Recent advances in polyoxometalate based delignification

Abstract

A new delignification technology based on the use of polyoxometalates (POMs) as delignification agents is under development at the USDA Forest Service, Forest Products Labotatory (FPL), in Madison, Wisconsin. 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 re-activated with oxygen under conditions that result in oxidation of the organic byproducts of the delignification process. Thus, they can be continuously recycled in a closed system that promises to 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. The first POM systems investigated included α-(PV₂Mo₁₀O₄₀]^5– α-[SiVW₁₁O₄₀]^5–. The former is effective in both delignification and wet oxidation, but it is not stable at pH levels greater than 4 and can result in unacceptable hydrolytic degradation of cellulose. The latter, α-[SiVW₁₁O₄₀]^5–, is more effective at delignification and is useable at pH levels up to 6 or 7, but it is more difficult to re-oxidize. A new group of POMs are now under development; they combine the most attractive features of each of the members of the first group. In addition, they are more easily synthesized and they have an inherent self buffering capacity. Three members of this new group of POMs are under investigation and each has been found to be effective in both delignification and the regeneration/wet-oxidation. The new generation of POMs includes α-[SiV₂W₁₀O₄₀]^6– and α-[A1VW₁₁O₄₀]^6-, as well as a mixed tungstatemolybdate system. The presentation will provide an overview of the development of these new delignification systems and their potential as the basis for new pulping and bleaching technologies. The POMs have now also been applied to linerboard pulps at kappa levels of 65. They reduced the kappa to below 10, while maintaining the viscosity above 20 mPa•s. Therefore, in addition to providing an alternative closed-mill technology for traditional delignification stages in bleaching, POM-based processes can also be considered as a basis for accepting the pulp in the bleach plant at higher kappa levels. This possibility allows for reduction of the load on the digesters and recovery boiler, thereby increasing the capacity of kraft mills at a significantly lower level of investment.

Citation

Atalla, R. H.; Weinstock, L. A.; Reiner, R. S.; Houtman, C. J.; Reichel, S.; Hill, C. G.; Hill, C. L. 1999. Recent advances in polyoxometalate based delignification. [10th International Symposium on Wood and Pulping Chemistry, Main Symposium, 1999 June 07-10, Yokohama, Japan]. Atlanta, GA : TAPPI Press, 1999.:pp. 408-412.