WO2004042138A1

WO2004042138A1 - Method for the electrochemical reduction of vat and sulphur dyes

Info

Publication number: WO2004042138A1
Application number: PCT/CH2003/000723
Authority: WO
Inventors: Walter Marte; Albert Roessler; Paul Rys
Original assignee: Tex-A-Tec Ag
Priority date: 2002-11-06
Filing date: 2003-11-05
Publication date: 2004-05-21
Also published as: AU2003273714A1; EP1563137A1

Abstract

The invention relates to a method for the electrochemical reduction of vat or sulphur dyes, by bringing the dye (1) into contact with a modified cathode (2), comprising a support (3) made from an electrically-conducting material with a large surface area on which a redox-active substance (4) is permanently immobilised. Substances with quinoid structures, in particular, an anthraquinoid structure are preferably applied in the above. No toxic contamination and greatly reduced salt loads in the effluent are achieved by means of the above method.

Description

Process for the electrochemical reduction of vat and sulfur dyes

The present invention relates to a method for the electrochemical reduction of vat and sulfur dyes according to claim 1 and to a use of the dyes reduced in this way according to claim 17.

Reducing agents play a crucial role in the application of vat and sulfur dyes, since their reduced form is water-soluble and has a substrate affinity. As a result of the oxidation carried out after dyeing, the dye is converted from its leuco form back into the water-soluble pigment structure.

The main reducing agent for vat dyes is sodium dithionite Na ₂ S ₂ 0 ₄ . However, this leads to a high sulfite and sulfate load in the wastewater, since it is a non-regenerable reducing agent. These salt loads are both toxic and corrosive and lead to the destruction of concrete pipes.

Even newer process variants could only partially solve these problems. Electrochemical processes were particularly promising. From WO 90/15182, a method is worth mentioning here in which electrochemical coupling is used with the aid of a mediator. The mediators are reversible redox systems that reduce the dye and are constantly regenerated at the cathode. Due to the high amounts used and the ecological concern of such mediators, there is still an acute environmental problem that can only be solved by additional investments in adequate wastewater technology or by a recycling process. Another disadvantage is the permanent medium metering to maintain the redox cycle in the continuous dyeing. The replenishment of the mediator system results from the discharge of the liquor proportional to the fabric or yarn flow. The problems mentioned led to a new solution which essentially allows the dye to be linked without reducing agents. This is an electrochemical reduction which, based on different starting mechanisms, does not require an additional reducing agent during the continuous operation of the reactor (WO 00/31334).

A disadvantage of both of the electrochemical processes mentioned (WO 90/15182; WO 00/31334) is the limited specific reactor power, for the increase of which very large electrode surfaces have to be made available.

Current process alternatives are based on the principle of electrocatalytic hydrogenation. From WO 01/46497, for example, a process for the electrochemical reduction of vat dyes is known, which is based on the process principle of the so-called 'precoat layer cell'. The dye is brought into contact with a cathode, comprising a porous, electrically conductive and filter-shaped support and an electrically conductive, cathodically polarized layer formed thereon by being floated thereon, in the presence of a base and electrochemically reduced by applying voltage. The catalytically active electrode is stabilized by the pressure loss at the other layer formed by floating. During operation, however, there are very large pressure losses and the electrodes designed as filters become blocked.

However, there is still an urgent need to improve the reactor performance and current efficiency of the process.

It is known that coal, graphite and similar materials are not absolutely inert to chemical reactions. The properties of carbon-containing products depend very much on the presence of functional groups on the surface. A wide variety of classes have been identified, whereby oxygen-carbon complexes, conjugated CC multiple bonds and intercalates are essential with regard to the redox properties of coal. The following classes can usually be observed: phenols, carbonyls, carboxyls, quinones and lactones. It is known that the redox properties of the carbon-containing products in the Essentially caused by the presence of quinoid groups. It is also state of the art to increase the proportion of redox-active quinoid groups by means of various pretreatments of the coal. The reactions with nitric acid, mixtures of nitric acid with sulfuric acid, potassium permanganate, potassium dichromate and oxygen are worth mentioning.

Furthermore, the functionalization of the surface with numerous organic and inorganic redox-active groups (US 4,595,479; US 4,836,904), in particular also quinoid groups, is known (P. Ramesh, S. Sampath, Analyst 2001, 136, S. 1872). The use in the case of quinoid groups has mainly been limited to oxidation reactions (e.g. ascorbic acid).

It is therefore an object of the present invention to provide a completely reducing agent-free linking process for producing fully reduced dye solutions while avoiding the disadvantages of known reduction processes mentioned.

The object is achieved by a method according to claim 1. The method is described below.

The dye is reduced in an oxygen-free, electrochemical reaction cell. In principle, many cell types that are common in electrochemistry can be used. However, work is preferably carried out in a fixed bed reactor in which the cathode material is in a flow channel and the electrical contact is ensured via a feed electrode.

According to the invention for the electrochemical reduction of a vat or sulfur dye by bringing a dye 1 into contact with a modified cathode 2, comprising a carrier 3 made of an electrically conductive, large-area material and a redox-active substance 4, which is permanently fixed on the carrier.

Basically, all electrically conductive, large-area materials that are stable in the alkaline range (pH 9 to 14) can be used as the carrier material for the carrier 3 become. Examples are metals such as platinum and gold, metal oxides such as tin dioxide, titanium dioxide and ruthenium oxide, semiconductors such as silicon and germanium, graphite and graphite-like materials, conductive composite materials and conductive polymers such as polyacetylene, polypyrrole, polyaniline, polyfuran or polyazulene. Doping of the polymer with alkali metal or halogens is also possible. Basically, it is also possible to use multi-layer systems. For example, a polymer film on another carrier material is conceivable.

Various redox-active substances 4 can be fixed on the carrier or on the carrier material and used for direct electrochemical dye reduction. Molecules with at least two carbonyl groups conjugated with one another are used as organic compounds with which the redox system can be implemented. In particular, quinoid and anthraquinoid structures are to be designated. Examples are benzoquinone, naphthoquinone, anthraquinone, acenphthalene quinone, 1, 8-dihydroxyanthraquinone.

In addition, metal salt complexes can also be immobilized on the carrier material. Examples are Ru (III) ethylenediaminetetraacetate or cobalt (II) tetrakis (p-aminophenyl) porphyrin.

According to the invention, these redox-active substances 4 are immobilized on the carrier 3 or on the carrier material, ie permanently fixed. This implies both covalent and coordinative bonds as well as irreversible adsorption. When a covalent bond is formed between the carrier material and the redox-active substance, it is possible, for example, to utilize surface functionalities of the carrier material, such as carboxylic acid groups, hydroxide groups or amino groups in the case of graphite or polymers, and the oxide or hydroxide groups in the case of metals, in order to use a chemical to mediator molecule To fix ties. It is also conceivable to use bifunctional anchor molecules, such as, for example, aminopropyltriethoxysilane, which covalently bond the support to the redox-active substances. Typical functional groups of such anchor molecules for the formation of covalent or coordinative bonds are amino, carboxyl, cyanide, sulfide and pyridine groups. Modification via activated carrier surfaces is also known. The activation can take place, for example, by vacuum pyrolysis or plasma treatment. In particular, substances with vinyl or amino groups can be bound in this way.

The irreversible adsorption of mediator molecules is less preferred because of the lower stability, but has advantages due to the simplicity of the preparation. Various techniques for immobilization on or in the carrier, such as sublimation or precipitation processes from the solution, are conceivable. For example, phthalocyanine systems and porphyrin systems can be irreversibly adsorbed on coal. The modification of platinum with functionalized vinyl components is also known. In this case too, the use of bifunctional anchor molecules, e.g. Phenanthrene substituted with pyridine is conceivable.

The system consisting of carrier 3 and redox-active substance 4 immobilized thereon is referred to as modified cathode 2.

In principle, all vat dyes and sulfur dyes - abbreviated to dye 1 - can be reduced within the scope of the method according to the invention. In particular, indigo dyes such as e.g. Indigo, 5,5'-dibromo indigo, δ.δ '' tetrabromo indigo and thioindigo, anthraquinone dyes such as e.g. Acylaminoanthraquinones, anthraquinonazoles, anthramides and other branched anthraquinones, anthrimidecarbazoles, phthaloylacridones, quinacridones, indanthrones and highly condensed ring systems such as e.g. Flavanthron, Violanthron, Isoviolanthron, Dibenzpyrenchinon, Anthron and Pyranthron to name.

According to the invention, the dye is thus reduced directly on the electrode or on the modified cathode and not via dissolved mediator molecules. Rather, these redox-active centers are permanently on the surface of the modified cathode.

In contrast to the method described in WO 00/31334, it is not necessary to carry out a start reaction since the linkage directly with the Dye can be used as a starting material. The radical species that occur during the reduction under certain conditions can be reduced in an analogous manner. In contrast to the method described in WO 00/31334, however, they are not necessary to maintain the reaction, since the dye itself can be reduced directly at the electrode.

The choice of anode material is not very critical. However, materials with low oxygen overvoltage are preferably used, e.g. Iron, nickel, platinum, titanium coated with platinum and titanium coated with ruthenium oxide.

In order to keep oxygen as far as possible from the cathode compartment, a divided cell must be used. Ion exchange membranes, diaphragms, glass frits, etc. are used as the separation medium. Ion exchange membranes, in particular cation exchange membranes, are preferably used, wherein, in turn, preference is given to using membranes which consist of a copolymer, tetrafluoroethylene and a perfluorinated monomer which contains sulfo groups.

The dye is introduced on the cathode side into an electrolysis vessel in an aqueous suspension containing various additives. The alkaline pH required for the reduction is pH 9 to 14, preferably 12 to 14, and is adjusted with alkali hydroxide, in particular sodium hydroxide. The acidic or alkaline anolyte spatially separated by a separator preferably consists of an aqueous solution of sulfuric acid or alkali hydroxide.

The following dye-related solubilizing or dispersing agents are used as additives:

Alcohols, e.g. Methanol, ethanol, isopropanol, with methanol being preferred,

Naphthalenesulfonic acid derivatives, e.g. Setamol WS,

- N, N-dimethylformamide and acetamide.

These additives are used in amounts of approximately 0.1 to 90%, preferably 1 to 30%, based on the dye composition used. The use of ultrasound has also proven itself to support the dispersion. According to the invention, surfactants and solvents (as described above) are also used as additives. Typical representatives are alcohol propoxylates such as Lavotan SFJ, alcohol sulfates such as Subitol MLF and alkyl sulfonates such as Levapon ML. The amounts used are in the range from 0.1 to 10 g / l, preferably between 1 and 5 g / l.

The voltage applied to the electrodes is a function of the hydrogen overvoltage of the respective electrode material and also depends on the reaction medium. Usually cell voltages between 1 and 5 V, preferably between 2 and 3 V, are applied.

The process is usually carried out at atmospheric pressure and temperatures between 20 and 100 ° C, preferably between 50 and 70 ° C.

Due to the fact that they are largely free of salt, dye concentrations of up to 200 g / l, but preferably 80 to 120 g / l, can be achieved in the stock vats.

The high level of dye solubility is of particular importance, as the overflowing of the dye in the dye baths can be prevented by means of more concentrated stock pools. This tying technique also leads to largely salt-free dyeing, which automatically ensures higher reproducibility and better fabric or yarn quality. Further advantages are the high stability of the reduced stem vat fleet in the oxygen-free electrolysis vessel, the high dye solubility of the linked species, the continuous dye reduction and thus the "just in time" preparation of the dye solution.

This reduction technique is suitable for both color master batches and dyeing liquors. The enormous economic advantage is thus the reduction in the consumption of chemicals (reducing agents and sodium hydroxide solution), the production of a better quality product and significantly lower wastewater costs due to the existing biocompatibility of the remaining wastewater constituents. There are no toxic loads on the waste water side, which makes it possible to recycle the waste water with considerably less effort than conventional dyeing systems. Furthermore, the present invention relates to the use of an electrochemically reduced dye produced by the process according to the invention for dyeing substrates. In principle, the term “substrates” in the context of the present invention encompasses all substrates which can be colored with the dye according to the invention. In addition to the fabric, knitted fabric and knitted fabric made of natural or synthetic fibers, wood, plastic, glass and metal objects can also be dyed. It is also possible to dye the skin and tissue.

The examples below explain the present invention in detail without claiming to have fully described the technical potential according to the invention.

Example 1 describes the electrochemical reduction of indigo on graphite electrodes modified with benzoquinone.

Manufacture of the electrode:

40 g of graphite (enViro-Cell Umwelttechnik GmbH, Oberusel, Germany) were boiled under reflux for 4 h at 100 ° C. with 150 ml of a mixture of sulfuric acid and nitric acid (3: 1 v / v). The mixture was then filtered off and the granules were washed with 2 l of distilled water. In order to neutralize the large proportion of acid, it was then rinsed with 10% sodium hydroxide solution and then washed again with distilled water until the filtrate had a pH of about 7. After drying overnight, the graphite was reduced with 20 g sodium borohydride (NaBH ₄ ) in 200 ml methanol at room temperature with stirring for 12 h. It was then filtered off and washed with 200 ml of methanol. Again after an overnight drying process, the graphite granules were refluxed with 200 ml of a benzoquinone solution (5% by weight) in benzene for 78 h. Zinc chloride in a 1: 1 molar ratio to benzoquinone was also added at the start of the reaction.

After filtering and washing the granules again with 100 ml of acetone, the granules were subjected to Soxhlet extraction with acetone for 60 h in order to remove any non-covalently bound quinone. Electrochemical reduction:

The electrochemical reactor consists of a bed electrode; the previously produced modified graphite granulate with a diameter of 2 - 4 mm serves as the electrode material. A centrally arranged platinum wire serves as the contact electrode. The balls are located in a flow channel made of glass (cross section 7 cm ² ) on a perforated glass plate. An anode (DeNora DSA: electrode area 20 cm ² ) is located in the anode space, which is spatially separated by a membrane (Nation 324, DuPont). Sodium hydroxide solution with a concentration of 40 g / l serves as the anolyte. In the catholyte tank, 0.4 g of indigo is dispersed in 2000 ml of water, which also contains 80 g of sodium hydroxide solution. The reduction of the dye suspension at 50 ° C in the reactor after appropriate degassing with nitrogen (99%) by simply applying a cathode potential of -1100 mV vs. Ag / AgCI reached in 3 M KCI solution. The working current is 10 mA. The catholyte flows vertically from bottom to top at 1.23 l / h. These conditions are maintained for 4 hours to completely reduce the dye.

Example 2 describes the electrochemical reduction of indigo on graphite electrodes modified with 1,8-dihydroxyanthraquinone.

Manufacture of the electrode:

40 g of graphite (TIMREX T1000-8000, TIMCAL, Bodio, Switzerland) were added for 4 h

100 ° C with 150 ml of a mixture of sulfuric acid and nitric acid (3: 1 v / v) under

Reflux cooked. It was then filtered off and the granules were distilled with 2 l

Washed water. In order to neutralize the large proportion of acid, it was then rinsed with 10% sodium hydroxide solution and then washed again with distilled water until the filtrate had a pH of about 7.

After drying overnight, the graphite was dried in dry THF for 24 h with N, N'-

Dicyclohexylcarbodiimide activated. This was followed by the addition of 1,8-dihydroxyanthraquinone and further stirring for 24 h. The concentration of quinone was 0.2 mol / l, the molar ratio quinone to N.N'-dicyclohexylcarbodiimide 1: 1.1.

After filtering and washing the granules again with 100 ml of methanol, the granules were subjected to Soxhlet extraction with methanol for 60 h in order to remove any non-covalently bound quinone. Electrochemical reduction:

The electrochemical reactor consists of a bed electrode; the previously produced modified graphite granulate with a diameter of 2 - 4 mm serves as the electrode material. A centrally arranged platinum wire serves as the contact electrode. The balls are located in a flow channel made of glass (cross section 7 cm ² ) on a perforated glass plate. An anode (DeNora DSA: electrode area 20 cm ² ) is located in the anode space, which is spatially separated by a membrane (Nation 324, DuPont). 1.5% sulfuric acid is used as the anolyte.

In the catholyte tank, 1 g of indigo is dispersed in 2000 ml of water, which also contains 80 g of sodium hydroxide solution. The reduction of the dye suspension at 50 ° C in the reactor after appropriate degassing with nitrogen (99%) by simply applying a cathode potential of -1100 mV vs. Ag / AgCI reached in 3 M KCI solution. The working current is 10 mA. The catholyte flows vertically from bottom to top at 1.23 l / h. These conditions are maintained for 8 hours to completely reduce the dye.

Example 3 describes the electrochemical reduction of Vat green 1 on graphite electrodes modified with acenaphthenequinone.

Manufacture of the electrode:

40 g of graphite (enViro-Cell Umwelttechnik GmbH, Oberusel, Germany) were boiled under reflux for 4 h at 100 ° C. with 150 ml of a mixture of sulfuric acid and nitric acid (3: 1 v / v). The mixture was then filtered off and the granules were washed with 2 l of distilled water. In order to neutralize the large proportion of acid, it was then rinsed with 10% sodium hydroxide solution and then washed again with distilled water until the filtrate had a pH of about 7. After drying overnight, the graphite was activated in dry THF for 24 h with N, N'-dicyclohexylcarbodiimide. This was followed by the addition of 5-amino-acenaphthenquinone and further stirring for 24 h. The concentration of quinone was 0.2 mol / l, the molar ratio quinone to N.N'-dicyclohexylcarbodiimide 1: 1.1. After filtering and washing the granules again with 100 ml of acetone, the granules were subjected to Soxhlet extraction with acetone for 60 h in order to remove any non-covalently bound quinone. Electrochemical reduction:

The electrochemical reactor consists of a bed electrode; the previously produced modified graphite granulate with a diameter of 2 - 4 mm serves as the electrode material. A centrally arranged platinum wire serves as the contact electrode. The balls are located in a flow channel made of glass (cross section 7 cm ² ) on a perforated glass plate. An anode (DeNora DSA: electrode area 20 cm ² ) is located in the anode space, which is spatially separated by a membrane (Nation 324, DuPont). Sodium hydroxide solution with a concentration of 40 g / l serves as the anolyte. In the catholyte tank, 0.4 g of Vat green 1 is dispersed in 2000 ml of water, which also contains 80 g of sodium hydroxide solution. The reduction of the dye suspension is carried out at 70 ° C in the reactor after appropriate degassing with nitrogen (99%) by simply applying a cathode potential of -1100 mV vs. Ag / AgCI reached in 3 M KCI solution. The working current is 10 mA. The catholyte flows vertically from bottom to top at 1.23 l / h. These conditions are maintained for 5.5 hours to completely reduce the dye.

Example 4 describes the electrochemical reduction of indigo on graphite electrodes modified with anthraquinone carboxylic acid.

Manufacture of the electrode:

1500 g of graphite (enViro-Cell Umwelttechnik GmbH, Oberusel, Germany) were boiled under reflux for 4 h at 100 ° C. with 6.21 of a mixture of sulfuric acid and nitric acid (3: 1 v / v). The mixture was then filtered off and the granules were washed with 10 l of distilled water. In order to neutralize the large proportion of acid, it was then rinsed with 10% sodium hydroxide solution and then washed again with distilled water until the filtrate had a pH of about 7. After drying overnight, the graphite was reduced with 0.8 kg of sodium borohydride (NaBH ₄ ) in 6 l of methanol at room temperature with stirring for 12 h. It was then filtered off and washed with 6 l of methanol. Again, drying takes place overnight.

Anthraquinone carboxylic acid was mixed in a molar ratio of 1: 1.1 with N, N'-dicyclohexylcarbodiimide in dry THF for 24 h, the quinone concentration being 0.2 mol / l. Then the graphite was added and the whole batch for more Stirred for 24 h.

After filtering again and washing the granules with 10 l of methanol, the granules were subjected to Soxhlet extraction with methanol for 60 h in order to remove any non-covalently bound quinone.

Electrochemical reduction:

The electrochemical reactor consists of a bed electrode; the previously produced modified graphite granulate with a diameter of 2 - 4 mm serves as the electrode material. The balls are located in an annular flow channel made of PVC (diameter 20 cm) around the central anode compartment on a perforated PVC plate. An anode (DeNora DSA: electrode area 20 cm ² ) is located in the anode space, which is spatially separated by a clay tube (d = 5 cm). Sodium hydroxide solution with a concentration of 40 g / l serves as the anolyte.

10 g of indigo are dispersed in the catholyte tank in 1000 ml of water, which also contains 40 g of sodium hydroxide solution. The reduction of the dye suspension at 50 ° C in the reactor after appropriate degassing with nitrogen (99%) by simply applying a cathode potential of -1100 mV vs. Ag / AgCI reached in 3 M KCI solution. The catholyte flows vertically from bottom to top at 1.0 l / h. These conditions are maintained for 13 hours to completely reduce the dye.

This stock vise is used as a coloring solution with a dye concentration of 10 g / l. The dyeing is carried out in the absence of oxygen using 10 g of cotton fabric at a temperature of 30 ° C. for 10 minutes. After the dyeing process is completed, the sample is oxidized in air, rinsed and finally washed at 50 ° C.

The sample produced in this way shows a brilliant shade of blue, the color depth is identical to that of a color sample produced by the conventional dyeing method with sodium hydrosulfite.

Claims

claims

1. A method for the electrochemical reduction of vat or sulfur dyes (1), characterized in that the dye (1) is brought into contact with a modified cathode (2), this cathode comprising a carrier (3) which consists of an electrically conductive , large-area material, and that a redox-active substance (4) is immobilized on the carrier (3) or is permanently fixed.

2. The method according to claim 1, characterized in that a substance with quinoid structures, in particular anthraquinone structure, is used as the redox-active substance (4).

3. The method according to claim 1, characterized in that a metal complex salt is used as the redox-active substance (4).

4. The method according to any one of claims 1-3, characterized in that a carrier (3) made of graphite or graphite-like material is used.

5. The method according to any one of claims 1-3, characterized in that a carrier (3) made of an electrically conductive polymer or an electrically conductive composite material is used.

6. The method according to any one of claims 1-3, characterized in that a carrier (3) made of a metal, a metal oxide or a semiconductor is used.

7. The method according to any one of claims 1-6, characterized in that a single-layer or a multi-layer carrier (3) is used.

8. The method according to any one of claims 1-7, characterized in that as the dye (1) indigoide and anthraquinone dyes, sulfur and other vat dyes and their mixtures are used.

9. The method according to any one of claims 1-8, characterized in that the concentration of the dye (1) is up to 200 g / l, preferably 80-120 g / l.

10. The method according to any one of claims 1-9, characterized in that the temperature is 20-100 ° C, preferably 50-70 ° C.

11. The method according to any one of claims 1-10, characterized in that the pH is 9-14, preferably 12-14.

12. The method according to any one of claims 1-11, characterized in that the cell voltage applied to the electrodes is 1 to 5 V, preferably 2 to 3 V.

13. The method according to any one of claims 1-12, characterized in that alcohols, preferably methanol and isopropanol, naphthalenesulfonic acid derivatives and acid amides are used as additives.

14. The method according to claim 13, characterized in that the amount of the additive, based on the amount of dye used, is 0.1-90%, preferably 1-30%.

15. The method according to any one of claims 1-14, characterized in that the surfactant concentration is 0.1-10 g / l, preferably 1-5 g / l.

16. The method according to any one of claims 1-15, characterized in that it is carried out as a batch process or continuously.

17. Use of the dye reduced according to claims 1-16 for dyeing substrates.