A novel production route for nylon-6: aspects of microwave-enhanced catalysis

Summary: Microwave irradiation was used for the amidation of a nitrile with an amine with a freshly prepared zirconium-based heterogeneous catalyst. Microwave irradiation selectively heats the catalyst which enhances its activity as compared to conventional heating. The difference between microwave heating and conventional heating disappears when Zr(OH)4 is used instead of ZrO2, indicating a microwave-induced shift in the hydrolysis equilibrium, i.e. the distribution of ZrO2, ZrO(OH)2 and Zr(OH)4, of the zirconium-based catalyst. The catalyst efficiently catalyzes the amidation of valeronitrile with n-hexylamine with conventional as well as with microwave heating. Zr(OH)4 was also used for the polymerization of 6-aminocapronitrile using conventional and microwave heating. With both heating methods a relatively low molecular weight polymer with a Mn of 4000¿g/mol was obtained in a sealed vessel, due to the presence of water and ammonia. A post-polymerization step under microwave irradiation, with active removal of water and ammonia shifts Mn to 10000¿g/mol. Pressure decrease to facilitate water removal resulted in products with higher molecular weights. A pressure reduction to 50¿Pa and operation in an argon atmosphere at 230¿°C resulted in nylon-6 with a Mn of 65000 in rather short reaction times. Lower pressures led to end-biting and evaporation of the volatile e-caprolactam at 230¿°C. As a consequence the resulting product has than a much lower molecular weight. The combination of a heterogeneous zirconium based catalyst and microwave heating is promising for process intensification for nylon-6 production