The conventional preparation method of adipic acid is divided into two steps. First, in the presence of cobalt catalyst and anhydrous metaboric acid, cyclohexane is oxidized by air into a mixture of cyclohexanol and cyclohexanone. The KA oil is then oxidized to adipic acid.
In the current industrial process, adipic acid is mainly synthesized by oxidation of KA oil, using 50-60% nitric acid as the oxidant and copper/ammonium metavanadate as the catalyst. However, this process emits nitrous oxide, which causes ozone consumption, acid rain and global warming. Furthermore, the application of phase transfer catalysts on an industrial scale is expensive. Therefore, we need to develop more sustainable hexanedioic acid manufacturing processes that avoid the use of toxic reagents and tedious product isolation.
One-step oxidation of cyclohexane
Using Using cyclohexane as raw material, acetic acid as solvent, cobalt and bromide as catalyst, react at 2MP and 90°C for 10-13h. The yield is about 75%.
Fractional Oxidation of Cyclohexane
The preparation of KA oil can be directly oxidized with air at 1.0~2.5MPa and 145~180℃, and the yield can reach 70%~75%. Metaboric acid can also be used as a catalyst, air oxidation at 1.0-2.0MPa and 165°C, the yield can reach 90%, and the ratio of alcohol to ketone is 10:1. The reactants are treated with hot water to hydrolyze and separate the esters, and the water layer recovers boric acid, which is dehydrated into metaboric acid for recycling. The organic layer is saponified with caustic soda to remove the acid, and distilled to recover cyclohexane to obtain a mixture of alcohol and ketone.
Oxidize KA oil at 60-80°C and 0.1-0.4MPa with an excess of 50%-60% nitric acid in a two-stage series reactor. The catalyst is copper-vanadium series, and the yield is 92% to 96% of the theoretical value. After the nitric acid is evaporated from the reactant, high-purity adipic acid powder can be obtained through two crystallization refinements.