New Catalysts for the Borohydride Dyeing Process N. Meksi,* ,†,‡ M. Ben Ticha, † M. Kechida, § and M. F. Mhenni † Research Unit of Applied Chemistry and EnVironment, Faculty of Sciences of Monastir, 5000 Monastir, Tunisia, Higher Institute of Fashion of Monastir, Stah JabeursRoute Korniche, 5019 Monastir, Tunisia, and Socie ´te ´ Industrielle des Textiles (SITEX), 5070 Ksar Hellal, Tunisia In the borohydride dyeing process, indigo cannot be reduced by sodium borohydride to its reduced form without the addition of catalyst. This catalyst, which is a metallic salt, is used to activate the reduction procedure of the reducing agent. So, the reduction reaction of indigo depends significantly on the nature of this catalyst. In this paper, the effect of 12 different metallic salts on the performances of the indigo reduction reaction has been discussed. These performances were evaluated by measuring the indigo reduction yield as well as the color yield (K/S) of the dyed samples of cotton. In these studies, it was found that the copper-based catalysts were the best and offered maximum performance. 1. Introduction Currently, indigo (C.I. Vat Blue 1) is a dye which is mainly used to color cotton yarns for denim fabrics. This dye is insoluble in water. Dyeing textile with indigo involves usually a dissolving step of the dye. This step consists of the reduction of indigo by a reducing agent in the presence of an alkali such as sodium hydroxide. 1-4 Thus, a water-soluble leuco form of indigo (leuco-indigo) is obtained which can be used to dye textile fibers. After soaking it in the reduced dye solution (dyeing bath), it is important to expose the textile out in the air to oxidize the dye back to its insoluble form. These two steps (immersion in the dyeing bath/exposure to air) is repeated many times to achieve a dark blue color. Classical processes for dyeing by indigo use generally sodium dithionite as reducing agent (Figure 1). These processes present several disadvantages: 5-8 (a) ecological problems resulting from the quality of the wastewaters (some of the byproducts formed in the decomposition of sodium dithionite are sulfur compounds which can heavily contaminate the environment, and the pH of wastewaters generated from these dyeing processes is very high (pH ) 12-14); so, this requires great quantities of acids for the neutralization); (b)technical problems such as the problem of the storage of the reagents, especially sodium dithionite, the difficulty of control of the dyeing bath, and the color variation of the dyed fabrics, etc. In a previous work, 9 we developed a novel process to reduce indigo by sodium borohydride in the presence of potassium nickel cyanide K 2 Ni(CN) 4 as catalyst. The present process gives the opportunity to reduce indigo and all kinds of vat dyes in total absence of alkali. The reduction of indigo and the cotton dyeing with the leuco-indigo form prepared in these conditions (pH range ) 9-10.5 and temperature ) 55 °C) could offer several economic and ecological advantages with a good preservation of cotton fiber qualities. In the borohydride dyeing process, it is necessary to use catalyst. This catalyst is a metallic salt. Without it, sodium borohydride fails to reduce indigo. This is attributable to the strong stability of the indigo carbonyl groups. As explained in the literature, 10-12 as vat dye, indigo has a conjugated molecular structure. The indigo atoms have sp 2 hybridization. These properties as well as the presence of intra- and intermolecular hydrogen bonds in its structure confer to the indigo molecule a great stability and consequently a low chemical reactivity of their carbonyl groups. So, the reduction of indigo carbonyl groups with hydride ion H - generated by sodium borohydride becomes very difficult in this case. However, it is very probable that the addition of metallic salt in the medium leads to the creation of chemical interactions between the metal atom of the catalyst and the oxygen atom of the indigo carbonyl group. These interactions provoke perturba- tion on the electronic cloud of the carbon atom of the indigo carbonyl group. Thus, the nucleophilic center existing in this carbon atom could be activated. So, this can facilitate consider- ably the attack of H - . Consequently, the reduction of the indigo carbonyl group becomes very possible. It appears here that the role of the catalyst consists probably of activating the reduction procedure of sodium borohydride. So, the performances of the indigo reduction reaction depend not only on the catalyst amount in the medium 9 but also on the nature of this metallic salt used, i.e., its cation and its anion. In this work, we try to study the effect of the catalyst nature on the reduction yield and the cotton dyeing quality. Then, we determine the best catalysts for the borohydride dyeing process which offer the most excellent performances. 2. Experimental Section 2.1. Chemicals and Materials Used. Indigo (C 16 H 10 N 2 O 2 , BEZEMA AG) was commercial grade. Sodium borohydride (NaBH 4 , Acros Organics) and sodium hydroxide (NaOH, Kaustik JSC) were laboratory grade. All of these chemicals were used for the reduction without further purification. Calcium chloride hydrate (CaCl 2 · 2H 2 O, PS Park), manga- nese(II) chloride hydrate (MnCl 2 · 4H 2 O, Riede-deHaen), iron(III) chloride (FeCl 3 , Riede-deHaen), cobalt chloride hydrate (CoCl 2 · 6H 2 O, Fluka), nickel chloride hydrate (NiCl 2 · 6H 2 O, Fluka), zinc chloride (ZnCl 2 , Aldrich), copper chloride hydrate (CuCl 2 · 2H 2 O, Acros Organics), copper sulfate hydrate (CuSO 4 · 5H 2 O, Fluka), copper nitrate hydrate (Cu(NO 3 ) 2 · 3H 2 O, Panreac Quimica SA), copper bromide (CuBr 2 , Fluka), and copper acetate hydrate (Cu(CH 3 COO) 2 · H 2 O, Riede-deHaen) were used as catalysts for the indigo reduction reaction. Potassium nickel cyanide (K 2 Ni(CN) 4 ) was synthesized using potassium cyanide and nickel sulfate as described in the * To whom correspondence should be addressed. E-mail: meksi_nizar@mailcity.com. † Faculty of Sciences of Monastir. ‡ Higher Institute of Fashion of Monastir. § SITEX. Ind. Eng. Chem. Res. 2010, 49, 12333–12338 12333 10.1021/ie100974d  2010 American Chemical Society Published on Web 11/08/2010