WO2015097459A1

WO2015097459A1 - Synthesis of polyoxometalates

Info

Publication number: WO2015097459A1
Application number: PCT/GB2014/053812
Authority: WO
Inventors: Nathaniel Martin; Matthew HERBERT
Original assignee: Acal Energy Limited
Priority date: 2013-12-23
Filing date: 2014-12-19
Publication date: 2015-07-02
Also published as: GB201322922D0

Abstract

The invention comprises a process for the preparation of a polyoxometalate, the polyoxometalate comprising a first component part having a first oxidation state; and a second component part having a second oxidation state, the process comprising reacting in suspension or solution in the presence of an oxoacid a first compound or material comprising the first component in a more reduced oxidation state than Its first oxidation state with a second compound or material comprising the second component such that the first component reduces the second component and is itself oxidised, resulting in the formation of the polyoxometalate.

Description

The present invention relates to a method of making polyoxometalates, a known class of heteropoly compounds which find particular appiication in fuel ceils and redox batteries as evidenced by our previous publications WO20Q7/1 10663, WO2009/040577, WO2012/085542, WO2013/132258 and PCT/GB2013/051676.

Heteropoly compounds are typically composed not only of the weak, oxygen containing metaiiie acids (such as W, Mo and V) but also of moderately strong to weak acids of non- metals (e.g. boric, phosphoric, silicic, arsenic, telluric, etc). Stable heteropoly compounds very frequently show non-metallic to metallic ratios of for example 1 : 12, 1 :8 and 1 :9.

The main literature method for the isolation of small quantities of such heteropolyacids concerns the extraction of the heteropoiyacid with diethyl ether from an acidic solution, as taught in the 'Handbook of Preparative Chemistry', Editor G. Brauer, Volume 2, Second Edition, 1983. We find that this method is relatively expensive, due to the large volumes of organic solvents used. Furthermore, yields and quality of product varied considerably from run to run.

Free heteropolyacids can also be prepared by ion exchange (W. P. Tbistlethwaite and H. Buchwaid, J. inorg. Nuct. Cho ,, 1958, 1 , p341 and L. C. W. Baker and T. P. fv Cutcheon, J. Amer. C ern. Soc, 1950, 72, p2374). The disadvantages of such methods are that one must begin with pure, crystalline alkali salts and, in addition, the heteropoiyacid solutions obtained by ion exchange are relatively dilute so that their concentration is time-consuming. There is also the possibility of unwanted exchange occurring between metal ions and the resins.

Another synthetic method (see for example J. H. Grate, D. R, Hamm and S. Mahajan in 'Catalysis of Organic Reactions', Editors J. R. Kosak and T. A. Johnson, Marcel Dekker, New York, 1994; US Patent 4148574, T, J. Hastings and H. A. Frediani, Anai, Chem., 1948, 20(4), p382, US Patent 4522934 and US Patent 4803187} prepares free phosphomolybdovanadic acids by dissolving o0₃ and V₂0₅ with stoichiometric H₃P0 in aqueous solution. These methods only appear to work satisfactorily, however, for low vanadium contents. Dissolution of V₂O_s info the polyoxoanion is very slow and requires long reaction times at reflux temperature, typically at least one week.

Free phosphomoiybdovanadic acids can also be synthesised by chemical activation of the vanadium with hydrogen peroxide (see V. F. Gdyakov, E.G. Zbizhina, R. I. Maksimovskaya, Applied Catalysis A: General, 2008, 342_f p126). This procedure requires careful control of reaction temperature and has the potential for the uncontrollable release of oxygen gas if the temperature rises. Furthermore the reaction is carried out at high dilution and requires extensive concentration once the reaction is complete to provide the polyoxometalate solution at useful concentrations, typically reducing the volume between 5 and 10 fold. This means the process is inefficient in terms of energy use and reactor throughput.

Heteropoly materials may also be prepared by synthesis using for example the reaction of aalV oC j and NaV0₃ with Na₂HP0₄ under acidic conditions (cone, H₂S0 or HCI) (see A. Tsigdinos and C. J. Haliada, Inorg. Chem., 1988, 7, p437). Typically, yields are of the order of 40% after the tedious and dangerous, on a large scale, diethylether extractions.

Observations that the known method of synthesis for heteropoly materials via a process according to etherate extraction is both laborious and dangerous led certain groups to investigate the use of reducing agents to soiubilise the vanadium (see K. I. Matveev, Klnet. Kat , 1977, 18, p882 and I. V. ozhevnikov, K. I. Matveev and V. E. Taraban'ko, Soviet J. Coord. Chem... 1978, p724). This work developed a process of reducing the V₂O_s (in the presence of H₃P o₁₂04o and Η₃ΡΌ₄ in water) with hydrazine hydrate, leading to the reduction of the vanadium(V) to vanadium(IV). This in turn, after prolonged heating for between 5-10 hours, was then re-oxidised with hydrogen peroxide or dioxygen.

Disadvantages of this procedure include the toxicity of hydrazine hydrate and the potential to mix hydrazine hydrate with solutions of hydrogen peroxide.

Consequently, it is an object of the present invention to provide an improved synthetic route to polyoxometalates.

According to a first aspect of the present invention, there is provided a process for preparing a polyoxometalate, the polyoxometalate comprising:

a first component part having a first oxidation state; and a second component part having a second oxidation state,

the process comprising reacting in suspension or soiution in the presence of an oxoacid a first compound or material comprising the first component in a more reduced oxidation state than its first oxidation state with a second compound or materiai comprising the second component such that the first component reduces the second component and is itself oxidised, resulting in the formation of the poiyoxometaiate.

This method allows the oxidation states of both the first and the second components to change during the reaction, thereby widening the number of possible first and second compounds that can be used in the reaction. In this way. cheaper compounds that include the components in a different oxidation state from that required in the poiyoxometaiate may be used, thereby potentially reducing the cost of the compounds used. Additionally or alternatively, this method provides a way of changing the oxidation state of one of the components so that it is in a state in which it can form a poiyoxometaiate. For example, the change in oxidation state may result in the solubiiising of at least one of the components.

Altering the oxidation state of the poiyoxometaiate components in this way means that no external reducing agent is required, although the process of the invention does not preclude the supplementary use of an external reducing agent when desired. However preferably the process of the invention may be effected in the absence of any reducing agent other than the first component mentioned herein, in this case the reaction is therefore traceless, unlike the reactions of the prior art, in the sense that at least some of ail of the reagents, or the component parts thereof, used in the process form part of the poiyoxometaiate structure at the end of the process. The redox reaction of the present invention may result in both first and second components being present in the oxidation state in which they exist in the poiyoxometaiate in at least one of its oxidised or reduced forms.

Further the process of the present invention may simplify poiyoxometaiate production as there is no need to concentrate the soiution after the reaction. Instead, the poiyoxometaiate can be produced at the desired concentration by suitable selection of the quantities and stoichiometric ratios of the reactive components. This is in contrast to the peroxide route of the prior art in which a lengthy and expensive water distiliation is required.

The reduction by the first component may solubilise the second component. This may result in the second component being in a state in which it can form part of the poiyoxometaiate. The first component may be a metal. Additionally or alternatively, the second component may be a metal. Metals are present in poiyoxometaiates and are well known to take part in redox reactions. The oxidation states required in the first and second compounds can be selected based on the required final oxidation states in the poiyoxometaiafe.

The first and/or second compounds may be metal oxides, Alternatively, the first compound may be the metal itself, i.e. the metal element at zero oxidation state. This provides a wider range of compounds that may be used to produce the desired polyoxomefaiate.

The second component may be vanadium. Vanadium Is commonly used in poiyoxometaiates. Often, the vanadium is present in the polyoxomefaiate as vanadium(IV). in this case, the second compound may include vanadium(V). The second compound may be V^O_s.

V₂0₅ is relatively cheap and readily available. However, it is poorly soluble in aqueous solution, which is necessary to form a poiyoxometaiate. It has been found that reducing the vanadium present in V₂O_s using the process currently claimed results in soluble vanadiumiJV) ions, which can then be incorporated directly into the poiyoxometaiate.

The first component may be molybdenum. Molybdenum is commonly used in poiyoxometaiates. Often, the molybdenum is present in the poiyoxometaiate as molybdenum(VI). In this case, the first compound may include moiybdenum(IV) and optionally may be molybdenum(IV) oxide.

Alternatively, the first compound may be molybdenum metal. Molybdenum metal may be preferred as it is cheaper than molybdenum(IV) oxide. Additionally, as molybdenum metal contains more reduced molybdenum than moiybdenum{IV) oxide, less will be required to reduce the second component by the same amount. The first compound may therefore include the first component in a highly reduced oxidation state.

The oxoacid may be selected from any suitable material or combination of materials and specifically includes precursors which combine together in situ to produce oxoacids. Suitably envisaged for use in the process of the invention are all oxoacids of the P block and D block which couid be envisaged as the heteroatom Z in the poiyoxometaiate structure identified below, Non limiting examples include oxoacids of periodic table groups 3, 4, 5 and 8, for example oxoacids of B, P, S, As, Si and Ge. H₃BG₃, H PQ , H₂SG , H Si0 _l H₃AsG H,,Ge0₄ may be specifically mentioned in this connection by way of non-limiting example.

The process of the invention may aiso be conducted in the presence of anciilary compounds, including "spectator" compounds which are not themselves oxidised or reduced during the process, but which form part of the poiyoxometaiate structure at the end of the process.

The process of the present invention may produce a poiyoxometalate in its reduced or partially reduced form. The reduced poiyoxometaiate may optionally be oxidised by a suitable oxidant; such as air, oxygen or hydrogen peroxide. Such oxidation may be preferable if the reduced or partially reduced poiyoxometaiate formed is less stable than when it is in. a more oxidised form.

The poiyoxometalate may have the structure: a[¾ vicG_d] wherein:

X is selected from hydrogen, alkali metals, alkaline earth metals, ammonium and combinations of two or more thereof;

Z is selected from B, P. S, As, Si, Ge, Ni, Rh, Sn, Al, Cu, I, Br, F. Fe, Co, Cr, Zn, H₂, Te, n and Se and combinations of two or more thereof;

is a metal selected from V, Mo, W, Mb, Ta, Mrs, Fe, Co, Cr, Ni, Zn, Rh, Ru, Ti, AL

Ga, in and other metais selected from the 1 ^st, '¹ and 3^rd transition metal series and the ianthanide series, and combinations of two or more thereof;

a is a number of X necessary to charge balance the i¾M_sO_d] anion;

b is from 0 to 20;

c is from 1 to 40; and

d is from 1 to 180.

In the formula above, b may be from 0 to 2. Additionally or alternatively, c may be from 5 to 20. Additionally or alternatively, d may be from 30 to 70.

In one embodiment, M comprises vanadium or molybdenum. Optionally. M may comprise vanadium and moiybdenum. Z may comprise phosphorus.

S Regarding the structure above, an embodiment of the invention provides a method in which the first and/or the second component are metals. In this case, the first and/or the second component are seiected from the possible species of M, as defined above.

More specifically, the poiyoxometalate may have the structure:

Hj+n+?.mPMOi2-n-_mV^v _nV ,0,4Q wherein n+m is from 1 to 4. Additionally or alternatively, the poiyoxometalate may have a Keggin structure. The poiyoxometalate may also have a Dawson-Wells structure.

The process of the invention will now be more particularly described and illustrated with reference to the following examples.

Example 1

Polyoxomefaiates were prepared in accordance with the following reactive equation: H₃PMO₁₂0₄G + 4 oOs + 4V₂0₅ + H₃PO ÷8H₂0 ~ *^* 2Hn P o₈V₄04o

The poiyoxometalate produced via this route was oxidised with oxygen gas at 80°C and analysed to see if it was the same as that produced via the peroxide route of the prior art. There were no significant differences detected by N R, mass spectrometry, CV or fuei eel! testing.

Example 2

Po!yoxometalates were prepared in accordance with the following reactive equation:

1 1 H₃PMo + 12Mo + 38V₂O_s + 7H₃PQ_A + 72H₂0 18H₁₁ PfVto_sV₄04o

The poiyoxometalate produced via this route was oxidised with oxygen gas at 80^'3C and analysed to see if it was the same as that produced via the peroxide route of the prior art. There were no significant differences detected by MMR, mass spectrometry, CV or fueS cell testing.

Example 3

Polyoxometaiates were prepared in accordance with the foilowing reactive equation:

22Mo0₃ + 2Mo + 8V₂O_s + 3H₃P0₄ + 12h G ►

The polyoxometalate produced via this route was oxidised with oxygen gas at 80°C and analysed to see if it was the same as that produced via the peroxide route of the prior art. There were no significant differences detected by NMR, mass spectrometry, CV or fuei ceil testing.

The Examples which follow illustrate In more detail a selection of synthetic routes available in accordance with this invention for the preparation of polyoxometaiates,

Example 4 ~ Synthesis of H₇PV₄f i0₈C¾_o (0,30 M) using Mo0₂

Reagents:

eagent Supplier Code Lot Purity Mass Ittoi

V₂O_s Alfa Aesar 1 1093 C29Z055 99.2% 109.13 g 0,60 I

6 H₂0 SAFC 221856-8 BBG436 3V 100% 334.81 g 0.15 I

H₃P<¾ SAFC W290017 KBH89K 3V 85% 17.30 g 0.15 I

MoO_z Alfa Aesar 481 17 LI 4X007 99% 76,74 g 0.60

Synthesis:

A 2 L round bottom flask equipped with a magnetic st rrer and a water cooled reflux condenser was charged with Η₃Ρ ο-!₂ ^₀·22.6 H₂0 (334.81 g, 0.15 moi) and uitrapure water (850 ml). The redox potential was measured at 692 mV at 20 "C vs Ag AgCI. H₃PC¾ (17.30 g, 0.15 mol} was added, followed by Mo0₂ (76,74 g, 0.60 moi) washed in with a further portion of uitrapure water (100 ml). The reaction was stirred under a blanket of nitrogen for 30 minutes during which time the yellow solution took on a dark blue-green colour, it is important to exclude oxygen as its presence leads to oxidation of the reduced poiyoxometalates, limiting the amount of V₂O_s reduced and hence solubiiised. The redox potential was measured at 458 mV at 20^"C vs Ag/AgCI.

V₂0₅ (109.3 g, 0.80 mol) was added portionwise and the suspension was heated to 80°C and stirred at this temperature for 18 hours. After this time, the solution appeared dark green and little or no solid V₂0₅ was visible. The redox potential was measured at 433 mV at 80 °G vs Ag/AgCI.

A sintered glass sparge was fitted to the reaction vessel and the reduced H₇PV₄Mo_gG ₀ solution produced was oxidised by the bubbling of oxygen gas through the solution at 80°C for 8 hours.

The reaction mixture was filtered through a glass sinter (porosity 3) to remove trace solids and the volume adjusted to 1 L by the addition of uitrapure water. The final redox potential was measured at 702 mV at 80 °C vs Ag/AgCI.

iesss of

powder

<150 pm_s

Synthesis:

A 2 L beaker equipped with a magnetic stirrer was charged with o0₃ (99.5%, 318.3 g), H₃PC¾ (85%, 34.8 g), V₂0₅ (99.2%, 1 10.0 g) and uitrapure water (800 ml). The resulting suspension was slurried and Mo powder (99,9% 19,3 g) added in one portion. On this scale and by adding the Mo metal to a cold suspension, no appreciable exotherm was detected. Care should be taken on larger scale or if adding the Mo whilst the suspension is hot. The reaction was heated to reflux and held at that temperature for 1 hour, during which time the solution took on a dark blue inky colour. The redox potential was measured at 408 mV at 108°C vs Ag/AgCi.

A sintered glass sparge was fitted to the reaction vessel and the reduced

solution produced was oxidised by the bubbling of oxygen gas through the solution at 80 for 8 hours. The final redox potential was measured at 702 mV at SOX vs Ag/AgCI.

The reaction mixture was filtered through a glass sinter (porosity 3) to remove trace solids and the volume adjusted to 1 L by the addition of uitrapure water.

E ample 8 - Synthesis of H₇PV₄I¾ So_g0 o (0.30 M) using V powder

0.8V + 1 .6V₂0₅ + 8IVI0O₃ + H₃P0₄ + 4H₂0→ H^PMo^V^Q^]

A 200 mi beaker equipped with a magnetic stirrer bar was charged with uitrapure water (80 ml), 85% phosphoric acid (3.48 g), MoG₃ (99.5%, 34.72 g), V₂0₅ (99.2%, 8.80 g) and V powder (99.5%, 1 ,229 g) and the mixture was slurried. Heat was then applied so that the temperature remained at approximately 100*0. When the volume decreased below 100 ml it was topped back up with uitrapure water. After 3 hours the heat was switched back off again, Only trace solids remained, which were filtered off before making the solution up to 100 ml as 0.3 H [P o₈V₄0_<}o], wherein ail of the vanadium was present as the reduced vanadium(IV).

Example 7 - Synthesis of

(0.30 M) sin Mo powder

1 .5V₂0s + CrO_s ÷ Mo + ? o0₃ + H₃PO< + 4.5H₂0 ^■■■■> H₁₂[P o^vl ₈Vⁱ ₃Cr^!!,G_JiC1]

A 250 ml round bottomed flask equipped with a magnetic stirrer bar was charged with uitrapure water (80 ml), 85% phosphoric acid (3.48 g), V₂0₅ (99.2%, 8,25 g), M0O₃ (99,5%, 30.38 g), Mo powder (99.5%, 2,89 g) and CrG₃ (99.0%, 3.0 g). A redox condenser was fitted and the reaction was stirred and heated at 100*0 for 48 hours. After this time the solids were not fully dissolved. The mixture was then stirred at ambient temperature for a further 72 hours after which a dark solution of H _{¾ 2}[PMo₈V₃OrQ o] formed. This was filtered to remove trace levels of soiids. Examp e 8 - Synthesis of H₁₁[Paflo^Vi ₈V^iV 04o3 (0,30 M) using V₂0₃

V₂0₃ + V₂0₅ + 8M0O3 + H3PO4 + 4H₂0→ HH

A 20Q ml beaker equipped with a magnetic stirrer bar was charged with uitrapure water (SO ml), 85% phosphoric acid (3.48 g), M0O₃ (99.5%, 34.72 g), V₂Q_S (99.2%, 5.50 g) and V₂0₃ (95%, 4.50 g) and the mixture was slurried. Heat was then applied so that the temperature remained at approximately 100°C. When the volume decreased below 100 mi it was topped back up with uitrapure water. After 3 hours the heat was switched back off again. Only trace solids remained, which were filtered off before making the soiution up to 100 mi as 0.3 M H iP oe 40₄o3 wherein ail of the vanadium was present as the reduced vanadium(IV).

Example 9 - Synthesis of H₁₃iP ^vs o ⁱ ₅ ο^ν ₂¥^ίν θ_4δ] (0,30 M) using W powder

2V;;O_g + 7Mo0₃ + W + H₃P0₄ + 6½H_K0→ H₁₃[PW^vlMo^Vi _s o^V^C

A 2 L beaker equipped with a magnetic stirrer bar was charged with ulirapure wafer (800 mi), 85% phosphoric acid (43.8 g), M0O₃ (99.5%, 384.8 g), V₂G₅ (99.2%, 139.3 g) and tungsten metal powder (99.9%, 12 micron, 89.9 g) and the mixture was siurried. Heat was then applied so that the temperature remained at approximately 100³C. When the volume decreased below 1000 ml it was topped back up with uitrapure water. After 4 hours the heat was switched back off again. Only trace solids remained, which were filtered off before making the solution up to 1 L as 0.38

wherein ail of the vanadium was present as the reduced vanadium(IV).

Example 10 - Synthesis of Η₁₁[Ρ¥¥^ν! ₈ν ^ν ₄0₄(3] (0.30 M) using W0₂

2V₂0s + 8W0₂ + 30₂ + H₃PO4 + 4H₂0→ Η₁.₁[ρνν^νι ₈ν^!ν ₄ο₄ο]

A 150 mi beaker equipped with a magnetic stirrer bar was charged with uitrapure water (50 ml), 85% phosphoric acid (870 mg). W0₂ (99.99%, 13.0 g) and V₂0₅ (99.2%, 2.75 g) and the mixture was slurried. Heat was then applied so that the temperature remained at approximately 100°C, When the volume decreased below 25 ml it was topped back up with ultrapure water. After 1 hour the heat was reduced to 80°C and 0₂ was bubbled gently through the reaction. After another hour the heat and 0₂ were removed and the reaction was allowed to cool, Only trace solids remained, which were filtered off before making the solution up to 25 mi as 0.3 H n[P ₈V₄04_C] wherein all of the vanadium was present as the reduced vanadium(IV).

Exampte 11 - Synthesis of

(0,30 ) ussng Co powder

2V₂G₅ + 2Co + 8 o0₃ + H3PO → H₇C^Q¾P o ^, ₈Vⁱ ₄0₄o]

A 200 ml beaker equipped with a magnetic stirrer bar was charged with ultrapure water (80 ml), 85% phosphoric acid (3.48 g), M0O₃ (99.5%, 34.72 g), V₂O_s (99.2%, 1 1 .0 g) and cobalt metal powder (99.8%, 1 .8 micron, 4.50 9} and the mixture was siurried. Heat was then applied so that the temperature remained at approximately 100°C. When the volume decreased beiow 100 mi it was topped back up with ultrapure water. After 1 hour the heat was switched back off again. The solution was made up to 100 ml as 0.3 H Co2 P o_sV₄04_Q], wherein all of the vanadium was present as the reduced vanadium(SV).

Example 12 - Synthesis of HrMn^PMo^eV^O^] (0,30 M) using IWn powder

V₂Q_S + 2Mn + 8Mo0₃ + H3PO4→

A 200 ml beaker equipped with a magnetic stirrer bar was charged with ultrapure water (80 mi), 85% phosphoric acid (3.46 g), 0O₃ (99.5%, 34.72 g) and V₂O_s (99.2%, 1 1 .0 g). The mixture was cooled in an ice bath, manganese metal powder (99.6%, <10 micron, 3,30 g) was added and the mixture was stirred for 1 hour. Heat was then applied to bring the temperature up to approximately 100°C. When the volume decreased below 100 ml it was topped back up with ultrapure water. After 1 hour the heat was switched back off again. The solution was made up to 100 mi as 0.3 M ^MnzIPMoaN^Q^], wherein ail of the vanadium was present as the reduced vanadium(IV). The summary table below outlines the redox species used in each of the Examples:

Claims

1 . A process for the preparation of a polyoxometalate, the polyoxometalate comprising:

a first component part having a first oxidation state; and

a second component part having a second oxidation state,

the process comprising reacting in suspension or solution in the presence of an oxoacid a first compound or material comprising the first component in a more reduced oxidation state than its first oxidation state with a second compound or material comprising the second component such that the first component reduces the second component and is itself oxidised, resulting in the formation of the polyoxometalate.

2. A process according to claim 1 wherein the first component reduces and thereby solubilises the second component.

3. A process according to claim 1 or 2 wherein the first component is a metal.

4. A process according to claim 3 wherein the metal is selected from V, Mo, W, Nb, Ta, Mn, Fe, Co, Cr, Ni, Zn, Rh, Ru, Tl, Al, Ga, In and other metals selected from the 1 ^st, 2^nd and 3^rd transition metal series and the lanthanide series, and combinations of two or more thereof.

5. A process according to claim 4 wherein the metal is Mo.

6. A process according to any one of claims 1 to 5 wherein the second component is a metal.

7. A process according to claim 6 wherein the metal is selected from V, Mo, W, Nb, Ta, Mn, Fe, Co, Cr, Ni, Zn, Rh, Ru, Tl, Al, Ga, In and other metals selected from the 1 ^st, 2^nd and 3^rd transition metal series and the lanthanide series, and combinations of two or more thereof.

8. A process according to claim 7 wherein the metal is V.

9. A process according to any one of claims 1 to 8 wherein the first compound or material is a metal, a metal containing species, or a mixture of two or more thereof.

10. A process according to claim 9 wherein the metal containing species is a metal oxide, a metal-containing acid, or a mixture of two or more thereof.

1 1 . A process according to any one of claims 1 to 10 wherein the second compound is a metal containing species, or a mixture of two or more thereof.

12. A process according to claim 1 1 wherein the metal containing species is a metal oxide or mixtures of two or more thereof.

13. A process according to any preceding claim wherein the polyoxometalate has the structure:

X_a[Z_bM_cO_d] wherein:

Z is selected from B, P, S, As, Si, Ge, Ni, Rh, Sn, Al, Cu, I, Br, F, Fe, Co, Cr, Zn, H₂, Te, Mn and Se and combinations of two or more thereof;

M is a metal selected from V, Mo, W, Nb, Ta, Mn, Fe, Co, Cr, Ni, Zn Rh, Ru, Tl, Al,

Ga, In and other metals selected from the 1 ^st, 2^nd and 3^rd transition metal series and the lanthanide series, and combinations of two or more thereof;

a is a number of X necessary to charge balance the [Z_bM_cO_d] anion;

b is from 0 to 20;

c is from 1 to 40; and

d is from 1 to 180.

14. A process according to claim 13, wherein b is from 0 to 2.

15. A process according to claim 13 or 14, wherein c is from 5 to 20.

16. A process according to any one of claims 13 to 15, wherein d is from 30 to 70.

17. A process according to any one of claims 13 to 16, wherein Z comprises phosphorus.

18. A process according to any one of claims 13 to 17, wherein M comprises vanadium.

19. A process according to any one of claims 13 to 18 wherein M comprises molybdenum.

20. A process according to claim 18 or claim 19 wherein M comprises vanadium and molybdenum.

21 . A process according to claim 20 wherein the polyoxometalate is of the structure:

H3+n+2m PMOi 2-n-m V^V _n V'^V _mO40

wherein n+m is from 1 to 4.

22. A process according to any one of claims 1 to 21 wherein the polyoxometalate has a Keggin structure.

23. A process according to claim 13 wherein the polyoxometalate is Hn PMo₈V₄0₄o.

24. A process according to claim 23 wherein the reaction that forms the polyoxometalate is:

H3PM012O40 + 4Mo0₂ + 4V₂0₅ + H3PO4 +8H2O ^► 2Hii PMo₈V₄04o

25. A process according to claim 23 wherein the reaction that forms the polyoxometalate is: 1 H3PM012O40 + 12Mo + 36V₂0₅ + 7H₃P0₄ + 72H₂0 ► 18Hn PMo₈V₄04o

26. A process according to claim 23 wherein the reaction that forms the polyoxometalate is:

22M0O₃ + 2Mo + 6V₂0₅ + 3H3PO4 + 12H₂0 ► 3Hn PMo₈V₄04o