Catalytic hydrolysis of waste nylon 6 to produce ɛ-caprolactam in sub-critical water

The wide use of nylon 6 (PA 6) resulting in the resource recovery of waste PA 6 is urgent and its hydrolysis in sub-critical water to produce ɛ-caprolactam is a very important method. Hydrothermal diamond anvil cell (HDAC) was used to in situ observe transformation of PA 6 in water up to 513 K. The result shows that PA 6 melts at 488 K, and then hydrolyzes to produce low molecular dissoluble compounds such as 6-aminocaproic acid and ɛ-caprolactam from cleavage of acyl–amido bond. Different from the microreactor (about 0.075 μl) of HDAC, degradation of PA 6 in hydrothermal vessel of 18 ml is far slow, so the higher temperatures of 553, 573, 583, and 603 K are tried. As for hydrothermal vessel runs, liquid products after degradation are mainly composed of ɛ-caprolactam. Phosphotungstic heteropoly acid (HPA) as catalyst can improve hydrolysis rate and yield of ɛ-caprolactam, and 3% HPA can attain preferable effect. The optimum hydrolysis conditions for ɛ-caprolactam are as follows: HPA amount is 3%, reaction temperature is 573 K and reaction time is 85 min, and the yield of ɛ-caprolactam is 77.96%. In the temperature range of 553–603 K, the non-catalytic and catalytic degradation kinetics are studied as pseudo-first-order reaction and apparent activation energy is 86.64 and 77.38 kJ/mol for no catalyst and 3% HPA, respectively.