Metal ligand containing bleaching compositions

Details Metal ligand containing bleaching compositions Metal ligand containing bleaching compositions

2000-05-04

2001-06-05

Anthony, Joseph D. (Department: 1714)

Bleaching and dyeing; fluid treatment and chemical modification

Bleaching

Chemical

C008S107000, C008S108100, C252S186330, C252S186430, C252S186390, C510S311000

Reexamination Certificate

active

06241779

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to the use of macrocyclic metal-ligand complexes as bleaching catalysts, and more particularly, to transition metal complexes of macrocyclic tetraamide ligands as catalysts for enhancing oxidative bleaching reactions.
The United States and Canada are the world's leading producers of wood pulp used for the production of paper and paper board. In 1994, the United States produced over 58 million tons of wood pulp. Pulp, which is made either mechanically or chemically from wood, contains 1) cellulose, a homopolysaccharide linear polymer of d-glucose of the formula —(C
6
H
10
O
5
)—; 2) lignin, a nonuniform three dimensional molecule having the following general composition, C
9
H
8.83
O
237
(OCH
3
)
0.96
; and 3) hemicellulose, a heteropolysaccharide polymer. See generally, W. G. Glasser and S. Sarkanen, eds. “LIGNIN PROPERTIES AND MATERIALS,” American Chemical Society Symposium Series 397.
Desirable qualities for paper include strength, whiteness and brightness. The strength of the paper is related to the viscosity of the pulp employed in its manufacture which, in turn, is related to the condition of the cellulose after the pulping operation Molecular cellulose, as explained above, is a linear chain of d-glucose which naturally forms long fibers. The longer the individual cellulose polymer chains, the higher the viscosity of the pulp, and in turn, the greater the strength of the paper. Thus, during processing, it is most desirable to avoid cleaving the cellulose polymers into smaller units.
Whiteness is based on the appearance of the paper to observers and its measure is therefore subjective. Brightness is a term used to describe the whiteness of pulp on a scale from 0% (absolute black) to 100% (relative to a MgO standard, which has an absolute brightness of ca. 96%) by the reflectance of blue light (457 nm) from the paper produced from the pulp. The more incident light that is reflected, rather than absorbed, the brighter the paper.
Brightness is obtained by bleaching. Pulp bleaching is defined as the treatment of cellulose fibers with chemicals to increase brightness. Bleaching chemicals increase brightness by removing and decolorizing the lignin in the pulp. Lignin exhibits a yellowish to a deep brown color, depending on the type of wood.
The most common bleaching chemicals are the oxidants chlorine, a source of hypochlorite ion, and chlorine dioxide. Oxygen gas in conjunction with NaOH may also be used, but requires expensive equipment and must be used in large amounts. Oxygen also results in loss of pulp strength resulting from free radical damage to the cellulose polymers, particularly when the lignin content of the pulp is low.
Chlorine and hypochlorite can result in loss of strength if used improperly, but in general are effective and relatively easy to use oxidants. Hypochlorite is an aggressive oxidant that is prone to attacking the cellulose, especially if nonoptimally employed. Chlorine dioxide achieves a high level of brightness without pulp degradation. However, it is an expensive oxidant and it is prone to explosive decomposition. All the chlorine based oxidants produce as effluent chlorinated byproducts that are hazardous to the environment and to health. Moreover, effluent that contains chlorine in any chemical form cannot be burned in the recovery boiler of a pulp mill. The chlorine produces corrosion of the recovery boiler. Moreover, as noted below, combustion of chlorine containing species can lead to the production of polychlorinated dioxins and dibenzofurans, 17 of which are considered toxic and carcinogenic. In addition, chlorine, for example, can react violently with combustible materials. It reacts with H
2
S, CO and SO
2
to form toxic and corrosive gases; and, in liquid form, causes bums, blistering and tissue destruction. In gaseous form, it causes severe irritation to eyes, nasal passages and respiratory tissue. In high doses, it can be lethal. Chlorine dioxide bleach decomposes into Cl
2
which is toxic and corrosive.
Polychlorinated aromatic compounds are environmental pollutants. The most well known examples are DDT, the polychlorinated phenols, dioxins, dibenzofurns and polychlorinated biphenyls (PCBs). These types of compounds can be formed when appropriate organic compounds are exposed to chlorine containing oxidants. The combustion of organic matter in the presence of chlorine in any form can produce dioxin. Even though dioxins and PCBs are no longer manufactured, there are chemical processes that form these compounds from polychlorinated phenol precursors. There is a need to prevent the unwanted formation of polychlorinated aromatic compounds and to remediate those that are present in the environment.
In the pulp and paper industry, chlorinated organics (monochlorinated and polychlorinated), collectively called “absorbable or adsorbable organic halogen” or “AOX”, are formed upon bleaching of wood pulp with chlorine based oxidants. One such compound is 2,4,6-trichlorophenol, TCP, which is produced, for example, during the bleaching process when chlorine is used as the bleaching agent. TCP ends up in the waste stream leaving the plant.
Notwithstanding the hazards to the environment, the chlorine-based oxidants are the most widely used for pulp bleaching in the United States. Commercial pulp and paper bleaching facilities actually use a combination of several methods. One widely used bleaching sequence begins with chlorination, followed by extraction with NaOH, treatment with chlorine dioxide, more NaOH extraction and then more chlorine dioxide treatment. A modification of that sequence adds a hypochlorite oxidation step between the first NaOH extraction and first treatment with chlorine dioxide. In another sequence, the second NaOH extraction and second chlorine dioxide treatment are eliminated.
On Nov. 14, 1997, the United States Environmental Protection Agency signed a Cluster Rule requiring the Pulp and Paper Industry to reduce chlorinated organics production. To meet the effluent reduction requirements, the industry is primarily expanding the use of what is called “elemental chlorine free” (ECF), which is a term used primarily for bleaching with chlorine dioxide. The important point is that chlorine dioxide bleaching produces considerably less toxic effluent than does bleaching with elemental chlorine, Cl
2
. Nevertheless, some AOX is produced and a further disadvantage is that the bleach plant effluent cannot be burned in the recovery boiler as noted above. In addition, the industry has been developing what is calls “totally chlorine free” (TCF) bleaching. The principal oxidants of TCF bleaching are oxygen and hydrogen peroxide, although ozone also has a position. Hydrogen peroxide oxidizes and brightens lignin and produces high yields of pulp. It is easier to use than oxygen and it does not require expensive equipment, one of the big disadvantages of oxygen bleaching. In use, it is generally believed that H
2
O
2
dissociates to produce the perhydroxyl ion, OOH—, which decolorizes lignin and does not attack cellulose. However, if H
2
O
2
decomposes, it produces free radicals which fragment the lignin as desired, but also degrades the cellulose. The principal offending radical is the hydroxyl radical, HO•, which is notoriously nonselective. Because the H—O bond of water is so strong (ca. 119 kcal.mol
−1
), the HO• radical will abstract H atoms rapidly from a wide variety of organic compounds and, indeed, from most H-atom sources. For this reason, pulp is generally treated with a sequestering agent prior to peroxide treatment. The purpose of the sequestering agent is to remove metal ions which decompose the peroxy compound producing radicals. Furthermore, peroxide leaching methods will often include the addition of further sequestering agent, again or shielding the peroxy compound from exposure to trace amounts of metal which can decompose it unnecessarily and lower its selectivity. While hydrogen peroxide itself is a strong oxidant which can burn skin and mucous membr

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