calcination/Na2CO3 cycle.

[0027] Figure 10. TG curves of the Zr-containing hydrotalcites prepared by method (i) and (iii). The insert shows the DTA profiles of the same samples.

[0028] Figure 11. DTA curves of Mg3AI1Zr compositions prepared by route (iii).

[0029] Figure 12. DTA curves of Mg2AI1Zr compositions prepared by 1 5 route (iii).

[0030] Figure 13. XRD patterns of the composition Mg3AI07Zr03, prepared by route (i), before and after pressure treatment at 120°C for 3 hours.

[0031] Figure 14. XRD patterns of the composition Mg3Al, prepared by route (i), before and after pressure treatment at 120°C for 3 hours.

[0032] Figure 15. DTA curves of Mg3AI0.9Zr0.1 hydrotalcites before and after pressure treatment at 120°C for 3 hours.

[0033] Figure 16. The variation of surface areas with zirconium content in the case of fresh made materials and the mixed oxides generated by heating at 450°C for 3hours.

[0034] Figure 17. TG and DTA profiles of Zn-containing materials prepared by route (ii) and (iii).

[0035] Figure 18. Sample Mg1.5Zn1.5A1-HT before and after pressure treatment at 12000 for 3 hours.

[0036] Figure 19. XRD patterns of Ti-containing hydrotalcites obtained by route (ii) and (iii).

[0037] Figure 20. Comparison of the XRD patterns of two hydrotalcites with and without ceria obtained through the route (ii).

[0038] Figure 21. Representation of the reduction of d50 values with the number of bead milling cycles.

[0039] Figure 22. A comparative view of the variation of both surface area and 002 capacity with zirconium content.

[0040] Figure 23. The evolution of 002 capacity of Mg3AI09Zr0.1 composition made by method (iii).

[0041] Figure 24. TPD-MS profiles of Mg3AI and Mg3AI095Ce005 indicating the temperatures where the H20 and 002 are desorbed.

[0042] Detailed description of the invention

[0043] Present invention refers to the preparation of hydrotalcite type compounds or layered double hydroxides whose compositions include zirconium and titanium among others and which are precursors for zirconium and titanium containing atomic level mixed oxides.

[0044] By selecting specific precursors and synthetic procedures various compositions can be generated, especially in terms of interlayered anions, impurities and water content.

[0045] As precursors for obtaining this sort of hydrotalcite-type compounds we have selected various nitrates, chlorides, carbonates and hydroxides, including but not limited to Mg(NO3)26H2O, Al(NO3)39H2O, ZrOCI2, ZrO(N03)2, Zr0003, NaOH and Na2CO3.

[0046] As synthetic procedures, we have approached three various procedures: (i)titration of the cationic solutions with a mixture of NaOH/Na2CO3, co-precipitation of the cationic solutions at constant pH (ii) with or (iii) without Na2CO3. The second method is a variation of the classical co-precipitation route where the cationic mixtures are coprecipitated using a mixture of NaOH/Na2CO3. Our method has proven a better control of the pH, by adding Na2003 in the reaction vessel before the co-precipitation reaction start. The initial solution has double role: act as a buffer during coprecipitation and provide a carbonate source for the resulted double layered hydroxides. Moreover, this second route allows a better control of the impurities when a chloride is used as cationic source.

Depending upon zirconium content, zirconium oxycarbonate can be used as carbonate source.

[0047] By selecting first synthesis method a series of various materials can be generated which contains carbonate as counterion. If ZrOCI2 is used as zirconium source we have noticed that chloride anion is very difficult to wash out, large amount of water being used to achieve this. This is of particularly concern when we talk about a post synthetic pressure treatment, the presence of chloride contribution to the pressure vessel corrosion. Figure 1 summarises the values of a and c cell parameters of these compounds function of zirconium content. The intercalation of zirconium at the level of brucite-type layers is translated in the XRD patterns through the increase of the d110 value (except one sample) and, correspondingly, the extension of a parameter due to partially replacement of the smaller Al3 cation (0.53A) by a bigger cation such as Zr4 (0.72A).

Despite the fact that Velu et al. (Chem. Mater. (1998), 10, 3451) have reported the increasing of c parameter with the increasing of zirconium content, we have observed an opposite behaviour (Fig. 2 and 3). Thus, as zirconium content increases we have noticed the shift of (003) and (006) to higher 2Theta values which indicates a constriction of the structure, mainly due to a strong interaction between brucite-type layers and the anions present in the interlayer space. This interaction is based on the enhancement of the positive charge density along these layers. Moreover, the introduction of zirconium in the brucite-type layers contributes to the decreasing of the resulted materials crystallinity, zirconium cation bringing a distortion at this level due to its bigger size. This distortion increases the number of the surface defects and consequently can contribute to the increase of surface areas.

[0048] If a different synthetic procedure is followed (iii) the trends (Fig. 4) of the evolution of cell parameters are similar, but we can notice an increase of the values of these cell parameters. If we compare samples having similar compositions, but made using different experimental procedures we can observe that the samples obtained by coprecipitation in the absence of carbonate anion shows (003) and (006) reflections positioned at lower 2 theta values which suggest a bigger c parameter and, correspondingly, a higher interlayer space (Fig. 5). This is due to the fact that all experiments have been carried on in the absence of Na2CO3 which allows the intercalation of nitrate anion as counterion. Due to its bigger size, this will contribute to the expansion of the interlayer space.

The elemental analysis of these materials suggested that nitrate anion is not the only one ion hosted by the cavities of these materials, its presence being accompanied by the presence of carbonate as well which is in far lower quantities.

[0049] An aspect already reported for this type of hydrotalcites is the decreasing of the crystallinity with the increase of zirconium content. This is completely confirmed by our experiments, as well, with the only one difference that the co-precipitation method leads to higher crystalline materials than the ones obtained by titration method. Not the same thing can be said about zirconium free materials (Fig. 5).

[0050] The decrease of Mg/Al atomic ratio from 3 to 2 reduces the a parameter, (110) reflection being shifted from 60.36° to 60.75°. This is due to the fact that the amount of aluminium increases, this cation being (0.53A) smaller than Mg2t (0.65A) and Zr4 (0.72A) (Fig. 6). The interlayer gallery decreases in size (003) reflection being shifted from 10.59° to 9.98°. This is due to the increases of the interaction between the positive charged layers and anions, the density of charge along brucite-type layers increasing as a bigger part of Mg2t cations is replaced by the trivalent cation Al3t. The same observations are valid for the Zr containing materials, as well, in this case (110) reflection being shifted from 59.91° to 60.63° and (003) reflection being shifted from 10.90° to 10.59°. While the (110) reflection shifts by only 0.39° in the zirconium free samples, in those where zirconium has replaced 3Oat% of aluminium this shift is almost double due to the incorporation of a bigger cation in the structure.

[0051] When a second divalent cation is introduced in the structure of Mg3AI-type hydritalcite to replace 50% of magnesium content a less crystalline material is obtained which shows the same cell parameters (Fig. 7), Zn2 being similar in size with Mg2. This structure contains nitrate as main counterion. If the same material is prepared using co-precipitation procedure, but in the presence of carbonate (ii), a higher crystalline material is obtained which shows a smaller interlayer gallery due to the incorporation of carbonate anion (Fig. 8). This is translated in the XRD pattern through the shift of (003) reflection from 10.87° to 11.36°.

[0052] The stability of the samples against thermal treatment and the memory form of these materials have been confirmed by calcinations at 500°C followed by the contact with a Na2CO3 aqueous solution. After calcinations at 500°C for 2 hours, the structure is converted to the corresponding mixed oxides. This is obvious in the XRD pattern, which shows the occurrence of periclase MgO reflections (Fig. 9). After being in contact with a diluted aqueous solution of Na2CO3 the initial structure of the material is recovered. Moreover, due to the replacement of nitrate anion by carbonate anion, which is smaller in size, the new structure will show (003) and (006) reflections positioned at higher 2 Theta values. This means a reduction of c parameter from 24.07A in the case of as made sample to 23.76A in the case of the material obtained through the contact of the calcined samples with Na2CO3 solution while a parameter remains constant (3.07A).

[0053] By incorporation of zirconium into the brucite-type layers of this materials not only that the temperatures of the anions removal from the interlayer space are significantly decreased, but, also, the surface area of the resulted mixed oxides is increased.

[0054] Despite the fact that both route (i) and (iii) don't imply a prolonged ageing stage the temperatures where the water content is released varies significantly being higher in the case of the samples prepared by titration method. Fig. 10 shows the profiles of the TG and DTA curves belonging to two samples having the same composition and prepared using routes (i) and (iii). Thus, while the anionic content is removed at similar temperatures (-400°C), the water content of the samples prepared by titration method (i) is released at 235°C while in the second case (iii) this content is released at 124°C. This suggest that during co-precipitation method using route (iii) water molecules will be weaker bound to the surface of the layers which allows an easier releasing at far lower temperatures.

[0055] While the temperatures corresponding to the releasing of physi-and chem i-sorbed water remains relatively constant along a series of samples having various zirconium contents, the temperature of dehydroxilation and carbonate traces removal decreases significantly from 380.6°C to 31 7.4°C (Table 1). The values reported in this table are obtained from TG data derivative. In the corresponding DTA curves (Fig. 11) we can observe the shifting of the second DTA peaks towards smaller temperature values accompanied by a pronounced asymmetry in the case of intermediary zirconium contents.

[0056] Similar behaviour was observed in the case of Mg2Al1Zr, as well, but here all the peaks are centred at about 400°C. The increase of zirconium content contributes only to the enlargement of these peaks towards small temperatures, which indicates a higher thermal stability of these structures.

[0057] In order to investigate whether a further thermal treatment will improve the samples crystallinity some of the reported materials have been subjected to pressure treatment in various conditions (Example 4).

This treatment has shown positive response in the case of the samples prepared by titration route (i) (Fig. 13) while in the case of the samples prepared by co-precipitation (iii) this sort of treatment doesn't show any change in the crystallinity (Fig. 14). The latest aspect is confirmed by DTA measurements, as well, these profiles not showing any major modification after the pressure treatment (Fig. 15).

[0058] The incorporation of a tetravalent cation in the structure of hydrotalcite-type compounds leads to the increase of the surface area of the mixed oxides obtained by calcinations (Fig. 16). This is due to the fact that the tetravalent cation being bigger in size than the other cations present in these structures will generate a high distortion along the octahedral layers. The increase of the number of defects at the surface of these materials will be directly responsible for the increase of the sites where these structures will break down during the heating.

[0059] When a second divalent cation, such as Zn2, is introduced in the structure of hydrotalcite-type compounds a pressure treatment will reduce the temperature where Zn2 cations are released for the octahedral layer.

By normal calcinations this phenomenon appears at about 300°C together with the dehydroxilation processes (Fig. 17). The XRD pattern of the sample obtained by route (iii) and pressure treated shows the increase of the sample crystallinity and the concomitant appearance of ZnO peaks, the resulted mixture being stable when treated with Na2CO3, which suggests the irreversibility of the process (Fig. 18).

[0060] If Zr-containing materials are easily obtained by following route (iii) not the same thing can be told about Ti-containing materials. It seems that the obtainment of these types of compositions are dependent by the presence of carbonate anion which makes route (ii) easily applicable in order to obtain phase pure materials (Fig. 19).

[0061] Cerium and lanthanum cations have been proven to be impossible to incorporate in the structure of octahedral layers of hydrotalcites. These cations are known from literature (AppL Catal. A: General (2005), 288, 185-193, Catat Today (2008), 133-135, 357-366, Fuel (2010), 89, 592- 603) as being too big in size, which make them unsuitable for such purposes, even by classical co-precipitation route. These always appear as dopants (Fig. 20) in the mother structures contributing to the enhancing of the catalytic properties of the final materials.

[0062] The bead milling process (Example 5) of the materials reported here led to a significant reduction of the particle size in aqueous solution.

Thus, by applying this procedure to a zirconium containing materials having the next composition Mg3AI078Zr014 the particles size evolution reported in Fig. 21 has been obtained.

[0063] The effect of the foreign cation in Mg/Al hydrotalcite type compositions has been assessed from CO2 sorption point of view. The investigations have conducted us at the conclusion that this type of materials can be used as CO2 sorbent in two different applications involving different activation temperatures.

[0064] Thus, one set of applications involves the using of high activation temperatures (500°C) and will allow capacities, which progressively increase with the amount of dopant, say zirconium (Fig. 22), and, correspondingly, with the surface area of the activated materials. Also, CO2 capacity varies with the nature of foreign cation. For example, the best CO2 capacity has been obtained for a low cerium-containing material

(Table 3).

[0065] The other set of applications targets the low temperatures. In this case, the physisorption of CO2 is exploited. Some of the materials reported here show a significant higher CO2 capacity when activated at lower temperatures. It has been concluded that these improved performances are related to the presence of physi-sorbed water. Once this water is removed from the system the capacity drops (Table 3). Figure 23 presents the stability of the CO2 capacity over a number of 160 cycles. During the first 19 cycles, when the activation has been carried on at 100°C, the capacity is constant, this being followed by a constant increase of this capacity when the material is activated at 130°C. By thermal treatment at 160°C the capacity fluctuates between 6-9wt%, the CO2 capacity dropping significantly when the sample is heated at 190°C. This behaviour can be related to the complete removal of water at temperatures higher than 160°C, which has been proven by thermogravimetric measurements. It can be concluded, thus, that the high CO2 capacity observed at lower temperatures is given weakly bonds formed by CO32 anions with water molecules. When this water is removed, the adsorption properties are drastically reduced.

[0066] The same sort of conclusion can be made for higher temperature applications as well. Basically, the CO2 sorption at high temperatures is directly related to the presence of hydroxyl groups in the hydrotalcitic structure. Once the dehydroxilation step is achieved, the CO2 absorption is drastically reduced. This can be investigated through TPD/MS, which will show the temperature where the water and CO2 are desorbed. The chemi-sorbed CO2 desorption can be rigorously controlled by modifying the hydrotalcites towards the reduction of dehydroxilation temperature (Fig. 24).

[0067] Example I

[0068] This first example is referred in the body of the text as titration method (i). Briefly, into a 5 L reactor was added a 1.86M aqueous solution (1.8Kg) containing the salts of the elements of interest. This solution was heated up to 50degC. When this temperature is reached, the pH of the solution is adjusted to 9.5-10 by using a 4.4M solution of NaOH and Na2CO3 (molar ratio 3.77:1, respectively). After the pH value is reached the reaction mixture is maintained at S0deg C for other 30 mm under vigorous stirring, followed by vacuum filtration and washing with deioninzed water till Na and other impurities are removed. The resulted cake is either pressure treated or dried in the oven at 11 OdegC over night.

[0069] In case of the samples prepared by this method, the metallic precursors have been selected from Mg(NO3fr6H2O, Al(NO3)39H2O, ZrOCI2, ZrO(N03)2 and Zr0003. A selection of the samples prepared using this method can be found in Table 2.

[0070] Example 2

[0071] This second example is referred in the body of the text as co-precipitation method in the absence of carbonate (iii). Into reaction vessel have been added 350m1 water and the pH is adjusted to the co-precipitation pH (between 9 and 11) using a 25M NaOH aqueous solution.

When the nitrates aqueous solution (1.86M nitrates) starts to be added the pH drops to lower values and is adjusted to the co-precipitation value through the addition of the 2.5M NaOH under vigorous stirring. During the reaction the temperature is maintained at 50degC. When the addition of acidic solution is finished the reaction mixture is maintained at SOdeg C for another one hour under vigorous stirring. In the next step, the slurry is vacuum filtered and washed intensely on the filter with water till sodium is removed. The cake is dried in the oven at 11 Odeg C over night. In order to investigate the effect of the pressure treatment on the samples crystallinity the fresh washed cake has been reslurried in water (200g cake in 800g distilled water) and pressure treated. A selection of the samples prepared using this method can be found in Table 2.

[0072] Example 3

[0073] This third example is referred in the body of the text as co-precipitation method in the presence of carbonate (ii) and is a variation of the Example 2. Into reaction vessel have been added 350m1 of 2.87M Na2CO3 aqueous solution its pH being 11.7. This value was lowered to the corresponding co-precipitation pH value using the acidic solution, which contains the cations. When this value is reached the concomitant addition of a 2.5M NaOH solution and I.86M salts solution starts and continued until the second solution is exhausted. As in Example 2, the temperature was maintained at 50°C, the reaction mixture being continuously stirred.

When the addition of acidic solution is finished the reaction mixture is maintained at SOdeg C for another one hour under vigorous stirring. In the next step, the slurry is vacuum filtered and washed intensely on the filter with water till sodium is removed. The cake is dried in the oven at I lOdeg C over night. In order to investigate the effect of the pressure treatment on the samples crystallinity the fresh washed cake has been reslurried in water (200g cake in 800g distilled water) and pressure treated. A selection of the samples prepared using this method can be found in Table 2.

[0074] ExamQle 4 [0075] Compositions prepared using all above-mentioned techniques have been subjected to a pressure treatment. In order to achieve this 200g fresh made cake has been reslurried into 800m1 deionized water followed by the pressure treatment at temperatures ranging between 120-170°C for 3 hours. A selection of the samples prepared using this method can be

found in Table 2.

[0076] ExamQleS [0077] Samples prepared using all above mentioned techniques have been subjected, both before and after pressure treatment, to 10 cycles of bead milling which aims to reduce the particles size in aqueous solution.

Briefly, 1.5Kg slurry containing 10% solid has been passed through the bead mill. Depending on the composition of the solid, the time of each cycles varied significantly. If, for example, in the case of Mg/Al/Zr compositions the first two cycles were quite fast, the other 8 cycles have been slower due to the modification of the slurry viscosity. In the case of Mg/Zn/Al compositions the slurry went thick from the first cycle. In all cases, in the end, highly stable colloidal solutions of hydrotalcites with d50<1 pm have been obtained. Particle size distribution in the final colloidal mixture has been measured using the laser light scattering method.

Table 1. TG derivative peaks occurrence for some Mg3Al1Zr compositions prepared by route (iii).

Temperature Temperature Temperature of the first of the of the third Composition mass loss second mass loss (°C) mass loss (°C) (°C) Mg3AI 98.9 380.6 448 Mg3AI0.9Zr0.1 100 363.4 444.3 Mg3AI0.8Zr0.2 108 364 446 Mg3AI0.7Zr0.3 93.1 317.4 442 Table 2. Samples prepared using the method of the preparative examples Synthetic Precursors XRD analysis Composition ________ ______ _________ ______ _______ procedure NaOH Carbonate a(A) c(A) Mg3Al1..Zr Mg3AI-HT iii, no PT -+ -3.0658 25.15 Mg3AI-HT i, no PT -+ + 3.059 23.41 Mg3AI-HT iii, PT -+ -3.0572 24.705 Mg3AI-HT i, PT -+ + 3.0624 23.34 Mg3AI09Zr0.1 iii, no PT ZON + -3.0716 24.49 Mg3AI09Zr0.1 i, no PT ZOC + + 3.0624 23.95 Mg3AI09Zr0.1 iii, PT ZON + -3.0716 24.18 Mg3AI0.3Zr0.2 iii, no PT ZON + -3.07 23.85 Mg3AI0.3Zr0.2 i, no PT ZOC + + 3.0456 23.28 Mg3AI0.7Zr0.3 iii, no PT ZON + -3.0862 24.4885 Mg3AI0.7Zr0.3 i, no PT ZOC + + 3.0864 23.87 Mg3AI0.7Zr0.3 i, PT ZOC + + 3.0794 23.8 Mg3AI0.78Zr014 iii, no PT ZON + -3.0829 24.0976 Mg3AI0.78Zr014 iii, PT ZON + -3.0726 23.8588 Mg3AI05Zr0.5 iii, no PT ZON + + 3.0862 24.1771 Mg2AI1 Mg2AI-HT iii, no PT 3.04 26.88 Mg2AI-HT iii, PT 3.0456 26.802 Mg2AI0.9Zr0.1 iii, no PT ZON + -3.0342 26.55 Mg2AI0.9Zr0.1 i, no PT ZOC + -3.0486 22.97 Mg2AI0.8Zr0.2 iii, no PT ZON + -3.0356 25.9923 Mg2AI0.8Zr0.2 i, no PT ZOO + + 3.0584 23.3208 Mg2AI0.8Zr0.2 iii, PT ZON + -3.055 26.6094 Mg2AI0.7Zr0.3 iii, no PT ZON + -3.0482 24.961 Mg2AI07Zr0.3 i, no PT ZOO + + 3.065 23.1724 Mg3..ZnAI Mg15Zn1.5A1 iii, no PT Zn(N03)2 + -3.065 23.4331 Mg1.5Zn1.5A1 iii, PT Zn(N03)2 + ---Mg1.5Zn1.5A1 ii, no PT Zn(N03)2 + + 3.0752 23.1361 Zn3AI iii,noPT Zn(N03)2 + -3.1118 24.5434 Zn3AI ii, no PT Zn(N03)2 + + 3.0768 22.8397 Mg3AI1..M (M=Ti, Ce, La) Mg3AI09Ti0.1 iii, no PT Ti0012 + -3.065 24.1268 Mg3AI09Tio.1 ii, no PT Ti0012 + + 3.0718 23.7811 Mg3AI0950e005 ii, no PT Oe(N03)3 + + 3.068 23.4705 Mg3AI0.9La0.1 iii, no PT La(NO3)3 + -3.0718 23.6628 aoations other than Mg2 and AI3 Table 3. 002 capacities of samples prepared using various preparative methods and activated in different conditions.

Maximum Activation Capacity Synthetic adsorption Composition temperature, a,b, procedure rate, °C mmol/g wt%/min Mg3AI-HT iii, no PT 500 o.85a 0.29 Mg3AI-HT i, PT 130 1.84' -Mg3AI-HT ii, PT 500 la 0.53 Mg3AI-HT iii, no PT 130 0.6W' Mg3AI0.9Zro.1-HT iii, no PT 500 0.83a 0.44 Mg3AI0.9Zr01-HT iii, no PT 130 1.71" -Mg3AI0.8Zr02-HT iii, no PT 500 0.88a 0.52 Mg3AI0.8Zro2-HT iii, no PT 130 0.66" -Mg3AI07Zr03-HT iii, no PT 500 1 0.99 Mg3AI0.7Zr03-HT iii, no PT 130 0.25" Mg3AI3.73Zr014-iii, PT 500 0.68a 0.33

HT

Mg3AI0.78Zr014-iii, PT 130 0.3"

HT

Mg3AI078Zr014-ii, PT 500 1.Ola 0.55

HT

Mg3AI0.95Ce005-ii, PT 500 1.08 0.62

HT

Mg3AI0.95Ce005-ii, no PT 130 047" -

HT

Mg3AI0.9Ti0.1 ii, no PT 500 0.98 0.5 Mg3AI.o9Ti0.1 ii, no PT 130 0.15" Mg2AI0.9Zr0.1 iii, PT 130 0.39" - 8adsorption -pure 002, 80CC; desorption -Ar "adsorption -moisturised CO2IO2IN2,80C; desorption -N2; initial activation carried out at 500CC.

Claims

Claims 1. A synthetic hydrotalcite of the general formula M(ll)xM(lll)yM'(lll)z(OH)aAbcH2O or M(ll)xM(lll)yM(lV)z(OH)aAbcH2O, wherein, M(ll) is a divalent cation selected from a series of Mg, Zn, Cu, Co, Fe and Ni.M(lll) is at least one trivalent cation selected from Al, Mn, Co, Ga, La, Ce and Ti, M(IV)canbeZrorTi A defines an anion which compensates the positive charge of the brucite type layers and can be at least one of the following: carbonate, nitrate, sulphate, chloride, hydroxide, phosphate.

Produced by a process containing the following steps: a. Preparation of a solution, said I, by mixing the metallic precursors other than carbonates to generate concentrations up to 3M b. Addition of a solution of an hydroxide and carbonate of an alkali element, said Ii having concentrations up to I OM, until a pH less than 12, at temperatures up to 70°C.and one or more of the following steps: c. Filtration d. Washing e. Hydrothermal treatment f Milling g. Drying h. Calcination
2. A synthetic hydrotalcite as claimed in any of the previous claims having particle sizes said between 0.1 and 50 micron.
3. A synthetic hydrotalcite of the general formula M(II)xM(III)yM'(III)z(OH)aAbcH2O compositions where 0«=z<ycx«=6 and a>x>c>b prepared as in Claim 1.
4. A synthetic hydrotalcite of the general formula M(l l)M(l I l)y(IV)z(OH)aAvcH2O compositions where 0«=z«=ycx«=6 and a>x>c>b prepare as in Claim 1.
5. A process to produce hydrotalcites as claim 1 where solution I and II are added in parallel to a third solution Ill to maintain a constant pH of 7-l2of that third solution.
6. A process as in claim 5 containing anion other than carbonate
7. A synthetic hydrotalcite as claimed in any of the previous claims containing transition metals, which show dehydroxilation temperature lower than 500°C.
8. A synthetic hydrotalcites as claimed in any of the previous claims containing transition metals, which show decarbonation temperature below 500°C.
9. A synthetic hydrotalcites as claimed in any of the previous claims containing transition metals, which show dehydration temperature bellow 200°C.
10. A synthetic hydrotalcite as claimed in any of the previous claims where the activation temperature (decarbonation) is lower than 160°C
11. A synthetic hydrotalcite as claimed in any of the previous claims with a partial CO2 capacity greater than Swt%.