Ultra-low formation of octahydrophenazine in the Beckmann rearrangement of cyclohexanone oxime using a microreactor†
Reaction solution circulation and control of heavy by-products are two crucial problems of the caprolactam industry with the consideration of energy conservation. To this end, the Beckmann rearrangement of cyclohexanone oxime, which was catalyzed by a mixed-acid catalyst containing trifluoroacetic acid (TFA) and oleum, was conducted in a microchemical system. The effects of experimental parameters on the heavy by-product, octahydrophenazine (OHP), were assessed, including the molar ratio of TFA to oleum (T/A ratio), molar ratio of oleum to oxime (A/O ratio), SO3 mass fraction, temperature, and TFA distribution between two fluids (D(TFA)). The content of OHP was decreased with the increase of the A/O ratio, T/A ratio, and SO3 mass fraction and a suitable D(TFA) and temperature within the range of experiments. Meanwhile, the adiabatic operation was found favorable and an ultra-low content (<1.0 ppm) of OHP was realized under the optimum conditions. Besides, potential reaction pathways of OHP formation were also proposed and discussed based on the experimental evidence. These results are therefore expected to contribute to the process design and optimization for the industrial production of CPL.