Commercial Manufacture of Nitroaromatic Compounds

Several variations of the conventional process have been proposed  

Single-phase nitration with a 2-4-fold excess of 98- - % nitric acid at a reaction temperature of 20-60°C and ambient pressure, which may be used for the nitration of benzene and mixtures of benzene and toluene. In the latter case NB and dinitrotoluene (DNT) are co-produced and total conversion is claimed to be high and very selective. 

Azeotropic nitration of benzene is conducted at 120-160 °C with excess benzene. Water generated during the reaction is removed as a water-benzene azeotrope and may be separated from the benzene, which is recycled to the reactor. The sulfuric acid remains dry and can be used for longer periods between regeneration. 

Nitration of toluene for production of mononitrotoluenes (MNT) is conducted either batch-wise or in continuous operation under somewhat milder conditions than those used for benzene nitration because of the activating effect of the methyl group. The nitrating acid usually has the composition 52-56% (w/w) H2SO4, 

28-32% (w/w) HNO3 and 12-20% (w/w) H2O, and the reaction temperature is ca 25-40 °C. The yield of MNT is typically ca 96%, the product contains 55-60% (w/w) 0-, 3-4% (w/w) m- and 35-40% (w/w) p-nitrotoluene (p-NT). Because the greatest demand is for p-NT, research has been direeted to the development of methods for increasing the p/o ratio of the product. An increased p/o ratio was, e. g., obtained by replacing sulfuric acid with other acids such as phosphoric acid or aromatic sulfonic acids. 

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