POLYOXOMETALATE DELIGNIFICATION AND BLEACHING

Abstract: A method for oxidative degradation of lignin and polysaccharide fragments dissolved during polyoxometalate delignification or bleaching of wood pulp, wood fiber or pulp or fiber obtained from a non-woody plant. The method comprises the steps of obtaining a spent polyoxometalate bleaching solution containing a polyoxometalate of the formula [V1MomWnNboTap(TM)qXrOs]<x-> where 1 is 0-18, m is 0-40, n is 0-40, o is 0-10, p is 0-10, q is 0-9, r is 0-6, TM is a d-electron-containing transition metal ion, and x is a heteroatom; which is p or d block element, provided that 1 + m + n + o + p >/= 4, 1 + m + q > 0 and s is sufficiently large that x > 0, and heating the solution in the presence of an oxidant under conditions wherein the dissolved organic compounds are oxidatively degraded to volatile organic compounds and water. The invention is further based upon the use of less caustic and less corrosive non-vanadium containing polyoxometalates for delignification or bleaching of wood pulp, wood fiber, or pulp or fiber obtained from a non-woody plant.

Description: POLYOXOMETALATE DELIGNIFICATION AND BLEACHING Field of the Invention The invention concerns the use of transition metal-derived agents in the delignification of wood or wood pulp and in the oxidative degradation of water soluble lignin and polysaccharide fragments. Specifically, the field of the invention is the use of non-vanadium containing polyoxometalates in the delignification or bleaching of wood pulp, and in the use of a variety of polyoxometalates and oxygen in the oxidative degradation of krafts lignin and polysaccharide fragments solubilized during polyoxometalate delignification or bleaching. Background Of The Invention The transition of a tree into paper involves several discrete stages. Stage one is the debarking of the tree and the conversion of the tree into wood chips. Stage two is the conversion of wood chips into pulp. This conversion may be by either mechanical or chemical means. Bleaching is the third stage. Delignification is the first step in the bleaching of chemical pulps. Lignin, a complex polymer derived from aromatic alcohols, is one of the main constituents of wood. During the early stages of bleaching, residual lignin, which constitutes 3-6% of the pulp, is removed. Currently, this is typically done by treatment of the pulp with elemental chlorine at low pH, followed by extraction with hot alkali. Once a significant portion of the residual lignin has been removed, the pulp may be whitened, by a variety of means, to high brightness. Chlorine dioxide is commonly used in the brightening step. Although chlorine compounds are effective and relatively inexpensive, their use in pulp mills results in the generation and release of chlorinated organic materials, including dioxins, into rivers and streams. Due to increasing regulatory pressures and consumer demand, new, non-chlorine bleaching technologies are urgently needed by manufacturers of paper-grade chemical pulps. Chlorine-Free Bleaching usinσ Polyoxometalates Polyoxometalates are discrete polymeric structures that form spontaneously when simple oxides of vanadium, niobium, tantalum, molybdenum or tungsten are combined under the appropriate conditions in water (Pope, M. T. Heteropoly and isopoly Oxo etalates Springer-Verlag, Berlin, 1983). In a great majority of polyoxometalates, the transition metals are in the d° electronic configuration which dictates both high resistance to oxidative degradation and an ability to oxidize other materials such as lignin. The principal transition metal ions that form polyoxometalates are tungsten(VI) , molybdenum(VI) , vanadium(V) , niobium(V) and tantalum(V) . Isopolyoxo etalates, the simplest of the polyoxometalates, are binary oxides of the formula [M n 0 y ] p" , where m may vary from two to over 30. For example, if m = 2 and M = Mo, then the formula is [Mo 2 0 7 ] 2 -; if m = 6, then [Mo 6 0 19 ] 2 -; and if m = 36, then [Mo 36 0 lla ] 8 ". Polyoxometalates, in either acid or salt forms, are water soluble. Heteropolyoxometalates have the general formula [X ^ M.O y ] " '- and possess a heteroatom, X, at their center. For example, in the α-Keggin structure, α-[P 12 0 40 ] 3" , X is a phosphorus atom: The central phosphorus atom is surrounded by twelve 0 6 octahedra. Removal of a (M=0) 4+ moiety from the surface of the α- Keggin structure α-[PM 12 0 40 ] 3" , where M is molybdenum or tungsten, creates the "lacunary" α-Keggin anion, α-[ RVL^O^] 7" . The lacunary α-Keggin ion acts as a pentadentate ligand for redox active transition-metal ions, such as vanadium(+5) in a-[PV ..θ ω ] " . Further substitution is also possible, giving anions of the form [X x M , n M n O y ] p" , such as - [ PV^>lo 10 0 40 ] s~ . In place of vanadiu (+5) , d-electron-containing redox active transition-metal ions (TM) , may also be used, giving complexes such as α-[SiMn(III) (H 2 0) 1 -0 39 ] 5" , which contains a manganese(III) ion. While stabilizing the active metal ions in solution and controlling their reactivity, the heteropolyanions are highly resistant to oxidative degradation (Hill, et a_l. , J. Am. Chem. Soc . 108:536-538, 1986). Despite the conventional innate preference for vanadium compounds, it has now been found that polyoxometalates containing manganese and other d-electron- containing transition metal-ions are preferable to the use of vanadium substituted ions in that: 1) in the event of accidental release into the environment many of these alternative ions are less toxic than vanadium ions; 2) these alternative compounds are less caustic or corrosive when compared to vanadium ions to kraft recovery boilers and related equipment; 3) many alternative ions are much less expensive than vanadium at the same level of effectiveness, and 4) many alternative ions, such as manganese, iron, cobalt and nickel, are more readily available than vanadium. Effluent Free Mill. During delignification or bleaching, whether by chlorine, chlorine dioxide, oxygen, hydrogen peroxide, ozone, or other methods, lignin and polysaccharide fragments are liberated as water-soluble organic compounds. After delignification or bleaching, these compounds remain dissolved in the liquor. At present, water soluble lignin and polysaccharide fragments removed from wood pulps during bleaching are generally treated in biological waste-treatment ponds prior to their release to rivers and streams. Unfortunately, biological remediation fails to remove or to sufficiently degrade all of the dissolved organic materials present. As a result, potentially harmful organic compounds, particularly those generated during chlorine bleaching, are released into the environment. Because some of the materials that survive the biological waste treatment may have deleterious environmental effects, there is a need for alternative and more effective methods for degrading these materials. Many in the U.S. pulp and paper industry expect that the release of any organic waste (other than carbon dioxide) into the environment will eventually be banned altogether. There is thus an additional need for the development of a "closed" bleach mill from which few or no chemical waste-products, other than carbon dioxide and water, are released (Pulp and Paper Mill of the Future — An Information Exchange, U.S. Department of Energy, Office of Industrial Technologies, Orono, ME, September 8-10, 1993). As discribed below, polyoxometalates are employed as reusable oxidizing agents or catalysts for selective bleaching of wood pulps. As reusable agents, the polyoxometalates are suitable for repeated use in a closed mill. During polyoxometalate bleaching, however, residual kraft lignin fragments, and some polysaccharide fragments, are dissolved by the polyoxometalate bleaching liquor. What is needed in the art of polyoxometalate bleaching is a method for achieving mill closure by removing dissolved lignin and polysaccharide fragments from the bleaching liquor. Summary Of The Invention It is an object of the invention to provide an improved catalyst and process for the oxidative degradation of dissolved lignin and polysaccharide fragments. It is an additional object of the invention to delignify wood fibers or other lignocellulosic fibers using a polyoxometalate. It is an additional object of the invention to employ an oxidant in the bleaching of pulp that may be regenerated by reoxidation of its reduced form. In a first aspect of the invention, a vanadium-free polyoxometalate of formula (A) defined below is used as a delignification and bleaching agent. At least one metal of the polyoxometalate must be sufficiently active to oxidize functional groups within lignin, residual lignin, and other chromophores of wood, wood pulp and other lignocellulosic fibers and pulp. The success of these polyoxometalates demonstrates that effective bleaching agents might be prepared by inclusion of a variety of d- electron-containing and other redox-active metal ions in the polyoxometalate structure. In addition, in the oxidative degradation of lignin and polysaccharide fragments solubilized during polyoxometalate bleaching of wood pulp vanadium(+4 or +5)- substituted, molybdenum(+5 or +6)-substitute , and other transition metal-substituted polyoxometalates are used as catalysts. A second aspect of the invention concerns a method of delignifying pulp comprising the steps of obtaining a wood pulp; exposing the wood pulp to a compound of the general formula (B) defined below, wherein the polyoxometalate is reduced and lignin and polysaccharide fragments within the pulp are dissolved; and then oxidizing the reduced polyoxometalate, under conditions capable of oxidatively degrading dissolved lignin and polysaccharide fragments, comprising the first steps of obtaining a pulp, and exposing the pulp to a polyoxometalate of the preferred formula, wherein the polyoxometalate is reduced and the reduced polyoxometalate bleaching liquor is exposed to an oxidant under conditions wherein the dissolved lignin and polysaccharide fragments are oxidatively degraded. Preferably, the polyoxometalate is oxidized and the resultant liquor is thus available for reuse in bleaching. The invention also concerns a method of using a polyoxometalate of the general formula (B) as a catalyst in the oxidative degradation of lignin and polysaccharide fragments solubilized during polyoxometalate delignification or bleaching of wood fibers to isolable low molecular weight compounds. Preferably, the degradation results in volatile organic materials, including carbon dioxide, and water. Preferably, the dissolved lignin and polysaccharide fragments are oxidatively degraded with air, oxygen hydrogen peroxide or other organic or inorganic peroxides (free acid or salt forms), or ozone which are more environmentally friendly than chlorine compounds. The oxidative degradation of dissolved lignin and polysaccharide fragments may be carried out prior to, simultaneously with, or after oxidative regeneration of the polyoxometalate bleaching agent. The polyoxometalate compound may be used as an oxidant in a repeated bleaching sequence. Other features, objects and advantages of the invention will become apparent upon examination of the specification, claims and drawings. Description Of The Figures Figs. 1A, B and C are polyhedral illustrations of three representative polyoxometalates. The light shaded octahedra are W VI ions and each polyhedron vertex is an O atom. Tetrahedral X0 4 units, where X is a main group or transition metal ion, are internal to all 3 structures. Fig. 1A is a Keggin structure, [XW 12 0 4O ] x " (the charge, x, depends on the heteroatom, X, shown in dark shading in the center of the structure). A transition metal- substituted Keggin anion is obtained when one of the twelve tunsgsten atoms is replaced by a d-electron-containing transition metal ion. Fig. IB is a trivacant Keggin derived sandwich complex, [(M II ) 2 (M II L) 2 (PW 9 0 34 ) 2 ] 1 °- and Fig. 1C is a trivacant Wells-Dawson derived sandwich complex, [ (M II ) 2 (M II L) 2 (P 2 W 15 0 S6 ) 2 ] 16 ", where M represent d-electron-containing transition metal ions (dark shaded octahedra) and L is an exchangeable ligand. Fig. 2 is a flow diagram for a closed mill polyoxometalate bleaching process including a bleaching reactor (Unit Operation A) and a reactor for wet oxidation of organics and oxidative regeneration of polyoxometalate bleaching agents (Unit Operation D). Fig. 3 is a plot of the ratios of integrated areas of the FT Raman bands at 1595 cm" 1 against those between 1216-1010 cm -1 for pulp samples removed after each stage M 1# M 2 , M 3 and E of the M X M 2 M 3 E bleaching sequence of Example 1, and from a pulp sample examined after completion of the entire Δ-Δ 2 Δ 3 E control sequence. The numbers at the bottom of the figure correspond to the four stages of the bleaching reaction with unity reserved for the unbleached pulp. The pulp sample examined after completion of the entire ΔiΔ 2 Δ 3 E control sequence is represented by an "x". Fig. 4 is a plot of COD values measured after successive cycles of polyoxometalate bleaching and wet oxidation. A least squares fit of the wet oxidation (COD) data was calculated using a mathematical model that assumes exponential decay to an asymptotic value. Detailed Description Of The Invention The invention concerns a method for removing substantial quantities of lignin from wood fibers or wood pulp and a method for oxidatively degrading lignin and polysaccharide fragments, dissolved during polyoxometalate delignification or bleaching of wood fibers or wood pulp, to volatile organic compounds and- water. Wood Pulp. The first step in the invention is the production of a wood pulp. Wood pulps may be produced by any conventional method, including both kraft and non-kraft pulps. Suitable pulp production methods are described in "Pulp and Paper Manufacture," 2nd Edition, Volume I, The Pulping of Wood. R.G. Macdonald and J.N. Franklin Eds., McGraw-Hill Book Company, New York, 1969. Wood pulps are generally divided into softwood pulps (e.g., pine pulps) and hardwood pulps (e.g., aspen pulps). Softwood pulp is the most difficult to delignify because lignin is more abundant in softwoods than in hardwoods. Due to structural differences, largely attributable to the lower average number of methoxy groups per phenyl ring, softwood lignin is less susceptible to oxidative degradation. The Examples below describe the efficiency of the method of the invention with softwood kraft pulp. However, the invention is suitable for delignification of hardwood pulps also. Another class of pulps for which the invention is suitable is that derived from non-woody plants such as sugar cane, kenaf, esparto grass, and straw, as well as plants producing bast fibers. The lignocellulosic constituents of such plants are usually susceptible to the same pulping methods as are applicable to wood, though in many instances they require less severe conditions than wood. The resulting pulps are usually less difficult to delignify or bleach than are those derived from softwoods by the kraft process. Closed Polyoxometalate Bleaching System The next step of the invention is the exposure of the pulp to a polyoxometalate. Polyoxometalates suitable for the invention may be applied as stoichiometric oxidants, much as chlorine and chlorine dioxide are cμrrently. The general formula (A) of the preferred vanadium-free polyoxometalate is [Mo B W n Nb 0 Ta p (TM) q X r O ε ] " - (A) where m is 0-40, n is 0-40, o is 0-10, p is 0-10, q is 0-9, r is 0-6, TM is a d-electron-containing transition metal ion, and X is a heteroatom, which is a p or d block element, provided that m + n + o + p > 4, m + q > 0, and s is sufficiently large that x > 0. X is typically Zn 2+ , Co 2+ , B 3+ , Al 3+ , Si 4+ , Ge 4+ , P 5+ , As 5+ or S 6+ . Preferably, the polyoxometalate used in the invention is one of five different formulas that are subsets of the general formula: Formula 1 , the transition metal-substituted Keggin structure, is [Mo-,W n (TM) o X p 0_] x" , where TM is any d-electron-containing transition metal ion, X is a heteroatom, which is a p or d block element, m + n + o = 12, p = l, o < 4 and m + o > 0. α-K 5 [SiMn(II) (H 2 0)W ι:ι 0 39 ] (compound 1) is an example of a potassium salt of this structure. Formula 2, the transition metal-bridged dimer of the Keggin structure, is [Mo n W n (TM) 0 X p O q ] x~ , where TM is any d-electron- containing transition metal ion, X is a heteroatom, which is a p or d block element, m + n + o = 22, o is 1-4 and p = 2. Formula 3, the transition metal-substituted Wells-Dawson structure, is [Mo n W n (TM) o X p 0 q ] *" , where TM is any d-electron- containing transition metal ion, X is either P s+ , As 5+ , or S 6+ , m + n + o = 18, o < 6, p = 2 and m + o > 0. Formula 4, the transition metal-bridged dimer of the tri¬ vacant Wells-Dawson structure, is [Mo B W n (TM) 4 X p 0 q ] x" , where TM is any d-electron-containing transition metal ion, X is either P 5+ , As 5+ or S 6+ , m + n = 30 and p = 4. Formula 5, the transition metal-substituted Preyssler structure, is [Mo n W n (TM) 0 P s C p Na q O r ]- " , where TM is any d-electron-containing transition metal ion, C is a di- or tri- valent main group, transition metal or lanthanide cation located in the center of the structure, m + n + o = 30, p + q = l and m + o > 0. A common feature of the structures described in the formulas above is the presence of a molybdenum ion in its +6 d° electronic configuration or of a d-electron-containing transition metal ion capable of reversible oxidation and that in one of its oxidation states is sufficiently active so as to oxidatively degrade lignin in wood or wood pulp leading to delignification and bleaching. This can occur via direct lignin oxidation by the d-electron- containing transition metal ion or molybdenum(+6) ion, leading to reversible reduction of the transition metal or molybdenum ion. In a subsequent step, the reduced polyoxometalate bleaching agent is regenerated to its active form by reaction with a chlorine-free oxidant such as oxygen, peroxide or ozone. Alternatively, the polyoxometalate complex can react with pulp in the presence of the chlorine-free oxidant. In either case, it is essential that a d- electron-containing transition metal or molybdenum(+6) 'ion be present in the polyoxometalate structure. The structures defined by the above formulas are all logical candidates for use in bleaching with chlorine-free oxidants because they all possess either d-electron-containing transition metal or molybdenum(+6) ions. Compound 1 of Formula 1 was chosen for the bleaching Examples given below because it is a well studied polyoxometalate and simple to prepare (Tourne, C. M. , et al. Journal of Inorganic and Nuclear Chemistry, 32:3875-3890, 1970). Formula 2 describes dimeric derivatives of compounds of Formula 1 (Finke, R. G. , et al. , Inorganic Chemistry , 26:3886-3896, 1987; Khenkin, A. M. , et a_l. , in The Activation of Dioxygen and Homogeneous Catalytic Oxidation. Barton, D. H. R. , ed. , Plenum Press, New York, 1993, 463; Gό ez-Garcia, C. J. , __, __.. , Inorganic Chemistry, 32:3378-3381, 1993; Tourne, G. F. , et. __.. , J. Chem. Soc , Dalton Trans . 1991, 143-155). Some of these derivatives are particularly well-suited for use in bleaching because they exhibit remarkably high selectivities and possess extremely high stabilities. Compounds of Formula 3 are structurally closely analogous to those of Formula 1, very similar in reactivity, and significantly more stable (Lyon, D. K. , _____ _____ , Journal of the American Chemical Society, 113:7209-7221, 1991). Compounds of Formula 4 are dimeric derivatives of those defined by Formula 3 (Finke, R. G., et al. , Inorganic Chemistry, 26:3886-3896, 1987; Khenkin, A. M. , et al. , in The Activation of Dioxygen and Homogeneous Catalytic Oxidation,. Barton, D. H. R. , ed. , Plenum Press, New York, 1993, 463). In the case of Formula 5, a number of main group-ion and lanthanide-ion derivatives, and one vanadium-ion-substituted structure, have been prepared and characterized (Creaser, I., et al. , Inorganic Chemistry, 32:1573-1578, 1993). The vanadium- substituted structure contains vanadium(+5) in place of one of the structural tungsten atoms. By analogy with the well-established syntheses of structures of Formulas 1 and 3, it is logical that, in addition to vanadium(+5) substitution, molybdenum(+6) or d- electron-containing transition metal ions could also be substituted in place of a structural tungsten atom. Based on the criteria outlined immediately following the introduction of Formulas 1-5 above, these complexes would be effective in bleaching.' Such derivatives of Formula 5 are likely to be extremely stable and thus particularly useful for commercial applications. Fig. 1 is a polyhedral illustration of three representative polyoxometalates of the formulas [XW 12 0 4O ] x" , [ (M II ) 2 (M II L) 2 (PW 9 0 34 ) 2 ] " °-, and [ (M- I ) 2 (M II L) 2 (P 2 W ls o se ) 2 3- 6 . The polyoxometalate of the invention is typically in an acid, salt or acid-salt form. Suitable cations for salt formation are Li + , Na + , K + , Cs + , NH 4 + and (CH 3 ) 4 N + which may be replaced in part (acid-salt form) or in full (acid form) by protons (H + ) . For example, compound 1 is in salt form and has potassium counter ions. The listed cations are sensible choices, but there are others that are available and cost-effective. Polyoxometalate salts are generally water soluble (hydrophilic) . However, hydrophobic forms can be made easily and are suitable for use in selective bleaching with solvents other than water. Some cations suitable for formation of hydrophobic forms are defined in U.S. patent 4,864,041 (Hill). An attractive feature of polyoxometalates is that they are reversible oxidants and, thus, could function as mediating elements in a closed-loop bleaching system in which used polyoxometalate solutions are regenerated by treatment with chlorine-free oxidants. Accordingly, the invention involves oxidative degradation of lignin in pulp by a polyoxometalate and regeneration of the polyoxometalate with chlorine-free oxidants. In the first step (eq. 1), mixtures of water, pulp and a fully oxidized polyoxometalate (P ox ), are heated. During the reaction, the polyoxometalate is reduced as the lignin-derived material within the pulp is oxidized. The reduced polyoxometalate (P rβd ) must be re-oxidized before it can be used again. This is done by treating the polyoxometalate solution with chlorine-free oxidants such as air, dioxygen, hydrogen peroxide and other organic or inorganic peroxides (free acid or salt forms), or ozone (eq. 2). Alternatively, reoxidation (eq. 2) could be performed at the same time as reduction (eq. 1). Pulp + P ox > Prβd + 0 2 + 4H + In addition to equations (1) and (2), a foreseeably 'useful method for using polyoxometalates as catalytic agents in delignification and bleaching would be to introduce a chemically- derived mediating agent. Such an agent would be chosen for its ability to selectively transfer electrons from specific functional groups in the lignin polymer to the polyoxometalate. For example, a thiol derivative mediating agent could be used, but many others are available and potentially usef l. Thiols, for example, are known to react with polyoxometalates under mild conditions, reducing the polyoxometalate and generating thiyl radicals. Thiyl radicals are known to selectively oxidize lignin at benzylic positions, a reaction known to result in fragmentation of lignin model compounds (Wariishi, et al., J. Biol . Chem. , 264:14185-14191, 1989) . Such an improvement on the invention might make the process more economical by allowing for significant reductions in the amount of polyoxometalate required for bleaching, and by allowing for simultaneous use of dioxygen and polyoxometalate under conditions mild enough to more easily avoid oxygen-radical degradation of cellulose fibers. A flow diagram of a typical, preferred polyoxometalate bleaching process is shown in Fig. 2. Unbleached kraft pulp, referred to as brownstock, is exposed to an aqueous polyoxometalate bleaching liquor (Unit Operation A) . During the bleaching reaction (Unit Operation A) , the polyoxemetalate is reduced (eq. 1) and residual lignin fragments and some polysaccharide fragments are solubilized and remain in the reduced (spent) bleaching liquor. After leaving the bleaching reactor, the pulp is concentrated to a preferable consistency of 30% solids, removing approximately 95% of the polyoxometalate laden liquor. The pulp then passes to a washing stage (Unit Operation B) . Although a washer is indicated in Fig. 2, high efficiency washers, such as diffusion washers, may be preferable. Preliminary washing studies demonstrate that the polyoxometalates are not adsorbed onto pulp fibers. This is a critically important result. It means that, unlike removal of caustic, removal of polyoxometalate from the pulp is controlled by diffusion phenomena alone, and that there is no adsorption limit. The polyoxometalates are negatively charged ions that should not normally bind to cellulose, which is also negatively charged. Still referring to Fig. 2, the wash water might be recycled by evaporation using heat provided by low grade steam. The concentrated liquor is then treated by a separation technology (Unit Operation C) to remove inorganic salts, such as those of manganese, iron and calcium, carried in with the pulp. Several separation technologies, using crystallization, ion-exchange columns or selective membranes may be appropriate here (McCabe, W. L- , et «__1. , Unit Operations of Chemical Engineering r McGraw-Hill, New York, 1985). We anticipate that some polyoxometalate will be removed at this or a separate point and re-refined. Still referring to Fig. 2, the spent liquor from both the reactor and evaporators, still containing polysaccharide and lignin fragments previously associated with the pulp, is then passed to a regeneration unit (Unit Operation D) . One purpose of this Unit Operation is to reoxidize the polyoxometalate to its active form (eq. 2). A second function is to oxidatively degrade dissolved lignin and polysaccharide fragments to volatile organic materials, carbon dioxide, and water (wet oxidation of the dissolved organic compounds) . Relationship Between Wet Oxidation and Polyoxometalate Regeneration. Polyoxometalate treatment involves two steps: Polyoxometalate bleaching (equation 1) , here described as Unit Operation A, and oxidative regeneration of the reduced polyoxometalates (equation 2), here referred to as Unit Operation D. The invention further expands Unit Operation D to include the use of polyoxometalates to catalyze the oxidative degradation (wet oxidation) of dissolved lignin and polysaccharide fragments either prior to, simultaneously with, or after the second step (eq. 2). The object here is not only the oxidative regeneration of the polyoxometalate to its bleaching-active form, but, in addition, the polyoxometalate catalyzed oxidative degradation (wet oxidation) of the dissolved lignin and polysaccharide fragments to volatile organic compounds, including carbon dioxide, and water. The wet oxidation of the lignin and polysaccharide fragments may be carried out simultaneously with the second step (eq. 2). However, the wet oxidation generally requires more severe conditions and longer reaction times than those required for catalyst regeneration (eq. 2) alone. It should be noted as well that significant wet oxidation would not likely occur under the conditions necessary for simply reoxidizing reduced polyoxometalates. However, wet oxidation, a method for removal of solubilized lignin and polysaccharide fragments is essential for mill closure. The invention is a method for achieving mill closure with the reusable polyoxometalate bleaching agents. Mill closure using polyoxometalate bleaching agents could be achieved by oxidative consumption (wet oxidation) of dissolved organic materials prior to, simultaneously with, or after oxidative regeneration of the reusable polyoxometalate bleaching agent. Effective removal of dissolved organic materials, the subject of the invention, does not require polyoxometalate catalyzed wet oxidation of the organic materials completely to carbon dioxide. What is required is that the dissolved organic materials are degraded to isolable low molecular weight compounds or to volatile compounds. These compounds might be fed into the kraft liquor recovery furnace to generate heat, and there converted to carbon dioxide, or collected by separation or condensation and used as a chemical feedstock. Polyoxometalates suitable for wet oxidation include all those suggested for use in bleaching, and additionally, vanadium containing compounds. For wet oxidation, unlike in bleaching, these polyoxometalates may either be in their fully oxidized or reversibly reduced forms. Although the polyoxometalates act with high selectivity in the bleaching reaction with pulp, the conditions in the wet oxidation unit will be significantly more aggressive. Under these conditions, the polyoxometalates act as catalysts for, and initiators of, the aerobic oxidation and autoxidation of dissolved organic materials. This is where the remarkable thermal stability and resistance to oxidative degradation of the polyoxometalates are used to their fullest advantage. The polyoxometalates are stable under conditions wherein even very robust synthetic metalloporphyrins (Dolphin, D. H., et al., US Patents 4,892,941 and 5,077,394) are susceptible to oxidative degradation. The general formula (B) of the preferred polyoxometalate for use in wet oxidation is [V 1 Mo n W n Nb o Ta p (TM) q X r 0 s ;r (B) where 1 is 0-18, m is 0-40, n is 0-40, o is 0-10, p is 0-1O, q is 0-9, r is 0-6, TM is a d-electron-containing transition metal ion, and X is a heteroatom, which is a p or d block element, provided that l + m + n + o + p > 4, l + m + q > 0, and s is sufficiently large that x > 0. X is typically Zn 2+ , Co 2+ , B 3+ , Al 3+ , Si 4+ , Ge* + , P 5+ , As 5+ or S 6+ . Preferably, the polyoxometalate used in wet oxidation is one of eight different formulas that are subsets of the general formula (B): Formula 1, the transition metal-substituted Keggin structure, is [V 1 Mo n W n (TM) 0 X p O < -] x ", where TM is any d-electron-containing transition metal ion, X is a heteroatom, which is a p or d block element, l + m + n + o = 12, p = l, o < 4 and 1 + m + o > 0. Formula 2, the transition metal-bridged dimer of the Keggin structure, is { ~ T 1 l ' o n Vl n ( ~~ ) o X p 0 g V ~ , where TM is any d-electron- containing transition metal ion, X is a heteroatom, which is a p or d block element, l + m + n + o = 22, 1 + o is 1-4 and p = 2. Formula 3, the transition metal-substituted Wells-Dawson structure, is [V 1 Mo B W n (TM) o X p 0 q ] x_ , where TM is any d-electron- containing transition metal ion, X is either P 5+ , As 5+ , or S 6+ , 1 + m + n + o = 18, o < 6, p = 2 and 1 + m + o > 0. Formula 4, the transition metal-bridged dimer of the Wells- Dawson structure, is [Mo m W n (TM) 4 X p O q ] x ", where TM is any d-electron- containing transition metal ion, X is either P 5+ , As s+ or S 6+ , m + n = 30 and p = 4. Formula 5, the transition metal-substituted Preyssler structure, is [V 1 Mo B W n (TM) c ,P_.CpNa q O r ~x~ , where TM is any d-electron- containing transition metal ion, C is a di- or tri-valent d, p, or f block cation located in the center of the structure, 1 + m + n + o = 30, p + q = l and 1 + m + o > 0. Formula 6, an isopolyvanadate, is [V n 0 r ] x" , where n > 4, r _> 12 and x = 2r-5n. Na 6 [V lo 0 28 ] is an example of a sodium salt of a polyoxometalate of this formula. Formula 7, a mixed-addendum Keggin structure, is [V n Mo-W o (MG) p (TM) ~ 0 r ] x" , where TM is any transition metal, MG is a main group ion, 1 < n < 8, n + m + o < 12 and p + q < 4. H 5 [PV 2 Mo lo 0,. o ] , compound 3, is an example of an acid of this formula. Na 4 [PVW lx 0 40 ] is an example of a sodium salt. Formula 8, a Wells-Dawson structure, is [V n Mo B W-(MG) p 0 r ] x" where MG is either P 5+ , As 5+ , or S 6+ , 1 < n < 9, n + m + o = 18, and p = 2. H 9 [P 2 V 3 W 1S 0 62 ] is an example of an acid of this structure. Preferred Formulas 1-8 are related to one another and to the general formula (B) in much the same way that the preferred formulas described above in relation to the general formula (A) are related to one another and to the general formula (A) . Oxidation State of the Polyoxometalate Anion Upon Initiation of Wet Oxidation. The criteria for polyoxometalate structures useful in anaerobic bleaching are that the complexes include vanadium ions in their highest, +5 d° electronic configurations, molybdenum ions in their highest +6 d° electronic configurations, or d-electron-containing transition metal ions that possess sufficiently positive reduction potentials, and that may be reversibly reduced. During use in anaerobic pulping or bleaching, a significant quantity of the polyoxometalate in question is reduced. The amount of polyoxometalate reduced will vary with conditions and with the nature of the lignocellulosic substrate. If oxygen or another oxidant is present in the bleaching reactor, the amount of reduced polyoxometalate emerging from the bleaching reactor might be significantly lower. Thus, in practice, used polyoxometalate bleaching liquors are likely to contain a mixture of oxidized and reduced complexes. The percentage of reduced polyoxometalate could vary from 0 to 100%. However, because the aerobic wet oxidation stage is catalytic and the polyoxometalate operates under turnover conditions, both reduced and fully oxidized forms of the polyoxometalate will be effective. This is demonstrated in Examples 10 and 11, below. At the elevated temperatures used during wet oxidation, fully oxidized polyoxometalate complexes may be expected to oxidize functional groups present within dissolved residual lignin and polysaccharide fragments. In the process, some percentage of the polyoxometalate present is reduced. Thus, even if the spent liquor entering the wet oxidation reactor initially contains only fully oxidized polyoxometalate complexes, some reduced polyoxometalate will be generated rapidly. The fate of these reduced species will be identical to that of the reduced species that might enter the wet oxidation reactor as a component of the spent bleaching liquor. In the presence of dioxygen gas, the reduced forms of the polyoxometalates are oxidized, generating hydroxyl and other oxygen-centered radicals, and hydrogen peroxide. These might then react with the organic compounds dissolved in the bleaching liquor. In addition, dioxygen can react directly with organic radicals generated either by reaction of the organic compound with the oxidized form of a polyoxometalate, or by reaction with an oxygen- centered radical. Thus, because the reduced forms of the polyoxometalates can be oxidized with oxygen producing additional oxygen-based oxidizing species, the reduced forms of the polyoxometalates described by the general formula are also useful in the invention. In addition, the reduced forms of the polyoxometalates can provide the added benefit of accelerating the initiation of radical-chain autoxidation of the dissolved lignin and polysaccharide fragments. Compound 3 of Formula 7, a subset of Formula 1 (Kozhevnikov, I. V., et a_L. , Russian Chemical Reviews , 51:1075-1088, 1982), was chosen for the wet oxidation Examples given below because it has been thoroughly studied and is simple to prepare. Using 31 P nuclear magnetic resonance (NMR) spectroscopy, we have observed that this compound, prepared according to the most widely cited procedure and originally described as having the composition H s [PV 2 Mo 10 0 40 ] (Tsigdinos, G. A., et al. , Inorganic Chemistry, 7:437-441, 1968), is actually a mixture of H 4 [PV_Mo_._0 4C ,] , H_.[PV 2 Mo_.o0 40 ] and H 6 [PV 3 Mo 9 0 40 ] , the latter two existing as mixtures of positional isomers, all still of Formula 1. In the invention, Compound 3 will thus refer to mixtures of these three compounds, in either acid or sodium-salt forms. Preferably, the oxidant used in the polyoxometalate catalyzed wet oxidation step will be air or dioxygen. Unlike degradative systems which use metalloporphyrins (Dolphin, D. H., et aJL. , US Patents 4,892,941 and 5,077,394) or simple transition metal salts or complexes (Huynh, V. B. , US Patent 4,773,966; Waldmann, H. , US Patents 4,321,143 and 4,294,703) which require the addition of costly organic or inorganic peroxides, extensive oxidative degradation of dissolved organic materials is achieved in the invention using oxygen alone. Nonetheless, any chlorine-free oxidant selected from the group consisting of air, dioxygen, hydrogen peroxide and other organic or inorganic peroxides (free acid or salt forms), or ozone might be useful in the invention. For example, small amounts of ozone might be used at the end* of wet oxidation to augment the catalytic oxygen treatment or to ensure complete polyoxometalate oxidation. An advantage of the polyoxometalates when compared to metalloporphyrins or other transition-metal complexes or salts, is that the polyoxometalates are uniquely stable to oxidative degradation under strongly oxidizing conditions, which include exposure to ozone. Bleaching: General Method. Aqueous polyoxometalate solutions, preferably 0.001 to 0.20 M, are prepared and the pH adjusted to 1.5 or higher. The polyoxometalate may be prepared by standard procedures. An organic or inorganic buffer may be added to maintain the pH within a desired range during the bleaching reaction. Pulp is added to the polyoxometalate solution to a preferable consistency of approximately 1-12%, although consistencies up to 20% may be useful. The mixture is heated either in the presence or absence of oxygen or other oxidants (M stage, "M" refers to a d-electron-containing transition metal substituted or a molybdenum(+6) substituted polyoxometalate). The temperature and duration of polyoxometalate treatment will depend upon variables such as the nature of the pulp, the pH of the polyoxometalate solution and the nature and concentration of the polyoxometalate. In the examples below, the reactions were run anaerobically, under nitrogen. Control experiments were carried out using identical conditions in parallel sequences, but with no added polyoxometalates. We call the control version of the M stage, in which no polyoxometalate was added, the Delta (Δ) stage. Sequential M stages are designated M 1# M 2 and M 3 . Sequential control stages are designated Δ- , Δ 2 and Δ 3 . The bleaching of. chemical pulps entails two inter-related phenomena: delignification and whitening. Once a significant amount of residual kraft lignin has been removed from a kraft pulp, the pulp becomes relatively easy to whiten by a number of means, including additional polyoxometalate treatment or treatment with hydrogen peroxide or other inorganic or organic peroxides. The effectiveness of the polyoxometalates in bleaching is demonstrated by their ability to delignify unbleached kraft pulp. It is understood, however, that to meet the requirements of specific grades of market pulp, additional polyoxometalate or other oxidative treatment, such as reaction with alkaline hydrogen peroxide, might be employed to achieve final pulp whitening. To oxidize the reduced polyoxometalate, the polyoxometalate solution may be separated from the pulp after the reaction is complete, and reoxidized. The oxidant is preferably air, dioxygen, a peroxide, or ozone. The pulps are washed with water and may be extracted for 1-3 hours at 60 - 85 °C in 1.0% NaOH (E stage). The cycle may be repeated in a MEME sequence, and may be followed by an alkaline hydrogen peroxide (P) stage. For the P stage, typically 30% aqueous hydrogen peroxide is added to a mixture of pulp and dilute alkali to give a final pH of approximately 9-11 and a consistency of 1-12%. The mixture is then heated for 1-2 hours at 60 - 85 °C. The quantity of hydrogen peroxide, defined as weight percent relative to the O.D. (oven dried) weight of the pulp may vary from 0.1-40%. To demonstrate the effectiveness of the d-electron containing- transition metal-substituted polyoxometalate, the amount of residual lignin remaining after the polyoxometalate treatment, and after subsequent alkaline extraction, was monitored. After each stage, the pulps were analyzed for lignin content both spectroscopically (FT Raman spectroscopy) and chemically (kappa numbers). Fiber quality was monitored by measuring the viscosities of pulp solutions according to TAPPI methods. Oxidation of a variety of d-electron-containing transition metal-substituted polyoxometalate complexes co their active oxidized forms can be accomplished using air, hydrogen peroxide or other peroxides (Tourne, C. M., et a_L. J. of Inorganic and Nuclear Chemistry, 32:3875-3890, 1970). The formation of active (oxidized) polyoxometalates can be monitored spectroscopically and titrametrically. A representative d-electron containing-transition metal- substituted polyoxometalate, α-K6[SiMn(II) (H 2 0)W 11 0 39 ] (compound 1) was evaluated. For activity in anaerobic bleaching, compound 1 must first be oxidized to α-K 5 [SiMn(III) (H 2 0)W_._0 39 ] (compound 2) by one electron oxidation at the manganese ion. Oxidation of compound 1 to compound 2 was accomplished with ozone, and the formation of compound 2 was monitored using UV-vis and FTIR spectroscopy, and by titration. In the bleaching of chemical pulps, the polyoxometalates react with lignin to solubilize it and to render it more susceptible to extraction with hot alkali. Since many pulping processes, including the kraft process, entail delignification brought about by cooking wood chips in hot alkali, we envision that polyoxometalates will be useful in commercial pulping because of the role that polyoxometalates play in the bleaching of kraft pulp. Thus, the invention also includes treating wood chips, wood fibers or wood meal or fibers or pulp from other lignocellulosic materials with polyoxometalates under conditions analogous to those used in the M stages of the bleaching process, and then pulping the chips or meal under alkaline conditions. The result is that greater reductions in lignin content are then found in polyoxometalate- treated wood or lignocellulosic material, than in wood or lignocellulosic material pulped under the same conditions, but with no polyoxometalate pre-treatment. Kappa numbers. Kappa numbers, obtained by permanganate oxidation of residual lignin, are 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. In the Examples, microkappa numbers were determined for each polyoxometalate treated pulp sample and for appropriate controls. The microkappa number determined for the unbleached kraft pulp used in the Example below was 33.6. Microkappa number determinations are used in Examples 1, 10 and 11 below to demonstrate that lignin- like material is effectively degraded or otherwise removed from the pulp during polyoxometalate bleaching. FT Raman spectroscopy. A published spectroscopic method (Weinstock, et a_L. , Proceedings of the 1993 TAPPI Pulping Conference; 1993 November 1-3; Atlanta, GA, 519-532.), using FT Raman spectroscopy, was used to monitor the oxidative degradation of residual lignin. FT Raman spectra of pulp samples were recorded using an RFS 100 Nd 3+ :YAG laser (1064 nm excitation) instrument, using a 180° reflective sample geometry. The bands observed in the FT Raman spectra of lignocellulosic materials correspond to both lignin and carbohydrate components of the pulp. Lignin content was calculated by measuring changes in the 1595 cm "1 band (1671-1545 cm 1 ), associated with one of the symmetric ring stretching modes of phenyl groups present in the residual lignin. The intensity of this band correlates well with the amount of residual lignin in the sample. Spectra acquired in all but the later stages of the process included substantial fluorescent backgrounds. Thus, for quantitative comparison, band areas were calculated as the peak above the baseline created by the fluorescence. For quantification, the band of interest must be compared to one that remains constant throughout the bleaching process. The cellulose band structure between 1216-1010 cm "1 was chosen for this purpose. Using these bands, changes in lignin content were quantified by measuring the ratios of integrated areas of the 1595 cm -1 bands against those of the band structure between 1216-1010 cm" 1 . In Example 1, FT Raman spectroscopy is used to demonstrate that phenyl groups, representing lignin, are effectively degraded or otherwise removed from the pulp during polyoxometalate bleaching. Selectivity and Pulp Viscosity. 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 during bleaching is one of several criteria indicating that cellulose fibers have not been cleaved or degraded during bleaching. In this regard, the relative rate of reaction of a bleaching agent with lignin vs. its rate of cleavage or degradation of cellulose fibers is referred to as the Selectivity of the agent. Bleaching agents highly selective for lignin are necessary for the commercial production of pulps that meet market specifications. In Example 2 below, it is demonstrated that d-electron-containing transition metal-substituted polyoxometalates are highly selective for lignin in bleaching. Before bleaching, the mixed-pine kraft pulp used in the Examples below had a viscosity (in solution with cupric sulfate and ethylene diamine according to TAPPI test method T230 om-89) of 34.2 mPa•s. Wet Oxidation: General Method. Aerobic, polyoxometalate- catalyzed wet oxidation of lignin and polysaccharide fragments dissolved in spent polyoxometalate bleaching liquors requires heating the spent liquor in the presence of oxygen. Key variables in the wet oxidation reaction are: concentration of dissolved oxygen, reaction temperature, reaction time, polyoxometalate concentration and pH. The concentration of dissolved oxygen is a function of its absolute pressure, temperature, the nature of the soluble ions present, ionic strength of the spent liquor and reaction rate. However, the rate and extent of the wet oxidation reaction will likely depend most heavily on three variables: oxygen pressure, temperature and time. As a result, only general limits may be assigned to any one of these parameters. Nonetheless, it is expected that absolute oxygen pressures of from 15 to 1000 pounds per square inch (psia) , reaction temperatures of from 100 to 400°C and reaction times of from 0.5 to 10 hours will encompass the most likely configurations of these variables. A preferable range of oxidation time is 1.0 to 5.0 hrs. Most preferably, the reaction time is 3.0 to 4.0 hrs. A preferable range of reaction temperature is between 125 °C and 225 °C. Most preferably, the reaction temperature is 150 °C. A preferable oxygen pressure during the heating step is 15 to 1000 psia. Most preferably, a pressure of approximately 100 psia is maintained. Polyoxometalate concentrations and pH values will likely be influenced by the requirements of the delignification and bleaching reactions. Nonetheless, dilution or concentration of spent bleaching liquors may be advantageous prior to the wet oxidation stage. However, because a buffer will probably be necessary for the bleaching reaction, the pH values encountered in the wet oxidation reactor are likely to be similar to those used in bleaching. Thus, for bleaching and wet oxidation in the continuous process, useful pH values are likely to range from one to 10. Preferably, pH values of between 1.5 and 3.5 will be obtained. Most preferably, pH values between 2.0 and 3.0 will be obtained. Polyoxometalate concentrations are likely to lie within an order of magnitude above or below those suggested above for use in bleaching. Thus, polyoxometalate concentrations of from 0.1 mM to 2.0 M are anticipated. The wet oxidation experiments described in the Examples below involved heating polyoxometalate solutions containing either model compounds, or spent polyoxometalate bleaching liquors,- to a temperature of 150 - 200 °C under 100 psia (pounds per square inch absolute pressure) of dioxygen gas for three to four hours in a glass lined, one liter, high pressure Parr reactor, which was fitted with a propeller for stirring. Total pressures, including those exerted by steam, were approximately 205 - 400 psia. COD values along with quantities of carbon dioxide generated as a result of wet oxidation, were then determined. The complex evaluated was a vanadomolybdophosphate , α-H 5 [PV 2 Mo lo 0_, 0 ] (compound 3, Formula 7), a representative of the α- Keggin structural class, Formula 1. To represent the lignin-like fragments and simple polysaccharides likely dissolved in spent bleaching liquors, two model compounds, veratryl alcohol (3,4- dimethoxybenzyl alcohol), a non-phenolic lignin model and D- glucose, a polysaccharide model, were used. In addition, a polyoxometalate bleaching liquor was prepared from compound 3, used to partially bleach a sample of kraft pulp and compared to solutions containing the lignin and polysaccharide model compounds. Stock solutions of model compounds were prepared by dissolving veratryl alcohol (200.3 mg/L) and D-glucose (375.1 mg/L) in purified water. Each stock solution had a theoretical COD value of 400 mg 0 2 /L. Measured COD values for these compounds were, for veratryl alcohol 405.4 ± 9.8 mg 0 2 /L and 416.1 ± 10.1 mg 0 2 /L (two different stock solutions) and 413.3 ± 10.1 mg 0 2 /L for D-glucose. Control experiments, without catalyst added, were performed on the undiluted stock solutions, adjusted to pH 3 by addition of cone, sulfuric acid. For experiments using compound 3 to catalyze the oxidation of model compounds, 0.048 M solutions of compound 3 were prepared by dissolving α-H 5 [PV 2 Mo lo 0 4O ].30H 2 0, 113.8 g/L, in the stock model compound solutions. The pH of each solution was adjusted to 3 using sodium bicarbonate. Final COD and C0 2 values were adjusted to account for the 5.0% increases in volumes that resulted from addition of compound 3 and sodium bicarbonate to the stock model compound solutions. At the end of each wet oxidation reaction, compound 3 was in its fully oxidized form and did not interfere with the COD determination. For experiments in which spent polyoxometalate bleaching liquors were subjected to wet oxidation conditions, a spent bleaching liquor was prepared by heating a sample of kraft pulp (microkappa number of 33.6) under nitrogen and with stirring, in a solution of α-H 5 [PV 2 Mo xo 0 4O ] (compound 3). At the end of the bleaching reaction, a significant portion of the vanadium(+5) in the solution had been reduced to vanadium(+4) . The concentration of reduced vanadium was determined titrametrically and subtracted from the COD values determined for an aliquot of the partially reduced, spent liquor. The spent liquor was then heated to 150 °C for four hours under 100 psia oxygen. At the end of this time, a second COD measurement was made to determine the extent of oxidation of the organic compounds present in the bleaching liquor. In one case, three bleaching-wet oxidation cycles were performed in succession to demonstrate how polyoxometalates might be employed as reusable agents in an effluent-free (closed) mill. At the end of the multi-cycle experiment, 31 P NMR was used to confirm that no degradation of the polyoxometalate bleaching agent/wet oxidation catalyst had occurred. After completion of each wet oxidation reaction, the Parr reactor was cooled to near room temperature and the headspace gases passed through a standard solution of barium hydroxide. The barium hydroxide solutions were located in a vertical glass chromatography column filled with glass beads. The headspace gases were introduced at the bottom of the column. During the course of the work, this method was altered to increase the efficiency of the reaction of C0 2 with barium hydroxide and to decrease the uncertainty present in calculated values (see the last two entries in Table 2, below). Instead of glass beads, a foaming agent (isopropanol, 2% by volume) was added and the volume of the barium hydroxide solution reduced by approximately 85%. After initial release of headspace gases into the barium hydroxide column, the reactor was purged with purified nitrogen and the nitrogen stream routed through the barium hydroxide solution. COD measurements were performed using 50 mL aliquots of model compound solutions or bleaching liquors, both before and after wet oxidation. Necessary titrametric standards and blanks were obtained and updated as necessary to minimize error in the COD and C0 2 measurements. Two methods were used to quantify the degradation of dissolved organic compounds during the wet oxidation reaction. The first method involved measurement of the chemical oxygen demand (COD) of the solutions .Standard Methods for the Examination of Water and Wastewater f 16th Ed., Franson, M. H, Managing Ed., American Public Health Association, Washington, DC, 532-535, 1985). The second involved measurement of the amount of carbon dioxide evolved during wet oxidation (Mohlman, F. W. , et _al. , Industrial and Engineering Chemistry, 3:119-123, 1931). Chemical Oxygen Demand .COD) . Determination of COD entails combining a measured volume of the sample to be tested with a known quantity of the oxidant potassium dichromate (K 2 Cr 2 0 7 ) and adding concentrated sulfuric acid (cone. H 2 S0 4 ) in which has been dissolved a catalytic amount of silver sulfate (Ag 2 S0 4 ). The solution is then heated to reflux (approx. 150 °C) for two hours. During this time the organic compounds in the sample are oxidized, reducing the dichromate to chromium(III) ions. Afterwards, the amount of unreacted dichromate is determined by reductive titration using ferrous ammonium sulfate ( (NH 4 ) 2 FeS0 4 , FAS). The number of electron equivalents by which the original dichromate solution has been reduced and the organic compounds in the sample oxidized is then mathematically converted into units of milligrams of dioxygen per liter of sample (mg θ 2 /liter) , each dioxygen molecule representing four electron equivalents. In theory, the COD is the mass of dioxygen consumed if the organic compounds in the sample are completely oxidized by dioxygen to carbon dioxide and water. In practice, the COD is a measure of the degree to which dichromate is reduced under the conditions of the COD test. Thus, if the organic compounds present in the sample are not quantitatively oxidized to carbon dioxide during the COD test, the COD value determined will be less than theoretical. This means that a zero COD value does not necessarily imply the absence of dissolved organic materials. Fortunately, the degree to which a COD value may be equated with quantitative oxidation to carbon dioxide and water (mineralization) can be ascertained by comparing measured COD values to theoretically determined ones. These comparisons are available as published data or can be readily determined experimentally for any known compound. It was found experimentally that D-glucose and 1,3-dimethoxybenzyl alcohol (veratryl alcohol) are quantitatively oxidized to carbon dioxide and water during the COD test. It is reasonable to expect that the products of partial oxidation of these compounds will also be quantitatively mineralized during the COD test. Thus, in the Examples, where the model compounds D-glucose or 1,3-dimethoxybenzyl alcohol are used, COD values may reasonably be taken to represent the total concentration of reducing equivalents of organic carbon present. It follows that reductions in COD values, brought about by polyoxometalate catalyzed wet oxidation of these model compounds, are a valid measure of the extent to which the model compounds have been oxidized. For actual bleaching liquors containing uncharacterized mixtures of lignin and polysaccharide fragments, the conditions of the COD test are expected to convert most of the dissolved organic compounds to carbon dioxide and water. This expectation is supported as follows. First, the COD test is performed in hot concentrated acid, where cellulose and other polysaccharides are rapidly hydrolyzed to glucose and other simple sugars, all of which may be expected, like D-glucose, to be completely oxidized by acidic dichromate to carbon dioxide and water (mineralized) . This assumption is supported by published data demonstrating that D-glucose and cellulose are completely mineralized, and lactose, the β-D-galactoside of D-glucose, is mineralized to 97.4% of theoretical (Moore, A.W. , et _ . , Analytical Chemistry, 21:953-957, 1949). Secondly, based upon the structure of lignin, and proposed structures of residual kraft lignin, dissolved lignin fragments are expected to contain oxygenated, substituted aromatic compounds, e.g. , possessing substituents such as hydroxyl and methoxyl groups, α-alcohols, α-ketones and α-acids. Furthermore, by virtue of their water solubility, the lignin fragments dissolved in spent bleaching liquors undoubtedly possess these polar functional groups. Based upon published data (Moore, A.W. , et a_L. , Analytical Chemistry, 23:1297-1300, 1951 and ibid., 35:1064-1067, 1963) these substituted lignin fragments, along with simpler aliphatic alcohols and organic acids that might be present, should be quantitatively, or nearly quantitatively, oxidized to carbon dioxide and water during the COD determination. One possible complication, however, is that the degree of acid dichromate mineralization of condensed aromatic structures, i.e., those containing carbon-carbon bonds between the carbon atoms that make up the C 6 units of aromatic rings, have not, to our knowledge, been reported. Because of this, and given uncertainty as to the precise composition of the various organic compounds present in spent bleaching liquors, care must be taken in equating COD values with the concentration of reducing equivalents of organic carbon present in spent bleaching liquor samples. Quantification of Evolved Gases. After completion of the wet oxidation reaction, gases evolved during the reaction were analyzed for carbon dioxide by adaptation of a published method (Mohlman, F. W. , et ~ _L. , Industrial and Engineering Chemistry, 3:119-123, 1931). This test involved passing the gas contained in the head space of the high pressure wet oxidation reactor through a standard barium hydroxide solution. As each equivalent of carbon dioxide consumes two equivalents of hydroxide, the amount of carbon dioxide was then determined by acid-base titration of the barium hydroxide solution. The mass of C0 2 detected was then divided by the volume of the solution subjected to wet oxidation and reported as mg C0 2 /L. Other volatile materials generated during wet oxidation might include low molecular weight alcohols, aldehydes, ketones and acids. To determine the amount of these organic compounds present in the head space after wet oxidation, the head-space gas was evacuated from the Parr bomb via a liquid nitrogen cold trap and the COD of the condensate was determined. Examples Example 1; α-K- ~ SiMn. IlDW u Q.,,,, " (compound 2): M.MJ1,E Sequence. For the Mi stage, 8.5 g (oven dried weight, O.D.) of unbleached kraft pulp was added to a solution of compound 2 in 0.20 M acetate buffer to give a final consistency (esc) of 3% (three weight- percent pulp) and a polyoxometalate concentration of 0.05 M. The pH after mixing was 5.02. The mixture was then placed in a glass lined Parr high pressure reactor and, while stirred, was purged with purified nitrogen for 40 minutes, sealed, and heated to 125 °C for one hour. During this time, the pH of the polyoxometalate solution dropped to 4.86. The polyoxometalate bleaching liquor was then recovered by filtration and the pulp washed with water. The amount of compound 2 reduced to compound 1 during the bleaching reaction (stage M x ) was determined by reaction of an aliquot of the bleaching liquor with an excess of potassium iodide and titration to a starch endpoint with sodium thiosulfate. Over the course of the bleaching reaction, more than 98.9% of the compound 2 present was reduced to compound 1. Upon cooling the bleaching liquor to 0 °C for three days, 21.02 g of orange crystalline compound 1, characterized by FTIR (KBr pellet), were obtained. The UV-vis spectrum of the supernatant was identical to that of compound 1. For the M 2 stage, 7.36 g O.D. of the M_. stage pulp was reacted as above (3% esc, 0.05 M compound 2, in 0.2 M acetate buffer) for 1.5 hours at 125 °C under purified nitrogen. At the end of this time, the pH had dropped from 5.14 to 4.95 and 89.2% of the compound 2 present had been reduced to compound 1. This was repeated for the M 3 stage using 5.99 g O.D. of pulp from the M 2 stage. The reaction was run for two hours during which the pH dropped from 5.16 to 4.89 and 66.8% of the compound 2 present was reduced to compound 1. The UV-vis spectra of the spent M2 and M3 bleaching liquors confirmed the presence of intact, unreacted compound 2. Division of the polyoxometalate treatment into three sequential applications was done here for convenience and to better monitor the bleaching reaction; it is not necessarily a preferential form of the invention. After the three sequential M stages, an alkaline extraction (E) was performed. 4.69 g O.D. of the M 3 stage pulp were heated for two hours under nitrogen at approximately 85 °C as a 2.0 % esc mixture in 1.0 % sodium hydroxide solution. A control experiment was performed by subjecting pulp to the same procedure as above, but with no polyoxometalate present. Microkappa numbers of pulp samples after each stage M lf M 2 , M 3 and E, and after the control sequence stages Δ-., Δ 2 , Δ 3 and E, are shown below in Table 1. Table 1. Microkappa numbers of pulps after each stage of the polyoxometalate bleaching and control sequences. Sample Microkappa number Unbleached kraft pulp 33.6 Polyoxometalate sequence M, . 25.5 M 2 19.6 M 3 13.7 E 6.5 Control Sequence Δ 2 32.0 Δ 3 31.4 E 29.4 FT Raman spectra were obtained from unbleached kraft pulp, from pulp samples removed after each stage M_., M 2 , M 3 and E of the MjM a a E bleaching sequence, and from a pulp sample examined after completion of the entire control sequence. Fig. 3 is a plot of the ratios of integrated areas of the FT Raman bands observed at 1595 cm" 1 against those between 1216-1010 cm "1 . The plot demonstrates that the concentration of lignin, as represented by the concentration of phenyl groups in the polyoxometalate bleached pulp, decreases dramatically over the course of the MiM-M s E bleaching sequence, while in the control, little change occurs. This demonstrates that the polyoxometalate treatment is cleaving or otherwise removing phenyl groups from the pulp and implies that kappa number determination is a valid criterion for delignification in the polyoxometalate process. Reoxidation Of Used Bleaching Liquors Containing Reduced Polyoxometalates All of the oxidants mentioned below are thermo-dynamically capable of reoxidizing all of the reduced vanadium-substituted polyoxometalates. Nonetheless, differences in rates have been observed, and no clear pattern of reoxidation rates is yet discernible. The most desirable oxidants are probably air, dioxygen or hydrogen peroxide, with air the most desirable. Example 2; Selectivity of Compound 2 for Lignin. The viscosity of the unbleached kraft pulp was 34.2 mPa«s. After completion of the four stages, the final viscosity of the polyoxometalate bleached pulp (microkappa no. 6.5) was 27.0 mPa«s, while that of the control (microkappa no. 29.4) was 31.3 mPa«s. These results compare favorably with those obtained using elemental chlorine (C), followed by extraction with alkali (E) (traditional chlorine-based bleaching sequence). Using the traditional CE sequence, the kraft pulp used in Example 1 was bleached to a microkappa number of 6.2, comparable to the microkappa no. of 6.5 achieved using compound 2. Notably, however, the viscosity of the CE delignified pulp had dropped to 17.9 mPa«s. The higher viscosity observed for the polyoxometalate treated pulp demonstrates that, as applied in Example 1, the d-electron- containing transition metal-substituted polyoxometalate (compound 2) is a more selective oxidant than elemental chlorine. Example 3; Oxidation of Compound 1 tb Compound 2 with Ozone. Compound 1, and other similar complexes useful in the invention, are reversible oxidants, able to sustain repeated reduction and reoxidation without undergoing degradative structural changes. This property is not shared by simple transition metal salts, such as those of copper, iron or manganese, that undergo irreversible hydrolysis reactions with water upon oxidation in aqueous media. Prior to bleaching, compound 1 was oxidized to compound 2 by treatment with ozone gas at room temperature. In a typical preparative reaction, 96.4 g, 0.0298 mol α-K 6 tSiMn(II)W 11 0 39 ] ' 22H 2 0 were dissolved in 150 mL water and the pH adjusted to approximately 2.5 by addition of 2.24 g of glacial acetic acid. The orange solution was then exposed to a dilute mixture of ozone and dioxygen gases (3.0 - 4.0 % 0 3 in 0 2 ) introduced via a sparger at a flow rate of approximately 1.0 L/min until the color of the solution had changed to dark purple. During the reaction the pH increased to 5.3. A very slight precipitation of metal oxide was observed in the sintered glass of the sparger. The UV-vis spectrum of the solution was identical to that reported in the literature for K 5 [SiMn(III) (H 2 0)W 0 39 ] , compound 2, and no evidence of permanganate was observed. The solution was then boiled in air to a volume of 50 mL and cooled to 0 * C overnight yielding 81.6 g dark purple crystals. The crystals were dried in a stream of air at room temperature. The Fourier Transform Infra-red (FTIR) spectrum of the crystalline material (KBr pellet) was consistent with that of compound 2. Titration to a starch endpoint using potassium iodide and sodium thiosulfate indicated an effective molecular weight of 3500 amu, which implied the presence of 32 molecules of water per α-[SiMn(III)W 11 0 39 ] 5 " (compound 2) anion in the crystalline material. When used in bleaching under anaerobic conditions, the active, oxidized form of the complex, compound 2 in this case, is added to the unbleached pulp. During bleaching, lignin acts as a reducing agent, converting compound 2 back to compound 1. Reduction to compound 1 was followed titrametrically and by isolation and characterization of compound 1 as reported in Example 1. Example 4: Regeneration of Compound 2 After Bleaching. To demonstrate the oxidative regeneration of compound 2, a 25 mL portion of polyoxometalate charged with spent bleaching liquor from the M .. stage of Example 1 was treated with ozone. During the K stage, better than 99% of the compound 2 originally present had been reduced to compound 1. Ozone (3.0% 0 3 in 0 2 ) was applied via a sparger to the 25 mL portion at a flow rate of 0.5 L/min for 100 seconds. During this time, the solution changed color from orange to dark purple and the pH rose from 4.9 to 5.5. Titration of the solution to a starch/iodine endpoint with sodium thiosulfate showed that 99% of the oxidizing equivalents expected for complete oxidation of compound 1 to active compound 2, were present. Upon sitting, however, some precipitation of dark brown material, probably hydrated manganese dioxide, was observed. This could mean that slight hydrolytic degradation of compounds 1 or 2 occurred during the M_. bleaching stage. If so, this would indicate that more hydrolytically stable d-electron-containing transition metal- substituted polyoxometalate structures, such as those defined by the general formula (B) or by formulas 2-5, derivatives of formula (B), might be required for commercial application. Example 5; Use of Compound 2 in Pulping. Compound 2 was examined for its ability to delignify wood fibers. 3.13 grams of 96% aspen wood meal (the remaining 4% being water) were added to a solution of compound 2 in 0.40 M acetate buffer to give a final a consistency of 3% and a polyoxometalate concentration of 0.20 M. The pH after mixing was 5.25. The mixture was then placed in a glass lined Parr high pressure reactor and, while stirred, was purged with purified nitrogen for 40 minutes, sealed, and heated to 125 °C for one hour (M stage). During this time, the pH of the polyoxometalate solution dropped to 4.46. The polyoxometalate bleaching liquor was then recovered by filtration and the wood meal washed with water. Over the course of the reaction, 96.5% of the compound 2 present was reduced to compound 1. A control (Δ) was performed by heating 3.125 grams of 96% aspen wood meal under identical conditions (0.40 M acetate buffer, initial pH = 4.75, final pH = 4.80) but with no polyoxometalates. The lignin content of each sample was then determined. Then, the two samples were each subjected to a short kraft cook after which the lignin content of each sample was again determined. The lignin contents of the two samples were analyzed gravimetrically according to TAPPI methods T222 and um-249 (Klason lignin) . The control sample was found to be 2% delignified after the Δ stage and 14% delignified after the short kraft cook. The sample treated with compound 2 was shown to be 8% delignified after the M stage and 19% delignified after the subsequent short kraft cook. Another embodiment of pulping using polyoxometalate compounds of the general formula is in the delignification of mechanical pulps. One preferred form is the surface delignification of high pressure mechanical pulp, wherein the energy consumed in preparation of the pulp is low, and the separation of the fibers occurs at the middle lamella between the fibers in the wood- chips. Such pulps have fibers with lignin predominant at the surface and, in the absence of delignification treatments, are incapable of sufficient interfiber bonding to allow formation of sheets with adequate properties. Application of a polyoxometalate treatment sufficient to delignify the surface of the fibers will liberate the surface polysaccharide component of the fiber wall and allow it to cause interfiber adhesion resulting in improved mechanical properties. Because high pressure mechanical pulp is prepared under conditions wherein the energy consumption is low, and internal damage to the fiber structure is more limited, it is anticipated that sheets formed from pulps partially delignified in the manner described above will have superior mechanical properties and will, therefore, be useful in many applications wherein only sheets containing large amounts of chemical pulps are currently used. Such applications include, but are not limited to, packaging, as in grocery bag stock, wrapping papers, corrugated containers and printing papers. More specifically, this preferred form of the pulping would begin with wood chips that are mechanically fiberized at steam pressures between 50 and 125 psig, depending on species, and treated with a solution of a polyoxometalate of the general formula under the conditions of consistency temperature, pH and polyoxometalate concentration for a period sufficient to remove 5 to 30% of the lignin, depending on species. The fibers would then be submitted to further refining prior to sheet formation. Another form preferred for other applications would have the delignification proceeding further, to remove more of the lignin and to provide fibers having a higher relative content of polysaccharide. Such fibers would have properties intermediate between those of the pulps described above and those of fully delignified pulps. Example 6: Catalytic Wet Oxidation of Veratryl Alcohol (1,3- dimethoxybenzyl alcohol, by α-H ~ PV.Mo...0,.- ~ (compound 3) and Oxygen: Temperature Profile A. A solution of compound 3 and veratryl alcohol was prepared as described above in the General Method. 150 mL of this solution were transferred to the Parr reactor, which was purged and pressurized to 100 psia with purified oxygen gas, heated to 150 °C, and stirred at this temperature for four hours. The final pH was 2.6, and all of the compound 3 present was fully oxidized. After cooling the reactor to room temperature, the amount of C0 2 in the headspace and the COD of the solution were determined as described above. The COD of the solution had dropped from 396 ± 17 to 114 ± 20 mg 0 2 /L and 63 ± 72 mg/L of C0 2 (13 ± 15 percent of theoretical, the large uncertainty is due to the use of excess barium hydroxide solution) were found in the headspace gas. The wet oxidation reaction was repeated using 100 mL of solution and the headspace gas was passed through a liquid nitrogen trap to condense volatile organic compounds. The COD values of both the reaction solution and of the headspace gas condensate were then determined. Of the original COD of the solution (396 ± 17 mg 0 2 /L), 95 ± 21 mg 0 2 /L were found in the reaction solution and 15 ± 4 mg 0 2 /L in the condensate of the headspace gas. The temperature of the reactor during release of the headspace gases was 50 °C. At this temperature, the partial pressures of water- soluble volatile organic compounds are likely to be small. A control experiment was performed using 100 mL of a stock veratryl alcohol solution and no added catalyst. The final pH, after wet oxidation, was 3.0. During the reaction, the COD dropped from 416 ± 10 to 384 ± 10 g 0 2 /L, and 30 ± 12 mg/L of C0 2 (6 ± 3 percent of theoretical, foaming agent method) were found in the headspace gas. Example 7; Catalytic Wet Oxidation of Veratryl Alcohol (1.3- dimethoxybenzyl alcohol, by α-H.. ~ PV,Mo...O,... ~ (compound 3) and Oxygen: Temperature Profile B. A solution of compound 3 and veratryl alcohol was prepared as described above in the General Methods. 102 mL of this solution was transferred to the Parr reactor, which was purged and pressurized to 100 psia with purified oxygen gas, heated to 150 °C for two hours followed by one hour at 200 °C. The final pH was 2.2, and all of the compound 3 present was fully oxidized. After cooling the reactor to room temperature, the amount of C0 2 in the headspace and the COD of the solution were determined as described above. The COD of the solution had dropped from 396 ± 17 to 89 ± 21 mg 0 2 /L and 199 ± 83 mg/L of C0 2 (42 ± 18 percent of theoretical, glass bead method) were found in the headspace gas. A control experiment was performed using 100 mL of a stock veratryl alcohol solution and no added catalyst. The final pH, after wet oxidation, was 3.2. During the reaction, the COD dropped from 416 ± 10 to 375 ± 10 mg 0 2 /L and -8 ± 103 mg/L of C0 2 (-2 ± 22 percent of theoretical, glass bead method) were found in the headspace gas. A variant of this control, in which the final temperature (200 °C) was maintained for one-half rather than one hour, gave a similar result: the final pH was 3.0 and the COD dropped from 416 ± 10 to 356 ± 10 mg 0 2 /L and 14 ± 11 mg/L of C0 2 (3 ± 2 percent of theoretical, foaming agent method) were found in the headspace gas. Example 8: Catalytic Wet Oxidation of D-glucose by α- H c fPV,Mo,-Q 1 -l (compound 3^ and Oxygen; Temperature Profile A. A solution of compound 3 and D-glucose was prepared as described above in the General Methods. 150 mL of the solution was transferred to the Parr reactor, which was purged and pressurized to 100 psia with purified oxygen gas, heated to 150 °C, and stirred at this temperature for four hours. The final pH was 2.3, and all of the compound 3 present was fully oxidized. After cooling the reactor to room temperature, the amount of C0 2 in the headspace and the COD of the solution were determined as described above. The COD of the solution had dropped from 396 ± 17 to 75 ± 20 mg 0 2 /L and 194 ± 60 mg/L of C0 2 (35 ± 11 percent of theoretical, glass bead method) were found in the headspace gas. A control experiment was performed using 100 mL of the stock D-glucose solution and no added catalyst. The final pH, after wet oxidation, was 2.9. During the reaction, the COD dropped from 413 ± 10 to 309 ± 10 mg 0 2 /L and 67 ± 54 mg/L of C0 2 (12 ± 10 percent of theoretical, glass bead method) were found in the headspace gas. Example 9; Catalytic Wet Oxidation of D-glucose by α- H.ΓPV-.MO T -O^-I (compound) and Oxygen; Temperature Profile B. A solution of compound 3 and D-glucose was prepared as described above in the General Method. 100 mL of the solution were transferred to the Parr reactor, which was purged and pressurized to 100 psia with purified oxygen gas, heated to 150 °C for two hours and to 200 °C for one hour. The final pH was 2.1, and all of the compound 3 present was fully oxidized. After cooling the reactor to room temperature, the amount of C0 2 in the headspace and the COD of the solution were determined as described above. The COD of the solution had dropped from 396 ± 17 to 46 ± 22 mg 0 2 /L and 233 ± 110 mg/L of C0 2 (42 ± 20 percent of theoretical, glass bead method) were found in the headspace gas. A control experiment was performed using 100 mL of the stock D-glucose solution and no added catalyst. The final pH, after wet oxidation, was 2.9 ' . During the reaction, the COD dropped from 413 ± 10 to 137 ± 10 mg 0 2 /L and 328 ± 12 mg/L of C0 2 (60 ± 4 percent of theoretical, foaming agent method) were found in the headspace gas. Results of the model compound studies are presented below in Table 2. TABLE 3 COD and C0 2 values of model compound solutions after wet oxidation by oxygen in the presence and absence of α-H 5 [PV z Mo 10 O 40 ] (compound 1). CA ¬ CTI Example 10: Catalytic Wet Oxidation of a Partially Spent α-H_. ~ PV-Mo...θΛ.. ~ (compound 3) Bleaching Liquor by Oxygen. To prepare a partially spent bleaching liquor, 6.2 g oven-dried (O.D.) weight of mixed-pine kraft pulp were added to a 0.05 M solution of compound 3, to a final consistency of 3.0% in a glass-lined one liter Parr high pressure reactor. The pH of the mixture was adjusted to 3.0. The reactor was purged with purified nitrogen and heated to 100 °C for four hours. During heating, the solution changed from orange to dark green-brown. The pulp was then collected on a Bϋchner funnel and the partially reduced polyoxometalate solution (pH = 2.9) was saved. The microkappa number of the partially bleached pulp was 29.5. A small aliquot of the partially reduced polyoxometalate solution was titrated to an orange endpoint with eerie ammonium sulfate. 29% of the vanadium(+5) present had been reduced to vanadium(+4) . The COD of the spent liquor, determined using a portion of the partially reduced liquor and subtracting the concentration of reduced vanadium, was 644 ± 17 mg 0 2 /L. 75 mL of the partially reduced spent liquor was than purged and heated under 100 psia oxygen gas for four hours at 150 °C. The final pH was 3.0, and all of the compound 3 present was fully oxidized. After the reaction, 383 ± 84 mg/L C0 2 (glass bead method) were found in the headspace gas and the COD of the solution had dropped to 232 ± 19 mg 0 2 /L. Example 11; Repeated Cycles of Bleaching and Catalytic Wet Oxidation of Dissolved Orqanics Using α-H_. ~ PV_Mo...0,_.. ~ (compound 31 and Oxygen. A partially spent α-H 5 [PV 2 Mo 10 0 40 ] bleaching liquor was prepared as described in Example 10 using 10.1 g O.D. weight of mixed-pine kraft pulp. During polyoxometalate treatment the microkappa number of the pulp dropped to 27.2. The final pH was 2.9 and 14% of the vanadium(+5) present was reduced. After bleaching, the COD of the partially reduced bleaching liquor was 793 ± 26 mg 0 2 /L. The pH of the solution was adjusted to 3.0 and 243 mL were heated under 100 psia oxygen for four hours at 150 °c. The final pH was 2.9. 419 ± 4 mg/L C0 2 (foaming agent method) were found in the headspace gas and the COD of the solution had dropped to 317 ± 30 mg 0 2 /L. The cycle was repeated two more times. Data pertaining to the conditions used in the bleaching and wet oxidation stages are summarized in Table 3a. Results of the bleaching and wet oxidation stages are presented in Table 3b. All C0 2 measurements were made using isopropanol as a foaming agent. TABLE 3a. Conditions used for each process step, bleaching and wet oxidation, where -H s CPVjJMOi-O,,-,] was used for three successive cycles of anaerobic bleaching and aerobic wet oxidation. TABLE 3b. Results obtained after each process step, bleaching and wet oxidation, where α-H 5 [PV 2 Mo 10 0 40 ] was used for three successive cycles of anaerobic bleaching and aerobic wet oxidation. The purpose of Example 11 was two-fold: to demonstrate the use of compound 3 in repeated cycles of bleaching and wet oxidation, and to determine whether additional more easily oxidized organic compounds introduced during bleaching might act as "sacrificial reductants" to bring about an eventual steady state COD value for the subsequently oxidized liquors. A least squares fit of the wet oxidation (COD) data, based on a mathematical model that assumes exponential decay to an asymptotic value, is presented in Figure 4. The equation used was COD = COD s exp{-aT(i) } where COD 8 = the final steady state COD value, 1/a represents the number of cycles for the COD to reach 63% of its steady-state value, and T(i) = number of bleaching/wet oxidation cycles. The initial COD value of zero (cycle 0) was included in the least squares fit. All available data are consistent with this model. Example 12: Use of Ozone to Augment Catalytic Wet Oxidation by α-H... PV.Mo,,.0,.. ~ (compound 31 and Oxygen. 50 mL of a solution having a COD after catalytic wet oxidation of 122 mg 0 2 /L and a pH of 2.4 was prepared using α-H 5 [PV 2 Mo_ o 0 4O ] (compound 3) as described in Example 10. It was then sparged with a hydrated mixture of ozone and oxygen gases (3.0% 0 3 in 0 2 ) at a rate of 1 L/min at room temperature for one hour. After exposure to ozone, the pH of the solution was 2.4 and its COD was 8 ± 22 mg 0 2 /L. The minimum quantity of ozone gas required to reduce the COD to this extent was not determined, but is probably much less than the amount applied here. Example 13: Phosphorus-31 Nuclear Magnetic Resonance Spectra of Polyoxometalate Solutions. The integrity of α-[PV 2 Mo lo 0 4O ] 5" (compound 3) after wet oxidation of model compounds, and after use in repeated cycles of bleaching and wet oxidation, was confirmed by 31 P NMR spectroscopy. The 31 P NMR spectra were acquired using solutions of α-H 5 [PV 2 Mo x0 0 4O ] diluted by addition of D 2 0, and were externally referenced to 85% phosphoric acid. Samples were placed in 5 mm NMR tubes and spectra acquired on a 250 MHz instrument. Spectra of solutions of α-[PV 2 Mo lo 0 4O ] 5" were recorded after each wet oxidation reaction described in Examples 6-9. In solutions obtained from Examples 6-8, no decomposition products were observed. In the solution obtained from Example 9, Catalytic Wet Oxidation of D-glucose-Temperature Profile B, a small unidentified signal (< 5% of the total integrated area of the spectrum) was observed at -0.3 ppm. Significantly, no decomposition was observed in the polyoxometalate solution that had been used for three successive cycles of bleaching and catalytic wet oxidation (Example 11). No polyoxometalate degradation was expected or observed when a partially reduced bleaching solution containing α-H s [PV 2 Mo 10 0 4O ] (compound 3) was heated to 100 °C and reoxidized by exposure to ozone (3.0% 0 3 in 0 2 ) at a rate of 0.1 L/min for one minute (initial pH = 2.18, final pH = 1.46). In the Detailed Description of the invention, polyoxometalate structures useful in anaerobic bleaching were defined as those containing molybdenum ions in their highest +6 d° electronic configurations, or d-electron-containing transition metal ions possessing sufficiently positive reduction potentials. Effective in anaerobic bleaching, these polyoxometalates must directly oxidize a range of organic functional groups. In addition, reduction of these polyoxometalates, all subsets of the general formula (A), is known to occur reversibly. Thus, at the elevated temperatures suggested for use in catalytic wet oxidation, these polyoxometalates probably oxidize functional groups present in dissolved lignin and polysaccharide fragments. In the presence of oxygen (aerobic wet oxidation) , they undoubtedly initiate a variety of radical-chain autoxidation reactions. Both processes, direct oxidation of organic functional groups and reversible reduction/reoxidation, likely occur during the reactions described in Examples 6 - 11 above. First, because compound 3 is capable of the anaerobic oxidation of organic functional groups under moderate conditions it is likely that the direct oxidation of a wider range of functional groups will occur more rapidly under the higher temperatures suggested for wet oxidation. A likely mechanism by which the wet oxidation reaction(s) can occur catalytically, as the data presented in Examples 6 - 11 suggest, requires that compound 3, at least initially, directly oxidize some organic functional groups. Secondly, because the wet oxidation reaction is catalytic in polyoxometalate, the vanadium ions in compound 3 probably cycle between more than one oxidation state, the most likely being oxidation states +5 (d°, fully oxidized) and +4 (d 1 , one electron reduced). The reversibility of the vanadium(+5)/vanadium(+4) couple likely plays an important role in the course of the radical- chain autoxidation reactions. Like compound 3, molybdenum(+6) (d° electronic configuration) and d-electron-containing transition metal ion substituted polyoxometalates of general formula (A) and useful in anaerobic oxidative delignification are reversible oxidants. Capable of direct oxidation of organic substrates and of reversible reduction, these materials are expected to be useful in wet oxidation because they meet the criteria most reasonably responsible for the demonstrated effectiveness of compound 3.

Claims: Claims We claim: 1. A method for delignifying a lignocellulosic material comprising the steps of: obtaining a lignocellulosic material; and exposing the lignocellulosic material to a solution of a polyoxometalate of the formula [Mo B W n Nb 0 Ta p (TM) q X r O ε ] J ' " where m is 0-40, n is 0-40, o is 0-10, p is 0-10, q is 0-9, r is 0-6, TM is a d-electron-containing transition metal ion, and X is a heteroatom, which is a p or d block element, where m + n + o + p > 4, m + q > 0 and s is sufficiently large that x > 0, under conditions wherein the polyoxometalate is reduced and delignification occurs, optionally followed by a step of reoxidizing the reduced polyoxometalate with an oxidant, the oxidant preferably selected from the group consisting of air, dioxygen, peroxide and ozone, and wherein the step of reoxidizing the reduced polyoxometalate is preferably simultaneous with the step of reducing the polyoxometalate. 2. The method of claim 1, wherein said lignocellulosic material is selected from the group consisting of wood pulp, wood fiber, non-woody pulp, and non-woody fiber. 3. The method of claims 1 or 2, wherein the polyoxometalate is [Mo B W n (TM) 0 X p O q ] x ", where TM is any d-electron-containing transition metal ion, X is a heteroatom, which is a p or d block element, wherein when p = l, m + n + o = 12, o < 4, and m + o > 0, and when p = 2, m + n + o = 22, and o is 1-4. 4. The method of claims 1 or 2, wherein the polyoxometalate is [Mo B W n (TM) 0 XpO q ] x ", where TM is any d-electron-containing transition metal ion, X is either P 5+ , As s+ , or S 6+ , and wherein when p = 2, m + n + o = 18, o < 6, and m + o > 0, and when p = 4, m + n = 30. 5. The method of claims 1 or 2, wherein the polyoxometalate is of the formula [Mo-W_(TM) o P s CpNa~0 r ] ~" , where TM is any d-electron- containing transition metal ion, C is a di- or tri-valent main group, transition metal or lanthanide cation located in the.center of the structure, m + n + o = 30, p + q = l and m + o > 0. 6. The method of claim 1 wherein the polyoxometalate is α-K 5 [SiMn(III)W xl 0 39 ] . 7. A method for oxidative degradation of lignin and polysaccharide fragments dissolved during polyoxometalate treatment of a lignocellulosic material, comprising the steps of: obtaining a lignocellulosic material; exposing the lignocellulosic material to a polyoxometalate of the formula [V 1 Mo B W n Nb 0 Ta p (TM) q X r O ε ] x" where 1 is 0-18, m is 0-40, n is 0-40, o is 0-10, p is 0-10, q is 0-9, r is 0-6, TM is a d- electron-containing transition metal ion, and X is a heteroatom, which is a p or d block element, where l + m -τ- n + o + p > 4, 1 + m + q > o and s is sufficiently large that x > 0, wherein the lignocellulosic material is delignified and the polyoxometalate is reduced and wherein a liquor is obtained that contains the polyoxometalate and dissolved lignin and polysaccharide fragments, and heating the liquor, preferably at a temperature of between 100 °C and 400 °C, more preferably between 125 °C and 225 °C, most preferably at 150 °C, in the presence of an oxidant under conditions wh «erein the dissolved lignin and polysaccharide fragments are catalytically and oxidatively degraded by the oxidant and the polyoxometalate to volatile organic compounds and water. 8. The method of claim 7, wherein the polyoxometalate is of the formula [V x Mo B W n (TM) o X p 0 q ]"", where TM is any d-electron- containing transition metal ion, X is a heteroatom, which is a p or d block element, wherein when p = 1, then l + m + n + o = l2, o < 4 and 1 + m + o > 0, and when p = 2, then l + m + n + o = 22, and 1 + o is 1-4. 9. The method of claim 7 wherein the polyoxometalate is of the formula [V x Mo_.W n (TM) o X P 0 q ] x" , where TM is any d-electron- containing transition metal ion, X is either P s+ , As 5+ , or S 6+ , wherein when p = 2, then l + m + n + o = 18, o < 6 and 1 + m + o > 0, and wherein when p = 4, then m + n = 30. 10. The method of claim 7, wherein the polyoxometalate is of the formula [V 1 Mo B W n (TM) o P s C p Na q 0 r ] x" , where TM is any d-electron-containing transition metal ion, C is a di- or tri- valent p, d, or f block cation located in the center of the structure, l + m + n + o = 30, p + q = l and 1 + m + o > 0. 11. The method of claim 7, wherein the oxidant is selected from the group consisting of air, dioxygen, hydrogen peroxide, ozone, and inorganic and organic peroxides and peracids. 12. The method of claim 7 wherein the heating step is performed at final pH of between 1.0 And 10.0, preferably between 1.5 and 3.5, most preferably between 2.0 and 3.0. 13. The method of claim 7, wherein the time of oxidation is between 0.5 hr and 10.0 hrs, preferably between 1.0 hr and 5.0 hrs, most preferably between 3.0 hrs and 4.0 hrs. 14. The method of claim 7, wherein the pressure of oxygen in the heating step is 15 to 1000 psia, preferably 100 psia. 15. The method of claim 7, wherein at least 14% of the polyoxometalate has been reduced. 16. The method of claim 7, wherein the reduced polyoxometalate is oxidized after exposure to an oxidant, and optionally additionally returning the oxidized polyoxometalate liquor to a reactor for reuse in bleaching. 17. The method of claim 7, wherein said lignocellulosic material is selected from the group consisting of wood pulp, wood fiber, non-woody pulp, and non-woody fiber.

Inventors: ATALLA RAJAI H, HILL CRAIG L, WEINSTOCK IRA A, ATALLA, RAJAI, H., HILL, CRAIG, L., WEINSTOCK, IRA, A.
Applicants: ATALLA RAJAI H, HILL CRAIG L, UNIV EMORY, US AGRICULTURE, WEINSTOCK IRA A, ATALLA, RAJAI, H., EMORY UNIVERSITY, HILL, CRAIG, L., THE UNITED STATES OF AMERICA, REPRESENTED BY THE S, WEINSTOCK, IRA, A.
Application year: 1995
Main classification: D21C3
IPC codes: C08H5, D21C3, D21C9