The Mechanochemical Beckmann Rearrangement over Solid Acids: From the Ball Mill to the Extruder

Beckmann rearrangement was carried out in the solid state in a ball mill using metal oxides as solid acids. After a comprehensive investigation of different reaction parameters, acids as well as further additives, a combination of aluminosilicate materials, phosphorus pentoxide, and para-toluenesulfonic acid was identified as the optimal system. This allowed the model compounds ϵ-caprolactam and acetanilide to be obtained in yields of 46 % and 94 %, respectively, while the robustness of the method was demonstrated by applying it to additional substrates. Finally, we scaled up our optimized reaction into a continuous process using a twin screw extruder. With this, yields beyond 90 % could be achieved in a residence time as low as seven minutes.