Indigo chemical process

Similarly, a catalytic route to indigo was developed by Mitsui Toatsu Chemicals (Inoue et al, 1994) to replace the traditional process, which dates back to the nineteenth century (see earlier), and has a low atom efficiency/high E factor (Fig. 2.15). Indole is prepared by vapour-phase reaction of ethylene glycol with aniline in the presence of a supported silver catalyst. The indole is selectively oxidised to indigo with an alkyl hydroperoxide in the presence of a homogeneous molybdenum catalyst. 

Vat Dyes. Vat dyes are insoluble in water. Indigo, for example, an ancient blue dye, is probably the best-known example of an ancient vat dye others include woad and Tyrian purple. Since the process of dyeing requires that the dye be in solution, dyeing with a vat dye (or vat dyeing, as the process is known) is possible only after the vat dye has been made soluble by a relatively long and somewhat complicated chemical procedure. The terms vat dye and vat dyeing are probably derived from the large tanks or "vats", in which the process was carried out in ancient times. 

Figure 5.13 The Genecor metabolic route to indigo [47], starting from glucose and using a genetically modified E. coli, is a complex yet clean multistep process which delivers a product identical to that of the century-old traditional chemical route.

Subsequently, indigo was successfully produced via fermentation on the 300000 L scale at a cost that was comparable with the price of chemical indigo [109]. Commercialization proved elusive, however, presumably because chemical indigo is marketed with a substantial profit margin. We note that it is common experience that the total costs of a new process, to compete, must be equal to (or lower than) the production cost of the existing process. In this particular case, it would seem that the STY of the fermentative process is too low to compete with a chemical procedure that encompasses only three steps from the basic chemicals aniline and acetic acid. 

One of the earlier processes for reducing Indigo, based on the use of chemicals instead of micro-organisms, was with copperas , the colloquial name for ferrous sulphate. The ferrous sulphate was dissolved in water and lime water added ferrous hydroxide was then precipitated which rapidly reacted with the oxygen with the formation of ferric hydroxide, accompanied by the liberation of hydrogen. 

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