A large-scale continuous-flow process for the production of adipic acid via catalytic oxidation of cyclohexene with H2O2

The demand for a clean production process of adipic acid (AA) can be achieved by developing a synthetic route using H2O2 as the oxidant. In this paper, a green process with a recyclable catalyst system consisting of H2WO4, H2SO4 and H3PO4 was developed for the production of AA via catalytic oxidation of cyclohexene. A continuous-flow reactor was set up for the optimization of the reaction parameters and developing the industrial operation of this green process. The mixture of H2SO4 and H3PO4 as acidic promoter displays a significant improvement in the activity of catalyst and the stability of H2O2. The catalyst could be recovered and reused 20 times, and no significant loss of catalytic performance can be observed. The effect of Fe3+ ion as a possible contaminant has no serious negative effect on this reaction, and the 316L stainless steel and glass-lined steel were selected as appropriate equipment material. Calorimetry and the scale-up in batch reactor demonstrate that the reaction could be operated safely on scale. The process was scaled up in a continuous-flow pilot plant, with excellent yield (94.7%) and purity (99.0%). Some advantages such as the solvent-, phase-transfer-catalyst-, and organic additive-free and low-cost light up the application of this process in the industrial production of AA.