Direct Electrochemical Reduction of Indigo

THE DEPARTMENT OF CHEMISTRY, ETH ZORICH, IN THE NEW HONGGERBERG BUILDINGS 879

CHIMIA 2001, 55, NO.1 0

Chimia 55 (2001) 879-882 © Schweizerische Chemische Gesellschafl ISSN 0009-4293

Albert Roesslera, Otmar Dossenbacha, Ulrich Meyera, Walter Marteb, and Paul Rysa*

Abstract: Increasing ecoefficiency of textile wet processes has become an important topic in our research group. Reducing agents required for the application of vat and sulfur dyes cannot be recycled, and they lead to problematic waste products. Therefore, modern aspects of economical and ecological requirements are not fulfilled.The application of direct electrochemical reduction of indigo as a novel route has been investigated by spectrophotometric and voltammetric experiments in laboratory cells. Experiments yield information about the reaction mechanism and the kinetics, and they show the possibility ofthis new route for production ofwater- soluble indigo, which offers tremendous environmental benefits. Keywords: Electrochemical reduction' Indigo' Indigo radical· Leuco indigo' Vat dyes

1. Introduction mental problems, particularly those origi- The disposal of dyeing baths and rins- nating from the textile industry. Wet ing water causes various problems, be- Research activities in the field of indus- processing in dyeing and finishing can cause the necessary reducing agents will trial chemistry are characterized by a lead to effluents which have been subject be ultimately oxidized into species that highly transdisciplinary cooperation be- to criticism for many decades. Forty can hardly be regenerated. Thus, sul- tween specialists of various scientific ori- years ago the dominant problems were phite, sulphate, thiosulphate and toxic entations and strengthen the links be- large amounts of foam in river beds and sulphide heavily contaminate waste wa- tween academic and industrial research effluent hazards sometimes leading to the ter from dyeing plants (worldwide ap- efforts. The objective of these activities is extinction of fish populations and other proximately 180000 tla). In addition, as a to reach a deeper understanding of the aquatic organisms. Due to the enforce- result of the considerable excess of re- basic principles governing the dynamic ment of environmental regulations this ducing agent required to stabilize the ox- behavior and the selectivity of chemical has changed considerably. idation-sensitive dyeing baths, the waste processes of industrial importance. Re- Nevertheless, there is a continuing water may contain excess dithionite search techniques are drawn from syn- need for improving the ecoefficiency of which affects aerobic processes in waste- thetic chemistry, from chemical engi- critical textile wet processes in dyeing water treatment. Therefore, many at- neering (e.g. laminar and turbulent reac- and finishing. For example dyestuffs tempts are being made to replace the tive flow) as well as from physical (e.g. such as sulfur and vat dyes, especially in- environmentally unfavorable sodium electrochemistry) and biological chemis- digo, play an important role in today's dithionite by ecologically more attractive try (e.g. aerobic and anaerobic degrada- dyeing industry (market about 120000 tla). alternatives. Previous investigations tion of organic wastes). The present use of this dye category is were focused on: Recently, process optimization and based on the application of sodium • the replacement of sodium dithionite waste management have become major dithionite to attain a water-soluble form by an organic reducing agent (e.g. topics for research groups involved in in- of the dye by reduction (= leuco dye). glucose, a-hydroxyketones [1]) of dustrial chemistry. Thus, a significant The reduced dyestuff has a high affinity which the oxidation products are bio- part of our efforts is focused on environ- to cellulosic fibers, will penetrate the fib- degradable; er and will remain fixed inside after hav- • the use of ultrasound to accelerate the ing been reoxidized back to the water- vatting procedure and to increase the insoluble form (Scheme 1). conversion [2][3];

'Correspondence: Prof. Dr. P. Rys" "Department of Chemistry Swiss Federal Institute of Technology ETH Honggerberg CH-8093 Zurich .. Tel.: +41 16323120 Fax: +41 16321074 E-Mail: [email protected] bTex-A-Tec AG Anlagen und Prozesstechnik Industriegebiet Farch CH-9630 Wattwil (SG) Scheme 1. The principle of dyeing with indigo (Vatting) THE DEPARTMENT OF CHEMISTRY, ETH ZORICH, IN THE NEW HONGGERBERG BUILDINGS 880

CHIMIA2001, 55, No. 10

• the electrochemical reduction em- aqueous solution [10][11], and the results [10]. In addition, the investigation of re- ploying a redox mediator (e.g. Fe(n)- are closely related to those obtained for dox reactions of its soluble derivate indi- triethanolamine [4-7]) which is indigo dissolved in solvents. However, if go-carmine in aqueous solution led to expensive and toxicologically not indigo is not immobilized but present in comparable results [12][13]. completely harmless. an aqueous suspension it shows a dis- The electrochemical activity of the tinctly different behavior, and it cannot soluble species in this system was inves- The above-mentioned alternatives be reduced electrochemically under these tigated by reducing indigo in aiM aque- can increase the biocompatibility of the conditions. On the other hand radicals are ous sodium hydroxide solution with sodi- vatting process, but there is still pot- usually electrochemically active species um dithionite, inducing the formation of ential for improvement. Electrochemistry and previous studies on the electrochem- the radical by adding an excess amount of would be an elegant way to reduce the istry of indigo dissolved in DMSO, DMF indigo, and then taking current-voltage dye because it minimizes the consump- or in pyridine have reported that indigo curves at nickel electrodes (Fig. 2). The tion of chemicals and the effluent load. In can be reversibly reduced to its leuco- voltammograms show that the radical can the following it is shown how a new al- form in two one-electron processes with be easily reduced, and that the reduction ternative for the reduction of indigo was the radical as an intermediate species product - leuco indigo - is stable under developed just on the basis of observations, and some preliminary results are discussed.

2. Development of a Novel Route to an Environmentally Friendly Reduction of Indigo

2.1. Preliminary Observations VIS-spectra of indigo dispersions in aqueous solutions show a small absorp- tion at about 700 nm corresponding to indigo particles in the system. After adding a reducing agent (e.g. dithionite) a peak is observed at 410 nm, which can be correlated to leuco indigo, the reduced form of the indigo pigment. If the molar ratio of dithionite to indigo is smaller than stoi- 347 347.2 347.4 347.6 347.8 348 348.2 348.4 348.6 348.8 349 chiometric, additional peaks can be B [mT] found at 540 and 495 nm. It is assumed that these peaks are due to the well- Fig. 1. X-band CW-EPR spectrum of the red colored indigo radical at room temperature in 1 M known cherry-red colored indigo radical NaOH, generated by sodium dithionite. The spectrum was recorded at room temperature on a [8][9], and that this species is formed in Bruker ESP 300 spectrometer (microwave frequency 9.76 GHz) with the use of a flat cell. A a comproportionation reaction between microwave power of 2 mW, a modulation amplitude of 0.01 mT and a modulation frequency of leuco indigo and indigo. An EPR-spec- 100 kHz was used. trum, together with simulation, clearly showed that the structure of the radical species is in fact identical to the one as- E [mV] vs. Ag/AgCI/KCI sat. sumed (Table 1 and Fig. 1). -1200 -11 00 -1000 -900 -800 -700 -600 -500 O.OOE+OO Table 1. Coupling constants measured & by EPR spectroscopy &&& • & -5.00E-06 & Related atoms Coupling [G] & • & • • -1.00E-05 & •• •••• ~ ...... •••••• N 5,5' lH 0.66 • ••• -1.S0E-OS 'E ••..- (.) • 7,7' 'H 0.66 • -2.00E-OS ~ • • 6,6' 'H 1.98 • • Indigoradical (1E-5 molll) & •• -2.50E-05 4,4' 'H 1.67 • • Leucodye (1E-4 moVI) -3.00E-05 1,1' 14N 0.66 • & Groundelectrolyte • • -3.S0E-OS 2.2. Electrochemical Reduction of -4.00E-OS Indigo It is possible to reduce solid indigo microcrystals immobilized on the surface Fig. 2. Voltammograms of different indigo derivates in 1 M NaOH, 19/1 Setamol, Ni-electrode, of several electrode materials in buffered 50 °C (reference electrode Ag/AgCI3 M). THE DEPARTMENT OF CHEMISTRY, ETH ZURICH, IN THE NEW HONGGERBERG BUILDINGS 881

CHIMIA 2001. 55. No. 10 these conditions. The curve exhibits a plateau, thus confirming the assumption of a diffusion-controlled electrode reaction. At potentials more negative than -900 mV (vs. Ag/AgCl 3 M) it appears that leuco indigo is also reduced to anoth- er substance, but this increase in current could be correlated to the reduction of sodium dithionite in the system, because this molecule shows a cathodic wave EI/2 at -1030 mV [14][15]. Beyond -1100 mV the current density increases in all three cases due to side reactions, e.g. the cathodic evolution of hydrogen from the reduction of water.

2.3. Principle of the Process The above-mentioned observations and results were the origin of a novel route for the reduction of indigo. The in- Scheme 2. Mechanism of the direct electrochemical reduction of indigo vention relates to a method for the direct electrochemical reduction of vat and sulfur dyes in aqueous solutions which does not require a soluble reducing agent, nor the permanent presence of a redox mediator [16]. The process is based on a reaction mechanism in which a radical anion is formed in a comproportionation reac- 2.00E-04 tion between the dye and the leuco dye, 1.aOE-04 and a subsequent electrochemical reduction of this radical. In order to start the 1.60E-04 ~ process, an initial amount of the leuco 1.40E-04 dye has to be generated by a conventional (5 reaction, e.g. by adding a small amount oS 1.20E-04 Leucolndlgo (J of a soluble reducing agent. However, c 0 1.00E-04 • Radical once the reactions have set in, it is not e • Indigo needed anymore, and the further process c a.00E-05 -Q) (..) is self-sustaining. Scheme 2 illustrates c 0 6.00E-D5 the mechanism of the direct reduction (,) under steady conditions. 4.00E-05 Online spectrophotometric inspection of the reduction process provided more 2.00E-05 • detailed information of reduction prod- • • • • • O.OOE+OO • ucts and reaction kinetics. Fig. 3 shows • 0 5 10 15 20 25 30 35 40 typical experimental concentration pro- files as obtained for a batch reduction Timet [min] reaction using a diode array spectro- photometer. The result is in perfect corre- Fig. 3. Concentration profile for the three indigo species during direct electrochemical reduction at 50°C, 1 M NaOH, 1 g/I Setamol, 0.005 mAlcm2, 400 cm2 Ni-electrode, total indigo lation with the suggested consecutive concentration 1E-4 M, molar ratio indigo/moles dithionite at t=O: 7/1. mechanism. The radical anion as a product of the comproportionation reaction between indigo and the leuco dye contin- uously undergoes the secondary electrochemical reduction and acts as an inter- terms of reaction rate. Titanium gives as current density and temperature on the mediate species only. only very low current densities and, reduction kinetics. Table 2 indicates an therefore, is not a useful material for this increase in reaction rate when the current process. Lead shows good catalytic prop- density is raised, but the experimental 3. Electrolysis Experiments erties for the radical reduction, but it is current efficiency decreased very fast. dissolved during electrolysis, thus pro- Current efficiency cD is an important pa- In order to optimize the process for ducing high background currents in com- rameter in electrochemical engineering industrial conditions, the choice of the parison to the other materials. and represents the fraction of electrical electrode material plays an important A series of galvanostatic reduction charge used for the desired reaction. Only role. Copper turned out to be the most runs was carried out in order to assess the in the case of the low current density of suitable of the materials tested so far in effect of other operating parameters such 0.001 mNcm2 can a region with a con- THE DEPARTMENT OF CHEMISTRY, ETH ZORICH, IN THE NEW HONGGERBERG BUILDINGS 882 CHIMIA 2001, 55, No.1 a

Table 2. Resultsofthe galvanostatic electrolysis experiments at different current densities (50 cC, Ni-net-electrode (200 cm2), concentration of the radical at t = 0: 2*10-5 M)

Current density J Conversion X Current efficiency $ [mAcm'2j [%] after 40 min [%]

0.001 65 80

0.0025 70 55

0.005 79 40

0.01 85 20

0.1 90 5

stant efficiency of approximately 80% be 1fT [K'1] reached. In all other cases the efficiency is much lower and shows an exponential 0.0028 0.0029 0.0030 0.0031 0.0032 0.0033 0.0034 0.003 decrease with time due to the side reac- -0.20 tion of hydrogen evolution. This behav- -0.30 ior is explained by the fact that - without adding further indigo pigment - after -0.40 some time the concentration of the radi- -0.50 cal is so small that the galvanostatic cur- CIl~ rent density exceeds the limiting current 'E -0.60 density. The electrode potential becomes ()