Oxidase process for pulp

Details Oxidase process for pulp Oxidase process for pulp

2000-10-10

2003-12-09

Alvo, Steve (Department: 1731)

Paper making and fiber liberation

Processes of chemical liberation, recovery or purification...

Gas, vapor or mist contact

C162S072000, C162S078000, C162S079000, C162S087000, C162S088000

Reexamination Certificate

active

06660128

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to an enzymatic redox catalytic process to delignify and bleach lignocellulosic materials, and oxidise dyes to remove colour, and more specifically a process involving phenol oxidases and a transition metal complex.
BACKGROUND ART
Bleaching of kraft pulp is traditionally performed by sequential reaction with chlorine or chlorine dioxide and sodium hydroxide. Environmental concerns and system closure requirements have opened up new opportunities for biotechnology to replace the use of chlorine or chlorine dioxide in pulp bleaching operations.
Enzyme application for pulp bleaching was first reported by Viikari et al., Proc. Third Intl. Conf. Biotechnol. Pulp Paper Industry, Stockholm, 67-69, 1986. The enzyme xylanase was found to enhance the effect of bleaching chemicals by hydrolyzing the xylan network surrounding lignin. The enzymatic treatment usually results in a 10 to 20% saving of bleaching chemicals. Larger chemical savings can be obtained by using a fungal culture of
Trametes
(
Coriolus
)
versicolor
, but the rate of biodelignification is too slow for commercial application (Paice et al., Tappi J. 72(5): 217-221, 1989, Reid et al., Tappi J. 73 (8): 149-15, 1990). The fungal bleaching effect is accompanied by the secretion of at least two lignin-oxidizing enzymes, laccase and manganese peroxidase (Bourbonnais and Paice, Appl Microbiol Biotechnol 36: 823-827, 1992; Paice et al., Appl. Environ. Microbiol. 59: 260-265, 1993).
Laccase is a multicopper oxidase which reduces oxygen to water and simultaneously performs one-electron oxidation of many aromatic substrates (Reinhammar et al, Boca Raton, Florida: CRC Press, Vol. 3, pp. 1-35, 1984). Laccase alone has a limited effect on pulp bleaching due to its specificity for phenolic subunits in lignin (Higuchi, NG Lewis, MG Paice, eds. Plant cell wall polymers: biogenesis and biodegradation. ACS Symposium Series, 399: 482-502, 1989). It has been reported that the substrate range of laccase can be extended to non-phenolic subunits of lignin by inclusion of a mediator such as 2,2′-azinobis-(3-ethylbenzthiazoline-6-sulfonate) (ABTS) (Bourbonnais and Paice, FEBS Lett 267: 99-102, 1990). Furthermore, the laccase and ABTS couple was also shown to effectively demethylate and delignify kraft pulp (Bourbonnais and Paice, Appl Microbiol Biotechnol 36: 823-827, 1992);
Over 50% delignification was reported with laccase/ABTS followed by alkaline extraction under conditions of time, temperature and consistency compatible with current bleaching technology (Bourbonnais and Paice, Tappi J 79(6): 199-204, 1996).
Since the initial report with the mediator ABTS, there has been intense research activity to discover a cost-effective laccase/mediator combination, and several nitrogen-containing aromatic compounds are now known to be at least as effective as ABTS. In a series of patent applications, Call describes a process for modifying, breaking down or bleaching lignin, where mediators of the family of N-hydroxy aromatic compounds such as 1-hydroxybenzotriazole (HBT) (WO94/29510), violuric acid (WO 97/36039) and N-hydroxyacetanilide (NHAA) (WO 97/36041) were used. Furthermore, Zing et al. (WO 97/06244) describe the use of nitroso-hydroxy aromatic compounds for enhancing the bleaching activity of laccase for pulp and textile applications. In WO 94/12619, WO 94/12620 and WO 94/12621 Schneider et al. proposed the application of peroxidase enzyme in the presence of hydrogen peroxide and several aromatic compounds to bleach lignin-containing material and textile dye. More recently, Schneider and Pedersen (WO 95/01426) proposed the application of several aromatic compounds to mediate laccase-catalyzed bleaching of textile dyes. Vaheri and Piirainen (International Application WO 92/09741) claim that the oxidizing enzyme laccase can be used in conjunction with manganese ions to reduce consumption of chlorine chemicals when applied in the later stages of bleaching. In their process manganese ions were used to control the redox potential of the pulp within a range between 0.05 to 0.3 V. However, delignification of pulp was not shown under these conditions. In U.S. Pat. No. 5,691,193 (1997) we have described a process for the bleaching of kraft pulp with non-chlorine chemicals where, in a first step, pulp is oxidized either with manganese peroxidase enzyme in the presence of Mn(II) ions and hydrogen peroxide or with laccase enzyme in the presence of 2,2′-azinobis-(3-ethylbenzthiazoline-6-sulfonate) (ABTS) and is followed by an alkaline peroxide bleaching step. Combinations of laccase and aromatic mediators have also been described for other applications such as degradation of polycyclic aromatic hydrocarbons (Johannes et al., Appl. Microbol. Biotechnol 46: 313-317, 1996) and chemical synthesis (Potthast et al., J. Mol. Catal. A: Chem 108: 5-9, 1996).
The. use of transition metal complexes as oxygen or peroxide catalysts has been reported for several applications. In the field of pulping and bleaching, ions of Fe, Mn, Co, Cr, Cu and V with various ligands were studied in oxygen bleaching experiments (Perng et al., Tappi J., 76(10), 139-147 (1993) and Tappi J., 77(11), 119-125 (1994)). From these, only iron complexes (ferricyanide, Fe/2,2′-dipyridyl and ferrate) were found to increase oxygen delignification of pulp. However the oxygen catalyzed reactions were not specific to lignin under the conditions claimed, and significant loss of pulp viscosity occurred. Sodium molybdate and sodium tungstate were proposed by Eckert (Canadian Patent 1,129,161 (1982)) to catalyze acidic H
2
O
2
delignification of krafi pulp. In a more recent study, Kubelka et al. (JPPS, 18(3), J108-114, i992) describe a process consisting of an acidic peroxide stage catalysed by sodium molybdate followed by an alkaline oxygen stage. This process was shown to increase pulp delignification by a factor of about 25% but with an equivalent loss in pulp viscosity. Furthermore, Agnemo (9
th
ISWPC Proceedings 1997, d2-1 -d2-4) proposed the addition of molybdates to reinforce oxygen and ozone delignification at pH 5.
None of the above described research reports or patent specifications mention the combined use of a transition metal complex with laccase to mediate the catalytic delignification of cellulosic materials and bleaching of pulp. Furthermore, in all applications previously described, there was no mention of the recyclability of the redox mediator.
SUMMARY OF THE INVENTION
It is an object of this invention to provide a process for oxidizing a substrate.
It is a particular object of the invention to provide a selective and catalytic process for pulp delignification or bleaching, driven by the enzymatic oxidation of a transition metal complex which can be regenerated for further pulp delignification or bleaching.
In accordance with the invention, there is provided a process for oxidizing a substrate comprising:
a) reacting a substrate to be oxidized with:
i) a phenol oxidase, and
ii) a transition metal complex redox mediator,
b) oxidizing said complex ii) with said oxidase i) from a reduced state to an oxidized state, and
c) oxidizing said substrate with said complex ii) in said oxidized state, said substrate being selected from lignocellulosic pulp, cellulosic pulp, paper dye and textile dye, whereby a lignocellulosic pulp is delignified by said oxidizing, and a cellulosic pulp, paper dye or textile dye is bleached by said oxidizing.
DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION
The process of the invention may be applied to the delignification and bleaching of kraft or sulphite pulps employing an oxidative enzyme and a transition metal complex which is a redox mediator and serves as a selective catalytic delignifying or bleaching agent. The principle of the invention is that a transition metal complex is oxidized by a phenol oxidase, for example, a laccase (E.C. 1.10.3.2) or a peroxidase (E.C. 1.11.1.7) and can then diffuse within the lignocellulosic fiber of the pulp to mediate selective catalytic lignin oxidation

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