Co-hydrothermal liquefaction of polyamide 6 and polycarbonate: Synergistic depolymerization mechanisms via experiments and DFT calculations

Green recycling of heteroatomic plastics, such as polyamide (PA6) and polycarbonate (PC), is essential for addressing plastic pollution and achieving carbon neutrality. Existing research has focused on a single-plastic treatment paradigm, limiting theoretical support for designing synergistic depolymerization pathways. This gap hinders the development of large-scale plastic recycling processes. In this study, we applied hydrothermal liquefaction (HTL) technology to treat PA6, PC, and their mixtures at 240–300 °C for 1 h. Solid residue and aqueous/oil-phase products were analyzed to uncover synergistic mechanisms, complemented by density functional theory (DFT) calculations. Experimental results showed that the degradation efficiency of PA6 and PC co-HTL at 270 °C reached 100 %, apparently higher than that of individual depolymerization tests (PA6: 75.0 %, PC: 34.2 %). Main products included PC-derived bisphenol A, 4-isopropenylphenol, phenol, and PA6-derived caprolactam (CPL), dimer, and oligomers. PA6 degradation products were water-soluble, while PC products (except phenol) were oil-soluble, enabling easy separation. DFT calculations verified that CPL or 6-aminocaproic acid (6-ACA) facilitates PC aminolysis via nucleophilic addition-elimination. Using diphenyl carbonate as a model compound, its depolymerization energy barrier was reduced to 162.2 kJ/mol in the presence of 6-ACA, a 62.1 kJ/mol reduction from the simple hydrolysis barrier (224.3 kJ/mol). Concurrently, carbonic acid (an intermediate from PC hydrolysis) functioned as an acidic catalyst, accelerating PA6 hydrolysis. These findings advance HTL technology for plastic recycling, contributing to a circular plastic economy and sustainable future.