Anthraquinone process side reactions

Redox behavior of anthraquinone is shown in Scheme 4. The quinone moiety may be reduced to the hydroquinone form and converted to a leuco salt under alkali conditions. In general, the leuco salt has a strong affinity for cellulose and is soluble in water. The hydroquinone form is insoluble in water and has low affinity to cellulose. The preferred dyeing procedure depends on the structure and properties of the vat dye. The variables that are used to control the process include, e.g., strength and amount of alkali, reduction temperature, and the presence of salts. During the process of reduction, some side reactions, such as overreduction, hydrolysis,... 

In the first step of the process the anthraquinone is hydrogenated to the hydroquinone with palladium as the preferred catalyst on carriers, such as gauze, or in suspension. The reaction is carried out at about 40°C and at pressures up to ca. 5 bar with cooling and only to ca. 50% hydrogenation to suppress side reactions (see below). 

The process has been briefly described by Turunen (1997) as an example of process intensification activities. Most of the hydrogen peroxide production is nowadays based on the anthraquinone method. The differences between the technologies include mainly differences in solvents, catalysts and equipment types and details. The process has less than ten main unit operations including two multiphase reactors, liquid-liquid extraction, gas desorption, distillation and filtration. The process conditions do not include high temperatures or pressures. The necessary properties are not readily available from literature because of the large number of components in the process liquid. However, the measurement of the most of the properties is relatively easy because of the mild conditions. The number of components which take part in the main production reactions and separation steps is small. Therefore it was possible to develop reliable models for most of the unit operations and to base the design on these nnodels. However, the side reactions and by-products involve complicated chemistry and... 

As already mentioned, the basic operations of dye chemistry utiUze simple chemical reactions. An intermediate can frequently be prepared in several entirely different ways and, in these cases, careful calculations must be made to determine which procediure is most advantageous. The least expensive process is often not necessarily the best when other factors are taken into account. For example, the question of apparatus may enter, and calculations may show that it is uneconomical to purchase an expensive apparatus for the process if a small quantity of the material is to be produced. Furthermore, consideration must be given to the usability of the side products formed. These often cannot be used at all (e.g., primuline), but may be valuable or even indispensable in another process (e.g, chromium sulfate in the production of anthraquinone). 

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