WO2002034842A1

WO2002034842A1 - Coated metal oxides and hydroxides

Info

Publication number: WO2002034842A1
Application number: PCT/GB2001/004777
Authority: WO
Inventors: Mark Green; Gareth Wakefield
Original assignee: Oxonica Ltd
Current assignee: Oxonica Ltd
Priority date: 2000-10-27
Filing date: 2001-10-29
Publication date: 2002-05-02
Anticipated expiration: 2003-04-27
Also published as: AU2002210729A1

Abstract

Process for preparing nanoparticles of a metal or metalloid oxide or hydroxide is described which comprises heating a decomposable salt of the metal in the presence of a capping agent which is a solvent for the salt to a temperature sufficient to convert the salt to the corresponding oxide and causing the oxide to precipitate by the addition of a non-solvent therefor. Nanoparticles of oxide or hydroxide of one or more metals and/or metalloids which possess a hydrophobic coating or an organic acid or anhydride or ester or Lewis base are also disclosed.

Description

COATED METAL OXIDES AND HYDROXIDES

This invention relates to coated metal oxides and hydroxides. Although many small particles of metal oxides are soluble or dispersible in water, they are generally not soluble or dispersible in organic solvents and this presents a significant disadvantage. By way of example, cerium oxide acts as a catalyst in automotive exhaust systems. Cerium oxide releases oxygen and is therefore capable of regulating the oxygen partial pressure in the exhaust system. With the engine working under lean conditions, cerium oxide removes excess oxygen from the exhaust gas and, catalysed by, for example, platinum, NO_x is reduced to nitrogen. During rich cycles, cerium oxide releases oxygen to oxidise carbon monoxide to carbon dioxide. It is believed that cerium oxide acts catalytically. As such, the magnitude of the effect should be directly related to the surface area of the particles. Accordingly it is advantageous that the particles are as small as possible. There is a need to make a cerium oxide readily available, especially as very small particles, to catalyst systems and this can be achieved most easily by incorporating the cerium oxide into the fuel i.e. as a fuel additive. Unfortunately, for this to be effective, the cerium oxide needs to be dispersible or soluble in the fuel. The present invention is aimed at rendering metal oxides and hydroxides soluble or dispersible in organic solvents. According to the present invention there is provided a nanoparticle of an oxide or hydroxide of one or more metals and/or metalloids, which possesses a hydrophobic coating of an organic acid or anhydride or ester or Lewis base.

Apart from cerium oxide, the present invention is applicable to other rare earth oxides and indeed other metal oxides and hydroxides including metals of Group II of the periodic table such as magnesium, calcium, strontium and barium, aluminium, zirconium e.g. Zr0₂, titanium e.g. TiO₂, nickel e.g. NiO, and iron as Fe₂0₃ and Fe₃0₄ and other transition metals as well as lanthanide and actinide metal oxides and metalloid oxides and hydroxides such as those of silicon. Particles of mixed oxides and hydroxides such as a mixture of cerium oxide and zirconium oxide can also be produced in according with the invention. The coating (or capping) agents are generally Lewis bases or an organic carboxylic acid or anhydride which typically possesses at least 8 carbon atoms, e.g. 10 to 25 carbon atoms, especially 12 to 16 or 18 carbon atoms, especially lauric acid. It will be appreciated that the carbon chain can be saturated or unsaturated, for example ethylenically unsaturated as in oleic acid. Similar comments apply to the anhydrides which can be used. A preferred anhydride is dodecylsuccinic anhydride. Other organic acids, anhydrides and esters which can be used in the process of the present invention include those derived from phosphoric acid and sulphonic acid. The esters are typically aliphatic esters, for example alkyl esters where both the acid and ester parts have 4 to 18 carbon atoms. Suitable Lewis bases generally possess an aliphatic chain of at least 8 carbon atoms and include mercapto compounds, phosphines, phosphine oxides and amines as well as long chain ethers, diols, esters and aldehydes. Polymeric materials including dendrimers can also be used provided that they possess a hydrophobic chain of at least 8 carbon atoms and one or more Lewis base groups, as well as mixtures of two or more such acids and/or Lewis bases.

Typical polar Lewis bases include trialkylphosphine oxides P(R₃)₃0, including trioctyl phosphine oxide (TOPO), which is particularly preferred, trialkylphosphines, P(R³)₃, amines N(R³)₃, thiocompounds S(R³)₂ and carboxylic acids or esters R³COOR⁴ and mixtures thereof, wherein each R³, which may be identical or different, is selected from C,_₂₄ alkyl groups, C₂.₂₄ alkenyl groups, alkoxy groups of formula -0(C,.₂₄ alkyl), aryl groups and heterocyclic groups, with the proviso that at least one group R³ in each molecule is other than hydrogen; and wherein R⁴ is selected from hydrogen and C_t.₂ alkyl groups, preferably hydrogen and C _{L H} alkyl groups. Typical examples of .,₄ and C,.₄ alkyl groups, C₂.₂₄ alkenyl groups, aryl groups and heterocyclic groups are described below.

It is also possible to use as the polar Lewis base capping ligand a polymer, including dendrimers, containing an electron rich group such as a polymer containing one or more of the moieties P(R³)₃0, P(R³)₃, N(R³)₃, S(R³)₂ or R³COOR⁴, wherein R³ and R⁴ are as defined above; or a mixture of Lewis bases such as a mixture of two or more of the compounds or polymers mentioned above.

As used herein a C| alkyl group is an alkyl group as defined above which contains from 1 to 4 carbon atoms. C alkyl groups include methyl, ethyl, i-propyl, n-propyl, n-butyl and tert-butyl. As used herein, a C₂.₂₄ alkenyl group is a linear or branched alkenyl group which may be unsubstituted or substituted at any position and which may contain heteroatoms selected from P, N, 0 and S. Typically, it is unsubstituted or carries one or two substituents. Suitable substituents include halogen, hydroxyl, cyano, -NR₂, nitro, oxo, -CO₂R, -SOR and -S0₂R wherein each R may be identical or different and is selected from hydrogen or C alkyl.

As used herein an aryl group is typically a C₆.₁₀ aryl group such as phenyl or naphthyl, preferably phenyl. An aryl group may be unsubstituted or substituted at any position, with one or more substituents. Typically, it is unsubstituted or carries one or two substituents. Suitable substituents include halogen, C₁ alkyl, C,_₄ alkenyl, each of which may be substituted by one or more halogens, hydroxyl, cyano, -NR₂, nitro, oxo, -CO₂R, -SOR and-SO₂R wherein each R may be identical or different and is selected from hydrogen and C_l alkyl.

As used herein a heterocyclic group is a 5- to 10- membered ring containing one or more heteroatoms selected from N, O and S. Typical examples include pyridyl, pyrazinyl, pyrimidinyl, pyridazinyl, furanyl, thienyl, pyrazolidinyl, pyrrolyl and pyrazolyl groups. A heterocyclic group may be substituted or unsubstituted at any position, with one or more substituents. Typically, a heterocyclic group is unsubstituted or substituted by one or two substituents. Suitable substituents include halogen, C_M alkyl, C,_.4 alkenyl, each of which may be substituted by one or more halogens, hydroxyl, cyano, -NR₂, nitro, oxo, -C0₂R, -SOR and-S0₂R wherein each R may be identical or different and is selected from hydrogen and C ₄ alkyl.

As used herein, halogen is fluorine, chlorine, bromine or iodine, preferably fluorine, chlorine or bromine.

The particles of the present invention can be prepared in more than one way but preferably by heating a decomposable salt of the metal (or metalloid) in the presence of a capping agent which is a solvent for the salt to a temperature sufficient to convert the salt to the corresponding oxide and causing the oxide to precipitate by the addition of a non-solvent therefor.

The salts which are used as starting materials must be such as will decompose at a convenient temperature to the corresponding oxide. Suitable salts generally include nitrates and carbonates, depending on the particular metal.

The process is conducted at a sufficient temperature to ensure the conversion of the salt to the oxide. In some instances, this will involve a change of charge on the metal ion. For example cerium nitrate, Ce(N0₃)₃ contains Ce³⁺ and this has to be converted to Ce⁴⁺ for the oxide. A temperature of 120° C to 220 °C is typical eg. 130°C to 205°C.

In general, this process of the present invention can be carried out in two ways. Either the process is carried out with the capping agent acting as solvent and reaction medium (which is therefore present in excess) and the metal salt is dissolved in a solvent which is miscible with the capping agent. Alternatively, the salt and the capping agent are mixed together in a mutual solvent.

In both instances the mixture is heated to the above mentioned temperature and then a non-solvent for the oxide is added to cause flocculation of the desired oxide. In general, it will be necessary to cool the mixture before the addition of the non-solvent in order to avoid evaporation of the latter.

In the first process, the metal salt is typically dissolved in a solvent therefor and then injected into the capping agent which may have already been heated. Typical solvents include aromatic solvents, for example heterocyclic compounds such as pyridine. It is desirable that the solvent also acts as a Lewis base such as 4- tert-butyl-pyridine. Other solvents which can be used include aliphatic alcohols, typically of 2 to 4 or 6 carbon atoms such as 1,4-butanediol.

In the second process typically roughly equimolar amounts of metal salt and capping agent are mixed together, for example from 10: 1 to 1 : 10, especially from 2:1 to 1 : 10, particularly about 1 : 1. An excess of the solvent, typically an aliphatic alcohol, for example having 2 to 4 or 6 carbon atoms such as 1 ,3-butanediol, is used. In general the molar ratio of solvent to metal salt is at least 50: 1, especially 50: 1 to 500: 1 , preferably 75:1 to 150: 1 , typically about 100: 1. If the concentration of metal salt is too high, the particle size increases and this makes dispersion more difficult. Methanol can be used to cause the oxide to come out of solution in the capping agent or in the alcohol. The resulting particles can then be recovered. They can, of course, be re-dispersed in a solvent and extracted again, for example for further treatment of the particles, by the addition of a non-solvent. Suitable solvent/non-solvent combinations for this purpose include toluene/methanol, hexane/ethanol, chloroform/methanol, pyridine/hexane and butanol/methanol. This process of the present invention which effectively involves a colloidal chemistry technique has the advantage that the capping agent can be added during the process thus rendering the particles perfectly dispersed. As a result, settling due to agglomeration is minimised.

Although this does not form part of the present invention, it is believed that the capping agent prevents the particle size from increasing. In this way it is possible to obtain particle over a wide range of sizes up to, say 40nm or 50nm. Indeed the particles of the present invention are nanometer sized, in general having a size not exceeding lOOnm. Typically they are from 2 or 3nm to 50nm, for example not exceeding lOnm. It will be appreciated that the actual size of the particles is less important than their ability to be dispersible. The resulting particles are generally fully dispersible in common solvents including aromatic solvents, aliphatic hydrocarbons and aliphatic alcohols such as those listed above.

The coated metal (or metalloid) oxides and hydroxides can also readily be obtained by dispersing particles of the material in water, adding the coating agent in an organic solvent therefor to it and then mixing the materials sufficiently to cause a phase transfer to occur. Typically, the organic solvent will be the solvent for which the particles are destined eg petrol or, for example, an aromatic hydrocarbon such as toluene. The amount of capping agent used is not critical provided there is sufficient to coat the particles. Thus an equimolar amount of capping agent or less is generally used. The present invention is particularly applicable to nanometer sized particles, in general particles having a size not exceeding 100 nm, typically 10 to 25 nm.

Although this does not form part of the present invention, it is believed that, unlike many other organic materials, the capping agents which can be used in the present invention are capable of reacting with the metal oxide or hydroxide. It is lαiown that cerium oxide, for example, possesses hydroxy groups on the surface (see, for example, J. Chem. Soc. Faraday T92: (23) 4669-4673, December 1996) and it is believed that these react with the acid grouping leaving the long chain, lipophilic, portion of the molecule pointing into the solvent. It should be added that the presence of these- hydroxy groups means that the particles are easily dispersed in water but this, of course, in itself makes them less readily dispersible in an organic solvent. By incorporating a long chain in the surface, the particle becomes lipophilic and therefore more susceptible to dissolution in organic media. Particles of cerium oxide obtained by the process of the present invention are particularly useful as fuel additives since they are fully dispersible in common solvents, diesel fuel and petrol.

The following Examples further illustrate the present invention.

Example 1 Commercially available CeO₂ (0.5g., 2.9 mmol) was dispersed in 100 ml de- ionised water. In a separate flask, a carboxylic acid namely stearic acid, 0.81 g, 3.84 mmol was dissolved in 100 ml toluene. The two solutions were mixed together and an immediate phase transfer occurred. The mixture was mixed vigorously for two hours. The resulting white powder was then isolated by centrifugation. The powder could then be dispersed in organic solvents such as toluene.

Example 2

(i) 21 g of trioctyl phosphine oxide (TOPO) is charged to a three neck flask, heated under vacuum to 140°C, then heated and stabilised at 204°C. The TOPO acts as a surface coating (capping agent). (ii) 0.49g of Ce(N0₃)₃. 6I-LO is dissolved in a 5ml 4-t butyl pyridine and injected into the TOPO. An immediate reaction takes place.

(iii) The flask is then cooled to 140°C and held at the temperature for 40 minutes.

(iv) The mixture is cooled to 60 °C and flocculation is then induced by addition of methanol.

(v) The material is centrifuged and washed. The resulting material dispersed easily in toluene and gave a pale yellow solution.

Figure 1 is an electronmicrograph of the resulting particles.

Example 3 (i) 4.72g Ce(NO₃)₃ 6H₂0 and 4.76g pure TOPO are mixed in 100ml 1 ,4- butanediol overnight in air (mole ratio 1 : 1 :100). This produced an opaque mixture after 18 hours stirring.

(ii) Gentle heating is then applied and the solution starts to clear at 50°C.

(iii) After 45 minutes stirring at 100°C the reaction mixture turns yellow. (iv) This is allowed to continue, heating for a further 16 minutes, reaching

130°C.

(v) Heat is removed and the temperature falls to 60 °C. At this point 10ml 0.05M NaOH/CH₃OH is added.

(vi) The dispersion is isolated as a yellow oil by centrifugation, dispersed in toluene and precipitated by CH₃OH giving a yellow/white powder.

The samples of Examples 2 and 3 have been confirmed to be the cubic phase of Ce0₂ by electron microscopy and electron diffraction. The particles of Example 2 are essentially cubes below 4nm in size while those of Example 3 which are below lOnm in size are spherical.

Claims

1. A nanoparticle of oxide or hydroxide of one or more metals and/or metalloids which possesses a hydrophobic coating of an organic acid or anhydride or ester or Lewis base.

2. A particle according to claim 1 in which the coating is of an organic acid or anhydride having an aliphatic chain of at least 8 carbon atoms.

3. A particle according to claim 2 in which the coating is of a carboxylic acid or anhydride which possesses 10 to 25 carbon atoms.

4. A particle according to claim 3 in which the coating is of lauric acid.

5. A particle according to claim 1 or 2 in which the organic acid or Lewis base is a polymer or dendrimer.

6. A particle according to claim 1 in which the Lewis base is a trialkyl phosphine oxide.

7. A particle according to claim 6 in which the Lewis base is trioctyl phosphine oxide.

8. A particle according to any one of the preceding claims in which the metal is cerium and/or zirconium.

9. A particle according to any one of the preceding claims_ hich has a size from 2 to 50nm.

10. A particle according to claim 9 which has a size from 10 to 25nm.

11. A particle according to claim 9 which has a size from 2 to lOnm.

12. A particle according to claim 1 substantially as hereinbefore described.

13. Process for preparing a nanoparticle as defined in any one of the preceding claims which comprises heating a decomposable salt of the metal or metalloid in the presence of the organic acid, anhydride or ester or Lewis base, which is a solvent for the salt to a temperature sufficient to convert the salt to the corresponding oxide and causing the oxide to precipitate by the addition of a non- solvent therefor.

14. Process according to claim 13 in which the salt is a nitrate or a carbonate.

15. Process according to claim 13 or 14 which is carried out in excess capping agent.

16. Process according to any one of claims 13 to 15 which is carried out in a solvent for the metal salt.

17. A process for preparing a nanoparticle as defined in any one of claims 1 to 12 which comprises dispersing the uncoated particle in water, adding the organic acid, or anhydride or ester or Lewis base in an organic solvent therefor, and mixing the two liquids sufficiently to cause a phase transfer to occur.

18. A process according to claim 17 in which the solvent is petrol or toluene.

19. Process according to claim 13 or 17 substantially as described in any one of the Examples.

20. A nanoparticle of an oxide or hydroxide of one or more metals and/or metalloids whenever prepared by a process as claimed in any one of claims 13 to 19.

21. A dispersion or solution of nanoparticles as claimed in any one of claims 1 to 12 and 20.

22. A dispersion or solution according to claim 21 in an aromatic solvent, an aliphatic hydrocarbon, or an aliphatic alcohol.

23. A dispersion or solution according to claim 22 in diesel fuel or petrol.