With Nitric Acid

The second step ofthe process from cyclohexane is the oxidation ofthe KA Oil with a large excess of 50-65% HNO3 (molar ratio HNO3/Ka Oil at least 7/1), and with a

Cu(II) and ammonium metavanadate catalyst [2a, b]. The formal stoichiometries are as follows (indeed, the reaction leads not only to N2O, but also to the formation of NO and NO2):

cyclohexanone to AA, DHR = —172 kcal mol 1 cyclohexanol to AA, Dhr = —215 kcal mol—1

The reaction is carried out in two (or more) in-series reactors, the first one operating at 60-80 °C, the second one being maintained at 90-100 °C and a pressure of 1-4 atm. The molar yield obtained for total KA Oil conversion is as high as 95%; the by-products are glutaric (selectivity 3%) and succinic (selectivity 2%) acids.

Figure 7.3 shows a simplified flow sheet of the process of KA Oil oxidation, with the main process units.

The reaction mechanism was discussed in detail by van Asselt and van Krevelen in 1963 [2e, f]; a complete analysis of the mechanism has also been reported ([2b] and

nitric acid fresh make-up catalyst Figure 7.3 Simplified flow sheet of the KA Oil oxidation process.

Nitric Acid Oxidizing Agent Mechanism
Scheme 7.2 Main reactions involved in the mechanism of KA Oil oxidation to AA with nitric acid. Source: elaborated from [2b, e, f].

references therein) (Scheme 7.2). Cyclohexanol is first oxidized to cyclohexanone, followed by nitrosation of cyclohexanone by nitrous acid (HNO2) to produce 2-nitrosocyclohexanone; the latter may undergo various transformations.

In the presence of HNO2, the nitrosoketone may be hydrolyzed to the a-diketone and hydroxylamine (Claisen-Manasse reaction), via intermediate formation of the ketoxime tautomer. Alternatively, the oxidation of the oxime by the stronger oxidant nitric acid also yields the a-diketone, with co-formation of NOx (the reaction shown in Scheme 7.2). Finally, the diketone is oxidized to AA; oxidation may occur directly either by the nitric acid (with the possible side co-formation of succinic acid and oxalic acid) or, with a suitable vanadium concentration, by VO2 + ; in the latter case, the reaction is very selective. HNO3 rapidly reoxidizes the resulting reduced V species, VO2 + or VO +, back to VO2 +. In this step, nitric acid is reduced to the regenerable oxides NO and NO2. Cu(II) helps to limit multiple nitrosation of cyclohexanone and the formation of glutaric acid. Only one mole of nitric acid is consumed per mole of cyclohexanone via this route. 2-Nitrosocyclohexanone may eventually yield 2-nitro-cyclohexanone in strongly oxidizing solutions.

The main pathway, however, is the reaction of 2-nitrosocyclohexanone to yield 2-nitro-2-nitrosocyclohexanone; the latter is hydrolyzed to 6-nitro-6-hydroximinohex-anoic acid (adipomononitrolic acid) that undergoes oxidative hydrolysis to AA via intermediate adipomonohydroxamic acid; in this case, HNO3 is completely reduced to N2O. Therefore, this pathway consumes two moles of nitric acid per mole of cyclohexanone.

Nitrosation may potentially also occur on cyclohexanol; in fact, cyclohexanol can be oxidized at much lower temperatures than cyclohexanone. The active reactant is HNO2; therefore, in this case, the first product of cyclohexanol oxidation is cyclohexyl nitrite. The latter is then rearranged into 2-nitrosocyclohexanone, which is also the key intermediate in the main reaction pathway involving cyclohexanone.

The nitric acid oxidation of cyclohexanol at low temperatures, for example, 10-15 °C, leads to the formation of both adipomononitrolic acid and the hemihydrate of cyclohexadione (predione) rather than the a-diketone. The predione is oxidized to AA in the presence of metavanadate.

For many years industries have shown an interest in the development of a new process that does not produce nitrogen oxides; this would not only be more environmentally sustainable than the current process but it would also not require denitration equipment.

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