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Lignin And Lignan Biosynthesis
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Craig L. Hill
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November 12, 2001
Volume 79, Number 46
CENEAR 79 46 p. 5
ISSN 0009-2347
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With cluster anion ensembles, O2 does selective oxidations in water


Searching for "greener" ways to remove lignin from wood, a process known as pulping, researchers have developed a solution that could be applied to the generic problems of catalytic oxidations using molecular oxygen in water.

The solution involves use of metal-oxide cluster anions, or polyoxometalates (POMs), in equilibrating ensembles that are thermodynamically stable in water [Nature, 414, 191 (2001)]. It was developed by chemists Ira A. Weinstock, at the USDA Forest Service, Forest Products Laboratory in Madison, Wis.; Craig L. Hill, at Emory University; and coworkers through a project funded in part by the pulp, paper, and allied industries.

Biological oxidations using O2 in aqueous media proceed readily and selectively. Industry would like to make use of similar reactions, but that's been hard to realize because of problems inherent in the use of metallic catalysts in water and in oxidations involving O2.

Water hydrolyzes--and therefore inactivates--metallic catalysts. Oxidations with O2 generate radicals that react indiscriminately. Also, oxidations and reductions in water produce H+ and OH ions, respectively, necessitating buffering of pH-sensitive processes.

To solve these problems, the researchers turned to POMs, which are known to be oxidatively stable and reversible oxidants. At specific pH and ionic strength values, they spontaneously assemble into clusters.

However, pure POMs are prone to hydrolysis. To achieve stability in water and impart buffering capacity, the researchers designed a new chemical entity--a thermodynamically stable ensemble of five POMs dominated by a heteropolytungstate anion.

The new entity is formed by mixing the ingredients required to form the heteropolytungstate anion in water and heating. The product is a mixture consisting of that anion in dynamic equilibrium with four other POMs, which were designed into the mix for pH control. Because the system is made with water, it is stable in water.

This approach was developed to address delignification speci- fically, but the idea of using water to develop thermodynamically stable inorganic systems has general applicability, Hill says.

In delignification, the challenge is not only to use O2 in water, but also to selectively remove lignin. Fine-tuning was key, Hill says. "POMs come in so many different combinations. But because they are easy to produce, modify, and evaluate, they are very tunable. You can make them extremely selective."

At present, paper production in the U.S. yields a lot of liquid waste containing chlorinated aromatic materials. The researchers believe that pollution-free delignification is possible with POM ensembles. The process they have developed to convert lignin in wood to carbon dioxide and water with POM ensembles is still inefficient. However, says Weinstock, "industry has given us criteria that, if met, they say, will make the process economically viable."

INGENIOUS Oxidation of lignin by the heteropolytungstate anion (olive green) produces a reduced anion (blue), cellulose, soluble lignin, and H+ ions. Reaction of H+ ions with the other cluster anions to form water shifts the equilibrium of the ensemble. Next, O2 oxidizes the reduced anion to the active form. Simultaneously, the heteropolytungstate anion catalytically converts the soluble lignin to carbon dioxide and water (red). Reduction of O2 forms "O2– equivalents," which in water become OH ions. These anions are taken up by the other cluster anions, and the equilibrium shifts again. The pH remains near neutral throughout.

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