METHOD FOR SELECTIVELY DELIGNIFYING LIGNOCELLULOSIC MATERIALS

Abstract: Disclosed is a method for delignifying lignocellulosic materials by first treating the material with an alkaline solution in the presence or absence of anthraquinone, and then exposing the material to a polyoxometalate. Inclusion of the alkaline pretreatment step allows delignification to take place at high efficiency and affords high residue yields and viscosities.

Description: METHOD FOR SELECTIVELY DELIGNIFYING LIGNOCELLULOSIC MATERIALS BACKGROUND OF THE INVENTION The production of paper from trees involves several discrete stages. First, the tree is debarked and converted into wood chips . The wood chips are converted into pulp by mechanical or chemical means, and the pulp is bleached. In the case of chemical pulps, delignification is the first step in bleaching. Lignin, a complex polymer derived from aromatic alcohols, is one of the main constituents of wood and makes up 3-6% of chemically pulped wood. During the early stages of bleaching, residual lignin is removed. Currently, this is typically done by treating the pulp with chlorine compounds at low pH, followed by extracting the chlorine treated pulp with hot alkali. Once a significant portion of the residual lignin has been removed, the pulp may be whitened by any one of a variety of means to high brightness . Chlorine dioxide and hydrogen peroxide are 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 . In recent years, the potential use of enzymatic processes associated with fungal degradation of lignin to develop environmentally friendly technologies for the pulp and paper industry has been given considerable attention. In many wood-rotting fungi, extracellular metalloenzym.es such as glyoxal oxidase, a copper-containing oxidase, in combination with lignin and manganese peroxidases, both of which contain iron in a protoheme active site, harness the oxidative capability of dioxygen and direct its reactivity to the degradation of lignin within the fiber walls. In this biochemical process, high valent transition metal ions serve as conduits for the flux of electrons from lignin to oxygen. Therefore, transition metal ions are known to possess redox properties that are useful in the delignification and bleaching of lignocellulosic materials. However, the behavior of transition metal ions in water is often difficult to control. In aqueous solution, complex equilibria are established between ionic hydroxides and hydrates, as well as between accessible oxidation states of the metal ions. In addition, many transition metal oxides and hydroxides have limited solubilities in water, where the active metals are rapidly lost from solution as solid precipitates . U.S. Patent No. 5,695,606 discloses a method of delignifying wood fiber pulp using bleaching agents comprising reusable, transition metal-derived polyoxo-metalates (POMs) . U.S. Patent No. 5,302,248 (the parent of U.S. Patent No. 5,695,606) discloses a method for delignification and pulp bleaching using vanadium (5) -substituted POMs. Bleaching agents comprising POMs are advantageous in that they are composed of relatively inexpensive, non-toxic materials. U.S. Patent No. 5,695,606 and U.S. Patent No. 5,302,248, as well as all other publications cited herein are incorporated by reference in their entirety. Polyoxometalates are discrete, polymeric structures that form spontaneously when simple oxides of vanadium (V) , niobium (Nb) , tantalum (Ta) , molybdenum (Mo) or tungsten (W) are combined under the appropriate conditions in water (Pope, M. T. Heteropoly and Isopoly Oxometalates Springer-Verlag, Berlin, 1983) . In a majority of polyoxometalates, the transition metals are in the d° electronic configuration, which confers high resistance to oxidative degradation and the 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). Polyoxometalates, in either acid or salt forms, are water soluble and highly resistant to oxidative degradation. The isopolyoxometalates, the simplest of the polyoxometalates , are binary oxides of the formula [M..0 ] p" , where m may vary from two to over 30. For example, if m=2 and M=Mo, then the formula is [M 2 0 7 ] 2" ; if m=6 , then [Mo 6 0 19 ] 2~ ; and if m=36, then [Mo 36 0 112 ] 8 - . The heteropolyoxometalates have the general formula [XJVl-P y ] p" and possess a heteroatom, X, at their center. For example, in the alpha-Keggin structure, alpha- [PW 12 0 40 ] 3~ , X is a phosphorus atom. The central phosphorus atom is surrounded by twelve W0 S octahedra . Removal of a (M=0) 4+ moiety from the surface of the alpha-Keggin structure, alpha- [PM 12 O 40 ] 3" , where M is molybdenum or tungsten, creates the "lacunary" alpha-Keggin anion, alpha- [PM 1X 0 29 ] 7" . The lacunary alpha-Keggin ion acts as a pentadentate ligand for redox active d° transition metal ions, such as vanadium(+5) in alpha- [PVW 11 O 40 ] 4" or molybdenum (+6) in alpha- [PMo 11 0 40 ] 3" , or for redox active, d-electron-containing transition metal ions (TM) , such as manganese (+3) in alpha- [P nW 1:L 0 39 ] 4" . In the case of vanadium, further substitution is common, giving anions of the form [X χ MJM- y ] p" , where m+n=12, such as alpha- [PV 2 Mo 10 O 40 ] 5" . The redox active vanadium (V), molybdenum (VI) or d-electron-containing transition metal (TM) ions are bound at the surface of the heteropolyanion in much the same way that ferric ions are held within the active sites of lignin or manganese peroxidases . However, while stabilizing the metal ions in solution and controlling their reactivity, the heteropolyanions , unlike enzymes or synthetic porphyrins, are highly resistant to oxidative degradation (Hill, et al . , J. Am. Chem. Soc. 108:536-538, 1986). Previously, polyoxometalates have been used as catalysts for oxidation under heterogeneous and homogeneous conditions, as analytical stains for biological samples, and for other uses still in development . BRIEF SUMMARY OF THE INVENTION The present invention is in part an improved method of delignifying lignocellulosic materials using polyoxo-metalates . The improvement involves contacting the lignocellulosic material with an alkaline solution, or an alkaline solution containing anthraquinone (AQ) prior to contacting the material with a polyoxometalate solution. The alkaline pretreatment enhances the removal of lignin, while at the same time reducing degradation of cellulose . The improved method for selectively delignifying lignocellulosic materials comprises the steps of: (a) contacting the lignocellulosic material with a solution comprising a strong alkali at a suitable temperature and for a period of time sufficient to reduce the lignin in the material; and (b) treating the residual material of step (a) with a polyoxometalate of the general formula (TM) g X-C x" under conditions wherein the polyoxometalate is reduced and delignification of the lignocellulosic material occurs. In a preferred embodiment, anthraquinone (AQ) is added to the alkaline solution used in the alkaline pretreatment step. In the present invention a transition metal-substituted polyoxometalate is used as a delignification and bleaching agent . The metal in question must be sufficiently active to oxidize functional groups within lignin, residual lignin, and other chromophores of wood, wood pulp and other ligno-cellulosic fibers and pulp. The efficacy of these polyoxo-metalates 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 poly-oxometalate structure. The general formula for a polyoxometalate useful in the present invention is [V 1 MoJflJϊb 0 'Σa 1? (TM) ^O 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, provided that 1+m+n+o+p > 4, l+m+q>0, and s is sufficiently large that x>0. Preferably, wood pulp or fibers are exposed to a polyoxometalate of the formula (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, and either l+m+n+o=12, o<4, p=3 and l+m+o>0, or l+m+n+o=22, l+o is 1-4 and p=2; or where X is either P 5+ , As s+ or S 6+ , and l+m+n+o=18, p=2 and l+m+o>0, or m+n=30, p=4 and o=4 and q is sufficiently large that x>0. More preferably, X is Zn 2+ , Co 2+ , B 3+ , Al 3+ , Si + , Ge 4+ , P 5+ , As 5+ , or S + . Still more preferably, X is Al 3+ or Si 4+ . Also preferably, wood pulp is exposed to a polyoxometalate of the formula [VYIo-.W n (TM) o P 5 C p Na q 0 r ] x~ , 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, l+m+n+o=30, p+q=l and l+m+o>0 and r is sufficiently large that x>0. Other preferable forms of polyoxometalates include polyoxometalates of the formula [V n O r ] x~ , where n>4 , r>12 and x=2r-5n, or [V n Mo m 0 (MG) p (TM) q 0 r ] x~ , where TM is any transition metal ion, MG is a main group ion, 1 n <8 , n+m+o < 12 and p+q 4, or [V n Mo m D (MG) p 0 r ] x" where MG is either P 5+ , As s+ , or S s+ , 1 < n < 9 , n+m+o=18 and p=2 . In the general and preferred formulas mentioned in U.S. Patent No. 5,302,248, heteroatoms are represented by the symbol "MG" , where MG is a main group element. However, a number of useful compounds introduced in U.S. Patent No. 5,695,606 contain heteroatoms that are ions of d block, rather than main group, elements. In U.S. Patent No. 5,695,606, as well as in the present invention, the symbol "X" is used to represent a heteroatom that may be either a p (main group) or d block element. Preferably, the reduced polyoxometalate is reoxidized with an oxidant selected from the group consisting of air, oxygen, hydrogen peroxide and other organic or inorganic peroxides (free acid or salt forms) , or ozone. Optionally, the method further comprises an extraction step following step (b) , wherein the lignin fragments are removed by extraction with a base. It is an object of the present invention to provide an improved method for delignifying lignocellulosic material including pulp or wood fibers from hardwood or softwood using a polyoxometalate . It is an advantage of the invention that by modifying known polyoxometalate delignification methods to include an alkaline or soda-AQ pretreatment step, enhanced delignification by the polyoxometalates is obtained, as measured by lignin removal, residue yield, or viscosity. It is a further advantage of the present invention that the improvement of adding an alkaline or soda-Aq step reduces the temperature and time needed to achieve delignification by polyoxometalates . It is an additional object of the present invention to delignify wood fibers or other lignocellulosic fibers using a polyoxometalate . It is an additional object of the present invention to employ an oxidant in the bleaching of pulp that may be regenerated by reoxidation of its reduced form. It is a feature of the present invention that suitable polyoxometalates may be reoxidized with an oxidant selected from the group consisting of air, oxygen, hydrogen peroxide and other organic or inorganic peroxides (free acid or salt forms) , or ozone . These oxidants are more environmentally friendly than chlorine compounds . It is another feature of the present invention that a polyoxometalate compound may be used as an oxidant in a repeated delignification or bleaching sequence. Other features, objects and advantages of the present invention will become apparent upon examination of the specification, claims and drawings. DETAILED DESCRIPTION OF THE INVENTION The present invention is directed to an improved process for selectively delignifying lignocellulosic materials. It was previously known that polyoxometalates may be used to bleach wood fiber pulp . The improved process involves pretreatment of the lignocellulosic material with a solution of a strong alkali or a strong alkali plus anthraquinone to partially delignify the material. Following the partial delignification step, at least a portion of the remaining lignin is oxidized by treatment with a solution of a polyoxometalate. Optionally, the oxidized lignin fragments may be extracted with a suitable agent . According to the method of the present invention, a lignocellulosic material is delignified by: (a) mixing the lignocellulosic material with a solution comprising a strong alkali at a suitable temperature and for a period of time sufficient to reduce the lignin concentration in the material ; and (b) contacting residual material of step (a) with a solution of at least one polyoxometalate of the formula (TM),_X r 0 S ] 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, provided that 1+m+n+o+p _> 4, l+m+q>0, and s is sufficiently large that x>0, wherein a solution is formed, under suitable conditions wherein at least a portion of the polyoxometalate is reduced and enhanced delignification of the alkali-treated material of step (a) occurs, relative to delignification by a comparable method lacking the alkali treatment step. In General The lignocellulosic material to be delignified may be first chopped or divided into relatively small pieces prior to the alkaline or soda-AQ pretreatment step. The lignocellulosic material be made into a pulp. For example, 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. As described below in the Examples, the present invention is suitable for delignification of both hardwood and softwood. Another class of pulps for which the present 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. Therefore, it is reasonably expected that the present invention may be practiced using any lignocellulosic material. Pretreatment with alkali or alkali/anthraquinone . As described in detail in the examples below, it has been discovered that "cooking" hardwood or softwood milled wood or chips with soda (NaOH) or soda-anthraquinone at a suitable temperature and for a suitable period of time prior to exposing the wood to a suitable polyoxometalate such as Na 5 [SiVW 11 0 40 ] enhances delignification. By "enhanced delignification" as used herein, it is meant that at least one of the following occurs: the residue lignin (%) is reduced; the efficiency of delignification (grams lignin removed/mole POM reduced) is increased; the residue yield (% * ) is increased; or the final viscosity (mPa.s) is increased. Pretreatment with alkali or alkali plus AQ reduces the extent of cellulose degradation (indicated by higher residue viscosities) relative to the cellulose degradation that occurs in materials not exposed to the preliminary cook (uncooked materials) . Although total residue yields were slightly reduced in some cases, the method produced residues with higher viscosities when a preliminary cook was employed. Pretreatment with alkali plus AQ has certain advantages over pretreatment with alkali alone. For example, the time and temperature of the pretreatment needed to achieve partial delignification is reduced when AQ is included in the pretreatment solution. The method of the invention is suitable for use in a wide variety of applications in which the removal of lignin from lignocellulosic materials is desired. Accordingly, the invention is intended to include, without limitation, methods for producing cellulosic pulps for making paper, regenerated cellulose products, enhancing the properties of recycled cellulosic fibers, and producing delignified animal feeds and other products. The method of the present invention depends on the pretreatment of the lignocellulosic material to be delignified with a strongly alkaline solution, including, but not limited to, solutions of sodium hydroxide or potassium hydroxide. Other examples of alkali salt solutions suitable for use in the practice of the present invention include solutions of salts of carbonate or borate, lithium hydroxide and any other strong alkali. The alkali is added to the lignocellulosic material in an amount effective to cause partial delignification of the lignocellulose. The amount of alkali required for partial delignification depends on a variety of factors , including the type of material to be delignified, the temperature, as well as on the cook length, and the particular alkali. Using the teachings of the present invention, it is expected that one could readily determine the concentration of alkali needed to reduce lignin. In the Examples below, sodium hydroxide was present in the cook at a concentration of 20% (NaOH/wood weight) in an aqueous solution that exceeded the wood weight by about 30-fold. It is expected that a larger or smaller volume of a NaOH solution of comparable normality would be equally effective. Considerable variations in the relative percentage of NaOH and wood are not expected to affect the practice of this invention. It is anticipated that alkaline solutions other than NaOH solutions would be useful in the practice of this present invention. Anthraquinone was added at a relative percentage of between about 0.5% (wood weight) in the case of aspen wood, and at a relative percentage of 0.06% (wood weight) in the case of white spruce wood. The relative amount of anthraquinone in the cook is not believed to be critical, provided that it is present in an amount sufficient to enhance delignification relative to a comparable method that lacks anthraquinone . Temperatures of between about 125°C and 171°C were used in the cook step. It is expected that the cook could be conducted at temperatures in the range from about 80°C to about 300°C. The length of the cook step may vary depending on the temperature employed and on the presence or absence of AQ. The next step of the present invention is the exposure of the pulp to a polyoxometalate . Polyoxometalates suitable for the present invention may be applied as stoichiometric oxidants, much as chlorine and chlorine dioxide are currently. Various groups of polyoxometalates suitable for use in delignification of lignocellulosic materials are discussed in detail in U.S. Patent Nos . 5,302,248 and 5,695,606. It is reasonably expected that the method of the present invention could be practiced using any suitable polyoxometalate . A common feature of the polyoxometalates found or expected to be useful (U.S. Patent Nos .5, 302 , 248 and 5,695,606) in oxidizing lignocellulosic materials is the presence of a vanadium ion in its +5 d° electronic configuration, 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 combination with chlorine-free oxidants such as oxygen, peroxides or ozone, complexes of this type oxidize functional groups within lignin, leading to delignification and bleaching. Delignification may occur via direct lignin oxidation by the d-electron-containing transition metal ion, or by a vanadium (+5) or molybdenum (+6) ion, leading to reversible reduction of the transition metal, vanadium, or molybdenum ion. In a subsequent step, the reduced polyoxometalate bleaching agent is regenerated to its active form by reaction with the chlorine-free oxidant. 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, vanadium (+5) , or molybdenum (+6) ion be present in the polyoxometalate structure. The polyoxometalates identified by structures defined by formulas 1-8 of U.S. Patent No. 5,695,606 are all logical candidates for use in bleaching with chlorine-free oxidants because they all possess either d-electron-containing transition metal, vanadium (+5) or molybdenum (+6) ions. The polyoxometalate of the present 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 + ) . The listed cations are suitable choices, but there are others that are available and cost-effective. 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. Another embodiment of the present invention has the additional step of regenerating the polyoxometalates 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 red ) 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, oxygen, 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) , thus omitting the necessity for two separate steps . Pulp+P ox → Bleached Pulp+P red (1) P red +0 2 +4H + → P ox + 2H 2 0 (2) As described below in the Examples , aqueous polyoxometalate solutions, preferably 0.001 to 0.6 M are prepared and the pH adjusted to 1.5 or higher. The polyoxometate may be prepared according to U.S. Patent No. 5,302,248 or 5,695,606 or by other standard procedures . An organic or inorganic buffer may be added to maintain the pH within a desired range during the delignification 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 in a sealed vessel either in the presence or absence of oxygen or other oxidants . The temperature and duration of polyoxometalate treatment will depend upon other variables, such as the nature of the pulp, the pH of the polyoxometalate solution and the nature and concentration of the polyoxometalate . The following nonlimiting examples are intended to be purely illustrative . Example 1 Preparation of polyoxometalates The Na 5 SiVW 11 O 40 was prepared by first synthesizing K B SiVW 11 O 40 according to previously described methods (P.J. Domaille, J. Amer. Chem. Socl , 106, 7677 (1984)) and converting it to the sodium salt using Purolite ion exchange resin (C100E) . The other compounds were prepared by mixing the quantities of the starting materials shown in Table 9 with 1100 mL of water in a 2 liter 316 stainless steel Parr reactor. The reactor was pressurized to 1480 kPa with 0 2 and heated to 210°C over the course of 1/2 hour and maintained at this temperature for 3 hours (with the exception of Na s AlVW 11 O 40 , which was maintained at temperature for 13 hours) . The solutions were filtered, if necessary, and concentrated for use as stock solutions. The concentrations of the solutions ([POM] in mol/L) were determined by measuring the density (P in g/cm 3 ) and applying the appropriate density factor (k p from Table 9) to equation 3. [POM = (p-l)k p Example 2 Delignification of aspen wood by Na 5 [SiVW 11 O 40) Soda or soda-anthraquinone precook of aspen wood Milled quaking aspen wood (Populus tremuloides Michx.) passed through a 40 mesh screen was subjected to soda cooking and soda- anthraquinone cooking using the conditions indicated in Table 1. The percent residue yields and residue lignin obtained are also given in Table 1. Potassium polyoxometalate delignification of aspen wood Untreated milled aspen wood and the cooked residues were delignified with the polyoxometalate using the conditions given in Table 2 using potassium as the cation associated with the polyoxometalate anion (runs 3 and 4) . A detailed description of delignification by polyoxometalates is provided in U.S. Patent No. 5,695,606. In comparing runs 3 and 4, Table 2, it can be seen that soda- cooked residue is much more easily delignified than untreated wood, as is evidenced by the lower reaction temperature and shorter reaction time needed to achieve delignification relative to untreated wood. In addition, when soda-cooked residue is used as the starting material, the resulting residue has a higher viscosity (indicating less cellulose degradation) at a lower lignin content relative to the residue obtained when untreated wood is used as the starting material (Table 2, runs 3 and 4) . However, the overall yield obtained from soda-cooked residue is lower than that obtained from untreated wood (55% versus 59%) . Polyoxometalate delignification of cooked or untreated aspen wood using sodium as the cation The untreated milled aspen wood and the cooked residues were subjected to polyoxometalate delignification under the conditions given in Table 2 using sodium as the cation (runs 36, 7, and 9) . Comparing the results obtained with runs 36, 7, and 9 (Table 2) it is apparent that the cooked residues are much more easily delignified and give much higher viscosities at lower lignin contents than uncooked wood. The soda-AQ residue is much more easily delignified than the soda residue and yet at a lower lignin content gives a slightly higher overall yield (56 versus 55) and a higher viscosity (57 versus 54) . Soda-AQ pretreatment appears to be a better pretreatment than pretreatment with soda alone. By comparing runs 4 and 7, it can be seen that slightly better results were obtained when potassium was used as the cation relative than when sodium was used as the cation. With the potassium polyoxometalate delignification, the residue lignin content is lower at the same overall yield and viscosity. Example 3. Delignification of sprucewood using Na [SiVW 11 O 40 ] Soda or soda-anthraquinone precook of white sprucewood Milled white spruce wood {Picea glauca) passed through a 40 mesh screen was subjected to soda cooking and soda-anthraquinone cooking using the conditions given in Table 3. The results are also given in Table 3. Polyoxometalate delignification of cooked or untreated spruce wood using sodium Untreated milled spruce wood and the cooked residues were delignified with the polyoxometalate using the conditions given in Table 4. A comparison of the results obtained with runs 37, 13 and 11 reveals that the cooked residues are much more easily delignified than the untreated wood (note the lower reaction temperature required with the cooked residue) . The final viscosity of the soda-cooked residue is just slightly lower than that of the untreated wood, despite a lignin content that is half that of the untreated wood. If the lignin content of the delignified cooked residue were equal to that of the delignified untreated wood, it follows that the viscosity of the cooked residue would be much higher than that of the delignified wood. As with the results obtained using aspen wood (Example 2) , the soda-AQ preliminary cook gave better results than did the soda cook. Both the yield and the final viscosity obtained with soda- AQ were much higher than the yield and final viscosity obtained with soda only. At equal residue lignin content, the soda-AQ overall yield would approach that of the uncooked wood. Example 4 Delignification of Loblolly Pine Soda AQ pulps using five different polyoxometalates Loblolly pine was pulped according to the conditions set forth in Tables 5 and 6 using 0.15 -1% anthraquinone and from 16- 20% NaOH for at 160-171°C for from about 55 min. to 2.5 hours. The pulped wood was treated with one of five polyoxometalates indicated in Tables 5-8 as described in the tables. The alkali extraction step was omitted from these delignification processes. The results of the experiments summarized in Tables 5-8 indicate that the soda-AQ pretreatment of lignocellulosic material and subsequent treatment with polyoxometalates is effective in delignifying loblolly pine, an important source of pulp wood that is generally difficult to delignify. The data further demonstrate that the method of the present invention is applicable to a variety of polyoxometalates, including Na 5 [SiVW 11 O 40 ] , Na 6 [SiV 2 W 10 O 40 ] , Na 6 , Na 5 [SiVMo 2 W 9 O 40 ] , or Na 5(+1 . 9) [SiV (0 9) MoW 10(+0 1 } O i0 ] . All publications cited in the specification are incorporated by reference herein. The present invention is not limited to the exemplified embodiments, but is intended to encompass all such modifications and variations as come within the scope of the following claims. TABLE 1 Soda and Soda-Anthraquinone Treatment of Milled Aspen Wood (through #40 mesh) TABLE 2 POM (Na 5 [SiVW 11 O 40 ] Delignification of 40 mesh Aspen Wood (20% lignin) versus Soda Cooked Aspen (16% lignin) versus Soda- AQ Cooked Aspen (10% lignin) * After the POM treatment the thoroughly washed residue was extracted with 1.0% NaOH at 50°C for one hour TABLE 3 Soda & Soda-Anthraquinone Treatment of Milled Spruce Wood (through #40 mesh) TABLE 4 POM (Na s [SiVW 11 O 40 ] Delignification of 40 mesh Spruce Wood (29% lignin) versus Soda Cooked Spruce (31% lignin) versus Soda-AQ Cooked Spruce (23% lignin) ♦After the POM treatment the thoroughly washed residue was extracted with 1.0% NaOH at 50°C for one hour TABLE 5 Na 5 [SiVW 1:l O 40 ] Delignif ication of Loblolly Pine Soda-AQ Pulps Soda Pulp No . P-44 . 1 1-11-99 #2 P-37 Pulping Conditions: Active alkali, % 20 1618 Anthraquinone , % 0.15 1.00.15 Temperature, °C171 171 160 Heatup Time, min. 60 6060 Cooking Time, min. 150 6555 Results : Yield. % 48 52 67 Kappa No . 47 49 120 POM Delig. Conditions : Run 92 108 85 POM, M 0.15 0.15 0.15 Na 2 W0 4 , M 0.15 0.15 0.15 H 2 S0 4 , M 0.02 0.02 0.02 Pulp Wt . , g 1.00 1.00 1.00 Solution, ml. 250 250 500 Reaction Temp. C140 140 145 Reaction Time,hr. 7.0 7.07.0 Initial pH 7.4 7.4 7.5 Final pH 6.9 6.9 6.9 Results : Residue Yield, %91 (44)* 90 (47)* 72 (48) * Lignin, % 0.2 0.2 0.5 Lignin Removed, %97 98 99 POM Reduced, % 25 27 36 Viscosity, mPa.sl4 23 28 Residue Color White+ White+ White+ Efficiency, 7.6 7.2 6.7 g.lig. rem./mole POM red. * Overall Yield No final alkali extraction was employed. TABLE 6 Na 6 Siv 2 W 10 O 40 and Na 6 AIVW, , 0 40 Delignif ication of Loblolly Pine Soda-AQ Pulp Soda Pulp No . P-37 P-37 P-37 Pulping Conditions : Active alkali, % 18 18 18 Anthraquinone , % 0.15 0.15 0.15 Temperature, °C 160 160 160 Heatup Time, min.60 60 60 Cooking Time, min.55 55 55 Results : Yield. % 67 67 67 Kappa No . 120 120 120 POM Delig. Conditions : Run 86 87 98 POM Used SiV2 AIV AIV POM, M 0.15 0.15 0.30 Pulp Wt . , g 1.00 1.00 1.00 Solution, ml. 500 500 500 Reaction Temp. . C145 145 160 Reaction Time, , hr .7.0 7.0 7.0 Initial pH 8.0 7.0 7.6 Final pH 8.4 7.6 7.6 Results : Residue Yield, %83 (56)* 88 (59)* 72 (48) * Kappa Number 47 62 9 Lignin Removed, %67 54 92 POM Reduced, % 19 15 18 Viscosity, mPa. , s -- 14 Residue Color Red Brown Red Brown+ Cream+ Efficiency 8.6 8.8 7.4 g.lig. rem. , /mole POM red. * Overall Yield No final alkali extraction was employed. TABLE 7 Na s [SiVMo 2 W 9 0 40 ] Delignif ication of Loblolly Pine Soda-AQ Pulp (Kappa No . 120 , Yield = 67% ) Run 99 100 Conditions : POM, M 0.50 0.50 Pulp Wt . , g 1.00 1.00 Solution, ml. 250 250 Reaction Temp. C 140 140 Reaction Time,hr. 3.0 6.0 Initial pH 5.0 5.0 Final pH 6.2 6.2 Results : Residue Yield, % 71 (48)* 68 (46)* Residue Lignin, % 3.9 1.3 Kappa Number 26 8.7 Lignin Removed, % 87 96 POM Reduced, % 26 29 Viscosity, mPa,s 30 27 Residue Color Tan- Cream Efficiency, 5.7 5.6 g.lig. rem./mole POM red. * Overall Yield No final alkali extraction was employed.

Claims: CLAIMS WE CLAIM: 1. A method for delignifying a lignocellulosic material comprising the steps of : (a) mixing the lignocellulosic material with a solution comprising a strong alkali at a suitable temperature and for a period of time sufficient to reduce the lignin concentration in the material; and (b) contacting the alkali-treated lignocellulosic material of step (a) with a solution of at least one polyoxometalate of the formula [V τ Mo m W n Nb 0 Ta p (TM) q X r 0 S ] 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, provided that 1+m+n+o+p > 4, l+m+q>0, and s is sufficiently large that x>0, wherein a solution is formed, under suitable conditions wherein at least a portion of the polyoxometalate is reduced and enhanced delignification of the alkali-treated material of step (a) occurs, relative to delignification by a comparable method lacking the alkali treatment step. 2. The method of Claim 1, wherein the solution of step (a) further comprises anthraquinone in an amount effective to enhance delignification relative to a comparable delignification method lacking anthraquinone . 3. The method of Claim 1, wherein the alkali of step (a) is NaOH. 4. The method of Claim 1, wherein the lignocellulosic material is hardwood or softwood. 5. The method of Claim 1, wherein the lignocellulosic material is a nonwoody plant . 6. The method of Claim 5, wherein the lignocellulosic material is sugar cane, begasse, kenaf, esparto grass, or straw. 7. The method of Claim 1, wherein the polyoxometalate heteroatom comprises Si (IV) or Al(III) . 8. The method of claim 2, wherein the anthraquinone of step (a) is present at a concentration of at least about 0.03% wood weight. 9. The method of claim 1, wherein the polyoxometalate is Na 5 [SiVW^O. , Na ε [SiV 2 W 10 O 40 ] , Na 6 [AlVW^O. , Na 5[ SiVMo 2 W 9 0 40 ] , or Na 5(+1 . 9) [SiVl^ . ^MoW^ o ^O,,,] . 10. The method of claim 1, further comprising the step of removing oxidized lignin present in the polyoxometalate-treated material of step (b) by extracting the material of step (b) with an alkaline solution. 11. The method of claim 1, further comprising the step of reoxidizing the reduced polyoxometalate with an oxidant. 12. The method of claim 9, wherein the oxidant is air, oxygen, peroxide or ozone . 13. The method of claim 1, wherein the lignocellulosic material is incubated with the alkaline pretreatment solution at a temperature of at least about 80°C.

Inventors: ATALLA RAJAI H, SPRINGER EDWARD L, WEINSTOCK IRA A, ATALLA, RAJAI, H., SPRINGER, EDWARD, L., WEINSTOCK, IRA, A.
Applicants: UNIV EMORY, US AGRICULTURE, EMORY UNIVERSITY, THE UNITED STATES OF AMERICA, REPRESENTED BY THE SECRETARY OF AGRICULTURE
Application year: 1999
Main classification: D21C1
IPC codes: D21C1, D21C3, D21C9