Efficient Hydrolytic Degradation of Polyamide 6 Using a Recyclable Solid Phosphoric Acid Catalyst

The recycling treatment of plastic products faces significant challenges. Polyamide 6 (PA6), as extensively researched and industrially recycled plastic material, has a high pollution emission rate from traditional treatment methods and can not solve the problem at the root, so it is necessary to develop a new catalyst for the chemical recycling of PA6. In this paper, the degradation effect of the new solid phosphoric acid (SPA) on PA6 was systematically investigated, and the results showed that, compared with other homogeneous catalysts, the SPA showed a better catalytic effect in the target monomer recovery and meanwhile avoided reaction device corrosion, which was more in line with green recycling and environmental protection needs. Compared with the noncatalyst condition, the complete hydrolysis time of PA6 was shortened from more than 10 to 6 h after adding SPA at 220 °C. The recovery of the monomer ε-caprolactam after hydrolysis catalyzed by SPA was also higher under the same reaction time, and the monomer yield reached 77% at 10 h. The catalyst demonstrated excellent recyclability after generation with poly(ethylene glycol) (PEG-400), maintaining high PA6 degradation rates (95%) and the monomer yield (65%) over several generations. Therefore, this solid phosphoric acid catalyst provides a different idea for the high-value recycling of PA6.