Transition metal complexes with polydentate ligands for enhancing the bleaching and delignifying effect of peroxo compounds

Abstract

Transition metal complex compounds of polydentate ligands with improved delignifying and bleaching performance. These polydentate ligands are organic ligands which, in aqueous solution and in the presence of atmospheric oxygen, or hydrogen peroxide, form a complex with a transition metal, in particular cobalt. The complexes are mono- or polynuclear and they have, when peroxo compounds are used, better delignifying and bleaching performances than conventional transition metal complex compounds. A delignifying and bleaching method, in which these transition metal complex compounds with polydentate ligands having improved delignifying and bleaching performance are used as catalysts, is also described.

Description

INTRODUCTION AND BACKGROUND[0001] The present invention relates to transition metal complexes with polydentate ligands for enhancing the bleaching and delignifying effect of peroxo compounds. In a further aspect, the present invention relates to the use of such complexes and to a method for the delignification of fibrous materials.[0002] The term "fibrous materials" will be used below to denote all lignin-containing fibers, which have either been mechanically and / or chemically pretreated by the process of lignin production or pulp production or are subjected to this method as chemically or mechanically untreated natural fibers. These fibers may also have undergone several stages of chemical and / or mechanical processing, for example chemical pulping and a first delignifying treatment after pulping.[0003] Lignin-containing fibers from wood or from annual plants should, as far as possible, be freed from lignin for most applications. The fibers should furthermore have high brightnesses,...

AI Technical Summary

Benefits of technology

[0010] It has now been surprisingly found that the above and other objects of the present invention can be achieved by using certain transition metal complex compounds with polydentate ligands which considerably surpass the delignifying and bleaching performance of hitherto known catalytic systems. In this case, increasing the amount of catalyst leads to an increase in the bleaching effect and not, as in the case of conventional catalytic systems, to uncontrolled degradation of the hydrogen peroxide. The consequences are better brightnesses, or decreased kappa numbers, and gentle treatment of the fibrous mass.

Problems solved by technology

In order to obtain approximately the same effect as with chlorine-containing bleaching agents, it is necessary to select more drastic conditions, such as for example higher reaction temperatures and longer reaction times. A disadvantage with oxygen-containing bleaching agents is that the reaction mechanisms of these chemicals are far less selective than is the case with chlorine or chlorine-containing chemicals, so that the delignification or bleaching entails greater damage to the cellulose.

It is more expensive than chlorine-containing bleaching agents, and significantly less selective.

Under more intense reaction conditions, for example higher temperature, greater use of chemicals and / or longer reaction time, although the residual lignin still present does become broken down to some extent, this is accompanied by increased damage to the fiber and yield losses.

A particularly undesirable feature is that the unselective reaction of hydrogen peroxide attacks the cellulose, so that the strength of the fiber is significantly reduced.

The results from the field of detergents, however, are scarcely applicable to the pulp and paper industry, since textile fibers are not comparable with lignin-containing fibers.

These compounds have the disadvantage that they are difficult to synthesize.

The consequences are inferior brightness, or increased kappa numbers, and significant damage to the fibrous mass.

These compounds also have the disadvantage that they are degraded under bleaching conditions (at least pH 10. 80.degree. C.).

Although this procedure is possible at the laboratory scale, it cannot be used industrially since additional mixing machines cannot be integrated into the bleaching towers.

In this case, increasing the amount of catalyst leads to an increase in the bleaching effect and not, as in the case of conventional catalytic systems, to uncontrolled degradation of the hydrogen peroxide.

However, the introduction of cobalt as a transition metal ion in the way described above would not have appeared expedient to the person skilled in the art.

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