Polyoxometalate-Based Closed Systems for Oxidative Delignification

Chapter 19

Polyoxometalate-Based Closed Systems for Oxidative Delignification of Wood Pulp Fibers 1,2

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R. H. Atalla , I. A. Weinstock , J. S. Bond , R. S. Reiner , D. M. Sonnen , C. J. Houtman , R. A. Heintz , C. G. Hill , C. L. Hill , M . W. Wemple , Yu. V. Geletii , and E. M. G. Barbuzzi 2

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Downloaded by COLUMBIA UNIV on September 17, 2012 | http://pubs.acs.org Publication Date: March 26, 2001 | doi: 10.1021/bk-2001-0785.ch019

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Forest Service, Forest Products Laboratory, U.S. Department of Agriculture, One Gifford Pinchot Drive, Madison, WI 53705 Department of Chemical Engineering, University of WisconsinMadison, Madison, WI 53706 Emory University, Department of Chemistry, Atlanta, GA 30322 2

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A new technology based on the use of polyoxometalates (POMs) as delignification agents is under development. These reagents are chlorine free and can be used under conditions wherein they oxidize lignin and chromophores in wood pulp fibers while leaving the cellulose undamaged. Their promise is enhanced by the fact that they can be reactivated with oxygen, in a separate stage, under conditions that result in oxidation of the organic byproducts of the delignification process. Thus, they can be continuously recycled in closed systems that can provide the basis for a new class of closed mill technologies in which the consumable oxidant is oxygen, and the primary byproducts are carbon dioxide and water. Such systems are inherently the product of well-integrated subsystems, each of which accomplishes a specific task. The two key tasks that have been the points of focus for the program are delignification of kraft pulps by POM solutions under anaerobic conditions, and regeneration of the oxidative capacity of spent POM liquors simultaneously

© 2001 American Chemical Society In Oxidative Delignification Chemistry; Argyropoulos, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.

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314 with oxidation of the organic byproducts to water and carbon dioxide. The first group of POMs investigated provided the basis for establishing the requirements for each of the tasks. They led to demonstration of selective delignification and effective re-oxidation and mineralization. The current generation of POMs have been optimized for effectiveness in both tasks. Furthermore they are readily synthesized and they are inherently self-buffering. Their performance in the context of traditional bleaching is indicated by the capacity to reduce the kappa levels of softwood kraft pulps from 30 to below 5 while retaining viscosity above 20. More recently it has been shown that they can effectively delignify kraft pulps at much higher kappa levels in the 100 to 110 range, and soda AQ pulps at kappa levels about 120. Thus POM delignification technology is applicable both for achieving TCF closed mill systems in the traditional bleaching context and, perhaps more significantly, for cost effective incremental expansions of the capacity of recovery-boiler-limited pulp mills.

Introduction The polyoxometalates (POMs) are a class of delignification agents that promise to provide the basis for a new class of closed-mill delignification technologies. The POMs used as delignification agents are transitionmetal-substituted cluster ions similar in structure to many mineral ores. They are entirely chlorine and sulfur free and can be used under conditions that make them very selective in their action on the constituents of lignocellulosics. They can be designed to be highly selective oxidants that can be targeted to the substructures that occur in lignin without also attacking the polysaccharide components of the wood fibers; in their active states, and when applied under anaerobic conditions, they can oxidize lignin and related chromophores, while leaving the polysaccharides undamaged. Their action on lignin results in reduction of the POMs so that their oxidative capacity has to be regenerated before they can be recycled. This is done in a separate stage wherein the reduced POMs are exposed to oxygen at elevated temperatures. The conditions for re-oxidation also result in oxidation of the organic byproducts of delignification. Thus, POM delignification is similar to kraft pulping processes in that the organic by

In Oxidative Delignification Chemistry; Argyropoulos, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.


315 products are oxidized rather than released to the environment; the POMs can be continuously recycled in a closed system. For these reasons, the POMs provide the basis for a new class of closed-mill delignification technologies in which the consumable oxidant is oxygen, and the primary byproducts are carbon dioxide and water. While the POMs were initially developed in the search for alternatives to chlorine based bleaching technologies, it has now become apparent that they can also complement kraft pulp mills by reducing the organic load on the recovery boilers. Thus they also present opportunities for expanding the capacity of kraft pulp mills with relatively low levels of capital expenditure. Eventually, together with soda-anthraquinone pulping, they may make possible entirely sulfur free pulping systems. We have previously reported on different generations of POMs that were investigated in our laboratory, and assessed them with respect to their suitability for use in economically viable and environmentally benign delignification systems (1-11). Here we present an overview of the evolution of closed-system delignification by POMs and the advances that were the basis for optimization. We also report on recent breakthroughs that have led to POMs that combine the most attractive characteristics of the POMs that were investigated in earlier stages. Because of these advances, we are now in a position to move toward development of pilotscale facilities for the use of POMs in delignification of commercial pulps. We would note here that a number of other investigations of the POMbased delignification have been reported (12), but they have focused on the use of POMs to promote oxidative delignification in a catalytic manner. As they do not address closed-system operation, we will not include them in our discussion.

Background The delignifying action of POMs was discovered as part of a new program on alternative pulping and bleaching technologies specifically directed at finding more selective oxidative systems. Selectivity was chosen as the point of emphasis because it was known that wood-degrading fungi use highly selective peroxidase enzymes to degrade lignin. These enzymes leave the cellulose intact so that it can later be hydrolyzed to glucose by a system of hydrolases; the fungi can then directly assimilate it as a nutrient. An interim goal of our program therefore, was duplication of the action of the peroxidase enzymes, but in inorganic systems that are useable in an industrial context.



316 We first recognized that the selectivity of the peroxidases is based on the way in which they use transition-metal ions in controlled environments to catalyze the oxidation of lignin. There is, in fact, a considerable literature on organic peroxidase analogs that are effective in selective oxidation of lignin; for example (13, 14). These systems, however, remain only of academic interest because they require complex organic platforms for the transition-metal ions, and the organic substructures are not stable at temperatures necessary to achieve the high rates required for an industrial process. The search turned to inorganic systems that can mimic the action of the peroxidases but have the stability prerequisite for application at elevated temperatures. It had earlier been reported that POMs provide controlled environments for transition metal ions, not unlike those provided by the porphyrins (15,16). They, therefore, seemed the logical candidates as a point of departure for our efforts. We recognized that polyoxometalate cluster ions provide a ligand environment that can mimic the function of the organic platform in many fungal enzyme systems. Our program thus began with the goal of simulating the action of enzymes, with the level of selectivity that we were seeking, by placing active metal ions in POM structures. The POMs are a class of oligomeric, metal-ion oxide clusters. The ones we have used in our program are among the most common and have structures identified as the Keggin type. In such structures, which are approximately spherical clusters, the anions typically include 12 structural transition-metal atoms, such as tungsten or molybdenum, surrounding a main-group atom, such as phosphorous, silicon, or aluminum; in most instances approximately 40 oxygen atoms are integrated into the structure. To make them active for delignification, one or two of the structural metal atoms of the cluster ion are replaced with a first-row transition-metal atom; the ones we have used most frequently are vanadium and manganese. In order to achieve the high degree of selectivity, the POMs have to be applied to the pulp in stoichiometeric amounts and under anaerobic conditions. It would have been preferable if the POMs could have been used catalytically, however, the re-oxidation of POMs by oxygen is accompanied by the generation of free radicals that are not selective in their action on substrates. Absent a component in the system that can act as a free radical scavenger, the re-oxidation of the POMs in the presence of pulp fibers can result in significant degradation of the polysaccharides. We have observed this both directly (17, 18) and indirectly (19). We have compared the application of POMs to pulp under oxygen with similar application under nitrogen. Application under oxygen resulted in significant reduction of pulp viscosity beyond that which can be attributed to hydrolysis. The



317 degree of viscosity reduction due to hydrolysis can be estimated from the loss when the POMs are applied under nitrogen (19). In the approach we have adopted, the re-oxidation of the POMs for recycling is carried out in a separate stage, after their separation from the pulps. In this stage, the spent delignification liquors are exposed to oxygen at elevated temperatures. The generation of free radicals during this stage has become the basis for mineralizing the lignin fragments dissolved in the liquor during delignification. The conditions are such that much of the organic material is oxidized to carbon dioxide and water; the POMs are stable since the structural metal ions in the clusters are in their highest oxidation states and the POM preparation is in a state of equilibrium (20). In the equilibrium state, a number of isomeric POM species coexist with oligomeric forms of the constitutive metal oxides. After re-oxidation the POM solution is recycled to delignifiy pulp again. The two steps of the process can be schematically represented in the following equations: Delignification: LigH + 2

^ Lig

POMQX

o x

+ POM ed + 2 H

+

r

(1)

Reoxidation and Catalytic Mineralization: Ligox + P O M d + 2 H + n02 +

re

POM

o x

+ mC02 + PH2O

(2)

When they are combined in a cyclic process, they can be represented as in Figure 1, where it is more clearly obvious that the two steps may be integrated into a two-stage cyclic process. The polysaccharides are not included in any of these representations as, apart from hydrolytic cleavage, they are not susceptible to oxidation under the conditions of unit operation A. In Figure 1, the delignification operation (A) is represented as the reaction of the POM, in its oxidized forms, with the lignin to produce the soluble oxidized lignin and the POM in its reduced form. The solubilized oxidized lignin, together with the reduced POM, is then fed into the reoxidation stage, where the POM is re-oxidized and the solubilized lignin is mineralized.



318

Figure 1. Net reactions of the POM process. Unit operation Λ denotes the delignifcation reactor under anaerobic conditions and unit operation D represents the wet-oxidation reactor. Further examination of equations 1 and 2 above reveals another important consideration that arises when the two steps are to be coupled. It is a consequence of the release of protons during the oxidation of lignin and their consumption during the re-oxidation by oxygen; the protons are incorporated into the H 0 formed by the reduction of the oxygen. For the coupling of these two stages, it is therefore necessary to have a buffer system. Yet another advantage of the POMs is that they are constituted from metal oxides that can occur in oligomeric forms in equilibrium with the Keggin anions and that can, through reorganization of the oligomeric distributions, provide the necessary buffering capacity. This will be illustrated below in the results of cyclic studies of delignification and re oxidation using one of the POM systems investigated. 2

Process Concept For use of the POMs in a delignification process that can be scaled up for commercial use, it is necessary to consider how the different chemical transformations can be organized in relation to each other. At the heart of the process, there are the two complementary operations indicated by the equations and schematic shown above. In the first one, an aqueous solution of POM in its oxidized form is applied to the pulp under anaerobic conditions. It oxidizes and solublilizes the lignin in the pulp and is itself reduced. When its oxidizing capacity has declined to a point determined by overall process parameters, the spent liquor, with all the soluble organics released during the delignification operation, is separated from the pulp. In



319 the second operation, the spent solution of POM and organics is oxidized with oxygen at an elevated temperature. Under these conditions, the reduced POM is reoxidized, and simultaneously, it catalyzes the oxidative degradation of the soluble organics to carbon dioxide and water. The process envisioned for POM delignification is shown in Figure 2. As noted earlier, the key unit operations are delignification in stage A and wet oxidation in stage D. Commercial installations will also require a number of additional complementary unit operations. The operation Β in Figure 2 designates the stages required for separating the pulpfromthe spent POM liquors. It begins with pressing to remove as much of the POM and soluble organic liquid phase as is feasible. The next stage is the washing of the pulp. It has been demonstrated that POMs can be effectively separatedfrompulp by washing. The high negative charge of the POM anions makes separation from the negatively charged pulp fibers quite easy. Preliminary laboratory studies have shown that the washing operation is such that it lends itself well to the use of commercially available washing equipment. Operation C then represents concentration of the POM and soluble organics separatedfromthe pulp during washing. Here again, evaporation of the wash water can be readily accomplished in commercially available evaporator systems. It is also anticipated mat a stage for removal of non-process elements will be incorporated into die overall scheme. Since multiple preliminary studies have demonstrated that operations Β and C, representing washing and recovery, can be accomplished with established technologies, the primary focus of our program has been on improvement of the delignification (A) and wet-oxidation stages (D) and on simplification of the processes for synthesis of the POMs. We are also seeking a deeper understanding of the physical and chemical processes that are the rate limiting steps in both A and D stages.

Figure 2. A simplified schematic representation of the POM delignification process.


320 Evolution of POM Development Early Studies In our first reports on POM-based delignification, we described successful operation in a low-consistency, multistage sequence at 125°C, using N a P V M o O as the POM (4,5). Hereinafter it will be designated as P V M o , and the same notation will be used in describing other POMs, as in most instances the counterion is Na and the complement of Ο atoms is similar. The work with P V M o was important in that it allowed us to demonstrate the high degree of selectivity of the oxidation that can be designed into POMs and it revealed to us their self buffering capacity. However, it is not stable at pH levels above 4, so that it could not be used at temperature levels that result in commercially feasible delignification rates. At such temperatures and pH levels, the hydrolysis of the cellulose occurred at unacceptable rates. It should be noted here that, unless otherwise noted, the pulps used in all trials were northern pine kraft from a commercial source. The next cycle of delignification trials were carried out with N a S i V W O , henceforth denoted by S i V W , which is stable at neutral pH levels and has a slightly higher oxidation potential than P V M o . It was possible to reduce the duration of the individual stages and to operate successfully at temperatures below 100°C. We also demonstrated effective operation at medium consistency (11%). Two representative examples are described. In the first, carried out at 125°C, the kappa number was reduced from 24.5 to 12.6 with a 30-minute POM stage, followed by caustic extraction. The corresponding loss in viscosity was from 28.7 to 22.2 m Pa s. This was accomplished by application of 0.5 M S i V W in 1.0 M phosphate buffer, which kept the pH in the 6 - 7 range. In the second example, carried out at 90°C, two successive POM stages, followed by caustic extraction, reduced the kappa number from 24.5 to 11.9, with a corresponding decline in viscosity from 28.7 to 27.3 m Pa.s. This was accomplished by application of 0.5 M S i V W in 0.5 M acetate buffer, which kep the pH at 5.5. The caustic extraction between the two stages utilized 1.0 % NaOH for 2 hours at 85°C and 10 % consistency. These two examples pointed to the possibility of further optimization of POM-based delignification. It was with the S i V W system, as well, that the possibility of achieving very good paper properties was first demonstrated. The measured paper properties that were demonstrated with S i V W are shown in Table I, where 5

2

2

10

40

10


2

5

n

40

10

u

2

10

n

H

H

u


321 they are compared with properties of the same pulp after a C/D E treatment and also with the properties of a control sample that was subjected to the same sequence of treatments as the POM-treated pulp, but without the presence of the POM; the control had a final kappa of 20.2 (9,10,11). The conditions of application of the S i V W differed somewhat from those described above in that the concentrations of the POM solutions were lower so that more than one application was necessary to fully accomplish the oxidation of the ligning. The POM delignified pulps the properties of which are reported in Table I were produced by applying 0.05 M S i V W in 0.2 M phosphate buffer. The first application was for 1 hour; this was followed by two 2 hour applications; all were at 125°C at 3 % consistency. The caustic extraction was with 1 % NaOH for 2 hours at 85°C at a consistenty of 2%. 30

n


u

Table L Paper Properties of P O M Delignified Pulps C/D E 30

P O M Control

Viscosity (mPa sec)

23.1

19.5

23.2

Burst Index (kN/g)

8.65

8.18

7.11

Tear Index (mN rriVg)

9.03

9.30

9.45

Tensile Index (N m/g)

109.0

103.7

96.5

Zero-Span (N m/g)

160.8

148.4

150.3

Density (kg/m )

796.1

792.3 769.9

3

• 5000 rev. PFl mill (360 ml CSF) • Initial Kappa of 24 • C/D Ε and POM bleached to a Kappa of 4.6

It is clear that the POM-treated pulp has properties that are comparable to those of the C/D E pulp. The Achilles heel of the S i V W system was, however, that it is not as easily re-oxidized with 0 at elevated temperatures, at least not at rates that were regarded as desirable in a commercial operation. We have reported that wet-oxidation can be carried out effectively and POM-containing liquors can be recycled for delignification (5). These wet oxidation studies, using P V M o , demonstrated the feasibility of achieving a low level of chemical oxygen demand (COD), together with complete re oxidation of the POM. They also demonstrated that the resulting liquors, 30

n

2

2

10


322 with the residual COD, can be used for delignification and are as effective as the fresh POM solutions. Although P V M o is not a feasible delignifcation agent from a commercial perspective, it remains an important POM in studies directed at characterizing the delignification liquors and the chemical transformations that occur during wet oxidation. It is anticipated that the results of these studies will be helpful in facilitating further optimization of the wet oxidation process. Although the first generation of POMs did not provide the basis for a commercial process, it did allow us to explore the behavior of POMs in the context of delignification and wet oxidation. The key to successful commercialization was the identification of POMs that are effective in both delignification and wet oxidation stages, and that are also self-buffering.


2

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The Second Generation

The development of the second generation of POMs was based on combining the characteristics of the two earlier systems. It was found that by shifting to variations on the S i V W we could move towards better performance. We report here the development of a new group of POMs that have a number of advantages over the first generation. In addition to being well suited to both delignification and wet oxidation, the new POMs are stable at pH levels above neutral, so hydrolysis of the cellulose is significantly reduced. Another important advance associated with the introduction of this new group of POMs has been the development of a new synthetic procedure that results in an equilibrium composition, which is inherently stable and, therefore, can be recycled repeatedly in a closed system (20). The new synthetic procedure is much more simple than traditional synthetic approaches. The availability of this new procedure should facilitate the design of POM production processes on an industrial scale, and it should make research in the field of POM delignification more accessible to other laboratories. A number of new POMs, all of which appear to be stable above pH 7 and are re-oxidized by oxygen, have been explored on the basis of the new synthetic approach. These include SiV Wi , A l V W , and a group that is described by the formula S i V M o W . O . Here it should be noted that the formula presented is intended to reflect the molar ratios in the final equilibrium mixture, which is likely to include a number of different Keggin structures, the majority of the isomers of the dominant species. The POM that has been studied most extensively is SiV W , which has been routinely used to reduce kappa number from about 32 to below 10 under several different conditions, including multiple stages at 10% u

2

x

n

x

0

n

40

2

10


323 consistency. The yield levels, in most instances, are above 95%. The effectiveness of this POM is illustrated in Figure 3, which shows the results of a cyclic experiment wherein the same solution of S i V W was used repeatedly first to delignify pulp fibers and then as the vehicle for mineralization of the organics removed as it was re-oxidized prior to re-use. In the experiment shown in Figure 3, the same 0.5M solution of SiV W O was used again and again, to delignify softwood kraft pulp from kappa of 33 to an average kappa of 8, and then re-oxidized with oxygen. The conditions of application were at a consistency of 3 %, for three hours at 150°C. The average drop in viscosity was from 30.4 to 19.4 and the average yield was 94.9 %. The experiment was repeated for twenty cycles; each cycle in the figure represents one bleaching reaction and one re-oxidation reaction. The left scale shows the % reduction of the POM, while the right scale shows the pH at the end of each cycle. The even oscillation of the reduction curve shows that the POM can be fully regenerated and used effectively in delignification over an indefinite number of cycles. The shifting of the pH within a very narrow band demonstrates the effectiveness of the POM as a self-buffering system. 2

10


2

0

5

10

15

10

40

20

CYCLE Figure 3. The results of cyclic experiments ofsuccessive delignification and oxidation reactions carried out with S1V2WJO. Left scale: % reduction of the POM. Right scale: pH.



324 When one considers that a proton is released into solution for each POM molecule reduced, it can be estimated that the pH of the solution would have dropped to 1 during the delignification reaction if theself-buffering capacity of the system were not avaialble. Of course, significant decline of the pH would and resulted in unacceptable degrees of hydrolytic degradation of the cellulose in the pulp fibers. The buffering capacity of the POM solutions is the result of shifting of the balance between the condensation and hydrolysis reactions of the oligomeric constitutive metal oxides present in the equilibrium solutions with the POMs (20). In addition to its extensive use in tests with pulps at kappa levels in the low 30s, this POM has been used to test the feasibility of applying POM technology to higher kappa pulps. This possibility is of interest in mills that have well-established delignification and bleaching stages but could benefit from increasing the kappa level at which the pulp is removed from the digesters. The results of some trials, with a southern pine linerboard pulp at a kappa of 65, are shown in Table II. It is clear that even if the POM stage was used to completely delignify the pulp, the viscosities are still acceptable. Beyond the studies with S i V W , other variations on the S i V W system have been explored. It has been shown that some are even more effective in meeting the criteria for commercial development. In particular it has been observed that addition of small amounts of molybdenum, as in S i V M o W , can enhance the effectiveness to the point where the properties of the pulps match those of commercial ECF pulps at acceptable residence times in the delignification reactor. It has also been demonstrated that SiV M n j M o W j is even more effective in the regeneration and mineralization steps during the wet-oxidation stage. 2

09

10

u

101

09

0

10

Table II: Delignification of Southern Pine Kaft Linerboard with 0.5 M SÎV2W10 Temp (°C)

Time (hr.)

Liquor to wood (mL/g)

Final pH

Final Kappa

145 7 100 9.3 7 145 100 14 9.3 2 initial kappa 65 ^estimated pulping yield of 55% on OD wood

1

Final Viscosity (mPa-s)

27 18

Yield on Pulp (%) 90 90

1


Yield on Wood 2

(%) 50 50

325

Conclusion


It is now clear that POM-based technology has the potential to outperform currently available technologies with respect to facilitating closed-mill systems and reducing environmental impact. It has also been shown that the technology is more energy efficient and quite competitive economically. The member companies participating in the consortium supporting this work, jointly with the USDA Forest Service and the U.S. Department of Energy, are now considering the establishment of a pilot facility to demonstrate the commercial feasibility of POM-based delignification technologies.

Acknowledgments We wish to acknowledge support of this work by the USDA Forest Service, the U.S. Department of Energy, Office of Industrial Technologies, and the member companies of the Polyoxometalate Bleaching Consortium.

References 1.

R.H.Atalla, I.A.Weinstock, R.S.Reiner, C.J.Houtman, S.Reichel, C.G.Hill, C.L.Hi11, M.W.Wemple, J.Cowan, E.M.G.Barbuzzi, Proceedings of the 10 International Symposium on Wood and Pulping Chemistry, Yokohama, June 1999, V.II, p. 408. Weinstock, I.A., Atalla, R.H., Reiner, R.S., Houtman, C.J., Hill, C.L.: Selective transition-metal catalysis of oxygen delignification using water-soluble salts of polyoxometalate (POM) anions. Part I. Chemical principles and process concepts. Holzforschung 1998, 52, 304-310. Weinstock, I.A., Hammel K.E., Moen, M.A., Landucci, L.L., Ralph, S., Sullivan, C.E., Reiner, R.S.: Selective transition-metal catalysis of oxygen delignification using water-soluble salts of polyoxometalate (POM) anions. Part Π. Reactions of a -[SiVW O ] with phenolic lignin-model compounds. Holzforschung 1998, 52, 311-318. Weinstock,I.A.,Atalla, R.H., Reiner, R.S., Moen, M.A., Hammel, K.E., Houtman, C.J., Hill, C.L., Harrup, M.K.: A new environmentally benign technology for transforming wood pulp into papers: Engineering polyoxometalates as catalysts for multiple processes. J. Mol. Cat. A: Chem. 1997, 116, 59-84. Sonnen, D.M., Reiner, R.S., Atalla, R.H., Weinstock, I.A.: Degradation of pulpmill effluent by oxygen and Na [PV Mo O ], a multipurpose delignification and wet air oxidation catalyst Ind. Eng. Chem. Res. 1997, 36, 4134-4142. th

2.

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5-

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4.

5.

40

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326 6.

7.


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12.

13. 14. 15. 16.

17. 18. 19. 20.

Atalla, R.H., Weinstock, I.A., Hill, C.L., Reiner, R.S., Houtman, C.J.: Polyoxometalate bleaching of kraft pulps: A highly selective closed mill technology. 1996 International Non-Chlorine Bleaching Conf., March 24-28, 1996, Orlando, FL, 9 p. Weinstock, I.A., Atalla, R.H., Reiner, R.S., Moen, M.A., Hammel, K.E., Houtman, C.J., Hill, C.L.: A new environmentally benign technology and approach to bleaching kraft pulp. Polyoxometalates for selective delignification and waste mineralization. New J. Chem. 1996, 20, 269-275. Atalla, R.H., Weinstock, I.A., Reiner, R.S., Houtman, C.J., Hill, C.G., Hill, C.L.: Polyoxometalate bleaching: A new effluent-free technology. Fourth European Workshop on Lignocellulosics and Pulp, September 8-11, 1996, Stresa, Italy, pp. 189-193. Weinstock, I.A., Atalla, R.H., Reiner, R.S., Hill, C.L., Houtman, C.J.: Polyoxometalate bleaching: A highly selective, closed-mill technology. 1995 TAPPI International Environmental Conf., May 7-10, 1995, Atlanta, GA, Book 2, pp. 1197-1199. Weinstock, I.A., Atalla, R.H., Reiner, R.S., Hill, C.L.: Highly selective TCF bleaching of kraft pulp by water soluble polyoxometalate complexes. Eighth International Symposium of Wood and Pulping Chemistry, June 6-9, 1995, Helsinki, Finland, Volume I, pp. 369-376. Weinstock, I.A., Atalla, R.H., Reiner, R.S., Moen, M.E., Hammel, K.E., Hill, C.L., Houtman, C.J.: Polyoxometalate bleaching of kraft pulp: A highly selective closed mill technology. 1995 TAPPI Pulping Conf., October 1-5, 1995, Chicago, IL, Volume I, pp. 153-156. Evtuguin, D.V., Neto, C.P., Proceedings of the 10 International Symposium on Wood and Pulping Chemistry, vol. 1, pp 330-334, Yokohama, Japan, Jappan TAPPI, 1999, and references cited therein. Habe,T., Shimada, M., Higuchi, T., Mokuzai Gakkaishi 1985, 31, 54. Shimada, M., Habe, T., Higuchi, T., Okamoto, T., Panijpan, B., Holzforschung 1987, 41, 277. Hill, C. L.; Brown, R. B., Jr., J. Am. Chem. Soc. 1986, 108, 536. Hill, C. L. "Catalytic Oxygenation of Unactivated C-H Bonds. Superior Oxo Transfer Catalysts and the 'Inorganic Porphyrin'," in Activation and Functionalization of Alkanes, Hill, C. L. Ed. Wiley: New York, 1989, Chapter VIII. Birchmeier, M., Doctoral Dissertation, Chemical Engineering Department, University of Wisconsin, Madison, 2000, to be published. Birchmeier, M., Atalla, R.H., Hill, C.G., to be published. Reiner, R.S., Weinstock, I.A., Atalla, R.H., unpublished. Reiner, R.S., Weinstock, I.A., Atalla, R.H., Hill, C.L., Wemple, M., Cowan, J., Heintz, R., Bailey, Α., pending patent application. th


Polyoxometalate-Based Closed Systems for Oxidative Delignification

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