Packed-bed microreactor for continuous-flow adipic acid synthesis from cyclohexene and hydrogen peroxide

The synthesis of adipic acid by means of a direct oxidation of cyclohexene by hydrogen peroxide was carried out in a continuous-flow packed-bed microreactor. The reaction uses Na2WO4·2H2O as a catalyst and [CH3(n-C8H17)3N]HSO4 as a phase transfer catalyst without additional solvent. A parametric process optimization, such as glass beads size, residence time, concentration of hydrogen peroxide, addition of acid, reaction temperature, and molar ratios of reactants and catalysts, was performed. It is demonstrated that smaller glass beads result in a better yield of adipic acid and that the addition of acid plays an important role in the oxidation of cyclohexene to adipic acid. Based on the concept of Novel Process Windows, the influence of elevated temperatures is investigated.