WO2007023299A2

WO2007023299A2 - Doped lithium zirconate

Info

Publication number: WO2007023299A2
Application number: PCT/GB2006/003189
Authority: WO
Inventors: Hazel Stephenson
Original assignee: Magnesium Elektron Limited
Priority date: 2005-08-25
Filing date: 2006-08-25
Publication date: 2007-03-01
Also published as: WO2007023299A3; GB0517343D0

Abstract

Alumina doped lithium zirconate and its preparation are described, which zirconate is capable of adsorbing carbon dioxide at a range of temperatures from room temperature to >550°C. The optimum temperature for carbon dioxide uptake depends on the composition and physical properties of the zirconate. Optional additional dopants are Na, Mg, Li, Y and Si.

Description

DOPED LITHIUM ZIRCONATE

This invention relates to an improved material for carbon dioxide removal comprising alumina-doped lithium zirconate, and a process for its preparation.

The use of lithium zirconate for gas purification, specifically carbon dioxide removal and/or collection, is known.

The reaction of lithium zirconate with carbon dioxide is a chemical rather than simply an adsorption reaction and is therefore highly specific for carbon dioxide. This specificity is of particular benefit when there are other gases present in a gas stream that do not need to be removed, such as nitrogen. The reaction of lithium zirconate with carbon dioxide is shown below:

Li₂ZrO₃ + CO₂ ** Li₂CO₃ + ZrO₂

1 mole of lithium zirconate removes from the gaseous phase 1 mole of carbon dioxide. This equates to over 400 times its own volume of carbon dioxide, giving an increase in volume of the lithium zirconate of about 34%. Since the reaction is reversible the lithium zirconate can be regenerated under appropriate conditions. This property is particularly useful in applications where carbon dioxide needs to be collected and later released in a controlled manner. Lithium zirconate may be regenerated by heating to elevated temperatures, typically in excess of 600⁰C. It is known that the kinetics of the carbon dioxide uptake of lithium zirconate and the carbon dioxide capacity of lithium zirconate may be altered by modifying the material. One such modification is the addition of potassium and/or excess lithium to increase the rate of carbon dioxide uptake. However, this has only previously been shown to start to be effective at temperatures in excess of 400⁰C. Modification of the particle size and crystalline structure/size of lithium zirconate has also been claimed to affect carbon dioxide uptake, but there are conflicting opinions in the prior art as to which form of modification provides the most benefit.

In a paper by Balagopal N. Nair, Takeo Yamaguchi, Hiroto Kawamura, and Shin-Ichi Nakao (Department of Chemical System Engineering, University of Tokyo) and Kazuaki Nakagawa (Toshiba R&D Centre) entitled "Processing of lithium zirconate for applications in carbon dioxide separation: structure and properties of the powders" (J. Am. Ceram. Soc . , 87 [1] 68-74 (2004)) lithium zirconates doped with potassium are analysed. Fast carbon dioxide removal is reported at 600⁰C. However, there is no mention of any carbon dioxide uptake at temperatures below 450⁰C. It is also noted in this paper that samples of lithium zirconate calcined at 700⁰C gave faster absorption of carbon dioxide than those calcined at 900⁰C, but that the total uptake of these samples was lower. It was concluded from this that crystal structure is a deciding factor in the absorption reaction. In particular, lithium zirconates with a tetragonal structure were found to absorb carbon dioxide faster than those with a monoclinic structure. Lithium zirconates doped with excess lithium and added potassium for separation of carbon dioxide from flue gas at high temperatures were proposed in a paper by Jerry Y. S. Lin and Jun-ichi Ida entitled "Novel ceramic membrane for high temperature carbon dioxide separation, technical progress report 09/01/00 to 02/28/01" (University of Cincinnati, Department of Chemical Engineering DE-FG26- 00NT40824) . The material in this case was monoclinic lithium zirconate. A carbon dioxide uptake of 20wt% after 270 minutes at 500⁰C was measured for the potassium doped material. There is no mention in this paper of removal of carbon dioxide at temperatures below 400⁰C.

US patent no. 6,271,172 in the name of Ohashi et al describes a carbon dioxide gas absorbent that contains lithium zirconate in the matrix. The doping of the lithium zirconate with oxides of lanthanide elements, such as yttrium oxide, and alkaline earth oxides, such as calcium and magnesium oxides, is disclosed. The total carbon dioxide uptake of the materials produced is between 0.1 and 17wt%.

A paper by Jun-ichi Ida and Y. S. Lin entitled "Mechanism of high-temperature CO₂ sorption on lithium zirconate" (Lin. Environ. Sci. Technol . 2003, 37, 1999-2004) discusses modification of lithium zirconate using potassium and lithium carbonates . The paper concludes that doping with potassium and lithium is more influential than particle size on sorption rates at 400⁰C and above, and that the monoclinic phase is preferred.

The doping of lithium zirconate with potassium is again discussed in the paper "Synthesis and CO₂ sorption properties of pure and modified lithium zirconate" by Jun-ichi Ida, Rentian Xiong and Y. S. Lin (Separation and Purification Technology, 36, 2004, 41-51) . The rate of carbon dioxide uptake of the potassium-doped samples was found to increase significantly from 400⁰C to 500⁰C. Particle size was also considered to be a factor in carbon dioxide uptake, smaller particles being preferred since they provide an increased surface area. Again, the monoclinic phase is described as the preferred crystal form.

WO 03/020283 by Valence Technology Inc describes preparation methods for the production of materials with the composition Li_nM'_aM' 'x-_aTi_bZri-_tAi (where n=0.01-l, a=0- 1, b=0-l, M'= one or more of V, Mn, Ti, Fe, Cr, Ni, Co and Mo, and M''= one or more of Al, B, In, Ga, Tl, Bi and At) for use in rechargeable batteries. The described method requires the use of reducing carbon and is a dry or non-aqueous process.

It has now been surprisingly found that doping lithium zirconate with alumina not only results in improved rates of carbon dioxide uptake, but that it also permits the use of lithium zirconate to remove carbon dioxide at significantly lower temperatures than previously used. Specifically, significant uptake of carbon dioxide at room temperature has been achieved with the alumina-doped material .

The level of alumina required in the product is above impurity level, preferably greater than 0.1% by weight and more preferably greater than 0.5% by weight. The alumina-doped lithium zirconate may contain one or more additional dopants, the preferred dopants being potassium oxide, magnesium oxide, sodium oxide, lithium oxide, yttria or silica. The most preferred additional dopants are potassium oxide and silica, particularly when present together. The preferred level of such additional dopants in the product is between 0.5% and 6.0% for each except sodium oxide, where the preferred level is above impurity level, preferably greater than 0.1% by weight and more preferably less than 0.5% by weight.

It has also been surprisingly found that when lithium zirconate is doped with alumina the crystal phase of the resulting material does not have a significant effect upon the uptake of carbon dioxide . There is no need to try to obtain a particular phase, such as monoclinic.

Although other methods can be used, the preferred process for the preparation of the alumina-doped lithium zirconates of the present invention involves the wet mixing of aluminium hydroxide-doped zirconium hydroxide with lithium carbonate, the zirconium hydroxide optionally including the hydroxides of further dopants as required. The reaction utilised in the present invention can take place in an aqueous medium, preferably water, or a non-aqueous medium. The zirconates formed by the process of this invention are inherently all of the type

Li₂ZrO₃ and therefore fall in the class Li_nM'₂-nZri-2nθ3

(where n=0.01-2, and M'= one or more of Al, Na, Si, K, Mg and Y) and therefore fall outside the range of materials described in the Valence patent publication. As an alternative to the addition of the dopants to the zirconium hydroxide, the required dopants may be added to the zirconium hydroxide-lithium carbonate mixture in salt form. The salt must be one that upon pyrolysis converts to the corresponding oxide. The preferred salt form is the carbonate salt, but other salts such as oxalate or nitrate may be used. Preferably the mixture is stirred and then the mixture is calcined to form the desired zirconate. Drying of the wet mixture may optionally take place before the mixture is calcined. Milling of the mixture before or after calcination can also be performed if desired.

It is preferred that the average particle size of the input zirconium hydroxide is between 0.6μm and 25μm, more preferably between 0.6μm and 1.5μm.

The calcination temperatures used are preferably greater than or equal to 700⁰C, more preferably greater than or equal to 775⁰C. They need not be as high as the prior art, such as 800⁰C or 900⁰C.

By using the process of the present invention the mean pore diameters of the zirconates formed can be less than 75nm and/or their surface areas can be at least 0.49m²/g and/or their total pore volumes can be at least 0.002cm³/g. These preferred material properties especially when combined, such as total pore volume and mean pore diameter, are believed to be unachievable by prior art production methods. It has been found that the carbon dioxide uptake of the zirconates formed by the present process can be at least 53% of theoretical and that these zirconates are capable of absorbing up to 4.85 mol/kg of carbon dioxide at a range of temperature from room temperature to at least 550⁰C.

Due to the unexpected improved ability of the lithium zirconates produced by the present process for absorbing carbon dioxide at relatively low temperatures, in order to obtain maximum capacity for carbon dioxide absorption it may be necessary to activate the zirconates prior to use to remove any pre-absorbed carbon dioxide, thus regenerating the material. Alternatively the zirconates can be stored after initial calcination in an environment free of carbon dioxide, e.g. packaged under an inert gas such as nitrogen.

The invention is further illustrated by the following examples, in which Examples 1, 3, 4, 5 and 7 are comparative:

Example 1 - Magnesium Oxide doped Lithium Zirconate calcined at 7OQ⁰C

415. Ig of magnesium hydroxide doped zirconium hydroxide having a d50 of ~15μm, containing the equivalent of 183.5g total oxides (96.6wt% zirconium oxide and 3.4wt% magnesium oxide) , was slurried in deionised water to bring the total weight up to 615. Ig. 110.3g of lithium carbonate was then slowly added and the resultant mixture was stirred for 2 hours. The mixture was then calcined at 700⁰C to yield 261.2g of lithium zirconate containing 2.3wt% MgO. Example 2 - Alumina, Yttria and Potassium Oxide doped Lithium Zirconate

478.4g of yttrium hydroxide and aluminium hydroxide doped zirconium hydroxide having a d50 of ~lμm, containing the equivalent of 155.Og of total oxides (94.45wt% zirconium oxide, 0.25wt% alumina and 5.3wt% yttria), was slurried in deionised water to bring the total weight up to 678.4g. 59.5g of lithium carbonate was then slowly added, followed by the slow addition of 16.35g of potassium carbonate, and the resultant mixture was stirred for 30 minutes. The mixture was then calcined at 775⁰C to yield 210.7g of lithium zirconate containing 3.4wt% yttria, 0.2wt% alumina and 5.1wt% potassium oxide.

Example 3 - Yttria and Potassium doped Lithium Zirconate from Zirconium Hydroxide having d50 of ~15μm 154g of yttrium hydroxide doped zirconium hydroxide having a d50 of ~15μm, containing the equivalent of 75.Og of total oxides (94.7wt% zirconium oxide and 5.3wt% yttria) , was slurried in deionised water to bring the total weight up to 225g. 42.9g of lithium carbonate was then slowly added, followed by the slow addition of 8.Og of potassium carbonate, and the resultant mixture was stirred for 30 minutes. The mixture was then calcined at 775⁰C to yield 75g of lithium zirconate containing 3.4wt% yttria and 5.1wt% potassium oxide.

Example 4 - Undoped Lithium Zirconate

211.2g of zirconium hydroxide having a d50 of ~15μm, containing the equivalent of 100. Og of zirconium oxide, was slurried in deionised water to bring the total weight up to 361.2g. 59.5g of lithium carbonate was then slowly added, and the resultant mixture was stirred for 30 minutes. The mixture was then calcined at 775°C to yield 96.7g of undoped lithium zirconate.

Example 5 - Potassium Oxide doped Lithium Zirconate 211.2g of zirconium hydroxide having a d50 of ~15μm, containing the equivalent of 100. Og of zirconium oxide, was slurried in deionised water to bring the total weight up to 361.2g. 59.5g of lithium carbonate was then slowly added, followed by the slow addition of 11. Ig of potassium carbonate, and the resultant mixture was stirred for 30 minutes. The mixture was then calcined at 775⁰C to yield 94.7g of lithium zirconate containing 5.3wt% potassium oxide.

Example 6 - Alumina doped lithium zirconate

391.7g of zirconium hydroxide having a d50 of ~0.8microns containing the equivalent of 131g of total oxides (95wt% zirconium oxide and 5wt% aluminium oxide) was slurried in deionised water to bring the weight up to 541.7g. 74.6g of lithium carbonate was then slowly added and the resultant mixture was stirred for 30 minutes. The mixture was then calcined at 775°C to form the zirconate.

Example 7 (Comparative) - Commercial Lithium Zirconate For comparison a sample of commercially available lithium zirconate (not produced by the process of present invention) was obtained and tested alongside lithium zirconates produced in accordance with the process of the present invention. Table 1

O

\

Notes on Table 1

*Measurement of CO₂ uptake was measured at 500⁰C using a gravimetric flow technique. The samples were placed in ceramic crucibles, brought to constant weight 500⁰C under atmospheric presser then exposed to a flow of pure CO₂ for 24hours . The weight was measured using a Sartorius microbalance with accuracy of ±50μg.

** Measurement of kinetics was done using an IGA (Isothermal gravimetric analysis) with a sample temperature of 490⁰C, using a flow of pure CO₂. Pressure steps from 0 to 500mbar and from 500 to 950mbar were used.

*** Measurements were also carried out using a TGA (thermal gravimetric analysis) method, again using a pure CO₂ stream at 500⁰C after first purging the sample with nitrogen.

* Measured using a TriStar 3000.

The invention is further described by reference to the attached set of Figures, in which:

Figure 1 shows the overall rate for, and the total of, the carbon dioxide uptake of a sample of the lithium zirconate doped with alumina, yttria and potassium oxide of Example 2.

Figure 2 shows the initial rate of carbon dioxide uptake of a sample of the lithium zirconate doped with alumina, yttria and potassium oxide of Example 2.

Figure 3 shows the improved rate of carbon dioxide uptake of a sample of the lithium zirconate doped with alumina, yttria and potassium oxide of Example 2 when compared with that for a sample of the lithium zirconate doped with only potassium oxide of Example 3.

Figure 4 shows XRD spectra of samples of the lithium zirconates formed in Examples 5, 2, 4 and 1. The peaks at 20 and 22 are for monoclinic lithium zirconate, the peaks at 23, 36 and 49 are for tetragonal lithium zirconate and the peaks at 28, 30, and 32 are for phases of zirconium. By comparing these traces with the results set out in Table 1 it is clear that crystal form is not the most important factor affecting the carbon dioxide uptake of the lithium zirconates produced by the process of the present invention.

Figure 5 shows the temperature programmed sorptions (using lbar CO₂ gas flow with a temperature ramp of l°C/minute) for samples of the zirconates of Example 2, 3, 6 and 7 after first heating at 750⁰C for 18 hours to remove any pre-adsorbed carbon dioxide. The sorption for the sample of the zirconate of Example 7 shows the typical sorption profile of material made by prior art preparation methods with no additional dopants. An unique property of lithium zirconates containing Al (Examples 2 and 6) is that they adsorb carbon dioxide at temperatures below 400⁰C, as shown by the -5% uptake in Figure 4. This relatively low temperature CO₂ sorption is believed to be due to phyisorption rather than any chemical reaction.

At temperatures above 750⁰C even in the presence of pure carbon dioxide all of the zirconates tested desorbed carbon dioxide .

Figure 6 is a graph showing the activation and regeneration of a sample of the zirconate of Example 2. The darker curved line shows the carbon dioxide uptake of the sample, whilst the lighter angular line shows the temperature profile experienced by the sample. The temperature was first ramped to 750⁰C and held for 4 hours to desorb the -8% of pre-adsorbed carbon dioxide, then lowered to 500⁰C which switches the sample to adsorption mode, whereat over 24wt% carbon dioxide was adsorbed. This was repeated for 4 more cycles with minimal decrease in total carbon dioxide uptake capacity as shown by the faint dotted line linking the peaks of the uptake curve.

Claims

CLAIMS :

1. A lithium zirconate doped with alumina.

2. A lithium zirconate as claimed in claim 1 wherein the alumina is present in an amount greater than 0.1% by weight .

3. A lithium zirconate as claimed in claim 2 wherein the alumina is present in an amount greater than 0.5% by weight .

4. A lithium zirconate as claimed in any of the preceding claims additionally doped with one or more of potassium oxide, magnesium oxide, sodium oxide, lithium oxide, yttria and silica.

5. A lithium zirconate as claimed in claim 4 wherein the additional dopant is potassium oxide and silica together.

6. A lithium zirconate as claimed in either claim 4 or claim 5 wherein the potassium oxide, magnesium oxide, lithium oxide, yttria and silica are each present in amount of between 0.5% and 6.0% by weight.

7. A lithium zirconate as claimed in any one of claims 4 to 6 wherein the sodium oxide is present in an amount greater than 0.1% by weight .

8. A lithium zirconate as claimed in claim 7 wherein the sodium oxide is present in an amount less than 0.5% by weight .

9. A process for the production of a lithium zirconate as claimed in any of the preceding claims comprising the steps of: wet mixing zirconium hydroxide with lithium carbonate, optionally drying the mixture, and calcining the mixture, wherein either the zirconium hydroxide is doped with aluminium hydroxide or an aluminium salt is added to the wet mixture of zirconium hydroxide and lithium carbonate, the salt being capable of undergoing pyrolysis to form aluminium oxide .

10. A process as claimed in claim 9 wherein the aluminium salt is aluminium carbonate .

11. A process as claimed in claim 9 or claim 10 wherein the zirconium hydroxide is additionally doped with one or more of potassium hydroxide, magnesium hydroxide, sodium hydroxide, lithium hydroxide, yttrium hydroxide and si1icon hydroxide .

12. A process as claimed in any one of claims 9 to 11 wherein one or more potassium, magnesium, sodium, lithium, yttrium and silicon salts are added to the wet mixture of zirconium hydroxide and lithium carbonate, the salts being capable or undergoing pyrolysis to form their corresponding oxides .

13. A process as claimed in claim 12 wherein the salts are carbonate salts .

14. A process as claimed in any one of claims 9 to 13 wherein the calcination temperature is at least 700⁰C.

15. A process as claimed in claim 14 wherein the calcination temperature is at least 775°C.

16. A process as claimed in claim 14 or claim 15 wherein the calcination temperature is less than 800⁰C.