STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT BACKGROUND OF THE INVENTION Catalyst stabilitv.

The usefulness and cost effectiveness of most catalyst systems is limited by their inherent thermodynamic instability. In almost all cases, catalysts composed of transition-and main-group ions are prepared and isolated under conditions very different from those in which they are designed to operate. As a result, most catalysts are only kinetically, rather than thermodynamically, stable. The catalysts, as prepared, lie far from thermodynamic equilibrium when placed in their operating environment, which can include solvents, additives, impurities, reactants, intermediates, products and byproducts under various conditions of temperature and pressure. The catalyst will gradually be converted into thermodynamic product (s) as the whole chemical system moves spontaneously toward equilibrium. What is more, this movement usually creates undesired changes in the catalyst itself.

These"catalyst degradation processes"determine the catalyst's operational lifetime as dictated by the rate the system approaches chemical equilibrium.

Once the deactivated catalyst is determined to be ineffective or too inefficient, it must be replace or regenerated, often an expensive proposition.

Limitations imposed by thermodynamic instability are particularly applicable to soluble (homogeneous) catalysts as they generally will decompose at faster rates than their solid-state (heterogeneous) counterparts. The process of homogeneous-catalyst degradation is often accelerated by the presence of water, which can react with transition-metal or main-group ions via hydrolysis or condensation reactions to give inactive metal oxides or hydroxides. Because these decomposition products typically possess low solubilities, elemental components key to the catalyst's performance are lost as they precipitate from the solution. Rates of decomposition increase when water is present in high concentration, as when it is used as a solvent. This is one reason why few reactions catalyzed by inorganic, metallo-organic or organometallic complexes are carried out in water even though it is the most desirable solvent with respect to both cost and environmental impact.

There is therefore a need for the development of catalysts, water- soluble ones in particular, that are thermodynamically stable under the conditions at which they will be ultimately utilized. In order to achieve such a mandate, the catalyst would have to be part of an equilibrium. Indeed, a catalyst that is at equilibrium with its precursors, synthetic intermediates or related compounds within its operating environment could posses the ability to regenerate itself if any degradation did occur. Such a catalyst system, in principe, would possess unlimited operational lifetimes.

Reactions of cations or anions with water or with its components, H+ or OH-, are referred to as speciation reactions. Catalysts and their components are also susceptible to speciation reactions. They possess useful kinetic stabilities in water over relatively narrow ranges of H'or OH-concentrations and are rapidly degraded when the pH deviates too greatly. There is a need to limit changes in H+ and OH-concentrations (pH control) in the design of

aqueous and other catalyst systems. For example, in aqueous systems, when electron-containing substrates are oxidized by electron acceptors (oxidants), the H+ concentration of the solution generally increases linearly with the number of electrons transferred from substrate to oxidant (a drop in the pH of the solution). Conversely, OH-anions are generated (an increase in pH) when POM solutions are used as reducing agents in the presence of oxidants such as dioxygen (02), ozone (03), hydrogen peroxide (H202) or other peroxides (i. e., oxidants that possess the O22-functionality). In many cases the substrate, the oxidizing or reducing agents or the desired product (s) are susceptible to decomposition by H+ or OH-ions.

Consequently, the addition of exogenous pH buffers is often necessary to maintain the integrity of all the species within the chemical system. However, in many cases the addition of chemical buffers induces undesirable side- reactions. Additionally, increased expenses are associated with the cost of the buffers themselves. It would be desirable to prepare soluble catalysts that are thermodynamically stable with respect to undesired speciation reactions in water and specifically there is a need for self-buffering catalyst systems.

Polyoxometalates.

Early-transition-metal oxygen-anion clusters, or polyoxometalates (POMs for short) are a large, structural diverse and rapidly growing class of inorganic compounds. They are composed of d° metal cations, especially W (VI), Mo (VI) and V (V), linked to one another by oxide anions (o2-) in varying combinations. The principal building blocks of POMs are MOX, x = 4-6, polyhedra, usually MO6 octahedra, typically linked together by one, two or three oxide anions. There are two generic classes of POMs: isopolyanions containing only the d° metal cations and heteropolyanions containing one or more d or p block"heteroatom"cations in addition to the metal cations.

Polyoxometalates in both of the classes may be mixed-addendum, meaning they contain more than one type of the d° metal cations. Isopolyanions have the general formula [MXOy]; examples include [W7O24]6- and [V2W4O19]4-.

Heteropolyanions have the general formula [XaMbOc]m- where Xn+ is a heteroatom; examples include [ColilW, and [SiVMoWioOJ'. The negative charges of POMs can be counterbalanced by hydrophilic cations, such as H+, Li+, Na+, K+, and NH4+, to provide neutral salts that are soluble in water. Additionally, hydrophobic cations, such as R4N+ (R = aliphatic or aromatic hydrocarbons of varying sizes) or Ph4P+ (Ph = phenyl group), can be used to render the complexes soluble in organic solvents and other hydrophobic media. POMs can range in size from 9A (0.9 nm) to over 30A (3 nm). Heteropolyanions are a larger, more versatile, and more easily modified class of POMs than the isopolyanions. The most common and most thoroughly investigated structure of heteropolyanions is the Keggin molecule (ca. 11 A) shown in Fig. 1.

POMs are being developed to catalyze a rapidly expanding library of chemical reactions. Many POM catalysts and catalyst systems have been developed as solid-state heterogeneous catalysts as well as a variety of homogeneous catalysts soluble in both water and organic solvents. Several attributes make POMs attractive for use in catalysis. First, POMs can be readily prepared in water from inexpensive, minimally toxic, and accessible compounds such as sodium tungstate (Na2WO4), sodium molybdate (Na2MoO4), sodium metavanadate (NaVO3), sodium metasilicate (Na2SiO3), and phosphoric acid (H3PO4). Second, and a point of particular pertinence to the use of POM catalysts for oxidation reactions, many POMs can be reversibly reduced, often by many electrons, while being simultaneously resistant to oxidative degradation. Such d° systems, which include common materials like sand, glass, and many ceramics, are already in the maximum

oxidation state attainable under any conventional reaction conditions. Third, POMs exhibit a tremendous amount of flexibility as a number of key physical properties, such as redox potential, acidity, charge, solubility, etc., can be controlled to a marked degree by rational design and synthesis. One or more of the d° metal ions in the parent POM structure can be replace by d- electron-containing metal ions or by main-group cations. Indeed, the profound control of the chemically significant properties of POMs vests, in part, in this rich substitution chemistry.

Most methods for the synthesis of POMs from their simples precursors, such as NaV03, Na2Mo04 and Na2WO4, involve condensation reactions brought about by the addition of protic (Bronsted) acids. This invariably results in the presence of salts containing the conjugate bases of the protic acids used in the synthetic procedure (e. g., Cl-from HCI, SO42-from H2SO4 or NO3-from HNO3). The exception is the use of phosphoric acid, H3PO4, for the preparation of vanadomolybdophosphates where the PO43- anion is incorporated into the POM structure. The presence of conjugate- base anions can lead to operational problems associated with such impurities, such as corrosion of reactor walls and interference with the chemical operations of the catalyst. Moreover, removal of the salts of the conjugate-base anions often involves laborious extractions with organic solvents or crystallizations, both of which add to the equipment, labor and material costs of the synthetic procedures. There is thus a need to produce POMs in high yield starting from simple hydroxide or neutral or anionic oxides without the addition of extraneous components. This goal would be achieved by preparing equilibrated POM-catalyst systems in a single step from component-element precursors.

As with most catalytic systems, POMs and their associated cations are also susceptible to speciation reactions. Although many POMs are

remarkably resistant to oxidative degradation, and thus excellent candidates for use as oxidation catalysts, they possess useful kinetic stabilities in water over relatively narrow ranges of H+ or OH-concentrations. Thus, most well- defined isopoly-or heteropolyoxometalates are rapidly degraded when used at the conditions often desired for catalysis, namely temperatures of 100°C or greater and neutral pH values. As is the case with most metal-based catalysts, thermodynamic instability severely limits their usefulness in water.

To overcome this limitation, it would be desirable to prepare soluble POM- catalysts that are thermodynamically stable with respect to undesired speciation reactions in water and which are additionally capable of buffering their own reactions.

Many solid-state POM catalysts and catalyst systems have been developed, as have a variety of water soluble and organic-solvent soluble catalysts. Most soluble POM-catalyst systems involve either heteropolymolybdovanadates, such as H5PV2Mo, 0040 (a water-soluble mixed- addendum Keggin molybdophosphate) or organic-solvent salts of a wide variety of isopoly-and heteropoly-molybdates or tungstates. Because tungstate-based POMs are generally more readily synthetically altered and less labile (more kinetically stable) than similar molybdate-based anions, isopoly-and heteropolytungstates by far constitute the largest class of soluble POM catalysts. Most of these, in turn, are prepared as salts of hydrophobic cations and used in organic solvents. The use of organic solvents stems primarily from practical considerations such as the need to solubilize chemical oxidants and organic substrates, many of which possess limited solubilities in water, and the need (in some cases) to avoid competition between substrate and water molecules for coordination sites on d-electron containing transition- metal addendum atoms.

Equilibrated mixtures of isopoly-and heteropolyoxomolybdates of the form IpVkMO12-koll]"+')- (phosphovanadomolybdates) can be prepared under mild conditions by combining MoO3 with NaVO3 and H3PO4 in water. While such systems have demonstrated the feasibility of using of POMs as water- soluble homogeneous catalysts, limitations have hindered their use.

Aqueous heteropolyoxomolybdate solutions are restricted to acidic conditions which may be undesirable for many chemical systems. An additional obstacle to their more expanded use in catalysis is that the substitution chemistry of heteropolyoxomolybdates is very limited.

Heteropolyoxotungstates, however, constitute a much more diverse class of soluble POM catalysts. Because they are generally less labile (i. e., more kinetically stable) than similar molybdate-based anions, it is easier to isolate a greater variety of tungsten-based POM compounds. Isopoly and heteropolyoxotungstates have been synthesized using a broader range of conditions than have the isopoly-or heteropolyoxomolybdates. The range over which heteropolytungstates are stable ranges from strongly acidic to weakly basic conditions for a vast array of d-electron-containing transition- metal-substituted and other derivatives. To date, however, catalysts from these classes of polytungstates complexes have only been isolated as kinetically-stable products which are thermodynamically unstable under the conditions at which they are used. They are susceptible to decomposition in water when heated to elevated temperatures or after prolonge use.

Therefore, in order to greatly expand the application of POMs as catalysts, it is necessary that thermodynamically (i. e., inherently) stable tungsten-based soluble polyoxometalate-catalyst systems be developed. One way to obtain thermodynamically stable systems and to additionally incorporate a capacity for self-buffering is to prepare soluble tungsten-based polyoxometalate catalysts from principal components as thermally equilibrated mixtures.

Until now, there was no reason to expect that useful equilibrated tungsten-based POM mixtures could be obtained or predictably modified.

Indeed, the available kinetic data suggested otherwise. As mentioned above, equilibrated mixtures of heteropolyoxomolybdates can be prepared under mild conditions. However, the kinetic barriers to attaining thermodynamic equilibria between polyoxomolybdates in aqueous solution are relatively small, much smaller than the kinetic barriers to attaining equilibria between polyoxotungstates. Half-lives for the equilibration of polyoxomolybdates are on the order of seconds to minutes in contrast to half-lives for polyoxotungstates which range from days to months. Such differences mean that polyoxomolybdates are orders of magnitude more labile than most polyoxotungstates. Consequently, while equilibrium mixtures of polyoxomolybdates could be readily prepared from metal-oxide precursors, no such expectations for equilibrated tungsten-based POM mixtures prepared by similar methods exist. It might have been argued that if sufficiently high temperatures were used, the approach to thermodynamic equilibrium would be more rapid. However, the paucity of data concerning the activation parameters associated with interconversions of polyoxotungstates rendered such arguments purely speculative. In addition, the exponential dependence of thermodynamic parameters on temperature would make it impossible to predict whether equilibrated tungsten-based POM mixtures might possess useful physical and chemical properties even if successfully prepared at elevated temperatures.

Very little information is available on mixed molybdenum and tungsten systems. While there are some similarities regarding the speciation chemistries of the cations of each element (i. e., Mo (VI), W (VI)), there are also some distinct differences. Arguably, the system might behave like other tungsten-based systems where the tungsten is the dominant component

(e. g., the molar ratio of tungsten to molybdenum is greater than one to one).

However, the limited amount of information available demonstrates that the properties of such mixtures are difficult to predict.

SUMMARY OF THE INVENTION The present invention is a homogeneous solution that contains one or more desired or useful (target) tungsten-based isopoly-or heteropolyoxometalate (POM) complexes present in equilibrium with all chemical species related to the complex or complexes by reactions between chemical components of the system. This solution comprises various soluble compounds that contribute specific whole-number or fractional ratios of any or all of the elements V, Nb, Ta, Mo, W, d-electron-containing transition-metal ions (TM) and main-group ions (MG) such that there exists one or more target polyoxometalates of the general formula [VkNbmTanMooWP (TM) q (MG) rOslZ- where TM is a d-electron-containing transition-metal ion and MG is a main- group ion; k is 0-18, m is 0-20, n is 0-10, o is 0-19, p is 1-150, q is 0-9 and r is 0-9; k<p, m<p, n<p and o < p provided that p. ; 1 and k+m+n+o+p> 4; and s is sufficiently large that z > 0. The useful POM of the general formula is present at an effective concentration for its intended purpose.

Preferably, all species present within the equilibrated aqueous solution remain dissolved during the application.

One specific embodiment of the present invention is an equilibrated, homogeneous solution containing target tungsten-based complexes that contain relatively high molar ratios of tungsten to molybdenum. Another specific embodiment of the present invention is an equilibrated, homogeneous solution containing target tungsten-based complexes that contain molar ratios of tungsten to molybdenum greater than five to one.

Another specific embodiment of the present invention is an equilibrated,

homogeneous solution containing target tungsten-based complexes that contain molar ratios of tungsten to molybdenum greater than four to one.

Another specific embodiment of the present invention is an equilibrated, homogeneous solution containing target tungsten-based complexes that contain molar ratios of tungsten to molybdenum greater than three to one.

Another specific embodiment of the present invention is an equilibrated, homogeneous solution containing target tungsten-based complexes that contain moiar ratios of tungsten to molybdenum greater than two to one.

Another specific embodiment of the present invention is an equilibrated, homogeneous solution containing target tungsten-based complexes that contain molar ratios of tungsten to molybdenum greater than one to one.

A preferred method for preparing the solutions of the present invention is to mix hydroxides or neutral or anionic oxides of transition-metal or main- group elements in water and to heat the mixtures to temperatures sufficiently high such that the hydroxides or neutral or anionic oxides of the transition- metal or main-group elements react to give the target polyoxotungstates of the general formula in thermodynamic equilibrium with additional chemical species or complexes also derived from the hydroxides or neutral or anionic oxides of the transition-metal or main-group elements.

In another embodiment, the present invention is a method of using a homogeneous, aqueous solution, comprising one or more catalytically or otherwise useful tungsten-based isopoly-or heteropolyoxometalate (POM) complexes present in thermal equilibrium for homogeneous or heterogenous oxidation of an organic or inorganic chemical substrate. One particularly advantageous application of this method is the application of the polyoxometalate solution in the present invention to wood pulp, wood fiber, lignocellulosic pulp or lignocellulosic fiber such that enhanced delignification occurs.

In another embodiment, the present invention is a method of preparing a self-buffering system, comprising preparing a homogeneous, aqueous buffer solution, comprising one or more tungsten-based isopoly-or heteropolyoxometalate (POM) complexes present in thermal equilibrium with all other related chemical species by reactions between chemical components of the system. The system pH is maintained within a pH range of 4 units, preferably 2 units and most preferably 1 unit. The buffer consists of various soluble compounds that contribute specific whole-number or fractional ratios of any or all of the elements V, Nb, Ta, Mo, W, d-electron-containing transition-metal ions (TM) and main-group ions (MG) such that there exists one or more useful polyoxometalates of the general formula [VkNbmTanMOoWp (TM) q (MG) rOs] Z-where TM is a d-electron-containing transition-metal ion and MG is a main-group ion; k is 0-18, m is 0-10, n is 0-10, o is 0-19, p is 1-150, q is 0-9 and r is 0-9; k < p, m < p, n < p and o < p provided that p 2 1 and k + m + n + o + p 2 4; and s is sufficiently large that z > 0. Preferably, the temperature of the system is 0°C to 400°C, the pH of the system is between 1.0 and 10.0.

An advantage of the present invention is that the ratios of the elements used can be modified along a continuum of values such that the resulting polyoxometalate mixture responds rapidly and reversibly upon reduction and oxidation of the target heteropoiyoxotungstates of the general formula such that the pH of the solution remains constant.

Another advantage of the present invention is that the ratios of the elements used can be modified along a continuum of compositions such that the resulting polyoxometalate mixture can be adjusted to control the pH of the prepared solution and the pH at which the solution acts as a pH buffer via changes in the relative concentrations of species present in the equilibrium mixture.

Another advantage of the present invention is that the ratios of the elements used can be modified along a continuum of values such that the concentrations of target heteropolyoxotungstates of the general formula along with other species present in the equilibrated polyoxometalate mixture can be adjusted to optimize the physical and chemical properties of the system for use in particular applications.

Another advantage of the present invention is to isolate target polyoxotungstates of the general formula from a polyoxometalate solution that was prepared by mixing of hydroxides or neutral or anionic oxides of transition-metal or main-group elements in water and heating the mixtures at temperatures sufficiently high such that the hydroxide or neutral or anionic oxide precursors of transition-metal or main-group elements react to give the target polyoxotungstates of the general formula.

An objective of the present invention is to delignify hardwood or softwood fibers, hardwood or softwood pulps, or fibers or pulps from other lignocellulosic materials using suitable equilibrated polyoxotungstate solutions.

Another objective of the present invention is to oxidize carbon monoxide (CO) to carbon dioxide (CO2) using suitable equilibrated polyoxotungstate solutions.

Another objective of the present invention is to oxidize reducing agents using suitable equilibrated polyoxotungstate solutions.

A feature of the present invention is that suitable equilibrated polyoxotungstate solutions may be reoxidized with an oxidant selected from the group consisting of air, oxygen, hydrogen peroxide and other organic or inorganic peroxides (free acid or salt forms), or ozone.

Another feature of the present invention is that equilibrated polyoxotungstate solutions may be oxidized using suitable oxidants.

Another feature of the present invention is that transition metals may be incorporated into the equilibrated polyoxotungstate solutions to increase the rates of reoxidation using an oxidant selected from the group consisting of air, oxygen, hydrogen peroxide and other organic or inorganic peroxides (free acid or salt forms), and ozone.

Another feature of the present invention is that equilibrated polyoxotungstate solutions may be used as an oxidant and be reoxidized in repeated cycles.

Another feature of the present invention is that suitable equilibrated polyoxotungstate solutions may be used wherein polyoxometalate complexes are reduced and reoxidized within the same process step.

Other features, objectives and advantages of the present invention will become apparent upon examination of the specification, claims and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1. A disubstituted, a-Keggin heteropolyanion displayed in polyhedral notation. Each polyhedron represents a main-group or transition- metal atom at its center with oxygen atoms at each of its vertices. The black tetrahedron in the center represents the oxide of the heteroatom, the ten gray octahedra represent the oxides of the structural atoms and the two white octahedra represent oxides of the substituted atoms. This figure represents one of the possible positional isomers of the [SiV2W, oO4o] 6- anion.

Fig. 2. 5'V NMR spectrum of K9[SiVW10O39]#14H2O in D2O (pD 8.4) prepared in a stepwise manner (Example 1 a) after treatment with excess Licol04 an removal of precipitated KCI04.

Fig. 3. 51V NMR spectrum of K6 [SiV2W, 0040] xH20 in D20 (pD 4.1) prepared in a stepwise manner (Example 1 b). The peaks shown represent different positional isomers of the [SiV2W10O40]6- anion.

Fig. 4. spectrumofK5[SiVMoW10O40]#xH2OinD2OpreparedNMR in a stepwise manner (Example 1 e). The peaks shown represent different positional isomers of the [SiVMoW10O40]5- anion.

Fig. 5. 51V NMR spectrum of a 0.1 M Na6(+2)[SiV2W10O40] solution prepared as an equilibrium mixture from the neutral and anionic elemental oxides (Example 3a); elemental bromine was added to the NMR sample to ensure full oxidation. Peaks labeled (1) are isomers of [SiV2W10O40]6-; the peak labeled (2) is [V2W4019] 4-present as part of the equilibrium mixture.

Fig. 6. 51V NMR spectrum of a 0.1 M Na6(+2)[SiV2W10O40] solution prepared as an equilibrium mixture from the neutral and anionic elemental oxides under mild reaction conditions (Example 3b); elemental bromine was added to the NMR sample to ensure full oxidation.

Fig. 7. 15V NMR spectrum of a 0.2 M NagJSiVoOJ soiution prepared as an equilibrium mixture from the neutral and anionic elemental oxides (Example 3c); elemental bromine was added to the NMR sample to ensure full oxidation.

Fig. 8.5'V NMR spectrum of a 0.5 M Na6(+2)[SiV2W10O40] solution prepared as an equilibrium mixture from the neutral and anionic elemental oxides (Example 3d); elemental bromine was added to the NMR sample to ensure full oxidation.

Fig. 9. 51V NMR spectrum of a K6 [SiV2W, o04o] solution isolated from a equilibrium mixture prepared using neutral and anionic elemental oxides (Example 4). The sample was first oxidized using elemental bromine.

Fig. 10. Stacked 5'V NMR spectra of 0.1 M Na6(+x)[SiV2W10O40] solutions prepared from the elemental oxides with various stoichiometries of Na2WO4 relative to W03 (Experiments A-D in Table 2 of Example 5); elemental bromine was added to each NMR sample to ensure full oxidation.

Fig. 11. Equilibrium solutions of Na6(+2)[SiV2W10O40] and Na6(+3)[SiV2W10O40] titrated with acid at 70°C demonstrating differences in buffering capacity. The horizontal plateaus indicate the buffering regions.

The dashed line is the pH if the acid had been added directly to water.

Fig. 12. Stability, as monitored by 5'V NMR, of a 0.1 M Na6(+2)[SiV2W10O40] equilbrium solution used in several cycles of reduction by wood pulp followed by reoxidation by oxygen (02) (Example 7): (A) Sample after initial synthesis, (B) after the first bleaching step, (C) after the first wet- oxidation/reoxidation step, and (D) after the last wet-oxidation/reoxidation step. Elemental bromine was added to the NMR samples to ensure full oxidation.

Fig. 13. Stability of a 0.1 M Na6(+2)[SiV2W10O40] equilibrium solution used in several cycles of reduction by wood pulp followed by reoxidation by oxygen (02) (Example 7). The stability of the equilibrium system is emphasized by the constant pH maintained throughout the bleaching and reoxidation steps.

Fig. 14. 51V NMR spectrum of a 0.25 M Na6 [AIVW"OQO) equilibrium solution, pH 3.9, prepared using neutral and anionic elemental oxides and hydroxides (Example 10b); elemental bromine was added to the NMR sample to ensure full oxidation. Peaks labeled (1) are isomers of [AIVW"040] 6-; the peaks labeled (2), (3) and (4) are [V2W401, 9]40, (V3W3O19]15- and [HV3W3O19]4-, respectively, present as part of the equilibrium mixture.

Fig. 15. 5'V NMR spectrum of a 0.25 M Na6(+2)[AIVW11O40] equilibrium solution at pH 8.6 prepared using neutral and anionic elemental oxides and hydroxides (Example 10c); elemental bromine was added to the NMR sample to ensure full oxidation. Peaks labeled (1) are isomers of [AIVW"040] 6- ; the peaks labeled (2) and (3) are isomers of rV2W40, 9] 4-present as part of the equilibrium mixture.

Fig. 16. 27Al NMR spectrum of a 0.25 M Na6(+2)[AIVW11O40] equilibrium solution at pH 8.6 prepared using neutral and anionic elemental oxides and hydroxides (Example 10c); elemental bromine was added to the NMR sample to ensure full oxidation. The peak labeled (1) is [AIVW"O4o] 6- ; the peaks labeled (2) are from [Al (Al) W"039] 6-present as part of the equilibrium mixture.

Fig. 17. Stability, as monitored by 5'V NMR of a 0.5 M Na6(+1.5)[Al1(+0.5)VW11(+1)O40] equilibrium solution used in several cycles of reduction by wood pulp followed by reoxidation by oxygen (02) (Example 13): (A) sample after initial synthesis, (B) after the first bleaching step, (C) after the first wet-oxidation/reoxidation step, (D) after the second bleaching step and (E) after the second wet-oxidation/reoxidation step. Elemental bromine was added to the NMR sample to ensure full oxidation.

Fig. 18. Stability of a 0.1 M Na6(+1.5)[Al1(+0.5)VW11(+1)O40] equilbrium solution used in several cycles of reduction by carbon monoxide (CO) followed by reoxidation by oxygen (02) (Example 14). The stability of the equilibrium system is emphasized by the constant pH maintained throughout the reduction and reoxidation steps.

Fig. 19.5'V NMR spectrum of a 0.5 M Na5(+1)[PV2W10O40] solution prepared as an equilibrium mixture from the neutral and anionic elemental oxides (Example 15); elemental bromine was added to the NMR sample to ensure full oxidation. Peaks labeled (1) are isomers of [PV2W, 0040] 5-, peaks labeled (2) are isomers of [PVgWgOr and the peak labeled (3) is [PW"039 '- or [PW, o036]' all present as part of the equilibrium mixture.

Fig. 20. 5'V NMR spectrum comparing the distribution of species in equilibrium solutions of 0.5 M Na5(+x)[SiVW11O40] for x = 1-3 (part of Example 17); elemental bromine was added to the NMR sample to ensure full oxidation. The peak labeled (1) is [SiVW1O40]5-, peaks labeled (2) are

isomers of [SiV2W10O40]6-; the peak labeled (3) is 'V2W40, 9] 4-present as part of the equilibrium mixture.

Fig. 21. Reoxidation rates using oxygen of a 0.5 M equilibrated Na5(+1.9)[SiV1(-0.1)MoW10(+0.1)O40] solution compared with 0.5 M equilibrated Na5(+1.5)[SiV1(-0.1)(TM0.1)MoW10O40] olutions where TM=Mn, Fe and Cr (Example 20).

Fig. 22. Comparison the visible spectra of a Na5(+0.26)[SiMn1(-0.2)(H2O)W11O39] equilibrium solution prepared using neutral and anionic elemental oxides and hydroxides (Example 22) to a literature "step-wise""step-wise"preparation of K5[SimN#@11O39].

DESCRIPTION OF THE INVENTION Preferred formulas of the present invention.

The present invention is a homogeneous solution that comprises one or more catalytically or otherwise useful tungsten-based isopoly-or heteropolyoxometalate (POM) complexes present at in thermal equilibrium with all chemical species related to the useful complex or complexes by reactions between chemical components of the system. This solution comprises various soluble compounds that contribute specific whole-number or fractional ratios of any or all of the elements V, Nb, Ta, Mo, W, d-electron- containing transition-metal ions (TM) and main-group ions (MG) such that there exists one or more useful polyoxometalates of the general formula rVkNbmTanMoOWp (TM) q (MG) rOSJZ- where TM is a d-electron-containing transition-metal ion and MG is a main-group ion; k is 0-18, m is 0-10, n is 0-10, o is 0-19, p is 1-150, q is 0-9 and r is 0-9; k < p, m < p, n < p and o < p provided that p 2 1 and k + m + n + o + p 2 4; and s is sufficiently large that z > 0. The useful or target POM of the general formula is present at a concentration suitable for its intended purpose. Depending on the

application, one may have a suitable concentration several orders of magnitude less than those shown in the Examples.

The homogeneous solutions of the present invention contain target POMs present as part of equilibrium distributions of chemically related species. Preferably, the target POM is one of eight different formulas that are subsets of the general formula. The following equilibrated solutions 2-10 each comprise an example of such a target POM that is a subset of the general formula: Equilibrated polyoxotungstate-catalyst solution 2 comprises a target isopolyoxotungstate or mixed-addendum isopolyoxotungstate present at a useful concentration as part of a homogeneous solution wherein it exists in thermal equilibrium with all chemical species present and related to one another and to the target isopoly-or mixed-addendum isopolyoxotungstate by reactions between chemical components of the system. This solution comprises various soluble complexes that each contribute specific whole- number or fractional ratios of any or all of the elements V, Nb, Ta, Mo and W d-electron-containing transition-metal ions (TM) and main-group ions (MG) to the total chemical composition of the system such that there exists one or more useful isopolytungstate with the formula [VkNbmTanMoOWpOs] Z~ where k< p, m < p, n < p and o < p provided that p 4; and s is sufficiently large that z > 0. The anions [W10O32]4- and [V2W4O19]4- are examples, respectively, of an isopolytungstate and of a mixed-addendum isopolytungstate of this formula.

Equilibrated polyoxotungstate-catalyst solution 3 comprises a target heteropolyoxotungstate possessing the Keggin structure present at a useful concentration as part of a homogeneous solution wherein it exists in thermal equilibrium with all chemical species present and related to one another and to the target heteropolyoxotungstate possessing the Keggin structure by

reactions between chemical components of the system. This solution comprises various soluble complexes that each contribute specific whole- number or fractional ratios of any or all of the elements V, Nb, Ta, Mo and W, d-electron-containing transition-metal ions (TM) and main-group ions (MG) to the total chemical composition of the system such that there exists one or more useful heteropolyoxotungstate of the Keggin structure with the formula [VkNbmTanMooWp (TM) q (MG) O,,] There TM is a d-electron-containing transition-metal ion and MG is a main-group ion; k < p, m < p, n < p and o < p provided that p 1 and k+m+n+o+p+q+r=13; 1 # r + q # 5 where one TM or MG atom acts as the heteroatom; and s is sufficiently large that z > 0. The anion [SiVMn"' (H20) W, o039] 6- is an example of a target Keggin heteropolyoxotungstate complex of this formula.

Equilibrated polyoxotungstate-catalyst solution 4 comprises a target transition-metal-bridged dimer of two tri-vacant Keggin heteropolyoxotungstate anions present at a useful concentration as part of a homogeneous solution wherein it exists in thermal equilibrium with all chemical species present and related to one another and to the target transition-metal-bridged dimer of two tri-vacant Keggin heteropolyoxotungstate anions by reactions between chemical components of the system. This solution comprises various soluble complexes that each contribute specific whole-number or fractional ratios of any or all of the elements V, Nb, Ta, Mo and W to the total chemical composition of the system such that there exists one or more useful transition-metal-bridged dimer of two tri-vacant Keggin heteropolyoxotungstate anions with the formula [VkNbmTanMO,) WP (TM) q (MG) rOs] Z-where TM is a d-electron-containing transition-metal ion and MG is a main-group ion; k < p, m < p, n < p and o < p <BR> <BR> <BR> <BR> <BR> providedthatp21andk+m+n+o+p+q+r=24; 4<k+q<12where four V or TM atoms act as bridging atoms; 2 < + q q : s : 12 where two TM or

MG atoms act as the heteroatoms; and s is sufficiently large that z > 0. The anion [Co4 (H20) 2 (PWg034) 2]'°~ is an example of a target transition-metal- bridged dimer of this formula.

Equilibrated polyoxotungstate-catalyst solution 5 comprises a target heteropolyoxotungstate with the Wells-Dawson structure present at a useful concentration as part of a homogeneous solution wherein it exists in thermal equilibrium with all chemical species present and related to one another and to the target heteropolyoxotungstate with the Wells-Dawson structure by reactions between chemical components of the system. This solution comprises various soluble complexes that each contribute specific whole- number or fractional ratios of any or all of the elements V, Nb, Ta, Mo and W, d-electron-containing transition-metal ions (TM) and main-group ions (MG) to the total chemical composition of the system such that there exists one or more useful heteropolyoxotungstate with the Wells-Dawson structure with the formula [VkNbmTanMooWP (TM) q (MG) rOslZ-where TM is a d-electron-containing transition-metal ion and MG is a main-group ion that is preferably P (V), As (V) or S (VI); k < p, m < p, n < p and o < p provided that p 2 1 and k + m + n + o + p+q+r=20; 2 < r+q < 8wheretwoTMorMGatomsactasthe heteroatoms; and s is sufficiently large that z > 0. The anion [P2V3W15062] 9-is an example of a target heteropolyoxotungstate with the Wells-Dawson structure of this formula.

Equilibrated polyoxotungstate-catalyst solution 6 comprises a target transition-metal bridged dimer of two tri-vacant Wells-Dawson anions present at a useful concentration as part of a homogeneous solution wherein it exists in thermal equilibrium with all chemical species present and related to one another and to the target transition-metal bridged dimer of two tri-vacant Wells-Dawson anions by reactions between chemical components of the system. This solution comprises various soluble complexes that each

contribute specific whole-number or fractional ratios of any or all of the elements V, Nb, Ta, Mo and W d-electron-containing transition-metal ions (TM) and main-group ions (MG) to the total chemical composition of the system such that there exists one or more useful transition-metal bridged dimer of two tri-vacant Wells-Dawson anions with the formula [VkNbmTanMoOWp (TM) q (MG) rOslZ-where TM is a d-electron-containing transition-metal ion and MG is a main-group ion that is preferably P (V), As (V) or S (V); k < p, m < p, n < p and o < p provided that p 2 1 and k + m + n + o + p + q + r 38; 4 < k + q < 20 where four V or TM atoms act as bridging atoms; 4 < r + q < 20 where four TM or MG atoms act as the heteroatoms; and s is sufficiently large that z > 0. The anion [Zn4 (H20) 2 (P2W, 5056) 2]'5~ is an example of a target transition-metal bridged dimer of two tri-vacant Wells- Dawson anions of this formula.

Equilibrated polyoxotungstate-catalyst solution 7 comprises a target heteropolyoxotungstate possessing the Preyssler structure present at a useful concentration as part of a homogeneous solution wherein it exists in thermal equilibrium with all chemical species present and related to one another and to the target heteropolyoxotungstate possessing the Preyssler structure by reactions between chemical components of the system. This solution comprises various soluble complexes that each contribute specific whole-number or fractional ratios of any or all of the elements V, Nb, Ta, Mo and W d-electron-containing transition-metal ions (TM) and main-group ions (MG) to the total chemical composition of the system such that there exists one or more useful heteropolyoxotungstate possessing the Preyssler structure with the formula [VkNbmTaMooWP (TM) q (MG) rCtOs] w where TM is a d-electron-containing transition-metal ion and MG is a main-group ion that is preferably P (V); C is a main-group ion or di-or tri-valent transition-metal or lanthanide ion located in the center of the structure that is preferably Na+; k <

p, m < p, n < p and o < p provided that p 1, k + m + n + o + p + q + r =35 and t = 1; 5 < r + q where five TM or MG atoms act as the heteroatoms; and s is sufficiently large that z > 0. The anion [NaP5W30O110]14- is an example of a target heteropolyoxotungstate possessing the Pressier structure of this formula.

Equilibrated polyoxotungstate-catalyst solution 8, a subset of equilibrated poiyoxotungstate-catalyst solution 3, comprises a target heteropolyoxotungstate possessing the Keggin structure present at a useful concentration as part of a homogeneous solution wherein it exists in thermal equilibrium with all chemical species related to the target heteropolyoxotungstate possessing the Keggin structure by reactions between chemical components of the system. This solution comprises various soluble complexes that each contribute specific whole-number or fractional ratios of any or all of the elements V, Mo, W, and main-group ions (MG) to the total chemical composition of the system such that there exists one or more useful heteropolyoxotungstate possessing the Keggin structure with the formula [VkMoOWp (MG) rOs] Z~ where MG is a main-group ion that is preferably AI (III), Si (IV) or P (V) acting as the heteroatom; k < p and o < p provided that p z 1, k + o + p = 12 and r = 1; and s is sufficiently large that z > 0. Target POM anions present in the equilibrated aqueous solutions used in the Examples include the sodium salts of [siVw11O40]5-, [SiV2W10O40]6-, and[PVW11o40]4-.[SiVMo0W11-0O40]5-,[AIVW11O40]6- Equilibrated polyoxotungstate-catalyst solution 9, a subset of equilibrated poloxotungstate-catalyst solution 3, contains a target heteropolyoxotungstate possessing the Keggin structure present at a useful concentration as part of a homogenous solutions wherein it exists in thermal equilibrium with all chemical species related to the target heteropolyoxotungstate possessing the Keggin structure by reactions

between chemical components of the system. This solution consists of various soluble complexes that each contribute specific whole-number of fractional ratios of any or all of the elements Mo and W, d-electron-containing transition-metal ions (TM) and main-group ions (MG) to the total chemical composition of the system such that there exists one or more useful heteropolyoxotungstate possessing the Keggin structure with the formula [MOoWP (TM) q (MG) rOs] 2~ where TM is a d-electron-containing transition-metal ion that is preferably Mn and MG is a main-group ion that is preferably AI (III), Si (IV) or P (V) acting as the heteroatom; m < p, n < p and o < p provided that p _> 1, o + p + q =12 and r = 1; and s is sufficiently large that z > 0. Target POM anions present in the equilibrated aqueous solutions used the Examples include the sodium salts of [SiMn (H20) W"039] 5~ and [AlMn (H20) W"039] 5~.

In another embodiment, the present invention is a solution comprising elemental ratios of Na: Si: V: Mn: Mo: W equal to 6.5: 1: 0.9: 0.1: 1: 10.

The negative charges of POMs can be counterbalanced by cations which are components of and varied by careful selection of the starting compounds used in synthesizing the equilibrated solutions. The degree of ion association is directed by the choice of solvent, cations and target and related POM anions and is dictated by the thermodynamic association constants commonly associated with anions and cations in solution.

Hydrophilic cations, such as H+, Li+, Na+, K+, and NH4+, may be selected to provide acids (isopoly-or heteropolyacids) and/or salts (salts of isopoly-or heteropolyanions) that are soluble in water. Additionally, hydrophobic cations, such as R4N+ (R = aliphatic or aromatic hydrocarbons of varying sizes) or Ph4P+ (Ph = phenyl group), can be used to render the complexes soluble in organic solvents and other hydrophobic media. The listed cations are sensible choices, but there are others available for particular applications.

A standard notation for describing the unprecedented equilibrated solutions of isopoly-and heteropolyoxometalates of the present invention is not available. In order to discuss the invention as clearly as possible while maintaining a certain degree of brevity, a concise nomenclature is herein devised. A single chemical formula will be used to refer to the equilibrium solutions, even though it is fully understood that the solutions contain complex distributions of species in equilibrium with one another. The formulas are written so as to identify the target POM while also attempting to adequately describe the relative ratios of the elements (other than water and its components-H+, HO-and 02-) present in the solution. This is done by indicating the difference between the ratios of elements present in the solution (i. e., the total chemical composition of the non-aqueous components of the solution) and the stoichiometric ratios defined by the empirical formula of the target POM. The differences between these ratios are provided as subscripted numbers in parentheses within the formula of the target POM.

For example, an equilibrated solution may be referred to as Nas+2 [SiV2W, o04o]: the target POM anion of this solution is the sodium salt of the Keggin anion [SiV2W, oO40J6 (i. e., Na6 [SiV2W, 0040]), that exists as one component of the complex equilibrium mixture. However, the solution contains two Na+ ions (and two equivalents of HO-) in excess of the six dictated by the empirical formula of the target POM. This excess is indicated by the subscripted"+2"in parentheses. As another example, an equilibrated solution may be referred to as Na6(+1.5)[Al1(+0.5)VW11(+1)O40]: the target POM anion of this solution is the sodium salt of the Keggin anion [AIVW, 1040] 6- (i. e., Na6 [AIVW"04o]). However, beyond the ratio of elements dictated by the empirical formula of the target POM anion, the solution contains 1.5 additional equivalents of Na+ ions as indicated by the subscripted" (+1.5)", one-half an additional equivalent of AI (III) ions as indicated by the subscripted" (+0.5)"

and one additional equivalent of W (VI) ions as indicated by the subscripted " (+1)". As a final example, an equilibrated solution may be referred to as Na5(+1.5)[SiV1(-0.1)(Mn0.1)MoW10O40]: in this case the target POM salt is defined as Na5 [SiVMoOW11 0040]. Beyond the ratio of elements dictated by the empirical formula of this POM anion, the solution contains 1.5 additional equivalents of Na+ ions as indicated by the subscripted" (+1. 5)", one-tenth of an equivalent less of V (V) than the number of equivalents dictated by the empirical formula (i. e., 0.1 equivalents less that the implied subscript of"1"for V (V) in the formula) as indicated by the subscripted" (-0.1)". Additionally this system contains one-tenth of an additional equivalent of Mn (Il or 111) ion which is not necessarily incorporated into the target POM anion as indicated by the " (mono.,)". Because both equilibrated solutions as well as specific POM complexes will be described below, every effort will be made to differentiate clearly between equilibrated solutions (the present invention) and specific anions.

Additionally, solution concentrations will be presented as if the solution contained only the target POM, although it is well understood that other species are present in the equilibrated solution. Use of the herein described notation facilitates clear definition of the total concentrations of individual elements present in each equilibrated solution. For example, an equilibrated 0.5 M solution of Na6(+1.5)[Al1(+0.5)VW11(+1)O40] contains (in total): 3.75 M Na+, 0.75 M AI (lit), 0.5 M V (V) and 6 M W (VI) distributed over all the species present in the equilibrium solution.

Because o2-is present as part of the solvent, water-55 M in its pure state, and because attainment of equilibrium requires reaction of transition- metal and main-group cations and of the oxide and hydroxide anions present in their salts with water, knowledge of the equilibrium distribution and association of all 02-anions with all cations present (transition-metal and

main-group, including H+) would require full characterization of the precise structure and concentration of each anionic complex present and of the degree of association between each of these anionic complexes and each cationic species (such as Na'or H+) also present. Characterization at this level is presently well beyond the state-of-the-art of physical and chemical methods currently available for even the most detailed characterization of considerably less complex systems than the ones described here. Therefore, the subscripted numbers associated with oxygen (O) atoms in the formulas used to describe the present inventions are limited to those defined by the empirical formulas of the target POMs-no subscripted parenthetical numbers are provided. At the same time, relative ratios of 02-and HO-added directly to water in the preparation of these solutions are explicitly and fully defined by reference to the nature and relative molar ratios of the transition-metal and main-group oxides and hydroxides used as synthetic starting materials.

POMs of the present invention are most usefully prepared as described below in the Examples.

In one embodiment, the present invention is a method for preparing the homogeneous, aqueous solutions that contain one or more catalytically or otherwise useful tungsten-based isopoly-or heteropotyoxometa ! ate (POM) complexes present in thermal equilibrium. Preferably, the method comprises the steps of (a) mixing hydroxides or neutral or anionic oxides of transition- metal or main-group elements in water, and (b) heating such that the hydroxide or neutral or anionic oxide precursors of the transition-metal or main-group elements react to give the target polyoxotungstate anions of the general formula in thermodynamic equilibrium with additional chemical species or complexes also derived from the hydroxides or neutral or anionic oxides of the transition-metat or main-group elements. The final solution will consist of various soluble compounds that contribute specific whole-number

or fractional ratios of any or all of the elements V, Nb, Ta, Mo, W, d-electron- containing transition-metal ions (TM) and main-group ions (MG) such that there exists one or more useful polyoxometalates of the general formula rVkNbmTanMoOWp (TM) q (MG) Rosi where TM is a d-electron-containing transition-metal ion and MG is a main-group ion; k is 0-18, m is 0-10, n is 0-10, o is 0-19, p is 1-150, q is 0-9 and r is 0-9; k < p, m < p, n < p and o < p provided that p: 1 and k + m + n + o + p 2 4; and s is sufficiently large that z > 0 and all species present within the equilibrated aqueous solution remain dissolved during the application. The temperature of the mixture in the heating step is 50°C to 700°C. The heating step is performed at a final pH of between 1.0 and 10.0, and the time of the heating step is between 0.1 and 24 hours. The heating step takes place in a vessel capable of withstanding pressures exceeding the vapor pressure of the solution during the reaction plus any additional applied gaseous pressures.

The temperature of the mixture in the heating step is preferably 100°C to 250°C, and more preferably 150°C to 200°C.

The heating step is performed at a preferable final pH between 4.0 and 10.0, more preferably between 5.0 and 9.0.

The time of the heating step is preferably between 0.5 and 6.0 hours, most preferably between 1.0 and 3.0 hours.

Preferred uses of the present invention.

The present invention, namely a homogeneous solution that comprises useful tungsten-based POM complexes as part of a thermal equilibrium with related species, has a number of distinct advantages over other homogeneous catalysts. Advantages include ease of preparation, responsiveness to process perturbations and the ease with which physical and chemical properties can be rationally altered. Indeed, the existence and use of equilibrated homogeneous solutions of isopoly-and

heteropolyoxotungstates described in the present invention provides additional degrees of freedom to an already flexible and systematically alterable class of compounds, thus dramatically increasing the scope of potential applications.

The thermally equilibrated tungsten-based POM solutions that constitute the present invention are preferably readily prepared from simple, component-element precursors as follows: Neutral transition-metal or main- group oxides or hydroxides, or salts consisting of cationic or anionic transition-metal or main-group oxides, hydroxides or aqua complexes and their counter-cations or-anions, that contribute specific whole-number or fractional ratios of any or all of the elements V, Nb, Ta, Mo, W, d-electron- containing transition-metal ions (TM) and main-group ions (MG), are mixed with or dissolved in water in either an open or sealed reactor and the resultant slurry or solution is heated with or without mechanical agitation under conditions of time, temperature and pressure such as are necessary to obtain a thermally equilibrated homogeneous solution that contains one or more useful polyoxometalates of the general formula. The useful POM of the general formula is present at a concentration suitable for its intended purpose. In addition, an effective system may include several compositionally and structural similar complexes that all perform equally well in a particular application. For example, an equilibrated solution may contain two target POM anions, such as [SiVMoW, 0040] 5~ or [SiVMo2Wg040] 5~.

The total concentrations of each component element and their relative ratios are important factors in determining the nature, composition and concentration of each species present at equilibrium in the solution. Because many possible pathways exist by which identical equilibria may be reached, many equivalent options exist with regard to the choice of synthetic precursors. For example, if two equivalents of Na+ and one equivalent of

W (VI) are required, a single equivalent of Na2WO4 might be used.

Alternatively, one equivaient of W03 and two equivalents of NaOH might be used if they are less expensive or more readily available. The only difference between these two options is that in the latter case, one equivalent of water (H20) is also included.

POMs themselves, such as sodium metatungstate (Na6 [(H2) W, 2040]), are also suitable for use as precursor compounds. Similady, POM solutions that require Na+ and V (V) might be prepared using NaVO3 or the combination of one-half an equivalent of VZOS with one equivalent of NaOH. The same principles apply to the addition of main-group heteroatoms such as AI (III). A single equivalent of AI (OH) 3 might be substituted by a combination of one-half equivalent of Al203 and 1.5 equivalents of H20.

It is not imperative that all the starting materials are soluble in water.

While different materials may react at different rates, it may be possible to increase their reaction rate through chemical activation by specific orders of chemical addition or through mechanical agitation. Such actions will have no bearing on the final state of the homogeneous equilibrium product.

Regardless of how the component elements are distributed among the precursor compounds actually used, at equilibrium, the concentration of each species present (precursors and their immediate speciation-reaction products, H+ and OH-ions, intermediate and byproduct isopoly-and heteropolyanions, and target, catalytically useful POMs) is a function of the stoichiometric ratios of the cations and anions initially added and of the total volume and density of the solution.

Thus, the manner in which the component elements were initially distributed among the precursor compounds has no bearing on the distribution of species present once the system has reached equilibrium. As a result, provided that the correct stoichiometry of component elements is

provided, no limitations need be placed on the distribution of these component elements among the synthetic precursor compounds used.

It is important to note that cations added to the solution must be associated with the speciation chemistry of the POMs. For example, to add sodium to the POM system, the simplest starting material would be NaOH.

One may also look to the salts of the POM building blocks as described above where Na2WO4 H20 can be seen as 2NaOH WO3. Additionally, OH- may be added by the exchange or removal of the counter-anion associated with the Na+ cation.

For example, sodium carbonate in water (Na2CO3H20) can be seen as sodium hydroxide and carbon dioxide (2NaOH CO2) where CO2 is a gas that can be readily driven from the solution leaving the net addition of NaOH.

Along the same lines, sodium acetate in water (NaC2H302 H20) can be seen as sodium hydroxide and acetic acid (NaOH HC2H302) where HC2H302 is a volatile compound that can be readily evaporated from solution at the proper pH or extracted with organic solvent leaving the net addition of NaOH.

Additionally, ion exchange could be used to obtain the net addition of NaOH.

Since water can be viewed as an ion pair consisting of hydrogen cations, H+, and hydroxide anions, OH-, it is possible to use the sodium form of a cation exchange resin (where sodium ions, Na+, are associated with an insoluble, functionalized polymer) to replace the hydrogen cations in solution with sodium cations from the resin. The sodium resin is mixed with the solution, the H+ and Na+ exchange takes place and the solution and resin are once again separated. The ion exchange resin will now have protons associated with the functionalized groups on the polymer resin and the solution will now contain sodium cations, Na+, and hydroxide anions, OH-, or NaOH.

These possibilities are among the most reasonable approaches, however, one trained in the art could conceive of a large number of alternatives involving ion exchange (with resins or membranes), precipitation and extraction that would accomplish the net synthesis of equilibrated POM solutions.

The following example is described here in order to help clarify the above synthetic description (from Example 3a). In synthesizing an equilibrium solution of 0.1 M Na6 +2) [SiV2W, o04o], the following were all mixed with water (25.0 mL) in a 50 mL Hastelloy C Parr Micro Reactor: Na2SiO3 (92%, 0.40 g, 3.0 mmol), NaVO3 (0.76 g, 6.2 mmol), Na2WO4 2H2O (1.98 g, 6.0 mmol) and WO3-H2O (6.00 g, 24.0 mmol). The reactor was pressurized with 02 (2000 kPa) and heated to 200°C over the course of 1/2 hour and held at this temperature for 3 hours with stirring to give 30 mL of a yellow-brown solution (final pH 8.20).

All the starting materials are simple oxides or salts of oxides that are relatively inexpensive and commercially available. While some of the starting materials are water-soluble compounds, namely Na2SiO3, NaVO3 and Na2WO4 2H2O, others are essentially insoluble in water, WO3*H2O. It is important to see that the eight equivalents of Na+ indicated in the solution formula (six from the target anion stoichiometry and two excess) are derived from several of the starting materials: two equivalents from the 3 mmol of Na2SiO3, two equivalents from the 6 mmol of NaVO3 and four equivalents from the 6 mmol of Na2WO4 2H2O. The same is true for the ten equivalents of W (VI) with two from the 6 mmol of Na2WO4 2H2O and eight equivalents from the 24 mmol of WO3 H2O.

The final color is an indication that there is a small amount of reduction in the final solution. This may be due to slight amounts of reduced compounds in the starting materials or by oxidation/reduction reactions taking

place during the synthesis with impurities or the reactor walls. Syntheses done at lower temperatures have produced final solutions with no indication of reduction. The purpose of oxygen over the solution is to help keep the POM solution in its most oxidized state during synthesis. The 5'V NMR shown in Fig. 5 demonstrates that the [SiV2W1oO40] 6-anions are present as a component of the equilibrated Na6(+2)[SiV2W10O40] solution. solution. This is clear when Fig. 5 is compared to Fig. 3, the 5'V NMR spectrum of a solution of pure [SiV2W, oO4oJ6- prepared using traditional multi-step synthetic methods.

Additionally, Fig. 5 shows that small amounts of isopoly-and heteropolyoxotungstates, such as [VWO', are present in equilibrium with the target POM anion.

The synthetic procedure described herein, where simple starting materials of an appropriate grade and purity are simply combined and heated in single step, has several positive implications for the general and commercial use of polyoxometalates. Rather than the multi-step synthetic procedures typically used to obtain kinetically stable catalysts, the equilibrated system is prepared in a single reactor in one step. The equilibrated system thus obtained can be used immediately without the need for isolation or purification of the catalytically or otherwise useful POM. Also, unlike kinetically stable catalysts prepared by hydrothermal synthesis, no crystallization or purification steps are needed. This minimizes the equipment and material capital that would have been required for past synthetic procedures. Finally, once the stoichiometric ratios of component elements necessary for preparation of a desired equilibrium solution have been determined, the catalyst system can be reproducibly prepared in yields of 100%.

While not all the precursor material is incorporated into the target POM, the other species present are needed to maintain the desired

(equilibrium) concentration of the target POM and, in addition, impart useful chemical properties to the system (see below). When a single, kinetically isolated, compound is used as a homogeneous catalyst, movement towards equilibrium will begin immediately. This usually results in a decrease in activity as the concentration of the original catalyst drops or it is chemically altered. The rates at which these changes occur dictate the useful lifetimes of many catalysts. Ultimately equilibrium might be achieved, although the catalyst might now be completely ineffective.

Additionally, expensive elements initially present in the catalyst coutd be irrecoverably lost via precipitation or by dissolution into liquid-effluent waste streams. However, by starting with a homogeneous POM catalyst that is present at an effective concentration and already in equilibrium with the other components in solution, these problems associated with catalyst instability are avoided entirely.

At equilibrium, in the absence of perturbations, the species in solution are continuously and dynamically interconverting such that the overall concentration of each species remains constant (this is the definition of chemical equilibrium). Therefore, each component of the equilibrated system is critical for maintaining a constant position of equilibrium; removal of any single component will thus induce changes in the system. Provided the composition of the system remains constant, such equilibrated catalyst systems thus possess, in principle, unlimited operational lifetimes.

An important factor in the stability of all homogeneous, transition-metal catalysts is the solution pH. In general, operation at inappropriate pH values may result in catalyst decomposition or lead to the precipitation of insoluble metal oxides. Operation at inappropriate pH values many also be detrimental to substrates and reaction products. Proper control of pH is particularly challenging when redox reactions are carried out in water. In aqueous

systems, when electron-containing substrates are oxidized by POMs acting as electron acceptors (oxidants), the H+ concentration of the solution generally increases linearly with the number of electrons transferred from substrate to oxidant (a drop in the pH of the solution). Conversely, OH- anions are generated (an increase in pH) when POM solutions are used as reducing agents in the presence of oxidants such as dioxygen (02), ozone (03), hydrogen peroxide (H202) or other peroxides (i. e., oxidants that possess the O22-functionality). In many cases the substrate, the oxidizing or reducing agents or the desired product (s) are susceptible to decomposition by H+ or OH-ions. Consequently, pH buffers are necessary to maintain the integrity of all the species within the chemical system. Exogenous buffers may be added, but introduce additional costs and can react in undesirable ways with the catalyst or with other components of the system. Therefore, redox-active catalysts that posses the ability to act as their own pH buffer have a distinct advantage over other systems.

The need for such a buffering system is demonstrated by the following example. A near-neutral (pH 6) aqueous solution of pure K5 [SiVW"040] was prepared at room temperature. The Keggin anion salt used to prepare this solution was prepared using traditional multi-step synthetic methods. Thus, unlike the equilibrated systems described in the present invention, the solution of K5 [SiVW"040] thus prepared was far from chemical equilibrium.

Wood pulp was then added to the solution and the mixture heated to 125°C.

During the reaction, 84% of the POM was reduced by lignin in the wood pulp; the final pH of the solution was 1.8. At elevated temperatures, the high H+ concentration will lead to severe degradation of the cellulose in the pulp and drastically reduce the strength properties of the final paper product.

(Additiona details are provided in Example 2.) Several candidates for exogenous buffers were considered for the pH range of 5-7. The use of

phosphate buffer (Na2HPO4/NaH2PO4; pKa 7.2), resulted in degradation of the [SiVW11040] 5- anion because P (V) can act as a heteroatom, forming new P (V)- based heteropoiytungstates. Carbonic acid/bicarbonate buffer, pKa 6.4, is impractical because its use would require that high carbon dioxide pressures be maintained throughout every step of the process. Finally, buffers prepared from acetic acid, pKa 4.8, and other organic acids are themselves inherently unstable under oxidizing conditions. Problems such as these are entirely eliminated by the present invention because the equilibrium solutions herein described provide their own buffering activity as an inherent property.

According to Le Chatelier's principe, a chemical system at equilibrium will adjust to perturbations by changes that serve to minimize or counteract the affects of the disturbance. Because the components in the thermally equilibrated tungsten-based POM systems are related to one another by speciation reactions (i. e., by reactions with H+ and OH-), these solutions can respond to transient additions of H+ or OH-by undergoing changes that reduce the concentration of the added species. These changes serve to maintain the pH of the solution during operation, i. e., the equilibrium solutions act as their own pH buffers. In the present invention, when the oxidation or reduction of the target polyoxometalate consumes or produces protons, the components of the polyoxometalate solution can respond in one of several ways. A new equilibrium distribution within the solution may be obtained by reversible condensation reactions that consume protons (Equation 1). Note that the species shown need only be present in small concentrations to consume a stoichiometrically large number of protons. Conversely, addition of hydroxide (OH-) might be compensated for by reversible hydrolysis reactions (Equation 2).

[SiWO-+[WOJ'-+6H-[SiWOF-+3HO(Eq. 1) [SiWOJ'-+6OH--[SiWOgg]+[WO-+3H,0(Eq. 2) [AIAI (H20) W"039] 6- + H+-H [AIAI (H20) W"03g] 5- (Eq. 3) Chemical species in the equilibrated solution may also act as proton donors or acceptors much in the same way as do traditional Bronsted acid-base buffers (Equation 3). The net consequence of the occurrence of these and related reactions is an inherently self-buffering system that can maintain the pH of the solution and the integrity of the system.

The ability of an equilibrated solution to provide pH management is demonstrated by the following example involving the repeated reduction and reoxidation of equilibrated Na6(+1.5)[Al1(1+0.5)VW11(+1)O40) using carbon monoxide (CO) as reducing agent and dioxygen (02) as oxidant. The reactions were carried out separately as shown by Equations 4 and 5 below. Note the change in oxidation state of the vanadium ion and that the distribution of species changes via condensation (Equation 4) or hydrolysis reactions (Equation 5) so as to avoid increases in H+ concentration during POM reduction or increases in OH-concentration during POM reoxidation.

Occurrence of the reactions shown in Equations 4 and 5 has been confirmed experimentally by use of 27AI and 51V NMR spectroscopy. (Synthesis of the equilibrated 0.1 M Na6(+1.5)[Al1(+0.5)VW11(+1)O40] solution is described in detail in Example 14.) 2 [A!VOJ-+[VW.O.s]'-+[At,v\0-+[W,0-+ 2 CO- -4[AtV'OJ'-+2CO (Eq. 4) O2#(Eq.5)4[AIVIVW11O40]7+ 2 [A)VO-+[VW.Or-+[A),v\0-+[W,0- For each reduction of the equilibrated solution by reaction with CO, the reactor was purged with Ar for at least 10 minutes and charged with CO to 550 kPa. The reactor, equipped with a gas entrainment impelter, was heated

to 130°C for 6 hours. To reoxidize the equilibrated solution, the reactor, equipped with a gas entrainment impeller, was charged with 2170 kPa 02 and heated to 210°C for 4 hours. Once synthesized, a single equilibrated solution was used throughout the experiment (ten two-step cycles of reduction by CO followed by reoxidation by °2)-Throughout the 20-step experiment, nothing (other than CO and °2) was added or removed from the system and no precipitation was observed. The results are summarized in Table 7 of Example 14 and presented in Fig. 18. Attention should be focused on the relatively constant pH value maintained throughout the series of reactions. In the absence of buffering activity, the pH would have fluctuated over a large range of pH values. In the reaction with CO (Eq. 4), one proton is introduced into the solution for each molar equivalent of vanadium reduced from V (V) to V (IV). Because a solution of nearly 0.1 M in V (IV) is produced by the reduction reaction, 0.1 M of protons are correspondingly introduced. Given an initial pH of 7.2 (an H+ concentration of 6.3 (10)-8 M using pH =-log [H+]), if no buffering activity occurred, the pH of the solution would have decreased to 1.0, instead, it remained near pH 7. Conversely, if no buffering activity had been present during POM reoxidation by OZ (Eq. 5), the pH of the solution would have increased to 13.0. However, through repeated introductions of high concentrations of H+ and HO- (from the reaction of reduced 02 molecules, 02-, with H20) the pH of the solution remained consistently near 7.5. This experiment clearly demonstrates the ability of equilibrated 0.1 M Na6 (+1. 5) [All (+0. 5) VW11 (+1) 0401 solutions to buffer their own reactions under conditions that introduce high concentrations of H+ and OH-.

Polyoxometates are frequently cited as compounds that are generally useful as oxidants and, more specifically, useful as intermediaries in the application of other oxidants, namely air, oxygen, hydrogen peroxide and other organic or inorganic peroxides, and ozone. Therefore, POM systems

that are more easily reoxidized by these oxidants are desirable. Such systems would reduce reaction times and conditions in order to minimize operating and capital costs. It is well established that many transition metals can be used to facilitate oxidation reactions with oxygen, peroxides and ozone (Fe, Mn and Cu are commonly cited examples). Under acidic conditions, many of these transition metals can be used homogeneously in aqueous systems, however, at neutral or basic pHs, many of these metals would have to be used heterogeneously. This may create handling and mass transfer problems. The breadth of substitution chemistry available to POMs allows one to incorporate a wide variety of transition metal into homogenous, aqueous solutions over a broad range of pH by incorporating them into the POM structures. Additionally, the use of equilibrated POM systems allows one to incorporate small amounts of the transition metals as needed to increase the rate of reactions of reduced POMs with oxygen or other oxides.

The advantage of incorporating transition metals into the equilibrated polyoxometalate systems is demonstrated by the results shown in Example 20. In this example, small amounts of manganese, iron and chromium are incorporated into equivalent POM systems where the base-case is Na5(+1.9)[SiV1(-0.1)MoW10(+0.1)O40]. One can see from Fig. 21 that the presence of certain of the transition metals improves the initial rate of oxidation of the reduced POM and lowers amount of reduced POM at the end of the reaction time. The addition of Mn is clearly the best of the three transition metals demonstrated. The addition of Cr shows little, if any, improvement and the addition of Fe shows an intermediate level of enhancement between the others. Other first-row, transition metals like Ti, Ni, and Cu (not presented) also showed various levels of improvement. Additionally, the results described in Example 21 demonstrate that the presence of these trace

amounts of transition metals does not compromise the ability for the target POMs to oxidize substrates, such as wood pulps.

Preferred engineering of the present invention.

The equilibrated POM solutions described in the present invention are truly engineering systems. By making small changes in the ratios of component elements used in their syntheses, the nature and relative concentrations of species present in the system are smoothly varied over a continuous range of equilibrium distributions. This flexibility provides a means to design for and control the physical and chemical properties of the catalyst system as needed for specific applications. For example, diverse applications might require different buffering capacities or that solutions operate at specific pH values. The relative concentration of the target POM within the equilibrium distribution of species can be maximized for specific processes or even changed for individual process steps to enhance overall performance.

The manipulation of POM solutions as equilibrium-state systems provides a unique opportunity by which catalyst performance can be finely tuned or otherwise optimized.

The buffering capacities of the equilibrated POM solutions can be systematically altered as required for desired applications. A large buffering capacity may be needed for systems where oxidation and reduction of the target POM occurs in separate process steps. On the other hand, a much smaller buffering capacity might be needed in a system wherein oxidation and reduction of the POM catalyst are simultaneous. One way the buffering capacity of the system can be systematically altered is to vary the relative amounts of acidic versus basic oxides used as synthetic precursors for introducing the transition-metal or main-group component elements (e. g., W03 vs. Na2WO4 for the W (IV)-note that Na2WO4 is a combination of W03 and two equivalents of NaOH). Although complex speciation reactions with

water make it difficult to predict the equilibrium distribution of 02-anions between polyoxometalates (tungsten oxides) and water (hydrogen oxide), the final relative Na+ content of the solution can be used as a guide to the excess or deficit of 0'-ions initially introduced as components of synthetic precursors. Excess or deficit is here defined in reference to the ratio of O atoms, relative to other elements, in the formula of the target POM. Thus, a convenient way to assess the relative concentrations of OH-used in the synthesis of a particular equilibrated solution is to refer to the subscripted value placed in parenthesis after the countercation in the formula that describes the solution.

For example, two nearly identical solutions were made with different levels of sodium relative to Si (IV), V (V) and W (VI): NagSiVWOJ and Na5 (+3) [SiVW"O40]. These two 0.5 M aqueous solutions were synthesized in a manner similar to that previously described (also see Example 6). Aliquots were diluted to 0.025 M in 160 mL of water and heated to 70°C. They were then titrated with 0.25 M HCI while monitoring the pH. One minute was allowed to pass between HCI additions and pH-meter readings to allow the solution to adequately stabilize. The results are shown in Fig. 11. The relatively constant pH values maintained as HCI is added shows that H+ ions are being consumed (i. e., buffering is occurring). The dashed line shows the calculated pH values expected had the added H+ ions not been consumed.

The pH plateau observed using Na5(+3)[SiVW11O40] clearly extends to higher volumes of added HCI than that observed using Nas (+2) [SiVW"04o], clearly demonstrating the useful correlation between a higher relative sodium content and a larger buffering capacity.

The pH at which the equilibrated POM solutions buffer can also be systematicaiiy altered for particular applications. In general, each polyoxometalate species possesses substantial thermodynamic stability only

within a well-defined range of pH values. If the pH of a POM solution is set beys. 2 a species'range of stability, it will eventually decompose via hydrolysis or condensation reactions until equilibrium is established. The pH of the equilibrated solution might be different from the initially set value. The new pH value may be one at which the POM species is stable or, the POM anion may no longer be present at a significant (measurable) concentration.

However, the pH can be adjusted and the desired distribution POM species in an equilibrated solution ensured by systematic alteration of the ratios of elements present.

In general, some trends regarding the pH ranges over which polyoxometalates of particular structures and composed of particular elements are thermodynamically stable have been described.

Polyoxotungstates of a particular structure and composition are generally stable at higher pH values that are isostructual polyoxomolybdates. For example, α-Keegin-[SiVW11O40]5- possesses greater thermodynamic stability at higher pH values than does a-Keggin-[SiVMo"040] 5~. Additionally, the nature of the heteroatoms within specific structures has a large affect. Higher charge and charge density of the POM anion, both altered by choice of heteroatom element or valence state, tends to correlate with greater stability at higher pH values. Stability is also influenced by the size and electronegativity of the heteroatom ion. For example, a-Keggin- [PW, 204o] 3- is stable at pH values between 1 and 2, a-Keggin- [SiW, z04o] 4- is stable to pH 4 and a-Keggin- [AIW, 2040] 5~ possesses substantial kinetic (and perhaps thermodynamic) stability to pH 6. Furthermore, specific polyoxometalate structures are themselves stable over different pH ranges. For example, tungstate is only present at relatively high pH values, paratungstates are stable at near-neutral pH values, metatungstate at lower pH values (ca. 3-6), and decatungstate at still lower pH values (ca. 0-3). Overall, knowledge of

these trends makes it possible to target a specific pH at which the equilibrated system will buffer by varying the ratios of the elements used. At the same time, as the complexity of the system increases, it becomes more difficult to precisely predict at what pH the system will buffer; this must then be determined experimentally.

One example of how knowledge of these trends can be used to advantage is provided by data concerning the ratio of tungsten, W (VI), to molybdenum, Mo (VI), within an equilibrated POM solution. According to the trends outlined above, the pH of the solution should decrease as molybdenum is substituted for tungsten. This has been verified for the equilibrated solutions Na5(+2)[SiVMo0W11-0O40] with 0 < o < 3 (see Table 8 of Example 17). From Table 8, the operating pH (both pH, and pHf, i. e., before and after the reduction of the solution) is higher for systems with higher W (VI): Mo (VI) ratios. Several examples also demonstrate the correlation between counter-cation content (i. e., associated with effective equivalents of OH-added as components of synthetic precursors) and the operating pH of the final equilibrated solution. In all cases, higher ratios of the counter-cation sodium, Na+, relative to other elements present correlate with higher equilibrated-solution pH values. For example, increases in x in the equilibrated solutions Na6 (lx) [S'V2WI00401 correspond to increases in pH (see Table 2 of Example 5). Additional data demonstrating the correlation between sodium counter-cation content and solution pH are presented in Tables 5 and 8 of Examples 11 and 17 respectively. Because of this correlation, it is convenient to associate larger Nation ratios with higher pH values at equilibrium.

However, very little information is available on mixed molybdenum and tungsten systems. While there are some similarities regarding the speciation chemistry based upon the two compounds, there are also some distinct

differences. The limited amount of information available demonstrates that the properties of such mixtures are difficult to predict. For example, the [SiVW11O40]5- anion has a higher oxidation potential than the [SiVMo"040] 5~ anion. One may expect systems containing [SiVW"04o] 5- to oxidize a substrate, such as lignin, more rapidly than systems containing [SiVMo"040] 5- Furthermore, one may expect that as Mo is substituted for W into the Nag (+2) [SiVWO] system that the delignification rate would decrease for a given set of reaction conditions. In fact, the opposite is true as demonstrated by Table 8 in Example 17. As Mo is added to the Na5(+2)[SiVW11O40] system, enhanced delignification occurs, evidence that properties of the equilibrated systems are difficult to predict as molybdenum is substituted into the polyoxotungstates.

The equilibrated solutions can be altered so as to increase or decrease the concentrations of specific POM species. If a particular species is responsible for a desired reaction, increasing its concentration may increase the rate at which the reaction occurs. There are several benefits of a faster reaction: one can decrease the reactor size to lower capital costs, decrease reaction temperatures to lower operating costs or decrease reaction times to increase throughput. Additionally, product quality can be improved if higher reaction temperatures or longer exposure times have degrading effects. The concentration of individual species in equilibrated solutions can be changed through systematic alteration of the ratios of elements present.

A number of different equilibrated aqueous solutions all containing the target [AIVW"04o] 6- anion were prepared by systematic variation in the ratios of Na: AI: V: W. The goal was to optimize for a specific application, the delignification of wood-pulp. The relative concentrations of the target and other key AI (lil)-and V (V)-containing species in the equilibrium solutions were determined by the integration of 51V and 27AI NMR signals. Samples of

individual complexes were painstakingly purified and fully characterized for use as NMR references. The data, summarized in Table 5 of Example 11, clearly shows that the ratios of the elements Na', AI (III), V (V) and W (VI) affects the distribution of species in solution. For the delignification of wood pulp, an equilibrated solution such as Na6(+1.5)[Al1(+0.5)VW11(+1)O40], which contains a large concentration of the target anion, [AIVW"04oJ6-, relative to the otherAI (III)-and V (V)-containing species might be chosen. Otherfactors, such as the solution pH, are also important.

The manipulation of component elements provides a unique means by which to optimize the effectiveness of individual chemical steps within a multi- step process. Table 1 shows the distributions of species present in two POM solutions that possess different relative ratios of sodium (Na+). To simplify discussion, only a directly pertinent subset of the total diversity of species present is listed. A qualitative comparison of the reduction potentials, Eo, of two specific anions is provided below the table.

Table 1: Simplifie distributions of species in solutions containing different relative ratios of sodium. Solution A Solution B Na5(+2)SiVW11O40Na5(+3)SiVW11O40 2 [SiVWOJ'-1 [SiVWOJ- 1 (SiV2W10O40]6-2 Order of Eo: [SiVW11O40]5- > [SiV2@10O40]6- In Solution A, the species possessing the higher reduction potential, [SiW11O40]5-, is present at a higher relative concentration (2: 1) than is the weaker oxidant, [SiV2W10O40]6-. Solution A should thus be more highly oxidizing. This expectation is confirmed experimentally by comparing the rates of wood pulp delignification (i. e., rates of lignin oxidation) obtained using the two solutions. The results in Table 8 of Example 17 show that for the

same reaction conditions, effectiveness decreases in the order : Naos [SiVW11O40] > Na5(+2)[SiVW11O40] > Na [SiVWOJ, as indicated by the final kappa number of the wood pulp. (The kappa number is a measure of the lignin content of the pulp; a pulp with a lower kappa number contains less lignin.) Once reduced, however, Solution B should be more readily oxidized by oxygen (02) under the same reaction conditions because the reduced form of the species with the less positive reduction potential (i. e., [SiV2W, o04o '-, the le--reduced form of [SiV2W, o04o7-) is present at a higher relative concentration. The results in Table 9 of Example 18 confirm this expectation: under the same reaction conditions, the reduced Na5(+4)SiVMo2W9O40 solution is reoxidized at a greater rate than is the reduced Na5 (12) S'VM02W9040 solution.

One could potentially devise a reversible"chemical switch"to optimize individual steps within a process. By adding or removing individual elements at critical times to adjust the distribution of species, the properties of the solution can be optimally matched to the requirements of the reaction.

Referring to the above example, after reaction of wood pulp with Solution A, the better delignification system, an appropriate amount of NaOH could be added and the solution heated to obtain Solution B, the more readily reoxidized system. After the solution has been reoxidized, ion exchange with an acid resin could be used to remove the added sodium and to convert Solution B back into Solution A for the next delignification step. By reversibly changing the solution between steps, the solutions would exist in states best suited for each reaction. Sodium bicarbonate (NaHCO3), sodium carbonate (Na2CO3) or other bases with different cations might also be used.

Another possibility is that after delignification with Solution A, one could add an appropriate amount of ammonia to the solution. Ammonia (NH3) reacts with water to form ammonium hydroxide, which would hydrolyze the

POMs in Solution A in a manner similar to the NaOH addition just described, to give NagNHJSiVVVO]. This solution should have a POM species distribution and reactivity similar to Solution B (although solubilities may be quite different with NH4+ cations). Once the POM has been reoxidized, the NH3 could be removed from solution and the system would revert to Solution A for the next delignification step.

It is also known that some POMs are more readily oxidized in organic solvents than in aqueous ones. It is therefore conceivable that a reduced POM solution could be transferred to an organic solvent using an appropriate, hydrophobic cation. Once reoxidized, the POM could be returned to an aqueous media using a hydrophilic cation for its subsequent reuse.

Many of the physical and chemical properties of equilibrated solutions of the present invention are interdependent. When the elemental composition of a POM system is altered, several properties are affected simultaneously.

Optimization, of course, depends upon the relative importance of these changes. A thorough knowledge of how composition affects solution properties will allow one to manage the distribution of species and maximize the performance of the equilibrated polyoxotungstate solutions.

In another embodiment, the present invention is a method of delignifying wood pulp, wood fiber, linocellulosic pulp, lignocellulosic fiber, comprising the step of degrading dissolved lignin and polysaccharide fragments to volatile organic compounds and water. Preferably the temperature of the degradation reaction is between 100°C and 400°C, the degradation time reaction is between 0.5 and 10 hours, the pressure of the oxidant is between 15 to 1000 psia, and the degradation reaction takes place in a vessel capable of withstanding pressures exceeding the vapor pressure of the solution plus any additional applied gaseous pressure.

EXAMPLES Nomenclature for equilibrated POM solutions.

A standard notation for describing the unprecedented equilibrated solutions of isopoly-and heteropolyoxometalates of the present invention is not available. In order to discuss the invention as clearly as possible while maintaining a certain degree of brevity, a concise nomenclature is herein devised. A single chemical formula will be used to refer to the equilibrium solutions, even though it is fully understood that the solutions contain complex distributions of species in equilibrium with one another. The formulas are written so as to identify the target POM while also attempting to adequately describe the relative ratios of the elements (other than water and its components-H+, HO-and o2-) present in the solution. This is done by indicating the difference between the ratios of elements present in the solution (i. e., the total chemical composition of the non-aqueous components of the solution) and the stoichiometric ratios defined by the empirical formula of the target POM. The differences between these ratios are provided as subscripted numbers in parentheses within the formula of the target POM.

For example, an equilibrated solution may be referred to as Na6(+2)[SiV2W10O40] : the target POM anion of this solution is the sodium salt of the Keggin anion [SiV2W, o04o] 6- (i. e., Na6 [SiV2W, 0040]), that exists as one component of the complex equilibrium mixture. However, the solution contains two Na+ ions (and two equivalents of HO-) in excess of the six dictated by the empirical formula of the target POM. This excess is indicated by the subscripted"+2"in parentheses. As another example, an equilibrated solution may be referred to as Na6(+1.5)[Al1(+0.5)VW11(+1)O40]: the target POM anion of this solution is the sodium salt of the Keggin anion [AIVW"O4o) 6- (i. e., Na6 [AIVW"O40]). However, beyond the ratio of elements dictated by the empirical formula of the target POM anion, the solution contains 1.5 additional

equivalents of Na+ ions as indicated by the subscripted" (+1.5)", one-half an additional equivalent of AI (III) ions as indicated by the subscripted" (+0.5)" and one additional equivalent of W (VI) ions as indicated by the subscripted " (+1)". As a final example, an equilibrated solution may be referred to as Na6(+1.5)[SiV1(-0.1)(Mn0.1)MoW10O40] : in this case the target POM salt is defined as Na5 [SiVMooW"_o040_ Beyond the ratio of elements dictated by the empirical formula of this POM anion, the solution contains 1.5 additional equivalents of Na+ ions as indicated by the subscripted" (+1.5)", one-tenth of an equivalent less of V (V) than the number of equivalents dictated by the empirical formula (i. e., 0.1 equivalents less that the implied subscript of"1"for V (V) in the formula) as indicated by the subscripted" (-0.1)". Additionally this system contains one-tenth of an additional equivalent of Mn (Il or 111) ion which is not necessarily incorporated into the target POM anion as indicated by the "(mono,)". Because both equilibrated solutions as well as specific POM complexes will be described below, every effort will be made to differentiate clearly between equilibrated solutions (the present invention) and specific anions.

Additionally, solution concentrations will be presented as if the solution contained only the target POM, although it is well understood that other species are present in the equilibrated solution. Use of the herein described notation facilitates clear definition of the total concentrations of individual elements present in each equilibrated solution. For example, an equilibrated 0.5 M solution of Na6(+1.5)[Al1(+0.5)VW11(+1)O40] contains (in total): 3.75 M Na+, 0.75 M AI (III), 0.5 M V (V) and 6 M W (VI).

Because 01-is present as part of the solvent, water-55 M in its pure state, and because attainment of equilibrium requires reaction of transition- metal and main-group cations and of the oxide and hydroxide anions present in their salts with water, knowledge of the equilibrium distribution and

association of all 02-anions with all cations present (transition-metal and main-group, including H+) would require full characterization of the precise structure and concentration of each anionic complex present and of the degree of association between each of these anionic complexes and each cationic species (such as Na+ or H+) also present. Characterization at this level is presently well beyond the state-of-the-art of physical and chemical methods currently available for even the most detailed characterization of considerably less complex systems than the ones described here. Therefore, the subscripted numbers associated with oxygen (O) atoms in the formulas used to describe the present inventions are limited to those defined by the empirical formulas of the target POMs-no subscripted parenthetical numbers are provided. At the same time, relative ratios of 0'-and HO-added directly to water in the preparation of these solutions are explicitly and fully defined by reference to the nature and relative molar ratios of the transition-metal and main-group oxides and hydroxides used as synthetic starting materials.

General procedures.

All chemicals were used as received; all water used was deionized with a Barnstead Ultrapure lon Exchange Cartridge (D0809). K5 [SiVW"040] (P. J. Domaille, J. Amer. Chem. Soc. 106: 7677,1984) and K7H [Nb6O, g] (C. M.

Flynn, Jr. and G. D. Stucky, ignora. Chem. 8: 178,1969; M. Filowitz, et al., ignora. Chem. 18: 93,1979) were prepared by literature methods. Elemental analyses were performed by E+R Microanalytical Laboratory, Inc. Infrared spectra (KBr) were recorded on a Nicolet model 510 spectrophotometer.

Oxidation potentials were determined using a Princeton Applied Research cyclic voltameter model 174A polarographic analyzer and model 175 universal programmer equipped with glassy-carbon, calomel and platinum electrodes. 5'V NMR (131.5 MHz), 27AI NMR (130.31 MHz) and/V NMR spectra were recorded on a General Electric GN500 spectrometer. An

aqueous solution of [PVMo11O40]4- was used as an external reference for 51V NMR; chemical shifts are quoted on the 5 scale (downfield shifts are positive) relative to neat VOC13. Chemical shifts for 2'AI NMR are quoted on the 5 scale (downfield shifts are positive) relative to AtCtg (AI (H20) 63+ in H2O or D2O) ; an aqueous solution of AICI3 was used as an external reference. Chemical shifts for 3'P NMR are quoted on the 5 scale (downfield shifts are positive) relative to H3PO4; 85% H3PO4 was used as an external reference. Chemical shifts for '83W NMR are quoted on the 5 scale (downfield shifts are positive) relative to WO42-; an aqueous solution of Na2WO4*2H2O was used as an external reference.

Often in the NMR spectra several peaks are identified with the same POM anion. This analytical technique can distinguish between positional isomers, i. e., compounds with the same chemical formulation and general structure but with different positional relationships between the metal ions (refer to Fig. 1). For example, a-Keggin- [SiV2W, 0040] 6~ has five distinct positional isomers with different relative positions of the two vanadium atoms distributed over the twelve available addendum atom positions. Additionally, there are also five possible Baker-Figgis isomers for the Keggin structure referred to as a, ß, y, b, and e where zero, one, two, three or all four of the M30, 3 triplets have been rotated by 60° relative to the a-isomer. The relative amounts of these isomers depend upon statistical probability, elemental composition and chemical environment (temperature, pH, etc.). While only a few of these isomers may be present at significant concentrations, there are a large number that may theoretically be present.

The extent to which POM solution were chemically reduced was measured using visible light spectroscopy. Two methods were employed. In one method, a sample of the solution was diluted and the solution absorbance measured at several different wavelengths. Mathematical

models, which have been developed using identical solutions of known extents of reduction, were used to calculate the extent of reduction from the measured absorbances. In another method, reduced POM solutions were mixed with an excess of [Co"'W, 204o16- in sulfate buffer at pH<2. The Co (III) complex is a very strong oxidant and oxidizes any reduced POMs present in the sample solution. The absorbance was measured at 600 nm to determine the concentration of reduced cobalt (i. e. Co (II) in the [Co"Wl2o4O'-] complex) present in the mixture.

For many of the examples, the delignification of wood-pulp is used to demonstrate the utility of the present invention. The kappa number, obtained by permanganate oxidation of residual lignin, is an index of how much lignin is present within a wood or pulp sample. Although difficult to measure accurately or to interpret when only small amounts of lignin are present, kappa numbers are a widely used and easily recognized index of lignin content. For relatively small pulp samples, microkappa numbers are determined. Microkappa numbers were obtained using TAPPI methods T236 om-85 and um-246. Microkappa number determinations are used in several examples below to demonstrate that lignin-like material is effectively degraded or otherwise removed from putp during reaction with equilibrated POM solutions.

The viscosity of a pulp sample is proportional to the average chain length of cellulose polymers within the pulp fibers. Consequently, retention of pulp viscosity is one of several criteria indicating that cellulose fibers have not been degraded during delignification. Viscosity values are commonly used in conjunction with kappa numbers to determine the selectivity of a delignification agent. TAPPI test method T230 om-89 is used to measure the viscosity of aqueous cupriethylene diamine solutions in which standard amounts of pulp samples have been dissolved.

Example 1.

"Stepwise"synthesis of K6 [SiV2W10040] and KstSiVMoWOJ.

The following are examples of"traditional"preparations of heteropolyoxotungstates of the general formula. Typically these preparative methods involve several synthetic steps with isolation and purification of intermediates. It is important to understand that the POMs discussed in Example 1 are kinetic products and are included here to demonstrate that the structures and compositions of the products present in thermodynamic equilibrium in later examples are well understood. By synthesizing pure compounds in a controlled, stepwise manner, one can obtain analytical references for characterizing systems that are more complex.

K9[SiVW10O39]#14H2O.Examplela.

K5 [SiVW11O40]#12H2O (100 g, 32 mmol) was dissolved in water (135 mL). Any undissolved solid was removed by filtration. Solid KOH (9.6 g, 150 mmol) was slowly added in portions to give a pale-yellow precipitate. After the addition of the KOH, the mixture was stirred for an additional hour. The yellow product was collecte by filtration and washed with water. The solid was allowed to air-dry for several days; yield 71% (71 g). Anal. Calcd. (found) for K9[SiVW11O39]#14H2O (fw = 3145.6): H, 0.90 K, 11.19 Si, 0.89 (0.73); V, 1.62 (1.24); W, 58.45 (58.46). IR (KBr, cm~'): 1631 (w), 984 (w), 940 (m), 864 (vs), 791 (s), 722 (s), 514 (w). Fig. 2 shows the"V NMR spectrum of the pure K9[SiVW11O39] in D2O. Several peaks are observed due to the different positional relationships between the structural defect and the vanadium atom.

K6$[SiV2W10O40]#xH2O.Example1b.

K9[SiVW10O39]#14H2O (36.0 g, 11.4 mmol) was slurried in water (36 mL). A 0.5 M NaVO3 (24.0 mL) solution was added to the slurry, and a 3 M

HCI solution (18.0 mL) was slowly added in a dropwise fashion to give an orange-red solution. This solution was stirred for an additional hour. The reaction was stored at 5°C overnight to give an orange crystalline solid, which was collecte by filtration, washed several times with cold water and allowed to air-dry: yield-60% (20.5 g). IR (KBr, cm-'): 1622 (w), 1009 (w), 962 (m), 911 (s), 776 (vs), 529 (w). Fig. 3 show the 5'V NMR spectrum of the pure, of K6 [SiV2W, o04o] (mixture of positional isomers) in D20.

Na6[SiV2W11O40]#xH2O.Example1c.

Due to cost, availability and solubility, it may be desirable to use the sodium counter-cations when preparing equilibrated POM systems. Here the kinetically derived product from Example 1 b is converted to the sodium salt using ion exchange. K6[SiV2W11O40]#xH2O (10.0g, #3. 3 mmol) was dissolved in water (25.0 mL). This solution was passed through a Na charged cation- exchange column (100 mL Amberlite IR-120 (plus) ; 1.9 meq per mL). The resultant solution (pH 4.9) was taken to dryness with a rotoevaporator: yield -98% (9.48 g). Anal. Calcd. (found) for Na6 [SiV2W, o04o] 12H20 (fw = 2962.57): H, 0.82 (0.68); Na, 4.66 (4.24); Si, 0.95 (0.75); V, 3.44 (3.41); W, 62.06 (61.88).

Example $forK7[SiV2W10O40]#13H2OK7[Si2VW10O39]#14H2O VIvOSO4 3H2O (3.91 g, 18.0 mmol) was dissolved in water (3 L) to give a blue solution (pH 3.2). Solid K2[SiVVW10O39]#14H2O (50.4 g, 16.0 mmol) was slowly added to the solution in portions to give a brown solution (pH 5.5).

The reaction was stirred for at least 1 additional hour. After this time, the solution was filtered to remove a small amount of precipitate. Solid KCI (400 g, 5.4 mol) was added to the filtrate to give a fine brown precipitate. The solid was allowed to air-dry overnight; yield 32.1% (18.0 g). Anal. Calcd. (found) for K7[SiV2W10O40]#13H2O (fw = 3116.3): H, 0.84 (0.76); K, 8.78 (9.04); Si,

0.90 (0.78); V, 3.27 (3.22); W, 59.00 (58.94). IR (KBr, cm-'): 1628 (w), 990 (w), 952 (m), 902 (s), 788 (s), 732 (w), 697 (w), 536 (w).

Example 1e. K5SiVMoW oDAo xH2O Na2MO04-2H20 (2.50 g, 10.4 mmol) was dissolved in 100 mL of water.

K9 [SiVW10O39]#14H2O (32. 7 g, 10.4 mmol) was slurried into this solution.

Concentrated HCI (5.2 mL, 62.4 mmol) was diluted to 20 mL and was slowly added in a dropwise fashion to give a yellow-orange solution. This solution was stirred for an additional hour. The pH was then adjusted up to 4.5 using a saturated solution of KHCO3. A small amount of precipitate was filtered from the solution. The solution was concentrated to 50 mL and 0°C overnight to give a yellow crystalline needles, which were collecte by filtration, washed several times with cold water and allowed to air-dry: yield-70% (23.0 g). IR (KBr, cm-'): 1615 (w), 1011 (sh), 966 (m), 919 (s), 778 (vs), 523 (w). Fig. 4 shows the 51V NMR spectrum of the pure K5[SiVMoW10O40] (mixture of positional isomers) in D2O.

Example 2.

Pulp delignification using kinetically-derived K5 [SiVW"04o] without a buffer.

K5 [SiVW04o]'12H20 (23.7 g, 7.5 mmol) was dissolved in water (139 mL). Kraft softwood pulp (4.6 g o. d. pulp containing 11 g water) was mixed with the POM solution. The pH of the mixture was 5.7 and was adjusted to 6.0 using a small amount of KHCO. The mixture was placed in a 1 L glass lined Parr reactor equipped with a 316 stainless-steel stirrer. The reactor was purged thoroughly with nitrogen. It was then heated to 125°C, taking one hour, and maintained at temperature for two hours. The reaction was then quenched by cooling the reactor in a water bath and the pulp filtered and

washed. The POM solution had been reduced by 84% and had a final pH of 2.1.

This example demonstrates that the system lacks pH control when kinetically isolated POMs are used to delignify pulp without a buffer; the pH of the solution dropped from 6 to 2. In this situation, the exposure of pulp fibers to low pH and elevated temperatures will hydrolyze the cellulose and significantly decrease the quality of the final pulp product.

Example 3.

Synthesis of equilibrated POM solutions targeting the IS'V2WlOo4o] 6- anion.

The following are examples of the single-step synthesis of equilibrated POM solutions containing target polyoxotungstates of the general formula.

The equilibrated solutions have been prepared under a variety of experimental conditions by mixing hydroxides or neutral or anionic oxides of transition-metal or main-group elements and water and heating the mixtures to temperatures sufficiently high such that they react to give solutions containing target heteropolyoxotungstates of the general formula present in thermodynamic equilibrium with additional compounds also derived from the starting materials.

Example 3a. Equilibrated aqueous solution of 0.1 M Na6(+2)[SiV2W10O40].

Na2Si03,92% (0.40 g, 3.0 mmol), NaV03 (0.76 g, 6.2 mmol), Na2WO4e2H2O (1.98 g, 6.0 mmol) and WO3-H2O (6.00 g, 24.0 mmol) were mixed with water (25.0 mL) in a stirred, 50 mL Hastelloy C Parr Micro Reactor. The reactor was pressurized with °2 (2000 kPa) and heated to 200°C over the course of 1/2 hour and held at this temperature for 3 hours to give a yellow-brown solution (final pH 8.20). Fig. 5 shows by 5'V NMR the presence of [SiV2W, 0040] 6~ isomers in solution (compare to Fig. 3, the 51V

NMR spectrum of pure [SiVoO') in equilibrium with smaller amounts of additional isopoly-and heteropolyoxotungstates, such as [V2W4O, 9] 4-.

Example 3b. Equilibrated aqueous solution of 0.1 M Na6(+2)[SiV2W10O40] Milder reaction conditions.

Na2SiO3,92% (0.40 g, 3.0 mmol), NaV03 (0.73 g, 6.0 mmol), Na2WO4 2H2O (1.98 g, 6.0 mmol) and W03 (5.56 g, 24.0 mmol) were mixed with water (25.0 mL) in a stirred, 50 mL Hastelloy C Parr Micro Reactor. The reactor was heated to 125°C over the course of 1/2 hour and held at that temperature for 5 hours to give a yellow-brown solution (pH 8.57). Fig. 6 verifies by 51V NMR that the same synthesis as described in Example 2a can be successfully carried out at lower temperatures and without pressurized oxygen.

Example 3c. Equilibrated aqueous solution of 0.2 M Na6(+2)[SiV2W10O40].

Na2SiO3,92% (2.043 g, 15.40 mmol), NaV03 (3.754 g, 30.79 mmol), Na2WO4 2H2O (10.156 g, 30.789 mmol) and W03 (30.773 g, 123.16 mmol) were mixed with water (75.0 mL) in a stirred, 100 mL Hastelloy C Parr Micro Reactor. The reactor was heated to 210°C over the course of 1 hour and held at that temperature for 3 hours to give a yellow-brown solution (pH 8.45).

Fig. 7 demonstrates using 5'V NMR that an equilibrated Na6(+2)[SiV2W10O40]. solution can be prepared by this method at a higher concentration, 0.2 M.

Example 3d. Equilibrated aqueous solution of 0.5 M Na6(+2)[SiV2W10O40].

Na2SioO3, 92% (2.00 g, 15.1 mmol), NaVO3 (3.80 g, 31.2 mmol), Na2WO4 2H2O (9.90 g, 30.0 mmol) and WO3-H2O (30.00 g, 120.1 mmol) were mixed with water (30.0 mL) in a stirred, 50 mL Hastelloy C Parr Micro Reactor. The reactor was pressurized with 02 (2000 kPa) and was heated to 200 °C over the course of 1/2 hour and held at temperature for 12 hours to give a yellow-brown solution (final pH 9.18). Fig. 8 demonstrates using 51V

NMR that an equilibrated 0.5 M Na6(+2)[SiV2W10O40]. solution can be prepared by this method at a higher concentration, 0.5 M. Although the distribution of species has changed slightly at this higher concentration, the same complexes (system components) are identifiable in Figs. 6,7 and 8 for the solutions.equilibrated$Na6(+2)[SiV2W10O40].

Example 4.

Isolation of K7 [SiVIVVVW10O41]#12H2O from an aqueous solution of 0.1 M Na) [SiV, W, oOJ.

This procedure was performed to confirm, through isolation via precipitation, that [SiV2W10O40]6. is present as part of the thermodynamic equilibrium.

Na2SiO3,92% (1.021 g, 7.70 mmol), NaV03 (1.877 g, 15.39 mmol), Na2WO4*2H2O (5.078 g, 15.39 mmol) and W03 (15.386 g, 66.36 mmol) were mixed with water (75.0 mL) in a stirred, 100 mL Hastelloy C Parr Micro Reactor. The reactor was heated to 210°C over the course of 1 hour and held at that temperature for 3 hours to give a yellow-brown solution (pH 8.28).

The solution was filtered; however, little or no precipitate was observed. The filtrate was diluted to 1 L. A 35% N2H4 solution (700 mL) was added dropwise to the POM solution in order to reduce one of the vanadium atoms in [SiVoOJ-, to give [SiV"WVOJ,which has a lower solubility. After about 30 minutes, solid KCI (150 g, 2.0 mol) was added to give a brown precipitate. After several hours, the solid was collecte by filtration, washed with water (3x25 mL) and allowed to air-dry overnight: yield 78.46% based on the available vanadium (18.71 g). Anal. Calcd. (found) for K7[SiV2W10O40]#12H2O (fw = 3098.3): H, 0.78 (0.70); K, 8.83 (8.87); Si, 0.91 (0.66); V, 3.29 (2.98); W, 59.34 (59.30). IR (KBr, cm-1) : 1628 (w), 990 (w), 956 (m), 902 (s), 788 (s), 732 (w), 539 (w), 501 (w). Comparison of the IR data with that reported in Example 1 d and the 51V NMR spectrum shown in

Fig. 9 confirms the presence of [SiV2W10O40]6- anions in the preparations of equilibrated solutions.

Example 5.

Equilibrated aqueous solution of 0.1 M Na6(+x)[SiV2W10O40]. : variation of Na2WO4 : WO3 H2O ratio.

The purpose of this experiment was to maximize the system pH for the application of POMs in the delignification of wood pulp. In the processing of wood pulp, it is critical to maintain a neutral to basic pH in order to minimize acid-catalyzed hydrolysis of the cellulose and maintain pulp strength. This example demonstrates the ability to engineer specific properties of the equilibrated POM solutions for optimization of their use in specific chemical process.

Na2SiO3, 92% (0.37 g, 2.8 mmol) and NaVO3 (0.73 g, 6.0 mmol) along with Na2WO4*2H2O and WO3*H2O, varied while maintaining a total W (VI) concentration of 30 mmol in each case, were mixed with water (30.0 mL) in a stirred, 50 mL Hastelloy C Parr Micro Reactor. The reactor was pressurized with 0 (1000 kPa) and heated to 200°C over the course of 1/2 hour and held at temperature for 18-20 hours to give a yellow-brown solution. See Table 2 for final pH values.

Table 2. Variation of Na2WOI : WO3-H2oStoichiometry Expmt. POM WO3#H2FinalpHNa2WO4 A Na6(+0)[SiV2W10O40] 0 99 g, 3.0 mmol 6.75 g, 27 mmol 7.71 B Na6(+1)[SiV2W10O40] 1. 48 g, 4.5 mmol 6.37 g, 25.5 mmol 7.91 C Na6 (+2) [Siv2wloo4o] 1. 98 9,6.0 mmol 6.00 g, 24 mmol 8. 15 D Na6(+3)[SiV2W10O40] 2.47 g, 7.5 mmol 5.62 g, 22.5 mmol 8. 03 Fig. 10 demonstrates by51V NMR that the concentration of target POM anions in the equilibrated solutions described herein can remain fairly constant over a range of pH values. However, small changes in the solution

pH do reflect changes in equilibrium distributions that are likely vital to the buffer capacity described in Example 6.

Example 6.

Buffering capacity: titration of equilibrated Na6(+x)[SiV2W10O40] solutions at 70°C.

Aliquoto of Na6(+3)[SiV2W10O40]synthesizedasand described in Example 17 were diluted to give 160 mL of 0.025 M solutions.

These were heated to 70°C and titrated with 0.25 N HCI while monitoring the pH. One minute was allowed to pass after each HCI addition before each pH reading to allow the solution to adequately stabilize. The results are shown in Fig. 11. The relatively constant pH as the HCI is being added verifies the solution is being buffered, especially when compared to the dashed line representing the theoretical pH that would have been observed had the acid been added to pure water (i. e. no buffering present). The pH plateau seen in the titration of the Na5 +3ySiVW"04o] solution clearly extends further than that seen in the titration of the NagSiVWO] sotution, indicating the system with a greater sodium content has a larger buffering capacity.

Example 7.

Wood-pulp delignification and wet oxidation/POM reoxidation using an equilibrated 0.1 M Na6(+2)[SiV2W10O40] solution.

This example demonstrates that equilibrated solutions containing a target heteropolyoxotungstate of the general formula prepared in the manner described above can be used to oxidize a substrate (for example, lignin in wood pulp). The solutions can then be reoxidized under conditions that also mineralize the soluble organic by-products of lignin oxidation. This example also demonstrates that the solution can be used repeatedly and that during all

the lignin oxidation and POM regeneration cycles, the pH is maintained at a constant level.

Na2SiO3,92% (1.021 g, 7.695 mmol), NaV03 (1.877 g, 15.39 mmol), Na2WO4 2H2O (5.078 g, 15.39 mmol) and WO3 (15.386 g, 66.362 mmol) were mixed with water (75.0 mL) in a stirred, 100 mL Hastelloy C Parr Micro Reactor. The reactor was heated to 210°C over the course of 1/2 hour and held at this temperature for 3 hours to give a yellow-brown solution (pH 8.25).

This mixture was used in the subsequent delignification of wood pulp without further purification. Experimental details and results of the delignification and wet-oxidation/reoxidation experiments can be found in Table 3. Kraft wood pulp used: kappa 31.8, viscosity of 31.4 mPa#s. No deliberate adjustment of the solution concentration was performed. Solutions were purged with argon at least 10 minutes before delignification. The pulp was isolated after each delignification step by filtration and washed with a copious amount of water.

Fig. 12 demonstrates the stability of the 0.1 M Na6 (+2) [SiV2W, 0040] cataiyst solution, as determined by 51V NMR, throughout the multicycle experiment: (A) Solution after initial synthesis. (B) Solution after the first bleaching step.

(C) Solution after the first wet-oxidation/reoxidation step. (D) Solution after the last wet-oxidation/reoxidation step. Fig. 13 demonstrates the ability of the equilibrated 0.1 M Na6(+2)[SiV2W10O40] catalyst solution to maintain a stable pH throughout multiple delignification and wet-oxidation steps.

Table 3. Reaction conditions and analytical data for the anaerobic delignification of wood pulp by 0.1 M Na6 (+2) [SiV2W, 0040] and the reoxidation of the reduced solution by 02. Cycle @meGas/PressurepH%red1Kappa@@@@@lemp % csc. °C hr kPa MPa#s 2103O2/21708.43.4synthesis- 1504N2/1708.429.5172411 3O2/22708.26.6-210 2 1 150 4 N2/170 8. 1 36. 5 17 20 3O2/2170845.9-210 3 1 150 4 N2/170 15 17 -210 3 0) 2170 8. 4 5. 0 4 1 150 4 N2/170 8.4 35.0 17 -21030/21708. 34"9 1504N2/1708.335.8-21851 -210 3 0 8. 5 5.5 'Determined by electron absorption spectroscopy (UV-Vis) 2Kappa not measured

Example 8.

Wood-pulp delignification and POM reoxidation/wet-oxidation using an equilibrated 0.5 M Na6(+2)[SiV2W10O40] solution.

This example demonstrates that equilibrated solutions containing a target heteropolyoxotungstate of the general formula prepared in the manner described above can be used to oxidize a substrate (for example, lignin in wood pulp). The solutions can then be reoxidized under conditions that also mineralize the soluble organic by-products of lignin oxidation. This example also demonstrates that the solution can be used repeatedly and that during all the lignin oxidation and POM regeneration cycles, the pH is maintained at a constant level. This experiment was carried out at a larger scale using a more concentrated POM solution than Example 7 and was extended to a larger number of oxidation/reduction cycles.

Na2SiO3 (97.53 g, 93.9%, 0.75 mol), NaVO3 (189.14 g, 96.7%, 1.50 mol), Na2WO4 2H2O (494.78 g, 1.50 mmol) and WO3 (1392.77 g, 99.88%,

6.00 mol) were mixed with water (1062 mL) in a stirred, 2 L 316 stainless steel Parr Reactor. The reactor was heated to 210°C over the course of 1/2 hour and heid at this temperature for 3 hours, the yellow-brown solution was adjusted to a volume of 1.5 L, density 2.157 g/mL. This mixture was used in the subsequent delignification of softwood Kraft pulp, kappa 31.8 and viscosity 31.4 mPas, without further purification. Solutions were purged with N2 at least 10 minutes before bleaching. Experimental details and results of the bleaching and wet-oxidation/reoxidation experiments can be found in Table 4. The concentration of the solution was held at 0.5 M by maintaining the density of the solution at 2.157 g/mL through the addition or evaporation of water. The pulp was isolated after each bleaching step by filtration and washed with water. The decreasing liquor volume was due to the removal of samples for analysis. V NMR spectroscopy was used to very the stability of the 0.5 M Na6(+2)[SiV2W10O40] catalyst solution throughout the multi-cycle experiment. Table 4 demonstrates the ability of the 0.5 M Na6 [SiV2W, 0040] catalyst solution to maintain a constant pH throughout multiple bleaching and wet-oxidation steps as well as its effectiveness as an oxidizing agent for the removal of residual lignin from pulp.

Table 4. Reaction conditions and analytical data for the anaerobic delignification of wood pulp by 0.5 M Na6(+2)[SiV2W10O40]and reoxidation of the reduced solution byO2. Pulplemp@meGas/PressurePhipHf%redfKappaViscosityycleVol. °ChrkPaPOMmPa#sL%csc. 1 1.50 3 150 3 N2/450 9.0 9.4 32.6 9. 6 19.3 1.49-2103020709. 49. 16.4 2 1.45 3 150 3 N2/450 - 9.0 32.7 10. 8 22.0 1.453O2/20708.79.26.4210 3 1.43 3 150 3 N2/450 8.5 9.4 33.6 10. 9 21.6 1.423O2/20709.09.55.4210 41. 4031503N4508. 99. 028. 110.1 21.0 1.39-2103020708. 59. 55.8 5 1.37 3 150 3 N2/450 8.7 92 30.6 10. 0 20.7 1.373O2/20708.99.25.6210 6 1.35 3 150 3 N2/450 8.7 9.4 30.7 9. 4 19.9 1.33-2103020708. 89. 45.5 71. 3231503N./4508. 49. 030. 99.8 20. 7 1.333O2/20708.79.55.5210 8 1.32 3 150 3 N2/450 8.5 9.2 31.9 9. 3 20.6 1.313O2/20708.79.55.2210 9 1.29 3 150 3 N2/450 8.5 9.0 32.3 9. 4 21.2 1.293O2/20709.09.24.8210 31503N2/4508.69.232.28.720.7101.27 1.26-2103020708. 99. 545 11 1.24 3 150 3 N2/450 8.4 9.2 32.5 8. 6 20.7 1.253O2/20708.99.64.5210 12 1.25 3 150 3 N2/450 8.4 8.8 32.2 8. 4 19.6 1.221.220 O2/20708.59.14.63 13 1.20 3 150 3 N2/450 8.4 9.0 33.8 8. 1 20.4 1.20-210 3 O2070 8. 9 9. 3 4.4 14 1.19 3 150 3 N2/450. 8.7 9.3 35.2 7. 1 19.9 2103O2/20708.89.64.41.20- 15 1.15 3 150 3 N2/450 8.7 9.3 36.0 6. 4 17.4 1.153O2/20708.99.24.5210 16 1.15 3 150 3 N2/450 8.9 9.1 36.8 5 9 17.1 1.143O2/20708.69.34.3210 17 1.11 3 150 3 N2/450 8.6 9.5 36.8 5. 9 17. 7 1.11-2103020709. 39749 18 1.10 3 150 3 N2/450 8.0 9.8 38.3 5. 0 16.3 10.93O2/20709.19.44.6210 19 1.09 3 150 3 N2/450 8.7 9.7 37.4 5. 4 15.7 10.93O2/20708.79.14.2210 20 1.08 3 150 3 N2/450 8.9 9.5 37.9 5. 6 16.4 1.073O2/20708.79.33.8210 Example 9.

"Stepwise"synthesis of K7[AIVIVW11O40] and related derivatives.

The following are examples of"traditional"preparations of heteropolyoxotungstates of the general formula. Typically these preparative

methods involve several synthetic steps with isolation and purification of intermediates. It is important to understand that the POMs discussed in Example 9 are kinetic products and are included here to demonstrate that the structures and compositions of the products present in thermodynamic equilibrium in later examples are well understood. By synthesizing pure compounds in a controlled, stepwise manner, one can obtain analytical references for characterizing systems that are more complex.

Example 9a. Aqueous solution of Na [AI) W"O 91.

Sodium tungstate dihydrate (Na2WO4 2H2O, 100 g, 0.304 mol) was dissolved in 400 mL of H20 in a 1000 mL 3-neck round bottomed-flask containing a magnetic stirring bar and fitted with an addition funnel and condenser. Hydrochloric acid (ca. 23.0 mL, 0.276 mol) was added to the solution dropwise with vigorous stirring to pH 7.7 (use of a calibrated pH meter in the solution during this procedure was necessary). After every several drops the addition was momentarily stopped to allow the local precipitate of tungstic acid to dissolve. The solution was then heated to reflux, and aluminum chloride hexahydrate (AICI3 6H2O, 13.32 g, 0.0552 mol), dissolved in 80 mL deionized water, was added dropwise by means of the addition funnel over ca. 90 minutes (approximately 5-6 drops/min) with constant stirring. During the addition, the solution became slightly cloudy.

However, addition was kept at a slow enough rate to prevent the mixture from becoming opaque. If this occurred, however, the addition was stopped and the solution stirred until it became clearer. After all the AICI3 had been added, the solution was kept at reflux for 1 hour, cooled to room temperature and filtered through a 0.5 inch thick pad of Celitee (diatomaceous earth) on a medium sintered glass frit. The final pH was approximately 7. The polyanion was not isolated from solution, but was characterized by 27AI NMR (73 ppm, A-vl/2 = 89 Hz; 8 ppm, ##1/2 = 256 Hz). The'83W NMR spectrum displayed 23

signals ranging from-43 to-200 ppm, assigned to two isomers of C, symmetry (tentatively ß, and ß3,6 signals expected from each) and one of C, symmetry (tentativeiy Ps, 11 signats expected).

Example 9b. Dodecatungstoaluminic Acid, H5 [AIW, 2X01.

The solution, which contained Na6[AIOH2)W11O39], was transferred to a 1000 mL round-bottomed flask fitted with a reflux condenser. The solution was acidified to pH 0 by careful dropwise addition of concentrated sulfuric acid (ca. 20 mL, 0.376 mol). After the pH reached 0, an additional 3 mL of conc. sulfuric acid was added and the solution was heated to reflux.

The solution became cloudy and slightly yellow as the acid was added, but usually cleared within 16 hours of the beginning of reflux. To ensure complete conversion to product, the solution was kept at reflux for 6 days.

Then, after cooling to room temperature, the solution was filtered (if cloudy) using a medium glass frit (this was typically not necessary if the two steps were performed in quick succession, i. e., provided that the [Al (AIOH2) W"O39] 5-was not allowed to stand in solution more than 24 hours).

The solution, which contained H5 [AIW, 204o] and 1.08 (i. e., 13/12) eq of soluble AI (III) salts was transferred to a 1000 mL beaker and cooled to 0°C.

(We have found that the following acidification and extraction procedure should be performed in a ventilated hood, while wearing appropriate safety clothing, including splash goggles.) Cold (0°C) conc. sulfuric acid (147 mL) was added carefully to avoid excessive heating. The solution was then cooled in an ice-water bath to 0°C and transferred to a 2000 mL separatory funnel. Diethyl ether (500 mL) was added and the mixture shaken very gently with frequent ventilation until rapid evaporation of diethyl ether subsided.

Then, the mixture was shaken more vigorously, still with frequent venting, and allowed to settle until three layers separated. The top clear colorless layer was diethyl ether, the middle somewhat cloudy layer was the aqueous phase

and the bottom layer (a dense, pale yellow, viscous liquid) was the etherate of HgtAtWO]. The bottom (etherate) layer was collected and the shaking and venting procedure was repeated until the etherate layer no longer formed (the aqueous layer appeared clearer as the extraction neared compietion). The combined etherate layers (about 20 mL) were concentrated to dryness by rotary evaporation. The crude product (69.2 g, 95%) was reprecipitated by dissolving in 20 mL of hot water, concentrating to a volume of 23 mL by gentle heating, and then cooiing to 0°C for 16 hours. Yield: 50.46 g, 64%.

Dodecatungstoaluminic acid is a water-soluble slightly yellow amorphous solid. To avoid reduction of the free acid (H5[AIW12O40], metal implements were not used in handling it as a solid or in solution. The polyanion was stable in water below pH 6 and characterized by NMR and IR spectroscopy.

2'AI NMR: ß-isomer 71.6 ppm (##1/2 = 4.6 Hz); a-isomer 72.1 ppm (avez = 1.3 Hz) ; 183W NMR: ß-isomer (integration)-110.8 (1),-118.7 (2), and-136.8 (1) ppm; a-isomer-110.1 ppm. IR (2-5 wt % KBr pellet, cm-'): 972 (s), 899 (s), 795 (broad, s), 747 (broad, s), 538 (m), and 477 (m). Anal. Calcd. (found) for (H5[AIW12O40]#15H2O : H 1.12 (1.15), W 70.07 (70.23), Api 0.86 (0.89).

Example 9c. Potassium Undecatungstoatuminate. K9[AIW11O39]#12H2O.

Dodecatungstoaluminic acid (H5 [AIW11O40]#13H2O, 43.76 g, 14.1 mmol) was dissolved in 100 mL of H2O and heated with stirring to 60°C. Three equivalents of potassium carbonate (K2CO3#1.5H2O, 6.97 g, 42.3 mmol) were added gradually as a solid. The pH should rise to about 2. Another 5 equivalents of potassium carbonate (K2CO3*1. 5H2O, 11.62 g, 70.5 mmol, dissolved in 20 mL of H20) were added dropwise carefully over about 60 minutes. The pH of the solution should not be allowed to rise above 8 until at least 80% of the potassium carbonate solution has been added and should at all times be kept below 8.5. A white precipitate began to form as addition of the potassium carbonate solution proceeded. The final pH of the mixture

should be near 8.25. After addition of the potassium carbonate solution was complete, the mixture was cooled to 0°C for several hours. The product, a fine white precipitate, was then collecte, washed three times with H2O and dried on a medium glass frit. Yield: 41.8 g, 92%. The potassium salt of the lacunary anion is a white amorphous solid, which is slightly soluble in water (2 g/100 mL at 22°C). 2'AI NMR (solubilized by addition of excess LiCl) : 63.3 ppm ##1/2 = 1735 Hz). IR (2-5 wt % KBr pellet, cm-'): 937,868,789,756 (sh), 704,524 and 493. Anal. Calcd. (found) for K9 [AIW11O39]#12H2O; H 0.75 (0.79), W 62.39 (62.05), Al 0.83 (0.92), K 10.86 (10.80).

K7[AIVIVW11O40].Example9d.

To a gently stirred slurry of Kg [AIWO39] (5.43 g, 1.80 mmol) in 10 mL of H20in a 50 mL beaker, vanadyl sulfate trihydrate (VOSO4#3H2O, 0.39 g, 1.80 mmol), dissolved in 5 mL of H20, was added dropwise rapidly at room temperature. The mixture was stirred for 30 minutes and filtered on a medium porosity sintered glass frit. The dark-purple filtrate was cooled to 0°C for 2 hours. The resulting dark-purple crystals were collecte on a sintered glass frit and recrystallized from a minimum of warm (60°C) H2O, Recrystallized yield: 3.5 g (61 %). Dark-purple crystalline K7 [AIW11O40]#15H2O was characterized in the solid state by IR (2-5 wt % KBr pellet, cm-'): 761,697,537,492 and 473. Anal. Calcd.

(found) for K7 [AIVW, 10401. 15H20 : H 0.92 (0.84), W 61.58 (61.66), Al 0.82 (0.97), V 1.55 (1.32), K 8.33 (8.17). The vandayl (VIV=O)2+ containing anion, [AIW11O40]7-, is paramagnetic. However, solutions of [AIVW11O40]7-, dark purple in color, are readily oxidized to [AIVW"040] 6-by addition of elemental bromine, which is bright yellow in solution. Diamagnetic [AIVW1140] 6-, prepared in situ, can be observed by 27AI, 51V and'83W NMR. 27AI: 72.5 ppm (##1/2 = 175 Hz) ; 51V :-535.5 ppm (##1/2 = 220 Hz);'83W (6 peaks):-79.7, -96.0,-116.2,-119.5,-121.4 and-141.2 ppm.

Example 9e, K7[AlCo(H2O)W11O39].

To a slurry of Kg [AIW O39] (10.0 g, 3.05 mmol) in 100 mL of H2O, cobalt (li) nitrate hexahydrate (Co (NO3) 2 6H2O, 0.89 g, 3.05 mmol), dissolved in 10 mL of H20, was added dropwise rapidly at room temperature. The mixture was heated over a water bath and stirred until all visible precipitate dissolved. The mixture was then cooled to room temperature, filtered, concentrated to one-half the volume and refrigerated at 0°C for several hours. The resulting red precipitate was collecte on a sintered glass frit and recrystallized from a minimum of warm (60°C) H2O. Crude yield : 7.31 g (74%). The cobalt (II) complex of K9[AIW11O39] is a red amorphous powder and is characterized in the solid state by IR (KBr pellet, cm-'): 936,879,796, 747,696,538 and 486. Anal. Calcd. (found) for K7 [AICoW"039] 14H20 : H 0.87 (0.91), W 62.07 (62.26), Al 0.83 (0.88), Co 1.81 (1.73), K 8.40 (8.16).

Example 9f. K7[AIMn(HwO)W11O39].

To a slurry of Kg [AIW O39] (10.0 g, 3.05 mmol) in 100 mL of H20, manganese (II) hydrate (MnSO4 H20), 0.516 g, 3.05 mmol), dissolved in 10 mL of H20, was added dropwise rapidly at room temperature. The mixture was heated over a water bath and stirred until all visible precipitate dissolves.

The mixture was then cooled to room temperature, filtered and refrigerated at 0°C for several hours. The resulting yellow precipitate was collecte on a sintered glass frit and recrystallized from a minimum of warm (60°C) H2O.

Crude yield: 8.51 g (86%). The manganese (II) complex of Kg [AIW O39] is a yellow amorphous powder and was characterized in the solid state by IR (KBr pellet, cm-'): 934,872,797,766,697,528 and 487. Anal. Calcd. (found) for K7[AIMnW11O39]#14H2O : H 0.87 (0.79), W 62.15 (62.34), Al 0.83 (0.96), Mn 1.69 (1.44), K 8.41 (8.20).

Example 9q. K6[AIMn (OH) W, 91.

To a slurry of K7 [AIMn(H2O)W11O39] (5.0 g, 1.6 mmol) (of exampte 9f) in water (15 mL), K2S208 (5.0 g. 18.5 mmol) was added. The mixture was stirred at 80°C until the solution had changed color from yellow to brown.

The mixture was then cooled to room temperature and the excess K2S208 filtered off. The solution was concentrated to dryness and the crude residue recrystallized three times, from pH 4.7 acetate buffer. The brown solid was characterized by sold state IR (KBr pellet cm-'): 954,876,798,768,728, 700,668,488. Anal. Calcd. (found) for K6 [AIMn (H20) W"039]. 10H20: K 7.41 (7.21), Al 0.85 (0.82), Mn 1.74 (1.68), W 64.0 (63.42). UV-VIS (Amax nm, E Mol-'dm3 cm-'): 488 (110).

Example 10.

Synthesis of equilibrated POM solutions targeting the [AIVW"04o] 6- anion.

The following are examples of the single-step synthesis of equilibrated POM solutions containing target polyoxotungstates of the general formula.

The equilibrated solutions have been prepared under a variety of experimental conditions by mixing hydroxides or neutral or anionic oxides of transition-metal or main-group elements and water and heating the mixtures to temperatures sufficiently high such that they react to give solutions containing target heteropolyoxotungstates of the general formula present in thermodynamic equilibrium with additional compounds also derived from the starting materials.

Example 10a. Equilibrated aqueous solution containing the sodium salt of the target POM anion [AIVW11O40]6-.

Al203 (0.051 g), NaV03 (0.121 g), Na2WO4 2H2O (1.154 g) and WO3-H2O (2.124 g) were placed in a 10 mL volumetric flask. The flask was filled with water to a total volume of 10 mL, and the solution was placed in a

Teflon liner inside a stainless steel pressure vessel. The pressure vessel was heated in a 200°C oven for 2 days (without stirring); during this time most of the solid dissolved. The solution was brown due to a small amount of reduction. The presence of the target POM anion, [AIVW11O40] verified by 5'V and 27AI NMR; before collecting the 5'V and 2'AI NMR spectra, it was fully oxidized to a clear-yellow solution by brief exposure to bromine vapors.

Example 1 Ob. Equilibrated aqueous solution containing the sodium salt of the target POM anion rAtVWO- (0. 25 M).

NaOH, 97% (1.55 g, 37.5 mmol), Al (OH) 3,83% (0.71 g, 7.5 mmol), WO3, 99% (19.4 g, 82.5 mmol), and NaVO3, 96% (0.95 g, 7.5 mmol), were mixed with water (30 mL) in a 50 mL titanium Parr Micro Reactor. The reactor was pressurized with 0 (1400 kPa) and heated to 200°C over the course of 1/2 hour and held at that temperature for 24 hours to give a yellow-orange solution (final pH 3.90). Fig. 14 demonstrates by 5'V NMR the presence of Na6 [AIVW"O40] in equilibrium with smaller amounts of additional isopoly-and heteropolyoxotungstates, such as [V2W4O, 9]4-, [V3W3O19]5- and [HV3W3O19]4-.

Example 10c. Equilibrated aqueous solution of 0.25 M Na6(+2)[AIVW11O40].

NaOH, 97% (2.16 g, 52.3 mmol), AI (OH) 3 83% (0.71 g, 7.5 mmol), WO3, 99% (19.4 g, 82.5 mmol), and NaVO3,96% (0.95 g, 7.5 mmol), were mixed with water (30 mL) in a stirred, 50 mL titanium Parr Micro Reactor. The reactor was pressurized with 02 (1400 kPa) and heated to 200°C over the course of 1/2 hour and held at that temperature for 24 hours to give a orange- brown solution (final pH 8.60). Figs. 15 and 16 demonstrate by 5'V NMR and 27AI NMR that these solutions can be prepared using a variety of experimental conditions.

Fig. 14 shows the 5'V NMR spectrum of [AIVW"04o) 6- synthesized from the elemental oxides described in Example 10b and Fig. 15 is the spectrum of the solution prepared using an excess of NaOH as described in Example 10c.

The narrow signal at-511 ppm (labeled (2)) in the 5'V NMR spectrum in Fig.

15 corresponds to IV2W40 19] 4- ; its area corresponds to 18% of the total amount vanadium in solution. The major product (82% of the total amount of vanadium) of the synthesis is the target anion, a- [AIVW"040] 6- at-536 ppm (labeled (1)). The other, smaller peaks labeled (1) correspond to structural isomers of a- [AIVW"O40] 6~. A comparison of Figs. 14 and 15 with the single 51V NMR peak at-536 ppm obtained from the stepwise synthesis given in Example 9d demonstrates that equilibrium solutions containing the target anion [AIVW"O40] 6-can be prepared in a single step from component-element oxides.

Example 11.

Equilibrated aqueous solutions containing the target anion [AIVW"O40] 6-. Variation of element ratios.

The purpose of this experiment was to maximize the system pH for the application of POMs in the delignification of wood pulp. In the processing of wood pulp, it is critical to maintain a neutral to basic pH in order to minimize acid-catalyzed hydrolysis of the cellulose and maintain pulp strength. This example demonstrates the ability to engineer specific properties of the equilibrated POM solutions for optimization of their use in specific chemical process.

Various ratios of starting compounds (NaOH, AI (OH) 3, NaV03, W03) were mixed and heated in water to give the equilibrated solutions listed in Table 5. The starting compounds were mixed in water to give 50 mL of 0.5 M equilibrated POM solution, placed in a stirred, 100 mL stirred titanium Parr reactor and heated to 200°C for 12 hours. The 5'V and 27AI NMR spectra of the solutions were obtained and the various signals integrated. The distributions of vanadium-and aluminum-containing species are listed in Table 5. Isopolyoxotungstates that may be present in the solution have not been listed.

Table 5. Concentrations of AI (Ill)- and V (V)-containing species in 0.5 M solutionsequilibratedvanadotungstoaluminate of various compositions as<BR> determined and27ANMR.51V

FormulationFormulationpH [V2W4O19][V3W3O19][VO3][A12W11O3 0.4950.0300.002Na6(+0)[AI1(+0.5)VW11O40]4.6 0.4750.0750.012Na6(+0.5)[AI1(+0.5)VW11O40]6.5 0.4500.1120.025Na6(+1)[AI1(+0.5)VW11O40]7.3 Na [A)VW,, OJ7. 50. 4300. 1000.035 0.4000.1120.050Na6(+2)[AI1(+0.5)VW11O40]7.6 Na A/WOJ8. 00. 4000. 1000. 040 Na +3)[AI1(+0.5)VW11O40] 8.5 0.355 0.186 0. 040 0.007 Na6(+4)[AI1(+0.5)VW11O40] 8.8 0.310 0.035 0. 035 0.040 0.4300.1000.035Na6(+1.5)[AI1(+0.5)VW11O40]7.5 Na Na6(+1.5)[AI1(+0.5)VW11O40] 8.2 0.355 0.150 0. 040 0.030 0.2150.0900.0050.018Na6(+1.54)[AI1(+0.5)VW11O40]8.2 Na +1.5)[AI1(+0.5)VW11(-2)O40] 8.5 0.200 0.100 0.025 0.070 0.022 Na OJ8. 00. 3650. 0750.050 0.012 0.005 Na 50. 4300. 1000.035 0.4800.0750.010Na6(+1.5)[AI1(+0.5)VW11(+1)O40]7.5 il 8. 2 0. 497 0. 040 0.012 *Some unreacted solids remained after the reaction Example 12.

Wood-pulp delignification using an aqueous solution of 0.5 M Na6(+1.5)[AI1(+0.5)VW11(+1)O40].

This example demonstrates that equilibrated solutions containing a target heteropolyoxotungstate of the general formula prepared in the manner described above can be used to oxidize a substrate (for example, lignin in wood pulp). The POM synthesis described here is carried out on a significantly larger scale than those previously described.

AI (OH) 3 (89.82 g, 97.7%, 1.125 mol), NaVO3 (94.57 g, 96.7%, 0.75 mol), Wo3 (2089.18 g, 99.88%, 9 mol) and NaOH (196.97 g, 99%, 4.875 mol) were mixed with water (1341 mL) in a stirred, 2 L 316 stainless steel Parr Reactor. The reactor was charged with 1480 kPa °2 and heated to 210°C for

13 hours. The solution was filtered to remove a small amount of solids. The final solution was 1.714 L in volume, 2.179 g/mL in density and 0.438 M in concentration (based on vanadium concentration), had a pH of 8.1 and 2.3% of the vanadium present had been reduced to V (IV). 1.23 L of solution was concentrated to 1 L. This solution was mixed with 33.29 g softwood Kraft pulp (dry equivalent), kappa 31.8 and viscosity 31.4 mPa's, which contained 0.08 L of water giving a pulp slurry of 3% consistency and a 0.5 M solution.

The delignification was carried out in a 2 L stainless steel Parr reactor, equipped with a tapered helical stirrer, that was purged with N2 for 5 minutes before heating. The reactor was heated to 150°C and held at this temperature for 3 hours. The reaction was then quenched and the pulp filtered and washed with water. The resulting pulp had a final kappa number of 4.8 and viscosity of 16.3 mPas. After the reaction, 41.2% of the vanadium present was reduced to V (IV) and the pH of the solution was 8.1.

Example 13.

Wood-pulp delignification and wet-oxidation/POM reoxidation using an equilibrated 0.5 M Na6(+1.5)[AI1(+0.5)VW11(+1)O40] solution.

This example demonstrates that equilibrated solutions containing a target heteropolyoxotungstate of the general formula prepared in the manner described above can be used to oxidize a substrate (for example, lignin in wood pulp). The solutions can then be reoxidized under conditions that also mineralize the soluble organic by-products of lignin oxidation. This example also demonstrates that the solution can be used repeatedly and that during all the lignin oxidation and POM regeneration cycles, the pH is maintained at a constant level.

AI (OH) 3 (11.04 g, 97.7%, 135 mmol), NaVO3 (11.70 g, 96.7%, 90 mmol), W03 (251.82 g, 99.88%, 1080 mmol) and NaOH (46.38 g, 50% solution, 585) were mixed with water (150 mL) in a stirred, 300 mL Hastelloy

C Parr Micro Reactor. The reactor was charged with 1480 kPa °2 and heated to 200°C for 12 hours. The resultant solution had a pH of 7.4 and 2.8% of the vanadium present had been reduced to V (IV). This mixture was used in the subsequent reaction with wood pulp without further purification.

Experimental details and results of the delignification and wet- oxidation/reoxidation experiments can be found in Table 6. Kraft wood pulp used: kappa 31.8, viscosity of 31.4 mPas. No deliberate adjustment of the solution concentration was performed. Solutions were purged with N2 for at least 10 minutes before heating. The pulp was isolated after each bleaching step by filtration and washed with water. Table 6 demonstrates the ability of the equilibrated 0.5 M Na6(+1.5)[AI1(+0.5)VW11(+1)O40] solution to maintain a constant pH throughout multiple delignification and wet-oxidation/reoxidation steps.

Table 6. Reaction conditions and analytical data for anaerobic delignification of wood pulp by 0.5M Na6(+1.5)[AI1(+0.5)VW11O40] and the reoxidation of the reduced solution boy 02. Cycle @meGas/Pressurelemp % csc. °C hr kPa Synthesis - 210 12 O2/2170 74 30 1 1 130 2 167 N2/170 77 133 - 120 3 O2/2170 75 34 2 1 130 2 167 N2/170 77 136 - 120 3 O2/2170 76 32 3 1 130 2 167 N2/170 77 146 - 120 3 O2/2170 76 34 4 1 130 2 156 N2/170 77 145 3O2/21707632-120 5 1 130 1 167 N2/170 77 148 - 120 3 O2/2170 76 34 6 1 130 2 167 N2/170 77 140 - 120 3 O2/2170 76 32 7 1 130 2 167 N2/170 77 140 - 120 3 O2/2170 76 28 8 1 130 2 167 N2/170 77 148 - 210 3 O2/2170 76 30 9 1 130 2 167 N2/170 77 145 3O2/21707.730-210 'Determined by electron absorption spectroscopy (UV-Vis) Example 14.

Reduction of 0.5 M Na6(+1.5[AI1(+0.5)VW11(+1)O40] using CO and reoxidation of the reduced POM solutions using °2- This example demonstrates that equilibrated solutions containing a target heteropolyoxotungstate of the general formula prepared in the manner described above can be used to oxidize a substrate (for example, carbon monoxide; see Equations 4 and 5). The solutions can then be reoxidized using 02. This example also demonstrates that the solution can be used repeatedly and that during all the oxidation/reduction cycles, the pH is maintained at a constant level.

AI (OH) 3 (11.04 g, 97.7%, 135 mmol), NaV03 (11.70 g, 96.7%, 90 mmol), WO3 (251.82 g, 99.88%, 1080 mmol) and NaOH (46.38 g, 50%

solution, 585) were mixed with water (150 mL) in a stirred, 300 mL Hastelloy C Parr Micro Reactor. The reactor was charged with 1480 kPa 02 and heated to 200°C for 12 hours. The final solution had a pH of 7.2 and 1.0% of the vanadium present had been reduced to V (V). This solution was used in the subsequent oxidation of CO without any purification. Experimental details and results of the CO oxidation and POM reoxidation experiments can be found in Table 7. No deliberate adjustment of the solution concentration was performed. The reactor was purged with Ar for at least 10 minutes and was then charged to 550 kPa CO and heated. Fig. 18 demonstrates the ability of the 0.5 M Na6(+1.5[AI1(+0.5)VW11(+1)O40] solution to maintain a constant pH throughout multiple reduction and reoxidation steps.

Table 7. Reaction conditions and analytical data for the anaerobic oxidation of CO by 0.5M Na6(+1.5[AI1(+0.5)VW11(+1)O40] and the reoxidation of the reduced solution by 02. TempGas/PressurepH%red1CyclePulp hrkPa%csc.°C 21012O2/21707.31Synthesis- 1302.167CO/5507.09.811 -210 3 0 2170 7. 3 2 1302.167CO/5506.910021 -210 3 0 2170 7. 4 1 1302.167CO/5506.910031 -210 3 0 2170 7. 5 2 4 l 130 2. 167 CO/550 7. 0 98 -210 3 0 2170 7. 5 1 5 l 130 2. 167 CO/550 7. 0 100 -210 3 0 2170 7. 6 2 1302.167CO/5506.99861 -210 3 0 2170 7. 6 2 7 1 130 2. 167 CO/550 6. 9 100 -210 3 0/2170 7. 7 1 1302.167CO/5506.99881 -210 3 0 2170 7. 7 2 9 1 130 2. 167 CO/550 6. 9 100 -210 3 0/2170 7. 7 1 1302.167CO/5506.999101 3O2/21707.711210 'Determined by electron absorption spectroscopy (UV-Vis) Example 15.

Equilibrated aqueous solution of 0.5M Na5(+1)[PV2W10O40].

The following is an example of the single-step synthesis of an equilibrated POM solution containing target polyoxotungstates of the general formula. The equilibrated solution was prepared by mixing hydroxides or neutral or anionic oxides of transition-metal or main-group elements and water and heating the mixtures to temperatures sufficiently high such that

they reacted to give solutions containing target heteropolyoxotungstates of the general formula in thermodynamic equilibrium with additional compounds also derived from the starting materials. This example further demonstrates the generality of the present invention.

Na2HPO4 (14.21 g, 100 mmol), NaV03 (25.26 g, 200 mmol), WO3 (232.15 g, 1000 mmol) and NaOH (50% soln, 16.00 g, 200 mmol) were mixed with water (143.0 g) in a stirred 300 mL 316 SS Parr Mini Reactor. The reactor was pressurized with O2 (1440 kPa) and heated to 201 °C over the course of 1/2 hour and held at this temperature for 3 hours. There were 3.5 g (~1 % of non-water materials) of material that were not incorporated into solution. The pH of the final solution was 5.0 and approximately 2% of the vanadium present in solution had been reduced to V (IV). Fig. 19 demonstrates by 3'P NMR the presence of [PV2W, 0040] 5- (29%) along with [PVgWOJs-(46%) and [PW"039)'-or [PW, o036)'- (24%) in solution. A very small amount of [V2W4O, 9] 4- (<1 %) can be seen by 51V NMR. Cyclic voltammetry of the solution revealed oxidation and reduction waves associated with [PV2W10O40]5- at E1/2=0. 67 V and [PV3W9O40]6- at E1/2=0 44 V versus normal hydrogen electrode (NHE).

Example 16.

Wood-pulp delignification using an equilibrated aqueous solution containing 0.5 M Na5(+1)[PV2W10O40].

This example demonstrates that equilibrated solutions containing the target heteropolyoxotungstates of the general formula prepared in the manner described above can be used to oxidize a substrate (for example, lignin in wood pulp).

Na2HPO4 (42.6 g, 300 mmol), NaV03 (75.8 g, 600 mmol), WO3 (232.15 g, 1000 mmol) and NaOH (24.5 g, 600 mmol) were mixed with water (400 g) in a stirred 1 L 316 SS Parr Reactor equipped with a glass liner. The reactor

was pressurized with 02 (700 kPa) and heated to 210°C over the course of 1/2 hour and held at this temperature for 14 hours. The pH of the final solution was 4.7. This solution mixed with 18.6 g softwood Kraft pulp (dry basis), kappa 31.8 and viscosity 31.4 mPas, which contained 0.04 L of water resulting in a pulp slurry of 3% consistency and a 0.5 M solution. The delignification was carried out in a 1 L stainless steel Parr reactor equipped with a tapered helical stirrer that was purged with N2 for 5 minutes before heating. The reactor was heated to 135°C and held at this temperature for 1 hour. The reaction was then quenched by cooling the reactor and the pulp filtered and washed with water. The resulting pulp had a final kappa number of 15.4 and viscosity of 23.0 mPas. After the reaction, 29% of the vanadium present was reduced to V (IV) and the pH of the solution was 5.0.

Example 17.

Wood-pulp delignificaton using equilibrated solutions of 0.5 M NaJSiV, Mo, WOJ.

The purpose of this experiment was to maximize the system pH and concentration of the [SiVMo0W11-0O40]5- species for use in the delignification of wood pulp. In the processing of wood pulp, it is critical to maintain a neutral to basic pH in order to minimize acid-catalyzed hydrolysis of the cellulose and maintain pulp strength. This example demonstrates the ability to engineer specific properties of the equilibrated POM solutions for optimization of their use in specific chemical process.

Various ratios of starting compounds (Na2SiO3, NaVO3, WO3, Na2WO4) were mixed and heated in water to give the solutions listed in Table 8. The starting compounds were mixed in water to give 550 mL of 0.5M equilibrated POM solution, placed in a stirred, 1 L 316 stainless steel Parr reactor, charged with 1400 kPa 02 and heated to 200°C for 3 hours. These solutions were then mixed with softwood Kraft pulp (Kappa 32, viscosity 31 mPas) to a consistency of 3% and heated in the Parr reactor to 140°C under N2 for 1 hour. The goal of delignification is to obtain a pulp with the lowest possible kappa number with the highest possible viscosity. The results are shown in Table 8 below.

Table 8. Delignificaton of softwood Kraft pulp by equilibrated polyoxotungstate solutions of various compositions.

pHfNet%KappatiscosityfFormutationpHi Red 7.9231415Na5(+2)[SiVW11O40]5.9 6.9281024Na5(+2)[SiVMoW11O40]5.9 6.527824Na5(+2)[SiVMoW9O40]5.3 6.431622Na5(+2)[SiVMo3W8O40]5.2 6.9281024Na5(+2)[SiVMoW10O40]5.9 7.130625Na5(+1.9)[SiV1(-0.1)MoW10(+0.1)O40]5.7 6.425522Na5(+1.75)[SiV1(-0.25)MoW10(+0.25)O40]4.8 4.626618Na5(+1.6)[SiV1(-0.4)MoW10(+0.4)O40]5.1 5.629721Na5(+1)[SiVW11O40]5.1 7.9231425Na5(+2)[SiVW11O40]5.9 8.6181827Na5(+3)[SiVW11O40]8.7 Example 18.

Comparison of reoxidation rates of solutions containing Na5(+x)[SiVMo2W9O40] that had been reduced by prior reaction with wood pulp.

The reduced solution liquor containing 0.5 M Na5(+2)[SiVMo2W9O40] from Example 17 was split into 150 mL aliquots. To one aliquot, NaOH (6.19 g, 150 mmol) was added now making it essentially 0.5 M Na5 (+4) [SiVMo2WgO40]. Nothing was done to the other bleach liquor aliquot.

The solutions were each, in turn, placed into a 300 mL 316 stainless steel Parr mini reactor equipped with a gas entrainment impeller. The reactor was purged with N2 and heated to 210°C at which time 1400 kPa 02 was injected into the system. Samples were taken at various times to monitor the rate at which the reduced polyoxometalate solutions were oxidized. The results,

shown below in Table 9, show that the reduced Na5 (+4) [SiVMo2W904o] so ! ution was reoxidized significantly faster than the reduced Na5(+2)[SiVMo2W9O40] solution. The results are explained by a change in the distribution similar to that described in Table 1 and its accompanying text. The changes in the species distribution are verified by the trend shown in Fig. 20.

Table 9. Reoxidation of equilibrated polyoxometalate bleach liquors with different levels of sodium

Time % reduced for % reduced for Na5(+4)[SiVMo2W9O40](hr.)Na5(+2)[SiVMo2W9O40] 0 51 49 1/2 15 1 37 10 2 30 8 3 27

Example 19.

Delignification of linerboard wood-pulp using 0.5 M Na5(+x)[SiVkMo0W12-k-oO40].

The purpose of this experiment was to maximize the system pH and concentration of the [SiVMooW"_oO40] for use in the delignification of wood pulp. In the processing of wood pulp, it is critical to maintain a neutral to basic pH in order to minimize acid-catalyzed hydrolysis of the cellulose and maintain pulp strength. This example demonstrates the ability to engineer specific properties of the equilibrated POM solutions for optimization of their use in specific chemical process.

Various ratios of starting compounds (Na2SiO3, NaVO3, W03, Na2WO4) were mixed and heated in water to give the solutions in Table 10. The starting compounds were mixed in water to give 250 mL 0.5 M of equilibrated POM solution, placed in a stirred, 300 mL 316 stainless steel Parr reactor, charged with 1400 kPa 02 and heated to 200°C for 3 hours. These solutions were then mixed with southern pine Kraft linerboard pulp to a consistency of 3% and heated in stirred, 1 1 L stirred 316 stainless steel Parr reactor to 145°C under N2 for 3 hours. The goal of delignification is to obtain a pulp with the lowest possible kappa number with the highest possible viscosity (the initial kappa number was 65, the initial viscosity could not be determined).

The results are shown in Table 10 below.

Table 10. Delignification of southern pine Kraft linerboard pulp (Kappa number 65) by equilibrated polyoxometalate solutions of various compositions.

Formulation pHj pH, Kappaf Viscosityf NaS, +2SIVM02W9O4o] 5.0 6. 6 18 36 5.82125Na5(+1.75)[SiV1(-0.25)MoW10(+0.25)O40]4.9 6.82041Na5(+1.9)[SiV1(-0.1)MoW10(+0.1)O40]4.9 Example20.

Oxidation of equilibrated aqueous solutions containing 0.5 M Na5(+1.5)[SiV1(-0.1)TMoW10O40] where TM=Mn, Fe and Cr.

The following is an example showing that various transition metals can be incorporated homogeneously into aqueous solutions containing equilibrated POM solutions containing target poloxotungstates of the general formula and that the presence of these transition metals increases the reoxidation rate by oxygen of these target polyoxotungstates.

Various ratios of starting compounds (Na2Si03, V205, MoO3, WO3, and NaOH along with MnO2, FeO (OH) or Cr (OH) 3) were mixed and heated in water to give the solutions listed in Table 11. The starting compounds were mixed in water to give 225 mL of 0.5M equilibrated POM solution, placed in a stirred, 600 mL 316 stainless-steel Parr reactor, charged with 1400 kPa CO and heated to 200°C for 5 hours. These solutions were then reduced with 2500 kPa CO (loaded at room temperature) at to 200°C for 6 hours. (This reduction step produced a small amount of precipitate that was filtered from all the solutions.) Finally, the solutions wer reoxidized using 2000 kPa 02 at to 210°C for 6 hours. Samples were taken periodically during reoxidation to produce the results shown in Fig. 21.

Tabel 11. Solution data for equilibrated POM solutions containing small amounts of various transition metals in order to enhance oxidation rates. Composition Equilibrated POM Synthesis CO 02 Solution Reduction Oxidation pH RedpH RedpH Red % % % 5.27.292.26.228.8Na5(+1.9)[SiV1(-0.1)MoW10(+0.1)O40]5.9 Na5(+1.5)[SiV1(-0.1)Mn0.1)W10O40] 5. 4 95.7 5. 5 9.4 Na5(+1.5)[SiV1(-0.1)MoW10(+0.1)O40] 5. 7 20.6 Na5(+1.5)[SiV1(-0.1)Cr0.1)W10O40] 5. 7 5.6 6.8 96.9 5. 8 28.2 Example 21.

Comparison of wood-pulp delignification using equilibrated aqueous solutions containing 0.5 M Na5(+1.9)[SiV1(-0.1)MoW10(+0.1)O40] versus Na5(+1.5)[SiV1(-0.1)(Mn0.1W10O40].

This example demonstrates that equilibrated solutions containing the target heteropolyoxotungstates of the general formula prepared in the manner described above can be used to oxidize a substrate (for example, lignin in wood pulp). The presence of a small amount of transition metals added to the systems, while improving reoxidation rates by oxygen, does not debilitate the oxidation of substrates such as lignin in wood pulp).

The correct ratio of starting compounds (Na2SiO3, V205, MoO3, W03 and NaOH along with MnO2) were mixed in water in a stirred, 20 L 316 stainless-steel Parr reactor, charged with 800 kPa 02 and heated to 200°C for 3 hours to yield 14 L of 0.5M equilibrated Na5(+1.5)[SiV1(-0.1)(Mn0.1W10O40] solution. A 0.5M equilibrated Na5(+1.9)[SiV1(-0.1)MoW10(+0.1)O40] solution was synthesized in a similar manner. Each solution was then mixed with softwood Kraft pulp (Kappa 27.6, viscosity 40.9 mPa s) to a consistency of 3% and heated in the Parr reactor to 140°C under N2 for 1.5 hours. Using the Na5(+1.5)[SiV1(-0.1)(Mn0.1W10O40] resulted in 30 percent net reduction of the

POM solution and a pulp with a Kappa number of 1.7 and a viscosity of 28.6 mPa s Using the Na5(+1.9)[SiV1(-0.1)MoW10(+0.1)O40] resulted in 34% net reduction of the POM solution a pulp with a Kappa number of 1.0 and a viscosity of 25.7 mPa s.

Example 22.

Equilibrated aqueous solution of 0. 37M Na5(+0.26)[SiMn1(-0.2)(H2O)W11O39].

The following is an example of the single-step synthesis of an equilibrated POM solution containing target polyoxotungstates of the general formula. The equilibrated solution was prepared by mixing hydroxides or neutral or anionic oxides of transition-metal or main-group elements and water and heating the mixtures to temperatures sufficiently high such that they reacted to give solutions containing target heteropolyoxotungstates of the general formula in thermodynamic equilibrium with additional compounds also derived from the starting materials. This example further demonstrates the generality of the present invention.

The correct ratio of starting compounds (Na2SiO3, MnO2, W03 and NaOH) were mixed in water in a stirred, 1 L 316 stainless-steel Parr reactor, charged with 800 kPa 02 and heated to 200°C for 3 hours to yield 0.6 L of 0.37M equilibrated Na5(+0.26)[SiMn1(-0.2)(H2O)W11O39] solution. This solution had a pH of 4.4, a density of 2.221 g/mL at room temperature, and was 2.7 percent reduced. Fig. 22 compares the visible spectra of this equilibrium solution to a literature"step-wise"preparation of K5 [SiMn"' (H20) W"039] (C. M.

Tourne, et al., J. Inorq. Nucl. Chem. 32: 3875,1970).

Example 23.

Wood pulp delignification using an equilibrated aqueous solution containing 0.32M Na5(+0.26)[SiMn1(-0.2)(H2O)W11O39].

This example demonstrates that equilibrated solutions containing the target heteropolyoxotungstates of the general formula prepared in the manner described above can be used to oxidize a substrate (for example, lignin in wood pulp).

The solution synthesized in Example 22 was then mixed with softwood Kraft pulp (Kappa 30, viscosity 30.5 mPa s) to a consistency of 3% and heated in the Parr reactor to 140°C under N2 for 2 hours. Using the Na5 (10. 26) [SiMnl (-0. 2) (H20) Wllo3g] resulted in a total reduction of 43% of the POM solution, a final pH of 4.0, and a pulp with a Kappa number of 6.8 and a viscosity of 9.1 mPa s.

Example 24.

Equilibrated aqueous solution of 0.1 MNa6 (+7) [AIMn (OH) W"O39].

The following is an example of the single-step synthesis of an equilibrated POM solution containing target polyoxotungstates of the general formula. The equilibrated solution was prepared by mixing hydroxides or neutral or anionic oxides of transition-metal or main-group elements and water and heating the mixtures to temperatures sufficiently high such that they reacted to give solutions containing target heteropolyoxotungstates of the general formula in thermodynamic equilibrium with additional compounds also derived from the starting materials. This example further demonstrates the generality of the present invention.

The correct rtion of starting compounds MnO2,WO3,NaoH, AI (OH) 3) were mixed in water in a stirred 50 mL Hastalloy Parr reactor, charged with 1800 kPa °2 and heated to 210°C for 3 hours to yield 30 mL of 0.1 M equilibrated Na6 (+7) [AIMn (OH) W"O39] solution. This brown solution had a pH of 9.0 and contained [AIMn (OH) W"039] 6- confirmed by UV-VIS spectroscopy (#max mn, # Mol-1 dm3 cm-1): 490 (107) (Reference Example 9g).