The Mechanochemical Beckmann Rearrangement over Solid Acids: From the Ball Mill to the Extruder
Daniel M. Baier, Tilo Rensch, Desislava Dobreva, Carolina Spula, Stephen Fanenstich, Marisol Fabienne Rappen, Konrad Bergheim, Sven Grätz, Lars Borchardt
Abstract
Abstract 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.