Enhanced hydrothermal conversion of caprolactam from waste monomer casting polyamide over H-Beta zeolite and its mechanism

- Zeolite H-Beta was applied in catalyzed hydrothermal decomposition of waste MC nylon and proved to be effective.
- The reaction pathway analyses revealed a less thermodynamically stable crystal phase appeared in the solid product.
- The water-soluble oligomers are intermediates which were further converted to the target monomer ε-caprolactam.
- The selective generation and consumption of the linear products by zeolite catalyst facilitated the hydrolysis.

The exploration of reaction process plays an important role in the mechanism study of plastic hydrothermal recovery. This work investigated the procedure of the efficient heterogeneous catalytic hydrolysis of waste monomer casting polyamide in subcritical water and then proposed the depolymerization mechanism. The solid products obtained before the complete hydrolysis became less thermodynamically stable than raw polyamide material as the degree of hydrolysis increased. According to the characterization of liquid phase products, the monomer casting polyamide was depolymerized to cyclic and linear oligomers, which were further converted to the target monomer ε-caprolactam by gradual chain scission. As firstly applied in the waste polyamide hydrolysis, the zeolite H-Beta was proved a promising solid catalyst which increased the degree of hydrolysis from 31% to more than 60% (300 °C, 50 min) and facilitated to produce the intermediates with specific structure due to the microporous topology. The kinetic analyses showed that compared to the formation step of oligomers, the hydrolysis of oligomers into the monomer was the critical step which could be accelerated by the zeolite H-Beta catalyst.