Adipic acid is widely used in perfume, food, pharmaceutical industry. The general applying of adipic acid is using as feed for different chemicals such as nylon, plasticizers and lubricating oils. Global production of adipic acid in 2011 was almost 2.8 million metric tons.
The most common process for adipic acid production is a two-stage process involving the initial oxidation of cyclohexane by oxygen to the mixture of cyclohexanol and cyclohexanone at a temperature of 142-145 °C and a pressure of 0,7 MPa followed by the oxidation with 40-60 % nitric acid at a temperature 55 °C with NH4VO3 catalyst to adipic acid. This process gives adipic acid in 90-98 % yield. But there is a disadvantage associated with the nitrous oxide (NOх) formation. The nitrous oxide known to cause global warming is emitted in large volumes despite the efficient recovery. On the first stage of this process aside from the desired products – cyclohexanone and cyclohexanol, the by-products – hydroxycaproic acid and ε-caprolactone, form in 30 % yield.
The research was aimed at increasing of adipic acid total yield by the oxidation of by-products. The aqueous hydrogen peroxide (30-35%) was used. It lets to carry out the oxidation in mild conditions. In addition, it is an economic and environmentally friendly process.
ε-Caprolactone known to transform to hydroxycaproic acid in aqueous solution. The oxidation of hydroxycaproic acid to adipic acid proceeds in the presence of peroxopolyoxo tungstates forming in situ in the system H3PW12O40 + H2O2 + H2O.
It was found that the catalytic oxidation of hydroxycaproic acid by aqueous hydrogen peroxide leads to the formation of adipic acid with a yield of up to 91 %.
The results obtained demonstrate the promise of this approach for adipic acid yield increase.