INTRODUCTION AND BACKGROUND
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.
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.
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, advantageously 90% ISO. These high brightnesses will be achieved only if lignin is substantially removed from the fiber or from the fiber surface. Using elemental chlorine and other chlorine-containing bleaching chemicals, it has been possible to delignify lignin-containing fibers efficiently and highly selectively in the past.
Since, when chlorine and/or chlorine-containing chemicals such as e.g. chlorine dioxide are used, it is not possible to avoid the formation of AOX (“adsorbable organically bound halogen”) in the waste water and OX (“organically bound halogen”) in the pulp, therefore, pulp manufacturers are making increased use of chlorine-free bleaching agents, such as for example oxygen and oxygen-containing chemicals, which are intended to brighten the fibers to the highest possible brightness (elemental chlorine-free bleaches (ECF bleaches) and totally chlorine-free bleaches (TCF bleaches). 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. There is therefore a great need for methods, and also therefore for chemicals, which selectively and mildly break down by oxidation the residual lignin still present after pulping.
The chlorine-free bleaching agents, which are used for this purpose in the pulp industry, also include hydrogen peroxide. Hydrogen peroxide is used, above all, for environmental protection reasons. It is more expensive than chlorine-containing bleaching agents, and significantly less selective. For these reasons, hydrogen peroxide has to date been used under the mildest possible conditions wherever fibrous materials are to be brightened but not delignified. 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.
In order to make best use of the brightening effect of hydrogen peroxide, attempts have for some time been made to find catalysts which suppress the many unselective side reactions and thereby make more hydrogen peroxide available for the removal of chromophoric groups and/or activate hydrogen peroxide for the delignification. To that end, substances which, for example, have been described for bleaching or brightening use in detergents are used time and time again. 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. The reason for this is that the chemical structure of stubborn stains in fabrics is very different from the structure of the wood lignin to be oxidized. Furthermore, the dirt is located on the textile fiber, whereas the majority of the lignin to be removed is embedded in the cellulose fiber (middle lamella).
The previously known methods, for instance that described in DE 19 620 241, WO 97/44520 and WO 99/64156, use manganese or iron complexes as catalysts to activate peroxo compounds. These compounds have the disadvantage that they are difficult to synthesize. Furthermore, they significantly show an H2O2 degradation effect (catalase activity), the result of which is that only certain amounts of the transition metal complex compounds can be used.
An increase in the amount of catalyst hence does not lead to an increase in the bleaching effect (see DE 19 620 241, Example 3), but rather to degradation of the hydrogen peroxide, which is thereby made unavailable for the bleaching. 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° C.). This instability of the complexes during bleaching makes it appropriate to add the catalyst portionwise, as described explicitly in APPITA Annual Conference 1999, pp. 455 to 461. 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 view of the prior art described and discussed above, it is an object of the invention to develop a method for the improved and selective delignification and/or bleaching of lignin-containing fibrous materials by using peroxo-containing and therefore environmentally friendly chemicals.
SUMMARY OF THE INVENTION
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.
This invention therefore provides a method for the delignification and/or bleaching of fibrous materials by treating or contacting
a) an aqueous suspension of fibers, which have a consistency of from 3% to 40%; with
b) a peroxo compound, which is used at from 0.1% to 10%, based on the bone-dry fibrous mass; and with
c) a mononuclear transition metal complex of the formula (1),
(LMXp)zYq Formula (1)
L is a polydentate ligand of the formula (a), (b), (c) or (d),
R1, R2, and R3 independently of one another represent hydrogen, linear of branched alkyl or alkenyl, or an optionally substituted aryl or arylalkyl;
R4 independently of one another represents an optionally N-substituted linear, branched or cyclic aminoalkyl, or an optionally substituted heteroaryl such as pyridyl, pyrazinyl, pyrazolyl, pyrrolyl, imidazolyl, pyrimidyl, triazolyl or quinolyl;
M is a transition metal ion, advantageously selected from the group consisting of iron in oxidation states (II) to (V); manganese in oxidation states (II) to (VII); and cobalt in oxidation states (II) to (IV); cobalt preferably being in oxidation states (II) or (III);
X is a coordinate species (neutral or anionic), such as for example CH3CN, CH3COO−, Cl−, Br−, H2O, OH−, HOO−, OCN−, SCN−, PO4 3−, NH3, NO3 −, NO2 −, NO, O2−, O2 2−;
Y is a counter-ion or counter-molecule, such as for example ClO4-, Br-, Cl-, PF6 −, NO3 −, BPh4 −, SO4 2−, CH3COO− or mixtures thereof;
p is an integer from 0 to 4;
z is a complex charge (+/0/−);
q is z/[charge of Y];
d) instead of (c), a mononuclear transition metal complex of the general formula (2),
(LmConXp)zYq; Formula (2)
L is a ligand of the formula (a), (b), (c) or (d);
Co stands for cobalt in oxidation states (II) to (IV) or mixtures of these oxidation states, the oxidation states (II) and (III) being particularly preferred;
X, Y, z and q are as described for formula (1);
m and n are integers from 2 to 4; and
p is an integer from 0 to 12;
the transition metal complex (c) or (d) being used in an amount of from 10 ppm to 5000 ppm, based on the amount of bone-dry (b.d.) fibers used.
Complex compounds of the formula (2), in which X=O2 2− and m, n=2 and p=1, are particularly preferred.
Examples of ligands of the formula (a) which can be used according to the invention are:
N,N-bis(pyridin-2-ylmethyl)-1,1-bis(pyridin-2-yl)-1-aminoethane (MeN4Py); and
Examples of ligands of the formula (b) which can be used according to the invention are:
Examples of ligands of the formula (c) which can be used according to the invention are:
Examples of ligands of the formula (d) which can be used according to the invention are:
In a first aspect of the invention, transition metal complex compounds of polydentate ligands with improved delignifying and bleaching performance will be described. These polydentate ligands are organic ligands which, in aqueous solution and in the presence of atmospheric oxygen and/or hydrogen peroxide, form a complex with a transition metal, in particular cobalt, characterized in that 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.
The transition metal complex compounds with polydentate ligands, which are described here, will also be referred to as bleaching catalysts. In this context, the term “polydentate ligands” means ligands which have at least 4 or more heteroatoms (so-called donor atoms), preferably nitrogen, which can coordinate with the transition metal ion. Pentadentate ligands are preferred. Pentadentate N-donor ligands are particularly preferably used in the present invention.
In a second aspect, a delignifying and bleaching method will be described, in which these transition metal complex compounds with polydentate ligands having improved delignifying and bleaching performance are used as catalysts. In this context, it has been demonstrated that mononuclear transition metal complex compounds of the general formula (1), in particular the mononuclear complexes of the formula (1a) with the pentadentate ligand N4Py and derivatives thereof, which are described for the low-temperature bleaching of textile fabrics in EP 0 909 809 and WO 00/12667, are useful as a bleaching catalyst in the bleaching of fibrous material and can be employed therein.
It has furthermore been found that polynuclear, in particular binuclear, cobalt complexes with polydentate ligands of the general formula (2) also activate peroxo compounds for the delignification or oxidative bleaching of fibrous materials.
It has been found that, when using the known iron or manganese complexes of the formula (1a), in particular with the ligand N4Py and derivatives thereof, in pulp bleaching or delignification with hydrogen peroxide, it is possible to achieve an improved effect compared with the reference test without the addition of catalyst.
It has furthermore been found that a cobalt complex of the formula (1a) can be obtained when the ligand N4Py known from the literature [M. Lubben, A. Meetsma et al., Angew. Chem. 1995, 107, 1610, EP 0 909809, WO 95/34628], as described in WO 00/12667, is reacted with a corresponding cobalt(II) salt instead of the iron(II) salt. This cobalt-containing complex shows a significantly better effect in the bleaching test than the said iron or manganese compound.
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. When following literature methods for the reaction of cobalt(II) salts with polydentate ligands [G. A. Lawrance, T. M. Manning et al., J. Chem. Soc., Dalton Trans., 1992, 1635; G. A. Lawrance, M. A. O'Leary et al. Aust. J. Chem. 1988, 41, 1533, D. A. Buckingham, P. J. Cresswell et al., Inorg. Chem. 1975, 14, 1485], with the ligand N4Py or derivatives thereof, a binuclear cobalt complex is obtained with a μ-peroxo (μ-O2) bridge similar to formula (2a), wherein X=O2 2−. This compound class, which has not to date been described, provides better results in pulp bleaching or delignification compared with the mononuclear compounds of the formula (1a) prepared according to WO 00/12667, in particular the said cobalt complex of the formula (1a) (see Table 1).
Peroxo compounds with a structure according to formula (2), in which X=O2 2−, can generally be prepared by reacting the polydentate ligand or its salt (in the “one-pot method”, see Example 2), dissolved in corresponding solvents such as methanol, water or mixtures thereof, with cobalt(II) salts and subsequent air oxidation. The results achieved with these compounds in pulp bleaching or delignification surpass the prior art discussed in the introduction.
The use, according to the invention, of the transition metal complex compounds substantially consists in creating conditions under which the peroxo compound and the bleaching catalyst can react with one another, for the purpose of obtaining consecutive products which have a more strongly oxidizing effect. Such conditions exist, in particular, when the two reaction partners encounter one another in aqueous solution. The aqueous fibrous mass already contains the bleaching catalyst according to the invention. The peroxo compound may preferably be added separately to the aqueous fibrous mass, and in substance or as an advantageously aqueous solution. The bleaching catalyst may advantageously be kneaded into the aqueous fibrous mass, optionally with other bleaching additives, such as for example sodium hydroxide.
The bleaching catalyst may alternatively be prepared “in situ” by separate addition of the metal salt and the ligand, or its salt, to the aqueous solution of the bleaching additives, which is subsequently mixed into the fibrous mass. In the case of such an “in situ” method, the bleaching catalyst need not be isolated.
With these transition metal complexes according to the invention, which are used in concentrations of from 10 ppm to 5000 ppm, preferably from 50 ppm to 3000 ppm, advantageously from 200 ppm to 2000 ppm, and particularly preferably from 200 ppm to 1500 ppm, based on the amount of bone-dry (b.d.) fibers used, the delignification of fibers with the method according to the invention can, quite surprisingly, be increased significantly compared with the known, merely brightening peroxide stage, with the cellulose being broken down only slightly. What is particularly astounding is that these increase factors are achieved with rather low residual lignin contents, where, according to the invention, there are lignin structures which are particularly difficult to break down, strongly condensed and therefore relatively unreactive.
This enormous delignification and/or bleaching is achieved by employing the transition metal complexes and process conditions according to the invention.
The method according to the invention can be used for a wide variety of fiber types, for instance mechanically and/or chemically pretreated fibers, including waste-paper fibers, but also untreated natural fibers.
As the peroxo compound, it is possible to use hydrogen peroxide or compounds which release hydrogen peroxide, but also organic or inorganic per-acids or salts thereof, for example peracetic acid, peroxymonosulfuric acid or percarboxylic acid and salts thereof. Mixtures of different peroxo compounds can be used in a delignifying stage. Adaptation to special process requirements is therefore possible.
The method according to the invention can be employed in a wide consistency range (the consistency is equal to the ratio of the bone-dry (b.d) fibrous mass to the total weight). The consistency may be between 3% and 40%, although it is advantageously a consistency of between 10% and 15%.
In order to achieve an optimum delignifying or bleaching effect, between 0.1% and 10%, although advantageously between 0.3% and 6%, of a peroxo compound should be used, in each case based on the bone-dry (b.d.) fibrous mass to be delignified.
The method according to the invention shows excellent delignifying or bleaching results when from 10 ppm to 5000 ppm, preferably from 50 ppm to 3000 ppm, advantageously from 200 ppm to 2000 ppm, and particularly preferably between 200 ppm and 1500 ppm, of the transition metal complex according to the invention are used, based on the bone-dry (b.d.) fibrous mass. The breakdown of the residual lignin takes place very efficiently when the pH at the start of the reaction is more than 10, preferably more than 11.
The reaction temperature can be selected in a wide range, according to the raw fibrous material in question. Between 20° C. and 130° C., although advantageously between 40° C. and 110° C., most fibers can be delignified. The temperature range between 50° C. and 98° C. is particularly preferred, because in this case it is still possible to delignify very selectively under mild conditions, and with relatively short reaction times.
The reaction time, like the reaction temperature, can also be selected in a wide range, between 5 and 240 minutes (min). However, a reaction time of from 30 to 150 min is preferred. The delignification is particularly extensive when the reaction is employed over a time period of from 45 to 120 min. These reaction times are shorter than in the case of customary peroxide stages.
The transition metal complexes used according to the invention for delignifying and/or bleaching not only improve the effect of a simple peroxide stage, but they also increase the delignification or bleaching of an oxygen stage, which is carried out with the addition of peroxide. If bleaching is carried out with the addition of oxygen, particularly good results are achieved when an overpressure of between 0.15 MPa and 1.5 MPa is applied. A reaction pressure of from 0.2 MPa to 0.9 MPa is preferably applied.
The complexing of transition metal ions has an advantageous effect. Diethylenetriamine pentaacetic acid (DTPA), diethylenetriamine penta(methylene phosphonic acid) (DTPMPA) or poly(α-hydroxyacrylic acid), which are even stable at quite high pH values, are advantageously used. In addition, or as an alternative, water glass and/or magnesium sulfate can be used.
DETAILED DESCRIPTION OF INVENTION
The method according to the invention will be described below with some exemplary embodiments:
The studies of catalyzed delignification stages with peroxide, and optionally also with oxygen-containing chemicals, were carried out using a kraft pulp (softwood, spruce/pine, kappa number 24.0), which was previously subjected to an acid wash (1.7% H2SO4, 70° C., 3% consistency, residence time 0.5 hours (h)) and was then characterized as follows:
kappa number: 23.5
brightness: 30.5% ISO
viscosity: 32.3 mPa*s
Unless otherwise indicated, all the tests were carried out with a 10% consistency, a reaction temperature of 80° C. and a reaction time of 90 min. Data in “%” are based on the amount of bone-dry (b.d.) fibers. The analyses were carried out according to the following standards:
The kappa number was established according to Zellcheming instruction sheet IV/37/80. The viscosity of the pulp was determined according to TAPPI instruction T230 om-82. The brightness was measured with an Elrepho 2000 (from Datacolor).
The invention will be explained by the following examples, without implying any limitation.