A modified mixed-acid catalytic system for Beckmann rearrangement of cyclohexanone oxime

ε-Caprolactam (CPL) is mainly produced through Beckmann rearrangement of cyclohexanone oxime catalyzed by oleum in industry nowadays, which suffers the problems like high viscosity and bad mixing, leading to multiple by-products. To this end, a modified mixed-acid catalytic system containing oleum and trifluoroacetic acid (TFA) was proposed to intensify the reaction. The viscosity of the reaction system was studied and the effects of several experimental parameters on the reaction rate and selectivity were also investigated, including oleum/oxime ratio, TFA/oleum ratio, temperature, and SO₃ concentration. Extremely high selectivity (>99.6%) was obtained under all conditions, revealing that by-products in industrial process were mainly formed owing to the insufficient mixing and poor mass and heat transfer rather than experimental parameters. A preliminary mathematical model considering the effect of CPL was developed to predict the reaction rate. These results are helpful to improve the industrial process of CPL production.