Hydrogenation Process Quinone to Hydroquinone

Most of the quinone/hydroquinone decomposition products are formed in the hydrogenation step of the AO process, which is usually carried out over Pd catalysts supported on alumina, silica, or sodium aluminum silicate, or as Pd black. Depending on the specific process, hydrogenation reaction temperatures range from 25 to 75 C, and hydrogen pressures up to 0.3 MPa are used. The reaction rate is apparently not affected by increasing pressures above 0.4 MPa. To minimize secondary reduction products (discussed later), conversions of quinone to hydroquinone are typically kept around 45 - 50% but may be performed up to 80%. 

An important redox equilibrium exists between anthrahydroquinone A and tetra (Eq. (14.9)). Because tetra is more readily reduced than the parent anthraquinone, this equilibrium lies almost entirely to the right. Thus, if tetra formation is not suppressed, eventually all the hydroquinone species present in the oxidation process will consist of the reduced form of tetra. Because the analogous tetra hydroquinone is oxidized more slowly, the autoxidation step will slow down significantly. 

If tetra concentration becomes high, further quinone degradation products are observed. Additional ring hydrogenation can occur to give an octa -hydroquinone (Eq. (14.10)). Autoxidation of the octa-hydroquinone is too slow to be relevant and thus constitutes a decomposition product from which quinone cannot be regenerated. 

While the chemical yield of hydrogen peroxide in the AO process is very high, the loss of quinone/hydroquinone via the formation of these by-products necessitates the regeneration of the reaction mediators, hydrogenation catalyst, and removal of organic by-products. Periodically, fresh anthraquinone and solvent are added to compensate for losses. 

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